The fire safety system on a brand-new Russian nuclear submarine accidentally turned on as the sub was tested in the Sea of Japan, spewing a gas that suffocated 20 people and sent 21 others to the hospital, officials said Sunday.
The Russian Navy said the submarine itself was not damaged in Saturday's accident and returned to its base on Russia's Pacific coast under its own power Sunday.
The accident also did not pose any radiation danger, the navy said.
It was Russia's worst naval accident since torpedo explosions sank another nuclear-powered submarine, the Kursk, in the Barents Sea in 2000, killing all 118 seamen aboard.
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Overcrowding may have been significant Saturday.
The submarine being tested had 208 aboard, including 81 seamen, according to Russian navy spokesman Capt. Igor Dygalo.
Russian news agencies said a sub of this type normally carries a crew of only 73.
“A submarine is the most vulnerable during trials. With both navy and civilian personnel on board, it's very difficult to keep such a large number of people organized,” Gennady Illarionov, a retired submarine officer, told the RIA Novosti news agency.
The victims suffocated after the submarine's fire-extinguishing system released Freon gas, said Vladimir Markin, an official with Russia's top investigative agency. He said forensic tests found Freon in their lungs.
Seventeen civilians and three seamen died in the accident, and 21 others were hospitalized after being evacuated to shore, Dygalo said.
Markin's agency, the Investigative Committee under the Prosecutor General's office, has launched a probe, which he said will focus on what activated the safety system and possible violations of operating rules.
Lev Fyodorov, a top Russian chemical expert, wondered why individual breathing kits that everyone on board is supposed to have did not keep people from dying.
“People on board the sub may have failed to use their breathing equipment when they found themselves in an emergency,” he told the AP.
Igor Kurdin, a retired navy officer who heads an association of former submariners, told Ekho Moskvy radio that the high death toll probably resulted from shipyard workers who lacked experience in dealing with the breathing kits.
A siren warning also may have failed, RIA Novosti quoted an unidentified navy official as saying, so those on board might not have realized Freon was being released until it was too late.
The submarine returned Sunday to Bolshoi Kamen, a military shipyard and a navy base near Vladivostok. Officials at the Amur Shipbuilding Factory said they built the submarine, called the Nerpa. Dygalo said it was to be commissioned by the navy later this year.
Construction of the Nerpa, an Akula II class attack submarine, started in 1991 but was suspended for years because of a shortage of funding, they said. Testing on the submarine began last month, and it submerged for the first time last week.