page_title: British hydrogen bomb programme
section_title: Decision
hierarchical_section_title: British hydrogen bomb programme / Decision
caption_attribution_description: English: The Castle Bravo nuclear test, the detonation of the most powerful thermonuclear device ever tested by the United States.
context_page_description: 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 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.
context_section_description: The successful test of an atomic bomb represented an extraordinary scientific and technological achievement. Britain became the world's third nuclear power, reaffirming its status as a great power, but hopes that the United States would be sufficiently impressed to restore the Special Relationship were soon dashed. On 1 November 1952, the United States conducted Ivy Mike, the first successful test of a true thermonuclear device (also known as a hydrogen bomb). Due to its physical size and use of cryogenic liquid deuterium, it was not suitable for use as a deliverable weapon, but the Castle Bravo test on 1 March 1954 used a much smaller device with solid lithium deuteride. Boosted by the nuclear fusion reaction in lithium-7, the yield of 15 megatonnes of TNT (63 PJ)  was more than twice what had been expected, and indeed was the largest detonation the Americans would ever carry out. This resulted in widespread radioactive fallout that affected 236 Marshall Islanders, 28 Americans, and the 23 crewmen of a Japanese fishing boat, the Daigo Fukuryū Maru (Lucky Dragon No. 5). Meanwhile, the Soviet Union tested Joe 4, a boosted fission weapon with a yield of 400 kilotonnes of TNT (1,700 TJ) on 12 August 1953. This was followed by Joe 19, a true two-stage thermonuclear weapon on 20 November 1954.
  Although the British Hurricane device was more advanced than the American Fat Man bombs of 1946, Britain was still several years behind in nuclear weapons technology, and while British and Soviet advances had taken much of the heat out of American opposition to renewed cooperation with the British, the United States Congress saw little benefit in it for the United States. The McMahon Act was amended by the Atomic Energy Act of 1954 on 30 August, which allowed for greater exchange of information with foreign nations, but it fell far short of what the British government wanted. Churchill, who had replaced Attlee as prime minister, turned to Lord Cherwell for advice on the prospect of producing a British hydrogen bomb. Cherwell reported that "We think we know how to make an H-bomb", but Penney did not agree with this sanguine assessment.
  A New Weapons Committee was established at Aldermaston on 15 October 1951 to examine improvements to their atomic bombs. John Corner, the head of the theoretical group at Aldermaston, suggested producing a device in the "megaton range"—one with a yield of 500 kilotonnes of TNT (2,100 TJ) or more. In this he was thinking not of a thermonuclear weapon, but of a large fission one. The idea was not pursued at that time, because the RAF wanted more, not bigger, atomic bombs. Meeting in Bermuda in December 1953 with Dwight D. Eisenhower, who had replaced Truman as president earlier that year, Churchill told him that the RAF had reckoned that fission bombs would be sufficient for most targets, and therefore that Britain had no intention of developing hydrogen bombs.
  
  On 12 and 19 March 1954, Penney briefed the Gen 475 Committee meetings, attended by the Chiefs of Staff, senior officials from the Ministry of Defence and Foreign Office, and Sir Edwin Plowden, about recent developments in thermonuclear weapons. Sir Frederick Brundrett, the chairman of the Chiefs of Staff's Working Party on the Operational Use of Atomic Weapons (OAW), then asked Penney on 25 May for a working paper for an OAW meeting on 31 May. In turn, OAW sent a report to the Chiefs of Staff, who recommended that the United Kingdom develop its own thermonuclear weapons. Admiral of the Fleet Sir Rhoderick McGrigor, the First Sea Lord, recalled that:
  The United Kingdom, as the recognised leader of the Commonwealth, and as a leading world power, had a position to maintain in world affairs. If our influence were to decline it would be virtually impossible to regain our rightful place as a world power. It was essential that the United Kingdom should have the ability to produce the H-Bomb in order that she could claim membership of the Allied H-Club. 
  There was another political consideration. The Lucky Dragon incident had touched off a storm of protest,
caption: The Castle Bravo test on 1 March 1954 was America's first test of a hydrogen bomb using solid thermonuclear fuel. Its yield was more than twice what had been expected, and was the largest ever American detonation.
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