Univesity of Ljubljana

Faculty of Mathematics and Physics

The Manhattan Project

Bor Gregorčič

Advisor doc dr Simon Širca

March 2011

Abstract

The Manhattan Project was an organized effort of the USA and its allies in the World War 2 towards creating a practical military weapon in which the energy is released by a fast neutron chain reaction in one of the then known materials that showed nuclear fission The tasks met by the Manhattan Project were separation of U-235 from natural uranium and the production of plutonium and making a disposable bomb The task of engineering the actual explosive device was given to a group of scientists at Los Alamos who had to design and test the first nuclear bombs before they were used in combat This paper discusses events discoveries and procedures that led to the creation of the atomic bomb It emphasizes the scientific aspects but also gives some attention to the organization and coordination of such a overwhelmingly big project

1 Discoveries that led to the discovery of fissionIn 1932 [1] James Chadwick Ernest Rutherfords colleague at Cambridge found a new sub-atomic particle and gave it the name neutron because it had no electrical charge therefore it was neutral The discovery of the neutron filled a big gap in understanding the atomic world The atomic number was determined by the number of protons in the nucleus and the mass number was determined by the number of protons and neutrons in the nucleus It became apparent that an element of same atomic number could exist with different mass number Isotopes were discovered and they turned out to be present in almost every naturally occurring substance For example hydrogen lithium nitrogen oxygen Isotopes of elements had the same atomic number but possessed a different number of neutrons The most abundant of the isotopes turned out to be the most stable There were some that were present in very small quantities because of their short decay time after which they decayed into some other element or just another isotope of the same element Some of these isotopes were so short lived that they could not be found in nature any moreAlso in 1932 Freacutedeacuteric Joliot and Irene Joliot-Curie discovered another peculiar phenomenon They were observing a flux of neutrons caused by bombardment of aluminum by alpha particles When they stopped the flux of incoming alpha particles the radiation detector did not stop counting The flux gradually diminished They have created a short lived radioactive isotope of phosphorus with a decay time of only few minutes It was the first man made radioactive substance [1]The energy released in a nuclear reaction was fairly large in comparison with chemical reactions But in the early 1930s achieving nuclear reactions in a laboratory was extremely difficult Energies needed to get a positively charged proton or an alpha particle past the electric barrier of the target nucleus were very large Also achieving an actual collision of two nuclei was very difficult The projectile particles would scatter away from the targets much rather then hitting them Only once in a million tries would such a projectile actually hit the nucleus and cause a nuclear reaction Therefore most scientists some of them very prominent like Einstein Rutherford and Bohr thought that using the energy bound in the nucleus is a very difficult task probably not achievable in the near future Einstein compared the particle bombardment to shooting at scarce birds in the dark Bohr believed that chances of taming the nuclear energy were remote and Rutherford called it moonshine in an interview he gave in 1933 [1] But this was about to change A young Italian physicist the youngest professor at the Roman university after Galileo Galilei started to experiment with a newly discovered particle ndash the neutronHe was bombarding nuclei of different elements with neutrons and discovered that they were more effective in causing nuclear reactions than the charged protons and alpha particles Possessing no electrical charge they penetrated with no resistance at all the electrostatic barrier of the nucleus and collided with it causing it to change in some way He found 4 ways in which a neutron can change the target nucleus One was a capture of the neutron the second was capture of the neutron and knocking out an alpha particle the third was capture of the incoming neutron while knocking out a proton and the fourth was knocking out a neutron reducing the target nucleus weight by one unit [2] Another discovery he made was that slow neutrons are in contrast to slow protons even more effective in causing reactions He found that carbon and hydrogen were good moderators for neutrons effective in slowing them down He and his coworkers created more than 30 new isotopes of different elements by bombarding them with slow neutrons [2]The heaviest of the then known elements was uranium It was a matter of scientific debate at that time what Fermi had produced by bombarding uranium with neutrons Some thought that the products were new transuranic elements but they seemed to resemble some much lighter elements by their chemical properties Fermi himself wasnt sure either The answer to that question came from Nazi Germany in December 1938

2 The discovery of fissionTwo German radio-chemists Otto Hahn and Fritz Strassmann were bombarding uranium with neutrons at their laboratory in Berlin when they discovered that uranium reacts rather unusual under neutron bombardment As other elements changed somewhat uranium changed a lot At first it seemed impossible but after careful chemical analysis they found that uranium nucleus when bombarded especially with slow neutrons splits into two approximately equal sized nuclei The substances Fermi had found in his experiments more than resembled lighter elements ndash they were lighter elements But the sum mass of the products of such a split did not equal the mass of the uranium nucleus [3] Therefore by the Einsteins

2

equation E = mc2 (1) this meant that the missing mass had to be transformed into energy that would be subsequently turned into heat Hahns former coworker in Vienna Lise Meitner who escaped from Nazi Germany to Sweden and her nephew Otto Frisch calculated that the energies released in such a reaction are of such scale that a new type of nuclear reaction has been observed Frisch named it fission after a biological process of cell division called binary fission [1]Fission of uranium nuclei soon proved to have another important property While the large uranium nucleus split into two smaller nuclei some neutrons were emitted during the split That meant that with each uranium nucleus splitting there were some additional neutrons acquired as a final product of fission These neutrons would propagate trough the material and could if they havent escaped from it cause another uranium nucleus to fission emitting even more neutrons If each splits yield of neutrons be greater than one that could cause in large enough mass of uranium a chain reaction in which the amount of released energy would increase exponentially with every generation of newly born neutrons [1] An idea of a chain reaction altered the prospects of harnessing the energy stored in the atomic nucleus A reaction that would be self sustaining and controlled at the same time could be a useful source of energy while an uncontrolled nuclear chain reaction would release increasingly large amounts of energy and would lead to great heating and possibly an explosionOne of the first scientists who saw the potential benefits and dangers of such a reaction was Leo Szilard[1]

Hahn and Strassmann informed Niels Bohr of their discovery in 1939 just before Bohr had left Denmark to travel to the USA where he announced the new discovery to some emigrant scientists from Europe and to the American scientific community at the opening session of a conference on theoretical physics on January 26 1939 in Washington DC [1] The Nazi regime forced many prominent scientists especially those of Jewish descent to migrate out of Germany Many of them found refuge in the United States including Einstein who played a major role in persuading the American government that research of the new phenomena especially nuclear fission is a matter of national security [1] The American physicists have throughout the 1930s developed some equipment that was vital for the research of nuclear phenomena One of the greatest and soon to be of essential importance was Lawrences cyclotron The American scientists started to actively participate in theoretical and experimental research of fission in uranium Niels Bohr and John A Wigner did some of the most important theoretical work in Princeton while Leo Szilard and Enrico Fermi started to investigate the possibility of a chain reaction in uranium on Columbia University [3] By March 1940 it was known that only one of uranium isotopes undergoes fission when bombarded with slow neutrons It was the less abundant U-235 which represented only 07 of natural uranium the majority being U-238 U-238 could also fission under very fast neutron bombardment but could not sustain a nuclear chain reaction The reason it couldnt was that too few neutrons produced by such fission possessed enough energy to cause further fission [2] An absorption of a neutron made available 53 MeV of binding energy while U-238 needed roughly 6 MeV to fission That meant that slow neurons could not cause it to fission and only neutrons of about 1 MeV of energy would do the job On the other hand U-235 also earned 53 MeV by capturing a neutron but gained

Figure 1 [5]

Representation of a fissioning nucleus A large nucleus gets hit by a neutron splitting in two roughly equally sized nuclei and emmiting 3 neutrons in the process The newly generated neutrons cause another fission if they encounter a fissionable nucleus [1]

3

some additional energy by turning from odd to even mass The sum of both was always greater than 6 MeV which meant that any neutron would cause U-235 to split [1] In such a reaction enormous quantities of energy about 200 MeV per fission would be released

4

Figure 2 [7]

The fission cross-sections as they were measured decades after the discovery of fission In the resonance region there is also the U-238 capture resonance (not shown on the graph) Thermal neutrons have energies of rougly 004 eV (far left on the graph) The U-238s neutron capture cross-section remains about 10 barns at that energies while the fission cross-section for U-235 is much higher about 2 orders of magnitude

Figure 3 [9]

An example of one of the many possible reactions when a neutron hits the U-235 nucleus

This discovery was of major importance since it meant that a nuclear explosion could only occur in U-235 if this was possible at all The slightly lighter isotope had to be in order to create such an explosion separated from the more abundant U-238 A task which presented a great challenge given the available technology and resourcesIn 1939 the second world war started with Hitlers army attacking Poland It became apparent that if a weapon employing nuclear fission as its energy source could be developed it would mean a huge advantage for anyone that possessed it Coming to the idea of a nuclear weapon themselves the American scientist couldnt help but to imagine that the Nazi Germany was already researching the possibilities of such a weapon A calculation showed that 1 kg of U-235 that fissioned would produce an explosive yield equivalent to about 20000 tons of TNT [3] The idea of Hitler having such a weapon at his disposal filled the American scientists many of them Jewish emigrants from Europe with dread Since the very discovery of fission came from Germany they imagined that the Germans already had some advantage in the research Having occupied Belgium it became available to them the high quality uranium ore from the Belgian Congo [1]Fermi and Szilard started to investigate the possibility of a sustained chain reaction in natural uranium They knew they had to slow down the neutrons that were emitted during fission to energies below 25 eV that being the capture resonance of neutrons by U-238 [3] This was important because in order to sustain a chain reaction as much neutrons as possible would have to cause further fissioning of U-235 In order to slow neutrons down but not diminish their number they had to find an appropriate moderator Fermi had previously found hydrogen and carbon to be of use in slowing neutrons Hydrogen proved to have too large capture cross-section which meant that it could not be used as a moderator for a natural uranium reactor The moderator of choice for Fermi and Szilard was graphite Szilard in particular spent a lot of time searching for a good source of graphite and testing it for impurities The Germans on the other hand have by occupying Norway gained a source of heavy water Heavy water contained the heavier isotope of hydrogen ndash deuterium which had much smaller capture cross-section for neutrons than ordinary hydrogen This was just another fact that worried the American scientistsThe threat of Germany producing a nuclear weapon was a real one decided Szilard So he visited Albert Einstein briefed him on the knowledge he had on the topic (most of it was kept secret among the American scientists in order to keep it from the Germans) and together they wrote a letter to the president of the United States Franklin D Roosevelt In the letter they urged him and his administration to start an organized effort one of national proportions towards creating an atomic bomb Though many scientists at that time were still unsure whether such a bomb was possible to Einstein and Szilard this was not of main importance If they found out in the procedure that such a weapon is indeed impossible to make this would be just as rewarding as making one before Hitler did However the risk America would be taking by not starting such a project would be unacceptable [1]

3 Manhattan Engineer DistrictThe bomb project fell under the Armys authority in mid 1942 and was led by Major General Groves United States Army Corps of Engineers officer who oversaw the construction of Pentagon [1] He gave the project the name Manhattan Engineer District later to be called The Manhattan Project In order to continue research on uranium fission enriched U-235 had to be obtained And if a bomb was to be made large amounts of U-235 would be needed Since the U-238 and U-235 are chemically identical they had to be separated physically All the separation methods relied on the small difference in mass of the two isotopes As their mass differed only by roughly one percent all physical means of separation would be extremely complicated and expensive The Manhattan Projects goal was to obtain fissile material for an atomic bomb by constructing necessary facilities

5

4 Isotope separationThere were 3 isotope separation processes that could prove to be of use at the time The first was gaseous diffusion This method would employ diffusion of UF6 gas trough a porous barrier The lighter isotope would diffuse through the barrier faster then the heavier isotope Continuous separation of the enriched gas trough thousands of such barriers each one enriching the UF6 a little further would result in high purity U-235 This was at the beginning the most promising method but the researchers encountered many difficulties before it became useful [3]

The second method was the electromagnetic separation It was discovered by Alfred O Nier and it was basically a mass spectrograph Charged particles flying trough a magnetic field deflect differently according to their mass Lawrence who was head of the electromagnetic separation research converted his cyclotron into a caultron a device created specially for separation of uranium for the Manhattan Project At first a yield of such a machine was very low but as with all the separation methods an immense amount of research and funds had to be spent on improving it in order to make it useful [3]

The radius of a charged particle in a homogenous magnetic field

r=mvqB

All the ionized particles are accelerated with the same voltage

E pot = V d = E kin = mv2

2

Therefore they have the same kinetic energy m 1 v 1

2

2= m 2 v 2

2

2

v 2 = m 1

m 2

v 1

r 2 = m 2 v 1

qB m 1

m 2

The radius of the second particle is larger if the mass of the second particle is larger This was the principle of electromagnetic isotope separation

r 2 = m 1 v 1

qB m 2

m 1

= r 1 m 2

m 1

Many scientist thought that centrifuge was the most prominent of the methods Centrifugal force in a barrel spinning around its vertical axis would separate the isotopes in a gaseous form The heavier U-238 would be drawn to away from the center leaving behind slightly enriched gas which would then have to be pumped from the top center of the cylindrical barrel This enriched gas would then be pumped into another spinning cylinder and further trough hundreds or maybe even thousands more to achieve the desired purity But this method soon encountered difficulties and was abandoned for the needs of the Manhattan Project [3]

