How many nuclear submarines have ever sank?

A total of 8 known nuclear submarines have sunk (all but one due to accidents) in the almost 55 years since they were created.

U.S. NAVY

USS Thresher (SSN-593) - Thresher-class (lead boat) Fast-Attack. April 10, 1963, during sea trials off Cape Cod. Major hull valve failure, leading to the SUBSAFE program.

USS Scorpion (SSN-589) - Skipjack-class Fast-Attack. May 22, 1968, returning home from Med deployment. Suspected torpedo hot-run incident and subsequent detonation before disarming.

SOVIET NAVY

K-27 - Experimental Soviet Fast-Attack, (2 lead-bismuth reactors in a modified November-class hull). Scuttled after a reactor control rod failure in the Kara Sea, September 6, 1982, in violation of International Regulations.

K-8 - November-class Fast-Attack, sank April 11, 1970 while being towed in rough seas following a major onboard fire.

K-219 - Yankee I class Ballistic Missile submarine. Sank East of Bermuda on October 3, 1986 while being towed after suffering major casualties and damage following an onboard missile explosion.

K-218 (Komsomolets) - Mike-class Fast-Attack, sank April 7, 1989 following a major onboard fire.

RUSSIAN NAVY (Post USSR)

K-214 (Kursk) - Oscar II class Fast-Attack. Sank August 12, 2000 in the Barents Sea following multiple onboard torpedo explosions (one triggered 2 others). The Kursk is the only nuclear submarine to sink after suffering major damage and later be salvaged. The History Channel and Discovery Channel often show documentaries on the raising of the Kursk.

K-159 - Decommissioned November-class Fast Attack. Sank August 28, 2003 after she broke from her mooring pontoons.

All boats are periodically checked for radation leakage of any type (reactors or weapons) and for any evidence of wreck tampering / attempted salvage. Most of the wrecks are in waters deep enough to preclude any salvage or tampering though.