Investigation on influence of the RPV depressurization on the PCV top head thermal load during the severe accident of BWR

• Residual risk of Boiling Water Reactor severe accident management is investigated. • All means of water injections are hypothetically assumed to be unavailable. • Vessel exterior thermal insulation assures no threat to the containment top head. • Radiation from the steam dryer is the dominant in-vessel heat transfer. • The analysis results support the validity of the current depressurization strategy. This study focuses on the BWR Reactor Pressure Vessel (RPV) depressurization management regarding thermal load on the Primary Containment Vessel (PCV) top head when all means of water injections have been lost including the reactor well. The primary concern is the potential threat to the PCV top head when the RPV dome heats-up to high temperature before and during the RPV depressurization until the RPV failure, which may lead to excessive thermal load on the PCV top head. The analysis model of BWR-4 with Mark-I containment for MELCOR 2.2 code has been developed, considering the exothermic core oxidation, core slumping, fission product (FP) release and depositions carrying decay heat, and conductive, convective and radiative heat transfers arising from such in-vessel events. Three key findings have been gained through the analyses. First, the thermal insulation of the RPV dome greatly reduces the thermal load on the PCV top head and its integrity is not threatened before the RPV failure. Second, the dominant heat transfer path to the RPV dome is the radiative heat transfer from the steam dryer, which is heated by in-vessel convective heat transfer and FP depositions (heat transport). Third, the in-vessel heat transfer and transport are highly dependent on the core degradation scenarios, which inevitably involve some randomness in nature and modeling uncertainties. Overall, these findings provide important insights into evaluating the PCV top head thermal load related to in-vessel accident progression prior to and during the RPV depressurization until the RPV failure.