ENOUGH enriched uranium to make more than 10 nuclear bombs went missing from Dounreay in just over three years, it was revealed yesterday.

The plant's own figures involving contents of the controversial Dounreay shaft show the loss of 170kg.

An independent nuclear expert reckoned there were two possible explanations for this loss, between 1965 and 68:

The missing material was ''covertly transferred'' into the military programme for bomb making at a time when the Cold War was at its height, which he believes the more likely;

It is still in the plant. In this case, it represents a huge safety risk with criticality (an uncontrolled chain reaction) possible which could threaten unprecedented environmental damage.

Dr John Large, who worked for years in the nuclear industry, said: the fact ''Dounreay is now willing to admit to such shoddy management of this plant is absolutely incredible. But no matter how incompetent they were, I cannot believe they lost all this material. It is simply inconceivable.''

The Ministry of Defence refused to comment, but Dounreay insisted it had not supplied material for the weapons programme.

In a statement it also maintained the ''Dounreay Shaft Inventory'' was prepared to establish a worst case estimate of its contents.

SNP environment spokeswoman Roseanna Cunningham called on the Government to launch a South-African style truth commission to determine what happened. ''How on earth can material as deadly as this just go missing. As usual one gets the feeling that we are not being told the whole story.''

She added: ''We are now in the ludicrous situation where we are hoping that the uranium was used to make British atom bombs because the alternatives are even harder to stomach.''

The inventory has just been published by the UK Atomic Energy Authority Waste Management Group, as part of the programme of preparatory work before the contents of the controversial shaft are removed.

It lists the totals of recorded nuclear material in the shaft, without any allowance for losses as: total uranium, 1119.7kg; Uranium 238, 1055.2kg; Uranium 235, 64.5kg; Plutonium 2.24kg.

It then continues: ''These values do not include any unacceptable losses to the shaft nor any margins for error. MUF (Material Unaccounted For) from DFR (Dounreay) fuel processing have been estimated to be up to 108.3kg of uranium, equivalent to approximately 80.2kg of U-235.

''Unaccountable losses are likely to be spread over various facilities, and are likely to be recovered during their decommissioning. As well as the DFR processing work, enriched uranium fuels from all over the world were processed at Dounreay in the 1960s. Concerns over losses from this processing work led to the addition of borated glass to the shaft in 1968.

''No MUF figures are available throughout the shaft lifetime, but in 1969 losses of 170kg U-235 were estimated over a 3.5 year period from 1/4/65 to 30/9/68.''

The authors estimate that, of the 170kg, at least 22 kg would have been put in the shaft.

Dr Large said last night this was the fifth inventory he had read and was the most worrying by far: ''I thought you were pulling my leg at first before I read it for myself. It is quite staggering. A nuclear plant in the 1960s was run like Hatton Gardens jewellers. You put brown paper on the floor to pick up all the diamond dust every night. That

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was the approach in nuclear plants.'' He added: ''In trying to solve the problems of the shaft, we are now seeing all the skeletons coming out the cupboard, and these are very disturbing skeletons.

''I suspect that much of it may well have been subject of covert transfer from the civil to the military stockpiles to make bombs. That is the only explanation that springs to my mind to account for such a large loss.''

However, he said it would be much more worrying if the material had not been sent for military use, and was still in the plant waiting to be uncovered during decommissioning. ''If you don't know where it is, then you have a criticality risk.'' He explained there is ''a lot of hydrogen, explosive vapours, kerosene and you have acids, so you definitely don't want an explosion with these sort of materials around.''

Mrs Lorraine Mann of Scotland Against Nuclear Dumping (Sand) said: ''It is clear there has been a military connection with Dounreay and we want to know when it started, if it is still continuing and whether that is what is behind this mad dash to get foreign spent fuel into Dounreay.'' She said the prospect of it still being in the plant was even more worrying.

The Dounreay statement claimed: ''The recently published inventory of the contents of the Dounreay shaft was prepared to establish a worst case estimate of its contents, as the basis for the production of a robust safety case for retrieval of the material in the shaft. It therefore makes pessimistic assumptions about the possible contents of the shaft.

''During the 1960s the Dounreay plants were processing material from a wide range of sources, including residues from other UKAEA sites which were difficult to analyse because of their variable nature and composition.'' In 1963 and 1973 inquiries were made into MUF from the different plants.

These inquiries confirmed that the MUF was likely to be due to uncertainty in estimates of the uranium input to the plants, ''... reflecting the imprecision of the analytical techniques available at the time, coupled with the difficult nature of the input material.''