Mod8 on Nuclear Energy-2 [Nuclear Power Stations] for Summer 2010-11

8/6/2019 Mod8 on Nuclear Energy-2 [Nuclear Power Stations] for Summer 2010-11

1/16

Page 1 of 16

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.


2/16

Page 2 of 16

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.


3/16

Page 3 of 16

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:


4/16

Page 4 of 16

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


5/16

Page 5 of 16

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


6/16

Page 6 of 16

(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


7/16

Page 7 of 16

(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.


8/16

Page 8 of 16

(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


9/16

Page 9 of 16

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.


10/16

Page 10 of 16

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.


11/16

Page 11 of 16

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.


12/16

Page 12 of 16

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.


13/16

Page 13 of 16

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


14/16

Page 14 of 16

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


15/16

Page 15 of 16

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.


16/16

Page 16 of 16

Date post:	07-Apr-2018
Category:	Documents
Upload:	jobayerme
View:	219 times
Download:	0 times

Mod8 on Nuclear Energy-2 [Nuclear Power Stations] for Summer 2010-11

Documents