Integrated modeling of debris bed heat and mass transfer in the PWR lower plenum for severe accident analysis

An integrated model, simulating the interactions between the core debris and the reactor pressure vessel (RPV) lower head and internal structures during a severe accident, has been developed for the Modular Accident Analysis Program Version 4 (MAAP 4). Typical internal structures include pressurized water reactor (PWR) grid plates and flow distributors, core barrel, and upper plenum internals. Heat transfer from the core debris to the RPV and these structures could increase the temperatures to levels where the integrity of individual structures could be jeopardized.