6

Figure 4 [8]

A scheme of a gaseous diffusion isotope separation process

(5)

(3)

(6)

(7)

(2)

(4)

5 PlutoniumHowever Lawrences mass spectrograph gave Glenn T Seaborg a researcher at Berkley encouraging results In February 1941 he identified a new trans-uranic element of mass 239 and atomic number 94 ndash plutonium Plutonium was a decay product of neptunium which itself was a decay product of U-238 that captured a neutron When U-238 captures a neutron it becomes unstable and decays with a half life of only 235 minutes into neptunium which also decays by another beta decay to plutonium the half-life of neptunium being 236 days Plutonium proved to be excellent fission material since its fission cross-section for slow neutrons was even larger than that of U-235 namely 17 times larger [1]This discovery gave Fermi-Szilards reactor a needed push A sustained and controlled nuclear chain reaction in natural uranium would produce plutonium since many neutrons would even if the reaction was well moderated get captured by U-238 The produced plutonium could then be used to make a fission bomb Plutonium had a huge advantage over U-235 Not being just another isotope of uranium it could be chemically separated from the other reaction products But to be sure that could be done they had to confirm that plutonium is indeed separable from uranium The research of this newly discovered element which was completely unknown was a very difficult task Seaborgs research team managed to separate a microscopic sample of pure plutonium from irradiated uranium in August 1942 This was a major chemical achievement The procedure would have to be rescaled by a billion times to get sufficient amounts of it to produce a plutonium weapon [1]

6 Chicago Pile 1The confirmation of separability of plutonium from uranium gave Fermi-Szilards uranium reactor another boost Fermi called the experiment uranium pile since it was essentially a pile of 60 tons of uranium oxide 6 tons of metal uranium and 400 tons of graphite arranged in an almost spherical configuration with long holes in which cadmium rods were placed [3] The cadmium rods would be drawn out of the pile to increase neutron activity to a level at which a chain reaction would be self sustaining and would be re-inserted into the pile to dampen the reaction The pile was originally supposed to be of roughly spherical shape but it

7

Figure 5 [10]

Plutonium is the most complicated and strange element an engineers nightmare It is extremely reactive reacts in many different ways and it changes its mechanical properties when heated or cooled The δ-phase is the most like normal metals and can be stabilized at room temperatures by chemical procedures

turned out in Fermis calculations that the critical mass of natural uranium was actually lower than first expected The pile therefore didnt have to be finished according to the original plan On December 2 nd 1942 at 325 pm under Fermis surveillance the pile went critical The cadmium rods were removed from the pile inch by inch with Fermi constantly watching the radiation counters The first ever man induced nuclear chain reaction ran at average power of 05 W for one half an hour Immediately after the reaction was terminated an encoded telephone call was made to the chairman of the National Defense Research Committee that was in charge of coordinating the nuclear research at the time [3] The news of a successful chain reaction made it to Roosevelt who approved on 28 of December 1942 05 million dollars for the construction of a full scale gaseous diffusion plant a plutonium plant and a smaller electromagnetic plant The caultrons still werent able to produce military significant amounts of U-235 but were still the only reliable source of it At the time it was estimated that the first bombs could be completed in the first half of 1945 but no sooner [3]

7 Clinton Engineer Works Oak Ridge TennesseeThe Manhattan Project moved its headquarters and most of the staff to a new location The army bought a piece of land roughly 230 km2 large just west of Knoxville Tennessee There a whole new town was built especially for the needs of the Manhattan project There were 2 uranium separation sites and one plutonium pile in Oak Ridge 15000 thousand construction workers were residing in a trailer camp building the biggest building in the world at the time ndash K-25 [3] K-25 was a gaseous diffusion plant Its construction started in 1943 and it was completed in early 1945 At the peak of construction in 1944 the population of Oak Ridge originally planned to be 13000 rose to 50000 The construction cost of the K-25 was 500 million dollars [3] (61 billion in 2009 dollars) It was the most promising of all the separation facilities but soon encountered problems The main problem was to find a suitable barrier that wasnt susceptible to highly corrosive UF6 The barrier crisis was so great that Groves ordered an expansion of the electromagnetic facility and to build another facility next to the K-25 power plant The new facility was a liquid thermal diffusion plant It utilized the fact that lighter atoms diffuse towards the hot surface while the heavier atoms towards the colder surface This was done in a vertically positioned cylinder with a steam pipe along its axis The lighter U-235 would diffuse towards the center of the cylinder and rise due to convection current The enriched liquid would be collected at the top of the cylinder [1] This was a cheap way of separating isotopes but it was not capable of producing highly pure U-235 Its slightly enriched product was used merely as a feed for the electromagnetic plant that was the

Figure 6 [5] A drawing of Chicago Pile 1 the first nuclear reactor to be constructed It was built under the stands of the University of Chicagos football stadium on a squash court Fermi Szilard Compton and other scientists and techincal personell were attending the December 2nd experiment when a critical chain reaction in natural uranium was established for the first time in history

8

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

1 Discoveries that led to the discovery of fissionIn 1932 [1] James Chadwick Ernest Rutherfords colleague at Cambridge found a new sub-atomic particle and gave it the name neutron because it had no electrical charge therefore it was neutral The discovery of the neutron filled a big gap in understanding the atomic world The atomic number was determined by the number of protons in the nucleus and the mass number was determined by the number of protons and neutrons in the nucleus It became apparent that an element of same atomic number could exist with different mass number Isotopes were discovered and they turned out to be present in almost every naturally occurring substance For example hydrogen lithium nitrogen oxygen Isotopes of elements had the same atomic number but possessed a different number of neutrons The most abundant of the isotopes turned out to be the most stable There were some that were present in very small quantities because of their short decay time after which they decayed into some other element or just another isotope of the same element Some of these isotopes were so short lived that they could not be found in nature any moreAlso in 1932 Freacutedeacuteric Joliot and Irene Joliot-Curie discovered another peculiar phenomenon They were observing a flux of neutrons caused by bombardment of aluminum by alpha particles When they stopped the flux of incoming alpha particles the radiation detector did not stop counting The flux gradually diminished They have created a short lived radioactive isotope of phosphorus with a decay time of only few minutes It was the first man made radioactive substance [1]The energy released in a nuclear reaction was fairly large in comparison with chemical reactions But in the early 1930s achieving nuclear reactions in a laboratory was extremely difficult Energies needed to get a positively charged proton or an alpha particle past the electric barrier of the target nucleus were very large Also achieving an actual collision of two nuclei was very difficult The projectile particles would scatter away from the targets much rather then hitting them Only once in a million tries would such a projectile actually hit the nucleus and cause a nuclear reaction Therefore most scientists some of them very prominent like Einstein Rutherford and Bohr thought that using the energy bound in the nucleus is a very difficult task probably not achievable in the near future Einstein compared the particle bombardment to shooting at scarce birds in the dark Bohr believed that chances of taming the nuclear energy were remote and Rutherford called it moonshine in an interview he gave in 1933 [1] But this was about to change A young Italian physicist the youngest professor at the Roman university after Galileo Galilei started to experiment with a newly discovered particle ndash the neutronHe was bombarding nuclei of different elements with neutrons and discovered that they were more effective in causing nuclear reactions than the charged protons and alpha particles Possessing no electrical charge they penetrated with no resistance at all the electrostatic barrier of the nucleus and collided with it causing it to change in some way He found 4 ways in which a neutron can change the target nucleus One was a capture of the neutron the second was capture of the neutron and knocking out an alpha particle the third was capture of the incoming neutron while knocking out a proton and the fourth was knocking out a neutron reducing the target nucleus weight by one unit [2] Another discovery he made was that slow neutrons are in contrast to slow protons even more effective in causing reactions He found that carbon and hydrogen were good moderators for neutrons effective in slowing them down He and his coworkers created more than 30 new isotopes of different elements by bombarding them with slow neutrons [2]The heaviest of the then known elements was uranium It was a matter of scientific debate at that time what Fermi had produced by bombarding uranium with neutrons Some thought that the products were new transuranic elements but they seemed to resemble some much lighter elements by their chemical properties Fermi himself wasnt sure either The answer to that question came from Nazi Germany in December 1938

2 The discovery of fissionTwo German radio-chemists Otto Hahn and Fritz Strassmann were bombarding uranium with neutrons at their laboratory in Berlin when they discovered that uranium reacts rather unusual under neutron bombardment As other elements changed somewhat uranium changed a lot At first it seemed impossible but after careful chemical analysis they found that uranium nucleus when bombarded especially with slow neutrons splits into two approximately equal sized nuclei The substances Fermi had found in his experiments more than resembled lighter elements ndash they were lighter elements But the sum mass of the products of such a split did not equal the mass of the uranium nucleus [3] Therefore by the Einsteins

2

equation E = mc2 (1) this meant that the missing mass had to be transformed into energy that would be subsequently turned into heat Hahns former coworker in Vienna Lise Meitner who escaped from Nazi Germany to Sweden and her nephew Otto Frisch calculated that the energies released in such a reaction are of such scale that a new type of nuclear reaction has been observed Frisch named it fission after a biological process of cell division called binary fission [1]Fission of uranium nuclei soon proved to have another important property While the large uranium nucleus split into two smaller nuclei some neutrons were emitted during the split That meant that with each uranium nucleus splitting there were some additional neutrons acquired as a final product of fission These neutrons would propagate trough the material and could if they havent escaped from it cause another uranium nucleus to fission emitting even more neutrons If each splits yield of neutrons be greater than one that could cause in large enough mass of uranium a chain reaction in which the amount of released energy would increase exponentially with every generation of newly born neutrons [1] An idea of a chain reaction altered the prospects of harnessing the energy stored in the atomic nucleus A reaction that would be self sustaining and controlled at the same time could be a useful source of energy while an uncontrolled nuclear chain reaction would release increasingly large amounts of energy and would lead to great heating and possibly an explosionOne of the first scientists who saw the potential benefits and dangers of such a reaction was Leo Szilard[1]

Hahn and Strassmann informed Niels Bohr of their discovery in 1939 just before Bohr had left Denmark to travel to the USA where he announced the new discovery to some emigrant scientists from Europe and to the American scientific community at the opening session of a conference on theoretical physics on January 26 1939 in Washington DC [1] The Nazi regime forced many prominent scientists especially those of Jewish descent to migrate out of Germany Many of them found refuge in the United States including Einstein who played a major role in persuading the American government that research of the new phenomena especially nuclear fission is a matter of national security [1] The American physicists have throughout the 1930s developed some equipment that was vital for the research of nuclear phenomena One of the greatest and soon to be of essential importance was Lawrences cyclotron The American scientists started to actively participate in theoretical and experimental research of fission in uranium Niels Bohr and John A Wigner did some of the most important theoretical work in Princeton while Leo Szilard and Enrico Fermi started to investigate the possibility of a chain reaction in uranium on Columbia University [3] By March 1940 it was known that only one of uranium isotopes undergoes fission when bombarded with slow neutrons It was the less abundant U-235 which represented only 07 of natural uranium the majority being U-238 U-238 could also fission under very fast neutron bombardment but could not sustain a nuclear chain reaction The reason it couldnt was that too few neutrons produced by such fission possessed enough energy to cause further fission [2] An absorption of a neutron made available 53 MeV of binding energy while U-238 needed roughly 6 MeV to fission That meant that slow neurons could not cause it to fission and only neutrons of about 1 MeV of energy would do the job On the other hand U-235 also earned 53 MeV by capturing a neutron but gained

Figure 1 [5]

Representation of a fissioning nucleus A large nucleus gets hit by a neutron splitting in two roughly equally sized nuclei and emmiting 3 neutrons in the process The newly generated neutrons cause another fission if they encounter a fissionable nucleus [1]

3

some additional energy by turning from odd to even mass The sum of both was always greater than 6 MeV which meant that any neutron would cause U-235 to split [1] In such a reaction enormous quantities of energy about 200 MeV per fission would be released

4

Figure 2 [7]

The fission cross-sections as they were measured decades after the discovery of fission In the resonance region there is also the U-238 capture resonance (not shown on the graph) Thermal neutrons have energies of rougly 004 eV (far left on the graph) The U-238s neutron capture cross-section remains about 10 barns at that energies while the fission cross-section for U-235 is much higher about 2 orders of magnitude

Figure 3 [9]

An example of one of the many possible reactions when a neutron hits the U-235 nucleus

