Multi-channel GOTHIC model of a postulated spent fuel pool leakage accident for the Maanshan PWR units

Two pressurized water reactor (PWR) units of the Maanshan Nuclear Power Plant are undergoing decommissioning. All spent fuel assemblies are currently stored in the spent fuel pools (SFPs) because the dry-cask interim storage facility has not yet installed. The SPF safety is a priority for this plant. This study develops a multi-channel thermal-hydraulic model for the Maanshan SFP using the GOTHIC code. Under normal condition with operable SFP Cooling and Cleanup System, the calculated bulk pool temperature is in good agreement with the plant’s Final Safety Analysis Report (FSAR), providing confidence in the modeling approach. Analyses for postulated leakage accidents with spray mitigation are then performed. Calculated results show that using coarse axial nodalization can produce artificial oscillations in the water level when the liquid-vapor surface crosses the interface between two vertical nodes. Furthermore, unusual temperature spike in the hot channel may be introduced with coarse axial nodes. Results show that this temperature spike can be eliminated when each fuel channel is discretized with more than 20 axial nodes. A leakage accident without spray mitigation will lead to fuel overheating. The Maanshan plant has installed the spray system above the SFP, and the calculated results show that the spray system can sufficiently suppress the peak cladding temperature in a leakage accident, maintaining the cladding temperature below regulatory limits.