A gas-cooled reactor (GCR) is a nuclear reactor that uses graphite as a neutron moderator and a gas (carbon dioxide or helium in extant designs) 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 to refer to this type of reactor.
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 in the 1950s only available from the United States or the Soviet Union. The Canadian CANDU reactor, using heavy water as a moderator, was designed with the same goal of using natural uranium fuel for similar reasons.
Historically thermal spectrum graphite moderated gas cooled reactors mostly competed with light water reactors, ultimately losing out to them after having seen some deployment in Britain and France. Heavy water reactors share some design considerations as both are capable in principle of using unenriched fuel but require online refueling to be viable power reactors.
There were two main types of generation I GCR:
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: two Magnox plants to Italy and Japan, and one UNGG to Spain. 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.
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.
Gas-cooled reactor types include: