The reputation of Russia’s accident-prone navy has suffered a major blow after it was forced to admit that 20 sailors and technicians on a nuclear submarine died after inhaling poisonous gas. The country’s worst naval disaster since the sinking of the Kursk eight years ago threatens to undermine Russia’s ambitions of restoring its reputation as a military superpower and could also test the popularity of Dmitry Medvedev, the new president.

For reasons that remain unclear, a fire extinguishing system aboard the Nerpa, an Akula II class nuclear powered attack submarine, was inadvertently triggered as the vessel was undergoing safety tests in the Sea of Japan. According to defence sources, personnel at the head of the submarine were unable to don oxygen masks after a sprinkler system in the roof of the vessel began to spray Freon, a poisonous gas used in refrigerators and air conditioning units. It was uncertain whether or not the submarine was equipped with oxygen masks, as it should have been for precisely such an event. Experts said that the submariners would have survived for about 15 minutes before falling into a coma — enough time to don the masks if they had been available.

The gas was contained after hatches dividing the vessel’s compartments sealed themselves automatically. The submarine’s atomic reactor, located towards the rear of the vessel, was unaffected.