British hydrogen bomb programme

Sir John Anderson, the minister responsible for Tube Alloys

The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932,[2] and in April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons.[3] In December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slow neutrons,[4] and discovered that barium had been produced, and therefore that the uranium nucleus had been split.[5] Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Robert Frisch, developed a theoretical explanation of the process.[6] By analogy with the division of biological cells, they named the process "fission".[7]

The discovery of fission raised the possibility that an extremely powerful atomic bomb could be created.[8] 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 1 to 10 kilograms (2.2 to 22.0 lb) would suffice, and would explode with the power of thousands of tons of dynamite.[9][10] The MAUD Committee was established to investigate further.[11] It reported that an atomic bomb was technically feasible, and recommended pursuing its development as a matter of urgency.[12][13] 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.[14]

James Chadwick (left), the head of the British Mission, with Major General Leslie R. Groves Jr., the director of the Manhattan Project; Richard C. Tolman, his scientific advisor; and Henry DeWolf Smyth

In July 1940, Britain offered the United States access to its scientific research,[15] and Cockcroft briefed American scientists on British nuclear weapons developments.[16] He discovered that the American S-1 Project (later renamed the Manhattan Project) was smaller than the British, and not as far advanced.[12] The two projects exchanged information, but did not initially combine their efforts,[17] ostensibly over concerns about American security. Ironically, it was the British project that had already been penetrated by atomic spies for the Soviet Union.[18]

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.[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.[20] 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.[21] The Quebec Agreement established the Combined Policy Committee and the Combined Development Trust to coordinate their efforts.[22] The 19 September 1944 Hyde Park Agreement extended both commercial and military cooperation into the post-war period.[23]

A British mission led by Akers assisted in the development of gaseous diffusion technology at the SAM Laboratories in New York.[24] Another, headed by Mark Oliphant, assisted with the electromagnetic separation process at the Berkeley Radiation Laboratory.[25] Cockcroft became the director of the joint British-Canadian Montreal Laboratory.[26] A British mission to the Los Alamos Laboratory was led by Chadwick, and later Peierls, which included several of Britain's most eminent scientists.[27][28] As overall head of the British Mission, Chadwick forged a close and successful partnership,[29] and ensured that British participation was complete and wholehearted.[30]

President Harry Truman and prime ministers Clement Attlee and Mackenzie King boarding USS Sequoia for discussions about nuclear weapons, November 1945

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".[31] The British government had trusted that America would share nuclear technology, which it considered a joint discovery.[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".[32]

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.[33][34] A Memorandum of Intention they signed replaced the Quebec Agreement.[35] The three leaders agreed that there would be full and effective cooperation on atomic energy, but British hopes were soon disappointed.[36] The Atomic Energy Act of 1946 (McMahon Act), which was signed into law by Truman on 1 August 1946,[37] ended technical cooperation. Its control of "restricted data" prevented the United States' allies from receiving any information.[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.[39] It was but the first of a series of spy scandals. The arrest of Klaus Fuchs in January 1950,[40] 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.[41] The remaining British scientists working in the United States were denied access to papers that they had written just days before.[42]

William Penney, Chief Superintendent Armament Research, was in charge of atomic bomb development.

Attlee set up a cabinet sub-committee, the Gen 75 Committee (known informally by Attlee as the "Atomic Bomb Committee"),[43] on 10 August 1945 to examine the feasibility of an independent British nuclear weapons programme.[44] The Chiefs of Staff Committee considered the issue of nuclear weapons in July 1946, and recommended that Britain acquire them.[45] A nuclear reactor and plutonium-processing facility was approved by the Gen 75 committee on 18 December 1945 "with the highest urgency and importance".[46] The decision to proceed was formally made on 8 January 1947 at a meeting of Gen 163, another cabinet subcommittee,[47] 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.[48][49] The project was given the cover name "High Explosive Research".[50]

Production facilities were constructed under the direction of Christopher Hinton, who established his headquarters in a former Royal Ordnance Factory at Risley in Lancashire.[51] These included a uranium metal plant at Springfields,[52] nuclear reactors and a plutonium processing plant at Windscale,[53] and a gaseous diffusion uranium enrichment facility at Capenhurst, near Chester.[54] 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,[55] which allowed for consultation on the use of nuclear weapons, and limited sharing of technical information.[56][57]

