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Chapter 8: Nuclear Power Plants
8.1 Introduction
8.2 Main parts of a nuclear power plant
8.3 Location of a nuclear power plant
8.4 Functional parts of a nuclear power reactor
8.5 Classification of nuclear power reactors
8.6 Nuclear radiations produced in a nuclear plant
8.7 Disposal of Nuclear Waste and Effluent
8.8 Radiation measurements and safety
8.9 Radiation effects on humans
8.1 Introduction
The energy need of a country cannot be met from a single source. Hydro electric stations produce
cheap power but need a thermal backing to increase the firm capacity. The coal reserves of the
world are fast depleting. Also energy suppliers need to ensure that they do not contribute to short
and long-term environmental problems. Governments need to ensure energy is generated safely
to that neither people nor the environment are harmed. The nuclear power is the only source
which can supply the future energy demands of the world. Nuclearhas a number of advantages
that warrant its use as one of the many methods of supplying an energy-demanding world. The
main advantages which nuclear power plants posses are:
y The amount of fuel used is small. Therefore, the fuel cost is low.
y Since the amount of fuel needed is small, there are no problems of fuel transportation,storage, etc.
y Nuclear power plants need less area than the conventional steam plants. A 2000 MWnuclear plant needs about 80 acres of land as compared to about 250 acres for a 2000MW coal fired steam plant.
y
Greater nuclear power production leads to conservation of coal, oil, etc.y Nuclear power plants need less fuel than ones which burn fossil fuels. One ton of uranium
produces more energy than is produced by several million tons of coal or several millionbarrels of oil.
y Waste is more compact than any source
y Extensive scientific basis for the cycle
y No greenhouse or acid rain effects.
y Well-operated nuclear power plants do not release contaminants into the environment.
Table1 compares the fuel requirements to produce 1 GW (1-million kilowatt) of electricity (i.e.,enough electricity for a city of 560,000 people) by common energy sources.
8.2 Main parts of a nuclear power plant
Most thermal power reactors work in a manner similar to that of fossil fuelled thermal power
plants. A nuclear plant consists of a nuclear reactor (forheat generation), heat exchanger (for
converting water into steam by using the heat generated in reactor), steam turbine, alternator,
condenser, etc. Thus it is similar to a steam station except that the nuclear reactor and heat
exchanger replace the boiler. Some of the auxiliaries are similar to those in the steam plant. As in
a steam plant, water for raising steam forms a closed feed system. The reactor and cooling circuit
have to be heavily shielded to eliminate radiation hazards.
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Table 1: Fuel requirement to produce 1 GW
Fuel Mass Required
Uranium 33 tons
Coal 2,300,000 tons
Oil 10,000,000 barrels
Natural Gas 64,000,000,000 cubic feet
Solar Cells 25,000 acres
Garbage 7,000,000 tons
Wood 3,000,000 cords
Fig. 1: World Electricity Generation
Therefore, a nuclear power reactor is nothing more than a steam electric generating station in
which the nuclear reactor takes the place of a furnace and the heat comes from the continuousfissioning of uranium atoms rather than from the burning of fossil fuel. To control the heat
production, control rods made of materials which absorb neutrons, are placed among the fuel
assemblies. When the control rods are pulled out of the core, more neutrons are available and the
chain reaction speeds up, producing more heat. When they are inserted into the core, more
neutrons are absorbed, and the chain reaction slows or stops, reducing the heat.
Two different light-water reactor designs are currently in use for producing steam from the heated
water,
(a) Pressurized Water Reactors (PWR) and
(b) Boiling Water Reactors (BWR)
Figure 2 shows the main parts of a nuclear power plant. A nuclear plant consists of:
(1) A nuclear reactor (forheat generation),
(2) Heat exchanger (for converting water into steam by using the heat generated in reactor)
(3) Containment structure
(4) Steam turbine,
(5) Condenser, etc.
8.3 Location of a nuclear power plant
In taking a decision on locating a new nuclear power plant, the following points have to be kept in
view:
1. A nuclear plant needs very little fuel. Hence it does not require direct rail facilities for fuel
transport. However, transport facilities are needed during the construction stage.
2. The cooling water requirements of a nuclear plant are very heavy. A nuclear plant needs
more than twice the water required for the same size coal plant. It is very rare that a river
with such a large flow through the year would be available. Hence cooling towers for
nuclear plants are larger than those for coal stations.
