SELLAFIELD, Britain's major nuclear site, has "lost" 30 kilograms of plutonium, according to figures due to be published today.
The annual audit of nuclear material at all of Britain's civil nuclear installations is expected to reveal that the plutonium - enough for seven or eight nuclear bombs - was classified as "material unaccounted for" last year.
The revelation that such a large amount of plutonium has apparently disappeared is likely to cause deep embarrassment at British Nuclear Group and the Department of Trade and Industry, which are weeks away from completing a huge shake-up in the nuclear industry, with the creation of an independent Nuclear Decomissioning Authority.
The discrepancy compares with a 19kg loss at Sellafield in 2003 and a cumulative loss of about 50kg at the Cumbrian plant over the past ten years. British Nuclear Fuels, which operates the plant, is expected to dismiss the figures as a "paper loss" and an "accounting issue."
But independent experts described the disclosure as deeply worrying. "They make this claim of an auditing problem but I would expect them to be overzealous in the current climate of fears about terrorism," John Large, an independent nuclear consultant, said.
Dr Frank Barnaby, a specialist in nuclear weapons, said: "There will always be some material unaccounted for but this is a dramatic development.
"This is a major reason for not reprocessing spent nuclear fuel because you can't tell what the material unaccounted for is."
All nuclear material at Sellafield, including the content of contaminated ponds and any emissions, are measured every year, according to guidelines endorsed by the International Atomic Energy Agency (IAEA).
"It's just like a balance sheet, we count what comes in and what goes out," one insider said.
The fact that the figures do not balance is embarrassing rather than sinister. They do not imply that any material has been improperly diverted, or that there has been a breach of security at the site.
Keeping track of just how much material is present at the plant at any one time is tricky, and subject to errors. Spent nuclear fuel rods, which have been inside nuclear reactors for about five years, are taken to Sellafield for reprocessing.
They are allowed to cool in ponds for up to four years before they are treated. The process then involves cutting up the fuel rods, dissolving them in acid and then separating the solution into three streams - uranium, plutonium, and high-level waste.
At each stage of the process the material is weighed and calculations made of the amounts of plutonium it contains. All this has to be done remotely behind shielding because of the radioactivity involved.
At the end of the process the weight of the plutonium recovered ought to balance with the estimates of the amount put in. They seldom do, but the discrepancy is rarely as large as it is this year.
Small measuring errors can accumulate to produce large discrepancies, as they have done this year.
The IAEA lays down rules for the permitted levels of "material unaccounted for," which must not exceed about 3 per cent of the plutonium throughput. Given the huge quantities reprocessed at Sellafield, this year's discrepancy will fall within these standards.
According to insiders the material unaccounted for is roughly 0.1 per cent of the material that went through Sellafield's facilities last year. "The point is none has gone missing and none has gone from the site," a source told The Times.
A spokesman for the DTI said that the 30kg "loss" was due to a "new accounting system."
Security at civil nuclear sites has been stepped up since September 11. But new figures showed security breaches on 45 occasions last year. They included a case of unauthorised access and the theft of sensitive information, according to documents released under the Freedom of Information Act.
The figures, from the Office for Civil Nuclear Security, indicate that human error has led to serious breaches.
The Health and Safety Commission is known to be concerned about the safety of nuclear sites as private companies bid to take over their management over the next three years.