In a severe accident at a nuclear power plant, a large amount of hydrogen can be generated from the chemical reaction of zircaloy and water vapor at high temperatures. As the hydrogen accumulates at the top of the containment building by buoyancy, it can be ignited with little energy and cause a catastrophic accident, as in Fukushima Daichi, 2011. Many safety and protection devices are developed and under research to relieve hydrogen concentration below 4%, which is the low flammability limit. One of the main safety measures in current pressurized water reactors is a passive autocatalytic recombiner (PAR). The catalytic reaction occurs on the Pt-coated plates inside a PAR, and hydrogen is oxidized to vapor during the exothermic process. Plates are installed vertically to allow heated reactant vapor and air to exit through the top outlet while the hydrogen-air mixture flows into the bottom inlet. An automatic buoyancy-driven flow is generated inside PAR whenever hydrogen is present. A PAR is used to reduce the concentration of hydrogen inside a nuclear reactor by using a catalytic reaction without an external power supply. In this study, we modeled the heat transfer, fluid flow, and chemical reaction on the surface of a PAR through a computational fluid (CFD) simulation. The exothermal reaction on the catalytic plate is modeled using detailed adsorption, surface, and desorption reactions concerning H₂, O₂, and Pt with different reaction rate coefficients of the Arrhenius relations. A comparison with an experimental result from the literature was made to verify the developed model and calculation.