Earlier this week the UK Atomic Energy Authority admitted that 170kg of weapons-grade enriched uranium had disappeared from Dounreay - enough to make a dozen atomic bombs.

The plant will now be run down

The revelation fuelled demands by the Scottish Nationalist Party for an immediate inquiry into Dounreay.

"We have been fighting for a general inquiry into Dounreay for some time, but now it is clear we need a truth commission where all the workers, past and present, are exempted from the Official Secrets Act," said the SNP's environment spokeswoman Roseanna Cunningham.

"Only by doing this can we ever hope to get to the bottom of Dounreay's history," she said.

The loss of the uranium was discovered as a result of another recent embarrassment at Dounreay, when the government announced in March that a contaminated waste shaft would need cleaning up.

Plant is in one of the most remote parts of Britain

The 25-year operation will cost up to £355 million. The shaft was sealed more than 20 years ago but is still leaking radioactivity.

It was discovered that the uranium was missing in an audit of the shaft which showed it still contains about 1,000 cubic metres of intermediate-level waste, about 4kg of plutonium and 100kg of uranium.

There have also been major worries about an explosion in the shaft in 1977 which official reports said had been caused by "a chemical reaction" between the cocktail of dumped materials.

Although experts said the blast had not caused "significant damage", pictures released to the BBC in October 1996 suggested the explosion was extremely powerful.

Campaigners fear much of the area was contaminated by the explosion

The blast blew a 12.5 tonne concrete plug four metres to one side. Shattered concrete was thrown some 60 metres beyond the perimeter fence and scaffolding poles were propelled up to 75 metres onto the foreshore.

Documents obtained by the BBC for a documentary on Dounreay last year showed that after the explosion, highly radioactive contamination was found around the waste shaft.

Some of it was found well outside the boundary fence of the nuclear plant.

During routine monitoring of the site in the weeks following the blast, contamination of the roads was around 100 times higher than normal, the programme reported.

The plant management have always maintained that another photograph, taken inside the shaft shortly after the blast, showed that although debris had fallen in, the waste itself was largely undisturbed by the explosion.

In May, the government's environmental watchdog in Scotland announced it was to curb radioactive pollution levels at Dounreay.

The Scottish Environmental Protection Agency (Sepa) acted after Dounreay officials admitted miscalculating radiation levels.

In April, officials admitted that levels of radioactivity at Dounreay over the previous six months had been up to 10 times higher than those reported. However, they stressed that they were still well below authorised limits.

That announcement came just a few weeks after Anthony Pointer, chief constable of the UK Atomic Energy Authority police force, resigned, claiming there were not enough officers to protect the country's nuclear installations.

Then in May the Health and Safety Executive revealed a sweeping safety review of the Dounreay plant.

The inquiry followed an incident when a digger cut through electrical cables, paralysing the plant after the emergency supply also failed, prompting concerns about the control of the plant.