The British hydrogen bomb programme was the ultimately successful British effort to develop hydrogen bombs between 1952 and 1958. During the early part of the Second World War, Britain had a nuclear weapons project, codenamed Tube Alloys. At the Quebec Conference in August 1943, British prime minister Winston Churchill and United States president Franklin Roosevelt signed the Quebec Agreement, merging Tube Alloys into the American Manhattan Project, in which many of Britain's top scientists participated. The British government trusted that America would share nuclear technology, which it considered to be a joint discovery, but the United States Atomic Energy Act of 1946 (also known as the McMahon Act) ended technical cooperation. Fearing a resurgence of American isolationism, and the loss of Britain's great power status, the British government resumed its own development effort, which was codenamed "High Explosive Research".

The successful nuclear test of a British atomic bomb in Operation Hurricane in October 1952 represented an extraordinary scientific and technological achievement. Britain became the world's third nuclear power, reaffirming the country's status as a great power, but hopes that the United States would be sufficiently impressed to restore the nuclear Special Relationship were soon dashed. In November 1952, the United States conducted the first successful test of a true thermonuclear device or hydrogen bomb. Britain was therefore still several years behind in nuclear weapons technology. The Defence Policy Committee, chaired by Churchill and consisting of the senior Cabinet members, considered the political and strategic implications in June 1954, and concluded that "we must maintain and strengthen our position as a world power so that Her Majesty's Government can exercise a powerful influence in the counsels of the world." In July 1954, Cabinet agreed to proceed with the development of thermonuclear weapons.

The scientists at the United Kingdom Atomic Energy Authority's Atomic Weapons Establishment at Aldermaston in Berkshire included William Penney, William Cook, Ken Allen, Samuel Curran, Henry Hulme, Bryan Taylor and John Ward. They did not know how to build a hydrogen bomb, but produced three designs: Orange Herald, a large boosted fission weapon; Green Bamboo, an interim thermonuclear design; and Green Granite, a true thermonuclear design. The first series of Operation Grapple tests involved Britain's first airdrop of a thermonuclear bomb. Although hailed as a success at the time, the first test of the Green Granite design was a failure. The second test validated Orange Herald as a usable design of a megaton weapon, but it was not a thermonuclear bomb, and the core boosting did not work. A third test attempted to correct the Green Granite design, but was another failure.

In the Grapple X test in November 1957, they successfully tested a thermonuclear design. The Grapple Y test the following April obtained most of its yield from nuclear fusion, and the Grapple Z test series later that year demonstrated a mastery of thermonuclear weapons technology. An international moratorium on nuclear tests commenced on 31 October 1958, and Britain ceased atmospheric testing for good. The successful development of the hydrogen bomb, along with the Sputnik crisis, resulted in the 1958 US–UK Mutual Defence Agreement, in which the nuclear Special Relationship was restored.

Background Tube Alloys

The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932, and in April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. In December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slow neutrons, and discovered that barium had been produced, and therefore that the uranium nucleus had been split. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Robert Frisch, developed a theoretical explanation of the process. By analogy with the division of biological cells, they named the process "fission".

The discovery of fission raised the possibility that an extremely powerful atomic bomb could be created. Frisch and Rudolf Peierls, both German refugee scientists working in Britain, calculated the critical mass of a metallic sphere of pure uranium-235, and found that instead of tons, as everyone had assumed, as little as would suffice, and would explode with the power of thousands of tons of dynamite. The MAUD Committee was established to investigate further. It reported that an atomic bomb was technically feasible, and recommended pursuing its development as a matter of urgency. A new directorate known as Tube Alloys was created to coordinate this effort. Sir John Anderson, the Lord President of the Council, became the minister responsible, and Wallace Akers from Imperial Chemical Industries (ICI) was appointed the director of Tube Alloys.

Manhattan Project

In July 1940, Britain offered the United States access to its scientific research, and Cockcroft briefed American scientists on British nuclear weapons developments. He discovered that the American S-1 Project (later renamed the Manhattan Project) was smaller than the British, and not as far advanced. The two projects exchanged information, but did not initially combine their efforts, ostensibly over concerns about American security. Unknown to the British, their project had already been penetrated by atomic spies for the Soviet Union.

