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Anyone with internet access can learn how to make a nuclear bomb. To turn theory into practice, however, they will need to beg, borrow or steal some plutonium. But for Britain at the end of the Second World War, the only way to obtain fissile materials was to make them. And that could only be done with an atomic pile. In 1946, work began on building two piles at a former Royal Ordnance factory at Sellafield in Cumbria. And with the new role came a new name: Windscale. By today's standards, the reactors were primitive, using uranium for fuel and graphite to keep the reaction under control. And whereas modern reactors use carbon dioxide for cooling, the Windscale piles used air - a big mistake, since air supports combustion.

By 1952, Windscale had produced enough plutonium to make Britain's first warhead. Unfortunately, the piles themselves had proved to be potential dirty bombs. The problem lay with the graphite, which unexpectedly grew in size when irradiated at relatively low temperatures.

And as it grew, it stored energy. At the time, nobody understood why. What they did understand, however, was that if the graphite took on too much energy, it would catch fire. Paradoxically, the only way of releasing this energy was to let the graphite get hotter by turning off the cooling system.

This procedure was carried out successfully 15 times. But on Thursday October 10, 1957, things went badly wrong. First the temperature was allowed to rise too high, then the energy was released too quickly. As a result, the graphite ignited, quickly followed by the uranium. In desperation, staff injected water into the pile. But while this extinguished the fire, it also released a plume of contaminated steam into the sky. An official report 25 years later suggested that 32 deaths and 260 cases of cancer resulted, although some independent experts have blamed the blaze for more than 1,000 deaths. One thing is certain. A belated victim of the fire, and subsequent lesser incidents, was the name Windscale, which was eventually changed back to Sellafield in the interests of public relations.

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