As the global demand for low-carbon energy accelerates, nuclear power is regaining attention as a reliable, independent, and sustainable energy source. High-temperature gas-cooled reactors (HTGRs), particularly pebble bed reactors (PBRs), are emerging as promising candidates for the next generation of nuclear systems due to their inherent safety features, high thermal efficiency, and ability to provide both electricity and high-grade process heat. Understanding and characterizing the void fraction distribution within the randomly packed pebble bed is critical for predicting coolant flow behavior, heat transfer, neutron flux distribution, and overall reactor performance. High-fidelity experimental benchmark data for validating void fraction predictions remain limited in literature, largely due to the challenges associated with measurement techniques. This work presents a combined experimental and numerical investigation of void fraction distributions in randomly packed pebble beds. Gamma-ray computed tomography (CT) was employed to non-invasively measure radial and cross-sectional porosity in two systems: (1) a bed of 6 cm graphite pebbles in a 44.4 cm diameter bed (D/dp = 7.4), capturing near-wall effects at full pebble scale, and (2) a scaled system using 0.95 cm pebbles to achieve D/dp ≈ 47, closely replicating the high-aspect-ratio geometry of actual PBR cores. These complementary experiments provide a unique combination of realistic fuel size and reactor-relevant geometry, providing insight into both local packing structure and overall void fraction behavior. DEM simulations of both systems were validated against the CT data and then extended to simulate void fraction distributions in the Xe-100 reactor core, which features a 2.4 m diameter, 9 m height, and approximately 220,000 pebbles. Moreover, existing void fraction correlations were tested and assessed against both the experimental measurements and the validated DEM simulation results. The findings advance the understanding of void fraction behavior in pebble bed reactors and provide much-needed experimental benchmarks and validated DEM results for high-aspect-ratio PBRs.
