Role of bulk condensation in buoyancy-driven gas flows: Insights from THAI HM2 experiment CFD simulations using containmentFOAM

Bulk condensation of steam within a nuclear containment vessel can significantly influence the evolution of pressure, temperature, and gas composition during post-accident transients. Depending on the accident sequence, it can occur at various stages, during which steam is released into the containment under near-saturation conditions. The associated localized latent heat release increases temperature and lowers density, amplifying the buoyancy driven flows that promotes mixing and erosion of stratified light gas layers. Understanding the bulk condensation phenomena interaction is essential for improved containment safety analysis. Recent developments in containmentFOAM CFD package based on OpenFOAM include the implementation and validation of bulk condensation models using saturation-temperature equilibrium (STE) and classical nucleation theory (CNT) approaches. This study presents a comprehensive numerical investigation of the OECD/NEA THAI HM2 benchmark experiment, focusing on bulk condensation influence on multi-component gas mixing dynamics. Steam injection into a pre-stratified vessel atmosphere is simulated with and without bulk condensation model to isolate its effect on hydrogen cloud layer dissolution and gas mixing. Results indicate that bulk condensation significantly amplifies local buoyancy forces through latent heat release, accelerating vertical flows and hydrogen cloud erosion. The local vertical velocities increases by 15% to 80%, and the hydrogen stratification dissolution occurs 65% faster compared to the simulations without bulk condensation. Strong spatial correlation between bulk condensing zones and enhanced upward flow confirms its decisive role. The simulations show good agreement with experimental temperature and concentration distributions, showing deviations below 10%. Both STE and CNT models predict similar vessel conditions, though CNT provides a more physical representation of fog volume and droplet size distributions. These findings highlight the vital role of bulk condensation in driving containment atmosphere mixing, with direct implications for hydrogen risk mitigation strategies.