Sensitivity Analysis on Critical Flow Model of RETRAN-3D code on OPR1000 ATWS Event

An Anticipated Transient Without Scram (ATWS) event could happen if the reactor protection system that provides a highly reliable means of shutting down the reactor (control rod insertion) fails to work during a reactor anticipated transient event. ATWS event is one of typical Beyond Design Bases Events (BDBE) due to low possibility of occurrence during the lifetime of NPP. Analysis of ATWS is performed based on the best estimate methodology using transient system analysis codes such as RETRAN-3D code. The major concern in this event is degree of overpressurization in primary reactor coolant system. The major factors that impact on the results of ATWS are behavior of Moderator Temperature Coefficient (MTC), capacity of primary pressure relief using pressurizer safety valve(PSV) and capacity of secondary heat removal using Auxiliary Feedwater(AFW) or Main Steam Safety Valves (MSSV). As a general, the capacity of PSV of OPR1000 is less than that of the Westinghouse-supplied plants in Korea. Most of thermal-hydraulic transient codes have various critical flow models. The capacity of PSV pressure relief might be affected by selection of critical flow model option. RETAN-3D code includes three critical flow models as follows ; Extended Henry- Fauske, Moody, and Isoenthalpic Expansion/Homogeous Equilibrium. In this paper, the effect of critical flow models on OPR1000 at ATWS event is investigated using RETRAN- 3D code