Thermal-hydraulic analysis of NSSS and containment response during extended station blackout for Maanshan PWR plant

Abstract

A thermal-hydraulic analysis has been performed with respect to the response of the nuclear steam supply system (NSSS) and the containment during an extended station blackout (SBO) duration of 24h in Maanshan PWR plant. Maanshan plant is a Westinghouse three-loop PWR design with rated core thermal power of 2822MWt. The analyses in the NSSS and the containment are based on the RELAP5-3D and GOTHIC models, respectively. Important design features of the plant in response to SBO are considered in the respective models, e.g., the steam generator PORVs, turbine driven auxiliary feedwater system (TDAFWS), accumulators, reactor coolant pump (RCP) seal design, various heat structures in the containment, etc. In the analysis it is assumed that the shaft seal in each RCP failed due to loss of seal cooling and the RCS fluid flows to the containment directly. Some parameters calculated from the RELPA5-3D model are input to the containment GOTHIC model, including the RCS average temperature and the RCP seal leakage flow and enthalpy. The RCS average temperature is used to drive the sensible heat transfer to the containment. It is found that the severity of the event depends mainly on whether the secondary side is depressurized or not. If the secondary side is depressurized in time (within 1h after SBO) and the TDAFWS is available greater than 19h, then the reactor core will be covered with water throughout the SBO duration, which ensures the integrity of the reactor core. On the contrary, if the secondary side is not depressurized, then the RCS pressures will remain high in conjunction with the higher RCP seal leakage flow. The accumulators will not be available due to high RCS pressure and the reactor core will eventually become uncovered since there is no any water make-up. In the aspect of the containment response, the high-energy RCP seal leakage fluid continues flowing into the containment and heats up the containment. The containment pressure and temperature will increase to high values, respectively. There exists no clear relationship between the available TDAFWS time and the maximum containment pressure and temperature. The response of the containment temperature is much worse than that of the containment pressure. The most severe containment temperature response occurs for the case with no secondary depressurization and the calculated maximum containment temperature is 336.8°F, which exceeds the design temperature of 300°F but is still below the inside-containment safety-related equipment environmental qualification temperature of 450°F.