Fuel: 92U235, 94Pu239, 92U233 are nuclear fuels used in nuclear reactor to release the heat energy. In uranium ore, 92U235 is available only up to 0.7% and the remaining is 92U238. The other two fuels 94Pu239 and 92U233 are formed in the reactor during the fission process of 92U235. 90Th232 is formed due to absorption of neutron by 92U235 without fission. In homogeneous reactors, the fuel (uranium) and moderator (carbon) are mixed homogeneously and used in the form of rods or plates in the reactor. In heterogeneous reactors, fuel is used in the form of rod and moderator surrounds it. The fuel rod is clad with aluminium, stainless steel or zirconium to prevent from oxidation.
Moderator: The moderator is a material which is used to reduce the kinetic energy of fast neutron (1 MeV or 13,200 km/s to 0.25 eV or 2,200 m/s) within fraction of second to maintain the fission chain reaction. H2, D2, N2, C, heavy water and Be are some important moderators. The ordinary water is used as moderator only with enriched uranium (enrichment is a process to increase the amount of 92U235) whereas graphite, heavy water, and beryllium can be used with natural uranium. The major characteristics of moderators are ability to slow down neutrons, resistant to corrosion, high melting point, high chemical stability, and non-absorbent of neutrons.
Control Rod: The function of control rod is to start the nuclear chain reaction when the reactor is started from cold, to maintain the chain reaction at steady state condition and to shut down the reactor automatically under emergency condition. The materials used for control rods must have very high absorbent capacity for neutrons. The common materials used for control rods are cadmium, boron or hafnium.
Shielding: Neutrons, gamma rays, and all other radiations are efficiently absorbed by the concrete and steel. The inner lining of the core is made of 50–60 cm thick steel plate and it is further thickened by few metres of concrete. The lining of steel plate absorbs these rays and becomes heated but prevent the adjacent wall of the reactor vessel from becoming heated. The thermal shield is cooled by circulating water.
Reactor Vessel: It consists of reactor core, reflector, and shield. It also provides the entrance and exit passage for directing the flow of the coolant. It should have withstanding capacity of more than 200 bar. At the top of the vessel a hole is provided to insert the control rods. The reactor core is generally placed at the bottom of the vessel.
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