Nuclear Power Plant Dr. Sikder Sunbeam Islam

Associate Professor

Department of EEE, IIUC.

Introduction The generating station in which nuclear energy is converted into

electrical energy is known as a Nuclear Power Station.

In nuclear power station , heavy elements such as Uranium (U235) or Thorium (Th232) are subjected to nuclear fission in a special apparatus known as a ‘reactor’.

Breaking up of nuclei of heavy atoms into two nearly equal parts with release of huge amount of energy is known as nuclear fission.

The heat energy thus release is utilized in rising steam at high temperature and pressure.

The steam runs the steam turbine which converts steam energy into mechanical energy. The turbine drives the alternator which converts mechanical energy into electrical energy.

Working Principle

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Nuclear Fuels

Fuel of a nuclear reactor should be fissionable material

which can be defined as an element or isotope (that take

part in nuclear fission). It can be one or all of following :

Natural Uranium found in earth crust (outermost solid

shell of our rocky planet) contains isotopes namely U234 ,

U235, U238 and their average percentage is as follows –

U238 99.3% ; U235 0.7% ; U234 Trace

Out of these U235 is most unstable and is capable of

sustaining chain reaction and has been given the name as

primary fuel .

U233 and Pu239 (Plutonium-239) are artificially produced from

Th232 and U238 respectively are called secondary fuel .

Uranium Oxide (UO2) also can be used as nuclear fuel.

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Nuclear Fission & Chain Reaction

The breaking up of nuclei of heavy atoms into two nearly equal parts

with release of huge amount of energy is known as ‘Nuclear fission’.

Nuclear fission is done by bombarding Uranium nuclei with slow

moving neutrons . This splits the uranium nuclei with the release of

huge amount of energy and emission of neutrons which is called

fission neutrons .

These fission neutrons cause further fission. If this process continues,

then in a very short time huge amount of energy will be released

which may cause explosion . This is known as explosive chain

reaction . This process is done in nuclear reactor.

All practical nuclear reactors are started with a startup neutron

source. The original startup source in a typical American light-water

power reactor is californium-252, a heavy synthetic element that has

a significant spontaneous fission (SF) mode.

Cf-252, a californium isotope with a half-life of 2.645 years, is a very

strong neutron source. Californium is a radioactive chemical element

with the symbol Cf and atomic number 98.

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But in a reactor , controlled chain reaction is allowed .

This is done by systematically removing the fission

neutrons from the reactor . The greater the number of

fission neutrons removed , the lesser is the intensity (i.e

fission rate ) of energy released and this controlling is done

by control rod .

However , the neutrons released have a very high velocity of

the order of 1.5 x 107 meters per second .The energy

liberated energy in the chain reaction is according to

Einstein law, E=mc2, Where

E= energy produced, m = mass in grams, c= velocity.

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Nuclear Fission & Chain Reaction (continues.)

Where does the Energy come from ? When an U235 atoms fission , we have the following example

equation,

235U + 1 neutron 2 neutrons + 92 Kr +142 Ba + Energy

You might have wondered , “where does the energy come from? “ . The mass to be some on both sides of the reaction;

235 + 1 = 2 + 92 + 142 = 232

Thus , it seems that no mass is converted into energy . However , this is not entirely correct . The mass of an atom is more than the sum of individual masses of its proton and neutrons .

Extra mass is a result of binding energy that holds the protons and neutrons of the nuclear together . Thus, when the uranium atom is split , some of the energy that held it together is released as radiation in the form of heat . Because, energy and mass are one and the same , the energy released is also mass released. Therefore, the total mass does decrease a tiny bit during the reaction.

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Advantages

The amount of fuel required is quite small.

A nuclear power plant requires less space as compared

to any other type of the same size.

It has low running charges as a small amount of fuel

is used for producing bulk electrical energy.

This type of plant is very economical for producing

bulk electric power.

It ensures reliability of operation.

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The fuel used is expensive.

The capital cost on a nuclear plant is very high as compared to other types of plants.

The fission by-products are generally radioactive and may cause a dangerous amount of radioactive pollution.

Nuclear power plants are not well suited for varying loads as the reactor does not respond to the load fluctuations efficiently.

The disposal of the by-products, which are radioactive, is a big problem. They have either to be disposed off in a deep trench or in a sea away from sea-shore.

Schematic Layout of Nuclear Power Plant

The whole arrangement can be divided into the following main stages :

(i) Nuclear reactor (ii) Heat exchanger (iii) Steam turbine (iv) Alternator.

