The Windscale fire of 10 October 1957 was the worst nuclear accident in the United Kingdom's history, ranked in severity at level 5 out of 7 on the International Nuclear Event Scale. The fire was in Unit 1 of the two-pile Windscale site (now Sellafield) on the north-west coast of England in Cumberland. 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. Windscale Pile No. 1 was operational in October 1950, followed by Pile No. 2 in June 1951.
The fire burned for three days and released radioactive fallout which spread across the UK and the rest of Europe. The radioactive isotope iodine-131, which may lead to cancer of the thyroid, was of particular concern at the time. It has since come to light that small but significant amounts of the highly dangerous radioactive isotope polonium-210 were also released. Calculations based on the amount of radiation released estimate that the accident may have caused 190 cases of cancer, with around 100 of these being fatal, however recent epidemiological studies failed to find any such increase.
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 destroyed for about a month afterward due to concerns about it being contaminated with iodine-131. 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.
The event was not an isolated incident; there had been a series of radioactive discharges from the piles in the years leading up to the accident. In early 1957, there had been a leak of radioactive material in which strontium-90 was released into the environment. Like the later fire, this incident was covered up by the British government. Later studies on the release of radioactive material due to the Windscale fire revealed that much of the contamination had resulted from such radiation leaks before the fire.
A 2010 study of workers involved in the cleanup of the accident found no significant long-term health effects from their involvement.
The December 1938 discovery of nuclear fission by Otto Hahn and Fritz Strassmann following its prediction by Ida Noddack in 1934 – and its explanation and naming by Lise Meitner and Otto Frisch – raised the possibility that an extremely powerful atomic bomb could be created. 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.
In response, the British government initiated an atomic-bomb project, codenamed Tube Alloys. The August 1943 Quebec Agreement merged Tube Alloys with the American Manhattan Project. As overall head of the British contribution to the Manhattan Project, James Chadwick forged a close and successful partnership with the Americans, and ensured that British participation was complete and wholehearted.
After the war ended, the Special Relationship between Britain and the United States "became very much less special". The British government had assumed that America would continue to share nuclear technology, which it considered a joint discovery, but little information was exchanged immediately after the war. The Atomic Energy Act of 1946 (McMahon Act) officially ended technical cooperation. Its control of "restricted data" prevented the United States' allies from receiving any information.
The British government saw this as a resurgence of United States isolationism akin to that which had occurred after the First World War. This raised the possibility that Britain might have to fight an aggressor alone. It also feared that Britain might lose its great power status, and therefore its influence in world affairs. The prime minister of the United Kingdom, Clement Attlee, set up a cabinet sub-committee, the Gen 75 Committee (known informally as the "Atomic Bomb Committee"), on 10 August 1945 to examine the feasibility of a renewed nuclear weapons programme.
The Tube Alloys Directorate was transferred from the Department of Scientific and Industrial Research to the Ministry of Supply on 1 November 1945, and Lord Portal was appointed Controller of Production, Atomic Energy (CPAE), with direct access to the prime minister. An Atomic Energy Research Establishment (AERE) was established at RAF Harwell, south of Oxford, under the directorship of John Cockcroft. Christopher Hinton agreed to oversee the design, construction and operation of the new nuclear weapons facilities, which included a uranium metal plant at Springfields in Lancashire, and nuclear reactors and plutonium processing facilities at Windscale in Cumbria. He established his headquarters in a former Royal Ordnance Factory at Risley in Lancashire on 4 February 1946.
In July 1946, the Chiefs of Staff Committee recommended that Britain acquire nuclear weapons. They estimated that 200 bombs would be required by 1957. 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, which was codenamed High Explosive Research. Penney contended that "the discriminative test for a first-class power is whether it has made an atomic bomb and we have either got to pass the test or suffer a serious loss of prestige both inside this country and internationally."
Through their participation in the wartime Tube Alloys and Manhattan Project, British scientists had considerable knowledge of the production of fissile materials. The Americans had created two kinds, uranium-235 and plutonium, and had pursued three different methods of uranium enrichment. An early decision had to be made as to whether High Explosive Research should concentrate on uranium-235 or plutonium. 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.
The scientists who had remained in Britain favoured uranium-235, but those who had been working in America were strongly in favour of plutonium. They estimated that a uranium-235 bomb would require ten times the fissile material as one using plutonium to produce half the TNT equivalent. Estimates of the cost of nuclear reactors varied, but it was reckoned that a uranium enrichment plant would cost ten times as much to produce the same number of atomic bombs as a reactor. The decision was therefore taken in favour of plutonium.
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 (12 inches) long encased in an aluminium canister to protect it from the air, as uranium becomes highly reactive when hot and can catch fire.
The cartridges were 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 burnup of the fuel would have been kept low to reduce production of the heavier plutonium isotopes like plutonium-240 and plutonium-241.