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Nuclear power plants

Introduction: There is a common trend throughout the world to use nuclear energy as a source of power. This is because of the rapid depletion of conventional energy sources. Transportation network and large storage facility are not required which is one of the major hurdle in coal based thermal power plants. How ever recently there is stiff opposition for the installation of nuclear power plants due to a fear of radiation hazards. Atomic structure:

All matter consists of unit particles called atoms. An atom consists of a relatively heavy , positively charged, electrons orbiting around the nucleus. The nucleus consists of protons and neutrons, which together are called nucleons. Protons are positively charged , while the neutrons are electrically neutral. The number of protons in the nucleus is called the atomic number, Z. The total number of nucleons in the nucleus is called the mass number, A. The atomic mass unit , is a unit of mass approximately equal to 1.66x10-27 kg. Mass of Neutron is 1.008665amu, Protons is 1.007277amu and Electron is 0.005486amu. ( The mass of Protons=1837xmas of Electron, Neutrons = 1839 x Electron. ). An element is distinguished by its atomic number . Some elements exist in more than one form, with the same atomic number but with different mass numbers. These are known as the isotopes of an element. For example Uranium exists in three isotopic forms 92U233, 92U235, 92U238. (Atoms which are having different number of neutrons than the number of protons are known as isotopes.) Binding energy: It is the energy required to keep the protons together in the nucleus of an atom or It is the energy required to overcome the binding forces of nucleus is called as binding energy. The binding energy is very large compared with chemical bond energy. When two nuclear particles are combined to form nucleus . It is observed that there is a different mass of the resultant nucleus and the sum of the masses of the two parent nuclear particles will be different. This decrement of mass is called mass defect Einstein’s theory of relativity shows that mass is convertible into energy and this energy is given by the formula . E = mc2 , E – Energy(J), m- mass defect(Kg), c – Velocity of light. (3 x 108 m/s) Energy can also be measured in electron volt ( 1. e.v = 1.6021 x 10-19 J). The energy equivalent of 1g of mass is E = 1x10-3 x (3 x 108 m/s) 2 = 9 x1013 J Similarly the energy equivalent of 1 amu of mass is E = 1.66x10-27 x (3 x 108 m/s) 2 = 9 x1013 J = 14.94 x 10-11 J = 9.31 x 10 8 eV = 931 MeV Therefore , if 1 amu of mass could be completely converted to energy , 931 MeV would be yielded. The amount of mass defect is directly proportional to the amount of energy released. Binding energy per nucleons increases with increase in number of nucleons. For example binding energy per nucleon for H2

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is 1.109MeV and for He4 it is 28.24 = 7.05 MeV. A curve representing the variation of nuclear binding energy per nucleon with the mass number is shown in figure. The curve indicates that peak value of about 8.8 MeV at nearly 60 mass number. As the mass number increases still further , the binding energy curve falls gradually to 7.6 MeV for U238. An atom with even number of protons of mass number is more stable because of pairing of protons and neutrons. Example 92U238 atom having 92 protons and 146 neutrons is quite stable and requires very high energy neutrons for fission, Where as 92U235 atom having 92 protons and 143 neutrons can be fissioned even by low energy neutrons. Radioactive decay and half life: All isotopes of heavier elements less stable emits radiation till a more stable nucleus is reached. Thus a spontaneous disintegration process , called radioactive decay occurs. For various elements decay time is different , which follows certain law . This law is known as radioactive decay law. The law states that the small amount of disintegration of the isotope in a small period is directly proportional to the total number of radioactive nuclei and proportionality constant. N= Number of radioactive nuclei present at any time t, No = Initial number of nuclei, = Proportionality constant. Then according to the decay law N = -Nt (1) dN/dt = -N (2) Negative sign indicates that during disintegration number of nuclei decreasing. Integrating the equation 2 No N dN/N = - o t dt (3) loge N – loge No = -t loge N/No = -t N/No = e-t N=No e-t (4) dN/dt = -N = - No e-t (5) Equation 5 shows that the decay scheme follows the exponential law. If A = Activity at time t, A1 = Initial activity, k = detection coefficient then A = k(-dN/dt) = kN = kNo e-t = A e-t (6) Half life: Half time represents the rate of decay of the radioactive isotopes. The half life is the time required for half of the parent nuclei to decay or to disintegrate. Using N =No/2 and t = t1/2 in equation 6 we get. No/2 = No e-t1/2 e-t1/2 = ½ t1/2 = log e 2 = 0.693 t1/2 = 0.693 / (7) Nuclear fission In this type of process heavy nucleus is divided in two equal number of fragments. Fission can be caused by bombarding with high energy particles, Protons, X-rays as well as neutrons. How ever neutrons are most suitable for fission, they require less kinetic energy to collide with nuclei. Two or three neutrons are released for each neutron absorbed in fission, and can thus keep reaction going . Isotopes like U233, U235

