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         Frisch-Peierls Memorandum
         UK Decides to Develop Nuclear Weapons
         Aldermaston Airfield Taken Over
         First UK Nuclear Device Successfully Detonated
         Blue Danube Nuclear Bomb Delivered to RAF
         Grapple Series Begins at Christmas Island
         UK/US Agreement
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         UK Mounts First Underground Nuclear Test (UGT)
         WE 177 Free-Fall Bomb Enters Service
         First Polaris Subarine - HMS Resolution - Operational
         UK Starts Chevaline Programme
         Pochin Report Recommends Improved Safety Procedures
         HELEN Laser Opened by HM the Queen
         Mogul-D Commissioned
         AWRE Becomes Atomic Weapons Establishment (AWE)
         Government Announces AWE to be 'contractorized'
         Hunting-BRAE Ltd. Awarded Phase 2 Management Contract
         Cray C98D Super Computer Installed
         Plutonium Facility A90 Fully Operational
         North Ponds Water Management System Commissioned
         Fiftieth Anniversary of Opening AWRE Aldermaston
         AWE Management Ltd Win Management Contract
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Grapple Series Begins at Christmas Island

The culmination of Britain's atmospheric test programme was the Grapple series of trials at Malden Island and Christmas Island in the Pacific which took Britain into the thermonuclear era.

The classic shape of the mushroom cloud rises above Christmas Island, flowing the Grapple-Y test on 28 April 1958. With a yield of 3 megatons, it was Britain's biggest nuclear explosion.
The classic shape of the mushroom cloud rises above Christmas Island, flowing the Grapple-Y test on 28 April 1958. With a yield of 3 megatons, it was Britain's biggest nuclear explosion.

The broad aim of the tests was to achieve the aim of a one megaton yield from a one ton warhead. A range of designs was tested on this series: a new concept of boosting the yield using solid thermonuclear 'fuel' and an external neutron source was first successfully tested with the development device known as Orange Herald.

The first two-stage design to be tested was Short Granite; on 15 May 1957, it was dropped from a Valiant bomber. But, at 300 kilotons, the yield was far short of the one megaton target. The problem was that a lack of computing power was limiting the designers' ability to predict the performance of the device, making intuition and guess-work an inevitable part of the process.

Nevertheless the shot was regarded by the designers as a successful proof of principle. Help was on hand with the acquisition of Aldermaston's first computers. Ferranti and English Electric machines installed in 1955 and 1956 were not powerful enough, but an IBM 704 , installed early in 1957, successfully evaluated alternative designs for the secondary, due to be tested at a further series of trials, known as Grapple-X.

The Grapple-X device was a great success. Fired on 8 November 1957 off the southwest coast of Christmas Island, it yielded 1.8 megatons. For the next trial - Grapple-Y - a single warhead would be tested.

The aim was a multimegaton yield more dependent on fusion than previous designs. Grapple-Y was dropped on 28 April 1958 giving 3 megatons. Even though the relative closeness of ground zero to the installations on Christmas Island meant deliberately limiting the yield, it was the largest ever British test.

Four months later, the final Christmas Island trial began. Code-named Grapple-Z, it tested a number of experimental designs, two of which were suspended from balloons, whilst the others in the series were freefall air drops. Grapple-Z culminated with a hollow gas-boosted device which was fired on 23 September with a yield in the kiloton range. It was the United Kingdom's final atmospheric test.