[en] The thermal behavior of the fuel and the cladding during off-normal operating conditions in light water reactors (LWRs) are of great importance to reactor safety evaluation. In the present work, fuel conditions during a hypothesized power-cooling-mismatch (PCM) accident in LWRs are characterized with regard to pellet cracking and fuel melting. Melting of UO₂ fuel at the center of the pellet during a PCM accident in which film boiling is present at the cladding surface, is assessed by employing a simplified steady state analysis. The induced pressure at the center of the pellet due to fuel melting, fission gas release, and UO₂ fuel vapor may force molten fuel to penetrate through radially open cracks in the outer unmelted portion of the pellet and relocate in the fuel-cladding gap. The contact of molten fuel with Zircaloy cladding, which is at high temperatures during film boiling, may initiate cladding melting at its inside surface and eventually result in a thermal failure of the cladding. An analytical model is developed to study the transient freezing of a superheated liquid penetrating an initally empty crack, maintained at a constant, subfreezing temperature