Best Estimate Plus Uncertainty analysis of a large break LOCA on Generation III reactor with RELAP5

Abstract The RELAP5/MOD3.3 code was used to simulate a Main Steam Line Break (MSLB) Loss of Coolant Accident (LOCA) on a Generation III type Reactor with passive safety features. The MSLB is a hypothetical multi-failure large break LOCA accident scenario. The results of the base case calculation have been compared with the available integral testing data to assess the code’s capability to predict the large break LOCA phenomena in this generic reactor, particularly with passive safety systems. Various flows such as break flow and gravity injection flow were simulated with a high level of accuracy. In addition, both sensitivity cases and uncertainty analysis were carried out to build the best estimate plus uncertainty practice. The sensitivity study was performed with different initial conditions and adjusted containment cooling capacities. The results confirmed that the code model held its stability and repeatability against a few variations of initial and boundary conditions. In the uncertainty quantification process, the 95/95 uncertainty bands of key output parameters were obtained using the Wilks statistic methods. Ten input parameters were selected with imposed uncertainty distribution to perform uncertainty propagation analysis. The contribution weight of input parameters for the uncertainty of minimum core level were identified with the Spearman rank correlation coefficient in sensitivity analysis.