[en] When core material reaches melting conditions severe degradation of the core geometry occurs. Data available on the core behavior in a severely degraded state suggest that extensive blockage of the flow channels would occur. If a sufficient bypass is available for the gas flow, such as in the LOFT LP-FP-2 test, severe retardation of the hydrogen and fission product sources from the degraded channel is suggested from the available data. This phenomena is expected to occur in an LWR core and should be considered by core models that are used for severe accident analysis. In the MAAP code it is done by preventing gas flow through molten core regions. Good agreement is obtained with all relevant data that are directly applicable to LWR accident conditions. A more mechanistic model for the freezing of core material and its effect on the coolant channel geometry is currently being investigated by the US Department of Energy Advanced Reactor Severe Accident Program (ARSAP). 8 refs., 2 figs