On the plant-specific impact of different pressurization rates in the estimation of containment failure mode probabilities

The explicit consideration of different pressurization rates (e.g. fast and slow) in estimating the probabilities of containment failure modes (e.g. leak, rupture, and/or catastrophic rupture) might have a profound effect on the confidence of the containment performance evaluation that is so critical for risk assessment of nuclear power plants. We have performed a bounding analysis for the impact of different pressurization rates on the leak and rupture mode probabilities using models for fast and slow pressurizations. The present models are based on the evaluations of probability distributions that are characterized with the median pressures and their standard deviations, for individual mechanisms of structural failure modes and the nominal break sizes. As a result, we have obtained a quantitative and plant-specific estimate of the impact of the pressurization rates on the leak and rupture containment failure probabilities. The present study showed that the impact was not significant for the specific cases considered in this study and confirmed that the treatment used in the specific cases was conservative.