Examination of SBO Emergency Operating Procedures of APR1400 using MARS code

For APR1400, the emergency operating procedure (EOP) for SBO is to maintain hot standby condition until power is recovered. With the Fukushima accident, the capability of NPPs against extended SBO becomes one of the important safety issues. To cope with the extended SBO, portable equipment is placed at sites. We have examined the optimum approach to cope with longer term SBO with easy transition to use portable equipment. Hwang et al., developed optimum mitigation procedure (OMP) which minimize operator action. Kim et al conducted sensitivity study to find the optimum range of auxfeed flow and atmospheric dump valve (ADV) opening for the secondary side feed and bleed operation. In this study, we compare the RCS condition for the case of following EOP with that of secondary feed and bleed operation (OMP). In this study, we examined the RCS behavior for SBO scenario for APR1400. Assuming that the DC power is available for 8 hours, MARS runs are made for 8 hours for the two cases: one following the current EOP and the other following the optimum procedure (OMP) developed for extended SBO. For the case of high RCP leakage (120gpm at RCS pressure of 155kg/cm²), EOP approach is applicable if the offsite power is recovered within one hour. The seal leakage makes it difficult to maintain RCS pressure and core would be uncovered after one hour. OMP approach shows that core is covered for 8 hours and it is ready to be connected to portable pumps with RCS pressure below 2.5MPa. RCP seal leakage under extended SBO is one of uncertain parameters. We used the highest value from the RCP manual. With the heightened interest in seal leakage, WOG reported that the seal leakage for Westinghouse-type pump varies from 21 to 480gpm (WOG 2000 Model).