Britain’s major nuclear site at Sellafield has "lost" almost 30 kilograms of plutonium, according to figures published today.

The annual audit of nuclear material at all of Britain’s civil nuclear installations revealed 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 has been described as merely an "accounting issue" by British Nuclear Fuels, which operates the plant.

However, it is likely to cause embarrassment as the British Nuclear Group and the Department of Trade and Industry are weeks away from completing a huge shake-up in the nuclear industry, with the creation of an independent Nuclear Decommissioning 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. A BNG spokeswoman said: "There is always a discrepancy between the physical inventory and the book inventory.

"Some years there is an apparent gain, some years there is an apparent loss."

The UK Atomic Energy Authority (UKAEA) said the figure was within international standards of expected measurement accuracies.

The International Atomic Energy Authority (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 falls within these standards. But independent experts have described the figure 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," said John Large, an independent nuclear consultant.

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."

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