05 Nuclear Energy

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Nuclear Energy


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Key Terms

Atoms: Atoms are made of three main parts: protons, neutrons, andelectrons. The protons and neutrons make up the center of the atomwhile the electrons orbit around the center.

Nucleus: The nucleus of an atom consists of protons and neutrons boundby the nuclear force (also known as the residual strong force).

Atomic number: The number of protons in an element that identifies it.

Isotope: If an atom has a different number of neutrons from protons.Isotopes, measured by their total weight called mass number are thesum of neutrons and protons.

Ion: if an atom has a different number of electron from protons.

Radioactive Elements: Some isotopes are unstable and will decay toreach a stable state. These elements are considered radioactive.


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Nuclear Energy

Synonymous with atomic energy

The nucleus of an atom is the source ofnuclear energy.

When the nucleus splits (fission),nuclear energy is released in the formof heat energy and light energy.

Nuclear energy is also released whennuclei collide at high speeds and join(fuse).


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Fission

Fission occurs when an atoms nucleus splits apart toform two or more different atoms.

The most easily fissionable elements are the isotopesof U 235 and Pu 239.

When fissionable elements are flooded with neutrons

causing the elements to split,they form new radioactiveelements and release extra neutrons that create achain reaction if other fissionable material is present.

While Uranium, atomic number 92, is the heaviestnaturally occurring element, many other elements canbe made by adding protons and neutrons with particle

accelerators or nuclear reactors. In general, the fission process uses higher atomic

numbered elements.


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Fusion

Fusion is the combining of one or moreatoms usually isotopes of hydrogen,which are deuterium and tritium.

Atoms naturally repel each other so fusion

is easiest with these lightest atoms. To force the atoms together it takes

extreme pressure and temperature, thiscan be produced by a fission reaction.

The suns energy is produced from anuclear fusion reaction in which hydrogennuclei fuse to form helium nuclei.


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Radiation

Radioactive decayis the process by which an unstableatomic nucleus loses energy by emitting ionizing particles orradiation.

There are currently 37 radioactive elements in the periodictable, 26 of them are manmade and include Plutonium (Pu)

and Americium (Am) (used in household smoke detectors). Half-life is the time taken for a given radioactive substance

to decay to half of its initial mass

Radiation is the release of particles or energy as aresult ofan unstable atom decaying to reach a stable state.

Natural radiation is everywhere, our bodies, rocks, water,sunshine. However, manmade radiation is much stronger.


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Radioactive Decay


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Types of Radiation

1. Alpha ( ) radiation,

2. Beta ( ) radiation

3. Neutron radiation

4. Gamma radiation

5. X-rays


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Alpha Radiation

Alpha radiation is a heavy, very short-range particle andis actually an ejected helium nucleus (4

2He).

Alpha radiation travels only a short distance (a fewinches) in air, but is not an external hazard.

Most alpha radiation is not able to penetrate human skin,clothing.

Alpha-emitting materials can be harmful to humans if thematerials are inhaled, swallowed, or absorbed throughopen wounds.

A variety of instruments has been designed to measurealpha radiation. A thin-window Geiger-Mueller(GM) probecan detect the presence of alpha radiation.

Alpha decay has been observed only in heavier elements(atomic number 52, Tellurium (Te) and greater). Radium,Radon, Uranium, Thorium are alpha emitters.


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Beta Radiation

Beta radiation is a light, short-range particle and is actually anemitted beta particle (an electron, 01 e or a positron orantielectron 01e).

Beta radiation may moderately penetrating and travel severalfeet in air.

Beta radiation can penetrate human skin to the "germinal layer,"where new skin cells are produced. If high levels of beta-emittingcontaminants are allowed to remain on the skin for a prolongedperiod of time, they may cause skin injury. Beta-emittingcontaminants may be harmful if deposited internally.

Most beta emitters can be detected with a survey instrument anda thin-window GM probe (e.g., "pancake" type).

Clothing provides some protection against beta radiation. Beta sources can be used in radiation therapy to kill cancer cells. Some beta emitters are Strontium-90, Carbon-14, Tritium, and

Sulfur-35.


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Neutron Radiation

Neutron radiation is a kind of ionizing radiation whichconsists of free neutrons.

Large neutron sources are rare, and are usually limited tolarge-sized devices like nuclear reactors or particleaccelerators

Uses Neutron radiation is most commonly used for scattering and diffraction

experiments in order to access the properties and the structure ofmaterials in crystallography, physics, biology, chemistry, materialsscience, geology, mineralogy and related sciences

Neutron radiation is also used in select facilities to treat cancerous

tumors due to its highly penetrating and damaging nature to cellularstructure. Neutrons can also be used forimaging of industrial parts termed neutron

radiography when using film, neutron radioscopy when taking a digitalimage, such as through image plates, and neutron tomography for threedimensional images.