This discovery was of major importance since it meant that a nuclear explosion could only occur in U-235 if this was possible at all The slightly lighter isotope had to be in order to create such an explosion separated from the more abundant U-238 A task which presented a great challenge given the available technology and resourcesIn 1939 the second world war started with Hitlers army attacking Poland It became apparent that if a weapon employing nuclear fission as its energy source could be developed it would mean a huge advantage for anyone that possessed it Coming to the idea of a nuclear weapon themselves the American scientist couldnt help but to imagine that the Nazi Germany was already researching the possibilities of such a weapon A calculation showed that 1 kg of U-235 that fissioned would produce an explosive yield equivalent to about 20000 tons of TNT [3] The idea of Hitler having such a weapon at his disposal filled the American scientists many of them Jewish emigrants from Europe with dread Since the very discovery of fission came from Germany they imagined that the Germans already had some advantage in the research Having occupied Belgium it became available to them the high quality uranium ore from the Belgian Congo [1]Fermi and Szilard started to investigate the possibility of a sustained chain reaction in natural uranium They knew they had to slow down the neutrons that were emitted during fission to energies below 25 eV that being the capture resonance of neutrons by U-238 [3] This was important because in order to sustain a chain reaction as much neutrons as possible would have to cause further fissioning of U-235 In order to slow neutrons down but not diminish their number they had to find an appropriate moderator Fermi had previously found hydrogen and carbon to be of use in slowing neutrons Hydrogen proved to have too large capture cross-section which meant that it could not be used as a moderator for a natural uranium reactor The moderator of choice for Fermi and Szilard was graphite Szilard in particular spent a lot of time searching for a good source of graphite and testing it for impurities The Germans on the other hand have by occupying Norway gained a source of heavy water Heavy water contained the heavier isotope of hydrogen ndash deuterium which had much smaller capture cross-section for neutrons than ordinary hydrogen This was just another fact that worried the American scientistsThe threat of Germany producing a nuclear weapon was a real one decided Szilard So he visited Albert Einstein briefed him on the knowledge he had on the topic (most of it was kept secret among the American scientists in order to keep it from the Germans) and together they wrote a letter to the president of the United States Franklin D Roosevelt In the letter they urged him and his administration to start an organized effort one of national proportions towards creating an atomic bomb Though many scientists at that time were still unsure whether such a bomb was possible to Einstein and Szilard this was not of main importance If they found out in the procedure that such a weapon is indeed impossible to make this would be just as rewarding as making one before Hitler did However the risk America would be taking by not starting such a project would be unacceptable [1]

3 Manhattan Engineer DistrictThe bomb project fell under the Armys authority in mid 1942 and was led by Major General Groves United States Army Corps of Engineers officer who oversaw the construction of Pentagon [1] He gave the project the name Manhattan Engineer District later to be called The Manhattan Project In order to continue research on uranium fission enriched U-235 had to be obtained And if a bomb was to be made large amounts of U-235 would be needed Since the U-238 and U-235 are chemically identical they had to be separated physically All the separation methods relied on the small difference in mass of the two isotopes As their mass differed only by roughly one percent all physical means of separation would be extremely complicated and expensive The Manhattan Projects goal was to obtain fissile material for an atomic bomb by constructing necessary facilities

5

4 Isotope separationThere were 3 isotope separation processes that could prove to be of use at the time The first was gaseous diffusion This method would employ diffusion of UF6 gas trough a porous barrier The lighter isotope would diffuse through the barrier faster then the heavier isotope Continuous separation of the enriched gas trough thousands of such barriers each one enriching the UF6 a little further would result in high purity U-235 This was at the beginning the most promising method but the researchers encountered many difficulties before it became useful [3]

The second method was the electromagnetic separation It was discovered by Alfred O Nier and it was basically a mass spectrograph Charged particles flying trough a magnetic field deflect differently according to their mass Lawrence who was head of the electromagnetic separation research converted his cyclotron into a caultron a device created specially for separation of uranium for the Manhattan Project At first a yield of such a machine was very low but as with all the separation methods an immense amount of research and funds had to be spent on improving it in order to make it useful [3]

The radius of a charged particle in a homogenous magnetic field

r=mvqB

All the ionized particles are accelerated with the same voltage

E pot = V d = E kin = mv2

2

Therefore they have the same kinetic energy m 1 v 1

2

2= m 2 v 2

2

2

v 2 = m 1

m 2

v 1

r 2 = m 2 v 1

qB m 1

m 2

The radius of the second particle is larger if the mass of the second particle is larger This was the principle of electromagnetic isotope separation

r 2 = m 1 v 1

qB m 2

m 1

= r 1 m 2

m 1

Many scientist thought that centrifuge was the most prominent of the methods Centrifugal force in a barrel spinning around its vertical axis would separate the isotopes in a gaseous form The heavier U-238 would be drawn to away from the center leaving behind slightly enriched gas which would then have to be pumped from the top center of the cylindrical barrel This enriched gas would then be pumped into another spinning cylinder and further trough hundreds or maybe even thousands more to achieve the desired purity But this method soon encountered difficulties and was abandoned for the needs of the Manhattan Project [3]

6

Figure 4 [8]

A scheme of a gaseous diffusion isotope separation process

(5)

(3)

(6)

(7)

(2)

(4)

5 PlutoniumHowever Lawrences mass spectrograph gave Glenn T Seaborg a researcher at Berkley encouraging results In February 1941 he identified a new trans-uranic element of mass 239 and atomic number 94 ndash plutonium Plutonium was a decay product of neptunium which itself was a decay product of U-238 that captured a neutron When U-238 captures a neutron it becomes unstable and decays with a half life of only 235 minutes into neptunium which also decays by another beta decay to plutonium the half-life of neptunium being 236 days Plutonium proved to be excellent fission material since its fission cross-section for slow neutrons was even larger than that of U-235 namely 17 times larger [1]This discovery gave Fermi-Szilards reactor a needed push A sustained and controlled nuclear chain reaction in natural uranium would produce plutonium since many neutrons would even if the reaction was well moderated get captured by U-238 The produced plutonium could then be used to make a fission bomb Plutonium had a huge advantage over U-235 Not being just another isotope of uranium it could be chemically separated from the other reaction products But to be sure that could be done they had to confirm that plutonium is indeed separable from uranium The research of this newly discovered element which was completely unknown was a very difficult task Seaborgs research team managed to separate a microscopic sample of pure plutonium from irradiated uranium in August 1942 This was a major chemical achievement The procedure would have to be rescaled by a billion times to get sufficient amounts of it to produce a plutonium weapon [1]

6 Chicago Pile 1The confirmation of separability of plutonium from uranium gave Fermi-Szilards uranium reactor another boost Fermi called the experiment uranium pile since it was essentially a pile of 60 tons of uranium oxide 6 tons of metal uranium and 400 tons of graphite arranged in an almost spherical configuration with long holes in which cadmium rods were placed [3] The cadmium rods would be drawn out of the pile to increase neutron activity to a level at which a chain reaction would be self sustaining and would be re-inserted into the pile to dampen the reaction The pile was originally supposed to be of roughly spherical shape but it

7

Figure 5 [10]

Plutonium is the most complicated and strange element an engineers nightmare It is extremely reactive reacts in many different ways and it changes its mechanical properties when heated or cooled The δ-phase is the most like normal metals and can be stabilized at room temperatures by chemical procedures

turned out in Fermis calculations that the critical mass of natural uranium was actually lower than first expected The pile therefore didnt have to be finished according to the original plan On December 2 nd 1942 at 325 pm under Fermis surveillance the pile went critical The cadmium rods were removed from the pile inch by inch with Fermi constantly watching the radiation counters The first ever man induced nuclear chain reaction ran at average power of 05 W for one half an hour Immediately after the reaction was terminated an encoded telephone call was made to the chairman of the National Defense Research Committee that was in charge of coordinating the nuclear research at the time [3] The news of a successful chain reaction made it to Roosevelt who approved on 28 of December 1942 05 million dollars for the construction of a full scale gaseous diffusion plant a plutonium plant and a smaller electromagnetic plant The caultrons still werent able to produce military significant amounts of U-235 but were still the only reliable source of it At the time it was estimated that the first bombs could be completed in the first half of 1945 but no sooner [3]

7 Clinton Engineer Works Oak Ridge TennesseeThe Manhattan Project moved its headquarters and most of the staff to a new location The army bought a piece of land roughly 230 km2 large just west of Knoxville Tennessee There a whole new town was built especially for the needs of the Manhattan project There were 2 uranium separation sites and one plutonium pile in Oak Ridge 15000 thousand construction workers were residing in a trailer camp building the biggest building in the world at the time ndash K-25 [3] K-25 was a gaseous diffusion plant Its construction started in 1943 and it was completed in early 1945 At the peak of construction in 1944 the population of Oak Ridge originally planned to be 13000 rose to 50000 The construction cost of the K-25 was 500 million dollars [3] (61 billion in 2009 dollars) It was the most promising of all the separation facilities but soon encountered problems The main problem was to find a suitable barrier that wasnt susceptible to highly corrosive UF6 The barrier crisis was so great that Groves ordered an expansion of the electromagnetic facility and to build another facility next to the K-25 power plant The new facility was a liquid thermal diffusion plant It utilized the fact that lighter atoms diffuse towards the hot surface while the heavier atoms towards the colder surface This was done in a vertically positioned cylinder with a steam pipe along its axis The lighter U-235 would diffuse towards the center of the cylinder and rise due to convection current The enriched liquid would be collected at the top of the cylinder [1] This was a cheap way of separating isotopes but it was not capable of producing highly pure U-235 Its slightly enriched product was used merely as a feed for the electromagnetic plant that was the

Figure 6 [5] A drawing of Chicago Pile 1 the first nuclear reactor to be constructed It was built under the stands of the University of Chicagos football stadium on a squash court Fermi Szilard Compton and other scientists and techincal personell were attending the December 2nd experiment when a critical chain reaction in natural uranium was established for the first time in history

8

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

equation E = mc2 (1) this meant that the missing mass had to be transformed into energy that would be subsequently turned into heat Hahns former coworker in Vienna Lise Meitner who escaped from Nazi Germany to Sweden and her nephew Otto Frisch calculated that the energies released in such a reaction are of such scale that a new type of nuclear reaction has been observed Frisch named it fission after a biological process of cell division called binary fission [1]Fission of uranium nuclei soon proved to have another important property While the large uranium nucleus split into two smaller nuclei some neutrons were emitted during the split That meant that with each uranium nucleus splitting there were some additional neutrons acquired as a final product of fission These neutrons would propagate trough the material and could if they havent escaped from it cause another uranium nucleus to fission emitting even more neutrons If each splits yield of neutrons be greater than one that could cause in large enough mass of uranium a chain reaction in which the amount of released energy would increase exponentially with every generation of newly born neutrons [1] An idea of a chain reaction altered the prospects of harnessing the energy stored in the atomic nucleus A reaction that would be self sustaining and controlled at the same time could be a useful source of energy while an uncontrolled nuclear chain reaction would release increasingly large amounts of energy and would lead to great heating and possibly an explosionOne of the first scientists who saw the potential benefits and dangers of such a reaction was Leo Szilard[1]

Hahn and Strassmann informed Niels Bohr of their discovery in 1939 just before Bohr had left Denmark to travel to the USA where he announced the new discovery to some emigrant scientists from Europe and to the American scientific community at the opening session of a conference on theoretical physics on January 26 1939 in Washington DC [1] The Nazi regime forced many prominent scientists especially those of Jewish descent to migrate out of Germany Many of them found refuge in the United States including Einstein who played a major role in persuading the American government that research of the new phenomena especially nuclear fission is a matter of national security [1] The American physicists have throughout the 1930s developed some equipment that was vital for the research of nuclear phenomena One of the greatest and soon to be of essential importance was Lawrences cyclotron The American scientists started to actively participate in theoretical and experimental research of fission in uranium Niels Bohr and John A Wigner did some of the most important theoretical work in Princeton while Leo Szilard and Enrico Fermi started to investigate the possibility of a chain reaction in uranium on Columbia University [3] By March 1940 it was known that only one of uranium isotopes undergoes fission when bombarded with slow neutrons It was the less abundant U-235 which represented only 07 of natural uranium the majority being U-238 U-238 could also fission under very fast neutron bombardment but could not sustain a nuclear chain reaction The reason it couldnt was that too few neutrons produced by such fission possessed enough energy to cause further fission [2] An absorption of a neutron made available 53 MeV of binding energy while U-238 needed roughly 6 MeV to fission That meant that slow neurons could not cause it to fission and only neutrons of about 1 MeV of energy would do the job On the other hand U-235 also earned 53 MeV by capturing a neutron but gained

Figure 1 [5]

Representation of a fissioning nucleus A large nucleus gets hit by a neutron splitting in two roughly equally sized nuclei and emmiting 3 neutrons in the process The newly generated neutrons cause another fission if they encounter a fissionable nucleus [1]

3

some additional energy by turning from odd to even mass The sum of both was always greater than 6 MeV which meant that any neutron would cause U-235 to split [1] In such a reaction enormous quantities of energy about 200 MeV per fission would be released

4

Figure 2 [7]

