In this article we will discuss about:- 1. Need of Nuclear Power Plants 2. Economics of Nuclear Power Plants 3. Merits and Demerits 4. Selection of Site 5. Constituents and Lay Out 6. Power Rating 7. Pollution and Its Disposal.
Need of Nuclear Power Plant:
The need of nuclear power plant lies in the fact that the hunger for electricity is virtually unending and after each decade the world demand for electricity is doubled owing to booming increase in the population and industrial growth. Moreover, the reserves of fossil fuels i.e., coal, oil and gas are fast depleting.
Again it would be better to utilise these minerals, particularly oil, for chemical industry as raw materials. Thus there is tendency to seek alternative source of energy and the nuclear power is the only alternative source which can meet the future energy demands of the world. One of its main attractions is the huge amount of energy that can be released from a small quantity of active material.
The energy obtainable by completely burning of 1 kg of uranium would give energy equivalent 3,000 tonnes of high grade coal i.e., uranium has 3 million lines the energy of coal. The possible energy reserves in the form of uranium and thorium is many times greater than that of fossil fuels.
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The findings of the committee appointed by the Govt., of India in 1984 for examining the comparative costs for nuclear and thermal power plants commissioned in 1983 are that the nuclear power would be cheaper by 8 to 10 paise per kWh than from thermal plants, at a distance of 800 km from pit heads. This difference goes on increasing as the distance from pit heads increases. The trend in favour of nuclear power becomes stronger as time passes.
The area’s most suitable for nuclear power plants are Western UP, Northern and Western Rajasthan, Punjab and Haryana.
Though the capital cost of nuclear power plant, because of heavy costs of land having strong substrata, foundation, nuclear reactors and nuclear fuel is pretty high as compared to that of a conventional thermal power plant, but the total operating costs per kWh of a nuclear power plants are less than those of a coal-fired thermal plant. Nuclear power is the cheapest non-hydroelectric power in India.
The development of nuclear power station is due to attention of scientists towards the peaceful applications of atomic energy. The world’s first atomic power plant was commissioned in U.S.S.R. on June 27, 1954 and after that a number of atomic power plants have been commissioned in many countries like U.S.A., Canada, Great Britain, Japan and France.
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It will be noteworthy here that steam turbines employed in nuclear power plants will be of design quite different from those used in coal-fired steam power plants. The reasons are operating pressure and temperature, in case of nuclear power plants, being quite different than those in case of conventional steam power plants (turbines for the modern coal-fired steam power plants are designed for operation with superheated steam at temperatures about 565° and pressure about 15 MPa).
Nuclear Fuel Reserves:
India domestic uranium reserves are small and the country is dependent on uranium imports to supply its nuclear power industry. Since early 1990s, Russia has been a major supplier of nuclear fuel to our country. Because of dwindling domestic uranium reserves, generation of electrical energy from nuclear power in India declined by 12.83% from 2006 to 2008.
Following a waiver from the Nuclear Suppliers Group in September 2008 which allowed it to commence international nuclear trade, India has signed bilateral deals on civilian nuclear energy technology cooperation with several other countries, including France, the United States, the United Kingdom, Canada and South Korea. India has also uranium supply agreements with Russia, Mongolia, Kazakhstan, Argentina and Namibia. An Indian private company won a uranium exploration contract in Niger.
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Large deposits of natural uranium, which promises to be one of the top 20 of the world’s reserve, have been found in the Tummalapalle belt in southern part of the Kadapa basin Andhra Pradesh in March 2011. The Atomic Minerals Directorate for Exploration and Research (AMD) of India which explores uranium in the country has so far discovered 4,400 tonnes of natural uranium (U308) in just 15 km of the 160 km long belt.
Economics of Nuclear Power Plants:
The economic analysis of nuclear power plants is in many respects similar to that for coal-fired thermal power plants and hydro power plants. However, the relative costs of different components are somewhat different in this case. Broadly speaking, the points for considerations are capital investment, plant life, fixed, operation and maintenance costs, plant operating condition such as plant load factor.
