Soviet submarine K - 431 Soviet submarine " K - 431 " ( originally the Soviet submarine " K - 31 " ) was a Soviet nuclear - powered submarine that had a reactor accident on 10 August 1985 . An explosion occurred during refueling of the submarine at Chazhma Bay , Vladivostok . There were ten fatalities and 49 other people suffered radiation injuries . TIME magazine has identified the accident as one of the world ' s " worst nuclear disasters " . Reactor refueling disaster . The K - 431 , completed around 1965 as unit K - 31 , was a Project 675 ( Echo II ) class submarine with two pressurized water reactors , each 70 MW capacity and using 20 % enriched uranium as fuel . On 10 August 1985 , the submarine was being refuelled at the Chazhma Bay naval facility near Vladivostok . The submarine had been refuelled and the reactor tank lid was being replaced . The lid was laid incorrectly and had to be lifted again with the control rods attached . A beam was supposed to prevent the lid from being lifted too far , but this beam was positioned incorrectly , and the lid with control rods was lifted up too far . At 10 : 55 AM the starboard reactor became prompt critical , resulting in a criticality excursion of about 5x10 fissions and a thermal / steam explosion . The explosion expelled the new load of fuel , destroyed the machine enclosures , rupturing the submarine ' s pressure hull and aft bulkhead , and partially destroyed the fuelling shack , with the shack ' s roof falling 70 metres away in the water . A fire followed which was extinguished after 4 hours , after which assessment of the radioactive contamination began . Most of the radioactive debris fell within 50 – 100 metres of the submarine , but a cloud of radioactive gas and particulate s blew to the northwest across a 6 km stretch of the Dunay Peninsula , missing the town of Shkotovo - 22 , 1 . 5 km from the dock . The contaminated forest area was later surveyed as 2 km in a swath 3 . 5 km long and 200 to 650 metres wide . Initial estimates of the radioactive release were about 74 PBq ( 2 MCi ) of noble gas es and 185 PBq ( 5 MCi ) of other fission product s , but most of this was short - lived isotopes ; the estimated release inventory one hour after the accident was about 37 TBq ( 1000 Ci ) of non - noble fission products . In part because the reactor did not contain spent fuel , the fraction of biologically active isotopes was far smaller than in the case of the Chernobyl disaster . M . Takano " et al . " suggest that only 29 GBq of I - 131 was released , but larger amounts ( 620 GBq of I - 133 and 1840 GBq of I - 135 ) of other isotopes . The same source suggests that the total release was about 259 PBq but due to radioactive decay this decreased to 43 TBq after 24 hours . The same source suggests that the fission yield was 5e18 fissions which would deliver 156 MJ of heat into the reactor . Ten naval personnel were killed ( 8 officers and 2 enlisted men ) , probably by the explosion itself and not from radiation injuries . Radiation injuries were observed in 49 people , with 10 developing radiation sickness ; the latter figure included mostly firefighters , some of whom sustained doses up to 220 rad ( 2 . 2 Gy ) external and 400 rem ( 4 Sv ) to the thyroid gland . Of the 2 , 000 involved in cleanup operations , 290 were exposed to high levels of radiation compared to normal standards . High - level waste gathered during clean - up operations was placed in temporary disposal sites . Due to the rapid decay of most of the fission products and the cleanup operations , some dockyard facilities were able to resume operations four days later . About two months post - accident the radioactivity in water in the cove was comparable to background levels , and 5 – 7 months post - accident the radiation levels were considered normal throughout the dock area . The damaged submarine was towed to Pavlovsk Bay and berthed there . Consequences . There were 10 fatalities and 49 other people suffered radiation injuries .