The fission cross-sections as they were measured decades after the discovery of fission In the resonance region there is also the U-238 capture resonance (not shown on the graph) Thermal neutrons have energies of rougly 004 eV (far left on the graph) The U-238s neutron capture cross-section remains about 10 barns at that energies while the fission cross-section for U-235 is much higher about 2 orders of magnitude

Figure 3 [9]

An example of one of the many possible reactions when a neutron hits the U-235 nucleus

This discovery was of major importance since it meant that a nuclear explosion could only occur in U-235 if this was possible at all The slightly lighter isotope had to be in order to create such an explosion separated from the more abundant U-238 A task which presented a great challenge given the available technology and resourcesIn 1939 the second world war started with Hitlers army attacking Poland It became apparent that if a weapon employing nuclear fission as its energy source could be developed it would mean a huge advantage for anyone that possessed it Coming to the idea of a nuclear weapon themselves the American scientist couldnt help but to imagine that the Nazi Germany was already researching the possibilities of such a weapon A calculation showed that 1 kg of U-235 that fissioned would produce an explosive yield equivalent to about 20000 tons of TNT [3] The idea of Hitler having such a weapon at his disposal filled the American scientists many of them Jewish emigrants from Europe with dread Since the very discovery of fission came from Germany they imagined that the Germans already had some advantage in the research Having occupied Belgium it became available to them the high quality uranium ore from the Belgian Congo [1]Fermi and Szilard started to investigate the possibility of a sustained chain reaction in natural uranium They knew they had to slow down the neutrons that were emitted during fission to energies below 25 eV that being the capture resonance of neutrons by U-238 [3] This was important because in order to sustain a chain reaction as much neutrons as possible would have to cause further fissioning of U-235 In order to slow neutrons down but not diminish their number they had to find an appropriate moderator Fermi had previously found hydrogen and carbon to be of use in slowing neutrons Hydrogen proved to have too large capture cross-section which meant that it could not be used as a moderator for a natural uranium reactor The moderator of choice for Fermi and Szilard was graphite Szilard in particular spent a lot of time searching for a good source of graphite and testing it for impurities The Germans on the other hand have by occupying Norway gained a source of heavy water Heavy water contained the heavier isotope of hydrogen ndash deuterium which had much smaller capture cross-section for neutrons than ordinary hydrogen This was just another fact that worried the American scientistsThe threat of Germany producing a nuclear weapon was a real one decided Szilard So he visited Albert Einstein briefed him on the knowledge he had on the topic (most of it was kept secret among the American scientists in order to keep it from the Germans) and together they wrote a letter to the president of the United States Franklin D Roosevelt In the letter they urged him and his administration to start an organized effort one of national proportions towards creating an atomic bomb Though many scientists at that time were still unsure whether such a bomb was possible to Einstein and Szilard this was not of main importance If they found out in the procedure that such a weapon is indeed impossible to make this would be just as rewarding as making one before Hitler did However the risk America would be taking by not starting such a project would be unacceptable [1]

3 Manhattan Engineer DistrictThe bomb project fell under the Armys authority in mid 1942 and was led by Major General Groves United States Army Corps of Engineers officer who oversaw the construction of Pentagon [1] He gave the project the name Manhattan Engineer District later to be called The Manhattan Project In order to continue research on uranium fission enriched U-235 had to be obtained And if a bomb was to be made large amounts of U-235 would be needed Since the U-238 and U-235 are chemically identical they had to be separated physically All the separation methods relied on the small difference in mass of the two isotopes As their mass differed only by roughly one percent all physical means of separation would be extremely complicated and expensive The Manhattan Projects goal was to obtain fissile material for an atomic bomb by constructing necessary facilities

5

4 Isotope separationThere were 3 isotope separation processes that could prove to be of use at the time The first was gaseous diffusion This method would employ diffusion of UF6 gas trough a porous barrier The lighter isotope would diffuse through the barrier faster then the heavier isotope Continuous separation of the enriched gas trough thousands of such barriers each one enriching the UF6 a little further would result in high purity U-235 This was at the beginning the most promising method but the researchers encountered many difficulties before it became useful [3]

The second method was the electromagnetic separation It was discovered by Alfred O Nier and it was basically a mass spectrograph Charged particles flying trough a magnetic field deflect differently according to their mass Lawrence who was head of the electromagnetic separation research converted his cyclotron into a caultron a device created specially for separation of uranium for the Manhattan Project At first a yield of such a machine was very low but as with all the separation methods an immense amount of research and funds had to be spent on improving it in order to make it useful [3]

The radius of a charged particle in a homogenous magnetic field

r=mvqB

All the ionized particles are accelerated with the same voltage

E pot = V d = E kin = mv2

2

Therefore they have the same kinetic energy m 1 v 1

2

2= m 2 v 2

2

2

v 2 = m 1

m 2

v 1

r 2 = m 2 v 1

qB m 1

m 2

The radius of the second particle is larger if the mass of the second particle is larger This was the principle of electromagnetic isotope separation

r 2 = m 1 v 1

qB m 2

m 1

= r 1 m 2

m 1

Many scientist thought that centrifuge was the most prominent of the methods Centrifugal force in a barrel spinning around its vertical axis would separate the isotopes in a gaseous form The heavier U-238 would be drawn to away from the center leaving behind slightly enriched gas which would then have to be pumped from the top center of the cylindrical barrel This enriched gas would then be pumped into another spinning cylinder and further trough hundreds or maybe even thousands more to achieve the desired purity But this method soon encountered difficulties and was abandoned for the needs of the Manhattan Project [3]

6

Figure 4 [8]

A scheme of a gaseous diffusion isotope separation process

(5)

(3)

(6)

(7)

(2)

(4)

5 PlutoniumHowever Lawrences mass spectrograph gave Glenn T Seaborg a researcher at Berkley encouraging results In February 1941 he identified a new trans-uranic element of mass 239 and atomic number 94 ndash plutonium Plutonium was a decay product of neptunium which itself was a decay product of U-238 that captured a neutron When U-238 captures a neutron it becomes unstable and decays with a half life of only 235 minutes into neptunium which also decays by another beta decay to plutonium the half-life of neptunium being 236 days Plutonium proved to be excellent fission material since its fission cross-section for slow neutrons was even larger than that of U-235 namely 17 times larger [1]This discovery gave Fermi-Szilards reactor a needed push A sustained and controlled nuclear chain reaction in natural uranium would produce plutonium since many neutrons would even if the reaction was well moderated get captured by U-238 The produced plutonium could then be used to make a fission bomb Plutonium had a huge advantage over U-235 Not being just another isotope of uranium it could be chemically separated from the other reaction products But to be sure that could be done they had to confirm that plutonium is indeed separable from uranium The research of this newly discovered element which was completely unknown was a very difficult task Seaborgs research team managed to separate a microscopic sample of pure plutonium from irradiated uranium in August 1942 This was a major chemical achievement The procedure would have to be rescaled by a billion times to get sufficient amounts of it to produce a plutonium weapon [1]

6 Chicago Pile 1The confirmation of separability of plutonium from uranium gave Fermi-Szilards uranium reactor another boost Fermi called the experiment uranium pile since it was essentially a pile of 60 tons of uranium oxide 6 tons of metal uranium and 400 tons of graphite arranged in an almost spherical configuration with long holes in which cadmium rods were placed [3] The cadmium rods would be drawn out of the pile to increase neutron activity to a level at which a chain reaction would be self sustaining and would be re-inserted into the pile to dampen the reaction The pile was originally supposed to be of roughly spherical shape but it

7

Figure 5 [10]

Plutonium is the most complicated and strange element an engineers nightmare It is extremely reactive reacts in many different ways and it changes its mechanical properties when heated or cooled The δ-phase is the most like normal metals and can be stabilized at room temperatures by chemical procedures

turned out in Fermis calculations that the critical mass of natural uranium was actually lower than first expected The pile therefore didnt have to be finished according to the original plan On December 2 nd 1942 at 325 pm under Fermis surveillance the pile went critical The cadmium rods were removed from the pile inch by inch with Fermi constantly watching the radiation counters The first ever man induced nuclear chain reaction ran at average power of 05 W for one half an hour Immediately after the reaction was terminated an encoded telephone call was made to the chairman of the National Defense Research Committee that was in charge of coordinating the nuclear research at the time [3] The news of a successful chain reaction made it to Roosevelt who approved on 28 of December 1942 05 million dollars for the construction of a full scale gaseous diffusion plant a plutonium plant and a smaller electromagnetic plant The caultrons still werent able to produce military significant amounts of U-235 but were still the only reliable source of it At the time it was estimated that the first bombs could be completed in the first half of 1945 but no sooner [3]

7 Clinton Engineer Works Oak Ridge TennesseeThe Manhattan Project moved its headquarters and most of the staff to a new location The army bought a piece of land roughly 230 km2 large just west of Knoxville Tennessee There a whole new town was built especially for the needs of the Manhattan project There were 2 uranium separation sites and one plutonium pile in Oak Ridge 15000 thousand construction workers were residing in a trailer camp building the biggest building in the world at the time ndash K-25 [3] K-25 was a gaseous diffusion plant Its construction started in 1943 and it was completed in early 1945 At the peak of construction in 1944 the population of Oak Ridge originally planned to be 13000 rose to 50000 The construction cost of the K-25 was 500 million dollars [3] (61 billion in 2009 dollars) It was the most promising of all the separation facilities but soon encountered problems The main problem was to find a suitable barrier that wasnt susceptible to highly corrosive UF6 The barrier crisis was so great that Groves ordered an expansion of the electromagnetic facility and to build another facility next to the K-25 power plant The new facility was a liquid thermal diffusion plant It utilized the fact that lighter atoms diffuse towards the hot surface while the heavier atoms towards the colder surface This was done in a vertically positioned cylinder with a steam pipe along its axis The lighter U-235 would diffuse towards the center of the cylinder and rise due to convection current The enriched liquid would be collected at the top of the cylinder [1] This was a cheap way of separating isotopes but it was not capable of producing highly pure U-235 Its slightly enriched product was used merely as a feed for the electromagnetic plant that was the

Figure 6 [5] A drawing of Chicago Pile 1 the first nuclear reactor to be constructed It was built under the stands of the University of Chicagos football stadium on a squash court Fermi Szilard Compton and other scientists and techincal personell were attending the December 2nd experiment when a critical chain reaction in natural uranium was established for the first time in history

8

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

some additional energy by turning from odd to even mass The sum of both was always greater than 6 MeV which meant that any neutron would cause U-235 to split [1] In such a reaction enormous quantities of energy about 200 MeV per fission would be released

4

Figure 2 [7]

The fission cross-sections as they were measured decades after the discovery of fission In the resonance region there is also the U-238 capture resonance (not shown on the graph) Thermal neutrons have energies of rougly 004 eV (far left on the graph) The U-238s neutron capture cross-section remains about 10 barns at that energies while the fission cross-section for U-235 is much higher about 2 orders of magnitude

Figure 3 [9]

An example of one of the many possible reactions when a neutron hits the U-235 nucleus

This discovery was of major importance since it meant that a nuclear explosion could only occur in U-235 if this was possible at all The slightly lighter isotope had to be in order to create such an explosion separated from the more abundant U-238 A task which presented a great challenge given the available technology and resourcesIn 1939 the second world war started with Hitlers army attacking Poland It became apparent that if a weapon employing nuclear fission as its energy source could be developed it would mean a huge advantage for anyone that possessed it Coming to the idea of a nuclear weapon themselves the American scientist couldnt help but to imagine that the Nazi Germany was already researching the possibilities of such a weapon A calculation showed that 1 kg of U-235 that fissioned would produce an explosive yield equivalent to about 20000 tons of TNT [3] The idea of Hitler having such a weapon at his disposal filled the American scientists many of them Jewish emigrants from Europe with dread Since the very discovery of fission came from Germany they imagined that the Germans already had some advantage in the research Having occupied Belgium it became available to them the high quality uranium ore from the Belgian Congo [1]Fermi and Szilard started to investigate the possibility of a sustained chain reaction in natural uranium They knew they had to slow down the neutrons that were emitted during fission to energies below 25 eV that being the capture resonance of neutrons by U-238 [3] This was important because in order to sustain a chain reaction as much neutrons as possible would have to cause further fissioning of U-235 In order to slow neutrons down but not diminish their number they had to find an appropriate moderator Fermi had previously found hydrogen and carbon to be of use in slowing neutrons Hydrogen proved to have too large capture cross-section which meant that it could not be used as a moderator for a natural uranium reactor The moderator of choice for Fermi and Szilard was graphite Szilard in particular spent a lot of time searching for a good source of graphite and testing it for impurities The Germans on the other hand have by occupying Norway gained a source of heavy water Heavy water contained the heavier isotope of hydrogen ndash deuterium which had much smaller capture cross-section for neutrons than ordinary hydrogen This was just another fact that worried the American scientistsThe threat of Germany producing a nuclear weapon was a real one decided Szilard So he visited Albert Einstein briefed him on the knowledge he had on the topic (most of it was kept secret among the American scientists in order to keep it from the Germans) and together they wrote a letter to the president of the United States Franklin D Roosevelt In the letter they urged him and his administration to start an organized effort one of national proportions towards creating an atomic bomb Though many scientists at that time were still unsure whether such a bomb was possible to Einstein and Szilard this was not of main importance If they found out in the procedure that such a weapon is indeed impossible to make this would be just as rewarding as making one before Hitler did However the risk America would be taking by not starting such a project would be unacceptable [1]