The capital investment of a nuclear power plant includes the cost of land, cost of nuclear reactor, heat exchanger, steam turbines, turbo-alternators etc., and the cost of design and planning.
The site for nuclear power plant should have substrata strong enough to support the heavy reactors which may weigh as high as 100,000 tonnes and impose bearing pressures around 50 tonnes/m2. The cost of foundation is also quite high.
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The capital cost of nuclear reactors is very high because very few countries have the technology of manufacturing nuclear reactors. The cost of cooling towers is also high because of very large requirements of cooling water in case of steam power plants. Nuclear fuel may remain in a reactor for more than 5 years. So, the cost of fuel injected initially is taken as a capital cost and may be a few crore rupees.
The nuclear power station has a much higher capital investment than a thermal power station. Some years back, this difference was much higher than it is today because it had to cover the initial development of reactors of the Calder Hall Type.
Fixed Costs:
The life of a reactor plant may be taken as between 15 and 20 years. For the other parts of the plant equipment, the life may be taken as 30 years. The fixed cost would be interest, depreciation, taxes and insurance charges. The depreciation would be determined on the life of the equipment in the plant as well as building etc. The insurance charges will be higher than for conventional power plants.
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Operation and Maintenance Costs:
The fuel, operation and maintenance costs will consist of the cost of the nuclear material, the cost of the processing material, the cost of fabricating the fuel elements, the cost of disposal of waste, and the salaries and wages of operating and maintenance staff. The total operating costs per kWh of a nuclear power plant are less than that of a coal-fired thermal power plant.
The overall efficiency of a nuclear power plant is around 30 to 40 per cent. The efficiency is higher at high load factors. Therefore, a nuclear power plant is always operated as a base load plant. Though the initial investment and capital cost of a nuclear power plant is higher than that of a conventional thermal power plant, but the cost of transport and handling of coal for conventional thermal power plant is much higher than the cost of nuclear fuel. With further developments, the cost of nuclear power plants likely to come down and they will soon be competitive.
Merits and Demerits of Nuclear Power Plants:
The chief merits of nuclear power plants over other conventional power plants are given below:
(i) The amount of fuel required is quite small; therefore, there is no problem of transportation, storage etc.
(ii) The demand for coal, oil and gas is reduced which are tending to rise in cost as the stocks are becoming depleted.
(iii) These plants need less area as compared to any other plant of the same size. A 2,000 MW nuclear power plant needs about 80 acres whereas the coal-fired steam power plant of the same capacity needs about 250 acres of land.
(iv) In addition to producing large amounts of power, the nuclear power plant can produce valuable fissile material, which is extracted when the fuel has to be renewed.
(v) These plants, because of the negligible cost of transportation of fuel, can be located near the load centres, therefore, primary distribution cost is reduced.
(vi) These plants are most economical in large capacity (100 MVA and more).
(vii) The output control is extremely flexible i.e., the output can be instantaneously adjusted from zero to an upper limit. The limit is set by the capacity of the heat removal system to prevent overheating of the pile.
(viii) There are large deposits of nuclear fuel available all over the world. Therefore such plants can ensure continued supply of electrical energy for thousands of years.
(ix) A coal-fired steam power plant needs thousands of tonnes of coal per day and usually looks like a mad house of trains, trucks, coal and ashes, whereas a nuclear power plant will be very neat and clean and is hospital quiet.
(x) The operating cost is quite low and once the installation is completed, the loading of the power plant will have no effect on the generation cost. Therefore, a nuclear power plant is always operated as a base load plant. The nuclear power plants are usually not operated at a load factor less than 0.8.
However the nuclear power plants have the following demerits:
(i) The initial capital cost is very high as compared to other types of power plants.
(ii) The erection and commissioning of the plant requires greater technical know-how.
(iii) The fission by-products are generally radioactive and may cause a dangerous amount of radioactive pollution.
(iv) Nuclear power plants are not well suited for varying loads since the reactor does not respond to the fluctuations of load efficiently.