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Gamma and X Radiation

Gamma radiation and x rays are electromagnetic radiation likevisible light, radio waves, and ultraviolet light. Theseelectromagnetic radiations differ only in the amount of energythey have. Gamma rays and x rays are the most energetic ofthese.

Gamma radiation or x rays are able to travel several feet in airand many inches in human tissue. They readily penetrate mostmaterials and are sometimes called "penetrating" radiation.

Dense materials are needed for shielding from gammaradiation. Clothing provides little shielding from penetratingradiation.

Gamma radiation is easily detected by survey meters with aSodium Iodide detector probe.

Gamma radiation and/or x rays frequently accompany theemission of alpha and beta radiation during radioactive decay.

Some Gamma emitters are Iodine-131, Cesium-137, Cobalt-

60, Radium-226, and Technetium-99m (metastable).


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Neutrino and Positron

Neutrino It is an elementary particle that usually travels close to the speed

of light, is electrically neutral, and is able to pass throughordinary matter almost undisturbed. This makes neutrinosextremely difficult to detect. Neutrinos have a very small, butnonzero mass. They are denoted by the Greek letter (nu).

Neutrinos are created as a result of certain types of radioactivedecay or nuclear reactions such as those that take place in theSun, in nuclear reactors, or when cosmic rays hit atoms.

Positron Thepositron or antielectron is the antiparticle or the antimatter

counterpart of the electron. The positron has an electric charge

of+1e, and the same mass as an electron. Positrons may be generated by positron emission radioactive

decay (through weak interactions), or by pair production from asufficiently energetic photon.


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Penetration of Radiation


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Doses

The effect of radiation (on a cellular level): changes functionality of thecell (causing cancer or inherited birth defects) or killing.

Radiation doses are measured in rems or sieverts

100 rem = one sievert = one J/kg. An exposure of

100 Sv will cause death within days 10-50 Sv will cause death from gastrointestinal failure in one totwo weeks

3-5 Sv will cause red bone marrow damage half of the time andmay have severe affects consist of burns, vomiting, hemorrhage,blood changes, hair loss, increased susceptibility to infection, and

death. The current recommended occupational dose by the InternationalCommission for Radiological Protection is 50 mSv per year.

The average radiation dose per year fornon-nuclear workers is about1 mSv.


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Energy from Fossil Fuels

Fossil Fuel (Coal) Energy Density: 2.9 x 107 J/kg

Fuel Consumed by 1000-MWe Plant: 7,300,000 kg/day

2007 Global Coal Consumption: 6.1 billion tons

1 eV=1.6 x10-19 J


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Energy from Nuclear Fission

Fission Fuel Energy Density: 8.2 x 10

13

J/kg Fuel Consumed by 1000-MWe Plant: 3.2 kg/day

neutron235

U

FISSION PRODUCT

neutron

neutron

fission

235U,

fission

activation

FISSION PRODUCT

ACTIVATIONPRODUCT

CHAINREACTION

239Pu, etc.

200 MeV


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Energy from Nuclear Fusion

Fusion Fuel Energy Density: 3.4 x 1014 J/kg

Fuel Consumed by 1000-MWe Plant: 0.6 kg/day

Deuterium Tritium

neutron

fusion

activation

HELIUM

ACTIVATIONPRODUCT

17.6 MeV


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The furnace in which the

controlled fission of nuclear

fuel take place is called reactor

or atomic reactor.


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Control of Fission

The nuclear fission produces a chain reaction, if such a

reaction occurs at uncontrolled rate it will become adivergent chain reaction resulting into an atomic bomb.

In order to control the chain reaction control rods, are

used, which are made of neutron absorbing material. Themovement of these control rods out of the reactor will

allow the neutron being captured by them (control rods)

to produce fission reaction at a brisk rate, thus,

increasing the power generated.

The absorbing controls (control rods) should have the

following properties:


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Properties of Control Rods

They must have high cross-sectional area for the

absorption. They must be adequately strong.

They must have low mass number in order to allowrapid movement with slight inertia effects.

They must be able to shut down the reactor almost

instantly under all conditions. They must stop any instability of the core power.

They must be able to provide adequate control of powerduring operation.

They must provide good resistance to corrosion.

They must be stable under heat and radiation.

They must have reasonable heat-transfer properties.

They must be economical.


