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

This study presents the development and verification of CUPID-MSR, an extended thermal-hydraulics code incorporating temperature-dependent properties of two chloride-based molten salts, KCl–UCl3 and NaCl–MgCl2–TRUCl3. Verification against the de Vahl Davis natural-convection benchmark across Rayleigh numbers 1,000–1,000,000 showed agreement within 0.4–3.9%, accurately capturing reference Nusselt numbers, flow structures, and thermal boundary layers. Additional temperature-variation studies confirmed stable and consistent performance of the implemented material correlations. The applicability of the Boussinesq approximation was assessed by comparing full variable-density and Boussinesq formulations, revealing that the approximation remains accurate for βΔT ≲ 0.1. Since this threshold depends only on relative density change, it is broadly relevant for natural-convection flows in Newtonian fluids.