SELLAFIELD nuclear plant cannot account for plutonium "missing" from its site.
Figures published by the British Nuclear Group (BNG) each year reveal an audit of nuclear material processed by plants around Britain.
According to reports, 30kg (66lb) of plutonium - a radioactive metal which can be used for bomb making - went astray at Sellafield.
A BNG spokeswoman said complex measuring procedures were the most likely cause.
"I wouldn't say we would be alarmed by it, because we are only talking about a book figure here."
"We work in tonnes rather than kilograms - hundreds of tonnes of nuclear materials are processed here."
In 2003, the processing plant revealed that there was 19kg (42lb) of plutonium which was material unaccounted for.