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The Windscale fire of 10 October 1957 was the worst nuclear accident in Great Britain's history, ranked in severity at level 5 on the 7-point International Nuclear Event Scale. The fire took place in Unit 1 of the two-pile Windscale facility on the northwest coast of England in Cumberland (now Sellafield, Cumbria). The two piles had been built as part of the British atomic bomb project.〔Gowing, M, Independence and Deterrence, Vol 2, p 386 ff.〕 Windscale Pile No. 1 was operational in October 1950 followed by Pile No. 2 in June 1951. The fire burned for three days and there was a release of radioactive contamination that spread across the UK and Europe.〔 Of particular concern at the time was the radioactive isotope iodine-131, which may lead to cancer of the thyroid, and it has been estimated that the incident caused 240 additional cancer cases.〔 No one was evacuated from the surrounding area, but there was a worry that milk might be dangerously contaminated. Milk from about 500 km2 of nearby countryside was diluted and destroyed for about a month. A 2010 study of workers directly involved in the cleanup found no significant long term health effects from their involvement. ==Windscale Piles== After the Second World War, the British government embarked on a programme to build nuclear weapons. Skipping the lower-performance uranium-based weapons in favour of those based on plutonium, a plutonium-breeding reactor system was designed to produce this material, which is not found in nature. The design was based on the graphite-moderated B Reactor built at the Hanford Site, which was known to British physicists who had been involved in the Manhattan Project during the war. The reactors were built in a short time near the village of Seascale, Cumberland. They were known as Windscale Pile 1 and Pile 2, housed in large concrete buildings a few hundred feet apart. The core of the reactors consisted of a large block of graphite with horizontal channels drilled through it for the fuel cartridges. Each cartridge consisted of a uranium rod about 30 cm long encased in an aluminium canister to protect it from the air, as uranium becomes highly reactive when hot and can catch fire. The cartridge was finned, allowing heat exchange with the environment to cool the fuel rods while they were in the reactor. Rods were pushed in the front of the core, the "charge face", with new rods being added at a calculated rate. This pushed the other cartridges in the channel towards the rear of the reactor, eventually causing them to fall out the back, the "discharge face", into a water filled channel where they cooled and could be collected. The chain reaction in the core converted the uranium into a variety of isotopes, including some plutonium, which was separated from the other materials using chemical processing. As this plutonium was intended for weapons purposes, the burn-up of the fuel would have been kept low to reduce production of the heavier plutonium isotopes (240Pu, 241Pu etc.). The design initially called for the core to be cooled like the B Reactor, which used a constant supply of water that poured through the channels in the graphite. There was considerable concern that such a system was subject to catastrophic failure in the event of a loss-of-coolant accident. This would cause the reactor to run out of control in seconds, potentially exploding. At Hanford, this possibility was dealt with by constructing a escape road to evacuate the staff were this to occur, abandoning the site. Lacking any location where a 30 mile area could be abandoned if a similar event were to occur in the UK, the designers desired a passively safe cooling system. In place of water, they used air cooling driven by convection through a tall chimney, which could create enough airflow to cool the reactor under normal operating conditions. The chimney was arranged so it pulled air through the channels in the core, cooling the fuel via fins on the cartridges. For additional cooling, huge fans were positioned in front of the core, which could greatly increase the airflow rate. During construction, Terence Price, one of the many physicists working on the project, began to consider what would happen if one of the fuel cartridges being pushed out the back of the core were to break open. This could happen, for example, if a new cartridge being inserted was pushed too hard, causing the one at the back of the channel to fall past the relatively narrow water channel and strike the floor behind it. In that event, the hot uranium could catch fire, with the fine uranium oxide dust being blown up the chimney to escape. When he raised the issue at a meeting and suggested that filters be added to the chimneys, the concern was dismissed as being too difficult to deal with and was not even recorded in the minutes. Sir John Cockcroft was alarmed enough to order that filters be installed, which required them to be constructed on the ground while the chimneys were still being built, and then winched into position at the top once the chimney's concrete had set. These became known as "Cockcroft's Folly" by workers and engineers. In the end, Price's concerns came to pass. So many cartridges missed the water channel that it became routine for staff to walk through the chimney ductwork with shovels and scoop the cartridges back into the water. On other occasions, fuel cartridges became stuck in the channels and burst open while still in the core. In spite of these precautions and the stack filters, Frank Leslie had discovered radioactivity around the site, but this information was kept secret, even from the staff at the station. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Windscale fire」の詳細全文を読む スポンサード リンク
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