1

Chapter-1

INTRODUCTION

1.1 PHYSICAL LOCATION

Rawatbhata is located at the bank of river Chambal near the Rana Pratap Sagar Dam. The

nearest city is Kota situated at a distance of 60 KMs from the plant.

There are four units of 220 MWe each and two units of 235 MWe newly constructed. There is

lush greenery around the site. For employee’s various colonies are constructed with all the

domestic facilities.

Fig.-1.1 RAPS 1&2

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Fig.-1.2 RAPS 3&4

Fig.-1.3 RAPS 5&6

1.2 ABOUT NUCLEAR ENERGY

Nuclear energy has turned out to be the achievement of the past century. The cleanest environmental

friendly and of less running cost mode of power generation is now in our hand.

At present it is estimated that our natural reserves of U3O8 is about 70,000 tones, but the long run

3

potentials depend upon the large reserves of Thorium which is about 3,60,000 tones. The optimum

usage of the available resources takes place via three stages namely: -

The first stage and perhaps used widely is using natural uranium as fuel.

The plutonium thus yields by first stage along with thorium is fed in Fast Breeder Reactors.

The third stage would employ the U-233 obtained from second stage together with thorium is

employed. Perhaps the third stage could either be a fast reactor or a thermal reactor.

In fast reactors high energy neutrons are required to bring about fission. It is most common with

element having even number of mass number.

In thermal reactors, thermal neutrons i.e. slow moving neutrons are required to being about the

fission. Those having mass number as an odd number possess this type of property.

1.3 NEED FOR NUCLEAR POWER

The exploration of natural resources for generation of electricity has been an evolutionary process.

Over the years, it has progressed from tapping the potential energy of falling water to burning of

fossil fuels. But the quest for more sources of electricity, which is the cleanest and most efficient

form of energy, is unending and the limits of the conventional sources have served to heighten man’s

anxious efforts in this regard. The discovery of fission and the promise of abundance which nuclear

energy came to hold subsequently turned man’s attention to utilize the potential of this source.

Considering the current population growth which has already crossed 100 crores in the 21st century

and improvements in standard of living of the forth coming generations, there will be a large increase

in the need of electrical energy particularly from clean, green and safe energy sources. The electrical

energy will play a vital role in sustainable development of the country. Among all the available

conventional and nonconventional energy sources, the nuclear energy is most efficient, abundantly

available, sustainable and cost effective energy sources. It does not emit obnoxious gases that cause

global warming, ozone hole and acid rain.

1.4 SO THE NUCLEAR POWER

It is thus evident that some new form of energy, such as nuclear, which is a large addition to our

energy resources, has to be developed in a big way. The currently known uranium reserves in the

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country can support a PHWR programme of 10,000MW for a design life of 30 years. Even though

there is every reasons can support an ultimate capacity of 350,000 MW(e) by fast breeder. the long

range potential of so, on thorium resources which exceed 360,000tonnes. when used in the breeder

reactors, the thorium reserves would be equivalent to 600 billion tons of coal. This is explained

below.

1.5 NUCLEAR POWER IS SAFE

Improving the quality of life has been the driving force for making to push ahead with the use of

modern technology. That these benefits carry along with them some risks, has been known for

sometimes and one has also to recognize that there is nothing like an absolutely safe technological

products be it the automobile, aircraft, Electrical industry, or for that matter, a nuclear reactor. If

mankind had decided to take a” zero-risk approach”, we would not have undertaken space

exploration or developed nuclear technology. They would have burnt more coal and oil, resulting

in more acid rain, pollution and scarce oil.

1.6 PRINCIPLE OF NUCLEAR REACTION

When a heavy nucleus split into smaller nuclei, a small amount of mass is converted into energy.

The amount of energy produced is given by Einstein mass energy relation (E=m*c2). this breaking

up of nuclei is called nuclear fission. Natural uranium has two types of isotopes, U238 and U235

isotope in the ratio of 139:1. The less abundant U235 isotopes that fissions when a U235 atom is struck

by allows (or thermal) neutron, it splits into two or refragments. This splitting is accompiled by

release of energy in the form of heat, radio-ability and two or three atom at high speed, are made to

slow down in the split atom at high speed, are made to slow down in a moderation, i.e. heavy water,

so that they have a high probability of hitting other U235 atoms which in turn release more energy

and further sets of neutrons. Attenuation of self-sustained stage of spilling of uranium atom is called

chain reaction. There is a particular size of fissionable material for which the neutron production by

fission is exactly balanced by leakage and absorption. This is called the critical size at which the

chain reaction is self-sustaining this the size of a reaction.

5

Fig.-1.4 Nuclear Defragmentation Reaction

In the above equation, (1) the total mass before fission, is the sum of the masses of U235 and

the neutrons. Mass after fission is the sum of fission fragments and neutrons.

n1

Sr90

Xe144 n1

- Ray

U235

U236

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Chapter-2

CLASSIFICATION OF POWER REACTORS

2.1 FAST REACTORS

The U-235 content of the fuel can be increased, i.e., the fuel is highly enriched in U-235 with

a substantial decrease in U-238. The U-235 fast fissions are thus, considerably increased in a

fast reactor. Some reduction in neutron energy does occur due to inelastic collisions of neutrons

with nuclei of the fuel and structural material but most of the fissions are caused by neutrons

of energies greater than 0.1Mev. The mass of U-235 required for the reactor to be critical varies

with a mount of U-235 enrichment. In all cases the critical mass of fissile material required

increases rapidly below 15% to 20% U-235 enrichment. To avoid large fuel inventories a

practical fast reactor, such as case C above, would require fuel containing at least 20% U-235

by volume. Incidentally the critical mass of U-235 in a fast reactor is considerably greater than

in a thermal reactor with the same fuel composition. The highly enriched fuel and absence of

moderator results in a small core therefore, fast reactors have high power density cores. The

average power density in a Fast Breeder Reactor (FBR) is 500 MW/m3 compared with 100

MW/ m3 for a Pressurized Water Reactor (PWR). It is therefore essential that a heat transport

fluid with good thermal properties be used. The choice is also limited to a non-moderating fluid

and liquid metals seem to satisfy both requirements. The capture cross-sections of most

elements for fast neutrons are small and since there is a relatively large mass of U-235 in the

reactor, the macroscopic capture cross-sections of structural material and fission products are

small compared with the macroscopic fission cross-section of the U-235.Consequently there is

more flexibility in the choice of materials and stainless steel can be used instead of aluminum

or zirconium. Fission product poisoning is not significant and for this reason, (and the fact that

temperature coefficient of reactivity is low), the excess reactivity required in a fast reactor is

small.

