An addendum to the great w/u by The Custodian: There were a couple of submarines lost to reactor failure, most notably the USS Thresher. While not actually lost due to reactor failure by-products (explosion, radiation, heat), the sub was lost due to a reactor shutdown.

The Thresher was designed to dive deeper than previous designs, and while on a test voyage, was operating at extreme depth when a leak developed in the piping in the reactor room. This caused a short-circuit, and the reactor shut down automatically. In the Navy's infinite wisdom, no back-up power supply was included in the submarine's design. It was this reactor shutdown that doomed the boat.

While attempting to restart the reactor (the sub was dead in the water due to a lack of power), the Thresher slipped below crush depth (the point where sea pressure exceeds the strength of the sub walls to resist it) and the ship was lost.