Modeling Hydrogen Explosion in Level 1 PSA

Extension of the operational lifetime beyond 40 years is currently a noteworthy project in the field of nuclear safety in France, especially for 900-MW(electric) reactors that will be the first ones to go through the fourth periodic safety review. For 1300-MW(electric) reactors, the safety studies have already been engaged. Probabilistic Safety Assessments (PSAs) play an important role in this process, including assessment of the robustness of the plants against internal and external hazards. Among the hazards, internal explosion induced by hydrogen accumulation due to leakage of pipes and/or singularities, or failure of venting systems in the battery rooms is now taken into account in the PSA developed by the licensee (EDF). The PSAs are then reviewed by IRSN, which is the French Technical support organization. To achieve this purpose, IRSN develops its own Level 1 PSA studies. This paper takes into account the most recent developments of the IRSN analyses, especially regarding the Bayesian quantification of the hydrogen leak frequency derived from an original method developed by Sandia National Laboratories.