At the time of the accident,
Nerpa was undergoing sea trials at the Russian
Pacific Fleet's test range in
Peter the Great Gulf, an inlet of the Sea of Japan adjoining the coast of Russia's
Primorski Krai province. The vessel had not yet been commissioned by the Russian Navy and was undergoing plant tests under the supervision of a team from the Amur shipyard. For this reason, it had a much larger than usual complement aboard, totaling 208 people, 81 military personnel and 127 civilian engineers from the shipyards responsible for building and outfitting the submarine. The accident occurred at 8:30 pm local time on 8 November 2008, during the submarine's first underwater test run. The gas, a
haloalkane refrigerant, is used in the Russian Navy's LOKh (
lodochnaya obyemnaya khimischeskaya – "submarine volumetric chemical") fire-suppressant system. Each compartment of a Russian submarine contains a LOKh station from which the gas can be delivered into that or adjacent compartments. The gas displaces
oxygen and chemically interferes with combustion, enabling it to extinguish fires rapidly in enclosed spaces. In high concentrations, it can cause
narcosis, which progresses by stages into excitation, mental confusion, lethargy, and ultimately
asphyxiation. but was later revised to 41 by the Amurskiy Shipbuilding Company, some of whose employees were among the injured. Many of the injured were reported to have suffered from
frostbite caused by the chilling effect of the gas. Following the incident, the and the rescue vessel
Sayany were dispatched from Vladivostok to provide assistance to the stricken submarine. The injured survivors were transferred to the destroyer and sent to military hospitals for treatment, while the submarine returned under its own power to Primorsky Krai. == Causes ==