Yen-Shu Chen

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.

The accident that occurred at the Fukushima Daiichi Nuclear Power Plant is a reminder of the danger of hydrogen explosion within a reactor building. Sufficiently high hydrogen concentration may cause an explosion that could damage the structure, resulting in the release of radioisotopes into the environment. In the first part of this study, a gas diffusion experiment was performed, in which helium was used as the working fluid. An analytical model was also developed using the GOTHIC code and the model predictions of the helium distribution were found to be in good agreement with the experimentally measured data. In the second part of the study, a model of the Mark III containment of the Kuosheng Plant in Taiwan was developed, and was applied to a long-term station blackout (SBO) accident similar to that of the Fukushima plant. The hydrogen generation was calculated using the Modular Accident Analysis Program and was used as the boundary condition for the GOTHIC containment model. The simulation results revealed that the hydrogen concentration at the first floor of the wetwell in the containment reached 4% 9.7 h after the accident. This indicated the possibility of dangerous conditions inside the containment. Although active hydrogen ignitors are already installed in the Kuosheng plant, the findings of this study indicate that it may be necessary to add passive recombiners to prolong an SBO event.