3. Areas remote from coal fields and hydro sites are preferable to improve the reliability of
supply over the whole area.
4. The substrata must be strong enough to support the heavy reactors which may weigh as
high as 100,000 tons and impose bearing pressures of around 50 tons per square metre.
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5. In the eventuality of an escape of radioactivity, proper monitoring, radiological control and
public safety can be more easily ensured in thinly populated areas. As such most of the
countries prescribe maximum permissible population densities within certain distances of
nuclear stations.
Fig. 2: Main parts of a nuclear power plant
8.4 Functional parts of a nuclear power reactor
Nuclear Power Reactor:
The need to keep keff " 1, to reduce thenumber of neutrons escaping from thereactor, to convert heat energy into electricenergy, and to keep the radiation out of theenvironment dictate the inclusion of severalcommon types of components in the basicreactor design as shown in Fig. 3.
(i) Reactor Core(ii) Moderator and Reflector(iii) Coolant(iv) Radiation Shielding(v) Control and Safety
Fig. 3: Functional parts of a nuclear power reactor
(i) Reactor Core:
The core consists principally of the fuel, moderator, and structural material. Because of design
and operating difficulties inherent in fuels consisting of naturally occurring uranium, enriched
uranium is used. The degree of enrichment depends on the design features of the reactor or vice
versa. The advantages of fuel enrichment are:
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Fuel:
Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium
dioxide (UO2) powder and pressed into small pellets about 0.6 in (1.5 cm) thick and 0.4 in
(1.0 cm) in diameter. These pellets are inserted into thin tubes , prevent chemical reactions
between fuel and moderator and provide structural support. Several physical factors considered
for cladding materials are: low neutron capture probability, structural strength at high
temperatures, good heat transfer, and non-corrodible characteristics. Commonly used cladding
materials that meet these requirements are aluminum, zirconium, alloys of the two, and stainless
steel.
Fuel rods tend to be about 12 ft (3.7 m) long and about 0.5 in (1.3 cm) in diameter. Fuel rods
are assembled into bundles called fuel assemblies. A fuel assembly consists of a square or
hexagonal array of 179 to 264 fuel rods, and 121 to 193 fuel assemblies are loaded into an
individual reactor. Some 25 ton s of fresh fuel is required each year by a 1000 MWe reactor.
Fig: A drum of yellowcake
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By far the most common type of nuclear fuels are:
UO2 (Uranium dioxide) : U(235
U+238
U)O2
ThO2 (Uranium dioxide) :232Th + O2
MOX (Mixed Oxide) : PuO2 + UO2
Enriched uranium:
Enriched uranium is a kind of uranium in which the percent composition of235
U has been
increased through the process of isotope separation. Natural uranium is 99.284%238
U isotope,
with235
U only constituting about 0.711% of its weight.235
U is the only isotope existing in nature
that is fissile with thermal neutrons.
Low-enriched uranium (LEU)
Low-enriched uranium'(LEU) has a lower than 20% concentration of235
U.
For use in commercial light water reactors (LWR), the most prevalent power
reactors in the world, uranium is enriched to 3 to 5%235
U. Fresh LEU used
in research reactors is usually enriched 12% to 19.75% U-235, the latter
concentration being used to replace HEU fuels when converting to LEU.
Highly enriched uranium (HEU)
Highly enriched uranium (HEU) has a greater than 20% concentration of235
U or233
U. The
fissile uranium in nuclear weapons usually contains 85% or more of235
U known as weapon(s)-
grade, though for a crude, inefficient weapon 20% is sufficient (called weapon(s)-usable); some
argue that even less is sufficient, but then the critical mass for un-moderated fast neutrons rapidly
increases, reaching infinity at 6%235
U.
(ii) Moderator and Reflector:
Slow neutrons have a higher probability of producing fission in 235U than do fast neutrons.
Neutrons emitted from the fission of235
U have a wide spectrum of energies from 0.025 eV to
approximately 7 MeV. Because the reactor needs slow neutrons, a moderator is used to slow (or
moderate) the neutrons down and enhance the fission process.
On the average, neutrons lose more energy per elastic collision with particles of equal mass (i.e.
hydrogen nuclei) than they do in colliding withheavier particles. For example, it takes less than 20
collisions to thermalize a neutron using ordinary water as a moderator, but more than 100
collisions with graphite. For this reason, materials with low atomic weight (generally hydrogen or
hydrogenous compounds) are used for moderators.