2.2 THERMAL REACTORS

Since a chain reaction cannot be maintained with fast neutrons without considerable

enrichment, the alternative is to reduce the neutron energy until the fission cross-section of U-

235 is sufficiently increased. If the neutrons are reduced to thermal energies, the U-235 fission

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cross-section is 580 barns whereas the radioactive capture cross-section is 106 barns. Thus,

even allowing for the low percentage of U-235 in natural uranium, the thermal neutron fission

cross-section in natural uranium is 4.2 barns whereas the radioactive capture cross-section is

3.5 barns. Thus, for every 77 neutrons captured in natural uranium about 40 will cause fission

and produce 40 x 2.5 or 100 new neutrons. For 77 neutrons out of every 100 to be captured,

fewer than 23 neutrons can be lost by escape or radioactive reaction could be sustained. In

thermal reactors the fission neutrons are thermalized by slowing them down in a moderator.

Most of the power reactors in existence are thermal reactors.

2.2.1 TYPES OF THERMAL REACTORS

In the previous lesson reactors were classified on the basis of neutron energy and the various

advantages and disadvantages of fast and thermal systems were enumerated. It was mentioned

that most of the reactor systems, at present in operation, are thermal reactors. Thermal reactors

will now be classified further on the basis of core structure, the moderator used and the heat

transport system used. Some reference will be made to the advantages and disadvantages of

each type, but some of these considerations will be discussed later when moderator and heat

transport system properties are discussed.

The moderator may be:

1. Light water

2. Heavy water

3. Graphite

4. Organic liquids

The heat transport system may be:

1. Pressurized light water

2. Pressurized heavy water

3. Boiling light water

4. Boiling heavy water

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5. Gases such as CO2 or helium

6. Liquid metals

7. Steam or fog

8. Organic liquids

2.2.2 HEAVY WATER MODERATOR REACTOR

Heavy water has a much lower neutron capture cross section than both light water and graphite.

The principal advantage of using heavy water as a moderator is, therefore, the neutron economy

that can be achieved with it. The thermal utilization factor, f, in the four factor formula, is

increased because of lower neutron capture in the moderator. Neutron economy is so much

improved that not only can natural uranium fuel be used, but that this fuel can be used in oxide

or carbide form. Thus, there is no longer any need for an enrichment plant. In addition, oxide

or carbide fuel improve the fuel integrity and the fuel in less susceptible to distortion.

2.2.3 PRESSURIZED HEAVY WATER REACTOR

PHWRs have established over the years a record for dependability, with load factors in excess

of 90% over extended periods. In the PHWR, the heavy water moderator is contained in a large

stainless steel tank (calandria) through which runs several hundred horizontal zircaloy

calandria tubes. The D2O moderator is maintained at atmospheric pressure and a temperature

of about 70°C. Concentric with the calandria tube, but separated by a carbon dioxide filled

annulus which minimizes heat transfer from fuel to the moderator, is the zircaloy pressure tube

containing the natural UO2 fuel assemblies and the heavy water coolant at a pressure of about

80 kg/cm² and a temperature of about 300°C. The term pressurized refers to the pressurized

D2O coolant which flows in opposite directions in adjacent tubes and passes its heat to the

secondary coolant via the steam generators. System pressure is maintained by a pressuriser on

one of the legs of a steam generator.

2.2.4 GRAPHITE MODERATED REACTORS

With a graphite moderator, a liquid or a gas must be used as the coolant. Although there is

water cooled graphite-moderated reactors, e.g., the Soviet Union’s RBMK series of power

stations, of which Chernobyl is one, only gas cooled reactors will be referred to here. Whilst

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the United States and Canada pioneered, respectively, the light and heavy water moderated

designs, France and United Kingdom undertook the early development of the graphite

moderated reactor, selecting carbon dioxide as the coolant because of its relative Electrical

inertness and low neutron activation. France abandoned this approach in favour of an extensive

PWR programme. The UK continued to be heavily committed to gas cooled reactors in the

form, initially, of magnox and subsequently the advanced gas cooled reactor.

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Chapter-3

PRINCIPLE OF OPERATION OF RAPS

3.1 THE PRIMARY HEAT TRANSPORT SYSTEM

3.1.1 PRINCIPLE OPERATION

Primary heat transport system provides the means for transferring the heat produced in the fuel

(located inside the pressure tubes of the reactor) to the Steam generators (boilers) in which the

steam to run the turbine is generated from ordinary DM water. The heat transport medium is

pressurized heavy water and is circulated through the main circuit by primary coolant pumps.

The principal feature of the system is to maintain continuous circulation of coolant through the

reactor at all times i.e. during normal & abnormal operation and shutdown condition The PHT

system provides continuous circulation of coolant through the reactor at all times by various

modes as listed below:

Normal operation: - By primary coolant pump.

Sudden loss of power to pumps: - By inertia of pump flywheels to avoid a sudden drop in

coolant flow.

Thermo siphoning: - By placing main equipment above the elevation of reactor core.

Loss of primary coolant: - By receiving emergency injection of heavy water from moderator

system after depressurization of primary heat transport system. In case of paucity of heavy

water from moderator system light water injection is initiated.

3.1.2 DESCRIPTION

The heavy water runs through the feeders into 306 coolant tubes, through the end fittings and

feeders to the reactor outlet headers. The reactor utilizes restriction orifices in selected inlet

feeders to achieve the flow required by the reactor channel ratings, commensurate with equal

temperature from all channels. The reactors outlet headers distribute the flow through 8 boiler

inlet valves, 4 on the north and 4 on the south, to the respective 8 boilers (in new PHWR it is

only 4 boilers 2 on each side). From the boilers through the boiler outlet valve the heavy water

arrives at the pumps. Each pump is associated with a respective boiler through an individual

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suction line. The pumps discharge the flow through pump discharge valves into the reactor

inlet header. No common suction header has been provided and pumps are attached directly to

the boilers, the only common connection being reactor inlet and outlet headers. This

arrangement allows the isolation of any of the circulating pumps and leads to the loss of a

boilers, the circuit has no spare pump. This situation is acceptable in view of the expected high

reliability of the heat transport pumps and also that the loss of a pump and a boiler does not

result in a substantial loss of plant capacity. From the reactor inlet headers, the heavy water

flows through the feeders and end fittings to the reactor coolant tubes.

Corrosion products and fission products are removed from the system by purification circuit.

Purification circuit also helps to achieve a pH value between 9.5 to 10.5 and to maintain the

conductivity of heavy water between 20 to 30 micromho/cm. In addition, it reduces radio lytic

decomposition of heavy water by controlling ionic impurities. The operating design pressure

in the reactor outlet headers is controlled at 87 Kg/cm2 (1237.5 psig). The pressure is

controlled by a feed and bleed system. In the event of a leak in the primary system, no matter

how large it is, cooling of the fuel can be maintained or restored by the emergency injection

system which is designed to pump heavy water from the moderator system into the primary

system. For cooling the system below 300F and for holding the system at low temperature

during plant maintenance, an auxiliary cooling system is provided which is known as standby

cooling system or shutdown cooling system. The system is connected between reactor outlet

and inlet header at each end of the reactor. If normal heat removal fails and normal pressure

control fails or their capacities are exceeded, the increase in coolant volume caused by the

reactor heat would be passed out of the primary system by relief valves. One relief line

connects the pressurized end of the north standby cooling loop, to the bleed condenser through

these instrumented safety relief valves in parallel. Isolated boilers are protected against

accidental high pressure by system relief valves. The PHT pumps are provided with flywheels

to provide better flow coast down after pump trip. The system layout as discussed above assures

adequate flow for decay heat removal from reactor during shutdown by thermos syphoning

action. A separate shutdown cooling system is provided to remove reactor decay heat during

cold shutdown conditions. This mode of cooling permits the draining of the steam generators

and pumps in the PHT system for maintenance. An emergency core cooling system provides

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adequate coolant flow to prevent overheating of the fuel in the unlikely event of loss of coolant

accident.

3.2 RANKINE CYCLE

Rankine cycle is a vapour power cycle having two basic characteristics

The cycle consists of a succession of steady flow processes, with each process carried out in a separate

component specially designed for the purpose. The working fluid used in the plant, i.e. water substance,

when passes through the cycle of operation undergoes changes in pressure and temperature (enthalpy).

It receives heat in various feed heaters and undergoes pressure change by pumps in the circuit. The

preheated water is converted into saturated steam inside steam generators and finally supplied to the

turbine, in which it undergoes a fall in pressure and increase in volume and gives up a certain amount

of energy to the turbine shaft. On reaching the lowest pressure in the system, in the condenser, heat is

extracted from it by the cooling water and it is thus restored to its original conditions as condensate. In

the simplest possible form of heat cycle for a steam turbine power plant, the process thus comprises

four steps.

1. Increase of pressure of the condensate in the feed pump, with a resultant very small absorption of

work.

2. The supply of heat by the combustion of fuel to produce steam in the steam generator.

3. The expansion of the steam in the turbine, with the production of work.

4. The rejection of heat by the steam to the cooling water at constant pressure in the condenser,

and the return of the water to its original condition. The cycle is rarely as simple as this and is

often complicated by such devices as regenerative feed heating and reheating. Under ideal

conditions of expansion in the turbine the above cycle is known as the Rankine cycle. The cycle

shown in figure represents a power station cycle without feed heating. 1-2-3-4-5-6 Feed water

receives the sensible heat 6-7 Feed water receives the latent heat 7- Adiabatic expansion 8 of

steam through high pressure turbine 8-9 Moisture removal and reheating 9-10 Adiabatic

expansion of steam through the low pressure turbine.10-1 Condensation of steam in condenser

at constant pressure.

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Fig.-3.1 RANKINE CYCLE

fig.-3.2 Rankine Cycle on P-V, T-S, H-S axis.

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Chapter-4

MAIN COMPONENTS OF NUCLEAR POWER PLANT

4.1 REACTOR BUILDING

A common spent fuel building is provided centrally between the two reactors building on the west

side. The orientation and location of the building is so decided as to reduce the total number of bends

traversed by the shuttle carrying spent fuel from each reactor building to the respective inspection

bay. This building is safety related and designed as class III. The common exhaust ventilation system

for Reactor Building, Reactor Auxiliary Building, Spent Fuel Building, Service building etc. is

located on the first floor of SFB at 106 m elevation.

4.1.1 PHT SYSTEM

1. Calandria

Calandria is a huge cylindrical structures which houses bundles. The specifications regarding

200 MWe reactors calandria are: -

Weight - 22 tons

Length - 4645 mm

Main Shield I.D. - 5996 mm

Small Shell I.D. - 4928 mm

Thickness of Shell - 25 mm

There are 306 channels each accommodating 12 bundles. The calendria is housed in steel lined

concrete. calandria vault filled with light water which provides shielding and cooling of vault

structure. calandria tubes made up of zircaloy.

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Fig.-4.1 Structure of Calandria

2. Control Rod

The control rods contain material that regulates the rate of the chain reaction. If they are pulled

out of the core, the reaction speeds up. If they are inserted, the reaction slows down.

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In RAPS, cobalt(Co-59) is used as control rods. The used cobalt is then processed and enriched.

The enriched cobalt is then used for different purposes such as cobalt therapy etc.

3. Fuelling Machine

Reactor fuel is moved into and out of reactor by a pair of fuelling machines that is clamped to

channels on north and south ends of the reactor. It consists of Head, which contains positioning

the mechanisms for manipulating the fuel, a carriage for Head in line with any desired fuel

channel, and numerous houses and cables, which supplies fluid and electrical services. A ram

and associated mechanism is provided for pushing reactor fuel and handling plugs in the reactor

channels. The ram is operated by the hydraulic pressure of Heavy water. The fuelling machine

is left in the vault when not in the use, unless maintenance operations are required on it.

The various plugs and fuel handled by the fuelling machines are stored in the various chambers

of the rotary magazine. The magazine has twelve chambers. Refuelling can be done in a number

of channels during one refuelling session.

