Optimization of passive autocatalytic recombiners (PARs) with respect to gas-phase ignition

In a Nuclear Power Plant (NPP) the containment represents the ultimate barrier against the release of fission products into the environment in case of a severe accident. Thus, an essential challenge of ensuring safety is the protection of the containment integrity. During a severe accident, the containment integrity is threatened by the risk of hydrogen combustion as a large amount of hydrogen can be produced by high temperature interactions of steam and metals inside the reactor pressure vessel. The potential combustion load is further increased by hydrogen and carbon monoxide released during the molten corium-concrete interaction. For mitigation of combustion risks in Light Water Reactors, containments are provided with Passive Autocatalytic Recombiners (PAR). This technology has been implemented in German, French and several more, mostly European NPPs. These devices recombine hydrogen in presence of oxygen to produce steam and heat on catalytic surfaces. In the wake of the Fukushima-Daiichi Nuclear accident in 2011, PARs are being considered in most of the countries operating NPPs. Several experimental programs in the last decade have demonstrated the gas-phase ignition potential of PARs at relatively high hydrogen concentrations (above 6 %-vol. H2 in air). Consequently, the resulting combustion loads may threaten the containment integrity in a more severe way. Also, PAR’s startup may be delayed due to the presence of materials such as radiolysis compounds, fire products or carbon monoxide. This delay may lead to the unhindered formationof an atmosphere with high hydrogen concentrations, and the possible consequence mentioned earlier. In this context, the goal of this work is to optimize PARs with respect to gas-phase ignition. While doing so, the conversion efficiency and natural buoyancy driven flow of the recombiner should be maintained. Also, the robustness of the recombiner against catalyst deactivation should not be compromised. For this purpose dedicated ignition experiments have been performed in the Hydrogen Laboratory of Forschungszentrum Juelich GmbH, Germany. The experimental data were used to validate two in-house codes, SPARK and REKO-DIREKT developed by research groups in France and Germany. The codes build the theoretical foundation for proposing a method to optimize the catalyst with respect to gas-phase ignition by introducing a heat sink of lower platinum concentration to optimize the catalyst surface temperature. The modified catalyst has been first tested in a flow tube reactor under forced flow conditions where it is demonstrated that catalyst induced gas-phase ignition in the optimized catalyst plates is delayed with respect to reference catalyst plates. Then the optimized catalyst was scaled up to a full length PAR scale and tested under realistic natural flow conditions in a pressured vessel, to demonstrate the delay in gas-phaseignition in optimized recombiner with respect to reference recombiner. Finally, a theoretical study on the effects of radiolysis compounds on the modified recombiner has been conducted to draw a conclusion that the optimized recombiner will still be as robust against catalyst deactivation as a reference recombiner