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.