The Windscale Meltdown

The Windscale fire on 10 October 1957 was the worst nuclear accident in UK history and one of the worst in the world, with a severity rating of 5 out of a possible 7 on the International Nuclear Event Scale. The fire started in Unit 1 of the two-pile Windscale site in Cumberland, on the northwest coast of England (now Sellafield, Cumbria). The two graphite-moderated reactors, known as "piles" at the time, were built as part of the British post-war atomic bomb project. Windscale No. 1 went into service in October 1950, and Pile No. 2 in June 1951.

The fire burned for three days, releasing radioactive fallout that spread throughout the United Kingdom and the rest of Europe. At the time, the radioactive isotope iodine-131, which has been linked to thyroid cancer, was of particular concern. It has since been discovered that trace amounts of the highly dangerous radioactive isotope polonium-210 were also released. It is estimated that the radiation leak resulted in 240 additional cancer cases, 100 to 240 of which were fatal. No one was evacuated from the area at the time of the incident, but milk from about 500 km2 (190 square miles) of the nearby countryside was diluted and destroyed for about a month due to radiation concerns. At the time, the UK government downplayed the events, and reports on the fire were heavily censored, as Prime Minister Harold Macmillan feared the incident would harm British-American nuclear relations.

The accident was not an isolated occurrence; there had been a series of radioactive discharges from the piles in the years preceding it. Only months before the fire, in the spring of 1957, there was a leak of radioactive material in which strontium-90 isotopes were released into the environment. The British government, like the later fire, covered up this incident. Subsequent research into the release of radioactive material as a result of the Windscale fire revealed that much of the contamination was caused by such radiation leaks prior to the fire.

Date: 10 October 1957
Location: Windscale, Seascale, Cumbria (now Sellafield)
Outcome: INES Level 5 (accident with wider consequences)
Deaths: Estimated 100 to 240 cancer fatalities in the long term
Cause: A loss of coolant, loss of coolant pressure, or low coolant flow rate or be the result of a criticality excursion in which the reactor is operated at a power level that exceeds its design limits.