Importance of Nuclear Energy

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Malik Muhammad Nasir Wirali

BSc Chemical Engineering

University of Gujrat,Pakistan.


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Importance of Nuclear Energy

There is no more sensible alternative than nuclear energy if we

really want to sustain our civilization.

In the next 50 years the global population will use more energy thanthe total consumed in all previous history.

Fossil resources-coal, oil and natural gas-are being consumed so fast

so as to be largely exhausted during the 21st Century.

Nuclear power plant do not emit green house gases. New reactor

design, radiation safety and transportation and improved more efficient

mining, is placing nuclear energy back in the scene. The UN report

climate change is important.

The global climate change is being changed by mankind.

It should at least be clear that nuclear energy significantly producesless atmosphere pollution than burning fossil fuels.


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Global Population on the rise

We live in a word that is just beginning to consume energy. China

and India are winning to Europe and America in the race for per capital

energy consumption.Humanity can not go backwards.

The rapidly growing world population will require vast amount of

energy to provide fresh water, energize factories, homes and

transportation and support infrastructure for nutrition, education and

health care.Meeting these needs will require energy from all sources

Reducing consumption of fossil fuel will preserve the environment and

irreplaceable resources for future generations.

The key is to generate vastly expanded supplies of electricity cleanly.


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Realism about energy

Clean energy from new renewable solar, wind, biomass and hydro

electric power-deserves strong support. But the collective capacity of these

technologies to produce electricity in the decades ahead is limited.Even with continued subsidy and research support, these new renewable can

provide only around 6% of world electricity by 2030.

Environmentalists have played a valuable role in warning that catastrophic

climate change is a real and imminent danger.

Even with maximum conservation and a landscape covered by solar panels and

windmills, we would still need large scale source for around the clock

electricity to meet much of our energy needs.

This is why nuclear energy is important in countries that do not use energy

themselves. We should encourage large industrialized countries to use clean

nuclear energy in safe manners as a means of limiting global pollution.

Nuclear power-like wind, hydro and solar energy, can generate electricity with

no carbon dioxide or other green house gas emissions.

The critical difference is that Nuclear Energy is the only option to produce

vastly expanded supplies of clean electricity on a global scale. Keep in mind

that sun not always shines and that the wind not always blows.


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Bio combustibles: fuel for cars or hunger

In a recent report, the united nations warns that the fever for bio

combustibles could bring hunger to the highest level in a short run if the

government do not think seriously about its extensive applications.It says that the last idea of converting the food like corn, sugar, palm oil

and wheat into combustibles is recipe for disaster.

A serious risk exists on creating a battle camp between food and

combustibles that could affect the poor of developing countries.

More than 20,000 people around the world die each day simply because

they are too poor to stay alive.

The UN report adds that the efforts for production of bio fuels are

important because it helps to control the climate change, but considers

unacceptable that the right for food for humanity be jeopardized.Higher prices for food are expected if the best land is used to nourish

cars instead of human beings.


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Let us place this technology in perspective:

The process of producing ethanol, for example, gives off large

amount of carbon dioxide and that is where ethanols green label

starts to brown. Most ethanol plants burn natural gas or,increasingly, coal to create steam that drive the distillation, adding

fossil fuel emissions to the carbon dioxide emitted by the yeast.

Some studies of energy balance of corn-ethanol-the amount fossile

fuel needed to make ethanol versus the energy it produces-suggestthat ethanol is a looser game.


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Why use the nuclear energy to make the steam?

Because it is clean, safe and usually competitive.

Nuclear energy has distinct environmental advantages over fossil fuels,

is that virtually all its wastes are contained and managed.Nuclear power stations do not cause any pollution

The fuel for nuclear power is virtually unlimited. That is to say, there is

plenty of uranium in the earths crust.

The safely record of nuclear energy is better that for any major

industrial technology.Safety of nuclear reactors has been a high priority in their design and

engineering. About one third of the cost of a typical reactor is due to

safety systems and structures.


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Superb Record of Nuclear Safety

Today, nuclear power plants have a superb safety record-both for plant

workers and the public.

In the transport of nuclear material, highly engineered containerscapable of withstanding enormous impact is the industrial norm.

The radiation produced with in the core of nuclear reactor is similar to

natural radiation but much more intense.

