Geometric fidelity, contact-point treatment, and thermal hydraulics in pebble-bed FHR: A review of pebble-bed modeling strategies

Fluoride-salt-cooled high-temperature reactors (FHRs) combine tri-structural isotropic (TRISO) fuel with low-pressure liquid-salt cooling, but their pebble-bed thermal hydraulics are unusually sensitive to how the packed geometry is generated and regularized for computation. This review connects five modeling decisions—packing generation, contact-point treatment, wall-effect representation, transport-model selection, and experimental validation—to the quantities used in reactor design. Ordered simple-cubic, body-centred-cubic and face-centred-cubic beds are distinguished from confined random beds because their channel connectivity, anisotropy and wall layering arise through different mechanisms. Contact regularization is assessed by separating global geometry changes from local gap, overlap, bridge, cap and hybrid treatments. In a published structured-bed benchmark, the selected treatment changed pressure drop by about 30% and the conduction-only maximum pebble temperature by 23%; the corresponding differences fell below 0.5% for full-power thermal metrics and to 0.3% when convection was included. These values are study-specific sensitivity indicators, not universal correction factors. Evidence from pressure-drop, velocity and heat-transfer experiments is mapped to the model scale and operating range that it can support. The synthesis shows that no single contact treatment is optimal for every quantity of interest: hydraulic screening, local hot-spot prediction and passive-loss-of-forced-circulation analysis require different levels of geometric and thermal-contact fidelity. A decision framework is therefore proposed to link geometry, regularization, physics, validation and engineering use while making transferability limits explicit.