DEMAND for a public inquiry into Dounreay grew yesterday as the confusion surrounding the missing 170kg of highly enriched uranium intensified with the chief executive of the UK Atomic Energy Authority appearing to dismiss his own agency's findings.

The controversy has centred on a document, The Dounreay Shaft Inventory, which was written by the UKAEA waste management group, which makes several references to ''losses'' of radioactive material.

But John McKeown, chief executive of the authority, said yesterday: ''The calculation that was done assumed that there was more material going into the plant to the extent of 1%. It is not a case of the material having been hidden, it is not a case of the material being lost, quite simply it is a case of the material never having existed.''

However, Dr John Large, an independent nuclear consultant who was previously a research fellow of the UKAEA, said he found this very difficult to accept.

''Their accounting procedures were far more precise than that.

''This stuff was literally like gold dust, it was worth millions of pounds and half a gram loss would have been regarded as very serious, a matter of great concern. So the idea that they were working on the basis of plus or minus 170kg is simply ludicrous to me.

''But I am not the one who is saying the 170kg is lost, it is in Dounreay's own investigators' report. Its authors quote, without challenge, estimates that 22kg went down the shaft and the rest is somewhere in the plant.

''But now we have the chief executive saying that is not true. I still believe that the only reasonable explanation is the military one. The irony is that we could all sleep easier in our beds if they would just say it went to make bombs.''

Meanwhile, the Scottish Tories entered the Dounreay debate with environment spokesman Struan Stevenson saying: ''There needs to be an investigation into this matter and if this missing uranium is a result of incompetence from the existing managerial team, then heads must roll.''

The issue of Dounreay has always been a thorny one for the Liberal Democrats with the Orkney and Shetland constituency of Jim Wallace being highly suspicious of the plant's effects on fishing and agriculture.

Across the Pentland Firth, the Caithness part of Robert Maclennan's Caithness and Sutherland seat has been one of the most pro-nuclear areas in the land.

But yesterday Mr Wallace issued a statement as leader of the Scottish party: ''Thirty years ago, terrorism was only a minimal threat, so it is unlikely that the disappearance of this uranium was due to intrigue.

''More likely the loss was due to incompetence and covered up by an industry whose obsession with secrecy at the time was only matched by its arrogance that it knew best. It is essential that a proper independent investigation takes place into the circumstances surrounding the loss.''

Friends of the Earth Scotland renewed its call for a full inquiry into Dounreay's operations and predicted that the #400m estimated cost for cleaning up the shaft would be a gross underestimation if the 170kg was still on the site somewhere.