At nuclear power plants, protective shielding isolates this radiation,

allowing millions of people to live in safety nearly.Typically the radiation people receive comes 90% from nature and 10%

from medical exposure. Radiation exposure from nuclear power is

negligible.


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Fuel for Nuclear power plants and waste

The great advantage of nuclear power lies in the vast amount of energy

that can be extracted from a more handful of the element uranium, which

is found in greater concentrations underground. The waste from nuclearpower is concentrated to tiny volume and can be safely returned to the

earth for underground storage. Because so much energy leaves only a

small amount of manageable waste, uranium has been called natures gift

to clean economic development.

In contrast, fossil fuel waste is too large and unmanageable to becontained and must be dispersed into the environment.

Due to effective shielding and containment, waste from nuclear power

has never caused any harm to any person or to the environment

For nuclear waste that is highly radioactive, well designed long termstorage is needed while its radioactivity decays to natural levels.


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Nuclear Competitiveness for the Future

Nuclear power plants currently cost more to build than power plants using coal or gas

This difference is narrowing, as long experience with nuclear power helps to shrink

construction periods and extend plant life times.

Due to low cost and improved efficiency, nuclear plants once built can be less expensive tooperate.

Today nuclear energy provides about 16% of world electricity. Fortunately, the uranium that

fuels nuclear power is found in great quantity in both earth and sea water. Uraniums

worldwide availability at economically viable cost is a key factor that would allow a

sharp/expansion in nuclear power.

Beyond producing clean electricity, the clean energy from nuclear power could be used todistill salt water on a massive scale. Desalination plants would help to meet the desperate

shortage of fresh water that could afflect more that half the world s people by 2025.

Nuclear Power and Sustainable development

Nuclear power is a sustainable development technology because its fuel will be available

for multiple centuries, its safety record is superior among major energy sources; its

consumption causes virtually no pollution.Its use preserves valuable fossil resources for future generations

Its costs are competitive and still declining.

Its waste can be securely managed over the long term.

Stabilizing the accumulation of atmospheric gases requires that worldwide emissions be cut

by 50%.


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NUCLEAR ENGINEERING


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Nuclear cross section and reaction rates

The efficiency of a nuclear reaction is usually expressed in terms of a cross section

which has the dimensions of an area. The cross section has a dimensions of length

squared (cm2). Fundamentally, it is the fraction of the reacting nuclei consumed by the

nuclear reaction per unit time per unit flux.Cross section for reactions with neutrons vary from a lower detectable limit

of around 1x 10-24 cm2 to a maximum of 2.65x 10-18cm2, which has been observed for135xe. To avoid using such large negative exponents, cross sections are usually

expressed in units of 10-24cm2, called barns (b). for instance, the Xenon cross section

is 2.65 x 10

6

b. The millibarn (mb) is 10

-27

cm

2

.There is different cross section for every different reaction of a nuclide with

neutrons. Example of cross section for low energy neutrons moving at a speed of 2200

m/s are given in table. (2.6)

The sum of the cross sections for all reactions in which neutron is absorbed is

called the absorption cross section, denoted by a

a 235u = 680.8 ba

14N = 1.88 b

The cross section generally varies with neutron speed, in many cases very strongly.


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ReactionRates

The number of nuclei reacting in a specified way with neutrons in unit time is

proportional to the number of nuclei present and to the concentration of neutrons. In

the language of chemical kinetics neutron reactions are first order with respect to

concentration of nuclei and neutrons, and it is because neutron reaction are simple firstorder irreversible processes.

The expression for the rate of change in the number of reacting nuclei N is

W

here n is the concentration of neutrons per unit volume, is the specific rateconstant.

It has become customary to express as the product of another constant , called the

cross section, and the neutron speed v, so that the equation 2.41 becomes:

The product vn is termed the neutron flues and is the measure most commonly usedto describe the neutron intensity in a reactor.

For a given neutron density and speed v, the product is the first order constant and

is the fraction of reacting nuclei consumed by the reaction for unit time. It plays the

same role in rate equation as the radioactive decay constant


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The energy released when one atoms of235U undergoes fission in the above reaction is:

Atoms of235U may undergo fission in a variety of ways, of which the reaction shown in

Fig. 1.4 is only one.

