Nuclear and Hydel Power Plants

8/2/2019 Nuclear and Hydel Power Plants

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Introduction

Life without electricity is unthinkable. This electricity is generated by power

plants using coal, water etc., Use of nuclear energy as a power source is becoming a

common trend in the world. This is due to the rapid depletion of conventional energy

source. Fuel transportation network and large storage facility in case of thermal

power plants and dependence on whether conditions in case of hydel power plants are

the major hurdles faced by today's power sectors. Utilization of nuclear power helps

to save a considerable amount of fossil fuels which can be used in other areas.

Nuclear power engineering is basically concerned with the phenomenon taking place

within the nucleus of the atoms.

One of the outstanding attractions is the large amount of energy that can be

released from a small mass of active material. Complete fission of one kg of uranium

92U235 results in energy equivalent to 4500 tones of high grade coal or 1900 tones of

oil. The economic advantage of nuclear power can be realized only at a load of

about 75%.

Nuclear energyThe energy released due to the split up of nucleus into two or more smaller

nuclei is known as atomic or nuclear energy. An atom consists of a relatively heavy,

positively charged nucleus and a number of much lighter negatively charged electrons

that revolves in different orbits around the nucleus. These electrons are held in their

orbits by electro static forces. The nucleus consist of sub particles nucleons which, in

turn, has electrically neutral charge neutrons and positively charged protons. It is

difficult to bring these protons together in a nucleus of an atom. There, it requires

same to bring and keep the protons together in the nucleus of an atom. This energy is

known as binding energy.

The forces that hold the protons and neutrons inside the nucleus are million

times as strong as the electrostatic forces holding the electrons to the nucleus. Thus,

the binding energy is very large compared with chemical bond energy. Therefore, if a

nucleus disintegrates then a very large amount of energy is released. This energy is

due to the fission of neutrons and is used for power production in nuclear plants.
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This energy is known as atomic energy or prompt energy or nuclear energy. In

other words, the amount of energy what released now is only at the time of fission. At

the time of fission, the neutrons are under fast moving conditions. If the reaction

proceeds at this rapid movement, then maximum utilization of energy is not possible.

So, fast moving neutrons are decelerated with the help of moderator to get more

amount of energy. Thus, more energy is produced due to slow decay of the fission

fragments into fission products and the non fission capture of excess neutrons in

reactions (by means of reflectors). This produces energy much lesser than that of

energy obtained by fission. So, the total energy produced per fission reaction is

greater than the prompt energy and is about 200 MeV.

Nuclear fission and fusion reactions

Nuclear reaction plays a vital role in power production. So, it is necessary to

study the basic concepts behind the operation of nuclear reactors. These reactions are

three types namely fission, fusion and radioactivity. The fission and fusion concepts

are well applicable to power production systems as these reactions will release

enormous amount of heat energy than radioactivity.

Nuclear fusion

In nuclear fusion, two or more lighter nuclei (of similar electric charge) are

made to collide to fuse them together in a plasma at high temperatures (above 108oC

i.e., high particle velocities). As a result of this, a heavier nucleus is formed and the

mass difference is transformed into enormous amount of heat which is much greater

than that was produced by fission reaction. To cause fusion, the nuclei are necessary

to accelerate to high kinetic energies by raising their temperature to hundreds of

millions of degrees in order to overcome electrical repulsive forces.

The tremendous energy produced in the sun and stars are by continuous fusion

reactions. In the sun, quintillions of hydrogen nuclei collide with each another and

fuse to become helium nuclei. During this, they lose a small amount of their original

mass which is converted into extremely energetic particles, radiation and heat. This

process is also known as thermo-nuclear fusion. For generating power by this process,
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a controlled, relatively slow and sustained fusion is needed. The nuclear fusion is

presently under experimentation and the results are expected by 2015.

Nuclear fission

Figure 1 : Nuclear fission reaction

In nuclear fission, a high energy neutron is made to struck with a heavy nuclei.

