Russian seamen line up in July on an Akula-class submarine at Vladivostok. On Saturday, the fire-safety system on a new Russian nuclear sub accidentally turned on as the vessel was being tested in the Sea of Japan, spewing a gas that suffocated 20 people and sent 21 others to hospital. Russian news agency RIA quoted a source as saying the accident occurred aboard the Nerpa, a Project 971 Shchuka-B attack submarine, known inside NATO as an Akula-class sub. The accident posed no radiation danger, according to the navy. It was Russia's worst naval accident since torpedo explosions sank the nuclear-powered submarine Kursk in the Barents Sea in 2000, killing all of the 118 seamen aboard.