3 Manhattan Engineer DistrictThe bomb project fell under the Armys authority in mid 1942 and was led by Major General Groves United States Army Corps of Engineers officer who oversaw the construction of Pentagon [1] He gave the project the name Manhattan Engineer District later to be called The Manhattan Project In order to continue research on uranium fission enriched U-235 had to be obtained And if a bomb was to be made large amounts of U-235 would be needed Since the U-238 and U-235 are chemically identical they had to be separated physically All the separation methods relied on the small difference in mass of the two isotopes As their mass differed only by roughly one percent all physical means of separation would be extremely complicated and expensive The Manhattan Projects goal was to obtain fissile material for an atomic bomb by constructing necessary facilities

5

4 Isotope separationThere were 3 isotope separation processes that could prove to be of use at the time The first was gaseous diffusion This method would employ diffusion of UF6 gas trough a porous barrier The lighter isotope would diffuse through the barrier faster then the heavier isotope Continuous separation of the enriched gas trough thousands of such barriers each one enriching the UF6 a little further would result in high purity U-235 This was at the beginning the most promising method but the researchers encountered many difficulties before it became useful [3]

The second method was the electromagnetic separation It was discovered by Alfred O Nier and it was basically a mass spectrograph Charged particles flying trough a magnetic field deflect differently according to their mass Lawrence who was head of the electromagnetic separation research converted his cyclotron into a caultron a device created specially for separation of uranium for the Manhattan Project At first a yield of such a machine was very low but as with all the separation methods an immense amount of research and funds had to be spent on improving it in order to make it useful [3]

The radius of a charged particle in a homogenous magnetic field

r=mvqB

All the ionized particles are accelerated with the same voltage

E pot = V d = E kin = mv2

2

Therefore they have the same kinetic energy m 1 v 1

2

2= m 2 v 2

2

2

v 2 = m 1

m 2

v 1

r 2 = m 2 v 1

qB m 1

m 2

The radius of the second particle is larger if the mass of the second particle is larger This was the principle of electromagnetic isotope separation

r 2 = m 1 v 1

qB m 2

m 1

= r 1 m 2

m 1

Many scientist thought that centrifuge was the most prominent of the methods Centrifugal force in a barrel spinning around its vertical axis would separate the isotopes in a gaseous form The heavier U-238 would be drawn to away from the center leaving behind slightly enriched gas which would then have to be pumped from the top center of the cylindrical barrel This enriched gas would then be pumped into another spinning cylinder and further trough hundreds or maybe even thousands more to achieve the desired purity But this method soon encountered difficulties and was abandoned for the needs of the Manhattan Project [3]

6

Figure 4 [8]

A scheme of a gaseous diffusion isotope separation process

(5)

(3)

(6)

(7)

(2)

(4)

5 PlutoniumHowever Lawrences mass spectrograph gave Glenn T Seaborg a researcher at Berkley encouraging results In February 1941 he identified a new trans-uranic element of mass 239 and atomic number 94 ndash plutonium Plutonium was a decay product of neptunium which itself was a decay product of U-238 that captured a neutron When U-238 captures a neutron it becomes unstable and decays with a half life of only 235 minutes into neptunium which also decays by another beta decay to plutonium the half-life of neptunium being 236 days Plutonium proved to be excellent fission material since its fission cross-section for slow neutrons was even larger than that of U-235 namely 17 times larger [1]This discovery gave Fermi-Szilards reactor a needed push A sustained and controlled nuclear chain reaction in natural uranium would produce plutonium since many neutrons would even if the reaction was well moderated get captured by U-238 The produced plutonium could then be used to make a fission bomb Plutonium had a huge advantage over U-235 Not being just another isotope of uranium it could be chemically separated from the other reaction products But to be sure that could be done they had to confirm that plutonium is indeed separable from uranium The research of this newly discovered element which was completely unknown was a very difficult task Seaborgs research team managed to separate a microscopic sample of pure plutonium from irradiated uranium in August 1942 This was a major chemical achievement The procedure would have to be rescaled by a billion times to get sufficient amounts of it to produce a plutonium weapon [1]

6 Chicago Pile 1The confirmation of separability of plutonium from uranium gave Fermi-Szilards uranium reactor another boost Fermi called the experiment uranium pile since it was essentially a pile of 60 tons of uranium oxide 6 tons of metal uranium and 400 tons of graphite arranged in an almost spherical configuration with long holes in which cadmium rods were placed [3] The cadmium rods would be drawn out of the pile to increase neutron activity to a level at which a chain reaction would be self sustaining and would be re-inserted into the pile to dampen the reaction The pile was originally supposed to be of roughly spherical shape but it

7

Figure 5 [10]

Plutonium is the most complicated and strange element an engineers nightmare It is extremely reactive reacts in many different ways and it changes its mechanical properties when heated or cooled The δ-phase is the most like normal metals and can be stabilized at room temperatures by chemical procedures

turned out in Fermis calculations that the critical mass of natural uranium was actually lower than first expected The pile therefore didnt have to be finished according to the original plan On December 2 nd 1942 at 325 pm under Fermis surveillance the pile went critical The cadmium rods were removed from the pile inch by inch with Fermi constantly watching the radiation counters The first ever man induced nuclear chain reaction ran at average power of 05 W for one half an hour Immediately after the reaction was terminated an encoded telephone call was made to the chairman of the National Defense Research Committee that was in charge of coordinating the nuclear research at the time [3] The news of a successful chain reaction made it to Roosevelt who approved on 28 of December 1942 05 million dollars for the construction of a full scale gaseous diffusion plant a plutonium plant and a smaller electromagnetic plant The caultrons still werent able to produce military significant amounts of U-235 but were still the only reliable source of it At the time it was estimated that the first bombs could be completed in the first half of 1945 but no sooner [3]

7 Clinton Engineer Works Oak Ridge TennesseeThe Manhattan Project moved its headquarters and most of the staff to a new location The army bought a piece of land roughly 230 km2 large just west of Knoxville Tennessee There a whole new town was built especially for the needs of the Manhattan project There were 2 uranium separation sites and one plutonium pile in Oak Ridge 15000 thousand construction workers were residing in a trailer camp building the biggest building in the world at the time ndash K-25 [3] K-25 was a gaseous diffusion plant Its construction started in 1943 and it was completed in early 1945 At the peak of construction in 1944 the population of Oak Ridge originally planned to be 13000 rose to 50000 The construction cost of the K-25 was 500 million dollars [3] (61 billion in 2009 dollars) It was the most promising of all the separation facilities but soon encountered problems The main problem was to find a suitable barrier that wasnt susceptible to highly corrosive UF6 The barrier crisis was so great that Groves ordered an expansion of the electromagnetic facility and to build another facility next to the K-25 power plant The new facility was a liquid thermal diffusion plant It utilized the fact that lighter atoms diffuse towards the hot surface while the heavier atoms towards the colder surface This was done in a vertically positioned cylinder with a steam pipe along its axis The lighter U-235 would diffuse towards the center of the cylinder and rise due to convection current The enriched liquid would be collected at the top of the cylinder [1] This was a cheap way of separating isotopes but it was not capable of producing highly pure U-235 Its slightly enriched product was used merely as a feed for the electromagnetic plant that was the

Figure 6 [5] A drawing of Chicago Pile 1 the first nuclear reactor to be constructed It was built under the stands of the University of Chicagos football stadium on a squash court Fermi Szilard Compton and other scientists and techincal personell were attending the December 2nd experiment when a critical chain reaction in natural uranium was established for the first time in history

8

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

This discovery was of major importance since it meant that a nuclear explosion could only occur in U-235 if this was possible at all The slightly lighter isotope had to be in order to create such an explosion separated from the more abundant U-238 A task which presented a great challenge given the available technology and resourcesIn 1939 the second world war started with Hitlers army attacking Poland It became apparent that if a weapon employing nuclear fission as its energy source could be developed it would mean a huge advantage for anyone that possessed it Coming to the idea of a nuclear weapon themselves the American scientist couldnt help but to imagine that the Nazi Germany was already researching the possibilities of such a weapon A calculation showed that 1 kg of U-235 that fissioned would produce an explosive yield equivalent to about 20000 tons of TNT [3] The idea of Hitler having such a weapon at his disposal filled the American scientists many of them Jewish emigrants from Europe with dread Since the very discovery of fission came from Germany they imagined that the Germans already had some advantage in the research Having occupied Belgium it became available to them the high quality uranium ore from the Belgian Congo [1]Fermi and Szilard started to investigate the possibility of a sustained chain reaction in natural uranium They knew they had to slow down the neutrons that were emitted during fission to energies below 25 eV that being the capture resonance of neutrons by U-238 [3] This was important because in order to sustain a chain reaction as much neutrons as possible would have to cause further fissioning of U-235 In order to slow neutrons down but not diminish their number they had to find an appropriate moderator Fermi had previously found hydrogen and carbon to be of use in slowing neutrons Hydrogen proved to have too large capture cross-section which meant that it could not be used as a moderator for a natural uranium reactor The moderator of choice for Fermi and Szilard was graphite Szilard in particular spent a lot of time searching for a good source of graphite and testing it for impurities The Germans on the other hand have by occupying Norway gained a source of heavy water Heavy water contained the heavier isotope of hydrogen ndash deuterium which had much smaller capture cross-section for neutrons than ordinary hydrogen This was just another fact that worried the American scientistsThe threat of Germany producing a nuclear weapon was a real one decided Szilard So he visited Albert Einstein briefed him on the knowledge he had on the topic (most of it was kept secret among the American scientists in order to keep it from the Germans) and together they wrote a letter to the president of the United States Franklin D Roosevelt In the letter they urged him and his administration to start an organized effort one of national proportions towards creating an atomic bomb Though many scientists at that time were still unsure whether such a bomb was possible to Einstein and Szilard this was not of main importance If they found out in the procedure that such a weapon is indeed impossible to make this would be just as rewarding as making one before Hitler did However the risk America would be taking by not starting such a project would be unacceptable [1]

3 Manhattan Engineer DistrictThe bomb project fell under the Armys authority in mid 1942 and was led by Major General Groves United States Army Corps of Engineers officer who oversaw the construction of Pentagon [1] He gave the project the name Manhattan Engineer District later to be called The Manhattan Project In order to continue research on uranium fission enriched U-235 had to be obtained And if a bomb was to be made large amounts of U-235 would be needed Since the U-238 and U-235 are chemically identical they had to be separated physically All the separation methods relied on the small difference in mass of the two isotopes As their mass differed only by roughly one percent all physical means of separation would be extremely complicated and expensive The Manhattan Projects goal was to obtain fissile material for an atomic bomb by constructing necessary facilities

5

4 Isotope separationThere were 3 isotope separation processes that could prove to be of use at the time The first was gaseous diffusion This method would employ diffusion of UF6 gas trough a porous barrier The lighter isotope would diffuse through the barrier faster then the heavier isotope Continuous separation of the enriched gas trough thousands of such barriers each one enriching the UF6 a little further would result in high purity U-235 This was at the beginning the most promising method but the researchers encountered many difficulties before it became useful [3]

The second method was the electromagnetic separation It was discovered by Alfred O Nier and it was basically a mass spectrograph Charged particles flying trough a magnetic field deflect differently according to their mass Lawrence who was head of the electromagnetic separation research converted his cyclotron into a caultron a device created specially for separation of uranium for the Manhattan Project At first a yield of such a machine was very low but as with all the separation methods an immense amount of research and funds had to be spent on improving it in order to make it useful [3]

The radius of a charged particle in a homogenous magnetic field

r=mvqB

All the ionized particles are accelerated with the same voltage

E pot = V d = E kin = mv2

2

Therefore they have the same kinetic energy m 1 v 1

2

2= m 2 v 2

2

2

v 2 = m 1

m 2

v 1

r 2 = m 2 v 1

qB m 1

m 2

The radius of the second particle is larger if the mass of the second particle is larger This was the principle of electromagnetic isotope separation

r 2 = m 1 v 1

qB m 2

m 1

= r 1 m 2

m 1

Many scientist thought that centrifuge was the most prominent of the methods Centrifugal force in a barrel spinning around its vertical axis would separate the isotopes in a gaseous form The heavier U-238 would be drawn to away from the center leaving behind slightly enriched gas which would then have to be pumped from the top center of the cylindrical barrel This enriched gas would then be pumped into another spinning cylinder and further trough hundreds or maybe even thousands more to achieve the desired purity But this method soon encountered difficulties and was abandoned for the needs of the Manhattan Project [3]

