The two graphite-moderated reactors, referred to at the time as "piles", had been built as part of the British post-war atomic bomb project.

[1][2][3] At the time of the incident, no one was evacuated from the surrounding area, but milk from about 500 km2 (190 square miles) of the nearby countryside was diluted and destroyed for about a month due to concerns about its radiation exposure.

The UK government played down the events at the time, and reports on the fire were subject to heavy censorship, as Prime Minister Harold Macmillan feared the incident would harm British-American nuclear relations.

[12] During the Second World War, Frisch and Rudolf Peierls at the University of Birmingham calculated the critical mass of a metallic sphere of pure uranium-235, and found that as little as 1 to 10 kilograms (2.2 to 22.0 lb) might explode with the power of thousands of tons of dynamite.

[18] The British government had assumed that America would continue to share nuclear technology, which it considered a joint discovery,[19] but little information was exchanged immediately after the war.

[25] The Tube Alloys Directorate was transferred from the Department of Scientific and Industrial Research to the Ministry of Supply on 1 November 1945,[26] and Lord Portal was appointed Controller of Production, Atomic Energy (CPAE), with direct access to the Prime Minister.

[32] The 8 January 1947 meeting of the Gen 163 Committee, a subcommittee of the Gen 75 Committee, agreed to proceed with the development of atomic bombs, and endorsed Portal's proposal to place Penney, now the Chief Superintendent Armament Research (CSAR) at Fort Halstead in Kent, in charge of the development effort,[23] which was codenamed High Explosive Research.

"[34] Through their participation in the wartime Tube Alloys and Manhattan Project, British scientists had considerable knowledge of the production of fissile materials.

While everyone would have liked to pursue every avenue, as the Americans had, it was doubtful whether the cash-strapped post-war British economy could afford the money or the skilled manpower that this would require.

They estimated that a uranium-235 bomb would require ten times the fissile material as one using plutonium to produce half the TNT equivalent.

[37] 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.

[40] Lacking any location where a 30-mile area could be abandoned if a similar event occurred in the UK, the designers desired a passively safe cooling system.

[41] Raising the issue at a meeting, he suggested filters be added to the chimneys, but his concerns were dismissed as too difficult to deal with and not even recorded in the minutes.

[45] In spite of these precautions and the stack filters, scientist Frank Leslie discovered radioactivity around the site and the village, but this information was kept secret, even from the staff at the station.

Hungarian-American physicist Eugene Wigner had discovered that graphite, when bombarded by neutrons, suffers dislocations in its crystalline structure, causing a build-up of potential energy.

[48] The sudden bursts of energy worried the operators, who turned to the only viable solution, heating the reactor core in a process known as annealing.

[50] Winston Churchill publicly committed the UK to building a hydrogen bomb, and gave the scientists a tight schedule in which to do so.

But by raising the temperature of the reactor beyond the design specifications, the scientists had altered the normal distribution of heat in the core, causing hot spots to develop in Pile 1.

"An inspection plug was taken out," said Tom Hughes in a later interview, "and we saw, to our complete horror, four channels of fuel glowing bright cherry red."

[48] The new gas-cooled Calder Hall reactors on the site had just received a delivery of 25 tonnes of liquid carbon dioxide and this was rigged up to the charge face of Windscale Pile 1, but there were problems getting it to the fire in useful quantities.

Tuohy once again hauled himself onto the reactor shielding and ordered the water to be turned on, listening carefully at the inspection holes for any sign of a hydrogen reaction as the pressure was increased.

It was thought that the remaining fuel could still reignite if disturbed, due to the presence of pyrophoric uranium hydride formed in the original water dousing.

[70] Partly because of this censorship, consensus on the extent of the long-term health impacts caused by the radiation leak has changed over time as more information on the incident has come to light.

[71] The release of the highly dangerous radioactive isotope polonium-210, which had been covered up at the time, was not factored into government reports until 1983, when it was estimated that the fallout had caused 33 cancer fatalities in the long-term.

Some considered that the determination and courage shown by Thomas Tuohy, and the critical role he played in the aversion of complete disaster, had not been properly recognised.

[59] The Board of Inquiry's report concluded officially that the fire had been caused by "an error of judgment" by the same people who then risked their lives to contain the blaze.

Epidemiological estimates put the number of additional cancers caused by the Three Mile Island accident at not more than one; only Chernobyl produced immediate casualties.

[citation needed] The accident at Windscale was also contemporary to the Kyshtym disaster, a far more serious accident[citation needed], which occurred on 29 September 1957 at the Mayak plant in the Soviet Union, when the failure of the cooling system for a tank storing tens of thousands of tons of dissolved nuclear waste resulted in a non-nuclear explosion.

[citation needed] The Windscale fire was retrospectively graded as level 5, an accident with wider consequences, on the International Nuclear Event Scale.

Titled Our Reactor is on Fire, the documentary featured interviews with key plant workers, including Tom Tuohy, deputy general manager of Windscale at the time of the incident.