Dozens of crewmen were fast asleep on a nuclear submarine when freezing Freon gas poured over them from a firefighting system, survivors said yesterday, in the first eyewitness accounts of the Russian submarine accident that killed 20 people.

The Nerpa submarine was undergoing sea trials in the Sea of Japan on Saturday when its fire-extinguishing system switched on, spewing liquefied Freon gas that asphyxiated the victims and sent 21 others to the hospital. The submarine returned to its home port under its own power on Sunday.

Survivors said they were completely caught off guard.

“A siren blared and Freon came streaming in immediately,” shipyard worker Viktor Rivk said on Russia’s NTV television, speaking from a hospital bed in the Pacific coast port of Vladivostok. Other survivors said a siren warning the crew the firefighting system was turning on may somehow have been delayed.

Navy experts have said that overcrowding and human error may have contributed to the accident. The submarine had 208 people aboard when the accident occurred, including 81 seamen. The rest were civilians, many from the shipyard that built the submarine. Akula-class subs normally carry a crew of 73.

Former submariners say sea trials always pose increased safety risks, because of the large number of unqualified people on board. They say every person on board must permanently carry an oxygen kit, but many civilian workers may have lacked experience in using them. Seventeen of the dead were civilians.