The nuclear reactors can be classified on the basis of neutron energy, fuel used, moderator used, and coolant used. On the basis of neutron energy, we can classify the nuclear reactor as fast reactor and thermal reactor; on the basis of fuel used, we can classify them as natural fuel reactor and enriched uranium reactor; on the basis of moderator, we can classify them as water moderated, heavy water moderated, graphite moderated, and beryllium moderated; on the basis of coolant, we can classify them as water cooled reactor and gas cooled reactor.
Pressurized Water Reactor (PWR): Fuel used in this reactor is enriched uranium (uranium oxide). Uranium oxide is highly resistant to irradiation damage and is very well adapted to minimize high burn-up. It is also highly resistant to corrosion by high pressure water in the event of break-up in the fuel cladding. The water becomes radioactive during passing from the reactor. Therefore, the entire primary circuit including steam generator must be shielded to protect the operating people. The radioactive coolant does not make the steam radioactive in the boiler.
Boiling Water Reactor (BWR): In this reactor, enriched uranium is used as fuel and water is used as coolant and moderator like PWR except the steam is generated in reactor itself.
Heavy Water Cooled and Moderated CANDU (Canadian Deuterium Uranium) Reactor: In this reactor, natural uranium is used as fuel and heavy water is used as coolant and moderator. The heavy water is passed through the pressure tubes in the reactor and heat exchanger in primary circuit in the same way as in PWR and the steam is raised in secondary circuit. Heat is transferred from heavy water to ordinary water in heat exchanger. The control of the reactor is achieved by level of moderator, i.e., heavy water in the reactor. Therefore, control rod is not required in this reactor.
Gas Cooled Reactor (GCR): The reactor is cooled by the gas and the heat carried away by the gas from the reactor is either used for generating steam in secondary circuit like PWR or it can be directly used in gas turbine.
Liquid Metal Cooled Reactor: The excellent heat transfer capacity of molten metals makes them attractive as reactor coolant. The liquid metal coolant is circulated through reactor at moderate pressure and high temperature (540°C). In this reactor, enriched uranium is used as fuel and graphite as moderator. The common metals which can be used as coolant may be sodium, potassium, etc. Sodium is most suitable coolant as it has low absorption, low melting point, high boiling point, high specific heat, high thermal conductivity, and considerably cheaper in cost.
Fast Breeder Reactor (FBR): In fast breeder reactor, enriched uranium or plutonium is kept in a casing without moderator. The casing is covered with a thick blanket of depleted fertile uranium. The ejected excess of neutrons are absorbed by the fertile blanket and it converts into fissile material. The heat produced in the reactor core is carried away by liquid metal. The major difficulty is to remove the large quantity of heat from the core as the power density is very high (430 kW) per litre of core volume which is 40 times greater than CANDU type reactor, 13 times greater than BWR and 20 times greater than gas cooled reactor.
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