Passive autocatalytic recombiners (PARs) are being used in most modern nuclear power plants (NPPs) for mitigating the hydrogen risk both in normal and accidental scenarios. Development of a systematic model of PARs based on computational fluid dynamics (CFD) is the subject of this paper. 2D simulations of AREVA (a nuclear technology company) recombiner have been performed to validate PARs data and then the developed methodology has been applied to a Chashma NPP-II (C-2) recombiners to assess their performance. Two cases have been discussed; one with constant velocity at inlet and other one is devoted to the startup response of the PAR. Within the recombiner, the hydrogen and oxygen recombine on catalytic plates surface by an exothermic reaction to produce steam. Reaction heat is dissipated among the plates, surroundings and air inside the PAR. Flow inside the PAR will continue downwards until the gas absorbs enough heat to become lighter in weight than the gas outside the PAR. In addition, hydrogen accumulation in containment dome of a C-2 has been modeled and the results have been compared with methods for estimation of leakages and consequences of releases (melcor) code results. Without PARs, hydrogen got buildup within the containment dome, but when PARs are activated, hydrogen concentration first started to rise until the recombination reaction activated at about ∼2% hydrogen concentration. The comparison shows that the results obtained by the model agree well with the melcor results.