|High Explosive Research ("HER") was the British project to develop atomic bombs independently after the Second World War. This decision was taken by a cabinet sub-committee on 8 January 1947, in response to apprehension of an American return to isolationism, fears that Britain might lose its great power status, and the actions by the United States to withdraw unilaterally from sharing of nuclear technology under the 1943 Quebec Agreement. The decision was publicly announced in the House of Commons on 12 May 1948.
HER was a civil project, not a military one. Staff were drawn from and recruited into the Civil Service, and were paid Civil Service salaries. It was headed by Lord Portal, as Controller of Production, Atomic Energy, in the Ministry of Supply. An Atomic Energy Research Establishment was located at a former airfield, Harwell, in Berkshire, under the direction of John Cockcroft. The first nuclear reactor in the UK, a small research reactor known as GLEEP, went critical at Harwell on 15 August 1947. British staff at the Montreal Laboratory designed a larger reactor, known as BEPO, which went critical on 5 July 1948. They provided experience and expertise that would later be employed on the larger, production reactors.
Production facilities were constructed under the direction of Christopher Hinton, who established his headquarters in a former Royal Ordnance Factory at Risley in Lancashire. These included a uranium metal plant at Springfields, nuclear reactors and a plutonium processing plant at Windscale, and a gaseous diffusion uranium enrichment facility at Capenhurst, near Chester. The two Windscale reactors became operational in October 1950 and June 1951. The gaseous diffusion plant at Capenhurst began producing highly enriched uranium in 1954.
William Penney directed bomb design from Fort Halstead. In 1951 his design group moved to a new site at Aldermaston in Berkshire. The first British atomic bomb was successfully tested in Operation Hurricane, during which it was detonated on board the frigate HMS Plym anchored off the Monte Bello Islands in Australia on 3 October 1952. Britain thereby became the third country to test nuclear weapons, after the United States and the Soviet Union. The project concluded with the delivery of the first of its Blue Danube atomic bombs to Bomber Command in November 1953, but British hopes of a renewed nuclear Special Relationship with the United States were frustrated. The technology had been superseded by the American development of the hydrogen bomb, which was first tested in November 1952, only one month after Operation Hurricane. Britain went on to develop its own hydrogen bombs, which it first tested in 1957. A year later, the United States and Britain resumed nuclear weapons cooperation.