An initial period of film boiling has been identified as a key element in the chain of events leading to a large-scale vapor explosion. From theoretical considerations, the conditions required for molten UO2 particles (assumed spherical) to undergo film boiling in water and sodium coolants are assessed. Results indicate that under most conditions when UO2 droplets are quenched in water or sodium, film boiling is expected, indicating that such systems could satisfy the initial coarse mixture requirement for explosive vaporization. In addition, it is shown that experimental vapor explosion results that have been interpreted in terms of spontaneous nucleation theory may likewise be viewed in terms of film boiling destablization.