Due to this effect the heavy nucleus will split into two almost equal fragments of

lighter atoms accompanied by release of heat plus two to three free neutrons. Fission

can be caused by bombarding with high energy a particles, protons, deutrons, X -rays

as well as neutrons. But, neutrons are most suitable for fission reaction because they

are electrically neutral and thus do not require high kinetic energy to overcome the

electrical repulsion from positively charged nuclei.

One of the elements whose nucleus easily fissions is 92U235 . All the other are

naturally available stable elements and hence splitting the nucleus of these atoms isnot as easy thing. Thus in a fission process, a high energy neutron is made to struck

with heavy unstable uranium ( 92U235) nuclei. During fission reaction, the following

results.

1. The immediate (prompt) products of a fission reaction, such as ( 54Xe 140) xenon

and strontium ( 38Sr94) are called fission fragments.

2. Fission fragments are released along with other decay products ( , , etc.)arecalled fission products.
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3. Two to three fast free neutrons accompanied by the release of heat for every

bombardment.

Controlled and uncontrolled chain reaction

Figure 2 : Chain Reaction

As discussed in nuclear fission reaction, when an unstable neutron enters the

uranium 92U235 nucleus, the nucleus splits into two fragments and releases 2 to 3

neutrons per fission accompanied with heat. The neutrons released during the fission

can strike other nuclei of 92U235 and causes further fission reactions under favorable

conditions. Thus the reaction becomes self-sustaining and results in continuous chain

reaction. Thus, a chain reaction is a self-sustained, continuing sequence of fission

reactions which is maintained under favorable conditions. This chain reaction may

either occur in controlled manner or uncontrolled manner shown in figure 2.

As discussed above, 2 to 3 neutron will be releasing as a result of fission

reaction. As an average, 2.5 neutrons are ejected per neutron absorbed. Out of 2.5neutrons, nearly 0.2 to 0.3 neutrons is lost due to escape at the surface and the

remaining is 2.2 neutrons. If these 2.2 neutrons are allowed to continue the chain

reaction, an exponential increase in reaction rate can be noticed. As a result of this,

enormous amount of heat energy will be evolved and such a type of chain reaction is

known as uncontrolled chain reaction. This extremely large amount of energy is

applied in atomic explosion.
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In other case, out of2.5 neutrons, about 0.9 neutron is absorbed by U238

converting it into fissionable material Pu239, 0.5 neutron is captured by control rod

material, coolant moderator and partly escape from the reactor and 0.1 neutron is lostfrom the reaction and the remaining is only one neutron. The only one neutron after

every fission is allowed to continue to cause further fission reaction. It is known as

controlled chain reaction. The energy produced by such a chain reaction is used in

nuclear power production.

Difference between fission and fusion reactions

S.No. Nuclear fission Nuclear fusion1 Nuclear fission is the process of

splitting up of heavy unstable

nucleus into two equal

fragments of equal mass and 2

to 3 neutrons accompanied by

the release of large amount of

heat.

Nuclear fusion is forming a

heavier nuclei by colliding two

or more nuclei so that they fuse

together into a heavier nuclei

accompanied with the large

amount of energy release.

2 Nearly about one thousandth of

the mass can get converted into

energy.

Nearly about four thousandth of

mass can get converted into

energy.

3 Fission process emits

radioactive rays.

Lesser or no more radioactive

rays emission in fusion process.

4 Process is possible even at

ordinary operating temperature.

Very high temperature is needed

for fusion process to occur.

5 It possibly gives rise to self-

sustained chain reactions in

controlled manner.

It does not give rise to chain

reaction in controlled manner.

6 Neutrons are emitted as a result

of fission reaction.

Protons are emitted as a result of

fusion reaction.
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Nuclear reactors

A nuclear reactor is an assembled apparatus in which, a self-sustaining nuclear fission

chain reaction is initiated and maintained at a steady controlled rate so that heat is

produced continuously. Its main function is to control the release and absorption of

neutrons during a chain reactor. It may be regarded as a furnace for burning nuclear

fuels like U 235 , U 233 or Pu 239 and produces useful products like heat, neutrons and

radio isotopes. The figure 3 shows the various essential elements of a typical nuclear

reactor. It consists of the following components.

