The technologies that have been investigated for a gas-cooled fast reactor concept have been reviewed by Mitchell et al. (2002). The basic idea has been to extend the thermal reactor concept but with a fast reactor core. The existing technology gas-cooled breeder reactor (ETGBR) was based on an AGR technology with a carbon dioxide-cooled system. The gas-cooled fast reactor (GCFR) design of General Atomics takes a helium technology as its basis. The gas breeder reactor (GBR) covered four different design concepts in respect of carbon dioxide and helium as possible coolants, oxide pins vs. particle fuel, etc., see below. These are surveyed in Table 12.4.