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.

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.

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.

[14] In July 1940, Britain offered the United States access to its scientific research,[15] and Cockcroft briefed American scientists on British nuclear weapons developments.

[19] 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.

[32] 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".

[38] 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.

To gain access to the stockpile, they reopened negotiations, which resulted in the 1948 Modus Vivendi,[55] which allowed for consultation on the use of nuclear weapons, and limited sharing of technical information.

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.

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.

[73] Churchill, who had replaced Attlee as prime minister,[74] turned to Lord Cherwell for advice on the prospect of producing a British hydrogen bomb.

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.

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.

[80] Thus, it was hoped that the development of thermonuclear weapons would shore up Britain's great power status and restore the special relationship with the United States, which would give the UK a prospect of influencing American defence policy.

[83] There was a third political consideration: the Lucky Dragon incident had touched off a storm of protest, and there were calls from trade unions and the Labour Party for a moratorium on nuclear testing,[84] resulting in an acrimonious debate in the House of Commons on 5 April 1954 in which Churchill blamed Attlee for the McMahon Act.

He was a strong supporter of the atomic energy programme,[89] but while he agreed with its size and scope, he was critical of its organisation, which he blamed for slower progress than its Soviet counterpart.

Within a month of assuming office, Cherwell had prepared a memorandum proposing that responsibility for the program be transferred from the Ministry of Supply to an Atomic Energy Commission.

[108] The Australian Minister for Supply Howard Beale, responding to rumours reported in the newspapers,[109] asserted that "the Federal Government has no intention of allowing any, hydrogen bomb tests to take place in Australia.

This was smaller than Green Granite I, and could fit into a Yellow Sun casing that could be used by the Blue Steel guided missile then under development; but it could not be made ready to reach Christmas Island before 26 June 1957, and extending Operation Grapple would have cost another £1.5 million.

The Minister of Defence, Duncan Sandys, queried Cook on the imperative to persist with thermonuclear designs, given that Orange Herald satisfied most military requirements, and the tests were very expensive.

Cook replied that megaton-range fission bombs represented an uneconomical use of expensive fissile material, that they could not be built to produce yields of more than a megaton, and that they could not be made small enough to be carried by aircraft smaller than the V-bombers, or on missiles.

Sandys was not convinced, but he authorised further tests, as did the Prime Minister,[142] now Harold Macmillan following Eden's resignation in the wake of the Suez crisis.

Corner and his theoretical physicists at Aldermaston argued that Green Granite could be made to work by increasing compression and reducing Taylor instability.

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).

Ken Allen had an idea, which Sam Curran supported, of a three-layer Dick that used lithium deuteride that was less enriched in lithium-6 (and therefore had more lithium-7), but more of it, reducing the amount of uranium-235 in the centre of the core.

Keith Roberts calculated that the yield could reach 3 megatonnes of TNT (13 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.

[159] 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.

[160] 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.

The United States Atomic Energy Commission (AEC) invited the British government to send representatives to a series of meetings in Washington, DC, on 27 and 28 August 1958 to work out the details.