The nuclear submarine *USS Thresher *sank on April 10, 1963. The US Navy periodically monitors the site, and so far no contamination has been detected.
The second USS Thresher (SSN-593) was the lead boat of her class of nuclear-powered attack submarines in the United States Navy. She was the U.S. Navy's second submarine to be named after the thresher shark.
On 10 April 1963, Thresher sank during deep-diving tests about 350 km (220 mi) east of Boston, Massachusetts, killing all 129 crew and shipyard personnel aboard. It is the second-deadliest submarine incident on record, after the loss of the French submarine Surcouf, in which 130 crew died....
http://www.history.navy.mil/danfs/t/thresher.htm
The USS Scorpion sank on June 30, 1968. So far, no contamination has been detected. The wikipedia article speculates that
The most likely scenario is that the plutonium and uranium cores of these weapons corroded to a heavy, insoluble material soon after the sinking, and they remain at or close to their original location inside the torpedo room of the boat. If the corroded materials were released outside the submarine, their large specific gravity and insolubility would cause them to settle down into the sediment.
USS Scorpion (SSN-589) was a Skipjack-class nuclear powered submarine that served in the United States Navy and the sixth vessel of the U.S. Navy to carry that name.
Scorpion was lost on 22 May 1968, with 99 crewmen dying in the incident. She is one of two nuclear submarines the U.S. Navy has lost, the other being USS Thresher. It was one of four mysterious submarine disappearances in 1968, the others being the Israeli submarine INS Dakar, the French submarine Minerve, and the Soviet submari...
Wikipedia lists six lost Soviet nuclear submarines. Details are sketchy.
http://en.wikipedia.org/wiki/Category:Lost_nuclear_submarines
