An explosion occurred during refueling of the submarine at Chazhma Bay in the far eastern Soviet port of Vladivostok.
The reactor explosion on-board Soviet Project 675 (Echo II) class submarine K-431 was caused by a technical error during it's reactor's refueling procedure.
The reactor lid was laid incorrectly and had to be lifted again with the control rods attached and the lid with control rods was lifted up too far.
The starboard reactor had a prompt criticality event and then a power excursion of about 5x1018 fission and a so a thermal/steam explosion the occurred.
The new load of fuel expelled by the blast that also destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the submarine re-fueling shack with such force the shack's roof fell 70 meters away in the near by bay! The fire that followed was extinguished after 4 hours. Most of the radioactive debris fell within 50–100 meters of the submarine, so only a small part of the dock needed to bey examined and decontaminated. The reactor did not contain spent fuel since it was being re-fueled and was partly filled with new fuel rods so reducing it's out put of the biologically active isotopes that were also limited a Chernobyl reactor accident.
A cloud of radioactive moderately gas, dust and particulates blew to the northwest, traveling across a 6 km stretch of the Dunay Peninsula, lucky missing the town of Shkotovo-22, which was only 1.5 km from the dock.
The high-level waste was placed in temporary disposal sites and later dumped safely. Decontamination was over after 6 month and the pace was safe to use again. The damaged submarine was towed to Pavlovsk Bay and then berthed there untill it was scrapped in 1987.
10 crewmen (8 officers and 2 enlisted men) died in the initial exploitation, 49 were injured in the accident and 10 were later made ill by the resultant radiation leak.
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- Atomic accidents and disasters