In the moderation process, some of the neutrons may be scattered at angles which project or
reflect them back toward where they came from (i.e., toward the core). Thus, some moderatingmaterials may also be suitable reflectors which serve to reduce neutron leakage. Usually such
reflector materials are of low mass number may be interspersed among the fuel elements where
they serve as a moderator and, when placed outside the reactor core area they can serve as a
neutron reflector. An important criterion is that the material used for the reflector and moderator
have a low probability for neutron capture. Most commonly used reflector and moderator
materials are:
The ideal moderator is of low mass, high scattering cross section, and low absorption cross
section. It should be inexpensive and chemically inert and should not corrode or erode.
Fig: A billet ofhighlyenriched uranium metal
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(iii) Control Rods:
Control rod is a rod made of chemical elements capable of absorbing many neutrons without
fissioning themselves. These are made with neutron-absorbing
material such as silver, indium and cadmium. Other elements that
can be used include boron, cobalt, hafnium, dysprosium, gadolinium,
samarium, erbium, and europium, or their alloys and compounds, e.g.
high-boron steel, silver-indium-cadmium alloy, boron carbide,
zirconium diboride, titanium diboride, hafnium diboride, gadolinium
titanate, and dysprosium titanate.
Control rods are usually combined into control rod assemblies typically 20 rods for a commercial Pressurized Water Reactor(PWR)
assembly and inserted into guide tubes within a fuel element.
The operation of a reactor can be described in terms of the multiplication factor, keff. Control rods
maintain the proper keff factor for various stages of reactor operation. Control rods are made of
materials which have a high capture cross section, removing them from the core region and
making them unavailable for further fissioning. The neutron population is controlled by moving the
rods in or out of the core region. With precise positioning of these rods, it is easy to maintain the
point where keff = 1 is reached and produce a stable, critical state in the reactor.
Control rods are classified as either coarse or shim rods. The names refer to their degree of
adjustment. Coarse control rods are used for making gross adjustments, while the shim rods areused for making finer adjustments in the number of fission events. Other rods called safety or
scram rods, are strategically positioned in the core. In the event of a drastic increase in k eff (i.e.,
super-criticality), these rods are inserted in the core immediately to shut down the reactor. Control
rods may also be used as safety rods. A unique concept of reactor control is t he adding of boron
directly to the coolant. The concentration of boron in the coolant is varied for routine control with
major reactivity changes controlled by the rods.
(iv) Coolant:
The great quantities ofheat produced in the reactor core must be removed to prevent the fuel
elements from melting. In a power reactor, the core heat is used to make steam which may turn a
turbine-generator to produce electricity or, in ships to turn propellers. The cooling system
removes the core heat by circulating a heat absorbing material through the core. The heat
generated in the fuel elements is transferred to this coolant and circulated out of the core.
Desirable coolant materials should have a low probability for neutron capture, have good heat
transfer capabilities, and be easy to move through the core. The most commonly used coolant
materials are:
(a) Water/Heavy Water
(b) Helium and carbon dioxide Gas
(c) Sodium and sodium-potassium Liquid Metal
Currently operating nuclear power plants
Moderator Reactors Design Country
graphite 30 AGR, Magnox, RBMK Britain, Russia, Lithuania
heavy water 42 CANDU Canada, India, South Korea, others
light water 359 PWR, BWR 27 countries
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(v) Radiation Shielding
Reactor shields may be designed for several functions. Shielding to reduce the radiation
exposure to persons in the reactor building is called biological shielding. Neutrons and gamma
rays emitted by the fission fragments produced in the fuel elements present the most serious
shielding problems. Alpha / beta particles and the recoil fission fragments are generally absorbed
by the fuel cladding and other materials used in reactor construction. Because the probability of
neutron capture/removal increases as the neutron kinetic energy decreases (i.e., becomes
thermal), a shield for neutrons must necessarily first moderate (i.e., slow down) the neutrons andthen remove them through capture reactions. Good neutron moderators are low density materials
with a highhydrogen content.
Good absorbers for gamma rays are high density materials such as lead or iron. Concrete is a
good compromise for shielding against both gamma rays and neutrons from reactors. It contains
both low and fairly high atomic weight materials (hydrogen and silicon). Besides good shielding
properties, concrete has good structural qualities and is relatively inexpensive. Iron punching and
boron can be added to enhance gamma shielding and neutron capture, respectively. Because of
these assets, concrete is the most often used shielding material in reactors. Waterhas been used
as a shielding material in special applications. Although the shielding properties of water are good,
its use presents considerable construction difficulties (e.g., no form).Th
us, in a reactor th
ecoolant provides shielding as an added benefit of its cooling role.
