Improved natural convection heat transfer correlations for reactor cavity cooling systems of high-temperature gas-cooled reactors: From computational fluid dynamics to Pronghorn

•Development of improved natural convection heat transfer correlations for system-code modeling LOFC conditions in the RCCS of different HTGR geometries using RANS CFD models.•The correlations account for local heat transfer via a local Nusselt number and for the influence of non-uniform temperature at the hot wall of the RCCS cavity.•The natural convection heat transfer correlations are valid in the range of the Rayleigh numbers from the HTR-10 to PBMR-400 modules.•The provided correlations are intended for use in system codes, enhancing their accuracy while keeping a low computational cost.