The United Kingdom did not have the manpower or resources of the United States, and despite its early and promising start, Tube Alloys fell behind its American counterpart. The British considered producing an atomic bomb without American help, but it would require overwhelming priority, disruption to other wartime projects was inevitable, and it was unlikely to be ready in time to affect the outcome of the war in Europe. At the Quebec Conference in August 1943, the Prime Minister, Winston Churchill, and the President of the United States, Franklin Roosevelt, signed the Quebec Agreement, which merged the two national projects. The Quebec Agreement established the Combined Policy Committee and the Combined Development Trust to coordinate their efforts. The 19 September 1944 Hyde Park Agreement extended both commercial and military cooperation into the post-war period.

A British mission led by Akers assisted in the development of gaseous diffusion technology at the SAM Laboratories in New York. Another, headed by Mark Oliphant, assisted with the electromagnetic separation process at the Berkeley Radiation Laboratory. Cockcroft became the director of the joint British-Canadian Montreal Laboratory. A British mission to the Los Alamos Laboratory was led by Chadwick, and later Peierls, which included several of Britain's most eminent scientists. As overall head of the British Mission, Chadwick forged a close and successful partnership, and ensured that British participation was complete and wholehearted.

End of American cooperation

With the end of the war the Special Relationship between Britain and the United States became, in the words of Margaret Gowing, "very much less special". The British government had trusted that America would share nuclear technology, which it considered a joint discovery. On 8 August 1945, the Prime Minister, Clement Attlee, sent a message to President Harry Truman in which he referred to both of them as "heads of the Governments which have control of this great force".

On 9 November 1945, Attlee and the Prime Minister of Canada, Mackenzie King, went to Washington, D.C., to confer with Truman about future cooperation in nuclear weapons and nuclear power. A Memorandum of Intention they signed replaced the Quebec Agreement. The three leaders agreed that there would be full and effective cooperation on atomic energy, but British hopes were soon disappointed. The Atomic Energy Act of 1946 (McMahon Act), which was signed into law by Truman on 1 August 1946, ended technical cooperation. Its control of "restricted data" prevented the United States' allies from receiving any information.

This partly resulted from the arrest for espionage of British physicist Alan Nunn May, who had worked in the Montreal Laboratory, in February 1946, while the legislation was being debated. It was but the first of a series of spy scandals. The arrest of Klaus Fuchs in January 1950, and the June 1951 defection of Donald Maclean, who had served as a British member of the Combined Policy Committee from January 1947 to August 1948, left Americans with a distrust of British security arrangements. The remaining British scientists working in the United States were denied access to papers that they had written just days before.

Resumption of independent UK efforts

Attlee set up a cabinet sub-committee, the Gen 75 Committee (known informally by Attlee as the "Atomic Bomb Committee"), on 10 August 1945 to examine the feasibility of an independent British nuclear weapons programme. The Chiefs of Staff Committee considered the issue of nuclear weapons in July 1946, and recommended that Britain acquire them. A nuclear reactor and plutonium-processing facility was approved by the Gen 75 committee on 18 December 1945 "with the highest urgency and importance". The decision to proceed was formally made on 8 January 1947 at a meeting of Gen 163, another cabinet subcommittee, and was publicly announced in the House of Commons on 12 May 1948. D notice No. 25 forbid the publication of details on the design, construction or location of atomic weapons. The project was given the cover name "High Explosive Research".

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. Uranium ore was stockpiled at Springfields. As the American nuclear programme expanded, its requirements became greater than the production of the existing mines. To gain access to the stockpile, they reopened negotiations, which resulted in the 1948 Modus Vivendi, which allowed for consultation on the use of nuclear weapons, and limited sharing of technical information.

As Chief Superintendent Armament Research (CSAR, pronounced "Caesar"), 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 on 3 October 1952. Britain thereby became the third country to test nuclear weapons. The first Blue Danube atomic bombs were delivered to Bomber Command in November 1953, although the V bombers to deliver them to their targets were not available until 1955. In the meantime, nuclear deterrence was provided by the United States Strategic Air Command, which had begun operating from British bases in 1949.

Decision