Research on Evaluation Method for MTBF of NPP I&C Cards during Operation

The evaluation of mean time between failure (MTBF) for NPP I&C cards during operation is significant for root cause analysis, NPP maintenance and aging management, and intelligent operation and maintenance technology research. However, due to the lack of unified technical standards in the nuclear power industry at the current stage, the simple averaging method is mainly used in practical engineering applications to calculate the operational MTBF. This algorithm does not consider the definition of liability failures or the fitting of failure distribution, and it cannot provide more instructive interval estimation. To address this issue, this study proposes an MTBF evaluation method for nuclear power I&C cards based on goodness-of-fit tests for multiple failure distributions. Firstly, the definition criteria for liability failures that need to be included in the evaluation scope are provided, and fault screening and statistics are conducted based on these criteria. Secondly, likelihood equations for parameter estimation of four failure distributions are established for timed truncated data. The goodness-of-fit test and optimal distribution selection are then conducted using the Pearson chi-square test and Bayesian information criterion (BIC). Based on this, the MTBF of the card under a certain confidence level is calculated. This method was applied to the relay output cards of a nuclear power unit in commercial operation. Practical applications have shown that the MTBF evaluation results obtained using this method, including point estimation and interval estimation, comprehensively consider the contributions of failure distribution, data sample characteristics, and information loss. These results are more reasonable and accurate compared to the conventional approach. Therefore, the MTBF evaluation method established in this study for nuclear power I&C card components can be applied to engineering-based fault analysis of NPP I&C cards.