Safety evaluation of an extended refilling mode operation after the permanent shutdown of a BWR/4 reactor

摘要

As limited by the capacity of the spent fuel pool (SFP), the last discharged full core will remain in the core for storage for an undetermined period. As a result, an extended refilling mode operation will be expected after the permanent shutdown of a BWR/4. During the extended refilling operation, the SFP, refilling cavity and reactor vessel are all connected. In the early transition phase, the residual heat removal (RHR) system will provide cooling to the reactor core, while the enhanced SFP cooling system will provide cooling to the SFP. While after the transition phase, the enhanced SFP cooling system will provide cooling to both the SFP and reactor core. An evaluation was performed to determine the duration of the transition phase containing both RHR and SFP cooling systems. Furthermore, systematic safety analysis was also performed to ensure the safety of the extended refilling mode operation. In this paper, advanced modeling techniques for RELAP5 and MELCOR were successfully developed to analyze the thermal hydraulic response of the extended storage pool. A postulated loss of cooling accident of the extended storage pool without any emergency mitigation was analyzed by RELAP5 and MELCOR, and good agreement between these two codes was observed.According to the steady state analysis of the extended storage pool, the early transition phase with RHR system cooling to the core will be three days, to assure the maximum water temperature in the core below the technical specification limits. Moreover, an assessment of a LBLOCA with rupture at recirculation line was performed by RELAP5 model. It was indicated that the existing makeup system cannot provide effective makeup coolant to the reactor core, unless the makeup coolant could be directed into the core shroud. It was also demonstrated by RELAP5 simulation that only the core spray can provide effective cooling to the reactor core, even under the worst recirculation line break scenario.