Validation of NekRS pebble bed modeling for low flow scenarios

This study validates the high-fidelity NekRS Large Eddy Simulation (LES) framework for modeling conjugate heat transfer in low-flow pebble bed reactor scenarios, using the SANA experiments as a benchmark. Verification began by comparing the MOOSE Net Radiation Method to a 1D analytical model for a tall-cavity case, yielding Nusselt numbers within 1% relative difference. Next, the NekRS P1 approximation was compared to MOOSE in a single-pebble case, where temperature and velocity profiles agreed within 5% relative difference. Validation proceeded with the 283-pebble bed experiments at Texas A&M, in which NekRS velocity profiles fairly matched experimental measurements despite uncertainties in pebble placements. Finally, full conjugate heat transfer simulations of the SANA pebble beds (5 kW helium–graphite and helium–alumina cases) were performed, with the optimized configuration producing a mean temperature error of 1.56%. The effective chamfer thermal conductivity (representing conductive contact between pebbles) introduced significant uncertainty in modeling the SANA experiments; further work will optimize this parameter based on a refined contact resistance model. Simulations will also be extended to higher-power conditions to continue validating conjugate heat transfer in NekRS for pebble-bed reactor designs.