In primary fission reaction shown at the top of this figure ( ) 235U splits into

two parts, the radioactive fission products, while at the same time giving off several fast

neutrons (2.418 on the average) and gamma radiation. One of these neutrons is used to

maintain the fission reaction. The remaining neutrons may either be used to bring about

the other desired nuclear reactions or lost through leakage from the reactor or through

capture by elements present in the reactor to produce unwanted or waste products.

Following the primary fission reaction, the radioactive fission products

undergo radioactive disintegration and ending up as stable fission products.

The total energy released in fission is the sum of the energy associated with thedifference particles as shown in Fig 1.5, 195-205 MeV. As upto 5 MeV of gamma

energy escapes from a typical power and is not utilized, a nominal figure for energy

released in fission is 200 MeV


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Nuclear Criticality safety

It is a field of nuclear Engineering dedicated to the prevention of self

sustaining nuclear reaction because of carelessness, additionally, nuclear

criticality safety is concerned with decreasing the consequence of anuclear criticality accident. A nuclear criticality accident occurs from

operations that involve fissile material and results in a tremendous and

potentially lethal release of radiation. Nuclear criticality safety

committee attempts to minimize the probability of a nuclear criticality

accident by analyzing normal and abnormal fissile material operationsand providing controls on the processing of fissile materials.

Contents

Principles

Calculations and analysis

Burn up credit


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Principles

Seven factors influence a criticality system.

Geometry or shape of a fissile material: If neutrons escape (leak from) the fissile

system they are not available to interact with the fissile material to create a fission

event. Therefore the shape of the fissile material influences the probability of creatinga fission event. A large surface area such as thin slab has lots of leakage and is safer

than the same amount of fissile material in a small, compact shape such as a cube or

sphere.

Interaction of Units: Neutrons leaking from one unit can enter another. Two units,

which by themselves are sub-critical, could interact with each other to form a critical

system. The distance separating and any material between them influence the effect.

Reflection: When neutrons collide with other atomic particles (primary nuclei) and

are not absorbed, they change direction. If the change in direction is large enough, the

neutron may travel back into the system, increasing the likelihood of interaction

(fission). This is called reflection. Good reflectors include hydrogen, beryllium,

carbon, uranium, water, polyethylene, concrete and steal.


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Moderation: Neutrons resulting from fission are typically fast (high energy).

These fast neutrons do not cause fission as readily slower (less energetic) ones.

Neutrons are slowed down (moderated) by collision with atomic nuclei. The most

effective moderating nuclei are hydrogen, deuterium, beryllium and carbon. Hence

hydrogenous materials including oil, polyethylene, water, wood, paraffin, and thehuman body are good moderators.

Note that moderation comes from collisions: therefore most moderates are also good

reflectors.

Absorption: Absorption removes neutrons from the system. Large amount of absorbers

are used to control or reduce the probability of a criticality. Good absorbers are boron,

cadmium, gadolinium silver and indium.

Enrichment: The probability of a neutron reacting with a fissile nucleus is influenced

by the relative numbers of fissile and non fissile nuclei in a system. The process of

increasing the relative number of fissile nuclei in a system is called enrichment.

Typically, low enrichment means less likelihood of a criticality and high enrichment

means a greater likelihood.Mass: The probability of fission increases as the total number of fissile nuclei

increases. The relationship is not linear. There is a threshold below which critically will

not occur. This threshold is called the critical mass.


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Conversion and Breeding

The fact that the number of neutrons produced per neutron absorbed exceeds 1.0 for

each fuel indicates that each will support a nuclear chain reaction. Neutrons in excess of

one needed to sustains the nuclear chain reaction may be used to produce new and

valuable isotopes, for example, to produce 239Pu form 238U or233U from thorium.When the number of neutrons produced per neutron absorbed in fissile

material is greater than 2.0, it is theoretically possible to generate fissile material at a

faster rate when it is consumed. One neutron is used to maintain the chain reaction, and

second neutron is used to produce a new atom of fissile material to replace the atom that

is consumed by the first neutron. The process is known as breeding.

The reaction taking place in breeding 239Pu from 238U are shown in Fig 1.6.238U is the only material consumed overall, 239Pu is produced from 238U and then

consumed in fission.