1. Reactor core

Reactor core is a chamber in which nuclear fission chain reaction occurs and as a

result of this, large amount of heat is liberated. It consists of assemblage of fuel

elements, control rods, coolant and moderator. Fuel elements are usually made of

plates or rods of uranium metal and are usually clod in thin stainless steel sheet to

provide corrosion resistance and structural support.

Figure 3 : Components of a nuclear reactor
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2. Moderator

Moderator is a device used to slow down the high energy fast moving neutrons

by reducing their kinetic energy so that neutrons are utilized completely before it

escapes. This increases the possibility of absorption of neutrons by the fuel to cause

further fission and hence the quantity of fuel required to maintain a chain reaction is

also reduced. The common moderators used are ordinary water, heavy water, graphite

and beryllium.

3. Control rods

Control rods are inserted to control the chain reaction and controlling the heat

production rate. To startup the reaction, the control rods are moved out whichincreases the heat production rate and the reaction are maintained by adjusting the

position of control rods. To shutdown the reaction under emergency, the control rods

are pushed in, which reduces the heat production rate. The common materials for

control rods are boron, cadium, indium or silver.

4. Reflector

The main purpose of a reflector is to conserve the neutrons by decreasing theloss of the same. This results in the reduction of fuel consumption. The neutrons

produced during the chain reaction will be partly absorbed by the fuel rods,

moderator, coolant or structural material etc., while the remaining will try to escape

from the reactor core. These escaping neutrons can be reflected back into the core by

reflectors to take part in the fission reaction.

5. Reactor vessel

It is a strong walled container that encloses the reactor core at its bottom,

reflector and thermal shield. It also has holes at the top for inserting the control rods

and has passages for entrance and exit of the coolant.

6. Thermal shielding

It surrounds the entire reactor core and absorbs some of the radiations in the

form of -rays, -rays and escaping neutrons. So, it gets heated and prevents the

reactor wall from getting heated. This shield is cooled by circulating water over it. It

is made up of steel plate lined with concrete.
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7. Coolant

The main purpose of coolant is to transfer the large amount of heat produced in

the reactor due to the continuous fission reaction. It also keeps the fuel assemblies at a

safe temperature to avoid their melting and destruction. The commonly used coolantsare ordinary water, heavy water, gas (He, Co 2), liquid metal (Na, K) and an organic

fluid.

8. Biological (or) External (or) Radiation shield

It surrounds the whole reactor unit and prevents the escape or leakage of fast

moving neutrons and slow neutrons. It also prevents the operating personnel from the

harmful effects of dangerous radiations like , , rays which tends to escape to theatmosphere.

Types of nuclear reactors

Nuclear reactors can be classified as follows

1. Depending on the way of energy released by fission.

a. Fast reactors (Fission is caused by high energy fast neutrons)

b.

Thermal or slow reactors (Fission is caused by slow neutrons)c. Intermediate reactors (Fission is caused by neutrons in course of slowing

down)

2. Depending on the type of fuel used

a. Fissile material (Natural and enriched uranium)

b. Fertile material (Pu 239 and U 233)

3. Depending on the moderator used

a. Ordinary water moderated reactorsb. Heavy water moderated reactors

c. Graphite moderated reactors

d. Beryllium moderated reactors

4. Depending on the type of cool and used

a. Water cooled reactors

b. Gas cooled reactors

c. Liquid metal (Na, K) cooled reactors d. Organic liquid cooled reactors
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5. Depending on the fuel moderator assembly (core type)

a. Homogeneous reactors (Fuel and moderator are mixed)

b. Heterogeneous reactors (Separate fuel rods are surrounded by moderators)

Pressurized Water Reactors (PWR)

Figure 4 : PWR power Plant

Water is an excellent moderator and coolant for the reactor and thus water

cooled reactor are developed in nuclear power plants. PWR power plant shown in

figure 4 uses enriched uranium as fuel. The water under pressure is used here for both

moderator and coolant. A constant pressure of about 150 bar is maintained in the

primary loop so as to avoid the boiling of water. In this reactor, a pressuriser is

mounted in which water and steam if filled and provided with electric heater at

bottom and water spray at the top. If the pressure in primary loop drops. The heater is

opened which generates steam and increases the steam content in the vessel so that

pressure is increased in the primary loop. If the pressure in primary loop is high, then

too high cold water is sprayed into the steam and so the steam gets condensed so that

pressure is reduced in the primary loop.

