Hastily built by the USSR in response to United States’ developments in nuclear submarines as part of the arms race, the Soviet K-19 submarine developed a complete loss of coolant to one of its two reactors on its first voyage in June 1961. To avoid a disaster, the engineering crew sacrificed their own lives to devise a secondary coolant system and keep the reactor from a nuclear meltdown.

Over the next two years, the K-19 experienced several other accidents (including two fires and a collision) that killed twenty-two crew members. The series of accidents inspired crew members to nickname the submarine “Hiroshima.”

Source: Wikipedia