In thermal reactors fueled with plutonium, the number of neutrons produced

per neutron absorbed is less than 2.0 and the breeding is impossible. For 233U, on the

other hand, the number is substantially greater than 2.0, and breeding is practicable in athermal reactor. In fast reactors, the number of neutrons produced per neutron absorbed

is close to the total number of neutron produced per fission, so that breeding is possible

with both 235U and plutonium. Table 1.1


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A fast reactor is one in which the average speed of neutrons is near that

which they have at moment of fission, around 15 million m/s.

At the high speeds, the probability of a neutrons being absorbed by a

fissionable atom is low, and neutron-absorption cross section which is a

measure of this probability is small.

A thermal reactor is one in which the neutrons have been slowed

down until they are in thermal equilibrium with reactor materials, in a

typical power reactor, thermal neutrons have speed around 3000 m/s. At

these lower speeds, neutron absorption cross sections are much largerthan for fast neutrons.

The critical mass of fissile material required to maintain the

fission process is roughly inversely proportional to the neutron

absorption cross-section. Thus the critical mass is lowest for plutonium

in thermal reactors, larger for the uranium isotopes in the thermalreactors, and much greater in fast reactors. For this reason, as well as

others, thermal reactors are preferred types except when breeding with

plutonium is an objective, then a fast reactor must be used.


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The multiplication factor

In order to establish a chain reaction in fissionable material, every nucleus

which undergoes fission must produce on the average at least one neutron which in turn

causes the fission of another nucleus. The ratio of number of neutrons in any one

generation to the number of corresponding neutrons in the previous generation istermed the multiplication factor or reproduction factor, represented by the symbol. If k

is less than unity, a chain reaction is not possible. If k is exactly equal to unity, a chain

reaction will proceed with constant neutron flux as in case of a controlled nuclear chain

reactor.

Since the multiplication factor k is the ratio of the number of neutrons in any generation

to the corresponding number in the previous generation, then

This value k is for a reactor of infinite size since we have not included any loss ofneutrons by leakage from the system.


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Note that and are the properties of the fuel over which we have no control

and that success of a chain reactor depends upon the values of p and f, which vary

with the geometry, the composition of the reactor elements, and ratio of fuel to

moderator.If k is greater than unity, the neutron flux will continually increase with time as in

case of atom bomb. We shall use the term multiplication factor to refer to a lattice of

infinite size, where loss of neutrons by leakage is ignored.

Let us consider the case of nuclear chain reactor using natural uranium as

fuel and graphite as moderator. For an infinite system, neutrons will be lost as a result

of non-fission capture by 235U and 238U including both thermal and resonance capture

and as a result of parasitic capture of neutrons by the moderator, coolant, structural

materials, fission product and any other material (poisons) present in the reactor.

Let us trace through the life history of n fast neutrons produced by thermal

fission of 235U. Some of these fast neutrons have high enough energy to produce

fission in238

U and there is a small amount of fast fission of235

U, so that to take intoconsideration this increase in the total number of neutrons we introduce the fast

fission factor which is defined as the ratio of total number of fast neutrons produced

by fissions due to neutrons of all energies Compared to the number resulting from

thermal neutron fission


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For a natural uranium-graphite reactor has a value of 1.029. The n fast neutrons are then

rapidly slowed down to thermal energies by collision with moderation nuclei, but during the

thermalizing process they will be susceptible to resonance capture until their energy is reduced

below a value of 5 ev. The probability that a fast neutron will reach thermal energies without

capture by 238U is given by resonance escape probability, designated by symbol p. thus we have

n p neutrons reaching thermal energies and of these neutrons some are absorbed by moderator,

coolant and other poisons while a fraction f are absorbed by the fuel to produce fission. The

factor f is called the thermal utilization factor and is defined as the ration of thermal neutrons

absorbed in the fuel to the total number of thermal neutrons absorbed by any process. If on the

average fast neutrons are produced for thermal neutron capture by 235U, the number of fast

neutrons produced must be npf

Reactor ControlA reactor which is to operate at any appreciable power level must have a k greater

than unity. This excess reactivity is necessary for overcoming temperature effects as the neutron

flux is raised to the operating level.

It is necessary to control any nuclear chain reactor. This is easily done by inserting in

reactor a material such as boron or cadmium, which has a large capture cross-section for

thermal neutrons.Adjustable control rods of boron or cadmium steel are inserted the proper distance

into the reactor to maintain a k of unity at the desired power level.


