Uncertainty Estimation of Henry-Fauske Critical Flow Model Considering Thermal-nonequilibrium Constant and Discharge Coefficient in MARS-KS

It is very important to evaluate critical flow in the safety assessment such as LOCA, MSLB etc. There are several models (HEM; Homogeneous Equilibrium Model, HFM; Homogeneous Frozen Model, Ransom Trapp, Henry Fauske Model) to simulate the critical flow in computer codes to analyze safety of the nuclear power plants. The default critical flow model in MARS-KS 1.4 used by KINS as independent regulatory assessment is the Henry-Fauske model, it is important to consider that the actual vapor quality is below the equilibrium quality due to thermal-nonequilibrium. Based on the Marviken experiment CFT-15 and 24 data, the uncertainty of Henry-Fauske critical flow model was evaluated. For the long nozzle (CFT-15, L/D=3.6), there was no range of uncertainty in the MARS-KS that could cover experimental data depending on the discharge coefficient and thermal non-equilibrium constant. In the case of the short nozzle (CFT-24, L/D=0.3), the uncertainty range of discharge coefficient 0.98 ⁓ 1.08 was derived for the region where the nozzle inlet pressure was greater than 3.3 MPa and the uncertainty range of thermal-nonequilibrium 0.1 ⁓ 0.3 was derived for the region above 3.1 MPa.