Extension of the operational lifetime beyond forty years is currently a noteworthy project in the field of nuclear safety in France, especially for 900 MWe reactors that will be the first ones to go through the fourth periodic safety review (PSR). Probabilistic safety assessment (PSA) are to play an important role in this process, especially in the frame of the assessment of the robustness of the plants against internal and external hazards (the PSA are performed by the licensee (EDF) and reviewed by the Institute of radioprotection and nuclear safety (IRSN), which is the French Technical safety organization (TSO)). Among these hazards, internal explosions induced by hydrogen accumulation following leakage of pipes and/or singularities, or due to the failure of venting systems in the battery rooms are now taken into account in the PSA developed by EDF. Independently, IRSN has therefore developed its own Level 1 PSA methodology and a case study to analyze the risk related to such an explosion.