Nuclear Power Station : Part 2

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.

See Nuclear Power Station : Part 1

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