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and Pu239 can be fissioned by neutrons of all energies , where as isotopes U238, Th232 and Pu240 are fissionable by high energy only. When neutron enters nucleus of U235 the nucleus splits in to two fragments and also releases 2 to 3 neutrons per fission. The difference in the binding energy between the products of fission and the original nucleus is evolved during the fission reaction. This is known as nuclear fission. The breaking of U235 can takes place in different ways , forming a variety of different products. Each way of splitting U 235 nucleus ejects different numbers of neutrons 1,2,3. As an average of 2.5 neutrons released per neutron absorbed. Out of 2.5 neutrons , nearly 0.2 to 0.3 neutron is lost due to escape at the surface and out of remaining 2.2 neutrons are allowed to continue chain reaction. The reaction rate will increase exponentially and enormous amount of heat energy will be released. Such reaction is known as uncontrolled chain reaction . When only one neutron after every fission is allowed to continue to cause fission reaction , it is known as controlled chain reaction. This is the type of nuclear fission reaction used for power production and energy evolved remains at constant level. For sustaining of the chain reaction at least there must be an one neutron available for absorption. This condition can be conveniently expressed in the form of multiplication product or reproduction factor of the system which may be defined as K = No of neutrons in any particular fission/ No.of neutrons in the preceding fission. If K > 1 , chain reaction will continue and if K<1 , chain reaction can not be maintained. When K<1 system is known as sub critical and when K>1 the system is known as super critical and when K=1 , the system is known as critical and this is the desirable requirement for power reactors.

U235IncidentNeutron

Prompt gama rays

Fission fragments

Prompt neutron

Fission fragment

Prompt neuttron

Chain reaction figure Nuclear fusion Nuclear fusion is the process of combining or fusing two lighter nuclei in to a stable and heavier nuclide. In this process also large quantity of energy released because mass of the product nucleus is less than the masses of the two nuclei which are fused. Several reactions between nuclei of low mass can be initiated by accelerating one or the other nucleus in a suitable manner. These are often fusion process accompanied by release of energy . How ever the nuclear fusion reaction can not be regarded as much significance for the utilisation of nuclear energy. To have a practical value fusion reaction must be self sustaining ,i.e., more energy must be released than is consumed in initiating the reaction. For initiating the nuclear fusion reaction very high stellar temperature of 30 million 0K is needed.

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Above figure shows the schematic diagram of futuristic deuterium-tritium fusion reactor . The plasma is contained inside an evacuated tube of 4m. The surrounding vacuum wall through which 14 MeV neutrons from the plasma pass, is maintained at about 7500C . Out side this wall are two concentric regions, viz, the lithium breeding moderator and magnetic shield. Tritium is manufactured in the lithium blanket. Large cryogenic superconducting magnets of 7 to 8 m diameter maintain the magnetic shield. The binary vapour power cycle consists of a potassium topping cycle and a conventional steam cycle. It includes tritium recovery system. Advantages of fusion power plants:

1) The supply of deuterium is almost inexhaustible. 2) Radioactive wastes are not produced. 3) It is very safe to operate. 4) High energy conversion efficiency can be achived. 5) Low heat rejection to the environment takes place per KW of electricity generated.

