Nuclear reactor containment flows are primarily driven by density differences in the long term of a severe accident. It is crucial to predict such flows from the perspective of reactor safety. The present work addresses the assessment of open-source CFD code OpenFOAM in modelling buoyancy-driven turbulent flows and heat transfer relevant for analyzing the flow and mixing in a nuclear reactor containment during severe accident scenarios. A fully compressible solver with the k-ω based shear stress transport (SST) turbulence model was utilized to simulate the two-dimensional turbulent flow of a differentially heated square cavity(h 0.75, Ra 109) and then against the technical scale THAI-TH21 natural convection test (60 m3, h 9 m, Ra 1012). The results from the current study, emphasize the importance of considering radiation phenomenon in buoyancy-driven flows and the grid resolution needed in the near-wall region to accurately predict wall heat flux. Overall, the simulation results of time-averaged temperature and velocity profiles in the quasi-steady state compare very well with experimental data. The predicted values of external wall heat transfer rates on the THAI vessel, the convective wall heat flux in the gas region, and the impact of buoyancy on turbulence are discussed. © 2020 Elsevier B.V.