1.9.2 Nuclear fission
The fuel inside the reactor is a metal called uranium. Uranium exists asan isotope in the orm of U 235 , U 234 and U 238 . Out of these isotopes U 235 is moreunstable. When a neutron is captured by a nucleus of an atom of U235, itsplits up roughly into two equal fragments and about 2.5 neutrons arereleased and a large amount of energy (nearly 200 million electron voltsMeV) is produced. This is called fission process. The neutrons so producedare very moving neutrons and can be made to fission other nuclei of U 235thus enabling a chain reaction to take place. When a large number of fissionoccurs, enormous amount of heat is produced. The following fig.1.8. showsthe chain reaction
It may be observed from the fig.1.8 that 2.5 neutrons are released infission of each nucleus of U235, out of these one neutron is used to sustain thechain eaction, 0.9 neutrons is absorbed by U238and becomes Pu239. Theremaining 0.6 neutrons escape from the reactor. Moderators are provided toslow down the neutrons from the high velocities but not to absorb them. Themoderators which are commonly used are ordinary water and Heavy water. The fig.1.9 shows how the reactor is put on and off.
Nuclear fuel • Nuclear reactor • Steam generator • Moderator • Control rods • Reflector • Turbine • Condenser • Shielding
Nuclear Fuel:
Fuel of a nuclear reactor should be fissionable material whichcan be defined as an element or isotope whose nuclei can be caused toundergo nuclear fission by nuclear bombardment and to produce a fissionchain reaction. It can be one or all of the following U235, U233and Pu239Nuclear reactor:
A nuclear reactor may be regarded as a substitute for theboiler furnace of a steam power plant. Heat is produced in the reactor due tonuclear fission of the fuel. During the fission process, the large amount of heat is liberated. This large amount of heat is absorbed by the coolant and itis circulated through the core. The various types of reactors used in nuclear power plant is1.Boiling water reactor2.Pressurised water reactor3.Fast breeder reactor
Steam generator:
The heat liberated in the reactor is taken up by thecoolant circulating through the core. The purpose of the coolant is to transferthe heat generated in the reactor core and use it for steam generation.Ordinary water or heavy water is a common coolant.
Moderator:
It is used to reduce the kinetic energy of fast neutrons into slowneutrons and to increase the probability of chain reaction. Graphite, heavywater and beryllium are generally used as moderator. A moderator shouldpossess the following properties:1.It should have high thermal conductivity2.It should be available in large quantities in pure form3.It should have high melting point in case of solid moderators and lowmelting point in case of liquid moderators. Solid moderators shouldalso possess good strength and machinability.4.It should provide good resistance to corrosion5.It should be stable under heat and radiation6.It should be able to slow down neutrons
Control rods:
They regulate the rate of a chain reaction. They are made of boron, cadmium or other elements which absorb neutrons. Control rodsshould posses the following properties:1.They should have adequate heat transfer properties2.They should be stable under heat and radiation3.They should be corrosion resistant4.They should be sufficient strong and should be able to shut down thereactor almost instantly under all conditions.5.They should have sufficient cross sectional area for the absorption.
Reflector:
The neutrons produced during the fission process will be partlyabsorbed by the fuel rods, moderator, coolant or structural material etc.Neutrons left unabsorbed will try to leave the reactor core and will be lost.Such loss is minimized by surrounding the reactor core by a material calledreflector which will send the neutrons back into the core. The returnedneutrons can then cause more fission and improve the neutrons economy of the reactor. Generally the reflector is made up of graphite and beryllium.
Turbine:
The steam produced in the steam generator is passed to theturbine. Work is done by the expansion of stem in the turbine.
Condenser:
The exhaust steam from the turbine flows to the condenserwhere cooling water is circulated. The exhaust steam is condensed to waterin the condenser by cooling. The condensate is pumped again into the steamgenerator by the feed pump.
Shielding:
The reactor is a source of intense radioactivity. These radiationsare very harmful and shielding is provided to absorb the radioactive rays. Athick concrete shielding and a pressure vessel are provided to prevent theradiations escaped to atmosphere.
1.9.4 Working of a Nuclear Power plant
The reactor of a nuclear power plant is similar to the furnace of steampower plant. The heat liberated in the reactor due to the nuclear fission of the fuel is taken up by the coolant circulating through the reactor core. Hotcoolant leaves the reactor at top and then flows through the tubes of steamgenerator (boiler) and passes on its heat to the feed water. The steamproduced is passed through the turbine and after work has been done byexpansion of steam in the turbine, steam leaves the turbine and flows tocondenser. Pumps are provided to maintain the flow of coolant, condensateand feed water
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