Britain had an atom bomb so big it could have gone off at any time

NUCLEAR bombs stored at British military bases in the late 1950s could have exploded by accident, according to declassified Royal Air Force papers. Just one going off would have been like 25 Hiroshima blasts.

Rushed into service as a stop-gap while Britain developed its own H-bomb, the gigantic devices held so much fissionable material that they risked going critical when armed, the documents reveal.

Up to a dozen of the huge fission weapons, based on a design codenamed Violet Club, were supplied to RAF bases including those at Finningley in South Yorkshire, Scampton in Lincolnshire and Wittering in Cambridgeshire between 1958 and 1960.

But the RAF was worried that when the bombs' safety mechanism was disabled there was a "risk of catastrophe". In one memo dated 12 January 1959, a Group Captain Tait wrote: "A high-yield nuclear explosion would be possible if the weapon were jettisoned, or in the event of a crash on return, or an accident in de-bombing."

The evidence was unearthed from the Public Records Office in London by Lesley Wright at John Moores University in Liverpool, David Wright of the University of Manchester and military historian Nicholas Hill. They say each bomb held around 70 kilograms of uranium-235, enough to create a 500-kiloton explosion.

But each weapon, made by the Atomic Weapons Research Establishment at Aldermaston, was packed with 450 kilograms of steel balls while on the ground to separate the sections of the uranium so that it could not accidentally form a critical mass.

To minimise the risk of stray neutrons from one radioactive mass triggering a chain reaction in another, the RAF initially stored the bombs in separate buildings, but later they had only to be 1.8 metres apart.

The bombs also took too long to arm. In the event of a nuclear attack, the RAF was meant to be able to launch a nuclear-armed aircraft within 15 minutes, yet Tait's memo warns that removing the metals balls that would arm a Violet Club bomb took at least 20 minutes.

Lesley Wright argues that the unwieldy weapon was imposed on the RAF because the British government wanted to keep up with the US and the Soviet Union, who were already testing fusion bombs with yields in the megaton range. "This is a prime example of how the pursuit of superpower status and fear overcame rational decision making," she says.

A Ministry of Defence spokesman says the bombs were supplied as a kit for Vulcan bombers "which could be assembled and deployed in a national emergency". But the MoD insists that there was "no risk" of an accidental explosion because it "took all the necessary precautions to ensure that [Violet Club] was transported and stored safely".