Investigation on response of HPR1000 under different mitigation strategies after SGTR accident

Steam Generator Tube Rupture (SGTR) accident was reclassified as Class III Design Basis Accident (DBA) in 1991 due to its higher frequency of occurrence. Under Class III condition, radioactive release safety criteria are even stricter than thermal hydraulic criteria. In this paper, HPR1000 SGTR accident was simulated with two kinds of mitigation measures after steady-state initialization. Key system responses (such as pressure, mass flow rate in primary and secondary system, SG level, short term phase terminating time and radioactive release) were analyzed and compared for the two strategies. No SG overfilled nor other thermal hydraulic limit occurred for both strategies, while lower SG overfill risk and higher radioactive release risk existed for strategy 1 and converse situation existed for strategy 2. Specially, it is recommended VDA to be actuated by SG level, and instant main feedwater and auxiliary feedwater isolations were strongly required for strategy 2. When designing SGTR mitigation measure, there should be a compromised balance between SG overfill risk and radioactive release. Optimized mitigation strategy can be obtained through more accurate activity signal and higher auxiliary feedwater mass flow rate.