The US Defense Department, which has been trying to persuade President Clinton to intervene to stop the plant opening, issued a report last week which says that exporting plutonium from Thorp would put some nations 'within days' of acquiring the nuclear bomb.
The study also concludes that operating the Sellafield plant - which would recover plutonium and uranium from used nuclear fuel and export them around the world - would increase the danger of terrorist groups obtaining nuclear weapons.
As the Independent on Sunday reported last month, two former CIA directors and two US weapon designers have already written to John Major to warn that Thorp would increase the dangers of nuclear proliferation.
The report, commissioned by the Pentagon from the Rand Corporation's National Defense Research Institution, directly contradicts repeated assertions by British Nuclear Fuels Ltd (BNFL), which owns Sellafield, and by the Government, that Thorp will not make nuclear conflict more likely. The Government is expected to give permission in the next few weeks for the pounds 2.8bn plant to start up.
Rand's experts say that civil reprocessing plants such as Thorp create an even greater threat of nuclear proliferation than the accumulation of plutonium from dismantled nuclear weapons. Thay conclude that it is 'critical' that governments should take notice of this risk.
Over the next 10 years such plants will produce enough plutonium for 47,000 bombs, even more than the 40,000 that could be made out of the old nuclear weapons, the report says.
'The diversion of even a tiny fraction of these materials will be enough to make many nuclear weapons,' it concludes. 'Countries with separated plutonium within their borders can, at will, produce materials for nuclear weapons use within days or weeks.'
BNFL insists that the plutonium produced by civilian reprocessing plants - which has a slightly different composition from the material used in bombs - 'cannot be converted into weapons-grade material without substantial expertise and equipment'.
The Government has said that the plutonium will be covered by safeguards laid down by the International Atomic Energy Agency (IAEA). But the US report says: 'No safeguard scheme, including that of the IAEA, can be effective if such sensitive materials are widely available in non-nuclear states.'
It concludes that the US should persuade Britain not to operate Thorp, adding to growing pressure on President Clinton to intervene. The House of Representatives has voted unanimously for an international halt to plutonium production.
The US could effectively kill off Thorp by stopping Japan, the plant's biggest customer, from using it. The US supplies Japan with its nuclear fuel under conditions that would enable it to prohibit reprocessing.
But Mr Clinton has accepted State Department advice that this would damage relations with Britain and Japan. In a letter to Congressmen he agreed that 'the continued production of plutonium . . . creates serious proliferation and security dangers'. But a ban would 'lead to a confrontation with our allies'.Reuse content