Experimental studies on PAR start-up under severe accident conditions

In the course of a severe accident in light water reactors, a large amount of hydrogen may be released due to the reaction of steam with the fuel rod claddings during core degradation. Consequently, the efficient hydrogen conversion by means of passive auto-catalytic recombiners (PARs) installed inside the containment is one key element of successful management of a severe accident. Several international reactor safety projects have been studying the conversion efficiency of PARs under diverse boundary conditions, e.g. OECD/NEA-THAI and THAI2, ERCOSAM. However, one relevant aspect of PAR operation, the fast start-up after hydrogen is released into the containment, hasn't been investigated in the past. In the framework of the German national project H2REKO, a data base for modelling the start-up behavior of PARs is being developed in collaboration of RWTH Aachen University and JUELICH. The research program is focused on start-up under adverse boundary conditions of a severe accident, including steam, carbon monoxide, oxygen starvation, and adverse flow conditions. For the experimental studies, the well-established REKO facilities have been used. Start-up of the hydrogen conversion reaction in the presence of carbon monoxide and under oxygen-lean conditions has been investigated in the REKO-3 flow channel. Inside the REKO-4 pressure vessel, experiments under counter flow conditions have been performed. For the study on the impact of steam, the new GRART facility has been taken into operation. The facility combines thermogravimetric analysis with the possibility to supply a catalyst sample with a gas mixture of air, steam and hydrogen. The paper gives an overview of the experimental results achieved so far and describes the numerical modeling approaches derived from the experimental findings. (author)