(v) The fuel used is expensive and is difficult to recover.
(vi) Maintenance charges are high owing to lack of standardisation. Salary bill of the maintenance staff is also high as specially trained personnel are required to handle the plant.
(vii) The disposal of the products, which are radioactive, is a big problem. They have either to be disposed off in a deep trench or in a sea away from seashore.
(viii) The cooling water requirements of a nuclear power plant are very heavy (more than twice the water required for the same size coal-fired steam power plant). Hence cooling towers required for nuclear power plants are larger and costlier than those for conventional steam power plants.
Selection of Site for Nuclear Power Plants:
The factors to be considered while selecting a site for nuclear power plant for economical and efficient generation are:
(i) Availability of Water Supply:
As sufficient water (more than twice the water required for the coal plant of the same size) is required for cooling etc., therefore, the site selected for the plant should be near a river or lake or by sea side.
(ii) Distance from Populated Area:
There should be a reasonable distance between the nuclear power plant and the nearest populated area from the point of view of safety as there is a danger of presence of radioactivity in the atmosphere near the plant. However, as precautionary measure, a dome is used in the plant which does not allow the radioactivity to spread by wind or underground waterways.
It is highly undesirable to choose a site adjacent to chemical industries, oil refineries, PWD works, hospitals and schools.
(iii) Transportation Facilities:
As nuclear power plant needs very little fuel, hence it does not require direct rail facilities for fuel transport. However, transportation facilities are required during the construction stage.
(iv) Nearness to Load Centre:
Such plants should be located as near to the load centre as possible, consistent with safety considerations in order to reduce the transmission costs.
(v) Availability of Space for Disposal of Waste:
The site selected for such power plants should have adequate space and arrangement for the disposal of radioactive waste.
(vi) Accessibility:
There should be reasonable accessibility for plant personnel, hauling in equipment, and dispatching and receiving heavily shielded radioactive materials.
(vii) Type of Land:
The substrata must be strong enough to support the heavy reactors which may weigh as high as 100,000 tonnes and impose bearing pressures around 50 tonnes/m2. Areas remote from coal fields and hydro sites are preferable so as to improve the reliability of supply over the area.
Generally hydel power plants are located far away from load centre. In conventional thermal power plants, enormous fuel shipment is required. Location of a nuclear power plant is practically independent of geographical factors, the only requirement being that there should be good supply of water. The ideal choice for a nuclear power plant would be near a sea, river or lake and away from thickly populated area.
Constituents of Nuclear Power Plants and Lay Out:
The concepts of nuclear power generation are much similar to that of conventional steam power generation. The difference lies only in the steam generation part i.e., coal or oil burning furnace and the boiler are replaced by nuclear reactor and heat exchanger.
Thus a nuclear power plant consists of a nuclear reactor (for heat generation), heat exchanger (for converting water into steam by using the heat generated in nuclear reactor), steam turbine, alternator, condenser etc. As in a conventional steam power plant, water for raising steam forms a closed feed system. However, the reactor and the cooling circuit have to be heavily shielded to eliminate radiation hazards.
The tremendous amount of heat energy produced in breaking of atoms of uranium or other similar metals of large atomic weight into metals of lower atomic weight by fission process in an atomic reactor is extracted by pumping fluid or molten metal like liquid sodium or gas through the pile.
The heated metal or gas is then allowed to exchange its heat to the heat exchanger by circulation. In heat exchanger the gas is heated or steam is generated which are utilised to drive gas turbine or steam turbine coupled to an alternator thereby, generating electrical energy.
While deciding the layout of a nuclear power plant due consideration should be given to safety, operating convenience and capital economy. One of the important operational areas in a reactor building is the charge hall which is used for the re-fuelling operation. This is located directly over the reactor core. As far as the main parts of a nuclear power plant are concerned the reactor, turbine and generator—the layout is quite simple.
