The analogy between thermal reactive and chemical reactive flows suggests that all propagating thermal explosions have a detonation-like (i.e., shock) structure. We show, however, that very high pressures and efficiencies need not necessarily be achieved. We consider a number of fragmentation mechanisms that could sustain these explosions, including hydrodynamic fragmentation, for which new results are presented; these results show the mechanism to be effective over a wide range of shock strengths. A vapor detonation model, which allows for thermal disequilibrium in the coolant, is developed. Predictions are in broad agreement with the characteristics of metal-water interactions. It is suggested that similar nonequilibrium effects may limit the efficiency of UO2-water detonations; this is less likely with the U02-sodium system, however, because of the high conductivity of the coolant. It may be difficult to achieve coarse intermixing with UO2-sodium; however, if this does occur, the possibility of a high efficiency interaction remains.