nuclear sub breach

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.

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

Wikipedia lists six lost Soviet nuclear submarines. Details are sketchy.

http://en.wikipedia.org/wiki/Category:Lost_nuclear_submarines