The BBC's Ian McDonald on the latest revelation from Dounreay

Thousands of tons of waste will have to be treated and reburied in a new silo at the Caithness plant at vast public expense.

The bunkers were built when processing first began at Dounreay in the 1950s and were designed to hold low level nuclear waste.

But it is thought some waste was too radioactive to have been put there.

The 40-year-old waste is stored in drums beneath Caithness

Dounreay is already committed to recovering highly radioactive debris dumped in a 200-foot waste shaft.

A spokesman for the plant said: "Dounreay is looking at options. Nothing has been decided - there are various options and it's early days.

"Dounreay is aware that something will have to be done about these low-level waste pits, but whether or not that means removing them remains to be decided."

The spokesman admitted there was a "potential risk that erosion of the cliffs could affect the pits but that is hundreds of years in the future, at the earliest, if at all".

Lorraine Mann, a spokesman for Scotland Against Nuclear Dumping (Sand), said: "I'm horrified about the negligence that led to this but I'm equally horrified that Dounreay's solution to the problem is simply to dig it all up and dump it elsewhere on the site again."

'Carefully monitored'

She said Sand would be opposing that as a solution and added: "It's not really a question of the stuff being exposed in hundreds of years if the cliffs fall into the sea - this stuff is going to be radioactive for thousands of years and it will affect future generations."

Mrs Mann said the waste should be stored above ground where it could be carefully monitored.

The Dounreay site was opened in 1955 for the testing of nuclear reactors but its main job now is decommissioning nuclear waste and reprocessing nuclear fuel.

Around 1,200 people work at the plant, which provides around £30m for the Caithness area's economy.