Reliability Assessment of the BWRX-300 Passive Isolation Condenser System : Addressing Uncertainties in Two-Phase Natural Circulation Flow Modeling

Le résumé fourni par la source

Passive safety systems are increasingly utilized in prospective nuclear power plant designs. The low magnitude of the forces involved in such systems, combined with the uncertainty inherent in the factors affecting them, poses a problem in assessing their reliability compared to active counterparts. The purpose of this paper 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 that provides emergency core cooling and pressure control for the BWRX-300. Using RMPS to analyze the effect of uncertainties in (a) the thermal characteristics of the fuel rods and (b) two-phase constitutive correlation factors on ICS operation, the reliability of natural circulation was quantified with a confidence of 95%, yielding 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 fault tree analysis of the ICS using failure mode and effect analysis (FMEA) of system components, including insufficient natural circulation as a failure mode. Analysis of sequences leading to failure resulted in system unavailability being determined as 1.62E-07 for the case of all three loops initially available and 2.91E-05 for the case when only two loops are initially available. Sensitivity analysis of the natural circulation failure probability with respect to ICS system unavailability was also performed to investigate the robustness of the design.

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