Passive safety systems in Small Modular Reactors (SMR) rely on gravity as the driving force for transferring heat from the core or a hot region into a large reservoir through natural circulation. They are designed to function without operator intervention for several hours or days. Given the large scale of the current SMR designs, the Rayleigh number is expected to be on the order of 10¹⁵. Currently, experimental data at such a high number is scarce, so new experiments are necessary to validate the thermal hydraulic codes used to predict such behaviour in an accident scenario. This paper presents preliminary calculations using Computational Fluid Dynamics (CFD) to help sizing a new experimental set up at the PANDA facility (PSI, Switzerland) within the OECD/NEA PANDA project.