Effect of power uprate on core damage frequency in a MBLOCA with multiple operator actions

Abstract

In the present study, the methodology developed by the Committee on the Safety of Nuclear Installations (CSNI) of Nuclear Energy Agency was used to quantify the changes in core damage frequency (CDF) owing to a power uprate. The Maanshan Nuclear Power Station of Taiwan Power Company was the plant selected for the analysis. The plant employs a 3-loop pressurized water reactor designed by Westinghouse. The power level was increased from 102% to 105%. The sequence analyzed corresponded to a medium-break loss-of-coolant accident with a break size of 3in. There were two operator actions in the accident mitigation. These actions were the emergency cooldown and depressurization (CND) of RCS and the lineup the high-head safety recirculation (HHSR) when RWST was empty.In the present study, the success of operator action was judged based on the results of the simulation of system thermal hydraulic code, RELAP5-3D. An uncertainty analysis of code calculation was performed. There were 20 parameters in PIRT table, and 124 sets of input data were generated using the Monte Carlo Sampling technique. The times for the operators to execute CND and HHSR were included as one of the parameter in the uncertainty analysis. The results demonstrated that, compared with the PSA results using the conventional HEP model, the CDF of MBLOCA increased by 188%. The dominant core damage sequence in MBLOCA was also changed.The simulation results showed that the CND failure probability increased by 23% after the power uprate. The sequence CDF increased by 16% after the power uprate. The results of the importance analyses indicated that the time to execute CND was not a significant factor in determining the success of CND. The uncertainty involved in the simulations of reactor coolant system thermal hydraulic responses is important in the determination of human error probability.