(vii) Containment Structure:
A containment building, in its most common usage, is a steel
or reinforced concrete structure enclosing a nuclear reactor. It is
designed, in any emergency, to contain the escape of radiation
to a maximum pressure in the range of 60 to 200 psi ( 410 to
1400 kPa). The containment is the final barrier to radioactive
release (part of a nuclear reactor's defence in depth strategy),
the first being the fuel ceramic itself, the second being the metal
fuel cladding tubes, the third being the reactor vessel and
coolant system.
The containment building itself is typically an airtight steel
structure enclosing the reactor normally sealed off from the
outside atmosphere. The steel is either free-standing or
attached to the concrete missile shield.
While the containment plays a critical role in the most severe nuclear reactor accidents, it is only
designed to contain or condense steam in the short term (for large break accidents) and long term
heat removal still must be provided by other systems. In the Three Mile Island accident the
containment pressure boundary was maintained, but due to insufficient cooling, some time after
the accident, radioactive gas was intentionally let from containment by operators to prevent over
pressurization. This, combined with further failures caused the release of radioactive gas to
atmosphere during the accident.
8.5 Classification of nuclear power reactors
Reactors may be classified by many categories, such as fuel-moderator arrangement, type of
coolant, reactor use, or a combination of these. The two major types of light water (i.e., H2O)
power reactors used, pressurized and boiling water, differ primarily in temperature and pressure
within the reactor core.
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(i) Fuel-Moderator Arrangement
In a homogeneous reactor, the fuel and moderator are in contact and intimately mixed with each
other. A heterogeneous reactor is one in which the fuel is lumped into rods surrounded by a
moderator and coolant.Powerreactors are heterogeneous reactors.
(ii) Reactor Use
There are four basic uses for reactors: research, power production, isotope production, and
breeder. Some of the salient features of each are:
Research - A reactor primarily used for research, either as a prototype or proving ground
for future reactor design or operated to produce neutrons for pure scientific research. A
homogeneous reactor would be an example of a research reactor.
Power - Heat produced in the core is removed by the coolant and put through various
heat exchanger subsystems and it is eventually converted to electrical or mechanical
energy.
Isotope Production - High neutron fluxes inside the reactor may be used to produce
radioisotopes or other products (e.g., colored gemstones, etc.) through neutron capture.
One of the more familiar reactions is the production of32
P by the absorption of a neutron
by 31P, the only naturally occurring isotope of phosphorus (i.e., abundance = 100%).
Similarly, the radiopharmaceutical most frequently used in Nuclear Medicine can beproduced by separating
98Mo (abundance = 24.13%) from natural Molybdenum and
bombarding the98
Mo with neutrons to produce99
Mo. The99
Mo is then placed in a
generator, which can be used to elute99m
Tc for diagnostic nuclear medicine.
Breeder / Converter - In addition to producing energy which may be used for power
generation, the breeder reactor produces more fissionable material than it consumes. The
reactor may be designed solely to produce fissionable material (e.g.,239
Pu) which may
then be processed and used at another facility.
(iii) Coolant
Reactors may also be classified by the type of coolant employed to remove the fission energy
from the reactor core and produce steam to turn the turbine-generator.
Boiling Water- The system is pressurized, but controlled boiling is allowed to occur in
the core. Steam is removed via a steam separator and sent to the turbine-generator or
heating system.
Heavy Water- Deuterium Oxide (D2O) is used instead of ordinary water.
Pressurized Water- The coolant system is pressurized to the extent necessary to
prevent boiling in the core. Steam is produced in a secondary system (i.e. steam
generator) at lower pressures.
Liquid Metal - Various liquid metals are used as coolants, primarily in fast breeder
reactors where no moderation of neutrons is wanted.
Gas - Inert gases or air serve as the heat removal material.
Fuelling a nuclear power reactor
Most reactors need to be shut down for refueling, so that the pressure vessel can be opened up.In this case refueling is at intervals of 1-2 years, when a quarter to a third of the fuel assembliesare replaced with fresh ones. The CANDU and RBMK types have pressure tubes (rather than apressure vessel enclosing the reactor core) and can be refueled under load by disconnectingindividual pressure tubes.
