Hydrogen safety is a relevant topic for both nuclear fission and fusion power plants. Hydrogen generated in the course of a severe accident may endanger the integrity of safety barriers and may result in radioactive releases. In the case of the ITER fusion facility, accident scenarios with water ingress consider the release of hydrogen into the suppression tank (ST) of the vacuum vessel pressure suppression system (VVPSS). Under the assumption of additional air ingress, the formation of flammable gas mixtures may lead to explosions and safety component failure. The installation of passive auto-catalytic recombiners (PARs) inside the ST, which are presently used as safety devices inside the containments of nuclear fission reactors, is one option under consideration to mitigate such a scenario. PARs convert hydrogen into water vapor by means of passive mechanisms and have been qualified for operation under the conditions of a nuclear power plant accident since the 1990s. In order to support on-going hydrogen safety considerations, simulations of accident scenarios using the CFD code ANSYS-CFX are foreseen. In this context, the in-house code REKO-DIREKT is coupled to CFX to simulate PAR operation. However, the operational boundary conditions for hydrogen recombination (e.g. temperature, pressure, gas mixture) of a fusion reactor scenario differ significantly from those of a fission reactor. In order to enhance the code towards realistic PAR operation, a series of experiments has been performed in the REKO-4 facility with specific focus on ITER conditions. These specifically include operation under sub-atmospheric pressure (0.2–1.0 bar), gas compositions ranging from lean to rich H2/O2 mixtures, and superposed flow conditions. The paper gives an overview of the experimental program, presents results achieved and gives an outlook on the modeling approach towards accident scenario simulation.