The allegations come as the Sunday Herald today uncovers how more than 600 tonnes of foreign nuclear waste is to be kept in Scotland despite repeated promises by governments and the nuclear industry that it would be sent back to the countries from which it came.

The Sunday Herald can reveal that the Scottish government has secretly proposed storing the waste at the Dounreay nuclear plant in Caithness. Moreover, under a “swap scheme”, waste from south of the border – from Sellafield in Cumbria, which has been stored in a different form – will be returned instead.

Anti-nuclear campaigners have responded by accusing the SNP of breaking its promises to prevent Scotland from becoming the world’s “nuclear dustbin”.

Campaigner Lorraine Mann said: “It is quite disgraceful that any Scottish government should acquiesce in turning Scotland and the Highlands into a dumping ground for vast quantities of other people’s radioactive waste.

“Solid undertakings were given by Tory and Labour administrations that all of this waste would be returned to the countries of origin. It is perhaps the ultimate irony that it is an SNP ­administration, whose members screamed so loudly about nuclear dumping when in opposition, which is reneging on these assurances now they’re in office.”

The waste comes from a series of money-making nuclear reprocessing deals agreed by the Dounreay plant in the 1990s. Some 800 fuel elements from research reactors in Belgium, Italy, Germany, the Netherlands, France and Australia were processed before a key chemical plant broke down in 1996.

Dounreay said at the time that the resulting liquid waste would be mixed with cement, solidified in drums and returned to the countries of origin, in line with government commitments.

When challenged on the issue in 1995, Ian Shepherd, Dounreay’s spokesman at the time, insisted: “We are quite clear that the waste will be returned to the country of origin and that means returned from Dounreay.”

But now internal memos released to the Sunday Herald under freedom of information legislation disclose the Scottish government’s plan to keep the foreign waste at Dounreay. Combined with cement, it fills about 500 drums and weighs some 625 tonnes.

In a memo, marked “restricted”, from the Scottish government to the UK government’s waste management steering group in March 2008, agreement was sought for “waste substitution arrangements”. These, it said, would enable waste from Sellafield to be returned in place of that from Dounreay.

The memo explained that this was necessary because reactor operators abroad had said they would prefer the Sellafield waste as it is vitrified – combined with glass – and much smaller in volume. “Dounreay does not have vitrified waste and it would need to come from Sellafield,” it said.

Crucially, the memo added: “We have consulted our cabinet secretary [Richard Lochhead], who is supportive of the proposal.”

A minute of the UK waste management steering group on March 26, 2008 reiterated that the plan “already had Scottish ministerial approval”.

Despite this, a spokeswoman for the Scottish government insisted yesterday that “no decisions have been taken to retain the waste at Dounreay”.

She added: “The Scottish government will be consulting on waste substitution at the end of the year … All comments received will be considered prior to making any decision.”

But this was angrily dismissed by Green MSP Patrick Harvie.

“The SNP has long said they don’t want Scotland to be Britain’s nuclear dumping ground, but now they’re in government that policy itself appears to have been buried,” he said.

“This waste should never have been imported, but now that it has been it would be a mistake to let the source countries ignore their toxic legacy. That would simply fuel the myth that the waste problem has been solved.”

A spokesman for Dounreay said the foreign waste only added 2% to the total amount of waste from the plant’s clean-up, adding: “Concluding these historical contracts is a small but important part of the overall site closure programme.”