6

Figure 4 [8]

A scheme of a gaseous diffusion isotope separation process

(5)

(3)

(6)

(7)

(2)

(4)

5 PlutoniumHowever Lawrences mass spectrograph gave Glenn T Seaborg a researcher at Berkley encouraging results In February 1941 he identified a new trans-uranic element of mass 239 and atomic number 94 ndash plutonium Plutonium was a decay product of neptunium which itself was a decay product of U-238 that captured a neutron When U-238 captures a neutron it becomes unstable and decays with a half life of only 235 minutes into neptunium which also decays by another beta decay to plutonium the half-life of neptunium being 236 days Plutonium proved to be excellent fission material since its fission cross-section for slow neutrons was even larger than that of U-235 namely 17 times larger [1]This discovery gave Fermi-Szilards reactor a needed push A sustained and controlled nuclear chain reaction in natural uranium would produce plutonium since many neutrons would even if the reaction was well moderated get captured by U-238 The produced plutonium could then be used to make a fission bomb Plutonium had a huge advantage over U-235 Not being just another isotope of uranium it could be chemically separated from the other reaction products But to be sure that could be done they had to confirm that plutonium is indeed separable from uranium The research of this newly discovered element which was completely unknown was a very difficult task Seaborgs research team managed to separate a microscopic sample of pure plutonium from irradiated uranium in August 1942 This was a major chemical achievement The procedure would have to be rescaled by a billion times to get sufficient amounts of it to produce a plutonium weapon [1]

6 Chicago Pile 1The confirmation of separability of plutonium from uranium gave Fermi-Szilards uranium reactor another boost Fermi called the experiment uranium pile since it was essentially a pile of 60 tons of uranium oxide 6 tons of metal uranium and 400 tons of graphite arranged in an almost spherical configuration with long holes in which cadmium rods were placed [3] The cadmium rods would be drawn out of the pile to increase neutron activity to a level at which a chain reaction would be self sustaining and would be re-inserted into the pile to dampen the reaction The pile was originally supposed to be of roughly spherical shape but it

7

Figure 5 [10]

Plutonium is the most complicated and strange element an engineers nightmare It is extremely reactive reacts in many different ways and it changes its mechanical properties when heated or cooled The δ-phase is the most like normal metals and can be stabilized at room temperatures by chemical procedures

turned out in Fermis calculations that the critical mass of natural uranium was actually lower than first expected The pile therefore didnt have to be finished according to the original plan On December 2 nd 1942 at 325 pm under Fermis surveillance the pile went critical The cadmium rods were removed from the pile inch by inch with Fermi constantly watching the radiation counters The first ever man induced nuclear chain reaction ran at average power of 05 W for one half an hour Immediately after the reaction was terminated an encoded telephone call was made to the chairman of the National Defense Research Committee that was in charge of coordinating the nuclear research at the time [3] The news of a successful chain reaction made it to Roosevelt who approved on 28 of December 1942 05 million dollars for the construction of a full scale gaseous diffusion plant a plutonium plant and a smaller electromagnetic plant The caultrons still werent able to produce military significant amounts of U-235 but were still the only reliable source of it At the time it was estimated that the first bombs could be completed in the first half of 1945 but no sooner [3]

7 Clinton Engineer Works Oak Ridge TennesseeThe Manhattan Project moved its headquarters and most of the staff to a new location The army bought a piece of land roughly 230 km2 large just west of Knoxville Tennessee There a whole new town was built especially for the needs of the Manhattan project There were 2 uranium separation sites and one plutonium pile in Oak Ridge 15000 thousand construction workers were residing in a trailer camp building the biggest building in the world at the time ndash K-25 [3] K-25 was a gaseous diffusion plant Its construction started in 1943 and it was completed in early 1945 At the peak of construction in 1944 the population of Oak Ridge originally planned to be 13000 rose to 50000 The construction cost of the K-25 was 500 million dollars [3] (61 billion in 2009 dollars) It was the most promising of all the separation facilities but soon encountered problems The main problem was to find a suitable barrier that wasnt susceptible to highly corrosive UF6 The barrier crisis was so great that Groves ordered an expansion of the electromagnetic facility and to build another facility next to the K-25 power plant The new facility was a liquid thermal diffusion plant It utilized the fact that lighter atoms diffuse towards the hot surface while the heavier atoms towards the colder surface This was done in a vertically positioned cylinder with a steam pipe along its axis The lighter U-235 would diffuse towards the center of the cylinder and rise due to convection current The enriched liquid would be collected at the top of the cylinder [1] This was a cheap way of separating isotopes but it was not capable of producing highly pure U-235 Its slightly enriched product was used merely as a feed for the electromagnetic plant that was the

Figure 6 [5] A drawing of Chicago Pile 1 the first nuclear reactor to be constructed It was built under the stands of the University of Chicagos football stadium on a squash court Fermi Szilard Compton and other scientists and techincal personell were attending the December 2nd experiment when a critical chain reaction in natural uranium was established for the first time in history

8

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

4 Isotope separationThere were 3 isotope separation processes that could prove to be of use at the time The first was gaseous diffusion This method would employ diffusion of UF6 gas trough a porous barrier The lighter isotope would diffuse through the barrier faster then the heavier isotope Continuous separation of the enriched gas trough thousands of such barriers each one enriching the UF6 a little further would result in high purity U-235 This was at the beginning the most promising method but the researchers encountered many difficulties before it became useful [3]

The second method was the electromagnetic separation It was discovered by Alfred O Nier and it was basically a mass spectrograph Charged particles flying trough a magnetic field deflect differently according to their mass Lawrence who was head of the electromagnetic separation research converted his cyclotron into a caultron a device created specially for separation of uranium for the Manhattan Project At first a yield of such a machine was very low but as with all the separation methods an immense amount of research and funds had to be spent on improving it in order to make it useful [3]

The radius of a charged particle in a homogenous magnetic field

r=mvqB

All the ionized particles are accelerated with the same voltage

E pot = V d = E kin = mv2

2

Therefore they have the same kinetic energy m 1 v 1

2

2= m 2 v 2

2

2

v 2 = m 1

m 2

v 1

r 2 = m 2 v 1

qB m 1

m 2

The radius of the second particle is larger if the mass of the second particle is larger This was the principle of electromagnetic isotope separation

r 2 = m 1 v 1

qB m 2

m 1

= r 1 m 2

m 1

Many scientist thought that centrifuge was the most prominent of the methods Centrifugal force in a barrel spinning around its vertical axis would separate the isotopes in a gaseous form The heavier U-238 would be drawn to away from the center leaving behind slightly enriched gas which would then have to be pumped from the top center of the cylindrical barrel This enriched gas would then be pumped into another spinning cylinder and further trough hundreds or maybe even thousands more to achieve the desired purity But this method soon encountered difficulties and was abandoned for the needs of the Manhattan Project [3]

6

Figure 4 [8]

A scheme of a gaseous diffusion isotope separation process

(5)

(3)

(6)

(7)

(2)

(4)

5 PlutoniumHowever Lawrences mass spectrograph gave Glenn T Seaborg a researcher at Berkley encouraging results In February 1941 he identified a new trans-uranic element of mass 239 and atomic number 94 ndash plutonium Plutonium was a decay product of neptunium which itself was a decay product of U-238 that captured a neutron When U-238 captures a neutron it becomes unstable and decays with a half life of only 235 minutes into neptunium which also decays by another beta decay to plutonium the half-life of neptunium being 236 days Plutonium proved to be excellent fission material since its fission cross-section for slow neutrons was even larger than that of U-235 namely 17 times larger [1]This discovery gave Fermi-Szilards reactor a needed push A sustained and controlled nuclear chain reaction in natural uranium would produce plutonium since many neutrons would even if the reaction was well moderated get captured by U-238 The produced plutonium could then be used to make a fission bomb Plutonium had a huge advantage over U-235 Not being just another isotope of uranium it could be chemically separated from the other reaction products But to be sure that could be done they had to confirm that plutonium is indeed separable from uranium The research of this newly discovered element which was completely unknown was a very difficult task Seaborgs research team managed to separate a microscopic sample of pure plutonium from irradiated uranium in August 1942 This was a major chemical achievement The procedure would have to be rescaled by a billion times to get sufficient amounts of it to produce a plutonium weapon [1]

6 Chicago Pile 1The confirmation of separability of plutonium from uranium gave Fermi-Szilards uranium reactor another boost Fermi called the experiment uranium pile since it was essentially a pile of 60 tons of uranium oxide 6 tons of metal uranium and 400 tons of graphite arranged in an almost spherical configuration with long holes in which cadmium rods were placed [3] The cadmium rods would be drawn out of the pile to increase neutron activity to a level at which a chain reaction would be self sustaining and would be re-inserted into the pile to dampen the reaction The pile was originally supposed to be of roughly spherical shape but it

7

Figure 5 [10]

Plutonium is the most complicated and strange element an engineers nightmare It is extremely reactive reacts in many different ways and it changes its mechanical properties when heated or cooled The δ-phase is the most like normal metals and can be stabilized at room temperatures by chemical procedures

turned out in Fermis calculations that the critical mass of natural uranium was actually lower than first expected The pile therefore didnt have to be finished according to the original plan On December 2 nd 1942 at 325 pm under Fermis surveillance the pile went critical The cadmium rods were removed from the pile inch by inch with Fermi constantly watching the radiation counters The first ever man induced nuclear chain reaction ran at average power of 05 W for one half an hour Immediately after the reaction was terminated an encoded telephone call was made to the chairman of the National Defense Research Committee that was in charge of coordinating the nuclear research at the time [3] The news of a successful chain reaction made it to Roosevelt who approved on 28 of December 1942 05 million dollars for the construction of a full scale gaseous diffusion plant a plutonium plant and a smaller electromagnetic plant The caultrons still werent able to produce military significant amounts of U-235 but were still the only reliable source of it At the time it was estimated that the first bombs could be completed in the first half of 1945 but no sooner [3]

7 Clinton Engineer Works Oak Ridge TennesseeThe Manhattan Project moved its headquarters and most of the staff to a new location The army bought a piece of land roughly 230 km2 large just west of Knoxville Tennessee There a whole new town was built especially for the needs of the Manhattan project There were 2 uranium separation sites and one plutonium pile in Oak Ridge 15000 thousand construction workers were residing in a trailer camp building the biggest building in the world at the time ndash K-25 [3] K-25 was a gaseous diffusion plant Its construction started in 1943 and it was completed in early 1945 At the peak of construction in 1944 the population of Oak Ridge originally planned to be 13000 rose to 50000 The construction cost of the K-25 was 500 million dollars [3] (61 billion in 2009 dollars) It was the most promising of all the separation facilities but soon encountered problems The main problem was to find a suitable barrier that wasnt susceptible to highly corrosive UF6 The barrier crisis was so great that Groves ordered an expansion of the electromagnetic facility and to build another facility next to the K-25 power plant The new facility was a liquid thermal diffusion plant It utilized the fact that lighter atoms diffuse towards the hot surface while the heavier atoms towards the colder surface This was done in a vertically positioned cylinder with a steam pipe along its axis The lighter U-235 would diffuse towards the center of the cylinder and rise due to convection current The enriched liquid would be collected at the top of the cylinder [1] This was a cheap way of separating isotopes but it was not capable of producing highly pure U-235 Its slightly enriched product was used merely as a feed for the electromagnetic plant that was the

Figure 6 [5] A drawing of Chicago Pile 1 the first nuclear reactor to be constructed It was built under the stands of the University of Chicagos football stadium on a squash court Fermi Szilard Compton and other scientists and techincal personell were attending the December 2nd experiment when a critical chain reaction in natural uranium was established for the first time in history

8

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

5 PlutoniumHowever Lawrences mass spectrograph gave Glenn T Seaborg a researcher at Berkley encouraging results In February 1941 he identified a new trans-uranic element of mass 239 and atomic number 94 ndash plutonium Plutonium was a decay product of neptunium which itself was a decay product of U-238 that captured a neutron When U-238 captures a neutron it becomes unstable and decays with a half life of only 235 minutes into neptunium which also decays by another beta decay to plutonium the half-life of neptunium being 236 days Plutonium proved to be excellent fission material since its fission cross-section for slow neutrons was even larger than that of U-235 namely 17 times larger [1]This discovery gave Fermi-Szilards reactor a needed push A sustained and controlled nuclear chain reaction in natural uranium would produce plutonium since many neutrons would even if the reaction was well moderated get captured by U-238 The produced plutonium could then be used to make a fission bomb Plutonium had a huge advantage over U-235 Not being just another isotope of uranium it could be chemically separated from the other reaction products But to be sure that could be done they had to confirm that plutonium is indeed separable from uranium The research of this newly discovered element which was completely unknown was a very difficult task Seaborgs research team managed to separate a microscopic sample of pure plutonium from irradiated uranium in August 1942 This was a major chemical achievement The procedure would have to be rescaled by a billion times to get sufficient amounts of it to produce a plutonium weapon [1]

