Experts are warning that sea water and intense cold are corroding a storage facility at Andreeva Bay, on the Kola Peninsula near Murmansk, which contains over 20,000 discarded fuel rods from nuclear submarines and some nuclear-powered icebreakers. A Norwegian environmental group, Bellona, says it has obtained a copy of a secret report by the Russian nuclear agency, Rosatom, which speaks of an "uncontrolled nuclear reaction".

John Large, an independent British nuclear consultant who has visited the site, said "The nuclear rods are fixed to the roof and encased in metal to keep them apart, and prevent any reactions from occurring. However, sea water has eroded them at their base, and they are falling to the floor of the tanks, where inches of saltwater have collected. "This water will begin to corrode the rods, a reaction that releases hydrogen, a gas that is highly explosive and could be ignited by any spark. When another rod falls to the floor and generates such a spark, an enormous explosion could occur, scattering radioactive material for hundreds of miles."

Large, who was decorated by Russia's President Vladimir Putin for his role in the salvage operation that retrieved nuclear material from the Kursk submarine in 2000, added: "This wouldn't be a thermonuclear or atomic explosion, as in a bomb, but the outcome is just as bad. Remember Chernobyl? Well, if you had the right weather conditions and wind pattern, this would mean a radioactive cloud drifting over the UK and Ireland."

The three storage tanks at Andreeva Bay contain over 32 tonnes of radioactive material. But the Kola Peninsula, which is littered with relics of Soviet nuclear facilities, has a total of over 100 tonnes of nuclear waste, the largest concentration in the world.

Experts have predicted that a major explosion at Andreeva Bay could destroy all life in a 32-mile radius, which would embrace Murmansk and a sliver of Norway, whose border is only 28 miles away.

But a much wider area of Norway, north-western Russia and Finland would be rendered uninhabitable for at least 20 years, and huge quantities of radioactive material would be dumped into the Barents Sea.

"In the best case a small, limited explosion in just one of the stored rods could lead to radioactive contamination in a 5km [three-mile] radius, " Aleksandr Nikitin, a Russian former submarine officer and nuclear safety inspector turned environmental activist, told the Norwegian newspaper, Aftenposten.

"In the worst case, such a single explosion could cause the entire tank facility to explode. We have no calculations for what that could lead to."

Nikitin, whose work on behalf of Bellona led to years of legal harassment and continuing treason charges in Russia, added: "We are sitting on a powder keg with a burning fuse, and we can only guess about the length of the fuse." Nils Bohmer, nuclear physicist and head of Bellona's Russian division, told the newspaper: "It will at least, at a careful estimate, hit northern Europe. There are enormous amounts of radioactivity stored in these tanks."

Other activists have voiced concern about the security of stored nuclear waste in the Kola Peninsula, amid reports that some is left in barrels in the open, protected by only a link fence and a couple of guards. Washington-based Global Security. org reported that in 1993 about 1.8kg of enriched uranium was stolen from the Andreeva Guba fuel storage area. Although the material was quickly recovered, the fact that some of the uranium is enriched to between 30 and 40%, much higher than the 2-3% used in civil nuclear reactors, could make it tempting to terrorists seeking to make a "dirty bomb".

Apart from the decay at the Andreeva Bay facility, said Ben Ayliffe, senior climate and energy campaigner at Greenpeace UK, "security is so lax that almost anyone who wants can just walk into the place. It's like Homer Simpson meets Dad's Army."

As the 1986 Chernobyl disaster showed, drifting atmospheric radiation can contaminate crops and water supplies more than 1,000 miles from the site of the explosion.