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 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
only available from the United States or Soviet Union. 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: 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 nuc