4. Dump Tank

Just below the calandria and connected to it by a transition section and expansion joint is the

dump tank. The purpose is to provide containment to the moderator when dumped through the

S-shaped dump ports. In normal operation the tank will be empty and contain helium at 24 psi

to support the moderator in the calandria.

5. Coolant Channels

Coolant channels are placed inside the calandria channel with air in between them as an

insulator. Coolant i.e. pressurized Heavy Water is paved through there coolant channels where

bundles are placed and thus carry vary the heat generated there in. It is called PHT (Primary

Heat Transfer).

The reason for using Heater Water as coolant is that its neutron capturing capacity is less than

light water. Coolant channels are made up of Zr-2.5% Niobium. This material is having very

lot neutron absorption cross-section and good mechanical strength.

In RAPS-2 all the coolant channels were replaced during 1994-98.

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6. Nuclear Fuel

The fuel used in a PHWR type reactor is sintered natural uranium di-oxide in the form of small

pellets. These pellets are kept in the zircaloy tubes and are 24 per tube. The tubes are known

as pencils and 19 pencils make a complete fuel bundle. The pencils are held between end plates

and zircaloy provide spacing between the tubes and zircaloy pads provide bearing action. This

help mixing of the coolant flow with the sub channels between the elements.

Fuel Bundle

Fig.-4.2 Fuel Bundle

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7. Steam Generators

Heat energy generated in the reactor is transported by the PHT heavy water to steam generators

(boilers). Heat transfer takes place in steam generator from primary to secondary ordinary

water in order to generate steam which in turn drives the turbine. Heavy water is flowing

through tube side (primary side) of steam generator and the ordinary water is circulated through

secondary side (shell side) of steam generator. Each steam generator comprises of ten hair pin

type heat exchangers and a common steam drum containing moisture separator. Each hair pin

heat exchanger has 195 tubes, 10 mm dia. and the tube material is made of Monel. Hot heavy

water from reactor outlet header enters in boiling leg of the heat exchanger and comes out

through the pre-heat leg. There is no provision for manual in service inspection for this type

heat exchanger. In case any tube leaks, titrated heavy water will come in secondary circuit.

Manual sampling of steam and feed water will monitor any tube leak. Provision is being made

to detect on line tube leak by N16, O19 activity monitor installed on blow down line of steam

generator.

All sides of each of the ten heat exchanger shell sides forming the boiler are connected to a

steam-drum through individual risers. There are two legs in each steam generator. One is called

preheat leg and the other is boiling leg, as shown. The pre-heated feed water of 173oC after HP

heaters enters in the pre-heat leg of the steam generator and rises through baffle plates. The hot

water after receiving heat from primary will go to the common steam drum through riser. The

water is circulated from drum through the down comer to the boiling leg. Boiling takes place

on the water surface of the drum and steam formation will be there above the drum water. The

steam is withdrawn from steam generator through peerless type four bank top outlet moisture

extractor. By removal of the end baffles from the Vth bank of the moisture extractor, provision

for increasing the steam capacity to 1.2 x 10E6 kg/hr. (3 x 106 lb./hr.) was made. The out let

steam line from all four SGs in each side combined together form the main steam line for north

and south side respectively.

SG failures are usually tube leaks and tube sheet cladding defects. Selection of tube material

involves variables such as good thermal conductivity, corrosion resistance, the long term build-

up of radiation fields etc. Tube material of SG is INCOLOY-800. Incoloy 800 (Ni 35 Cr 23

Fe) is a relatively recent material for use in nuclear SGs and is reported to be highly corrosion

resistant in water and steam services and has good resistance against stress corrosion cracking

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with controlled water chemistry, low stress level etc. Incoloy 800 with a limited cobalt of

0.03% (max.) is a better material than Inconel-600 or Monel-400 due to its lower Ni content so

far as cobalt release into the system is concerned.

4.1.2 MODERATOR SYSTEM

The following are the parts of moderator system-

1. Calandria

2. Coolant Channel

3. Over Pressure Rupture Disc

4. Dump Tank

5. Expansion Joint

6. Dump Port

7. Moderator Pumps

8. Heat Exchanger

9. Control Valves

Heater water moderator is filled in calandria serving the essential purpose of slowing down the

fast neutrons as well as acting as heat sink in case of an emergency.

For the cooling of moderator another cycle runs through heat exchangers where heat is

transferred to process water system.

The specification of 220 MWe

No. of pumps: 05

Heat exchangers: 02

In Unit 1&2 moderator is filled up to 98.6% and rest is filled with Helium gas. This proposal

is necessary for shutdown of the plant.

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In Unit 3&4 moderator is filled up to 100% of moderator as the shutdown mechanism is entirely

different. It has got primary shut off rods which gets inserted into calandria and absorbs

neutrons thus causing breakage of chain reaction.

For this there are 14 shut off rods made up of Cadmium sandwiched in SS.

It was seen that the fission cross-section for thermal neutrons is so much greater than the

radioactive capture cross-section that the high fuel enrichment, required in fast reactors, is no

longer necessary. In heterogeneous thermal reactor systems, little or no enrichment is required.

The slowing down of fission neutrons to thermal energies takes place in two stages:

Inelastic scattering by the heavier nuclei, such as U-238, which are already present in the fuel.

During this stage the neutron energy is only reduced to about 0.1 MeV and so, further slowing

down of the neutrons is required.

Further slowing down of neutrons, below 0.1 MeV, occurs by elastic scattering of the neutrons

by the lighter nuclei of the moderator. The basic requirements of moderators will now be

discussed at greater length and the suitability of substances, as moderators, will be considered.

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Fig.-4.3 Moderator System

4.2 STEAM TURBINE

Steam turbine is a rotating machine in which heat energy of steam is converted into mechanical

energy.

4.2.1 WORKING PRINCIPLE OF STEAM TURBINE

The steam is caused to fall in pressure in a passage or nozzle; due to this fall in pressure a

certain amount of heat energy is converted into mechanical kinetic energy, and the steam is set

moving with a greater velocity. The rapidly moving particles of steam enter the moving part of

the turbine and here suffers a change in direction of motion which gives rise to a change of

momentum and therefore to a force. This constitutes the driving force of the machine.

