Development and Verification of CUPID-MSR Using the de Vahl Davis Natural-Convection Benchmark

This study provides the first systematic verification of the CUPID-MSR thermal–hydraulic code for molten-salt reactor applications, incorporating temperature-dependent thermophysical properties of two chloride-based molten salts, KCl–UCl3 and NaCl–MgCl2–TRUCl3. Verification against the de Vahl Davis benchmark for Rayleigh numbers 103–106 shows agreement within 0.4–3.9%, with the simulations accurately reproducing the reference Nusselt numbers, velocity fields, and thermal boundary layers. Additional temperature sensitivity studies confirm stable and accurate predictions using the implemented thermophysical property correlations over a broad temperature range. Furthermore, the applicability of the Boussinesq approximation is assessed by comparing the full variable-density formulation with the Boussinesq formulation, revealing that the approximation remains accurate when the relative density variation is below approximately 10% (βΔT≲0.1). The obtained threshold is consistent with classical Boussinesq criteria and confirms their relevance for molten-salt flows.