Research on Effect of Main Coolant Pump Two-phase Degradation on Large Break LOCA

During the large break LOCA process, the working range of main coolant pump covers single liquid, liquid-vapor two-phase and single vapor conditions. Under the two-phase condition, the head and torque of main coolant pump degrade. For the AP1000, WCOBRA/TRAC was applied to analyze the large break LOCA and pump two-phase degradation data came from Westinghouse W93A pump. In order to simulate the thermal-hydraulic characteristics of AP1000 main coolant pump during large break LOCA correctly, the research on the pump two-phase degradation behavior was needed. In this paper, pump two-phase degradation data of SEMISCALE and EPRI/CE were applied to the analysis of cold leg double ended guillotine accident for AP1000, and the results were compared with that of W93A pump. The results show that pump two-phase degradation characteristics of AP1000 have little influence on reactor coolant system pressure, break flow and accumulator flow. Compared to two-phase degradation data of SEMISCALE and EPRI/CE, current W93A results result in a lower core cooling flow and a higher maximum peak cladding temperature, and thus proves that the results obtained by W93A data are more conservative.