Date: 12 February 1965
Location: Severodvinsk, USSR, aboard K-11 nuclear submarine
Type of event: reactor accident during refueling
A criticality excursion took place during refueling of the submarine K-11 at Severodvinsk. The K-11 was a Project 627 (November) class submarine commissioned on 23 December 1961; it had two pressurized water reactors, each 70 MWt capacity using 20% enriched uranium as fuel. Around 7-10 February 1965 the submarine was being refueled 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 the beam was positioned too high. The lid with the control rods was lifted too far and the reactor went critical. Some radioactive steam was emitted and was detected by radiation monitors, prompting the termination of operations and withdrawal of personnel. The nature of the event was not identified, however, and on 12 February operations resumed, with the lid again being lifted too far. This time when the reactor went critical it overheated, ejecting radioactive steam from the reactor, causing the lid to fall on the reactor tank at an angle, and starting a fire in the reactor compartment. Water used to fight the fire spread to other compartments in the submarine. Seven crew members suffered radiation injuries. The reactor compartment had to be replaced, and the two reactors were dumped in Abrosimova Bay.
Consequences: 7 injuries.