Probabilistic estimation of containment failure modes in Level 2 PSA: Realistic approach and risk implications

A realistic assessment of the probabilities associated with containment failure modes—such as leaks and ruptures—can significantly influence the quality and credibility of Level 2 probabilistic safety assessments (PSAs) for nuclear power plants (NPPs). From this perspective, this paper provides technical backgrounds on the impact of two different approaches to estimating these probabilities: the Rupture-Before-Leak (RBL) assumption, adopted in many existing Level 2 PSAs, and the more realistic Leak-Before-Break (LBB) assumption. Fundamentally, both approaches rely on probabilistic fragility curves developed independently for two distinct failure modes—more specifically leaks and ruptures, whose nominal sizes are predefined. In some situations, a leakage mode smaller than the leak size may be explicitly considered to more realistically assess the Cs-137 100 TBq release-related probabilistic safety goals. The findings from relevant case studies are presented along with key insights, particularly regarding to their implications for Level 2 safety and performance goals such as Large (Early) Release Frequencies (LERF/LRF), and the Cs-137 risk.