As Chief Superintendent Armament Research (CSAR, pronounced "Caesar"), Penney directed bomb design from Fort Halstead.[58] In 1951 his design group moved to a new site at Aldermaston in Berkshire.[59] The first British atomic bomb was successfully tested in Operation Hurricane on 3 October 1952.[60] Britain thereby became the third country to test nuclear weapons.[61] The first Blue Danube atomic bombs were delivered to Bomber Command in November 1953,[62] although the V bombers to deliver them to their targets were not available until 1955.[63][64] In the meantime, nuclear deterrence was provided by the United States Strategic Air Command,[65] which had begun operating from British bases in 1949.[66]

Implicit in the creation of a hydrogen bomb was that it would be tested. Eden, who replaced Churchill as prime minister after the latter's retirement,[105] gave a radio broadcast in which he declared: "You cannot prove a bomb until it has exploded. Nobody can know whether it is effective or not until it has been tested."[106]

The survey ship HMNZS Lachlan

Testing of boosted designs was carried out by Operation Mosaic in the Monte Bello Islands in Western Australia in May and June 1956.[107] This was a sensitive matter; there was an agreement with Australia that no thermonuclear testing would be carried out there.[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. Nor has it any intention of allowing any experiments connected with hydrogen bomb tests to take place here."[110] Since the tests were connected with hydrogen bomb development, this prompted Eden to cable the Prime Minister of Australia, Robert Menzies, detailing the nature and purpose of the tests. He promised that the yield of the second, larger test would not be more than two and a half times that of the Operation Hurricane test,[108] which was 25 kilotonnes of TNT (100 TJ). Menzies cabled his approval of the tests on 20 June 1955.[111] The yield of the second test turned out to be 60 kilotonnes of TNT (250 TJ),[107][112] which was larger than the limit of 50 kilotonnes of TNT (210 TJ) for tests in Australia.[113]

Another test site was therefore required. For safety and security reasons, in light of the Lucky Dragon incident, a large site remote from population centres was required. Various remote islands in the South Pacific and Southern Oceans were considered, along with Antarctica. The Admiralty suggested the Antipodes Islands,[114] which are about 860 kilometres (530 mi) southeast of New Zealand.[115] In May 1955, the Minister for Defence, Selwyn Lloyd, concluded that the Kermadec Islands,[114] which lie about 1,000 kilometres (620 mi) northeast of New Zealand, would be suitable. They were part of New Zealand,[116] so Eden wrote to the Prime Minister of New Zealand, Sidney Holland, to ask for permission to use the islands. Holland refused, fearing an adverse public reaction in forthcoming elections. Despite reassurances and pressure from the British government, Holland remained firm. The search for a location continued, with Malden Island and McKean Island being considered.[114] The former became the frontrunner. Three Avro Shackletons from No. 240 Squadron RAF were sent to conduct an aerial reconnaissance and Holland agreed to send the survey ship HMNZS Lachlan to conduct a maritime survey.[117]

The test series was given the secret codename "Operation Grapple". Air Commodore Wilfrid Oulton was appointed task force commander,[118] with the acting rank of air vice marshal from 1 March 1956.[119] He had a formidable task ahead of him. Nearby Christmas Island was chosen as a base. It was claimed by both Britain and the United States, but the Americans were willing to let the British use it for the tests.[118] With pressure mounting at home and abroad for a moratorium on testing,[120] 1 April 1957 was set as the target date.[121] Oulton held the first meeting of the Grapple Executive Committee on New Oxford Street in London on 21 February 1956.[122] The RAF and Royal Engineers would improve the airfield to enable it to operate large, heavily loaded aircraft, and the port and facilities would be improved to enable Christmas Island to operate as a base by 1 December 1956. It was estimated that 18,640 measurement tons (21,110 m³) of stores would be required for the construction effort alone.[123] The tank landing ship HMS Narvik would reprise the role of control ship it had for Operation Hurricane; but as it was also required for Operation Mosaic, it had very little time to return to the Chatham Dockyard for a refit before heading out to Christmas Island for Operation Grapple.[121]

Having decided on a location and date, there still remained the matter of what would be tested. John Challens, whose weapons electronics group would have to produce the assembly, wanted to know the configuration of Green Granite. Cook ruled that it would use a Red Beard Tom, and would fit inside a Blue Danube casing for dropping. The design was frozen in April 1956. There were two versions of Orange Herald, large and small. They had similar cores, but the large version contained more explosive. The designs were frozen in July. Green Bamboo was also nominally frozen, but tinkering with the design continued. On 3 September, Corner suggested that Green Granite could be made smaller by moving the Tom and Dick closer together. This design became known as Short Granite. By January 1957, with the tests just months away, a tentative schedule had emerged. Short Granite would be fired first. Green Bamboo would follow if Short Granite was unsuccessful, but be omitted as unnecessary otherwise. Orange Herald (small) would be fired next. Because Short Granite was too large to fit into a missile or guided bomb, this would occur whether or not Short Granite was a success. Finally, Green Granite would be tested. In December 1956, Cook had proposed another design, known as Green Granite II. 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.[124]

