npcil rawatbhata kota report
TRANSCRIPT
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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
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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.
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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.
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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
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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)
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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.
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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