American Experience on PBS. It's the story of the Titan II ICBM which blew up in it's silo in 1980. Luckily the 9 megaton thermonuclear warhead, though ejected out of silo, did not detonate.
I was a college freshman at the time. Never forget it.
There are good reasons why the warhead didn't detonate: the fission component (the pit) has to be uniformly inploded inward by a shaped-charge explosion for the pit to achieve critical mass and trigger the primary fission reaction. On older American nukes, there were cryptographical codes in PALs, permissive action links, needed to enable the mechanisms for setting up the possible shaped-charge and fission reactions needed for nuclear detonation: in some weapons, if unauthorized attempts were made to tamper with or detonate a nuke, the warhead disabled itself. Without the primary exploding, the fusion component, the secondary, fusion reaction cannot occur. There can't be a thermonuclear reaction without the primary fission component triggering the secondary fusion component. Naval submarine missiles never had PALs for obvious reasons: hard to steal a warhead in a submerged sub. Russians never bothered with PALs.
Fire is not enough to trigger fission, but radiation leakage from a damaged or burned warhead is a hazard.
Second-generation explosives needed to compress the pit in the primary are much more stable.
To understand thermonuclear weapons, look up the TellerUlam design: all modern thermonuclear weapons are of this design. Some countries with nuclear arms may have only fission or boosted-fission warheads, not thermonukes.