A main control room is provided in a central location and consists of all the necessary equipment for controlling normal and emergency operation of the reactors as well as controls of boilers and turbo-alternators. All other ancillary rooms such as charge room, maintenance room, store, switchyard, railway siding, machine shop, instrument shop, office etc. are suitably located for convenient operation.
Power Rating of Nuclear Power Plant:
Nuclear power plant reactor power outputs are quoted in three ways:
Thermal MW, which depends on the design of the actual nuclear reactor itself, and relates to the quantity and quality of the steam it produces.
Gross electrical MW indicates the power produced by the attached steam turbine and generator, and also takes into account the ambient temperature for the condenser circuit (cooler means more electric power, warmer means less). Rated gross power assumes certain conditions with both.
Net electrical MW, which is the power available to be sent out from the plant to the grid, after deducting the electrical power needed to run the reactor (cooling and feed water pumps, etc.) and the rest of the plant.
The relationship between these is expressed in two ways:
i. Percentage thermal efficiency is the ratio of gross electrical MW to thermal MW. This relates to the difference in temperature between the steam from the reactor and the cooling water. It is often 33-37%.
ii. Percentage net efficiency is the ratio of net electrical MW achieved to thermal MW. This is a little lower, and allows for plant usage.
In WNA papers and figures and WNN items, generally net MW is used for operating plants and gross MW for those under construction or planned/proposed.
In nuclear power stations, the combustion of fuel is low. Harmful effluents from such power stations in the atmosphere are insignificant. However, isotopes formed in nuclear power reactors have a high toxicity and their effect on living organisms may be accumulative. That is why, the problems of disposal, transport and storage of liquid radioactive wastes are extremely significant. Such power plants will produce practically no harmful effect on the biosphere provided the radioactive waste storage problem is safely solved.
A nuclear reactor produces α-rays, β-rays, γ-rays and neutrons which can disturb the normal working of living organisms, and thus calls for special safety measures, α- rays are heavy particles carrying positive charge and can cause internal hazard if ingested. β-rays have greater penetrating power, as compared to α-rays, due to their smaller size.
Overexposure to β-rays can cause skin burns and repeated overexposure may result in malignant growth, γ-rays are electromagnetic radiations of very short wavelength, have high energy and are very penetrating. They are capable of causing considerable-damage, especially to organic materials. Overexposure to γ-rays causes the blood diseases, undesirable genetic effects, anaemia etc. Larger exposure may cause death within hours, of exposure.
However, the effects of slow exposure may become apparent after several years. Neutrons are produced in fission with a very wide range of energies up to about 10 MeV. Though they have no charge but are highly penetrating. Their effects are similar to those of γ-rays.
The biological effects of nuclear radiations depend upon:
(i) Amount of dose absorbed
(ii) Time duration of exposure
(iii) Sensitivity and recovery of recipient organism, and
(iv) Distribution of active material within body.
A long time exposure to even a small dose may not cause any immediate effect but leads to delayed effects such as shortening of life, leukemia, genetic effects etc
Pollution from Nuclear Power Plants and Its Disposal:
Nuclear power stations are surrounded by a sanitary protective zone to minimize the risk of irradiation of the population within such a zone. It is prohibited to build residential buildings, children houses and auxiliary buildings not related to the concerned power plant. The level of radiation of this territory is checked periodically.
In nuclear power plants, there are three main sources of radioactive contaminating of air. The first source is the fission of nuclei of solid or gaseous nuclear fuels. Gaseous fission fragments which are more likely to enter the air include inert gases, such as Xenon, Crypton etc., and radioactive iodine. The second source is due to the effect of neutron fluxes on the heat carrier in the primary cooling system and on the ambient air.
Among the components of air, an inert gas, argon-40 is the most prone to activation. It may form a radioactive isotope argon-41 with the half-life period of 1-82 hours. Induced activity can appear in the dust present in the air. The third source is damage of shells of fuel elements or the presence of activated inert gases and aerosols in heat carrier leakages.