Inside the reactor, continuous chain reaction takes place and is accompanied by

the liberation of a large amount of heat which is absorbed by the coolant (water). This

hot pressurised coolant flows to heat exchanger (steam generator) through pressuriser.
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Here, heat exchanger acts as steam generator. The steam is generated by transferring

the heat from the coolant to the feed water of secondary loop. Then, the pump returns

the coolant into the reactor again through primary loop. The steam generated in heat

exchanger is expanded in the turbine which is coupled with the generator. From the

turbine exit, the condensate passes to condensor and it condenses. The feed pump

delivers the same as feed water to the heat exchanger.

Advantages

1. Single fluid used here is water which is cheaper, available in plenty and multi-

purpose (coolant, moderator and reflector).

2. Reactor is more compact and higher power density.3. Lesser number of control rods are required.

4. Steam is free from contamination by radiation and hence normal turbine

maintenance is enough.

5. Fuel is utilized efficiently and thus minimizes the fuel cost.

Disadvantages

1.

High primary loop pressure requires strong and leak proof pressure vessel and sohigh capital cost.

2. Low pressure in the secondary circuit leads to poor thermodynamic efficiency of

the plant.

3. Use of high pressure, high temperature water causes severe corrosion problems.

4. Reactor must be shut down for fuel recharging.

5. Fuel reprocessing is difficult as fuel suffers radiation damage.

6. Preparation of fuel element is expensive.

Boiling Water Reactor (BWR)

Boiling water reactor also uses enriched uranium as a primary fuel. Here, the

reactor pressure is substantially reduced to allow the boiling of coolant (feed) water.

Figure 5 illustrates the BWR power plant:
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Figure 5 : BWR Power plant

This system is also known as direct cycle boiling water reactor power plant. In

this reactor, the steam is generated inside the reactor itself.

Coolant ( water) enters the reactor at the bottom and gets heated by the heat

liberated due to the fission reaction. Inside the reactor itself, the water is completely

converted into steam. The steam is leaves the reactor top and supplied to the steam

turbine and gets expanded. Exhaust steam from the turbine passes through the

condenser and condensed. The condensed water is rerouted again by using feed

pump.

Advantages

1. Elimination of heat exchanger results in reduction in cost.

2. Maintenance of comparatively lower pressure inside the reactor makes the

reactor much lighter and reduces the cost.

3. Low metal surface temperature.

4. Thermal efficiency is high of about 30% due to single loop operation.
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Disadvantages

1. Steam leaving the reactor is slightly radioactive.

2. Safety is a major problem in BWR plants than PWR plants.

3. Steam wastage during part load operation results in lower thermal efficiency.4. Lower power density makes to use larger vessel.

Fast Breeder Reactor (FBR)

Figure 6 : Fast breeder reactor power plant (Liquid metal cooled reactor)

Fast breeder reactor is a small vessel in which the chain reaction is sustained

primarily by fast moving neutrons (without using moderator). In fast breeder reactor,

fuel such as enriched uranium and plutonium or a mixture of these is kept inside the

reactor core without using moderator. This active core region is surrounded by a

blanket of breeding material (fissible material) such as fertile uranium to absorb the

excess neutrons otherwise those neutrons would possibly lost by leakage. The reactor

core is cooled commonly by liquid sodium. Fast breeder reactors are normally

designed to produce more amount of fissible material than that is being consumed by

it.

The figure 6 shows the fast breeder reactor power plant which is more similar to

liquid metal ( sodium graphite) cooled reactor plant. When the enriched uranium

undergoes fission reaction, heat will be liberated with the release of fast moving

neutrons. Excess neutrons are absorbed by the surrounded uranium blanket and are
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converted into fissionable material (Pu 239) which is capable of sustaining chain

reaction continuously. Heat evolved in the reaction is carried by the primary coolant

(sodium) and given to the primary heat exchanger. In primary heat exchanger the heat

is transferred to the secondary coolant (sodium, however different coolant could be

used). The hot secondary coolant passes through the secondary heat exchanger (steam

generator) and the feed water coming from the condenser is heated and converted into

steam. It is then supplied to steam turbine for mechanical power. The electric power

is produced by coupling an electric generator to the steam turbine.

