K-431, completed around 1965 as unit K-31, was a Project 675 -class submarine with two pressurized water reactors, each 70 MWt 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 5·1018fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured 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 of the submarine, but a cloud of radioactive gas and particulates blew to the northwest across a stretch of the Dunay Peninsula, missing the town of Shkotovo-22, from the dock. The contaminated forest area was later surveyed as in a swath long and wide. Initial estimates of the radioactive release were about 74 PBq of noble gases and 185 PBq of other fission products, but most of this was short-lived isotopes; the estimated release inventory one hour after the accident was about 37 TBq 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 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 5·1018 fissions which would deliver 156 MJ of heat into the reactor. Ten naval personnel were killed, 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 external and 400 rem 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.