The processes in nuclear power plants include various types of technological blow-off of gases. At some periods (say, during overload operation) blown-off gases may have an elevated activity and need special decontaminating measured. Other potential sources of radioactivity are various auxiliary structures and elements (cooling ponds, reactor blow- off system, tanks for collecting radioactive leakages etc.) They may evolve radioactive inert gases.
Within any of the groups, elements may be ejected in any proportion, but their sum should not exceed the given value. A simultaneously daily emission of all groups of isotopes is also allowed, provided that the given values for each of the groups are not exceeded.
Disposal of Nuclear Waste and Effluent:
Solid radioactive wastes arise from used filters, sludge from the cooling ponds, pieces of discarded fuel element cans, splitters etc. These, along with discarded items of plant such as control rods have to be stored on site in shielded concrete vaults.
There are many ways for disposing of the solid fission products. The storing in shielded storage vaults consists in fixing the solid waste in borosilicate glass and then storage of this glass in leak tight capsules. These capsules or vaults can then be stored in deep salt mines or in deep wells drilled in the stable ocean floor.
Deep salt mines are suggested because the presence of the salt pockets indicates that there has been no ground water in the vicinity for thousands of years. Sometimes, suitable containers are filled with radioactive waste and sunk to the bottom of seas and oceans. However, this method does not completely prevent the radioactivity from leading into the water.
Another way of disposal is the separation and transmutation of the long-lived isotopes to short-lived or stable products following neutron absorption in a breeder or fusion reactor. The possibility of firing these longs- lived products into the sun or into a long-term stable orbit is also being considered.
Radioactive liquid effluents arise from the laundry, personal decontamination, etc. together with activity accumulating from the corrosion of the irradiated fuel elements in the storage ponds, before discharging to sea, where enormous dilution takes place, the effluent is passed through ion exchange resins which absorb a large proportion of activity. The final levels of any particular isotope in the sea will be well below the maximum drinking water level.
It is safe enough to store radioactive waste underground in liquid form in suitable tanks or in reduction to clinker. Clinkering serves a two- fold purpose of improving the protection and reducing the volume of waste. A promising method is known as “solidifying” the liquid radioactive waste through heat up and evaporation. The current technology enables 1,000 litres of highly radioactive liquid waste to be processed into less than 0.01 m3 of solid waste. The solid waste is put into sealed metal containers suitable for storage in deep salt mines.
Gaseous effluents are filtered before discharging into atmosphere. Moreover, the filtered gas is discharged at high levels so that it is dispersed properly. The probability of fire in the reactor fuel channel is extremely low. However, if fire breaks out, large volumes of gaseous fission products may be released. So it is necessary to have a clean-up plant through which these products can be passed for removal of radioactive iodine which is the major hazard.
It is essential to monitor the loss of CO2 from the reactor to ensure that this loss does not exceed about 1 ton per day. It is also necessary to check the concentration of CO2 in the atmosphere near the reactor. Proper precautions against toxic and radiological hazards are necessary.
Shielding:
Adequate shielding is necessary to guard personnel and delicate instruments. The various materials used for shielding are lead, concrete, steel and cadmium. Lead is a common shielding material and is invariably employed due to its low cost. Concrete is another shielding material having efficiency lesser than that of lead.
Steel is not an efficient shielding material but has good structural properties and is sometimes employed as an attenuating shield. Cadmium is capable of absorbing slow neutrons by a nuclear reaction. The effectiveness of a shielding material depends mostly on the density of material (lead-11,300 kg/m3; concrete-2,400 kg/m3; steel-7,800 kg/m3; cadmium- 8,650 kg/m3).
No single material is effective in shielding radiations of different kinds. A material containing hydrogen, e.g., water or polythene is used to slow down fast neutrons, boron or steel is employed for absorption of thermal neutrons. A heavy material like lead is required to act as a thermal shield and to absorb gamma rays.
In nuclear power reactors a thermal shield of thickness of several cms of steel surrounded by about 3 m thick concrete is used. Water, in concrete, slows down fast neutrons while iron, barium or steel turnings are mixed in concrete to attenuate gamma rays and absorb thermal neutrons.