After the war, fearing that Britain would lose its great power status, the British government resumed the atomic bomb development effort, now codenamed High Explosive Research.

The successful test of an atomic bomb in Operation Hurricane in October 1952 represented an extraordinary scientific and technological achievement, but Britain was still several years behind the United States, which had developed the more powerful thermonuclear weapons in the meantime.

The second test, Grapple 2, was of Orange Herald; its 720-to-800-kilotonne-of-TNT (3,010 to 3,350 TJ) yield made it technically a megaton-range weapon, and the largest ever achieved by a single stage nuclear device.

The design of Grapple Y was notably successful because much of its yield came from its thermonuclear fusion reaction instead of fission of a heavy uranium-238 tamper—the dense material surrounding the core that kept the reacting mass together to increase its efficiency.

The revelation of a Canadian spy ring that included British physicist Alan Nunn May while the bill was being prepared caused the United States Congress to add the death penalty for sharing "restricted data" with foreign nations.

[8] Efforts to restore the nuclear Special Relationship with the United States over the following decade were dogged by repeated spy scandals, including the arrest of Klaus Fuchs in 1950,[9] and the defection of Guy Burgess and Donald Maclean in 1951.

[10] Fearing a resurgence of American isolationism and Britain losing its great power status, the British government restarted its own development effort,[11] now codenamed High Explosive Research.

[13] In November 1952, the United States conducted Ivy Mike, the first successful test of a true thermonuclear device or hydrogen bomb, a far more powerful form of nuclear weapons.

Anthony Eden, who replaced Churchill as prime minister after the latter's retirement on 5 April 1955,[20] gave a radio broadcast in which he declared: "You cannot prove a bomb until it has exploded.

[22] The Australian Minister for Supply, Howard Beale, responding to rumours reported in the newspapers,[23] asserted that "the Federal Government has no intention of allowing any hydrogen bomb tests to take place in Australia.

For safety and security reasons, in the light of the Lucky Dragon incident, in which the crew of a Japanese fishing boat were exposed to radioactive fallout from the American Castle Bravo nuclear test,[28] a large site remote from population centres was required.

[40] A United States Air Force (USAF) special weapons monitoring team was based there, and the airstrip was improved to allow its supporting Douglas C-124 Globemaster II to use it.

[41] South Pacific Air Lines (SPAL) had been granted permission by the United States and British governments to operate a flying boat service from Christmas Island.

[21] Makins reported in March 1956 that Admiral Arthur W. Radford, the Chairman of the Joint Chiefs of Staff, was willing to help so long as the dormant American claim to the island was not prejudiced.

[52] The light aircraft carrier HMS Warrior was the operation control ship, and the flagship of Commodore Peter Gretton, the overall Naval Task Group commander.

[54] HMS Narvik reprised the role of control ship it had in Hurricane; but it also participated in Operation Mosaic, and had very little time to return to the Chatham Dockyard for a refit before heading out to Christmas Island for Grapple.

At Singapore she embarked 55 Field Squadron, which came from Korea, having been left behind there when the rest of 28 Engineer Regiment had returned to England after supporting the 1st Commonwealth Division in the Korean War.

RAF aircraft were allowed to overfly the United States, even when carrying radioactive or explosive materials, thereby obviating the need for winterisation for the more northerly journey over Canada.

Amid growing public concern about the dangers of fallout, particularly from strontium-90 entering the food chain, a committee chaired by Sir Harold Himsworth was asked to look into the matter.

The Red Beard Tom was given an improved high explosive supercharge, a composite uranium-235 and plutonium core, and a beryllium tamper, thereby increasing its yield to 45 kilotonnes of TNT (190 TJ).

[117][118] As the final preparations were being made for the test on 8 November, Oulton was advised at 01:00 that a Shackleton had sighted the SS Effie, an old Victory ship now flying the Liberian flag, in the exclusion zone.

Keith Roberts calculated that the yield could reach 3 megatonnes of TNT (12.6 PJ), and suggested that this could be reduced by modifying the tamper, but Cook opposed this, fearing that it might cause the test to fail.

The cargo ship SS Tidecrest arrived at Christmas Island on 20 July, but the firing harness was lost at San Francisco International Airport on 1 August, and a replacement had to be flown out.

The third test was of Halliard, an unusual three-stage design with two nuclear-fission components followed by a thermonuclear stage that was supposedly immune to exposure from another bomb despite its not using boosting.

Air Chief Marshal Sir Harry Broadhurst, the head of Bomber Command, wished O'Connor luck; his XD827 would make the drop, with Squadron Leader Tony Caillard in XD827, the grandstand aircraft.

The Aldermaston weapon makers had now demonstrated all of the technologies that were needed to produce a megaton hydrogen bomb that weighed no more than 1 long ton (1.0 t) and was immune to premature detonation caused by nearby nuclear explosions.

[147] At the suggestion of Harold Caccia, the British Ambassador to the United States, Macmillan wrote to Eisenhower on 10 October urging that the two countries pool their resources to meet the challenge.

[148] British information security, or the lack thereof, no longer seemed so important now that the Soviet Union was apparently ahead and the United Kingdom had independently developed the hydrogen bomb.

[166] Various veterans' organisations then filed a class action lawsuit against the UK Ministry of Defence following the publication of the study, with many media outlets reporting on it at the time.

[177] A group of 1,011 British ex-servicemen were denied permission to sue the Ministry of Defence by the Supreme Court in March 2012, on the grounds that too much time had elapsed since they became aware of their medical conditions, under the terms of the Limitation Act 1980.