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
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
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