Dounreay was thought a possible repository site

The drums are stored above ground while the search continues for a safe subterranean repository.

Nirex, the company charged with finding a suitable repository site, was ordered by the UK Government in 1997 to stop investigating rock formations beneath Sellafield, which it had hoped could house the waste.

The Sellafield geology was reported to be unsuitable for safe long-term disposal, and the search continues.

High-level waste is vitrified (made into blocks of glass). Because it gives off so much heat, it has to be continuously cooled.

Some of its constituent isotopes decay fairly quickly, in decades or centuries, to the point where they can be reclassified as intermediate-level waste.

But plutonium itself has a half-life of 24,000 years (the time it takes to lose half its radioactivity), and other isotopes take millions of years to reach the same point.

Dr Helen Wallace, of Greenpeace, told BBC News Online: "There are no facilities for disposing of high-level waste. All you can do is vitrify it and hope for the best.

Long search

"There is considerable evidence that if you put it underground it would eventually leak into the groundwater. The science is very poorly understood.

"Waste disposal is one of the major question marks over the nuclear industry. In the UK there simply is no policy.

"They've been looking for a solution for decades, and they'll be looking for decades yet.

"And while that's true of the UK, it's also true worldwide."

Nirex had to stop drilling at Sellafield

Some countries believe they have found at least part of the answer. Sweden puts LLW and ILW in a huge rock chamber carved out of the rock beneath the bed of the Baltic sea.

Its critics say the policy is dangerously short-sighted: they claim that geological changes could mean the repository rises above the seabed within a few centuries.

Optimism

And the nuclear industry itself remains hopeful. The Uranium Institute, a London-based worldwide network of people working in the production of nuclear-generated electricity, thinks the waste problem is soluble.

It says: "Many milestones have been accomplished with the establishment of research facilities for HLW repositories in Belgium, France, Germany, Canada, Sweden and Switzerland . . . The disposal of HLW is surmountable."