Comparison between nuclear fission and fusion Fission Fusion Heavy nucleus splits in to two nuclei of equal mass and energy released.

Lighter nuclei fuse together to form heavy nucleus with the release of energy.

About one thousandth of the mass is converted in to energy.

It is possible to have four thousandth of mass converted in to energy.

Nuclear reaction residual problem is great Residual problem is much less. Amount of radioactive material in a fission reactor is high.

Radioactive material produced is much less than that of the fission reaction.

Health hazards are high in the event of accidents. Health hazards is much less. It is possible to construct self sustained chain It is extremely difficult to construct controlled

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reaction reactors. fusion reactors. Manageable temperatures are obtained Un manageable temperatures Raw fissionable material is not available in plenty Reserves of deuterium, the fusion element is

available in large quantity. Fuels used in the reactor: The fuels which are commonly used are natural uranium containing 0.7% U235 or enriched uranium containing 1.5 to 2.5 % U235. In addition to natural nuclear fuels some of artificial or man made fuels such as Pu239, Pu241,U 233 are also used. Considering the necessary requirement of fission process and its availability economically the fuels used in reactors are uranium, plutonium and thorium. U235 is easily available nature with concentrations up to 0.7% and its content increases up to 90% in enriched uranium. The nuclear fuels is available in three states solid, liquid and gas. In reactors fuel is mostly used in solid state or in the form of solution dissolved in water. The liquid metal reactors are in practical use. The fuel used in the reactors is in the form of rods or plates . The fuel rods are surrounded by the moderator. The fuel rods are clad with stainless steel or zirconium to prevent oxidation. The minimum amount of fuel required to maintain chain reaction is known as critical mass. The fuel core must contain at least the critical mass and more often, slightly larger than the critical mass in order to maintain the chain reaction. Elements of the nuclear reactor: The essential components of nuclear reactor are as follows: 1) Fuel rods 2) Control rods 3) Moderator 4) Reflector. 5) Coolants. 6) Shielding 7) Control mechanisms 8) Measuring systems.

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1)Fuel rods Fuels which are commonly used are natural uranium and enriched uranium cast in the form of rods and plates. The fuel rods are clad with stainless steel to prevent the oxidation. The fuel rods are surrounded by the moderator. The minimum amount of the fuel must be maintained in the reactor in order to sustain the chain reaction this is known as the critical mass. The fuel rods must contain at least the critical mass and slightly larger than the critical mass in order to maintain the chain reaction. 2) Control rods The purpose of the control rod is to maintain the value of multiplication factor as one this is the minimum condition required to maintain the nuclear fission. This maintains the steady state heat generation in the reactor. The control rod helps to vary the out put according to the load and shut – down the reactor under emergency conditions. When the shutting down of the reactor is required the control rods, absorb more number of neutrons than emitted and the fission reaction dies out. The material which are commonly used for control rods are cadmium, Boron etc. The control rods are automatically operated. 3) Moderator The function of the moderator is to reduce the energy of the neutrons evolved during fission from 2Mev to 0.25 Mev in order to maintain the chain reaction. By the slowing down of high energy neutrons, possibility of escape of neutrons is reduced and possibility of absorption of neutrons to cause further fission is increased. This also reduces the quantity of the fuel required to maintain the chain reaction. The common moderators used are ordinary water , heavy water , graphite and beryllium. 4) Reflector The neutrons which may escape from the surface of the core without taking part in fission can be reflected back in to the core to take part in the chain reaction . This is done by a reflector. The required properties of a good reflector are low neutron absorption , high capacity to reflect and resistance to oxidation and radiation. The moderators which are commonly used also work as reflectors. A blanket of reflector can reduce the critical mass required to maintain the chain reaction. 5) Coolants The purpose of the coolants is to transfer the heat generated in the reactor core and use it for steam generation. The coolant circulated in the reactor core keeps the temperature of the fuel below safe level by continuous removal of energy from the core. The coolant used must have very high specific heat to carry more heat per kg of coolant used. It should not absorb neutrons, It must be non corrosive , non oxidizing and non toxic. Ordinary water , heavy water, sodium, potassium and carbon dioxide are the common coolants used in power generating reactors. 6) Shielding The reactor is source of intense radio activity and these radiations are very harm full to the human life. Therefore it is necessary to prevent the escape of these radiations to the atmosphere. The inner core is made of 50 to 60cm thick steel plate and it is further thickened by few meters using concrete. The thermal shield is cooled by circulation of water. 7) Control mechanisms The control system is also necessary to prevent the chain reaction from becoming violent and consequently damaging the reactor. It is an essential part of a reactor and serves the following purposes i) Starting the reactor , ii) Maintaining the reactor at that level ,iii) Shutting down of the reactor during emergency conditions. The control system works on the principle of absorbing the excess neutrons with the help of control rods either made up of boron steel or cadmium strips.

