Reliability Assessment of Passive ICS in an SMR as part of the PSA Analysis

Abstract: Passive safety systems are increasingly being utilized in prospective nuclear power plant designs. Indeed, the use of safety systems driven by natural phenomena might be seen as an unmitigated virtue. However, the low magnitude of the forces involved in such systems, combined with the uncertainty inherent in the factors which affect them, pose a problem in the assessment of their reliability when compared to their active counterparts. Hence, the purpose of this thesis is to investigate and apply a state-of-the-art technique in passive reliability assessment, known as the Reliability Methods of Passive Systems (RMPS) methodology, to the isolation condenser system (ICS) of the prospective BWRX-300 small modular reactor (SMR) design. The ICS is a safety system driven by natural circulation which provides emergency core cooling, residual heat removal, and pressure control for the BWRX-300. Using RMPS to analyze the effect that uncertainties in thermal characteristics of the fuel have on ICS operation, the reliability of natural circulation was quantified with a confidence of 99%. This yielded an immeasurably small failure probability. Considering residual uncertainty, an engineering judgment assigned a failure probability of 1.00E-07. This finding was integrated into a Level 1 probabilistic safety assessment, involving analysis of initiating events, event tree analysis, and failure mode and effect analysis (FMEA) of safety systems, including natural circulation. Analysis of sequences leading to core damage resulted in a core damage frequency of 1.23E-07 yr-1.