A methodology for Level 1 PSA Success Criteria during Mid-loop operation for Low Power Shutdown Probabilistic Safety Assessment

The primary objective of the Level 1 PSA Success Criteria during Lower Power Shutdown (LPSD) operation is to provide timing considerations for the use or restoration of support systems which enable the front-line systems to perform with regard to accident progression. The ANS LPSD PRA Standard (Reference 1) presents a specific set of requirements concerning the development of success criteria for systems and actions that are modeled in accident mitigation. Development of success criteria makes use standard computer codes, e.g., MAAP, and models developed specifically for APR1400 NPPs reactor and systems. The Thermal Hydraulic Analyses have been performed to determine success criteria and the available time to recover SCS operation when RCS status are mid-loop operation during scheduled outage. According to the results, if SCS operation is recovered within shutdown cooling limits such as shutdown water level or the core boiling time, success criteria are satisfied. The time to shutdown cooling limits in POS 5 and POS 11 are 598 and 1,315 seconds as time to start core boiling, respectively. These are conservative assumption. The time to shutdown cooling limits is increased significantly due to low decay heat when core in reactor vessel is refueled. The time to shutdown cooling limit in POS 5 before off load is half of that of POS 11 because the decay heat in POS 5 is still relatively high when it is compared to that of POS 11 after refueling.