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4.2.2 IMPORTANT ELEMENTS OF TURBINE

1. The Nozzle

This is the element in which the steam expands from a high pressure and a state of comparative

rest to a lower pressure and a state of comparatively rapid motion.

2. The Blades or Deflector

This is the element in which the stream of steam particles strikes and experience a change in

momentum due to change in direction resulting in a tangential force for rotation of turbine. The

blades are attached to the rotating element of the machine, or rotor; whereas, in general the

nozzles are attached to the stationary part of the turbine, which is usually termed the stator,

casing or cylinder.

4.2.3 TYPES OF TURBINES

1. Impulse Turbine

In this, steam is expanded in turbine nozzle and attains a high velocity, also complete expansion

of steam takes place in the nozzle & steam pressure during the flow of steam over turbine

blades remains constant. The blades have symmetrical profile.

2. Reaction Turbine

In this, only partial expansion takes place in nozzle and further expansion takes place as the

steam flows over the rotor blades.

4.2.4 COMPOUNDING IN IMPULSE TURBINE

Several problems crop up if the energy of steam is converted in one step, i.e.

in a single row of nozzle-blade combination. With all heat drop taking place in one row of

nozzles, the steam velocity becomes very high and even supersonic velocity. The rotational

speed of the turbine also becomes very high and impracticable which may result in failure of

blades due to centrifugal force.

So, in order to convert the energy of steam within practical speed range, it is

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necessary to convert it in several steps and thus reducing the velocity of steam and rotor speed

to practical levels. In addition to above there will be a high leaving loss.

Following are the various types of compounding.

1. Velocity Compounded Impulse Turbine

Like simple impulse turbine this has also only set of nozzles and entire steam pressure drop

takes place there. The kinetic energy of high velocity steam issuing from nozzles is utilized in

a number of moving row of blades with fixed blades in between them (instead of a single row

of moving blades in simple impulse turbine). The role of the fixed guide blades is just to change

the direction of steam jet and guide it to next row of moving blades. This type of turbine is also

called Curtis turbine.

2. Pressure Compounded Impulse Turbine

This is basically a number of simple impulse turbines in series on the same shaft the exhaust

of one steam turbine entering the nozzle of the next turbine. The total pressure drop of the

steam does not take place in the first nozzle ring, but is divided equally between all of them.

Steam is passed through the first nozzle ring in which it is only partially expanded. It then

passes over the first moving blades wheel where most of its velocity is absorbed. From this

ring it exhausts into the next nozzle ring and is again partially expanded. The velocity obtained

from the second nozzle ring is absorbed by the next wheel of moving blades. This process is

repeated in the remaining rings until the whole of the pressure has been absorbed. This type of

turbine also called Rateau turbine after its inventor.

3. Pressure-velocity Compounded Impulse Turbine

Pressure Velocity Compounding is a combination of both the previous methods and has the

advantage of allowing a bigger pressure drop in each stage and so less stages are necessary.

Hence, for a given pressure drop the turbine will be shorter, but the diameter of the turbine is

increased at each stage to allow for the increasing volume of steam. This type was once very

popular, but it rarely used as efficiency is quite low.

4.2.5 IMPULSE VS REACTION-PRESENT TREND

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The hard and fast distinction between the impulse reaction is becoming progressively less

important. The trend is to have some percentage of reaction for an impulse turbine or to have

some percentage of impulse for a reaction turbine. It can be mathematically proved that

efficiency of reaction stage is greater than efficiency of impulse stage. A pressure difference

exists across the reaction type moving blades, therefore, the changes of leakage of steam from

around the blade is more in a reaction stage. The advantage of efficiency is off set by the inter

stage leakage of steam which flows without doing useful work. Hence a reaction stage should

be located in the low pressure region of turbine.

There is a general rule to use a greater percentage of impulse on the HP end and greater

percentage of reaction on the L. P. end. The percentage of reaction progressively increases as

we go towards L. P. end.

In actual turbines it is common for the best feature of various types to be incorporated in one

machine. For example, a turbine may have a velocity compounded (Curtis) first stage followed

by pressure compounded impulse (Rateau) stages and at the low pressure end of the machine,

reaction balding

4.3 CONDENSOR

The condenser has thousands of small tubes. On-line cleaning systems inject small balls during

operation. Periodically, the tubes must be cleaned manually. During outages, the condenser

tubes may be non-destructively tested to determine if wear is occurring. Tube leakage cannot

be tolerated because the chemicals, e.g. sodium and chlorides can concentrate in the reactor (if

a BWR) or steam generator (if a PWR).

4.3.1 FUCTION

1. To provide lowest economic heat rejection temperature for the steam. Thus saving on steam

required per unit of electricity.

2. To convert exhaust steam to water for reuse thus saving on feed water requirement.

3. Deaeration of make-up water introduced in the condenser.

4. To form a convenient point for introducing make up water.

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5. To provide means for venting and draining of associated equipment of feed water system.

4.3.2 TYPES OF CONDENSER

Condenser is basically a heat exchanger and hence can be of two types:

1. Direct Contact

In this type, condensation of steam takes place by directly mixing exhaust steam and cooling

water. Requirement of cooling water is much less here compared to surface type. But cooling

water quality should be equal to condensate quality

2. Surface Contact

The condenser essentially consists of a shell, which encloses the steam space.

Tubes carrying cooling water pass through the steam space. The tubes are supplied cooling

water from inlet water box on one side and discharged, after taking away heat from the steam,

to the outlet water box on the other side.

Instead of one inlet and one outlet water boxes, there may be two or more pair of separate inlet-

outlet water “boxes, each supplying cooling water to a separate bundle of tubes. This enables

cleaning and maintenance, of part of the tubes while turbine can be kept running on a reduced

load.

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Fig.-4.4 Condenser

4.3.3 MATERIALS FOR CONDENSER TUBES

Selection of tube material depends mainly on the quality of cooling water and the cost. Copper

bearing alloys are preferred as copper has very high heat transfer coefficient. But as copper has

very little mechanical strength; it has to be reinforced by alloying with other metals. Copper

alloys are basically of three categories:

1. Brasses

2. Cuprous-nickel

3. Bronzes

Stainless steel tubes have also been used and has good corrosion resistance though heat transfer

co-efficient is quite lower than the copper alloys. Because of high cost, stainless steel is used

only where water is highly corrosive. Some sea side power plants are also using Titanium

despite high cost, because of high corrosive environment. Now a days Monel material is also

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preferred as one of the high corrosion resistant material in RAPS 3& 4, Cu-Ni alloy (70-30) is

used as material for condenser tubes.

