Nuclear Power Station

A generating station which converts the nuclear energy into an electrical energy is called nuclear power station.

       In such a power station, heavy radioactive elements like uranium (U²³⁵), Thorium (Th²³²), are subjected to the nuclear fission. The fission is breaking of nucleus of heavy atom into the parts by bombarding neutrons. This is carried out in a special nuclear reactor. During the nuclear fission, huge amount of energy is released.

       The heat energy that released is used in rising the steam at high pressure and temperature. The steam turbines are operated using the high temperature steam. The turbines converts the heat energy into a mechanical energy. The turbine drives the alternator which converts mechanical energy into an electrical energy.

      
The energy conversion involved in the nuclear power station is shown in the Fig.1.

Fig. 1 Energy conversion

1.1 Conversion of Nuclear Energy

       According to Einstein's hypothesis, the relation between the energy released by the nuclear reaction of the mass given by,

                          E = mc²
where        E = Energy released in jouls
                 m = Actual mass converted into energy in kg
                 c = Velocity of light = 3 x 10⁸ m/s

       There are three types of nuclear reaction, radioactive decay, fission and fusion. Out of this, only fission is used to produce the energy.

       The fission reaction is achieved by bombarding an electrically neutral neutron, on the positively charged nucleus of radioactive element. This results in the sustained reaction to release two or three neurons for eacj one absorbed in fission.

       The immediate products of fission reaction such as xenon (Xe¹⁴⁰) and strontim (Sr⁹⁴) are fission fragments and are the decay products. The complete fission of 1 gm of U²³⁵ nucleus produces 0.948 MW energy per day.

1.2 Factors selection of site

       The following factors are to be considered for the selection of site for the nuclear power station.

1. Availability of water : Water is a secondary working fluid and used as a coolant for the cooling purpose, in the nuclear power station. A huge amount of water is necessary for this purpose. Hence site must be near the river or canal so that abundant quantity of cooling water is availabe.

2. Disposal of waste : The immediate products of fission reaction are the waste products which are radioactive in nature. These can cause problems to the health of the people and hence must be disposal quickly. Such a waste is either burried in deep pits or disposal off in the sea. Hence the site should be selected so that their is sufficient arrangement for disposal of such radioactive waste products.

3. Distance from populated area : The radioactive elements are hazardous to the health of the people around. There is always danger of presence of radioactivity in the atmosphere near the plant. Hence as a safety measure the site itself must selected to far away from the populated area. Practically a dome is used in the plant, which restricts radioactivity to spread in the atmosphere.

4. Transportation facilities : For transporting the equipments and the machinery required, there must be adequate transportation facilities. The site must accessible by a rail or road so that it is easy for the movement of the workers, working in the plant.

5. Nearness to the load centres : Though the site should be away from the populated area near the river or sea, it should not be too large distance, due to which transmission cost may increase tremendously.

6. Cost and type of land : The land price must be reasonable and the bearing capacity of the land should be good enough to withstand the forces due to heavy equipments of the plant.

        All these factors affects the selection of site for the nuclear power station.

1.3 General Arrangement of Nuclear Power Plant

       The Fig. 2 shows the schematic arrangement of a nuclear power plant.
        The entire arrangement can be divided into following stages.
1.Nuclear reactor      2. Heat exchange (Steam generator)
3. Steam turbine         4. Alternator               5. Cooling water circuit.

Fig.2    Schematic arrangement of nuclear power station

1.4 Nuclear Reactor 

       This represents that part of a nuclear power plant where U fuel is subjected to a controlled fission chain reaction, during which tremendously energy is generated.

       The Fig. 3 shows the various components of a nuclear reactor and a heat exchanger.

       The following are the components of the nuclear reactor.

