The Windscale Piles produced weapons-grade plutonium for Britain's atomic bomb programme. Pile No. 1 went critical in October 1950; Pile No. 2 followed in June 1951. They were air-cooled, graphite-moderated reactors built fast under the pressure of the Cold War. The British had little experience with how graphite behaved under sustained neutron bombardment. Eugene Wigner had discovered in America that graphite, hit by neutrons, accumulates dislocations in its crystal structure - and stores potential energy that can release as a sudden rush of heat. American advice had warned that such releases could ignite a reactor. By 1957, Windscale staff were managing Wigner energy through periodic anneals - deliberately heating the core to bleed off the stored energy slowly. To meet a tight deadline for hydrogen bomb production, the reactor fuel had been modified with lithium-magnesium cartridges and the cooling fins reduced, raising core temperatures beyond design. Hot spots developed in places the thermocouples could not see. The ninth Wigner release of Pile No. 1 began on 7 October.

Channel 20/53 began behaving oddly during that ninth anneal. The temperature there rose while others fell. On 8 October, a second nuclear heating was applied to coax the release across the core - too soon and too rapidly, the inquiry later concluded. By the early morning of 10 October, core temperatures were rising when they should have been falling. The cooling fans were sped up to try to chill the reactor; instead, they fanned a fire that operators did not yet know existed. Tom Hughes, second in command to the Reactor Manager, climbed to the charge face in protective gear. "An inspection plug was taken out," he said in a later interview, "and we saw, to our complete horror, four channels of fuel glowing bright cherry red." Tuohy then climbed up himself. By the morning of 11 October, about 120 channels were involved. Men in protective suits used steel rods to push fuel elements out the back of the reactor. The rods came out red-hot. They switched to scaffolding poles. They cleared surrounding channels to create a firebreak. Carbon dioxide was brought in from Calder Hall; it had no noticeable effect. At 08:55 on 11 October, knowing the risk of a hydrogen explosion, they started hosing water into the reactor at 300 gallons per minute, eventually 1,000 gallons per minute. The shutdown fans were switched off at 10:10. The fire began to come under control. It was finally out by 15:10 on 12 October.

When the Pile chimneys were being built in the late 1940s, physicist Terence Price had warned that burst fuel cartridges could release uranium oxide dust up the chimneys and into the air. He suggested filters. His concerns were dismissed and not even recorded in the meeting minutes. Sir John Cockcroft, leading the project, was sufficiently alarmed to order the filters built anyway - but the chimneys were already under construction, so the filters had to be installed at the top, hoisted up after the concrete had set. The cost and delay were mocked: people called them Cockcroft's Folly. During the 1957 fire, those filters trapped about 95 per cent of the radioactive dust the fire would otherwise have sent across northern England. "The word folly," Terence Price said afterwards, "did not seem appropriate after the accident." There is no precise estimate of how much worse the contamination would have been without Cockcroft's filters. The honest answer is: much.

The reactor released approximately 740 terabecquerels of iodine-131, 22 TBq of caesium-137, 12,000 TBq of xenon-133, and about 8.8 TBq of polonium-210 - the polonium being a particularly dangerous, highly radioactive alpha-emitter whose release was covered up at the time. The radioactive plume spread across the UK and Europe. No one was evacuated. Milk from about 500 square kilometres of nearby Cumbrian countryside was collected, diluted a thousandfold and dumped in the Irish Sea for about a month - a quiet, enormous act of food-safety triage that affected hundreds of farms. Prime Minister Harold Macmillan ordered original reports heavily censored, fearing damage to British-American nuclear relations on the eve of the 1958 Mutual Defence Agreement. The polonium release was not factored into government estimates until 1983. The most recent UK government estimate, from 1988, attributes about 100 long-term fatal cancers to the fire, plus 90 non-fatal cancers and 10 hereditary defects across forty to fifty years. A 2024 epidemiological study of people born in Cumbria during 1950-1958 found no elevated thyroid cancer risk from childhood iodine-131 exposure. Children born in Seascale between 1950-2006 showed significantly elevated rates of leukemia and non-Hodgkin lymphoma compared to the national average - a phenomenon that the Committee on Medical Aspects of Radiation in the Environment found "unlikely to be due to chance," but for which there is no confirmed cause. The Penney Report blamed "an error of judgement" by the same men who then risked their lives to contain the fire. Pile No. 1 is not scheduled for final decommissioning until 2037.