Disadvantages

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Schematic Arrangement of Nuclear Power Station

Fig.1. The schematic arrangement

of a nuclear power station

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Schematic Layout of Nuclear Power Plant

(Continues.)

Nuclear Reactor

• It is an apparatus in which nuclear fuel (U235) is subjected to nuclear fission.

• It controls the chain reaction that starts once the fission is done. If the chain reaction is not controlled, the result will be an explosion due to the fast increase in the energy released.

Fig.2:Nuclear Reactor

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Schematic Layout of Nuclear Power Plant

(Continues.): Nuclear Reactor

A nuclear reactor is a cylindrical pressure vessel. It houses fuel

rods of Uranium, moderator and control rods .

Fuel Rods: The fuel rods constitute the fission material

and release huge amount of energy when bombarded with slow

moving neutrons.

Moderator : The moderator consists of graphite rods which

enclose the fuel rods. The moderator slows down the

neutrons before they bombard the fuel rods.

Control Rods: The control rods are of cadmium and are

inserted into the reactor. Cadmium is strong neutron

absorber and thus regulates the supply of neutrons for fission.

When the control rods are pushed in deep enough, they absorb

most of fission neutrons and hence few are available for chain

reaction which, therefore, stops. Similarly, withdrawing control

rods increases the intensity of chain reaction (or heat

produced).

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Fig.3

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Therefore, by pulling out the control rods, power of the nuclear reactor is increased, whereas by pushing them in, it is reduced.

In actual practice, the lowering or raising of control rods is accomplished automatically according to the requirement of load.

Coolant: The heat produced in the reactor is removed by the coolant, generally a sodium metal. The coolant carries the heat to the heat exchanger.

Schematic Layout of Nuclear Power Plant

(Continues.): Nuclear Reactor

Fig.4

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In this reactor enriched uranium (enriched uranium contains more

fissionable isotopes’ U235 then naturally occurring percentage 0.7%

) is used as nuclear fuel and water is used as coolant.

Water enters the reactor at the bottom .It takes up the heat

generated due to the fission of fuel and gets converted into steam.

Steam leaves the reactor at the top and flows into the turbine.

Water also serves as moderator .Heavy water, carbon and

beryllium are the best moderators.

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Schematic Layout of Nuclear Power Plant (Continues.):

Nuclear Reactor: Boiling Water Reactor

Fig.5. Boiling Water Reactor

It uses enriched Uranium as fuel .Water is used as coolant and moderator .

Water passed through the reactor core and takes up the heat liberated due to the nuclear fission of fuel .In order that water may not boil (due to its low boiling point 212oF at atmospheric condition) and remain in liquid state.

It is kept under a pressure of about 1200 p.s.i.g by the pressurizer .This enables water to take up more heat from the reactor .From the pressurizer water flows through tubes of steam generator where it passes its heat to the feed water which in turn gets converted into steam.

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Schematic Layout of Nuclear Power Plant (Continues.):

Nuclear Reactor: Pressurized Water Reactor (P.W.R)

Fig.6. Pressurized Water Reactor

In this reactor two liquids metal coolant are used. Liquid sodium (Na) serve as the primary coolant and alloy of sodium potassium (NaK) as secondary coolant.

Liquid sodium as coolant can transfer its heat very easily .The only problem in this system is that sodium becomes sometimes radioactive while passing through the core and reacts chemically with water . So, a secondary coolant NaK is used .

The primary coolant while passing through the tubes of inter mediate heat exchanges(I.H.X) transfer its heat to the secondary coolant .The secondary coolant then flows through the tubes of steam generator and passes its heat to the feed water .Graphite is used as moderator in this reactor.

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Schematic Layout of Nuclear Power Plant (Continues.):

Nuclear Reactor: Sodium Graphite Reactor(SGR)

Fig.7. Sodium Graphite Reactor

In this reactor the core containing U235 is surrounding by a blanket( a layer of fertile material placed outside the core) or fertile material U238 .

In this reactor no moderator is used. The fast moving neutrons liberated due to fission of U235 are absorbed by U238 which gets converted into fissionable material Pu239 which is capable of sustaining chain reaction.

Thus ,this reactor is important because it breeds fissionable material from fertile material U238 available in large quantities.

