Boiling Water (B.W.R.), Pressurized Water (P.W.R.) and Fast Breeder Reactor (F.B.R.)
Boiling Water Reactor (B.W.R.):
Fig. shows nuclear power plant using B.W.R. In this reactors enriched uranium (enriched uranium contains more fissionable isotope U235 then the naturally occurring percentage 0.7%) is used as nuclear fuel and water is used as coolant. Water enters the reactors at the bottom. It takes up the heat generated due to the fission of fuel and gets converted into steam. Steam leaves the reactor at the top and flows into the turbine. Water also serves as moderator. India’s first nuclear power plant at Tarapur has two reactors (each of 200 MW capacity) of boiling water reactor type.
Pressurized Water Reactors (P.W.R.)
A P.W.R. nuclear plant is shown in Fig. It uses enriched U as fuel. Water is used as coolant and moderator. Water passes through the reactor core and takes up the heat liberated due to nuclear fission of the fuel. In order that water may not boil (due to its low boiling point 212 F at atmospheric conditions) and remain in liquid state it is kept under a pressure of about 1200 р.s..g. by the pressurizer. This enables water to take up more heat from the reactors. Form the pressurizer water flows to the steam generator where it passes on its heat to the feed water which in turn gets converted into steam.
Fast Breeder Reactors (F.B.R.)
Fig. shows a fast breeder reactor system In this reactors, the core containing U235 is surrounded by a blanket (a layer of fertile material placed outside the core) or fertile material U238. In this reactors no moderator is used. The fast-moving neutrons liberated due to fission of U235 are absorbed by U238 which gets converted into fissionable material Pu239 which is capable of sustaining chain reaction. Thus this reactor is important because it breads fissionable material from fertile material U238 available in large quantities. Like sodium graphite nuclear reactors this reactor also uses two liquid metal circulated through the tubes of intermediate heat exchange transfers its heat to
secondary coolant sodium-potassium alloy. The secondary coolant while flowing through the tubes of steam generator transfer its heat to feed water. Fast breeder reactors are better than conventional reactor both from the point of view of safety and thermal efficiency. For India which already is fast advancing towards self-reliance in the field of nuclear power technology, the fast breeder reactor becomes inescapable in view of the massive reserves of thorium and the
finite limits of its uranium resources. The research and development efforts in the fast breeder reactors technology will have to be stepped up considerably if nuclear power generation is to make any impact on the country’s total energy needs in the not too distant future.
Coolants for Fast Breeder Reactors:
The commonly used coolants for fast breeder reactors are as follows:
(i) Liquid metal (Na or NaK)
(ii) Helium (He)
(iii) Carbon dioxide
Sodium has the following advantages
(i) It has very low absorption cross-sectional area.
(ii) It possesses good heat transfer properties at high temperature and low pressure.
(iii) It does not react on any of the structural materials used in primary circuits.
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