4.3.4 TECNICAL SPECIFICATION OF CONDENSER

1. Type : Surface condenser

2. No. of Pass : Single

3. Heat load at MCR (Kcal/hr.) : 4.452 x 10-8

4. Effective heat transfer area : 19,500 m2

5. Cooling water flow (m3 /hr.) : 55,740

6. Design cooling water inlet temperature (0C) : 33

7. Design shell pressure [kg/cm2 (g)] : 2.0

8. Design water box pressure [kg/cm2 (g)] : 2.0

9. Design Temp. - shell (0C) : 100

10. Design Temp. - water box (0C) : 100

11. Design code: HEI & ASME Sec. VIII : Div. - 1

12. Tube Material : St. steel TP 7161

13. Tube outside dia./ thickness : 22.225 mm : 0.711 mm

14. Effective length between tube sheets : 13.5 m

15. Hot well capacities (m3 ) Normal level : 47.0

Higher level : 58.0

Lower level : 31.5

4.4 DEAERATOR

A deaerator is a device that is widely used for the removal of oxygen and other

dissolved gases from the feed water to steam-generating boilers. In particular,

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dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems

by attaching to the walls of metal piping and other metallic equipment and

forming oxides (rust). Dissolved carbon dioxide combines with water to form carbonic

acid that causes further corrosion. Most deaerators are designed to remove oxygen down to

levels of 7 ppb by weight (0.005 cm³/L) or less as well as essentially eliminating carbon

dioxide.

4.4.1 FUNCTIONS

The presence of certain gases like Oxygen, carbon dioxide and ammonia, dissolved in water is

harmful because of their corrosive attack on metals, particularly at elevated, temperatures. Thus

in modern high-pressure boiler, to prevention internal corrosion, the feed water should be free,

as far as practicable, of all dissolved gases, especially oxygen. This is achieved by embodying

into the fled system a deaerating unit, apart from this; a deaerator also serves the following

functions:

1. Heating incoming feed water.

2. To act as a reservoir to provide a sudden or instantaneous demand.

4.4.2 PRINCIPLE OF DEAERATION

1. The solubility of any gas dissolved in a liquid is directly proportional to the partial pressure

of the gas. This holds within close limits for any gas which does not react electrically with the

solvent.

2. Solubility of gases decreases with increase in solution temperature and or decrease in

pressure.

4.4.3 A TYPICAL DEAERATOR

A constant pressure deaerator, pegged at 7 kg/ cm2 (abs.) is provided in turbine regenerative

cycle to provide properly deaerated feed Water for boiler, limiting gases (mainly oxygen) to

0.005 cc/litre. It is a direct contact type heater combined with feed storage tank of adequate

capacity.

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The heating steam is normally supplied from turbine erections but during starting and low load

operation the steam is supplied from auxiliary source.

The deaerator comprises of two chambers:

1. Deaerating column

2. Feed storage tank.

Deaerating column is a spray cum tray type cylindrical vessel of horizontal construction with

dished ends welded to it. The tray stack is designed to ensure maximum contact time as well

as optimum scrubbing of condensate to achieve efficient deaeration. The deaerating column is

mounted on the feed storage tank is fabricated from boiler quality steel plates. Manholes are

provided on deaerating column as well as on feed storage tank for inspection and maintenance.

The feed water is admitted at the top of the deaerating column and flows downwards through

the spray valves and trays, the trays are designed to expose to the maximum water surface for

efficient scrubbing to affect the liberation of the associated gases. Steam enters from the

underneath of the tray and flows in counter direction of condensate. While flowing upward

through the trays, scrubbing and heating is done. Thus the liberated gases move upwards along

with the steam. Steam gets condensed above the trays and in turn heats the condensate.

Liberated gases escape to atmosphere from the orifice opening meant for it. This opening is

provided with a number of deflectors to minimize the loss of steam.

In some deaerator designs, a vent condenser is also located above the deaerator. A portion of

feed water is first passed through the vent condenser before it enters the deaerator. This water

is heated by remaining steam after steam has passed through the deaerator. Thus only gases

escape to atmosphere.

4.4.4 DEAERATOR SPECIFICATION

1. Type : Spray-Cum-Tray

2. Design Code : ASME sec VIII Div. - I

3. Design Temperature : 6 kg/cm2 (g) and full vacuum

4. Design Temperature : 1650C

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5. Condensate flow rate at MCR :1017735.5 kg/hr.

6. Extraction steam flow MCR : 5791.0 kg/hr

7. Condensate out let temperature at MCR : 156.50C

8. Capacity of deaerator storage Tank at normal Level :235m2

9. Dissolved oxygen in Effluent Feed water : 0.005 cc/litre

10. Steam dumping condition : 0.60C

11. Deaerator

Outside diameter : 2.60m

Overall length : 9.00m

Thickness : 16.00mm

Material : SA 515/516 Gr

No. of spray nozzles : 10

12. Storage tank

Outside diameter : 4.0 m

Thickness : 18.0 m

Overall length : 23350 mm

Material : SA 515/516 Gr

4.5 DRAIN COOLER

This is a shell and tube heat exchanger using the main condensate as cooling water. The main

condensate passes through the tube side and the drains from the LP heaters pass on the shell

side, give away the heat to the main condensate before being drained to the condenser hot well.

4.6 FEED WATER HEATER

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A feed water heater is a power plant component used to pre-heat water delivered to

a steam generating boiler. Preheating the feed water reduces the irreversibility’s involved in

steam generation and therefore improves the thermodynamic efficiency of the system. This

reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when

the feed water is introduced back into the steam cycle

4.6.1 HIGH PRESSURE FEED WATER HEATER

1. Functional needs

The structural design of high-pressure (HP) feed water heaters are determined by two1main

needs:

1. To contain the steam and HP feed water at the appropriate cycle conditions.

2. To provide the heat transfer surface to raise the feed- water temperature by the specified

amount.

