On 10th October 1957 a nuclear accident at the Windscale reactor (now known as Sellafield) in Cumbria occurred when an aluminium cannister burst inside Unit (Pile) 1.
The resulting fire burned for three days, released radioactive fallout which spread across the UK and the rest of Europe and was the worst case of accidental release of radioactive materials in British history. Nobody died as a direct result of the accident, but it is estimated that it later resulted in several hundred cancer fatalities. As bad as this was, it could have been worse if John Cockcroft had not insisted that the Windscale reactor chimney stacks be fitted with filters. At the time these were mocked as "Cockcroft's Folly" because they were thought to be a hugely expensive and totally unnecessary addition.
If graphite has not been sufficiently annealed, its exposure to neutrons in a reactor can cause a very rapid, very significant build-up of heat. This is known as the Wigner Effect and was the cause of the accident. At the time, the UK government played down the accident because Prime Minister Harold Macmillan feared the incident would harm British-American nuclear relations.
Windscale - Britains Biggest Nuclear Disaster (YouTube. Originally BBC, 2007)
Staff at Harwell were asked to help determine the cause of the fire, and to characterise the Wigner effect. Bob Henson was one of the first engineers on the scene; he later wrote about his career at Harwell and that trip to Cumbria in his 2019 memoirs:
• Bob wrote these memoirs for his family. Chilton Computing gratefully acknowledges the cooperation of his wife, Alison, who has made a slightly redacted version of them available to us in the hope that the work Bob did becomes better known and appreciated. We also acknowledge the work of his friend and colleague Bob Loader who helped bring us all together.
In 1958 RTL made a film about Harwell's use of BEPO to characterise the Wigner Effect. There is no sound but Bob Loader has provided a brief description of what is seen.