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Reactor components and their characteristics

Reactor fuel elements

Fuel elements are exposed to radiation damage. Since all radiations are generated by

the fissionable material the total energy available to produce damage is considerable.

Type ofFuels.

Metal Fuels: Metal fuels have the advantage of a much high heat conductivity than oxide

fuels but cannot survive equally high temperatures

Uranium: Metallic uranium can exist in three different allotropic forms, depending on

the temperature. The most spectacular effect of radiation on uranium is the radical change in

dimensions as the material is irradiated. In spite of radical changes in dimension and shape during

irradiation, the volume of uranium increases only slightly.

Uranium alloys

In a heterogeneous reactor, it is desirable to have narrow coolant channels to increase the

heat transfer rate and the decrease the parasitic absorption of neutrons by the coolant. Also in a

power reactor it is necessary to have high uranium burn ups to produce economic power. Because

it has been impossible to eliminate all dimensional changes in pure uranium, considerable effort

has been directed toward the development of suitable uranium alloys for use in heterogeneous

reactors.There are three basic ways by which an alloying element can decrease the radiation

damage of uranium. In the high uranium alloys it is possible to bring desired improvement in the

physical properties of the uranium by proper heat treatment.

Among the most satisfactory alloys are the aluminum-uranium. The changes in dimensions

were less than 1% with radiation. Beryllium-uranium and zirconium-uranium alloys also have

high stability. The zirconium-uranium alloys have been widely used in pressurized water reactors.


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Oxide Fuel

UOX

The thermal conductivity of uranium dioxide in low, it is effected by porosity and burn

up. The low thermal conductivity can lead to overheating of the centre parts of the pellets during

use.

MOX

Mixed oxide, or MOX is a blend of plutonium and natural and depleted uranium. MOX

fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors.

Ceramics Fuels

Ceramics fuels other than oxides have the advantage of high heat conductivities and

melting point, but thy are prone to swelling than oxide fuels.

Uranium nitrideOne advantage is that UN has a better thermal conductivity than UO2. This fuel has the

disadvantage that unless 15N was used that a large amount of14C would be generated from 14N

(n,p).

Uranium carbide

The high thermal conductivity and high melting point make uranium carbide asalternate fuel. Uranium carbide could be the ideal fuel candidate for reactor such as gas cooled

fast reactors.


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Moderators

A material which rapidly slows down fast neutrons is called a moderator, and the process of

slowing down neutron is called moderation or thermal zing. The moderators which have best nuclear

properties are water, heavy water, graphite, beryllium and beryllium oxide. Heavy water is best because

of its low cross-section and good slowing down power, but it is expensive and does undergo

dissociation. In case of water and heavy water the problem of radiation damage exists. Ordinary waterhas a fairly high cross-section and can only be used in an enriched fuel reactor. Beryllium and

beryllium oxide are expensive, toxic and have poor mechanical properties. Graphite is readily available

and given good results where a solid moderator is required.

Reactor Coolants

For low-power reactors where there is no attempt to recover heat, air has been widely used

as a primary coolant. The one serious disadvantage of air is the high cross-section of nitrogen. Tominimize absorption of neutrons by coolant, it was proposed to use helium as coolant. But difficulties

are involved in handling large quantities of helium gas.

Water and heavy water are also being used as primary coolants. Since heat transfer

coefficients of a liquid are better than in case of a gas, they can be used for high power operation.

Ordinary water has a fairly high cross-section so that it can not be present in too large amounts in

reactor.

The interest in heat transfer fluids has been shifted to these materials adaptable to hightemperature. Of chief importance are the liquid metals which are stable to radiation, can be used at

high temperatures without generating high pressure, and have very high heat transfer coefficients.

The most satisfactory liquid metal coolant appears to be sodium or sodium-potassium alloy.

The only advantage of N a k is its lower melting points. Pure sodium has a higher specific heat, higher

thermal conductivity, and smaller cross-section.


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Structural materials

The one absolute requirement that any structural material going into a reactor

must meet is that its cross-section must be sufficiently low that it does not reduce

critically factor below unity. The only common structural material on the basis of low

cross-section is aluminum. But aluminum is not particularly good material from thestand point of corrosion resistant.

