Plans to refurbish Trident nuclear weapons had to be put on hold because US scientists forgot how to manufacture a component of the warhead, a US congressional investigation has revealed.

The US National Nuclear Security Administration (NNSA) "lost knowledge" of how to make a mysterious but very hazardous material codenamed Fogbank. As a result, the warhead refurbishment programme was put back by at least a year, and racked up an extra $69 million.

According to some critics, the delay could cause major problems for the UK Trident programme, which is very closely tied to the US programme and uses much of the same technology. The US and the UK are trying to refurbish the ageing W76 warheads that tip Trident missiles in order to prolong their life, and ensure they are safe and reliable. This apparently requires that the Fogbank in the warheads is replaced.

Neither the NNSA nor the UK Ministry of Defence would say anything about the nature or function of Fogbank. But it is thought by some weapons experts to be a foam used between the fission and fusion stages of a thermonuclear bomb. US officials have said that manufacturing the material requires a solvent cleaning agent which is "extremely flammable" and "explosive". The process also involves dealing with "toxic materials" hazardous to workers.

Over the last year the Government Accountability Office (GAO), which reports to the US Congress, has been investigating the W76 refurbishment programme. An unclassified version of its final report was released last week. The GAO report concluded: "NNSA did not effectively manage one of the highest risks of the programme - the manufacture of a key material known as Fogbank - resulting in $69m in cost over-runs and a schedule delay of at least one year that presented significant logistical challenges for the navy."

For the first time, the report described the difficulties faced by the NNSA in trying to make Fogbank. A new production facility was needed at the Y-12 National Security Complex at Oak Ridge, Tennessee, because an old one had been demolished in the 1990s.

But vital information on how Fogbank was actually made had somehow been mislaid. "NNSA had lost knowledge of how to manufacture the material because it had kept few records of the process when the material was made in the 1980s, and almost all staff with expertise on production had retired or left the agency," the report said.

The GAO report also accused the NNSA of having an inconsistent approach to costing the W76 refurbishment programme. The total cost was put at $2.1 billion in 2004, $6.2bn in 2005 and $2.7bn in 2006.

To John Ainslie, the co-ordinator of the Scottish Campaign for Nuclear Disarmament, it was "astonishing" that the Fogbank blueprints had been lost. "This is like James Bond destroying his instructions as soon as he has read them," he said. "Perhaps the plans for making Fogbank were so secret that no copies were kept. The British warhead is similar to the American version, and so the problems with Fogbank may delay Aldermaston's plans for renewing or replacing Trident."

The NNSA's principal deputy administrator, William Ostendorff, said that the agency "generally agrees" with the findings of the GAO report. He stressed that NNSA was strengthening its management procedures. He added: "As with many processes that implement increased rigour, there is a need for identification of increased funding in order to increase the fidelity in project risk assessment."

UK sources suggested, though, that the US and UK designs were not identical. All the details of exactly how nuclear weapons are put together are classified as top secret in both countries.

A spokesman for the Ministry of Defence told the Sunday Herald: "It is MoD policy not to comment on nuclear warhead design. To do so would, or would be likely to, prejudice national security."

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