The innovative design of passive containment cooling system (PCCS) is one of the most significant safety improvements for the third generation of advanced pressurized water reactors (APWRs), however, its impact on the hydrogen risk in severe accidents for nuclear power plants (NPPs) has not been determined yet. On the one hand, natural convection through liquid film condensation on the inner wall of the containment may mitigate the hydrogen risk, but on the other hand, rapid steam condensation may also ruin the existing inerting environment within the containment. Brownian diffusion is one of the essential mechanisms which drives the steam condensation near the wall. In this paper, the distributions of the velocity field, temperature field, and concentration field in the gas mixture boundary layer are firstly obtained through the build-up of a two-dimensional mathematical model allowing for the Brownian diffusion behavior of the mixing gas under the presence of non-condensable gas. Compared with the COPAIN test data and simulation results from the GASFLOW code obtained from previous literature, the applicability and prediction error of the present model are evaluated. Besides, the comparison with the prediction results based on Nusselt’s classical liquid film condensation equation demonstrates that the Brownian diffusion of the mixing gas, including steam and air, will reduce the heat transfer rate through the liquid film by 1–2 orders of magnitude. The theoretical model from the present work can support the development of new CFD codes and help get new understandings of complex thermal-hydraulic phenomena in hydrogen risk under various containment cooling conditions.

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