AISI 304 stainless steel is recommended for entire pressure vessel rather than that more

commonly used steel lined carbon steal structure.

Zircaloy

In order to meet corrosion resistance at high temperatures, Zircaloy-4 (an alloy of

zirconium +Tin +iron +chromium) is presently being used as a material of construction

for many power reactors. It has good structural properties, good corrosion resistance,

and undergoes only slight radiation change.

Shielding materials

The shield for a reactor must effectively reduce the strength of neutrons and gammas

which are generated by the fission process. This means that both heavy and lightelements must be present in the shield.

If a material such as hydrogen is present in the shield will reduce the energy of

neutron to thermal energy and is absorbed before it can diffuse much further in the

shield.


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Heavy concrete

Ordinary concrete has been widely used as a shielding material. It is cheap and reliable

and readily available. From a practical stand point, it must be realized that ordinary concrete is a

better neutron shield than gamma shield, so the gamma dose will determine the shield. In use of

heavy concrete, it must be remembered that any heavy concrete will be more expensive than

ordinary concrete pound per pound.Several different types of heavy concrete have been used for shielding reactors and

chemical processing facilities. Bayrite concrete is made by substituting a cheap barium mineral

(crude barium sulphate) for the coarse and fine aggregate of ordinary concrete. Baryte concrete

has a density about 50 per cent greater than ordinary concrete, and the increased cost is very little.

A density of about twice that of ordinary concrete can be obtained by use of ferrophosphorous

aggregate, but the material cost is about four times as great. A still higher or density can beobtained by use of iron aggregate, but the material cost and the cost of installation are much

higher.

Thermal Shield

A thermal shield is an inner wall, often of steel, which is placed between the reactor and the

biological shield. Its function is to remove most of the heat energy of gamma and thermal

neutrons leaking from the reactor and thereby to protect the biological shield from damage due toheat generation in the shield. The need for a thermal shield depends on the reactor power, the

amount of cooling around the reactor.

An iron thermal shield has a problem of added cost, also when thermal neutrons are

captured by iron there is production of capture gamma up to 10 Mev which must be shielded out

by the biological shield. The problem of capture gamma can be minimized by the some boron

containing thermal shield. For example, an aluminum-boron carbide complex known as Boralhas been develo ed which contains 35 er cent boron.


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Type ofReactors

ResearchReactors

Research reactors are nuclear reactors that serve primarily as a neutron source. They are

called non-power reactors, in contrast to power reactors that are used for electricity

production, heat generation, or submarine propulsion.Purpose

The neutrons produced by research reactors are used for non destructive testing,

analysis and testing of materials, production of isotopes, research and education.

Research reactors that produce radioisotopes for medical or industrial use are

sometimes called isotopes reactors.Technical Aspect

Research reactors are simpler than power reactors and operate at low temperatures.

They need far less fuel, and for less fission products build up as the fuel is used. On the

other hand their fuel is more highly enriched uranium, typically upto 20% 235U,

although some use 93% 235U while 20% enrichment is not generally considered usable

in nuclear weapon. 93% is commonly referred to as bomb grade. Like power reactors,the core needs cooling, typically natural or forced circulation with water and a

moderator is required to slow down the neutrons and enhance fission. As neutron

production is their main function.


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Classes of research reactors

Aqueous homogenous reactor

Argonaut class reactor

DIDO class-six High flux reactors world wide

TRIGA A high successful class with > 50 installation world wide

SLO POKE Reactor class developed by AEC, Canada

Miniature neutron source reactor, Based on the SLOW POKE design, developed by AECL.

A common design (67 units) in the pool type reactor where core is a cluster of fuel elements

sitting in a large pool of water. Among the fuel elements are control rods and employ channels for

experimental materials. The water both moderates and cools the reactor, and graphite or

beryllium is generally used for the reflector.

The TRIG A reactor is another common design (40 units). The core consists of 60-100 cylindricalfuel elements about 36mm diameter with aluminum cladding enclosing a mixture of uranium fuel

and zirconium hydride (as moderator). Ii sits in pool of water and generally uses graphite or

beryllium as a reflector. This type of reactor can safely be pulsed to very high power level (e.g.

25,000MW) for a fraction of a second. The rapid increase in power is quickly cut short by a

negative reactivity effect of the hydride moderator.