6 Chicago Pile 1The confirmation of separability of plutonium from uranium gave Fermi-Szilards uranium reactor another boost Fermi called the experiment uranium pile since it was essentially a pile of 60 tons of uranium oxide 6 tons of metal uranium and 400 tons of graphite arranged in an almost spherical configuration with long holes in which cadmium rods were placed [3] The cadmium rods would be drawn out of the pile to increase neutron activity to a level at which a chain reaction would be self sustaining and would be re-inserted into the pile to dampen the reaction The pile was originally supposed to be of roughly spherical shape but it

7

Figure 5 [10]

Plutonium is the most complicated and strange element an engineers nightmare It is extremely reactive reacts in many different ways and it changes its mechanical properties when heated or cooled The δ-phase is the most like normal metals and can be stabilized at room temperatures by chemical procedures

turned out in Fermis calculations that the critical mass of natural uranium was actually lower than first expected The pile therefore didnt have to be finished according to the original plan On December 2 nd 1942 at 325 pm under Fermis surveillance the pile went critical The cadmium rods were removed from the pile inch by inch with Fermi constantly watching the radiation counters The first ever man induced nuclear chain reaction ran at average power of 05 W for one half an hour Immediately after the reaction was terminated an encoded telephone call was made to the chairman of the National Defense Research Committee that was in charge of coordinating the nuclear research at the time [3] The news of a successful chain reaction made it to Roosevelt who approved on 28 of December 1942 05 million dollars for the construction of a full scale gaseous diffusion plant a plutonium plant and a smaller electromagnetic plant The caultrons still werent able to produce military significant amounts of U-235 but were still the only reliable source of it At the time it was estimated that the first bombs could be completed in the first half of 1945 but no sooner [3]

7 Clinton Engineer Works Oak Ridge TennesseeThe Manhattan Project moved its headquarters and most of the staff to a new location The army bought a piece of land roughly 230 km2 large just west of Knoxville Tennessee There a whole new town was built especially for the needs of the Manhattan project There were 2 uranium separation sites and one plutonium pile in Oak Ridge 15000 thousand construction workers were residing in a trailer camp building the biggest building in the world at the time ndash K-25 [3] K-25 was a gaseous diffusion plant Its construction started in 1943 and it was completed in early 1945 At the peak of construction in 1944 the population of Oak Ridge originally planned to be 13000 rose to 50000 The construction cost of the K-25 was 500 million dollars [3] (61 billion in 2009 dollars) It was the most promising of all the separation facilities but soon encountered problems The main problem was to find a suitable barrier that wasnt susceptible to highly corrosive UF6 The barrier crisis was so great that Groves ordered an expansion of the electromagnetic facility and to build another facility next to the K-25 power plant The new facility was a liquid thermal diffusion plant It utilized the fact that lighter atoms diffuse towards the hot surface while the heavier atoms towards the colder surface This was done in a vertically positioned cylinder with a steam pipe along its axis The lighter U-235 would diffuse towards the center of the cylinder and rise due to convection current The enriched liquid would be collected at the top of the cylinder [1] This was a cheap way of separating isotopes but it was not capable of producing highly pure U-235 Its slightly enriched product was used merely as a feed for the electromagnetic plant that was the

Figure 6 [5] A drawing of Chicago Pile 1 the first nuclear reactor to be constructed It was built under the stands of the University of Chicagos football stadium on a squash court Fermi Szilard Compton and other scientists and techincal personell were attending the December 2nd experiment when a critical chain reaction in natural uranium was established for the first time in history

8

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

turned out in Fermis calculations that the critical mass of natural uranium was actually lower than first expected The pile therefore didnt have to be finished according to the original plan On December 2 nd 1942 at 325 pm under Fermis surveillance the pile went critical The cadmium rods were removed from the pile inch by inch with Fermi constantly watching the radiation counters The first ever man induced nuclear chain reaction ran at average power of 05 W for one half an hour Immediately after the reaction was terminated an encoded telephone call was made to the chairman of the National Defense Research Committee that was in charge of coordinating the nuclear research at the time [3] The news of a successful chain reaction made it to Roosevelt who approved on 28 of December 1942 05 million dollars for the construction of a full scale gaseous diffusion plant a plutonium plant and a smaller electromagnetic plant The caultrons still werent able to produce military significant amounts of U-235 but were still the only reliable source of it At the time it was estimated that the first bombs could be completed in the first half of 1945 but no sooner [3]

7 Clinton Engineer Works Oak Ridge TennesseeThe Manhattan Project moved its headquarters and most of the staff to a new location The army bought a piece of land roughly 230 km2 large just west of Knoxville Tennessee There a whole new town was built especially for the needs of the Manhattan project There were 2 uranium separation sites and one plutonium pile in Oak Ridge 15000 thousand construction workers were residing in a trailer camp building the biggest building in the world at the time ndash K-25 [3] K-25 was a gaseous diffusion plant Its construction started in 1943 and it was completed in early 1945 At the peak of construction in 1944 the population of Oak Ridge originally planned to be 13000 rose to 50000 The construction cost of the K-25 was 500 million dollars [3] (61 billion in 2009 dollars) It was the most promising of all the separation facilities but soon encountered problems The main problem was to find a suitable barrier that wasnt susceptible to highly corrosive UF6 The barrier crisis was so great that Groves ordered an expansion of the electromagnetic facility and to build another facility next to the K-25 power plant The new facility was a liquid thermal diffusion plant It utilized the fact that lighter atoms diffuse towards the hot surface while the heavier atoms towards the colder surface This was done in a vertically positioned cylinder with a steam pipe along its axis The lighter U-235 would diffuse towards the center of the cylinder and rise due to convection current The enriched liquid would be collected at the top of the cylinder [1] This was a cheap way of separating isotopes but it was not capable of producing highly pure U-235 Its slightly enriched product was used merely as a feed for the electromagnetic plant that was the

Figure 6 [5] A drawing of Chicago Pile 1 the first nuclear reactor to be constructed It was built under the stands of the University of Chicagos football stadium on a squash court Fermi Szilard Compton and other scientists and techincal personell were attending the December 2nd experiment when a critical chain reaction in natural uranium was established for the first time in history

8

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

only one that could achieve high degrees of separation until a proper barrier was found for the K-25 The barrier problems continued and the K-25 was not fully operational until the end of the war By 1945 it produced about 50 enriched uranium that was fed to the caultrons [1]

The Lawrences caultrons had crisis of their own There has been a major shortage of copper in the United States during the war So Groves asked the United States Treasury to lend the Manhattan Project 15000 tons of silver to fabricate wire for the coils used in the caultron electromagnets After the war the magnets were disassembled and the silver was returned to the treasury with only 0035 of it lost in the procedure [1] The processing of uranium in Y-12 as the electromagnetic separation facility was called took place a few kilometers from the K-25 and the thermal diffusion plant In late 1943 the caultron magnets had to be opened and repaired as the wires were short-circuiting and this additionally delayed the separation The separation went through two stages in Y-12 The alpha racetracks as the arrangement of caultrons was called were producing roughly 10 enriched uranium that was then fed to the beta racetracks When the thermal and gaseous diffusion plants were running the partly enriched uranium was fed directly to the beta racetracks Their efficiency improved and by 1945 they were producing bomb grade U-235 in quantities of few hundred grams per day [3]

Figure 9 [5] Caultron operators at work The girls operating the caultrons proved to be more effective than the scientists because they didnt pay much attention to minor fluctuations of counters The scientists who at first operated these machines were easily distracted by unusual behaviour of the instruments and were therefore inefficient

Figure 8 [5]

A scheme of Lawrences caultron which was bassically a mass spectrograph with improvements Tuballoy was a codename for uranium

Figure 7 [5] An aerial photograph of K-25 at the time the worlds largest building It was a gaseous diffusion plant separating U-235 from natural uranium by means of gaseous diffusion through a porous barrier The procedure had to be repeated thousands of times therefore the UF6 gas was flowing through thousands of tanks The building was 800 m long and 300 m wide and it employed 12000 people

9

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

8 Hanford Engineer Works WashingtonAfter Fermis successful chain reaction and Seaborgs extraction of Plutonium from the irradiated uranium planning began for a plutonium production facility A site in the north-western United States was selected Hanford in Washington state was the most appropriate place It was well isolated and inland the area was flat and the ground was rocky and could sustain the massive reactors The plutonium production had to take place in an unpopulated area since it was still considered highly risky If the chain reaction ran away they had to make sure there were as few casualties as possible For that same reason Fermis Chicago Pile 1 was disassembled and moved southwards into a less populated area There were enormous challenges ahead of the plutonium production project The piles had to be rescaled for a factor of about 20 and the power at which they would be running would be much higher too Fermis experimental pile didnt need cooling since it was designed for low power reactions But if the new plutonium production piles were to produce significant amounts of plutonium they would have to be cooled An experimental plutonium production plant was necessary in order to confirm if the chemical separation processes were good enough for industrial application The experimental reactor was built in Oak Ridge The X-10 was its name and it was an air cooled reactor that was very successful in producing plutonium By the first half of 1944 it had produced enough plutonium that research of its fission characteristics could be done [3] Those characteristics proved to be essential for the plutonium bomb design The construction of the first of the three piles to be built in Hanford began in the summer of 1943 Hanford became the newest boomtown of the Manhattan Engineer District By the summer 1944 50000 people were living there constructing 3 piles and 3 chemical separation plants [3]The piles had to be cooled shielded and automatized Water cooling was applied because it was the most familiar and was in comparison to helium cooling technically less complicated After the completion of construction another few months were spent inserting graphite piping and wiring for the numerous monitoring and control devices The irradiated uranium slugs would be pushed trough the pile and be dropped into water filled pools behind the pile where it would cool off and wait for the most powerful radioactivity to cease Then they would be moved using remote control as the radiation levels were dangerous to the human body to rail carts which drove them to one of the three chemical separation plants These facilities were scaled-up versions of those at X-10 They were 250 m long 20 m wide and 25 high buildings nicknamed Queen Marrys In these ship-cruiser-like buildings workers were placed behind thick concrete walls using periscopes and television monitors to control the remotely guided equipment Bismuth phosphate would carry the plutonium trough a succession of process pools At the end it had to be concentrated and then extracted The extraction took place in a more conventional laboratory setting as the radioactivity levels were not so high anymore The final product of Hanford Engineer Works was plutonium nitrate Pure plutonium was extracted from the nitrate in Los Alamos [3]

Figure 10 [5] X-10 the experimental plutonium production reactor in Oak Ridge

Figure 11 [5] Queen Marry as the construction workers called it was the plutonium separation facility almost entirely remotely controlled

10

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

9 Los AlamosIn Los Alamos New Mexico a research laboratory was established and was led by Robert Oppenheimer a prominent theoretician who was involved in the uranium research from the very beginning of the American uranium project Los Alamos was boy scout ranch in a desolate remote New Mexico on a mesa located some 30 km northwest of Santa Fe The main idea was to bring there all the scientists theoreticians and experimentalists to join forces and resume the research and designing of nuclear weapons in a more organized fashion There were benefits of such an isolated location One of them was to reduce distracting factors and enable the scientists to engage in the research more thoroughly The second was to control the outgoing information But the most important reason was to bring scientists together and enable them to communicate with each other on a regular basis [1]Security measures at Los Alamos were extremely strict No one except only few officials was allowed to make telephone calls and mail was being censored and controlled Being so and in order to keep the scientists in normal psychological state their families moved with them to Los Alamos It was another boomtown of the Manhattan Project Over 1500 construction workers and hundreds of scientists among which there were many Nobel laureates moved to Los Alamos to assist in making the atomic bomb The Los Alamos task was to discover means of efficiently liberating the nuclear energy in an explosive manner at the right place and at the right moment No-one knew exactly how much of the fissionable material would have to be brought together to achieve such a reaction but they knew that the reaction would not occur if the amount was insufficient If the surface area of the fuel mass was great in comparison with the volume of the mass the neutrons would escape out of the material before causing further fission It was clear that an atomic weapon would apply fast neutron fission since it was the only way to release sufficient amounts of energy before the explosion completely scattered the fissionable material In order to determine the critical mass density shape and other characteristics of the materials had to be known One of them was how many neutrons are released by each fission This number was still unknown when Los Alamos commenced operation in 1943 Another unknown characteristic was how likely it is for a neutron to cause fission in a nucleus of U-235 or plutonium This likelihood is described with a term fission cross-section and is expressed in terms of effective target area of the nucleus Only precise and extensive measurements of bombarding these elements with neutrons of different energy could give the scientists at Los Alamos the needed results For that purpose cyclotrons were brought and installed and samples of the fissionable materials were provided to them [4]Surrounding the explosive mass with another material would serve 2 purposes The tamper would reflect back the escaping neutrons effectively reducing the critical mass and it would with its inertia slow down the expansion of the reacting material allowing more of the material to fission before it is blown apart Tamper materials too needed extensive study [4]If the explosion was to be efficient they could not rely on spontaneous fission or a cosmic ray to start the chain reaction A source of neutrons that would release millions of neutrons at the same time and even more importantly at the right time was needed Such a source was called an initiator and it had to be developed especially for the bomb project Another problem was that the critical mass was incapable of not exploding since a random cosmic ray or a spontaneous fission event would certainly set it of in matter of a fraction of a second Though it might not be the most efficient way of exploding it would be more than enough to destroy the bomb and everything around it So a critical mass could not be assembled in advance It had to be done on the exact place of wanted explosion It had to be done very fast since the mass could prematurely explode due to a random cosmic ray if the parts of the mass were approaching each other too slowly That would cause the explosion to be highly inefficient and wasteful since the parts would still not be in an optimal configuration [4] There were two ways of achieving the super fast assembly Both of them used high explosives since this was the only way to achieve large enough speeds First of them was the so-called ldquogunrdquo method A sub-critical projectile of fissionable material is fired using a high explosive gun into a sub-critical target This method was well understood and few technical problems were still to be solved including how the two pieces should be shaped The gun itself would have to be enclosed in the bomb shell as well as the target The other method was implosion Implosion used high explosives to squeeze the material increasing its density and thus rendering it critical It was somehow more problematic since it was still not clear how to avoid the material bursting out of the imploding bomb The hydrodynamics of the problem were very complicated The decision of which method to use was heavily dependent on the U-235s and plutoniums characteristics In 1944 came a confirmation of scientists fear that plutonium was not appropriate for gun assembly [1]