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8) Measuring systems Main instruments required in nuclear reactor are thermocouples for measuring temperatures instrument for determining the thermal neutron flux. Types of Nuclear reactors: 1) Pressurised water reactor. (PWR)

In pressurized water reactor , heat generated in the nuclear core is removed by water circulating at high pressure through the primary circuit. The heat is transferred from primary to secondary circuit in a heat exchanger , or boiler, there by generating the steam in the secondary circuit. As such the steam in the turbine is not radioactive and need not be shielded. The pressure in the primary circuit maintained high using pressuriser so that boiling of water will not takes place. In order to vary the pressure in the primary circuit electric heating coils are used in the pressuriser. PWR produces only saturated steam. By providing separate furnace steam formed from the reactor could be super heated. Advantages: 1) The coolant used is cheap and easily available. 2) The reactor is compact, small in size and power density is high. 3) Fission products remain in the reactor and are not circulated. 4) There is a complete freedom to inspect and maintain the turbine, feed water heaters, and condensers during the operation. 5) Small number of control rods are required. 6) The fuel costs are less as the reactor extracts more energy per unit weight of fuel Disadvantages: 1) High primary circuit pressure requires strong pressure vessel and so high capital costs. 2) Severe corrosion problems.

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3) Reprocessing of fuel is very difficult. 4) The reactor must be shut down for recharging. 5) Fuel fabrication is very difficult. 6) Thermal efficiency of secondary loop is very poor. 7) Designing of the vessel against the thermal stresses is very difficult. Boiling water reactor (BWR)

Apart from heat source the BWR generation cycle is similar to that found in the thermal power plants. In this type of reactor , enriched uranium is used as fuel and water is used as coolant, moderator and reflector like PWR except the steam is generated in the reactor itself instead of separate steam boiler. The plant can be safely operated using natural convection within the core or forced circulation as shown . Advantages: 1) The cost of the pressure vessel is less compared to vessel required for PWR. 2) This reactor does not requires separate steam generator therefore the cost is further reduced. 3) The metal temperature remains low for given out put conditions. 4) The reactor is capable of meeting the small fluctuating load requirements. 5) Thermal efficiency is high compared to PWR. 6) BWR is more stable than the PWR. Disadvantages: 1) Steam leaving the reactor is slightly radioactive therefore shielding of turbine and piping is required. 2) Power density of the reactor is only 50% of PWR. 3) Part of the steam is wasted at low loads. 4) Enrichment of the fuel for the reactor is extremely costly process. 5) More biological protection is required. 6) Possibility of burn out of fuel is more in this reactor than PWR

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3) CANDU ( Canadian-Deuterium-Uranium ) Reactor.

