Simulations of the Pressurizer Safety Valve Stuck Open Accident for the Development of the Accident Sequences in the Low Power/Shutdown PSA

We have simulated the pressurizer safety valve (PSV) stuck open accident in a Korea Standard Nuclear power Plant (KSNP). The purpose of the simulation is 1) to investigate the mitigation function available for the important accident sequence having considerable core damage frequencies and 2) to support the determination of the accident sequences with relevant success criteria for the Low Power Shutdown (LP&S) Probabilistic Safety Assessment (PSA). The analysis showed that the PSV stuck open accident in a KSNP has both the characteristics of a small and medium LOCA in the sense that the primary system pressure decreases slowly but the break flow is sufficient enough to uncover the core in the early stages of the accident. we found that, in the accident sequence of a high pressure safety injection (HPSI) failure, core damage could occur earlier before reaching the actuation set-pressure of the safety injection tank (SIT) provided that no operator action is considered. We also performed the simulation of a rapid cool-down by a steam generator for this accident sequence to investigate the feasibility of the SIT injection before core damage occurs. From these simulations, if the operators start the rapid cool-down operation within 15 minutes, it was shown that SIT could be injected and core damage could be prevented if the subsequent Low Pressure Safety Injection (LPSI) and Shutdown Cooling System (SCS) is successfully operated.