Assessment of hydrogen risk in French pressurized water nuclear reactors

During the course of a severe core accident, hydrogen evolving, mainly due to zircaloy cladding-steam reaction, may form early a flammable mixture in the containment. The risk of a short-term containment failure due to a hydrogen explosion, which would result in a large radioactivity release into the environment, is currently being assessed for the various types of large dry containments existing for French PWRs. In this framework, comparisons between pressure peaks, due to the most severe conceivable hydrogen deflagration, and the realistic ultimate strength of French containment buildings have led to the conclusion that containment integrity should not be questioned. However the problem of the likehood of local detonations and of their impact on structures is still examined. Another aspect of the hydrogen risk is the possible impairment of safety-related equipment in the containment during an eventual hydrogen combustion or explosion. Information coming from the EPRI research program on hydrogen combustion and control, in which the safety body (CEA/IPSN) and the utility (EDF) jointly took a participation, is to back up our own studies on hydrogen risk analysis. Up to now all the results gathered as regards the integrity of the large, dry containments of the French PWRs against hydrogen explosion tend to relegate such a risk to the level of a residual risk. Although the studies are going on a drastic change of this trend in the future is not expected. Therefore no hydrogen specific design modification is presently required by the safety authorities