Abstract: A device for the catalytic conversion of hydrogen in a containment of a reactor containing a gas mixture. The device includes a recombiner which is disposed in the containment and has a plurality of catalytically active catalyst elements, a first cooling unit which is disposed upstream of the recombiner and cools the gas mixture and a second cooling unit disposed downstream from and behind the recombiner. The device effectively eliminates hydrogen resulting from an accident in the containment, without flashback. The device also suppresses heating of the atmosphere in the containment.