The UK has a long history of using graphite as a moderator, from the early Magnox reactors to the advanced gas-cooled reactors (AGRs) such as the B-1 and B-2 reactors at Hunterston, which came online in 1976 and 1977.

The moderator slows down the speed of neutrons produced during nuclear fission and helps to sustain the chain reaction so that the heat can be used for electricity production. In both of these reactor designs, the core is constructed from thousands of interlocking graphite bricks which also form a large number of important channels. These channels contain the nuclear fuel, the reactor control rods and allow the passage of carbon dioxide coolant gas to remove heat from the reactor fuel and core.