An uncontrolled chain reaction in the engine compartment of the Russian nuclear submarine Kursk was "the worst case scenario" being contemplated by Russian experts yesterday.
If that happened the fuel would heat up to 1,800-2,000 degrees, enough to melt the engine compartment and outer casing, releasing molten fuel on to the seabed.
Dr Mils Bohmer, a Russian physicist and expert on nuclear submarines, said this would release fission products like plutonium and uranium into the fertile fishing grounds of the Barents sea. However, the true state of the reactors is not yet known.
"We were told that the first stage of shutting the reactors down had been completed, but there is no word on what happened after that," he said. "If the crew managed to shut them down completely then they should remain safe, but if not then the fuel elements may start to heat up and begin an uncontrolled chain reaction," he said.
Dr Bohmer was speaking in Oslo, where he has been helping the Norwegian environmental group the Bellona Foundation.
He said because there had been an explosion in the forward area it was not possible to know the effect on the crew in the engine compartment at the rear had been or how much time they may have had to act.
The reactor in the submarine was of a new design with a "natural" cooling system which works even if there is no electrical power, but it might not be effective if there was damage or the reactor had not been fully shut down.
A Greenpeace disarmament campaigner, Mr William Peden, said the automatic cooling system worked on convection and its efficiency would be badly affected because the vessel was lying on its side. "There is a real fear of a meltdown if the cooling system does not work properly. To protect the area from serious radioactive pollution the Russians have no alternative but to raise the sub," he said.
Dr Bohmer believed the Russians would want to raise the sub anyway because it was in relatively shallow water - 108 metres - and, as a relatively new boat, it contains military secrets the Russians would be keen to protect from other powers.
In one sense the sea is the best place for damaged nuclear equipment because of the dilution and cooling effect of vast quantities of sea water, however, the Kursk is in important fishing waters.
Yesterday the news from the area of the Kursk was good. Tests of water and air samples by the Norwegian Nuclear Protection Authority said five water samples and two air samples gathered through large filters on Tuesday showed no trace of abnormal radiation.