Nuclear power has a long history of safety, but people tend to remember the tragic accidents that have caused immense ecological damage. Unfortunately, nine nuclear submarines have sunk, either due to accidents or intentional sinking. The Soviet Navy lost five, the Russian Navy two, and the United States Navy two. The causes of sinking ranged from fires to floods to bad weather.
The question remains: are these sunken submarines a source of radiation? The nuclear cores of these submarines, made of uranium and plutonium, are not soluble in water and are likely to oxidize. This means that the risk of radiation poisoning is low, as long as the nuclear reactor is properly secured. The K-278 Komsomolets nuclear submarine sank in 1989 and had cracks in its titanium hull. Losing a nuclear submarine is not something any navy wants to experience, but if it does, the risk of radiation is much lower than if it were on land or in the air.
This is because all possible precautions are taken to ensure that the crew is safe from radiation poisoning due to the nuclear reaction that powers the vessel. Three nuclear submarines have been lost with all hands on board: two by the United States Navy (129 and 99 lives lost) and one by the Russian Navy (118 lives lost). The second USS Thresher, a nuclear-powered attack submarine of the United States Navy, sank in 1963 during deep dive tests about 220 miles east of Boston, resulting in more than a hundred casualties. The Soviet submarine K-129 was carrying nuclear ballistic missiles when it was lost with all hands, but since it was a diesel-electric submarine, it is not included in this list. The nuclear submarine Scorpion is getting a lot of attention due to its powerful nuclear reactor and two Mark 45 torpedoes with a nuclear tip.
It currently rests on the floor of the Barents Sea, a mile deep. In conclusion, while losing a nuclear powered ship is not ideal, the risk of radiation poisoning is low as long as all safety measures are taken and the reactor is properly secured.