Effectiveness re-evaluation on the intentional primary system depressurization during Zion-like Westinghouse PWR station blackout considering pressure dependence of radionuclides release

Abstract

Previous studies have successfully resolved the issue of high-pressure melt ejection (HPME) followed by direct containment heating (DCH) during a total station blackout of Zion-like Westinghouse pressurized water reactor (PWR). This resolution is crucial, as the earliest occurrence of hot-leg creep failure can cause a decrease in the pressure difference between the reactor pressure vessel (RPV) and the primary containment vessel (PCV), falling below the cut-off pressure for HPME/DCH. As a recommended accident management strategy, intentional depressurization of the reactor coolant system (RCS) has been proposed. Depressurization leads to a delayed accident progression due to accumulator injection and maintains a pressure difference at the time of RPV failure below the cut-off pressure. In the present analyses using MAAP5, it was observed that depressurization before core heat-up, achieved by opening 2 power-operated relief valves (PORVs), resulted in the most delayed RPV failure, consistent with previous studies. However, present sensitivity analysis considering the pressure-dependent release of radionuclides from the fuel revealed significant changes in both the accident progression and the point of creep failure. In addition, the creep-failure point shifted from the RPV bottom to the RPV sidewall, and the pressure difference at that location exceeded the cut-off pressure. To prevent sidewall failure of the RPV, it may be advisable to employ a depressurization rate higher than that achieved by using 2 PORVs, even if it slightly accelerates the accident progression.