Database of radiological incidents and related events--Johnston's Archive

K-431 submarine reactor accident, 1985

compiled by Wm. Robert Johnston
last modified 10 July 2022

Date: 10 August 1985

Location: Chazhma Bay, near Vladivostock, Russia, USSR, aboard USSR submarine K-431

Type of event: reactor accident during refueling


An explosion occurred during refueling of the submarine K-431 at Chazhma Bay, Vladivostok. 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 MWt capacity and using 20% enriched uranium as fuel. (Note that some sources confuse this submarine with K-314, a Project 671 or Victor I class submarine launched in 1972 and withdrawn from service after a reactor accident in December 1985.) On 10 August 1985, the submarine was being refueled at the Chazhma Bay naval facility near Vladivostock. The submarine had been refueled but one reactor lid was leaking so the reactor tank lid was being removed 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 were lifted too far up. At 10:55 AM the starboard reactor became supercritical, resulting in a criticality excursion of about 5x1018 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 meters 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 meters of the submarine, but a cloud of radioactive gas and particulates blew to the northwest across a 6-km stretch of the Dunai Peninsula, missing the town of Shkotovo-22, 1.5 km from the dock. The contaminated forest area was later surveyed as 2 square km in a swath 3.5 km long and 200-650 meters wide. Estimated initial radioactive release was about 2 MCi of noble gases and 5 MCi of other fission products, but most of this was short-lived isotopes; the estimated release inventory one hour after the accident was about 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 reactor accident.

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 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 were placed in temporary disposal sites. Due to the rapid decay of most of the reactor 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: 10 fatalities (probably due to non-radiation injuries), 49 injuries.


© 2004-2007, 2022 by Wm. Robert Johnston.
Last modified 10 July 2022.
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