I spoke with one of the key investigators of the tragedy; he was from Babcock and Wilcox. The initial failure was a braised joint in a line which was a hull penetration. Upon failure, sea water leaked into a circuit breaker room and shorted most of the electrics and shut down the reactor. Some key personnel were not on this dive; their presence may have averted the disaster. At any rate, trying to blow the ballast with emergency methods froze the valves and prevented the sub from surfacing.
Methods were later developed to ultrasonic test brazed joints - there was no good method when Thresher was built.