2. Construction

The heater has both integral drain cooling and de-superheating sections. The DE superheating

section is placed on the outlet end of the U-tubes in order that the incoming super- heater steam

can raise the feed water near to or above the saturation temperature of the body pressure before

it leaves the heater. The drain cooling section is placed at the inlet end of the tubes to allow the

outgoing drains to be cooled to as near to the incoming feed water temperature as needed.

Steam enters the de-superheating section and is reduced in temperature by transferring its heat

to the feed water to within 27°C of the temperature of saturation of the condensing section

pressure. The steam then flows to the condensing section, where it leaves as water at saturation

temperature to enter the drain cooling section. A water seal is maintained at the inlet to the

drain cooling section by a level control system to prevention loss of prime in the section. In the

drain cooling section, the condensate is cooled to the drain outlet temperature and then

discharged to the next lowest pressure heater.

Each section within the heater is provided with baffles to ensure flow across the outside of the

tubes by the heating medium. As the heating steam is condensed in the heater, non-condensable

gases are released. Unless correctly vented these would rapidly blanket the heat transfer surface

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and would impair the heater thermal performance. To remove these gases, vents connected to

the condenser are provided at strategic points throughout the heater tube-nest

Fig.-4.5 High Pressure Feed Water Heater

4.6.2 LOW PRESSURE FEED WATER HEATER

1. Functional needs

Because LP heater extraction points are normally on the LP turbine cylinders, the superheat

(even on the highest-pressure LP heater) does not justify the provision of de-superheating

section within the heaters.

Drain cooling section can be provided but the complication and the cost of a drain level control

system can seldom be justified. It is usual practice to have the LP heaters and to provide a drain

cooler upstream of the lowest pressure heater to recover some of the heat in the drains.

2. Construction

The construction of vertical and horizontal LP heaters is very similar. The following

descriptions are for horizontal heaters but any significant points of dissimilarity between

horizontal and vertical heaters are included.

The maximum head that the condenser extraction pump can generate occurs at the no flow

condition and is sometimes called the ‘closed valve head’. The LP heaters are designed on the

feed water side to withstand the extraction pump ‘closed valve head’. The general form of the

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LP heater is similar to HP heaters but, because the feed water side pressure is modest, the water

header can be of cylindrical design with a dished end. The shell side pressures are also modest,

so again the shell is cylindrical in section with a welded dished end. A fixed and a sliding foot

are provided to support the heater. The shell, tube plate and water headers are all made of mild

steel.

An all-welded construction is used and it is accepted that in the unlikely event of access being

required to the heater internals, the shell will have to be removed by cutting close to the back

of the tube plate. The tubes are roller-expanded into the tube plate. The tubes of LP heaters

may be of 70/30 brass or stainless steel as dictated by steam temperature or boiler feed water

chemistry requirements. Brass, cupronickel may be used in LP heaters where the steam

temperature is not greater than 150°C, above this temperature stainless steel is used.

Fig.-4.6 Low Pressure Feed Water Heater

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4.7 PUMPS

In RAPS, mainly pumps are categorized into five groups

4.7.1 MODERATOR CIRCULATING PUMP

Heavy water used as moderator inside the Calendria gets heated up due to neutron moderation and

capture, attenuation of gamma radiation as well as due to transfer of heat from other reactor

components in contact. This heat is transported to moderator system heat exchanger outside of the

core where it is transferred to Active L.P. process water system which in turn transfers this heat to

the induced draft cooling system. Circulation of moderator through moderator heat exchangers is

accomplished by moderator pumps. These pumps are installed at 95 m elevation in Reactor

Building.

4.7.2 PRIMARY HEAT TRANSPORT CIRCULATING PUMP

Primary Circulating Pumps (PCPs) are located at the downstream of each steam generator and

pump and coolant into the respective reactor inlet header. These are vertically mounted

centrifugal pump. Pump casing is at 114.6 in Elevation and motor top touches 121.20 m

Elevation. PHT pumps circulate heavy water through the reactor and steam generators; hence

directly affect the availability of the station. The pumps are on the downstream side of the

steam generators and thus located at a point of lowest temperature in the circuit. Each of the

PHT pumps equipped with a fly wheel located at the motor top. The energy stored in the fly

wheel keeps the pump operating for 2 minutes after loss of power and with the specified

slowing down rate, the coolant flow inadequate at all times. Natural gravity circulation

(Thermos phoning) starts after the pump comes to rest and this will suffice to remove about

6% of the full power. The actual heat input to the coolant after the pump in down is

approximately 6% of full power.

4.7.3 BOILER FEED PUMP

There (3) nos. 50 % boiler feed pumps 4321-P-1003, 1004 & 1005 each of capacity 716 MR/hr

located in the Turbine Building ground floor take suction from the deaerator storage tank by

means of independent suction lines of size 350 mm. The pump common discharge passes

through the HP heater No. 6 located on the mezzanine floor to the roof of the DG building

where it bifurcates into four headers-going to four steam generators.

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4.7.4 CONDENSATE EXTRACTION PUMP

Condensate extraction pumps are normally multistage, vertical, centrifugal pumps. They are

generally required to operate on minimum net positive suction head NPSH. The condensate

pumps operate on few inches of suction submergence.

A vent line connects the hot well, from where the condensate pumps take suction with the

condenser. This equalizes the vapour pressure of condenser and hot well. No. of stages in the

pump is determined by the discharge pressure required for the condensate cycle. In 220 MW

unit, two condensate extraction pumps, each having 100% capacity, are provided for pumping

the condensate to deaerator.

4.8 COOLING TOWERS

Mainly there are two types of cooling towers: -

IDCT : Induct Draft Cooling Towers

NDCT : Natural Draft Cooling Towers

The main purpose of these cooling towers is to bring down the temperature of circulating water.

This is light water that circulates through the heat exchanger and carried away the heat generated by

the DM water. This DM water condenses the steam. Hence the application of cooling towers

enhances the efficiency of the plant.

Following is the description of the types of cooling towers: -

4.8.1 IDCT

As the name indicates it requires induced draft for cooling the active process water. Big fans are

used to produce the draft. The active water is used in Reactor Building to cool various process

equipment etc.

4.8.2 NDCT

The inductive water that is used to condense water is further cooled by natural draft. They are 150M

high with hyperbolic shape atomizing action.