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(Thermopiles)


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REACTOR TYPES

Nuclear Reactors are classified by several methods

Fission reactors can be divided roughly into two classes,depending on the energy of the neutrons that are used to sustain the

fission chain reaction:

1.Thermal reactors use slow or thermal neutrons2.Fast neutron reactors

Classification by moderator material

Graphite moderated reactors Water moderated reactors

1.Heavy water reactors

2.Light water moderated reactors


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Classification by coolant

Water cooled reactor

Pressurized water reactor (PWR)

Boiling water reactor (BWR)

Liquid metal cooled reactor

Sodium-cooled fast reactor

Lead-cooled fast reactor

Gas cooled reactors

Classification by phase of fuel

Solid fueled

Liquid fueled

Gas fueled

REACTOR TYPES contd


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Pressurized Water Reactor

uss an


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uss an(High Power Channel-type

Reactor)


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Russian RBMK


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urren ower eac or


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urren ower eac orTypes

Reactor Type Moderator Coolant Comments

Gas Cooled Reactor Graphite Light Water CO2 Coolant. Heat Exchangers

(GCR) Primarily Built in UK

Pressurized Water Reactor Light Water Light Water >50% Reactors in 24 Countries

(PWR) Water Pressure = 2000 psi

Boiling Water Reactor Light Water Light Water 2nd most common, >10% of World

(BWR) Water Pressure = 1000 psi

Canadian Deuterium U. Heavy Water Heavy water Uses natural U fuel (


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Characteristics of an Atomic PowerStation

The power is economical only if the output isvery high.

The initial cost of the project is very high (likethat of hydro-electric stations), but the runningcost may be low. However due to several

international sanctions the scenario is different. The power output is controlled by means of heat

removal methods from the nuclear reactor toprevent the overheating of the reactor.

Fuel transportation is less expensive, as the

fuel is used in terms of kg and not in tonne.


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Points to consider whiledeveloping atomic power plants

To make the casing of the thermopile suchas to withstand high temperature, corrosionand intense neutron bombardment.

To devise a fluid to transfer the heat

energy to gas or to feed water. To protect radiation and heat from heattransferring system.

To dispose the radioactive waste of theplants which give out large quantity as gamma

rays etc. at high energy level. To shield the plant from emitting radiations.


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Energy release by Fission

One of the fission products is represented as:

U235

+ n1

U236

La148

+ Br85

+ 3n

i.e., Uranium235 + neutron Uranium236

Uranium236 Lanthanum148 + Bromine85 + 3 free neutrons.

The mass equation for the above reaction is given as:

235.124 + 1.009 147.96 + 84.938 + 3.027

The mass deficiency on the right hand side of above mass equationis 0.208 a.m.u. (atomic mass unit) or 0.3452 x 10 -27 kg.

The energy release can also be represented in electron volts. Theelectron volt is defined as the energy possessed by an electronafter it has fallen through a potential difference of 1 volt.

1 electron volt = 1.6 x 10-19 J

1 a.m.u. = 931 MeV (Million electron volt)

Equivalent energy released

= 0.208 x 931

= 193.65 MeV ~ 200 MeV

1 amu = 1.6597 x 10-27 kg


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Equivalent energy E = mc2

= 0.3452 x 10-27 x 9 x 1016

= 3.1068 x 10-11 Joules/Atom

No. of atoms in 1 kg Uranium

= 26.029 x 1023 (approx.)

Energy released= 3.1068 x 10-11 x 26.029 x 1023

= 80.87 x 1012 J/kg

The above energy released is approx.equivalent to 2.8 million kg of coal with acalorific value of 28.36 x 106 J/kg


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Nuclear Fuel Cycle


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Mining

All of the material is removed in trucksfrom Opencast Mines


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From mine to mill

The mined material is

now transported tomill in Dumpers


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Milling

The mill crushes the rock to powder

The powder is then treated with sulphuric acid to dissolve

the uranium, leaving the rock (depleted ore) behind

Hard rock needs more


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Hard rock needs moreenergy

Hard rock ores areapproximately 3 to 4times more energy-intensive than soft

rock ores to crush.

Neutralisation


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Neutralisation

The depleted ore is

washed andneutralised usinglime

Lime is made by

roasting limestonewith fossil fuels todrive off the CO2

The slurry ispumped to thetailings ponds


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Tailings ponds

Tailings ponds beingcompacted


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Yellow cake

The uranium solution is filtered, and thengoes through a solvent extraction process thatincludes kerosene and ammonia to purify theuranium solution. After purification the

uranium is put into precipitation tanks. Theresult is a product commonly calledyellowcake.


