Sensitivity analysis to improve the gap conductance uncertainty for KINS-REM

KINS has been using the Best Estimate Plus Uncertainty(BEPU) methodology to analyze the LBLOCA that is the design basis accident of emergency core cooling system(ECCS). KINS-REM(Realistic Evaluation Methodology) is the currently used for LBLOCA analysis methodology and has been improved continuously. One of the important issue of the improvements is the consideration about the uncertainty parameters related to fuel rod behaviors during LBLOCA.. Effect of Thermal Conductivity Degradation(TCD) of fuel rod has been studied to be considered in KINS-REM. For this purpose, the sensitivity analysis has been performed to improve the gap conductance uncertainty parameter in this study. The OPR1000 plant, Hanul unit 3 and 4, was selected as the reference plant. LBLOCA transient calculations have been performed by MARSKS. As the method of uncertainty quantification for gap conductance, the controls of the cladding roughness parameter(B) is changed to the controls of the global variable, effective gap conductance(h_g) that is physically reasonable manner. The sensitivity analysis has been performed on the uncertainty multiplication coefficient of h_g corresponding to the previous uncertainty range of cladding roughness parameter B in PCT calculations of LBLOCA. Through the comparison and analysis of the PCT values and behavior trends for reflood and blowdown, the range of uncertainty of the multiplication coefficient of the global variable h_g, 2.34 - 0.66 and mean value 1.5 are reasonable to replace the local variable called the cladding roughness