If graphite orheavy water is used as moderator, it is possible to run a power reactor on naturalinstead of enriched uranium. Natural uranium has the same elemental composition as when itwas mined (0.7% U-235, over 99.2% U-238), enriched uranium has had the proportion of the
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fissile isotope (U-235) increased by a process called enrichment, commonly to 3.5 - 5.0%. In thiscase the moderator can be ordinary water, and such reactors are collectively called light waterreactors. Because the light water absorbs neutrons as well as slowing them, it is less efficient asa moderator than heavy water or graphite.
During operation, some of the U-238 is changed to plutonium, and Pu-239 ends up providingabout one third of the energy from the fuel.
In most reactors the fuel is ceramic uranium oxide (UO2 with a melting point of 2800C) and most
is enriched. The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are typically arrangedin a long zirconium alloy (zircaloy) tube to form a fuel rod, the zirconium being hard, corrosion-resistant and permeable to neutrons.* Numerous rods form a fuel assembly, which is an openlattice and can be lifted into and out of the reactor core. In the most common reactors these areabout 3.5 to 4 metres long.
*Zirconium is an important mineral for nuclear power, where it finds its main use. It is thereforesubject to controls on trading. It is normally contaminated withhafnium, a neutron absorber, sovery pure 'nuclear grade' Zr is used to make the zircaloy, which is about 98% Zr plus tin, iron,chromium and sometimes nickel to enhance its strength.
Burnable poisons are often used (especially in BWR) in fuel or coolant to even out theperformance of the reactor over time from fresh fuel being loaded to refueling. These are neutron
absorbers which decay under neutron exposure, compensating for the progressive build up ofneutron absorbers in the fuel as it is burned. The best known is gadolinium, which is a vitalingredient of fuel in naval reactors where installing fresh fuel is very inconvenient, so reactors aredesigned to run more than a decade between refueling.
Nuclear power plants in commercial operation
Reactor type Main Countries Number GWe Fuel Coolant Moderator
Pressurised Water Reactor(PWR)
US, France,Japan, Russia,
China265 251.6
enrichedUO2
water water
Boiling Water Reactor(BWR)
US, Japan,Sweden 94 86.4
enrichedUO2 water water
Pressurised Heavy WaterReactor 'CANDU' (PHWR)
Canada, SouthKorea
44 24.3naturalUO2
heavywater
heavywater
Gas-cooled Reactor (AGR& Magnox)
UK 18 10.8
natural U(metal),enriched
UO2
CO2 graphite
Light Water GraphiteReactor (RBMK)
Russia 12 12.3enriched
UO2water graphite
Fast Neutron Reactor(FBR)
Japan, Russia 2 1.0PuO2 and
UO2liquid
sodiumnone
Oth
er Russia 4 0.05
enriched
UO2 water graph
ite
TOT AL 439 386.5
GWe = capacity in thousands of megawatts (gross)Source: Nuclear Engineering International Handbook 2010
For reactors under construction: see paper Plans for New Reactors Worldwide.
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The power rating of a nuclear power reactor
Nuclear power plant reactor power outputs are quoted in three ways:
Thermal MWt, which depends on the design of the actual nuclear reactor itself, and relates to thequantity and quality of the steam it produces.
Gross electrical MWe indicates the power produced by the attached steam turbine and generator,and also takes into account the ambient temperature for the condenser circuit (cooler means
more electric power, warmer means less). Rated gross power assumes certain conditions withboth.
Net electrical MWe, which is the power available to be sent out from the plant to the grid, afterdeducting the electrical power needed to run the reactor (cooling and feed-water pumps, etc.) andthe rest of the plant.
* * footnote: This (as also actual gross MWe) varies slightly from summer to winter, so normallythe lower summer figure, or an average figure, is used. If the summer figure is quoted plants mayshow a capacity factor greater than 100% in cooler times. Some design options, such aspowering the main large feed-water pumps with electric motors (as in EPR) rather than steamturbines (taking steam before it gets to the main turbine-generator), explains some gross to netdifferences between different reactor types.The EPR has a relatively large drop from gross to net
MWe for this reason.
8.6 Nuclear radiations produced in a nuclear plant
In physics, radiation describes any process in which energy emitted by one body travels through
a medium or through space, ultimately to be absorbed by another body.
The radiation one typically encounters is one of four types: alpha radiation, beta radiation, gamma
radiation, and x radiation. Neutron radiation is also encountered in nuclear power plants and high-
altitude flight and emitted from some industrial radioactive sources.