CANDU is pressurized heavy water reactor first developed in Canada. The coolant heavy water is passed through the fuel pressure tubes and heat exchanger. The heavy water is circulated in the primary circuit in the same way as with a PWR and steam is raised in the secondary circuit transferring the heat in the heat exchanger to the ordinary water. The reactor is controlled by the moderator level hence control rods are not required. In CANDU reactor refueling is carried out even as the reactor is in operation. The high temperature coolant leaving the reactor passes out of the outlet header to a steam generator of conventional inverted U tube and is then pumped back in to the reactor through the inlet header .The steam is generated at temperature about 2650C. The reactor vessel and the steam generator system are enclosed by a concrete containment structure. A water spray in the containment would result from large break in the coolant circuit. Advantages: 1) The fuel need not be enriched one. 2) The cost of vessel is less. 3) No control rods are required. 4) Low moderator level increases the effectiveness in slowing down of neutrons. 5) Construction time required is less compared to BWR and PWR. 6) The cost of moderator used is less. Disadvantages: 1) The power density is considerably low compared to BWR and PWR. 2) It requires high standard of design , manufacture and maintenance. 3) The leakage is a major problem. 4) The cost of heavy water is extremely high. 5) The size of the reactor is very large.

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Sodium Graphite reactor.(Liquid metal cooled reactor)

Sodium graphite reactor is one of the typical liquid metal reactor. In this reactor sodium works as a coolant and graphite works as moderator. It consists of three circuits, primary circuit, secondary circuit and Steam circuit. In primary circuit liquid sodium which circulates through the reactor core and gets heated. This heated liquid sodium gets cooled in the intermediate heat exchanger and returns to the reactor core . The secondary circuit has an alloy of sodium and potassium in liquid form. This coolant absorbs heat from the sodium circulating in the primary circuit in the intermediate heat exchanger. The heated coolant then passes through the boiler and supplies heat required for generation of steam. The steam generated in this boiler is super heated. The sodium potassium liquid in the secondary circuit from the boiler is supplied back in to the intermediate heat exchanger with the help of pump. Advantages: 1) The thermal efficiency is high . 2) The cost of graphite moderator is low. 3) Excellent heat removal capability. 4) The size of the reactor is small. 5) High temperatures are available at low pressure. 6) Super heating of steam is possible. 7) High conversion ratio. 8) The coolant sodium need not be pressurized. Disadvantages: 1) Sodium reacts violently with water in the air. 2) Heat exchanger must be leak proof. 3) The problem of thermal stresses can not be maintained.

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4) Intermediate system is necessary to prevent the reaction of sodium with water. 5) The leak of sodium is very dangerous as compared with other coolants. 6) It is necessary to shield the primary and secondary circuits with concrete blocks as sodium is highly radioactive. Fast breeder reactor.

The fast breeder reactor derives its name from its ability to breed , that is to create more fissionable material than it consumes. When U235 is fissioned it produces additional heat and neutrons. If some U238 is kept in the reactor , part of additional neutrons available , after reaction with U235 convert U238 in to fissionable plutonium. The general arrangements of the sodium fast breeder reactor is as shown in the figure. In fast breeder reactor, enriched uranium or plutonium is kept in reactor core without moderator. The vessel is surrounded by thick blanket of depleted fertile uranium. The ejected excess neutrons are absorbed by the fertile blanket and converts it in to fissile material. The heat produced in the reactor core is carried by liquid metal sodium. Advantages: 1) High breeding gain is possible. 2) High power density. 3) It has high boiling point. 4) It has low vapour pressure at most temperatures. 5) Absorption of neutrons is low. 6) High burn – up of fuel is achievable. 7) Small core is sufficient. 8) The moderator is not required.

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Disadvantages: 1) Requires highly enriched fuel . 2) Neutron flux is high at the centre of the core. 3) The specific power of the reactor is low. 4) Handling of hot radioactive sodium is major problem. 5) Safety must be provided against the melt down. Homogeneous graphite reactor and gas cooled reactor.(HGGCR) In gas cooled reactors most commonly inert gases such as helium and carbon dioxide are used as coolants and graphite as moderator . The graphite tubes fitted with fuel rods or fuel tubes fitted in tubes or rods made up of graphite and fuel mixed together are used. The gas is passed through the tubes and carry the heat . The fuel used is either enriched uranium or natural uranium. Two types of reactors are used . a) Indirect circuit gas cooled reactor.

