|The Windscale Reactor|
To produce plutonium, the essential fission material for the bomb, they build the Windscale reactor, the first ever nuclear reactor in Britain. Political pressure for an urgently needed success and the risks they had to take to meet the deadline had their inevitable effects on the security.
To keep up with the United States, who by then already had their own hydrogen bomb, Britain cranked up the production of plutonium and tritium. The aluminium cooling fins of the fuel cartridges had been reduced to dissipate less heat. The resulting higher temperature increased reaction in the core, producing more of the badly needed fission material.
Not designed to operate under these conditions, the graphite core increasingly suffered from so-called Wigner energy, which caused sudden local heat releases at irregular intervals. On 10 October 1957, some of the refitted cartridges, containing enriched uranium and lithium-magnesium, caught fire and overheated the reactor's graphite core. In an attempt to cool down the reactor, the operators increased the airflow, causing the fire to spread throughout the reactor core. The fire was eventually extinguished after 48 hours by pumping water into the fuel channels.
In contrast to modern closed-circuit water-cooled reactors, the Windscale design used airflow to control the reactor core temperature, evacuating excessive heat through a large chimney into the air. Consequently, the fire caused a release of nuclear material across Britain and Europe, making it both the first and worst ever nuclear incident in Western Europe, rated 5 on the 7-point INES scale (Chernobyl in Eastern Europe rated 7). The air-cooled core design, used for the first time in Windscale, has been abandoned since.
The truth about the cause of the Windscale incident was kept secret for political reasons. It was one of the more sinister episodes of the Cold War race for the bomb, and hardly mentioned in history. The dismantling of Windscale's iconic chimney started last September. More about this technically challenging work on the Sellafield website which includes the complete demolition program, and on World Nuclear News. The BBC website has some historical images of Windscale.
There's an excellent BBC documentary about the Windscale nuclear disaster that you can watch here below or alternatively via this youtube link.