Calder Hall, the world’s first true nuclear power station, arose from the British military need for plutonium. Having decided in 1947 to build atomic weapons, Britain opted for a version of the plutonium-based bomb dropped on Nagasaki. The alternative required enriched uranium, which the UK had then no ability to manufacture.
With water-cooled reactors seen as too dangerous, Britain opted for two air-cooled ‘piles’ at Windscale − later renamed Sellafield. These primitive reactors produced enough plutonium for ten bombs a year and provided the fuel for the successful test of a UK atomic bomb at the Monte Bello islands, off Australia, in October 1952. But their heat was simply vented to the atmosphere.
With the worsening of the Cold War and the start of the Korean War in 1950, the British military wanted enough material for 200 bombs. The Windscale piles were not up to this task so a new reactor was needed.