The first test of the series was Grapple 1, of Short Granite. This bomb was dropped from a height of 14,000 metres (45,000 ft) by a Vickers Valiant bomber of No. 49 Squadron RAF piloted by Wing Commander Kenneth Hubbard, off the shore of Malden Island at 11:38 local time on 15 May 1957.[125] It was Britain's second airdrop of a nuclear bomb after the Operation Buffalo test at Maralinga on 11 October 1956, and the first of a thermonuclear weapon.[126] The United States had not attempted an airdrop of a hydrogen bomb until the Operation Redwing Cherokee test on 21 May 1956. Their bomb had landed 6.4 kilometres (4 mi) from the target;[127] Hubbard missed by just 382 metres (418 yd). The Short Granite's yield was estimated at 300 kilotonnes of TNT (1,300 TJ), far below its designed capability. Penney cancelled the Green Granite test and substituted a new weapon codenamed Purple Granite. This was identical to Short Granite, but with some minor modification to it; additional uranium-235 was added, and the outer layer was replaced with aluminium.[128] Despite its failure, the test was described as a successful thermonuclear explosion, and the government did not confirm or deny reports that the UK had become a third thermonuclear power.[129] When documents on the series were declassified in the 1990s, the tests were denounced as a hoax,[130] but the reports were unlikely to have fooled the American observers.[131]

The next test was Grapple 2, of Orange Herald (small). This bomb was dropped at 10:44 local time on 31 May by another 49 Squadron Valiant, piloted by Squadron Leader Dave Roberts.[132] It exploded with a force of 720 to 800 kilotonnes of TNT (3,000 to 3,300 TJ). The yield was the largest ever achieved by a single stage device,[133] and made it technically a megaton weapon, but it was close to Corner's estimate for an unboosted yield, and Hulme doubted that the lithium-6 deuteride had contributed at all.[134] This was chalked up to Taylor instability, which limited the compression of the light elements in the core.[133] The bomb was hailed as a hydrogen bomb, and the truth that it was actually a large fission bomb was kept secret by the British government until the end of the Cold War.[135][136]

An Operational Requirement (OR1142) had been issued in 1955 for a thermonuclear warhead for a medium-range ballistic missile, which became Blue Streak. This was revised in November 1955, with "megaton" replacing "thermonuclear". Orange Herald (small) could then meet the requirement.[137] A version was created as an interim megaton weapon in order to provide the RAF with one at the earliest possible date.[138] Codenamed Green Grass, the unsuccessful fusion boosting was omitted, and it used Green Bamboo's 72-lens implosion system instead of Orange Herald's 32. This allowed the amount of highly enriched uranium to be reduced from 120 kilograms (260 lb) to 75 kilograms (165 lb). Its yield was estimated at 0.5 megatonnes of TNT (2.1 PJ).[133] It was placed in a Blue Danube casing, and this bomb became known as Violet Club.[139] About ten were delivered before Yellow Sun became available.[138]

The third and final shot of the series was Grapple 3, the test of Purple Granite. This was dropped by a Valiant piloted by Squadron Leader Arthur Steele on 19 June.[140][132] The yield was a very disappointing 300 kilotonnes of TNT (1,300 TJ), even less than Short Granite. The changes had not worked.[134] "We haven't got it right", Cook told a flabbergasted Oulton. "We shall have to do it all again, providing we can do so before the ban comes into force; so that means as soon as possible."[141]

A re-think was required. Cook had the unenviable task of explaining the failure to the government. Henceforth, he would take a tighter grip on the hydrogen bomb programme, gradually superseding Penney. The scientists and politicians considered abandoning Green Granite. 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.[143] The earliest possible date was November 1957 unless the Operation Antler tests were cancelled, but the Foreign Office warned that a moratorium on nuclear testing might come into effect in late October.[142]

Hydrogen bombs produce tritium in situ and thus burn deuterium and lithium. Under conditions of sufficient temperature and pressure the reactions form a loop chain reaction known as Jetter's Cycle.

