This study extends previous investigations on the Pre-Defueled (PD) phase of the Decommissioning Transition Stage for a reference BWR-4/Mark-I plant by examining internal flooding risk under the same transitional plant configuration. During the PD phase, spent fuel remains in the reactor core, and the reactor cavity is hydraulically connected to the Spent Fuel Pool (SFP). Decay heat removal is maintained by the Residual Heat Removal (RHR) system or the SFP additional cooling system (SFPACS), depending on system alignment.
While earlier work focused on internal initiating events and the evolution of fuel risk during the PD phase, the present study addresses internal flooding as a hazard that may challenge electrical distribution and decay heat removal capability. An internal flooding Probabilistic Risk Assessment (PRA) is developed in accordance with the Electric Power Research Institute (EPRI) methodology, incorporating flood initiating sources, propagation pathways, and their impacts on safety-related functions.
Quantification results indicate that the dominant contributor to flooding risk is a flood propagation scenario originating in the 4.16-kV switchgear room. The associated flood area exhibits substantially higher risk significance than other evaluated zones, primarily due to its influence on power supply availability and the integrity of decay heat removal functions.
The results provide additional risk insights into hazard management and configuration control during the PD phase, complementing prior internal event analyses and further clarifying the risk profile of the Decommissioning Transition Stage.