1. Alpha Radiation
Alpha radiation is a heavy, very short-range particle and is actually an ejected helium nucleus.
Some characteristics of alpha radiation are:
o Most alpha radiation is not able to penetrate human skin.
o Alpha-emitting materials can be harmful to humans if the materials are inhaled,
swallowed, or absorbed through open wounds.
o A variety of instruments has been designed to measure alpha radiation. Special
training in the use of these instruments is essential for making accurate
measurements.
o A thin-window Geiger-Mueller (GM) probe can detect the presence of alpha radiation.
o Instruments cannot detect alpha radiation through even a thin layer of water, dust,
paper, or other material, because alpha radiation is not penetrating.
o Alpha radiation travels only a short distance (a few inches) in air, but is not an
external hazard.
o Alpha radiation is not able to penetrate clothing.
Examples of some alpha emitters: radium, radon, uranium, thorium.
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2. Beta Radiation
Beta radiation is a light, short-range particle and is actually an ejected electron. Some
characteristics of beta radiation are:
o Beta radiation may travel several feet in air and is moderately penetrating.
o Beta radiation can penetrate human skin to the "germinal layer," where new skin cells
are produced. Ifhigh levels of beta-emitting contaminants are allowed to remain on
the skin for a prolonged period of time, they may cause skin injury.
o Beta-emitting contaminants may be harmful if deposited internally.
o Most beta emitters can be detected with a survey instrument and a thin-window GM
probe (e.g., "pancake" type). Some beta emitters, however, produce very low-energy,
poorly penetrating radiation that may be difficult or impossible to detect. Examples of
these difficult-to-detect beta emitters are hydrogen-3 (tritium), carbon-14, and sulfur-
35.
o Clothing provides some protection against beta radiation.
Examples of some pure beta emitters: strontium-90, carbon-14, tritium, and sulfur-35.
3. Gamma and X Radiation
Gamma radiation and x rays are highly penetrating electromagnetic radiation. Some
characteristics of these radiations are:
o Gamma radiation or x rays are able to travel many feet in air and many inches in
human tissue. They readily penetrate most materials and are sometimes called
"penetrating" radiation.
o X rays are like gamma rays. X rays, too, are penetrating radiation. Sealed radioactive
sources and machines that emit gamma radiation and x rays respectively constitute
mainly an external hazard to humans.
o Gamma radiation and x rays are electromagnetic radiation like visible light,radiowaves, and ultraviolet light. These electromagnetic radiations differ only in the
amount of energy they have. Gamma rays and x rays are the most energetic of these.
o Dense materials are needed for shielding from gamma radiation. Clothing provides
little shielding from penetrating radiation, but will prevent contamination of the skin by
gamma-emitting radioactive materials.
o Gamma radiation is easily detected by survey meters with a sodium iodide detector
probe.
o Gamma radiation and/or characteristic x rays frequently accompany the emission of
alpha and beta radiation during radioactive decay.
Examples of some gamma emitters: iodine-131, cesium-137, cobalt-60, radium-226, andtechnetium-99m.
4. Neutron Radiation
Neutron radiation is a kind of non-ionizing radiation which consists of free neutrons.
o Due to the high kinetic energy of neutrons, this radiation is considered to be the most
severe and dangerous radiation available.
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o Neutrons are uncharged particles; as a result they are more penetrating than alpha
radiation or beta radiation. In some cases they are more penetrating than gamma
radiation, which is impeded in materials of high atomic number. In hydrogen, a low
energy neutron may not be as penetrating as a high energy gamma.
o The most effective materials are for example water, polyethylene, paraffin wax, or
concrete, where a considerable amount of water molecules are chemically bound to
the cement.
o The light atoms serve to slow down the neutrons by elastic scattering, so they can
then be absorbed by nuclear reactions. However, gamma radiation is often produced
in such reactions, so additional shielding has to be provided to absorb it.
o In living tissue, neutrons have a relatively high relative biological effectiveness, and
are roughly ten times more effective at causing cancers compared to photon or beta
radiation of equivalent radiation exposure.
Examples of neutron emitters are nuclear fission, nuclear fusion, very high energy
reactions such as in the Spallation Neutron Source and in cosmic ray interactions, or from
other nuclear reactions such as the historically significant (,n) reaction.