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Chapter-5

ZONE CLASSIFICATION

Depending on contamination level the entire plant is divided into four zones. This

classification is as follows:

ZONE 1 zero contamination (admin. buildings, official buildings etc.)

ZONE 2 zero contamination (shop floors)

ZONE 3 little contamination (service buildings)

ZONE 4 very high contamination (reactor building)

5.1 METHODS OF MEASURING DOSE

For measuring dose absorbed by a person, devices known as dosimeters are used. Generally,

there are two types of dosimeters these are:

5.1.1 Direct Reading Dosimeters (DRD)

This device measures the dose directly and is used for day to day dose control. It is a pen shaped

device and lenses are fitted on both the ends. On bigger lens, a scale is marked which directly

tells about the dose absorbed. For reading the DRD it is so held that the bigger lens should face

the light source and it is seen from the smaller lens. This dosimeter is used in Third and Fourth

Zone only.

5.1.2 Thermo Luminescence dosimeter (TLD)

This is a badge type device and is used to dose absorbed during one month’s time. TLD badge

consists of a TLD CARD loaded in a CASSETTE. The dose measured by TLD is based on the

phenomenon of THERMOLUMINISCENSE. TLD cassette has a dual metallic filter and an

open window to distinguish between doses received due to different type of radiation (alpha,

beta & gamma) and provides energy dependence correction. Personal data such as Name, TLD

No., Service months etc. are written on it. The person has to wear his TLD badge at his chest

level when entering the operating island. After one month, the TLD card is sent to the TLD

laboratory where the absorbed dose is measured.

37

Chapter-6

RADIOACTIVE WASTE MANAGEMENT

6.1 GENERAL

Operation of a nuclear facility like nuclear power station inevitably leads to the production of

low level radioactive wastes which are collected segregated to select best processing method,

and conditioned for either interim site storage or for disposal. The design of facilities is such

that the average public exposure from radioactive materials at the exclusion boundary is a small

fraction of the recommended AERB limits.

The radioactive wastes produced at the site may belong to one of the following categories:

Spent Fuel, Solid Wastes, Liquid Wastes & Gaseous Wastes.

Spent fuel is stored in a pool of water until it is ready for shipping for reprocessing at special

facilities.

6.2 SOLID RADIOACTIVE WASTE MANAGEMENT SYSTEM

Solid radioactive waste in segregated into three general categories based on contact dose.

Category -1 wastes.: Largely originates

Protective clothing, contaminated metal parts and miscellaneous items as it may contain no

radioactivity. This waste will be collected in unshielded standard drums.

Category-II & III Wastes.: filter cartridges and ion exchanges resins

Typically, this waste has an unshielded radiation field greater than 1 R/hr. on contact. These

require additional shielding and greater precautions than for Category-I during transportation,

handling and storage operation.

6.3 LIQUID RADIOACTIVE WASTE MANAGEMENT SYSTEM

The Liquid Radioactive Waste Management System provides for collection, storage, sampling

and necessary treatment and dispersal of any liquid waste produced by the station. The system

is designed to control the release of radioactivity in the liquid effluent streams so that radiations

dose to members of the public is within those stipulated by the regulatory board. This system

38

handles radioactive wastes that are carried in liquid streams from the laundry active floor

drains, decontamination centre and Electrical laboratories.

6.4 GASEOUS RADIOACTIVE WASTE MANAGEMENT SYSTEM

An extensive ventilation system collects potentially active exhaust air from such areas as the

Reactor Building, the spent fuel handling and storage area, the decontamination centre and the

heavy water management area. The active and potentially active exhaust air and gases are all

routed to a gaseous effluent exhaust duct. This exhaust flow is monitored for noble gases,

tritium, iodine and active particulate before being released. Facilities for filtration are provided.

Signals from the iodine, wide range beta-gamma and particulate monitors are recorded in the

control centre. Tritium monitoring is carried out by laboratory analysis.

39

Chapter-6

RADIATION SAFETY

In a Nuclear reactor the Radiation is produced in following ways:

Directly in fission reaction

By decay of fission products

Following types of radiations are encountered:

Alpha radiation

Beta radiation

Gamma radiation

Neutron radiation

Out of the above types of radiations Alpha radiation is practically zero, whereas Beta and

Gamma radiation fields may be present almost everywhere inside the reactor building and

in negligible amount even outside the reactor building. Neutron radiations are mainly present

inside the reactor vault. It is worth noting that the secondary side of the plant i.e. feed water

and steam cycle etc. are completely separate from the nuclear systems and are therefore not

supposed to be and neither they are to carry any sort of radioactive particle and therefore

free of contamination and radiation. It is also worth noting that all radiations are emitted from

the nucleolus of every radioactive nuclide which will always have a tendency to become stable

by emitting radiations through disintegration.

Following methodologies are used to control the exposure to the radiation and therefore

receive of the radiation dose.

Administrative Control

Zoning Technique

Design Control

Operation Control

40

Maintenance and house keeping

Exposure to any kind of radiation can be controlled by an individual by following methods:

1. Distance

2. Shielding

3. Decay (Time to Decay)

41

CONCLUSION

The practical training at R.A.P.S. has proved to be quite faithful. It proved an opportunity for

encounter with such huge components like 220MW generators, turbines, transformers and

switchyards etc.

The way various units are linked and the way working of whole plant is controlled make the students

realize that engineering is not just learning the structure description and working of various

machines, but the greater part is of planning, proper management.

It also provides an opportunity to learn technology used at proper place and time can save a lot of

labour for example almost all the controls are computerized because in running condition no any

person can enter in the reactor building.

But there are few factors that require special mention. Training is not carried out into its tree spirit.

It is recommended that there should be some projects specially meant for students where the

presence of authorities should be ensured. There should be strict monitoring of the performance of

students and system of grading be improved on the basis of the work done.

However, training has proved to be quite faithful. It has allowed as an opportunity to get an exposure

of the practical implementation to theoretical fundamental.

42

BIBLIOGRAPHY

[1]. Nuclear Training Centre(NTC), RAWATBHATA

[2]. NPCIL Rawatbhata Manual

[3]. www.powershow.com/search/presentations/npcil

[4]. https://en.m.wikipedia.org/wiki/Nuclear_Power_Corporation_of_India

[5]. http://www.npcil.nic.in/main/AboutUs.aspx