The UK government has assured the Minister for the Environment, Mr Roche, that the missing 30 kg of plutonium in Sellafield was "material unaccounted for", the Dáil was told.
The Minister for Education, Ms Hanafin, speaking on behalf of Mr Roche, said the term represented the difference between measured stock and book account.
"It arises as a consequence of the accounting process for these nuclear materials and mainly from measurement uncertainties," she added.
"The Minister received assurances from the UK government that the figures in this case related to a 'book' discrepancy arising from measurement uncertainties and that there is no evidence to suggest that any of the apparent losses reported are real losses of nuclear material."
Ms Hanafin said that, apparently, it was not unusual for the accounting process to indicate material unaccounted for and it could have a positive or negative value.
She said the figures published yesterday related to last year and the previous year and had all long since been reported to EURATOM and the International Atomic Energy Agency under the UK's various nuclear safeguards obligations.
"The Minister for the Environment understands that the figures returned are not the subject of further inquiry by these bodies and that they are all within international standards of expected measurement accuracies for closing a nuclear balance at the type of facility concerned."
The Minister was replying to the Green Party leader, Mr Trevor Sargent, who said they were dealing with material for weapons of mass destruction.
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