To mitigate the combustibility and detonation hazards of hydrogen in nuclear containment, Passive Autocatalytic Recombiners (PARs) are widely employed as an effective strategy. To optimize PAR placement, this study evaluates how PAR elevation influences both hydrogen removal efficiency and mixing performance under two representative release modes: premixed uniform distribution and continuous jet injection. A porous‑medium PAR model is developed in the CFD code STAR‑CCM+ and validated against the THAI‑HR2 experimental data. The validated model is then applied to compare high and low PAR positions under both source conditions. In the premixed scenario, the lower PAR position yields a higher removal efficiency because it promotes more uniform convective transport and facilitates nearly complete hydrogen depletion. Under the injected condition, the upper PAR triggers earlier onset of catalytic recombination, while the lower PAR exhibits distinctly stronger pumping action and enhances bulk mixing, thereby reducing local stratification risk. The results demonstrate that PAR elevation is a decisive factor not only for depletion rate but also for spatial distribution of hydrogen, and they confirm that the present CFD approach can effectively guide pretest positioning decisions. Overall, this work underscores that a well‑chosen PAR elevation can significantly improve both removal effectiveness and mixing uniformity, which are critical for containment safety.

Passive Autocatalytic Recombiners (PARs); PAR elevation; Hydrogen removal efficiency; Mixing performance; Containment safety