Until 1990, the Soviet Navy routinely dumped radioactive waste in Far Eastern and Arctic waters. There were 13 areas of nuclear waste dumps in Arctic seas and 10 areas off-shore in the Russian Far East.The Russian navy simply sank 57 vessels filled with nuclear waste. Sixteen decommissioned reactors were also sent to the deep, including six with unloaded fuel.
If my memory serves me correctly, the Russians would cut the reactor out of the bottom of their aging nuclear subs and deposit them in the same freezing ocean water.