Other designs are moderated by heavy water (12 units) or graphite.Homogenous type reactors have a core comprising a solution of uranium salts as liquid contained

in tank about 300mm diameter. The simple design made them popular early on, but Russia has

most research reactor (62), followed by USA (54), Japan (18), France (15), Germany (14) and

China (13).


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N l P Pl t


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Nuclear Power Plants

Nuclear power plants are an important source of electrical energy. At the moment there are more than 400

nuclear power plants (NPP) all over the world, which produce about 17% of the world electricity. The

share can range from just few per cent in some countries and to 75% as in France. The KRSKO nuclear

power plants produce almost 40% of the electrical energy in Slovenia.

Different types of nuclear power plantsThere are a number of different types of nuclear reactors currently in operation throughout the world.

Some of the most common types are:

Pressurized Water Reactor

Fuel

Usually pellets of uranium oxide (UO2) arranged in tubes to form fuels rods. These rods are

arranged into fuel assemblies in the reactor core.

This is the most common types with over 230 in use for power generation and several hundred moreemployed for naval propulsion. The design of PWR originated as a submarine power plant. PWRs use

ordinary water as coolant and moderator. The design is distinguished by having a primary cooling circuit

which flows through the core of the reactor under very high pressure and a secondary circuit in which

steam is generated to drive the turbine. In Russia these are known as WER types (Water Moderated and

Cooled)

A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor

would have about 150-250 fuels assemblies with 80-100 tons of uranium.Water in the core reaches about 325Co, hence it must be kept under about 150 times atmospheric pressure

to prevent it boiling. Pressure is maintained by steam in a pressurizer. In the primary cooling circuit the

water is also the moderator and if any of it turned to steam the fission reaction would slow down. The

negative feed back is one of the safety features of the type. The secondary shut down system involves

adding boron to the primary circuit.


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Pressurized heavy water reactor (PHWR or CANDU)


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Pressurized heavy water reactor (PHWRor CANDU)

The PHWR reactor design has been developed since the 1950 S in Canada as the CANDU and

more recently also in India. It uses natural uranium (0.79%235U) oxide as a fuel, hence needs a

more efficient moderator, in this case heavy water (D2O).

The moderator is in a large tank called a calendria penetrated by several hundred horizontal

pressure tubes which form channels for the fuel cooled by the flow of heavy water under highpressure in the primary cooling circuit reaching 290oC. As in the PWR the primary coolant

generates steam in a secondary circuit to drive the Turbines. The pressure tube design means that

the reactor can be fueld progressively without shutting down by isolating individual pressure

tubes from the cooling circuit.

A CANDU fuels assemble consists of a bundle of 37 half meter long fuels rods (ceramic fuel

pellets in Zircaloy tube) with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calanderia vertically, and a secondary shut down system involves adding

gadolinium to the moderator.

Advanced Gas Cooled Reactor

These are the second generation of British gas-cooled reactor using graphite moderator and

carbon dioxide as coolant. The fuel is uranium oxide pellets, enriched to 2.5 to 3.5% in stainless

steel tubes. The carbon dioxide circulation through the core reaching 650oC and then pas steamgenerator tubes outside it. But still inside the concrete and steel pressure vessel. Control rods

penetrate the moderator and a secondary shut down system involves injecting nitrogen coolant.

The AGR was developed from the Magnox reactor, also graphite moderated and CO2 cooled.

They use natural uranium in metallic form. This is a Sovint designed. It employs long (7meter)

vertical pressure tubes running through graphite moderator, and is cooled by water which is

allowed to boil in the core at 290 oC. Fuel is low enriched uranium oxide made up into assemblies


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The super-phenix was the first large scale breeder reactor It was put into


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The super-phenix was the first large scale breeder reactor. It was put into

service in France in 1984.

The reactor core consists of stainless steel tubes containing a mixture of

uranium and plutonium oxides about 15-20% fissionable plutonium-239.

Surrounding the core a region called breeder blanket consisting of tubes

filled only with uranium oxide. The entire assembly is about 3 X 5 meter

and is supported in a reactor vessel in molten sodium. The energy from

the nuclear fission heats the sodium to about 500oC and it transfers that

energy to a second sodium loop which in turn heats water to producesteam for electricity production. Such a reactor can produce about 20%

more fuel than it consumes by the breeding reaction. Enough excess

fuels is produced over about 20 years to fuel another such reactor.

