Most high explosives will not detonate when burnt. They need to have a shock wave running through them for the detonation to occur. The initial shock wave is us

Most high explosives will not detonate when burnt. They need to have a shock wave running through them for the detonation to occur. The initial shock wave is usually created using a detonator, which uses a very small amount of a much more risky explosive. You can get a lump of C-4, and use it as fuel for a fire to cook over without any danger, and in fact soldiers certainly used to do this. You can also hit it with a hammer - the shock wave caused by this isn't strong enough to cause a detonation. In terms of a nuke, you need to detonate a set of independent lumps of explosive with very precise timing, in order to get a compression lens that will squash the fissile core sufficiently to get a nuclear explosion. Heat is unlikely to cause this to happen with a sensibly-designed bomb.