Abstract

Accurate characterization of void-fraction distributions in pebble-bed reactors (PBRs) is essential for predicting flow, heat transfer, and neutronic behavior. High-fidelity experimental benchmark data for validating such predictions remain scarce, largely due to the challenges of non-invasive measurements. In this study, gamma-ray computed tomography (CT) was employed to measure radial and cross-sectional porosity in a laboratory-scale pebble bed containing 6cm graphite pebbles. A Discrete Element Method (DEM) simulation was implemented and validated against these measurements, then applied to the full-scale geometry of the Xe-100 high-temperature gas-cooled pebble-bed reactor. Analyses included radial and axial void-fraction profiles in the cylindrical section and conical base, with particular attention to near-wall oscillations at multiple axial levels. Both axially averaged profiles, integrating over extended bed sections, and locally resolved profiles, capturing fine-scale oscillations, were evaluated. Additional analyses examined cross-sectional void distributions and the effect of pebble recirculation. The DEM results reproduced the expected near-wall oscillatory layering with a characteristic wavelength of ∼1 dp and bulk void fractions near 0.40 and further showed that oscillatory patterns persist into the conical region, where the first trough shifts outward, and a broader near-wall gap develops. Recirculation studies, corresponding to 5,10, and 15 complete bed inventory cycles, showed that structural rearrangements occur mainly during the initial passes, after which the bed attains a quasi-steady configuration. Recirculation intensified near-wall oscillations, particularly in the lower regions, but had negligible impact on bulk porosity in the cylindrical section. In the cone, however, the void fraction was elevated during dynamic operation due to pebble drainage and upward void propagation. The findings support improved neutronic and thermal-hydraulic modeling and contribute to the design and safety assessment of next-generation pebble-bed systems.

Department(s)

Chemical and Biochemical Engineering

Publication Status

Full Text Access

Keywords and Phrases

DEM simulation; Gamma-ray computed tomography; Pebble bed reactor; Void fraction

International Standard Serial Number (ISSN)

0029-5493

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2026 Elsevier, All rights reserved.

Publication Date

01 Apr 2026

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