Kairos Power (KP) is developing a proprietary reactor systems code, KP-SAM, for use in fluoride salt–cooled high-temperature reactor (FHR) safety analysis, including the low-power test FHR, Hermes. KP-SAM differs from Argonne National Laboratory’s SAM (System Analysis Module) code in key ways, one of which is the use of different heat transfer closure models based on data collected at KP. While work is ongoing for the development of a reliable downcomer mixed-convection Nusselt number correlation for laminar and turbulent flows, an effective framework has been developed for the selection of closure models in a complicated heat transfer regime map.

Rather than allowing for overly sharp transitions that may challenge the fully implicit solver, the KP-SAM vertical parallel plate heat transfer coefficient selection relies on a series of weighting functions based on hyper tangents to enforce smooth transitions and maintain high performance in the Jacobian-free Newton-Krylov solver. This framework invokes a preliminary mixed convection heat transfer correlation for high Prandtl number fluids, replacing an erroneous correlation published previously. The new framework works effectively in a demonstration loss-of-forced-circulation simulation of a preliminary Hermes design (design not detailed in this work). While this new framework will continue to evolve, the overall strategies that have gone into making it numerically stable and all encompassing shall be maintained.