Methodology of core integrity and radiation dose assessment for postulated LOCA scenario during long-term shutdown period in Kuosheng NPP

Abstract The Kuosheng Nuclear Power Plant, operated by the Taiwan Power Company, is currently undergoing decommissioning. Since the spent fuel pool has reached full capacity and the dry storage system has not yet received an operational license, spent fuel assemblies currently residing in the reactor core cannot be relocated. This study establishes an analytical methodology utilizing the RELAP5-3D code to evaluate the reactor pressure vessel (RPV) water level and peak cladding temperature in a postulated loss-of-coolant accident (LOCA) occurring under these specific constraints. In addition, MicroShield software is employed to assess the radiation dose received by control room personnel, considering the shielding effects of the plant’s structural components in scenarios where a LOCA leads to partial fuel uncovering. The objective is to determine whether the resulting dose exceeds regulatory safety limits. Results from the RELAP5-3D analysis indicate that, in the event of a recirculation pipe double-ended break, the reactor water level would decrease to approximately two-thirds of the active core fuel height and then stabilize there. Furthermore, the MicroShield analysis reveals that, for the scenario that LOCA occurs one year after shutdown and one-third of the core fuels are uncovered, the highest radiation dose occurs for the case with only the drywell wall providing shielding. Nevertheless, the calculated dose remains significantly below the regulatory limits, indicating that personnel safety can be assured even under these conservative assumptions.