Which was the first nuclear submarine lost to an accident?

USS Thresher (SSN-593), lead ship of her class, was lost on April 10, 1963, during post-shipyard sea trials. The loss of the Thresher (believed to be the result of a chain of events beginning with a pipe joint failure and frozen air lines) led to the SUBSAFE Quality Assurance program, as well as re-engineering of critical systems and emergency procedure changes.