The K-19, the first Soviet submarine to carry ballistic missiles, was on its first training voyage in the neutral waters of the North Atlantic in July 1961 when its reactor cooling system sprang a leak, sending the core temperature skyrocketing and threatening a meltdown.
The craft's captain and 139-man crew stayed on board to repair the system, exposing themselves to severe radiation.
Eight crew members died within weeks, 12 died over the next two years and 20 others suffered long-term illnesses.
"It could have been another Chernobyl near America ... but on a much greater scale," said Alexander Nikishin, an organizer of the anniversary ceremony, referring to the explosion and fire at a Soviet nuclear power plant in Ukraine in 1986.
Some experts say an explosion in the submarine's reactor could have caused ballistic missiles and torpedoes aboard the submarine to also explode, which could have been interpreted as a pre-emptive strike by the United States and could have triggered a third world war.
The incident was not publicized in the Soviet era because the operation was regarded as a failure and the crew members seen as not deserving recognition, Nikishin said.
The first crew members to die were buried secretly in lead coffins in order to contain the radiation from their bodies, Nikishin said.
But the crew's heroism was finally recognized following the Soviet collapse, and last year former Soviet leader Mikhail Gorbachev called for K-19 crew members to be awarded the Nobel Peace Prize.
The accident was the basis for the 2002 Hollywood movie "K19: The Widowmaker" starring Harrison Ford and Liam Neeson.
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