8.7 Disposal of Nuclear Waste and Effluent
The disposal of solid, liquid and gaseous waste and effluent from nuclear power plants needs
special attention because of the danger of radiation. It is necessary to measure the radioactivity in
the gaseous and liquid effluents and keep the records. Gaseous effluents are filtered before
discharging into atmosphere. Moreover, the filtered gas is discharged at high levels so that it is
discharged properly. The probability of fire in the reactor fuel channel is very low. However, if fire
does take place, large volumes of gaseous fission products may be released. It is necessary to
have a clean up plant through which these products can be passed to remove radioactive iodine
which is the majorhazard.
It is essential to monitor the loss of carbon-dioxide from the reactor to ensure that this loss does
not exceed about 1 ton per day. It is necessary to ch
eck th
e concentration of carbon-dioxide inthe atmosphere near the reactor. Proper precautions against toxic and radiological hazards are
necessary, specially during scheduled blowing down operations.
At most of the nuclear power stations, the liquid effluents are discharged after filtration, pH
adjustment and dilution by mixing with the discharged cooling water. However, at some stations it
may be necessary to remove the radioactivity from the liquid effluents by ion-exchange process.
Proper records are maintained for all potentially radioactive liquids discharged from the plant.
These records should indicate the quantities of such effluents discharged. The samples of
discharge are also kept so that these samples may be checked by Government agencies.
It is necessary to take special precautions regarding leakage of radioactive liquid effluents to
ground. These precautions include double containment of drains and design of concrete storage
tanks.
The solid wastes like rejected control rods, pieces of fuel cans, etc. have to be stored in shielded
concrete vaults. It is necessary to separate chemically incompatible and combustible materials.
The most highly radioactive solid wastes are irradiated fuel elements. These waste elements are
stored under water or air cooled shielded area for about 100 days so that radioactivity may decay
to a sufficiently low level. The spent fuel storage chambers have capacities to cool, shield and
store such materials for many years. After this time, these wastes are disposed to underground
places. Vacated coal mines are also used for this disposal.
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8.8 Radiation measurements and safety
It is necessary to ensure that environment in and around a nuclear plant is safe for personnel and
delicate instruments. The radiations from the nuclear plant have to be monitored and safety
ensured under all conditions of operation.
Radiation Measurement:
When scientists measure radiation, they use different terms depending on whether they arediscussing radiation coming from a radioactive source, the radiation dose absorbed by a person,
or the risk that a person will sufferhealth effects (biological risk) from exposure to radiation.
Units of Measure
Different units of measure are used depending on what aspect of radiation is being measured.
For example, the amount of radiation being given off, or emitted, by a radioactive material is
measured using the conventional unit curie (Ci), named for the famed scientist Marie Curie, or
the SI unit becquerel (Bq). The radiation dose absorbed by a person (that is, the amount of
energy deposited in human tissue by radiation) is measured using the conventional unit rad or the
SI unit gray (Gy). The biological risk of exposure to radiation is measured using the conventional
unit rem or the SI unit sievert (Sv).
Measuring Emitted Radiation
When the amount of radiation being emitted or given off is measured the conventional unit Ci or
the SI unit Bq. The Ci or Bq is used to express the number of disintegrations of radioactive atoms
in a radioactive material over a period of time. Also Ci or Bq may be used to refer to t he amount
of radioactive materials released into the environment. For example, one Ci is equal to 37 billion
(37 X 109) disintegrations per second. One Bq is equal to one disintegration per second, one Ci is
equal to 37 billion (37 X 109) Bq.
Measuring Radiation Dose
Wh
en a person is exposed to radiation, energy is deposited in th
e tissues of th
e body.Th
eamount of energy deposited per unit of weight of human tissue is called the absorbed dose.
Absorbed dose is measured using the conventional rad or the SI Gray (Gy). The rad, which
stands for radiation absorbed dose, was the conventional unit of measurement, but it has been
replaced by the Gy. One Gy is equal to 100 rad.
Measuring Biological Risk
A person's biological risk (that is, the risk that a person will sufferhealth effects from an exposure
to radiation) is measured using the conventional unit rem or the SI unit Sievert (Sv) (1 Sv = 100
rem).
