http://en.wikipedia.org/wiki/Violet_Club
Violet Club was a nuclear weapon deployed by the United Kingdom during the cold war. It was Britain’s first operational “high yield” weapon, and was intended to provide an emergency capability until a thermonuclear weapon could be developed from the 1956-1958 Operation Grapple thermonuclear tests conducted on Kiritimati (Christmas Island).
In 1953, shortly after the Americans tested a thermonuclear weapon in 1952, followed by the Soviets with Joe 4, and before the UK government took a decision in July 1954 to develop a thermonuclear weapon, the Atomic Weapons Research Establishment at Aldermaston was asked about the possibilities for a very large pure fission bomb with a yield of one megaton. This study referred to the Zodiak Mk.3 bomb, but progressed no further than a rudimentary study. At this time studies were also started that ultimately led to a decision in 1954 to develop a thermonuclear weapon, and the design studies were split into two tracks because the British at that time had not yet discovered the Teller-Ulam technique necessary to initiate fusion. One track led to an intermediate design, the so-called Type A thermonuclear design, similar to the Alarm Clock and layer cake hybrid designs of other nuclear powers; although these designs are now regarded as large boosted-fission weapons, and no longer regarded as thermonuclear weapons that derive a very large part of their energy from a fusion reaction, designated by the British as Type B, but as hybrids. The British hybrid weapon was known as Green Bamboo, weighed approx 4,500 lb (2,045 kg) and its spherical shape measured approx 45 inches diameter with a 72-point implosion system. This Green Bamboo weapon was intended as the warhead for all projected British strategic delivery systems of the period; the Yellow Sun Stage 1 air-dropped bomb, and the Blue Steel air-launched stand-off missile. The large girth of these weapons was designed to accommodate the diameter of Green Bamboo’s implosion sphere.
Design features
Schematic of the steel ball bearing arrangement common to Green Grass, Green Bamboo and Orange Herald warheads. The internal and external diameters can be computed from the 450 kg weight of the steel balls. First published in Synergy Magazine, published Southampton, UK, No3, 2003
Violet Club (and to a lesser degree Yellow Sun Mk.1) was not considered a satisfactory design and suffered from numerous design defects. An implosion design, the fissile core of the weapon was a hollow sphere of highly-enriched uranium (HEU) which was surrounded by a High Explosive supercharge and 72-lens implosion system. The HEU core was greater than one uncompressed critical mass and to maintain it in a sub-critical condition AWRE fashioned it into a hollow thin-walled sphere. The HEU sphere was collapsed inwards by the supercharge and 72 explosive lenses. However, a fire in the bomb store or a traffic collision on the airfield could easily lead to a partial crushing or collapse of the unremovable uranium shell, and in turn a spontaneous nuclear chain reaction. AWRE responded by inserting (though a hole in the shell) a rubber bag, rather akin to an outsize female condom, and filled this with 20,000 steel ball bearings of 0.375-inches diameter (9.5 mm), weighing 70 kg. Subsequently, the number of these steel balls was increased to 133,000, with a reduction in size to approx 5 mm diameter. The balls were retained in the device by sealing the hole with a plastic bung. The steel balls were intended to prevent a nuclear detonation even if the explosives fired accidentally, or in any conceivable accident. The ball bearings had to be removed through the hole in the bomb casing during flight preparation, and after the bomb was winched into the aircraft. The ball bearings then had to be re-inserted into the lowered and upturned bomb before transport back to the bomb store. Quite obviously, without the ball bearings installed, these weapons were armed and live, and too dangerous to allow to be flown on exercises. Bomber Command exercises demonstrated that flight preparation followed by a scramble take-off could not be reduced below thirty minutes, and on exercises in bad weather and at night a ninety minute scramble was the norm. At least one accident dated 1960 was reported in the press when the plastic bung was removed and 133,000 steel ball bearings exited onto the aircraft hangar floor, leaving the bomb armed and vulnerable. The Royal Air Force were so nervous of the outcome of a fire in storage, that permission was sought to store the bombs inverted, so that a loss of the plastic bung could not end with the steel balls on the floor, leaving the HEU unprotected against a subsequent explosion. Even without the partial nuclear detonation feared by the RAF, there was “a risk of catastrophe”