Fig. 3 Nuclear reactor and heat exchanger

1. Fuel : The commonly used fuel is uranium containing 0.7 % U²³⁵ or enriched uranium containing 1.5 - 2.5  U²³⁵. The fuel is used in the form of rods or plates which are surrounded by the moderators. The fuel rods are arranged in cluster and the entire assembly is called core. The minimum amount of the fuel required to maintain the chain reaction is called the critical mass.

2. Moderators : The main function of the moderators is to reduce the energy of neutrons evolved during fission. By slowing down the high energy neutron, the possibility of escape of neutrons is reduced while possibility of absorption of neutrons by fuel to cause further fission is increased. This also educes the amount of fuel required for the chain reaction. The commonly used moderators are graphite, beryllium and heavy water. Some other functions of moderators include prevention of corrosion of fuel element, retain the radioactivity and to provide structural support.

3. Reflector : The reflector is placed around the core to reflect back some of the neutrons which may leak out from the surface of the core, without taking part in the fission. A blancket of reflector can reduce the critical mass required.

4. Control rods : The cadmium rods are used as control rods which are strong neutron absorber. Thus control rods can regulate the supply of neutrons for chain reaction. If the number of neutrons are not controlled, there is a possibility of explosion due to large amount of energy released. By pushing or pulling out of these rods, the rate of chain reaction and hence the heat produced can be controlled. The control rods are operated automatically as per the next requirement. The other material used for the control rods is boron or hafinium.

5. Coolant : The main purpose of the coolant is to transfer heat generated in the reactor core and use it for the system generation. he coolant in the reactor keeps the temperature of fuel below safe level by continuous removable of the energy from the core. The liquid metals like sodium or potassium are used as coolants.

6. Radiation shield : The radiation of a radioactive substances are harmful to the human life. Hence radiation shield is used to prevent the escape of these radiations to the atmosphere. Generally 50 to 60 cm thick steel plate and few meters of the concrete outside are used as the radiation shield.

1.5 Heat exchanger

       It is a device which is used to exchange the heat from the primary circuit to the secondary circuit. The coolant carries the heat in the reactor to the exchanger where it is exchanged to the water, to convert water to steam. Thus the heat exchanger is nothing but a steam generator. Once the heat exchanged, the coolant is fed back to the reactor, using the coolant recirculating pump.

1.6 Steam turbine

       The steam generated from the water in the secondary circuit is taken to the steam turbine through a main valve, where it is expanded. Due to this, turbine starts rotating and thus the heat energy is converted to a mechanical energy.

1.7 Alternator

       The shaft of an alternator is coupled to the turbine shaft. Thus when the turbine rotates, the alternator starts rotating. The alternator converts mechanical energy into an electrical energy. The energy output of an alternator is given to the bus bars through transformer, circuit breakers and isolators.

1.8 Cooling Water Circuit

       The expanded steam from the turbine is the exhausted steam which is taken to the condenser. In the condenser, the steam is condensed into water. For the condensation of steam , a flow of natural cold water is circulated through the condenser. This water takes heat from the exhaust steam. This hot water is passed through cooling tower, where it is again converted to cold water. The it is recirculated through the condenser by pump. The condensed steam is then recirculated through the secondary circuit of exchanger, using the feed water pump.

1.9 Advantages

1. The amount of fuel required is very small
2. Three is saving in the transportation cost of fuel as fuel required is less.
3. It requires less space compared to any other type of the power plant.
4. The running cost per unit energy generated is lower than the thermal power plant.
5. It is very much economical.
6. There is a lake of environmental problems which are associated with the thermal power plant.
7. Large deposits of nuclear fuels are available so such plants can ensure continued supply of the fuel.
8. It ensures reliability of the operation.

1.10 Disadvantages 

1. The fuel is very expensive.
2. The fuel is difficult to recover.
3. The capital cost is very high compared to other types.
4. The waste products are radioactive and can cause pollution.
5. The waste disposal problem is severe.
6. The maintenance charges are very high.

7. It is not suitable for the varying load conditions, as the reactor can not respond instantly to the load fluctuations.

8. The fuel may be misused in weapons.