Operators of the Dounreay nuclear plant in northern Scotland have found 2.29kg of HEU while dismantling an old fuel fabrication plant.

Over many years, ‘MUF’ (material unaccounted for) figures have shown discrepancies in the former reprocessing site’s nuclear materials balance, as well as at other sites across the UK, such as Sellafield in England.

In the past, the UKAEA has stressed that the Dounreay discrepancies are not a result of ‘lost’ nuclear material but are due to the accounting process.

In contrast, antinuclear campaigners have cited the figures as a demonstration that procedures are not tight enough to prevent illicit trafficking of bomb-grade material.

But it has transpired that some of the HEU in question was, in fact, trapped in ventilation ducts and equipment used to cast and machine fuel elements in a site fuel fabrication plant.

It was removed during the decommissioning process and consigned as waste.

A spokesperson for the UKAEA said: “2.29kg of HEU U-235 was recovered from the former fuel fabrication plant during decommissioning operations.

“This was material recovered during the decommissioning of active plant equipment and containment that was involved in the casting and machining of fuel elements.

“The project team was aware that the material was trapped in the equipment, but it was inaccessible until decommissioning began. The material was recovered from the casting line, active lathe, powder fuel line and redundant ventilation ductwork.

“No discrete fuel items were found.”