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Calcining

The yellowcake isroasted at 800C inan oil-fired furnacecalled a calciner

The Ammonium di-uranate is convertedto 98% pureUranium oxide

(U3O8)which is a darkgreen powder


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Packing for shipment

The Uranium oxide is packed into drums andtransported to a shipping port

The drums are then shipped, often half wayaround the world


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UF6 handling

The Uranium hexafluoride gas isthen compressed and transported incylinders to be enriched


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Enrichment

Uranium enrichment increases the amount of U235 in comparisonto U238 . Domestic power plants use a mixture that is 3-5% U235 ,

while highly enriched uranium is generally used for weapons,some research facilities, and naval reactors. Domestic reactorsusually require fuel in the form of uranium dioxide and weaponsuse the enriched mix in the form of a metal. The conversion andenrichment process is very dangerous because not only is theuranium hexafluoride radioactive, it is also chemically toxic. Inaddition, if the uranium hexafluoride comes in contact withmoisture it will release another very toxic chemical calledhydrofluoric acid. There have been numerous accidents during

the conversion and enrichment process. Depleted uranium is thewaste that is generated from the enrichment process.


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Transport

Low-enriched (3.6%) Uranium hexafluoride gasis then transported to the fuel fabrication plant


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Fuel Fabrication

After being enriched, the UF6 is taken to afuel fabrication facility.

The UF6 gas is converted to Uraniumdioxide (UO2) powder and pressed intopellets

They are then baked in an oil-fired furnaceto form a ceramic material.

The pellets are put into long tubes. Thesetubes, made of a zirconium alloy, arecalled fuel rods.

A fuel assembly is a cluster of these sealed

rods. Fuel assemblies go in the core of thenuclear reactor. It takes approximately 25tonnes of fuel to power one 1000 MWereactor per year.

The picture on the right is a fuel assembly.


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Fuel assemblies

Fresh fuel is only mildly radioactive and canbe handled without shielding.

The fuel assemblies are then transported to

the reactor by truck or train


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Transportation

Radioactive materials are transported fromthe milling location to the conversionlocation, then from the conversion location to theenrichment location, then from the enrichment locationto the to the fuel fabrication facility, and finally to the

power plant. These materials are transported inspecial containers by specialized transport companies.People involved in the transport process are trained torespond to emergencies. In the US, Asia, and WesternEurope transport is mainly by truck, and in Russiamainly by train. Intercontinental transport is usually by

ship, and sometimes by air. Since 1971 there has beenover 20,000 shipments with no incidents and limitedoperator exposure.


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Nuclear Reactors

There are usually several hundred fuelassemblies in a reactor core. There are severaltypes of reactors, but they all use a controlledfission process with a moderator like water

or graphite. During the fission process, plutonium iscreated and half of the plutonium also fissions accountingfor a third of the energy. The fission process makes heatthat is converted to energy.

S t f l


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Spent fuel

Spent fuel rodsnormally spend sixmonths in coolingponds located

within the reactorbuilding, so thatshort-lived radio-activity can decay,

making the materialsafer to handle

W t l t it


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Waste leaves reactor site

Reactor waste is moved by road and rail

It is highly radioactive

There are only a very small number ofreprocessing plants in the world, so thedistances involved are long.

Short term storage


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Short-term storage

This is The Pond at Sellafield in theUK. Spent fuel is kept under water untilit is reprocessed. This keeps it cool and

acts as a radiation shield


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Long-term storage

There are no long-term storage facilitiesoperating anywhere in the world

This is the one military bunker at a Mountain. Thisis what one might look like if one was ever to bebuilt

H d


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Hazard

Degree of retention of radioactive material in the

body. If the material is retained for longer time, itis capable of doing more harm.

The fraction of the radioactive material which ispassed to the critical tissues by the blood stream.

The greater the fraction conveyed, the greater will

be the harm.The radio sensitivity of the tissues involved for

example bone, lymph, glands, ovaries, testes aremore vulnerable to energetic radiation.

Size of organ involved. If the organ is small the

concentration of the radioactive material is highwhich can cause more damage.Essentiality of organ. The most essential organ

damaged by radioactivity can cause early death.Type of radiation.


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Energy Release by Fusion

Fusion of two heavy hydrogen atoms (1H2) (sometimescalled as Deuterium atoms represented as D) toproduce a helium atom.

1H2 + 1H2 = He3 + n + energy

The mass of heavy hydrogen atom is 2.01473 a.m.u.

mass of helium atom is 3.016997 a.m.u. and the massof free neutron is 1.00898, thus, mass equation is

D+D = He3 + n

2.01473 + 2.01473 = 3.016997 + 1.00898

It will be observed that the sum of mass on the left andright side of above equation are not same. There is amass deficiency on the right hand side equal to0.003483 a.m.u. or equivalent heat energy released is0.003483 x 931 MeV or 3.24 MeV.


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Future Fusion Reactors

1.The fusion technology is still at

infancy

2.Several fusion reactors have been

built, but as of yet, none hasproduced more thermal energy than

electrical energy consumed. Despite

research having started in the 1950s,

no commercial fusion reactor isexpected before 2050.


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05 Nuclear Energy