The scientists at Aldermaston had created a design incorporating staging, radiation implosion, and compression, but they had not mastered the design of thermonuclear weapons. Knowing that much of the yield of American and Soviet bombs came from fission in the uranium-238 tamper, they had focused on what they called the "lithium-uranium cycle", whereby neutrons from the fission of uranium would trigger fusion, which would produce more neutrons to induce fission in the tamper. However, this is not the most important reaction. Corner and his theoretical physicists at Aldermaston argued that Green Granite could be made to work by increasing compression and reducing Taylor instability. The first step would be achieved with an improved Tom. 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). The Dick was greatly simplified; instead of the 14 layers in Short Granite, it would have just three.[142][144] This was called Round A; a five-layer version was also discussed, which was called Round B. A third round, Round C, was produced, for diagnostics. It had the same three layers as Round A, but an inert layer instead of lithium deuteride. Calculations for Round B were performed on the new IBM 704, while the old Ferranti Mark 1 was used for the simpler Round A.[142]

The next trial was known as Grapple X. To save time and money, and as Narvik and the light aircraft carrier HMS Warrior were unavailable,[142] the bomb would be dropped off the southern tip of Christmas Island rather than off Malden Island, just 20 nautical miles (37 km; 23 mi) from the airfield where 3,000 men would be based.[145] This required a major construction effort to improve the facilities on Christmas Island, and those that had been constructed on Malden Island had to now be duplicated on Christmas Island.[142] Works included 26 blast-proof shelters, a control room, and tented accommodation.[146] Components of Rounds A and C were delivered to Christmas Island on 24, 27 and 29 October. Round B would not be available; to get the calculations for Round A completed, the IBM 704 had to be turned over to them, and there was no possibility of completing the Round B calculations on the Ferranti. On inspection, a fault was found in the Round A Tom, and the fissile core was replaced with the one from Round C.[147]

Round A was dropped by a Valiant bomber piloted by Squadron Leader Barney Millett at 08:47 on 8 November 1957.[148][149] This time the yield of 1.8 megatonnes of TNT (7.5 PJ) exceeded expectations; the predicted yield had only been 1 megatonne of TNT (4.2 PJ). But it was still below the 2 megatonnes of TNT (8.4 PJ) safety limit. This was the real hydrogen bomb Britain wanted, but it used a relatively large quantity of expensive highly enriched uranium. Due to the higher-than-expected yield of the explosion, there was some damage to buildings, the fuel storage tanks, and helicopters on the island.[150]

Vickers Valiant XD818 at the RAF Museum Cosford was the aircraft that dropped the bomb in the Grapple 1 test in May 1957.

The physicists at Aldermaston had plenty of ideas about how to follow up Grapple X. Possibilities were discussed in September 1957. One was to tinker with the width of the shells in the Dick to find an optimal configuration. If they were too thick, they would slow the neutrons generated by the fusion reaction; if they were too thin, they would give rise to Taylor instability. Another was to do away with the shells entirely and use a mixture of uranium-235, uranium-238 and deuterium. 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. This proposal was the one adopted in October, and it became known as "Dickens" because it used Ken's Dick. The device would otherwise be similar to Round A, but with a larger radiation case. The safety limit was again set to 2 megatonnes of TNT (8.4 PJ). 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. Because of the possibility of a moratorium on testing, plans for the test, codenamed Grapple Y, were restricted to the Prime Minister, who gave verbal approval, and a handful of officials.[151]

Air Vice Marshal John Grandy succeeded Oulton as Task Force commander.[152] The bomb was dropped off Christmas at 10:05 local time on 28 April 1958 by a Valiant piloted by Squadron Leader Bob Bates.[153][154] It had an explosive yield of about 3 megatonnes of TNT (13 PJ), and remains the largest British nuclear weapon ever tested.[154] The design of Grapple Y was successful because much of its yield came from its thermonuclear reaction instead of fission of a heavy uranium-238 tamper, making it a true hydrogen bomb, and because its yield had been closely predicted—indicating that its designers understood what they were doing.[144][155]

On 22 August 1958, Eisenhower announced a moratorium on nuclear testing, effective 31 October 1958. This did not mean an immediate end to testing; on the contrary, the United States, the Soviet Union and the United Kingdom all rushed to perform as much testing as possible before the deadline, which the Soviets did not meet, conducting tests on 1 and 3 November.[156] A new British test series, known as Grapple Z, commenced on 22 August. It explored new technologies such as the use of external neutron initiators, which had first been tried out with Orange Herald. Core boosting using tritium gas and external boosting with layers of lithium deuteride were successfully tested in the Pendant and Burgee tests, allowing a smaller, lighter Tom for two-stage devices.[157] The international moratorium commenced on 31 October 1958, and Britain ceased atmospheric testing for good.[158]