Advantages

1. It does not require moderator.

2. High breeding is possible.3. High power density makes to use small core.

4. More fuel is produced than consumed.

5. Absorption of neutrons is low.

Disadvantages

1. It requires highly enriched fuel and thus initial cost is very high.

2. Safety is essential against melt-down.3. Circulation of special coolants are essential to carryout large quantity of heat

from the reactor core.

4. Handling of sodium is difficult as it becomes highly radioactive.

Advantages and Disadvantages of nuclear power plants

Advantages of nuclear power plants

The following are the merits of nuclear power plant over conventional power plants:-1. Power generation is more economic when compared with thermal plant.

2. Fuel transportation, handling and storage charges are absent.

3. No ash disposal problems.

4. Power production is not affected by weather conditions.

5. Space requirement is less.

6. Capital cost is low for bigger unit sizes.

7. Less number of workers is needed than thermal plant.

8. Water quantity required is very less.
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9. Enormous amount of heat can be generated from small quantity of fuel

consumption.

10. It is well suited for large power requirements.

11. Highly reliable operation.

12. Nuclear power plants have no effect on atmosphere by pollution.

Disadvantages of nuclear power plants

1. Disaster is the major safety problem faced by nuclear power plants due to

nuclear explosions.

2. Radioactive wastes may affect the health of workers and other surrounding

people.

3. High initial cost and maintenance cost.

4. Nuclear waste disposal is a major problem.

5. Nuclear plants require well trained personals for its operation.

Waste disposal and safety

Disposal of waste materials is a major and common problem in nuclear power

industry. All the wastes being highly radioactive and are injurious to human, animals

and plant life. Therefore, proper care must be taken to dispose the waste safely

without polluting the atmosphere. The waste materials produced in the nuclear plants

are in the form of solids, liquids and gaseous.

1. Solid waste

Solid waste consists of discarded control rods, fuel cans, scrap material or

disordered objects contaminated with radioactive matter. The solid containing the

combustible matter is burnt and the resulting gases are disposed to atmosphere afterdilution. The remaining is mixed with concrete and buried deep in the ground. It is

necessary to keep the radioactive solid waste first in water as nearly SOOIo of the

radioactive material disappear during cooling.

2. Liquid waste

Liquid wastes are diluted by adding water to remove most of the activity in the

form of solid precipitate and then discharged into the ground if the activity level islow. However, ground water may get contaminated, if dilution is insufficient. So, the
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treated liquids are kept in hold-up tanks before discharge for a period to allow part of

the radioactivity to decay and then buried in ground. The tank should be leak proof

and has long term strength.

3.

Gaseous wasteGaseous waste could easily pollute air and hence the gas should be treated in a

clean-up plant to remove radioactive iodine which causes major health hazards. They

are also commonly diluted with air and after passing through the filters and then

released to the atmosphere through a high chimney.

Site selection of nuclear power plant

i)

A vailability of waterThe modern nuclear power plant requires plenty of water for condenser cooling

and makeup water requirement. Therefore it should be nearer to a river, reservoir or

sea.

ii) Distance from populated area

The nuclear power plant should be constructed far away from the populated area

to avoid the radioactive hazard.

iii) Distance from load centreIt should be located nearer to the load centre to reduce the tranmission loss and

power loss.

iv) Waste disposals

The nuclear wastes are radioactive. Therefore the sufficient safely space for

disposal is required.

v) Safe against Earth Quakes

The country divided in to five seismic zone codes (zone 1,2,3,4 and 5) by

seismic survey of geological survey of India. The zone 5 will not be the most suitable

because of frequently occurred tremors arid earth-quakes. The zone 1 will be highly

suitably place for nuclear plant.

vi) Soil condition

The bearing capacity of the soil place very important role in selection of foundation.

vii) AccessibilityThe power plant should be connected with excellent transport facility.
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Documents

Nuclear and Hydel Power Plants