<p>Catalytic recombiners have been deployed worldwide in containment buildings of water-cooled nuclear power reactors to mitigate the risks posed by hydrogen generated during a postulated accident. In a plate-type recombiner, the plates are coated with platinum/palladium and arranged in parallel at the lower section of the recombiner box. The box is open at the bottom and top to establish natural circulation. Hydrogen recombines with oxygen in the presence of catalyst-coated plates, generating heat. This heat raises the gas and plate temperatures, which is a function of hydrogen concentration and recombiner geometry. For higher concentrations of hydrogen, the recombiner may itself become a source of ignition due to higher plate temperatures, which needs to be investigated. In reported experiments, ignition has been observed to occur at hydrogen concentrations above 5.5% v/v in dry air in the recombiner. To investigate this ignition behavior, a computational fluid dynamics (CFD) model has been developed and verified with available experimental data. A detailed 16-step surface chemistry model has been used in the CFD code. For gas-phase combustion, a 19-step reaction mechanism has been used. Analysis has been carried out at different hydrogen concentrations (4–10% v/v in dry air) to investigate gas-phase ignition in REKO geometry. The volumetric concentration of OH radical, hydrogen, and steam, as well as the temperature in gas phase, have been predicted. The effect of inlet velocity and temperature has been investigated on the minimum hydrogen concentration required for the onset of self-ignition in the recombiner. This CFD model will be extended to perform detailed self-ignition characterization for indigenous recombiner at different hydrogen and steam concentrations.</p>