Loss of coolant accident analysis using best estimate plus uncertainties considering the change of fuel thermal conductivity with burnup

In nuclear engineering field, the credible audit calculation to evaluate the safety of the nuclear power plant is very important. Recently, many audit calculation of loss of coolant accident (LOCA) is performed using best estimate plus uncertainty (BEPU) method. However, there is a problem where the change of thermal conductivity of UO2 nuclear fuel with burn-up is not considered. Since the fuel thermal conductivity significantly affects the internal energy and the temperature of the nuclear fuel, it is very important parameter to estimate the safety of nuclear system. In the present study, thermal hydraulic (TH) analysis was carried out using the MARS code to simulate the large break LOCA in the Westinghouse 3-Loop type reactor with the KINS-REM (KINS Realistic Evaluation Methodology) as a BEPU method. First, default BEPU calculations were performed without considering the fuel conductivity degradation with burnup. The default calculation results were compared with the calculation results considering the fuel thermal conductivity degradation. It was found that the calculation results considering the fuel conductivity degradation were more conservative. As a result, it was shown that consideration of the fuel conductivity degradation is essential to the BEPU calculations. (authors)