The arrangement of this type of reactor is as shown in the fig. The gas is passed through the reactor to carry the heat generated by fission and the hot gas is further used for generating the super heated steam. The Hinkley power station in England is working on this principle. Advantages: 1) Fuel processing is simple. 2) There is no need for limiting the fuel element temperature. 3) Graphite remains stable even at high temperatures under high intensity radiation. 4) There is chances of explosion in the reactor due to the use of carbon dioxide as the coolant. 5) There is no corrosion problem. 6) It gives better neutron economy. Disadvantages: 1) Power density is too low. Therefore reactor vessel is very large. 2) The leakage of gas is the main problem. 3) The loading of the fuel is more elaborate and costly. 4) The coolant circulation absorbs as large as 10 to 20% of plant capacity where as only 5% is required in water cooled reactor. 5) The critical mass is high. 6) The control is more complicated.

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b) Direct circuit gas cooled reactor.

Direct gas cooled reactor is as shown in the fig. The high pressure , high temperature gas coming out of the reactor is directly fed in to the gas turbine for power generation. This is similar to the closed Brayton Cycle except that heat required to heat the fluid is generated in the reactor instead of in the combustion chamber. Advantages: 1) Thermal efficiency is high. 2) The capital cost is low. 3) The reactor can be made more compact as high density gas can be used. 4) The use of gas turbine offers greater flexibility for selection of site Disadvantages: 1) The system design is more complicated. 2) The components must be designed to bear higher stresses as high pressure gases are used. This increases the capital cost of the plant. Advantages of Nuclear power plants 1) Nuclear power plants need less space compared to other types of power plants. 2) Better performance at higher load factors. 3)There is saving in cost of the fuel transportation. 4) The operation is more reliable. 5) Nuclear power plants operation is independent of the weather conditions. 6) Advantage is more with large size power plants. 7) The expenditure on metal structures piping , storage mechanisms is much lower for a nuclear power plant than a coal burning power plant. 8) The nuclear power plants, besides producing large amount of power , produce valuable fissile material which is produced when the fuel is renewed. Disadvantages: 1) The capital cost is high. 2) The danger of nuclear radiations always persists in the nuclear plants. 3) The maintenance cost is high. 4) The disposal of fission products is major problem.. 5) Working conditions in the power plants always detrimental to heath of workers.

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Selection of site for Nuclear power plants:

1) Proximity to load. 2) Population distribution. 3) Land use. 4) Geology. 5) Hydrology 6) Seismology. 7) Safety

