In this reactor the core containing U235 in surrounded by a blanket (a layer of fertile material placed outside the core) of fertile material U238.
In this reactor 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 breeds fissionable material from fertile material U238 available in large quantities like sodium graphite nuclear reactor this reactor also uses two liquid metal coolant circuits.
In this reactor, the primary coolant is liquid sodium. heat exchange transfers its heat to secondary coolant(sodium potassium alloy), when circulated through the tubes.
When the secondary coolant flowing through the tubes of steam generator transfers its heat to feed water. Fast breeder reactors are better than conventional reactors both from the point of view of safety and thermal efficiency.
The fast breeder reactor becomes inescapable in view of the massive reserves of thorium and the finite limits of its uranium resources.
The commonly used coolants for fast breeder reactors are as follows
Liquid metal (Na or NaK).
Helium (He)
Carbon dioxide
Sodium has the following advantages:
It has very low absorption cross-sectional area.
It possesses good heat transfer properties at high temperature and low pressure.
It doesn’t react on any of the structural materials used in primary circuits.
Gas cooled reactor
A gas-cooled reactor (GCR) is a nuclear reactor that uses graphite as a neutron moderator and carbon dioxide (helium can also be used) as coolant.
Although there are many other types of reactor cooled by gas, the terms GCR and to a lesser extent gas cooled reactor are particularly used.
The GCR was able to use natural uranium as fuel, enabling the countries that developed them to fabricate their own fuel without relying on other countries for supplies of enriched uranium, which was at the time of their development only available from the United States or Soviet Union.
There were two main types of generation I GCR
The Magnox reactors developed by the United Kingdom.
The UNGG reactors developed by France.
The main difference between these two types is in the fuel cladding material. Both types were mainly constructed in their countries of origin, with a few Export sales: Magnox plants to Italy and Japan, and a UNGG to Spain.
Both types used fuel cladding materials that were unsuitable for medium term storage under water, making reprocessing an essential part of the nuclear fuel cycle.
Both types were in their countries of origin, also designed and used to produce weapons-grade plutonium, but at the cost of major interruption to their use for power generation despite the provision of online refuelling.
In the UK, the Magnox was replaced by the advanced gas-cooled reactor (AGR), an improved Generation II gas cooled reactors. In France, the UNGG was replaced by the pressurized water reactor (PWR).
More recently, GCRs based on the declassified drawings of the early Magnox reactors have been constructed by North Korea at the Yongbyon Nuclear Scientific Research Center.
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