Optimum breeding allows about 75% of the energy of the natural

uranium to be used compared to 1% in the standard light water reactor.


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Reactor Designs


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Reactor Designs

As of 2006, all large scale FBR power station have been liquid metal fast

breeder reactors (LMEBR) cooled by liquid sodium.

These have been one of two designs:

Loop type, in which primary coolant is circulated through primary heat

exchangers external to the reactor tank (but within the biological shield

owing to the presence of radioactive sodium-24 in the primary coolent)

Pool type, in which primary heat exchangers and circulators are

immersed in the reactor tank.FBRs usually use a mixed oxide fuel core of up to 20% plutonium

dioxide (PUO2) and at least 80% uranium dioxide (UO2). Another fuel

option is metal alloys, typically a blend of uranium, plutonium and

zirconium.


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Nuclear Fusion


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If fusion reactions are to be practical method of generating energy on earth, other means

than gravitational attraction must be found to confine the reacting atoms.

The confinement principle on which most work is being done depends on the fact that

atoms heated to the extremely high temperatures required for fusion are fullydissociated into positively charged ions and negatively charged electrons. Such a

reaction mixture of positive and negative ions is called a thermo nuclear plasma. By

placing a plasma in a strong magnetic field , its positively and negatively charged

particles are constrained to travel in helical path around the magnetic lines of force. By

proper shaping of the magnetic field, the charged particles can be confined for

substantial periods of time, long enough to permit some fusion reactions to take place.The fusion reaction easiest to bring is between a deuterium ion (bydrogen of

mass 2) and a tritium ion (hydrogen of mass 3) to produce a helium of mass 4 and a

neutron.

2D + 3T 4He + 1no

Deuterium Tritium helium neutron

This reaction is favoured because it occurs at an appreciable rate at a lower


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pp

temperature (20,000,000 R) than other possible fusion reactions. For use n fusion

reaction tritium must be made by reaction of lithium isotope of mass 6 with a neutron.

6L I +1n 4He +-

3T

Lithium Neutron Helium Tritium

The energy release in these two reaction may be calculated from decrease in mass

between reactants and the products

Function Reaction

Reactant, amu Products, amu Difference

amu2D 2.014102 4He 4.0026033T 3.016050 1n 1.008665

Total 5.030152 5.011268

0.018884

With the conversion factor 931.480 MeV/amu this fusion reaction release 17.6 MeV

per pair of atoms fused.

Fusion here on earth

h fi h l f i i k l h d i k A ll O b 31


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The first thermonuclear fusion reaction to take place on earth occurred at Eniwetok Atoll on October 31,

1952, when united states exploded a fusion device, generating an energy release equivalent to 10 million tons

of TNT. The high temperature needed to initiate the reaction triggered by a fission bomb.

A sustained and controllable source of fusion power, a fusion reactor, is considerably harder to

achieve. The goal is, however, is being pursued vigorously in many countries around the world because many

look to the fusion reactor as the power source of future, at least as for as generation of electricity is

concerned.

The three requirements for a successful thermonuclear reactor are:

A high particles density the density of interacting particles must be great enough to ensure that collision rate

is high enough.

A high plasma temperature. The plasma must be hot otherwise the colliding particles will not be energetic to

penetrate the electrical barrier that tends to keep them apart.A long confinement time. A major problem is containing the heat plasma long enough to ensure that its

density and temperature remain sufficiently high for enough of fuel to be fused. It is clear that no solid

container can with stand the high temperature that are necessary.

Thermal Nuclear Power

The binding energy curve shown that energy can be released if two light nuclear combine to form

a single larger nucleus. The process is called nuclear fusion. The process is hindered by the electrical

repulsion that acts to prevent the two particles from getting closer enough to each other to be within rangeand fusing

To generate useful amount of power, nuclear fusion must occur in bulk matter. That is, may atoms

need to fuse in order to create a significant amount of energy. The best hope of bringing this about is to raise

the temperature of the material so that particles have energy-due their motion alone to penetrate the electrical

repulsion barrier. This process is known as thermonuclear fusion. Calculation show that these temperatures

need to be close to the suns temperature of 1.5 X 107K.


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Importance of Nuclear Energy

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