Radiation hazards. Human beings are continuously exposed to radiation from cosmic rays and various radioactive materials in the earth and air. Small amounts of radiation can be tolerated but exposure to radiations above certain level is dangerous to health and life. Living tissues are affected in three different ways when exposed to radiations they are i) Ionization: The formation of ion – pair in tissue requires 32.5 MeV of energy. About 3100 ion – pairs are formed when single 1MeV beta particle is stopped by tissue. This absorption results in complete damage of tissues in the body man, or beast or bird. ii) Displacement: If the energy of the impinging particle is sufficiently high, an atom in the tissue is displaced from its normal lattice position with possible adverse effects. iii) Absorption: Absorption of neutron by a tissue nucleus results in forming a radioactive nucleus and change the chemical nature of the nucleus. This severe alteration of the tissue causes malfunctioning of the cell and cell damage may have severe biological disorders including genetic modifications. Ultimate effect of all these hazards on human being is to damage the living cells of body by ionization. The result of such damage may be immediate , effects like burns, even death, or delayed effects like lukaemia , a anemia or cancer or may be genetic giving hereditary effects. Shielding: The common nuclear radiation emitting from nuclear reactors are in theform of -rays,neutrons,X-rays,-Rays and -Rays. The and radiations are absorbed in a smaller thickness of the shielding. radiations require higher thickness shielding because of their higher high level of energy and frequency they can penetrate more . Neutrons have high power of penetration and do not follow any defined path through the shield materials. The shield should be designed to absorb or reduce and neutron radiations. The nuclear radiation if it is not prevented , it will have very bad effects on the human life and biological plants. The desirable properties of the good shielding materials are. 1) It must have ability to absorb more radiation with minimum thickness. 2) It must be fire resistant. 3) The strength of the material should remain constant under the influence of radiations. 4) It must have high density and it must contain light materials. 5) Density of the material must remain constant. The use of best neutron absorber shield is beneficial. The combination of light and heavy elements in the shield is best , the use of laminated construction or the use of iron concrete. The latter consists of iron mixed in barytes concrete, or alternatively limonite is used partially to replace barytes in the mix. Example for shielding materials include Water, Iron, cement and concrete, Tantalum, Lead, Bismuth and Boron. Nuclear waste disposal Used fuel in a nuclear power plant is highly radioactive and can contaminate air or water and if absorbed by a living organisms, it can cause biological damage. Disposal of radioactive waste is therefore a problem which requires consideration right from the planning stage.

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The nuclear wastes from the reactor are classified as i) High level waste( above 1000 Curie) ii) Medium level waste (100 to 1000 Curie) iii) Low level waste ( below 100 Curie). The spent fuel is withdrawn from the reactor and placed in a water pond where heat is removed. The pond water is treated to remove radiations. The spent fuel is then transferred to the processing plant where cladding that contains the fuel is removed and the fuel is dissolved in the nitric acid. The U235 (20 to 90% ) and Pu239 are then removed leaving the solution the solution known as highly active liquid waste. The separated U235 and Pu239 are further purified and either stored for future use or fabricated in to fresh fuel for reactor. The waste from the cooling fond is the transferred to intermediate storage and kept there for a period of about 30 to 100 years where most of radioactive nature is reduced to a considerably low level. Then waste is permanently shifted to the final storage . Various methods used for the disposal of radioactive waste are given below. a) Storage in tanks on site. Solid and liquid wastes are stored in concrete or stainless steel tanks at site . During storage period the radioactivity decays and then the waste is disposed of either in the sea or buried under the ground. b) Dilution: Disposal of liquids after dilution to safe limits, in the rivers or sea is also done. Gases are also left off in air after dilution. Before disposal in the dilutent the radioactivity of the gas or liquid being discharged is reduced to acceptable levels. c) Sealed containers: Radioactive liquid and solid wastes are put in sealed containers which prevent the radioactive contamination . These sealed containers are then disposed of at sea where they are quickly and completely covered with mud in the bottom. d) Underground burial. Another alternative is the burial of wastes direct in the ground . How ever burial ground must be isolated from the public and water must not be able to seep through as it may cause radioactive contamination of drinking water supplies. Nuclear power plants in India: 1. Tarapur power plant: Located in Maharastra , has a capacity of 380 MW with the steam pressure and temperature of 35 bar and 2400C. 2.Rana Pratap Sagar power plant: Located near Kota in Rajasthan, has a capacity of 400 MW with steam pressure and temperature of 40 bar and 2500C. 3. Kalpakkam power plant: Located near Chennai in Tamilnadu, has a capacity of 470 MW. 4. Narora power plant : located near narora in UP , with , a capacity of 470 MW , steam pressure temperature of 40 bar and 2500C. 5. Kakrapar atomic power plant : Located near the Surat in Gujarat with a capacity of 470 MW, steam pressure temperature of 40 bar and 2500C. 6. Kaiga atomic power plant: Kaiga situated near Karwar in Karnataka. With a capacity of 440 MW , steam pressure temperature of 40 bar and 2500C.