John McKeown, chief executive of the United Kingdom Atomic Energy Authority, has corrected figures for its existing commercial reprocessing contracts at the Dounreay site in northern Scotland. In letters to MEP Winifred Ewing and MP Roseanna Cunningham, he says that information lodged in the House of Commons library last autumn “contained a number of inaccuracies, which I seek to correct for the public record”.

The inaccuracies were due to typographical errors and included 393 kg of thorium from a contract to recover material from irradiated fuel pins and remnants, being recorded as 343 kg. Other errors included 15 kg of reprocessed material being recorded as 9 kg and 117 kg as 110 kg.

In a separate development, the Nuclear Safeguards Office has published a report into material accounting aspects of the Dounreay shaft. The report concludes that “significantly less than 86 kg” of HEU from work which took place in the 1960s, was unaccounted for. This contrasts with a figure of 170 kg produced last June by a working group which the UKAEA set up to address the issue.

The report put leakage of HEU into concrete floors as one of the reasons for the apparent loss. Other factors include uranium in the air filtration systems, corrosion and erosion of fuel and uncertainties about the residues awaiting recovery.

Andy Munn, a spokesman for the UKAEA, said that part of the reason for the discrepancies is that material accounting systems in the 1960s were far less sophisticated than they are today as the technology did not exist to accurately measure material throughput. Another reason is that some HEU would have been burnt in the reactor.

The spills of HEU into concrete has been known for a long time.

“It was discovered in 1973” said Munn. “The floors were dug up and removed. Some of the material still remains in drums on site.” The Safeguards Office report emphasises that there is no suggestion that material has actually been lost.

“The review found no evidence to support suggestions that high enriched uranium was actually lost from Dounreay,” it said, “ie that it had been stolen, diverted for military purposes or somehow removed from the site using an unidentified route. The impression that this might have been the case was the result of a misinterpretation of the nuclear materials accountancy terminology used in 30-year old reports.”