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Nuclear power plants have same working principle as that of coal fired power plants i.e. saturated water is converted into superheated steam which converts heat energy into kinetic energy to do mechanical work. They have the same three main components as in coal fired power plant i.e. feed pump, turbine and condenser but the boiler used in coal fired plant is replaced by nuclear reactors in nuclear plant. In nuclear power plants the heat energy is produced in nuclear reactor by nuclear fission reaction.
Nuclear fission reaction involves the splitting up of an atom into smaller parts, producing free neutrons and lighter nuclei (which may produce photons in form of gamma rays). The fuel used in nuclear power plants is mainly uranium which occurs naturally in earth crust and is composed of two slightly different isotopes called uranium-235 (0.7% of all natural uranium) and uranium -238 (99.3%of all natural uranium).when an atom of uranium-235 is struck by a slow moving neutron it under goes nuclear fission reaction and the frequent products of this reaction are an atom of krypton, an atom of barium, three more neutrons and a significant amount of energy.
+ N = + +3n + 200meV
The three neutrons produced during the reaction could cause three more atoms of uranium - 235 to split which would lead to a rapidly accelerating reaction called as chain reaction resulting in production of enormous amount of energy.
4.2 Nuclear reactor:
The operation of a nuclear reactor is based on the idea that a nuclear chain reaction can be controlled so that process continues indefinitely, but is never allowed to get out of control and become a chain reaction .A reactor in which each nuclear reaction produces one further nuclear reaction is known as critical, if the product of nuclear reaction is more than one further reaction, the reactor is described as super critical. Following are the main components which are common to most type of reactors:
Fuel: The main fuel being used in current nuclear reactors is UO2 (uranium dioxide) or a mixture of PuO2 (plutonium dioxide) and UO2 called as MOX fuel (mixed oxide). Pellets of UO2 or MOX fuel are arranged in tubes to form fuel rods which are further arranged into fuel assemblies in the reactor core.
Moderator: The main function of moderator in a nuclear reactor is to reduce the speed of fast neutrons, therefore making them thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235. The most commonly used moderators are substances of low atomic weight such as graphite, deuterium oxide (heavy water) etc. In some specialized reactors hydrocarbons, beryllium and beryllium oxide have also been used as moderators.
Control rods: Control rods are solid cylinders of metal which are made up of materials with an ability to absorb neutrons without fissioning themselves .Its main purpose is to maintain the ratio of neutrons produced during nuclear fission to the neutrons used up in fission at about 1:1.They are usually combined into control rod assemblies (about 20 rods for pressurized water reactors) and are inserted in guide tubes within a fuel element. In order to control neutron flux, increase or decrease the number of neutrons which will split the uranium atoms further, control rods are inserted or removed from central core of the nuclear reactor. This whole process can affect the amount of steam produced, thermal power of the reactor and hence the electricity generated. Materials used in making control rods include boron, hafnium, gadolinium, samarium etc .
Coolant- It is used in nuclear reactor to transfer heat from reactor core to electrical generators and environment. Normally a chain of two coolant loops used as the primary coolant loop takes on a short-term radioactivity from the reactor. Most of the nuclear plants operating with light weight reactors use normal water under high pressure as coolant.
Pressure vessels - Pressure vessel is generally a robust steel vessel used to contain the reactor core, moderator and coolant, although they can be series of tubes conveying the coolant through the moderator and holding the fuel. These vessels are designed and built to withstand high pressure conditions in the system.
Steam generator - It is a heat exchanger (also part of cooling system) which is used to convert water into steam for the turbines, from the heat produced in nuclear reactor core. Steam generators contains about 3000 - 6000 tubes (which are 3/4 inch in diameter each), weighs about 800 tons and can be 70 feet in height.
Containment: Containment building is an airtight structure that encloses the nuclear reactor and is made up steel or reinforced concrete. In case of emergency ,it is the final barrier to radio active release (first being fuel ceramic followed by metal fuel cladding tube, reactor vessel and coolant system and can contain the escape of radiation to a maximum pressure in range of 4.136 to 13.789 bar.
Fig: 4.3 a Schematic Diagram of Advanced gas cooled reactor 
Advanced gas cooled reactors (AGR) are the most commonly used reactors in nuclear power plants situated in UK. The AGR were developed from the earlier used Magnox reactor (which are still being used in some power plants). They have improved efficiency (thermal efficiency of 41%) and can operate at higher temperatures than Magnox reactors.
In AGR reactors, CO2 act as coolant and circulates through the core at a temperature of 640°C and pressure of 40 bar, after which it passes through the steam generator .The primary shut down system for AGR reactors is penetration of graphite moderator by control rods followed by injection of nitrogen into the coolant. The tertiary system for shut down system of reactor involves
Injection of boron balls into the reactor.
4.3 WORKING -
Enriched uranium is formed into 2.66 cm long pellets (fuel rods) which are collected together into bundles. These bundles are then submerged inside the water (which also acts as coolant) contained in pressure vessel. The submerged bundles are kept in slightly supercritical condition so that reactor could start working .To prevent overheating and melting of uranium(in case it is left in supercritical condition), control rods are inserted into bundles .The nuclear reaction is controlled by lowering and raising of control rods from the bundle . In case the operator wants uranium core to produce more, the control rods are raised out of uranium bundle (thus absorbing few neutrons) and in case less heat is needed from the uranium core the control rods are again inserted in uranium bundle (absorbing more neutrons). The heat energy produced by the uranium bundle is then used to heat feed water (pumped by water circulator) through the heat exchanger. The feed water turns into superheated steam at high temperature which flows into the turbine where the expansion of steam takes place in the nozzle located at the turbine entry resulting in the steam pressure to drop and the steam velocity is finally converted into kinetic energy. The turbine blades and shaft are turned (rotated) during this process and the kinetic energy is converted into mechanical workout put which results in production of electricity.