When Britain began planning to build nuclear reactors to produce plutonium for weapons in 1946 , it was decided to build a pair of air-cooled graphite reactors similar to the X-10 Graphite Reactor at Windscale . Natural uranium was used as enriched was not available , and similarly , graphite was chosen as a neutron moderator because beryllia was toxic and hard to manufacture , while heavy water was unavailable . Use of water as a coolant was considered , but there were concerns about the possibility of a catastrophic nuclear meltdown in the densely populated British Isles if the cooling system failed . Helium was again the preferred choice as a coolant gas , but the main source of it was the United States , and under the 1946 McMahon Act , the United States would not supply it for nuclear weapons production , so , in the end , air cooling was chosen . Construction began in September 1947 , and the two reactors became operational in October 1950 and June 1951 . Both were decommissioned after the disastrous Windscale fire in October 1957 . They would be the last major air-cooled plutonium-producing reactors ; the UK 's follow-on Magnox and AGR designs used carbon dioxide instead
Windscale fire
Windscale fire
X-10 Graphite Reactor