11

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

An isotope of plutonium Pu-240 proved to have a high rate of spontaneous fission thousands of times greater than that of U-235 and it was constantly emitting large numbers of neutrons Since the plutonium produced in Hanford reactors contained a significant fraction of Pu-240 the gun method was too slow The plutonium bomb would react long before optimal conditions were achieved producing only a fizzle a blast many orders of magnitude less powerful Plutonium weapons had to be implosion triggered George Kistiakowsky a Ukrainian-American chemist professor at Harvard was in charge of the troubling calculations concerning the implosion The problem needed extensive computation which demanded the use of IBM punched-card computers More than 600 people were working on the problem by 1945 [3] He used shock wave lensing to produce a spherically symmetrical shock wave that compressed the plutonium core simultaneously from all directions

This was the main reason why the plutonium bomb was first tested on American ground before it was used against the enemy They had to make sure that implosion worked [1]

They were however much more confident in the gun-type assembly methods success The critical mass of U-235 was experimentally determined by Otto Frisch in Los Alamos in 1944 He used dozens of 3 cm bars of enriched uranium hydride and piled them together measuring increased neutron activity This was extremely dangerous work as the reaction could easily run off and would have killed anyone near by a high flux of neutrons and possibly a mechanical explosion The hydrogen in the hydride slowed the advancing reaction sufficiently to observe its progression Richard Feynman referred to this experiments as ldquotickling a sleeping dragons tailrdquo [1] At one point Frisch almost set of a dangerous reaction by leaning over the pile of

Figure 14 [5] The implosion type weapon (Fat Man) was a great challenge Achieving a spherical converging shock wave was the main problem

Figure 13 [5] A gun-type weapon Using highly purified U-235 this type of weapon was relatively easy to create and detonate

12

Figure 12 [5] The explosive shell was made of three basic layers (drawn in purple) The outermost layer was fast explosives (ignited by the detonators ndash the red dot) then followed a slower explosive that lensed (curved) the shock wave in such a way that it set of the inner layer of fast explosives in a spherically symmetrical way This was achieved by a very complicated arrangement of the explosives and triggers and numerous configurations had to be tested before selecting the best one and using it in an actual nuclear test explosion

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

uranium hydride The hydrogen in his body reflected the neutrons back into the pile causing it to fission at a fast rate His coworkers warned him immediately and he scattered the bars quickly with his hands receiving a full daily dose of neutrons (by the Los Alamos standards) in a matter of seconds [1]

10 TrinityIn 1944 preparations began for the very first nuclear weapons test in the history of mankind The test would not only be the ultimate test of the implosion method its purpose was also to obtain detailed and quantitative data of the effects of a nuclear blast The data would then be used for strategical planning and for future improvements Its obvious that such quantitative data would not be available if the bomb was used only in combat The next question was how much energy would such an explosion release The prediction of the explosive force and the effects were very diverse from scientist to scientist Some predicted a zero yield still being skeptical towards Kistiakowskis methods and others believed that a catastrophe of unimaginable proportions was possible They said that the Earths atmosphere will ignite causing a worldwide catastrophe These claims were certainly studied by the Los Alamos scientists and were after careful calculations rejected as being nearly impossible But the fact was that little was known about how a nuclear explosion would look like and what would be its effects So a test was necessary to clear up these disputes and help the Manhattan Project to advise the military an effective way of using such a weapon Trinity was a monstrously big project The test was to be performed about 300 km south of Los Alamos 56 km southeast of Soccoro New Mexico The location was remote and desolate Prior to the test hundreds of kilometers of delivery roads had to be made and hundreds of kilometers of wires for different measuring instruments had to be installed Bunkers for cameras and camps for working personnel had to be built In 1944 plutonium was so scarce that it was of great importance to make sure that an unsuccessful implosion would not scatter the valuable nuclear fuel all over the desert A big steel tank called Jumbo was manufactured for the purpose of containing the implosion blast Later plutonium became more available and the big container was not used It was placed near the explosion site just to see what the explosion would do to it Prior to the nuclear explosion a rehearsal was performed using 100 tons of classical explosive This way instruments for measuring the force of the blast seismic waves and other machinery could be calibrated allowing the scientists to perform better measurements at the real test Also the timing of the cameras and other instruments had to be precisely controlled since the development of events after the activation of the bomb is very rapid The test was rescheduled many times as much of important test had to be done prior to the final blast On 16th of July 1945 at 445 am the weather report was favorable and at 510 the countdown began The countdown was planned to start hours earlier but rain and lightning postponed it It was a nervous time for most of the scientists as they were afraid lightning could accidentally set off the Gadget as the bomb was nicknamedAt 530 am the first nuclear blast was observed 32 detonators went off at the same time The explosives compressed the outer tamper shell the 62 kg plutonium core (39 of bare sphere critical mass) inside it and

the initiator in the very center Neutrons from the initiator entered the plutonium and started a chain reaction producing more and more neutrons 80 generations in millionths of a second The immensely hot core starts emitting soft x-rays which get absorbed by the surrounding air This is called radiation transport Only the boundaries of this sphere of very hot air can be seen by an observer As the hot air cools to half a million degrees by radiation transport a shock wave forms and moves outwards faster then the radiation transport The opaque shock front leaves behind the slowly growing nearly isothermal radiation transport sphere When the shock wave cools down enough to become transparent the second of the

13

Figure 15 [5] The fireball of the Trinity explosion 16 ms after detonation The fireball measures about 200 m in diameter The dark objects in the foreground are trees

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

two flashes of a nuclear bomb is observed This second flash is caused by the revealed scorching interior of the fireball about 01 s from detonation After few seconds the buoyant fireball starts to rise upward This is seen as the mushroom cloud [1] The explosion had huge impact on the scientists Some of them were overwhelmed with unpleasant feelings other felt relieved some were laughing and some were crying The Gadgets yield was equal to about 21 kilotons of TNT Roughly 20 of the plutonium underwent fission The flash was thousands of times brighter than sunlight [3] The desert was instantly illuminated by a flash more powerful than anything witnessed by man so far It was the beginning of the atomic age

The dynamic of the Trinity blast summarized

10-6 s 10-4 s 10-3 s 10-2 s 2 10-2 s 10-1 s 1 s 30 s

Radiation transport of energy from the centre outwards by x-rays

Shock wave leaves behind the isothermal sphere heated by radiation transport

Shock wave moves outwards at the speed of about 30 kms

The first minimum in thermal radiation (the shock-front is opaque to light)

Shock front becomes transparent revealing the hot interior(8000 K)

The thermal radiation reaches its second maximum

Mushroom cloud starts to form(cooling by convection)

Shock wave reaches the closest observers (mechanical blast)

11 Hiroshima and NagasakiNews of a successful test reached president Truman who had replaced Roosevelt after his death in April 1945 But the public was not yet informed of the success The war in Europe was already over with Hitler dead and the German uranium stocks confiscated A secret operation went ahead of the Western front to investigate whether Germans have already constructed a nuclear weapon but found only an abandoned heavy water reactor The Germans were not close to building a nuclear weapon which was a great relief But the war at the Pacific continued The American policy was to surprise the Japanese by this powerful new force As the Japanese did not unconditionally surrender the first uranium bomb - Little Boy - was dropped from about 10 km high and detonated 580 m above Hiroshima after 48 s of falling on the 6 th of August 1945 at 815 local time The blast immediately killed 70000 people injuring 70000 more In few months tens of thousands more died due to radiation poisoning and in the next 5 years the toll rose to 200000 [3] A city of 300000 people was demolished The radiation blast ignited houses and caused extreme skin burns and the blast wave tore down buildings completely leveling with the ground 13 km2 of the city leaving the rest of it in flames [3] Within hours it was announced that America has used a completely new weapon on Japanese city of Hiroshima The yield of the bomb was roughly 13 kilotons while there was 60 kg of U-235 in Little Boy [3] That meant that only 138 of its material underwent fission (1) Thus uranium bomb was very inefficient Nevertheless the effects were unimaginably devastating 90 of all medical personnel in the area was killed or injured by the blast meaning there was no help available for the wounded Hours passed before first planes arrived from Tokyo to see why all the telephone connections had been cut off Two days later another bomb this time an implosion type was dropped on Nagasaki Fat Man the plutonium device dropped on Nagasaki on 8 th of August 1945 had a yield of 21 kilotons of TNT and immediately killed 40000 and injured over 60000 people The total death toll of Fat Man was 140000 people Majority of them civilians [3]

The cities selected for the bombing were one of few that hadnt been damaged considerably by constant air raids that the US Air Force had been performing for months The Americans did not want to bomb an already destroyed city as there would be little to learn about the weapons destructiveness presuming that the city had already been damaged Kyoto was also among the possible targets but luckily the cultural capital of Japan was spared of the destruction [1]The fact is that the Japanese refused to surrender until 2 atomic bombs were dropped onto their homeland and that an invasion of the Japanese islands would be in terms of American lives more costly Also there was great pressure on the American government to end the war in the Pacific and bring hundreds of thousands of soldiers home But the moral question remains up to today Was this mass slaughter of innocent civilians justified Can this kind of mass murder ever be justified

14

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15

12 ConclusionThe Manhattan Project performed 2 more nuclear tests on the Bikini Atoll in South Pacific before it was replaced by the civilian Atomic Energy Commission Journalists scientists military officers congressmen and foreign observers witnessed two explosions in 1946 These were the final weapon tests performed by the Manhattan Project Its influence on the outcome of the war was unmistakable It employed at its peak more than 130000 people an equivalent of the American automobile industry at the time [3] The organization and the fast pace at which things were built and researched was incredible It is a great scientific triumph to develop in just few years such a powerful device initially knowing almost nothing about it One could blame the scientists of the Manhattan Project for the deaths of so many civilians but at the start of the war it was evident that the atomic bomb if possible would be created if not by Americans by someone else The fact is that USA had it first and thus held a somewhat privileged position among the post war countries The nuclear arming race that took place in the second half of the 20 century increased the risk of total annihilation of the planet by nuclear war Its hard to imagine how the world would have looked by now if the Soviet Union was the first to have the bomb but the mere fact that not a single atomic bomb has been dropped in combat after the Fat Man tells us that things could have been a whole lot worse Niels Bohr once said that the complementarity of the bomb lays in its destructiveness Its destructiveness could in fact bring peace to the world No one dares to use an atomic bomb as long the other side has it too [1] This has proved to be a major safety mechanism against a nuclear war so far But subjecting the faith of the world to human error is the risk we are taking by allowing thousands of nuclear warheads to be kept and maintained by different governments

13 Literature[1] R Rhodes The making of the atomic bomb (Penguin Books 1986)[2] M F LAnnunziata Radioactivity Introduction and history (Elsevier Amsterdam 2007)[3] F G Gosling Manhattan Project Making the Atomic Bomb (Diane Publishing Co 1999)[4] Los Alamos Historical Society Los Alamos The beginning of an era (Los Alamos Historical Society 2008)[5] wwwwikipediacom (1032011)[6] httpmemagazineasmeorgArticles2010DecemberBelow_Horizoncfm (532011)[7] httpelectron6physutkeduphys250modulesmodule205nuclear_energyhtm (2032011)[8] httpwwwchemcasescomnuclearnc-07html (2032011)[9] httphyperphysicsphy-astrgsueduhbasenuceneu235chnhtml (2232011)[10] S S Hecker Los Alamos Science 26 16 (2000)

Figure 16 [5] Little Boy (left) and Fat Man (right) mushroom clouds photographed from the bomber airplanes

15