The huge storage tanks in Russia's Kola Peninsula contain spent nuclear-fuel rods that were used to power Russian nuclear submarines.
Bellona said salt water has leaked into the tanks and could cause an "uncontrolled chain reaction."
The group cited a report by the Russian Federal Atomic Energy Agency, Rosatom, describing the danger.
Nuclear Power's Resurgence
Nuclear-fuel pellets being produced in Kazakhstan (TASS)
LOOKING FOR ALTERNATIVES. Despite warnings from scientists, the world seems unable to stem its insatiable appetite for fossil fuels, which are producing the greenhouse gases blamed for global warming. And emissions are set to rise dramatically in the next few years. Is it time for the environmental movement to drop its opposition to nuclear power? (more)