Annual limit on intake (ALI)
OrganNRC Limit
(mrem/year)
Whole Body 5000
Lens of the Eye 15,000
Extremities 50,000
Skin 50,000
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The intake in the body by inhalation, ingestion or through
the skin of a given radionuclide in a year which would result
in a committed dose equal to the relevant dose limit. The
ALI is the smaller value of intake of a given radionuclide in
a year by the reference man that would result in either a
committed effective dose equivalent of 5 rems (0.05 Sv) or
a committed dose equivalent of 50 rems (0.5 Sv) to any
individual organ or tissue. In general, a yearly dose of
0.36 rem (360 millirem) from all radiation sources has notbeen shown to cause humans any harm. The Nuclear Regulatory Commission (NRC) have
established dose limits which are based on recommendations from national and international
commissions are shown in the table.
Doses from Medical Procedures
In addition to natural background radiation, we receive
an average dose of about 0.06 rem (60 mrem) per year
from man-made sources of radiation, including medical,
commercial, and industrial sources. Among these
medical procedures, x-rays, mammography, and CT useradiation or perform functions similar to those of
radioisotopes.
Radioactivity in Food
All organic matter (both plant and animal) contains some
small amount of radiation from radioactive potassium-40
(40
K), radium-226 (226
Ra), and other isotopes. In addition,
all water on Earth contains small amounts of dissolved
uranium and thorium. As a result, the average person
receives an average internal dose of about 30 mrem ofthese materials per year from the food and water that we
eat and drink, as illustrated by the following table.
8.9 Radiation Effects on Humans
Certain body parts are more specifically affected by
exposure to different types of radiation sources. Several
factors are involved in determining the potential health effects of exposure to radiation. These
include:
1. Amount of dose absorbed
2. Duration of exposure
3. The ability of the radiation to harm human tissue
4. Which organs are affected
Long Term Effects on Humans
Long after the acute effects of radiation have subsided, radiation damage continues to produce a
wide range of physical problems. These effects- including leukemia, cancer, and many others-
appear two, three, even ten years later.
Embryo/Fetus500 (for the
entire pregnancy)
Occupational
exposure of a
minor
10% of the limitsabove
Member of the
general public100
Medical Procedure Doses
Procedure Dose (mrem)
X-Rays
Abdomen 40
Chest 6
Pelvis 60Dental 3
Mammography 170
CT (full body) 130
NuclearMedicine
400
Natural Radioactivity in Food
Food40
K
(pCi/kg)
226Ra
(pCi/kg)
Bananas 3,520 1
Carrots 3,400 0.6 - 2
White Potatoes 3,400 1 - 2.5
Lima Beans
(raw)4,640 2 - 5
Red Meat 3,000 0.5
Brazil Nuts 5,600 1,000 - 7,000
Drinking Water --- 0 - 0.17
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Blood Disorders
According to Japanese data, there was an increase in anemia among persons exposed to the
bomb. In some cases, the decrease in white and red blood cells lasted for up to ten years after
the bombing.
Cataracts
There was an increase in cataract rate of the survivors at Hiroshima and Nagasaki, who were
partly shielded and suffered partial hair loss.
Malignant Tumors
All ionizing radiation is carcinogenic, but some tumor types are more readily generated than
others. A prevalent type is leukemia. The cancer incidence among survivors of Hiroshima and
Nagasaki is significantly larger than that of the general population, and a significant correlation
between exposure level and degree of incidence has been reported for thyroid cancer, breast
cancer, lung cancer, and cancer of the salivary gland. Often a decade or more passes before
radiation-caused malignancies appear.
Keloids
Beginning in early 1946, scar tissue covering apparently healed burns began to swell and grow
abnormally. Mounds of raised and twisted flesh, called keloids, were found in 50 to 60 percent of
those burned by direct exposure to the heat rays within 1.2 miles of the hypocenter. Keloids are
believed to be related to the effects of radiation.
Table: Short term effect of radiation on human
Dose-Rems Effect
5 - 20 Possible late effects; possible chromosomal damage.
20 - 100 Temporary reduction in white blood cells.
100 - 200 Mild radiation sickness within a few hours: vomiting, diarrhea, fatigue;
reduction in resistance to infection. 10% fatal in 30 days.
200 - 300 Serious radiation sickness effects as in 100-200 rem and
hemorrhage; 35% fatal in 30 days.
300 - 400 Serious radiation sickness; also marrow and intestine destruction;
50% fatal in 30 days.
400 - 500 Hair loss, fever, hemorrhaging in 3wks.
500 - 600 Internal bleeding. 60% die in 30 days.
600- 1,000 Intestinal damage. 100% lethal in 14 days.
5,000 Delerium, Coma: 100% fatal in 7 days.
8,000 Coma in seconds. Death in an hour.
10,000 Instant death.
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