EPRI MAAP5 Fukushima Daiichi Analysis

The study presented in this paper summarizes work conducted as part of the Electric Power Research Institute (EPRI) Fukushima Technical Evaluation project. This effort was designed to develop a representation of the core damage events that occurred at Fukushima Daiichi Units 1, 2, and 3 using the analytical capabilities provided by the EPRI Modular Accident Analysis Program, Version 5 (MAAP5). The analytical investigations of Fukushima Daiichi performed with MAAP5 indicate that core-melt progressions at Units 1, 2, and 3 likely span a range of core damage conditions. The core status at Unit 1 is likely consistent with a large fraction of core debris having relocated into the containment. By contrast, the MAAP5 evaluations indicate that there is a reasonable potential for a significant fraction of core debris to be retained inside the reactor pressure vessel (RPV) at Unit 2. The corresponding Unit 3 simulations, however, highlight the important role that degraded high-pressure coolant injection at low RPV pressure may have played in promoting some relocation of core debris out of the RPV and into containment. The detailed containment evaluations conducted as part of this study also highlight the critical role played by thermal stratification phenomena (either in the suppression pool or in the drywell) in influencing the magnitude of containment pressure and thermal challenges. These simulations highlight the potentially critical role that thermal stratification in the upper drywell may have played in accelerating the onset of leakage through the drywell head flange due to thermal degradation of the drywell head gasket. Finally, these simulations provide good representations of the occurrence of flammable conditions in the Units 1 and 3 reactor buildings, supporting the nature and timing of the observed reactor building combustion events.