Catalytic combustion of hydrogen for mitigating hydrogen risk in case of a severe accident in a nuclear power plant: study of catalysts poisoning in a representative atmosphere

In case of a severe (beyond design basis) accident in a nuclear power plant, a large amount of hydrogen could be generated by reaction of water of the primary coolant circuit with the fuel rods inside the reactor pressure vessel, and eventually released into the air-filled reactor building. For mitigating the risk of an explosion within the containment, a catalytic combustion of this hydrogen is considered as one of the most efficient counter-measure. The difficulty which is to be overcome is a possible poisoning of the catalyst by fission products and other components released by the damaged core, notwithstanding the fact that most of them enter the containment building as non-reactive large oxide particles. The main vapors which are suspected to have an inhibiting or poisoning effect are indeed di-iodine and methyl iodide, both potentially present in the containment atmosphere. We report on the possible effect of these molecules on Pt, Pd and Pt–Pd model catalysts at lower temperatures and somewhat higher iodine or iodide concentrations, as compared to inferred catalyst operational parameters in a reactor building during a severe accident. In these particular experimental conditions, platinum is substantially poisoned by both vapors. On the other hand, palladium, about 400 times less active than platinum, is much less altered by I2 and ICH3 vapors. A marked beneficial effect was found by alloying the two noble metals: the alloys show only a threefold decrease in activity with respect to platinum, and undergo a much weaker deactivation.