U238 + neutron Pu239

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Schematic Layout of Nuclear Power Plant (Continues.):

Nuclear Reactor: Fast Breeder Reactor(FBR)

Like, sodium graphite nuclear reactor this reactor also uses two liquid metal coolant circuits. Liquid sodium is used as primary coolant when circulated through the tubes of intermediates heat exchange transfer its heat to secondary coolant sodium potassium alloy .

The secondary coolant while flowing through the tubes of steam generator transfer its heat to feed water.

Fast breeder reactors are better than conventional reactor both from the point of view of safety and thermal efficiency.

Fig.8. Fast Breeder Reactor

The commonly used coolants for FBR are :

• Liquid metal (Na or NaK) (ii) Helium (He) (iii) Co2 .

Sodium has following advantage in this case –

• It has very low absorption .

• It possess good heat transfer properties at high

temperature and low pressure.

• It does not react on any of the structural materials

used in primary circuits.

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Schematic Layout of Nuclear Power Plant (Continues.):

Nuclear Reactor: Fast Breeder Reactor(FBR)

Schematic Layout of Nuclear Power Plant

(Continues.):

• Heat Exchanger: The coolant gives up heat to the heat exchanger which is utilized in raising the steam. After giving up heat, the coolant is again fed to the reactor.

• Steam Turbine: The steam produced in the heat exchanger is led to the steam turbine through a valve. After doing a useful work in the turbine, the steam is exhausted to condenser. The condenser condenses the steam which is fed to the heat exchanger through feed water pump.

• Alternator: The steam turbine drives the alternator which converts mechanical energy into electrical energy. The output from the alternator is delivered to the bus-bars through transformer, circuit breakers and isolators.

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Waste Disposal • Importance: Wastes from atomic energy installation are

radioactive, create radioactive hazard and so, require

strong control to ensure that radioactivity is not released

into the atmosphere to avoid atmospheric pollution.

Nuclear wastes are likely liquid, gas or solid and each is

treated in different manner.

• Liquid Wastes:

• (i) Dilution: Liquid wastes are diluted with proper factor

with large quintiles of water and released into the

ground. The drawback of this method is that there is a

chance of contamination of underground water if dilution

factor is not adequate.

• (ii) Concentration to small volumes and storage: Here

liquid wastes are concentrated to small volume and

stored in underground tanks.

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• Gaseous Waste: Gaseous wastes causes atmospheric pollution. This wastes are passes through filters and then released to atmosphere.

• Solid Wastes: Solid wastes included scrap material or discarded objects contaminated with radioactive matter.

• This waste is burnt if combustible. Non-combustible wastes are always buried deep (around 500 meters deep) in the ground.

• The radioactive matter is mixed with concrete, drummed and shipped for burial.

• High-level waste is delayed for 40-50 years to allow its radioactivity to decay, after which less than one thousandth of its initial radioactivity remains, and it is much easier to handle. Hence containers of vitrified waste, or used fuel assemblies, are stored under water in special ponds, or in dry concrete structures or casks, for at least this length of time.

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Waste Disposal (continues)

Problems

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Examples-1: Calculate the fission rate of U235 for producing power of one watt if 200 MeV of energy is released per fission of U235.

Soln : Given, P = Power = 1 Watt E = Energy released per fission of U235 nucleus = 200 MeV = 200 x 1.6 x 10-13 j

= 3.2 x 10-11 watt. Sec [ `.’ 1 MeV = 1.6 x 10-13 J ]

Fission rate (fission/sec) of producing one watt of Power,

= 𝑃

𝐸 =

1

3.2 × 10−11 = 3.1 x 1010 fissions/ sec ,

Ans.

Examples-2: Determine the energy released by the fission of 1.5 gm of U235 in Kwh , assuming that energy released per fission is 200MeV. .

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Soln :

We know , Avogadro number = 6.025 × 1023

Mass of Uranium = 235 a.m.u. [ 235 gm of U235 contain 6.02×1023 numbers]

n = Number of atoms in 1.5gm of U235

= 1.5 × 6.025 × 1023

235

E1 = Energy released per fission = 200 MeV = 200 × 106 ×

1.6 × 10-19

= 3.2 × 10-11 J

E = Energy released by 1.5 gm of U235

= n × E1 = 1.5 ×6.025 ×1023

235 ×

3.2 ×10−11

103 ×3600 Kwh = 3.42 × 104 Kwh . Ans.

References

1. Principles of Power Systems, V.K.Mehta & Rohit

Mehta

2. Power Plant Engineering, G.R.Nagpal

3. Power Station Engineering &Economy, William

A Vopat