Influence of convective and radiative heat transfer on effective thermal conductivity of solid breeder pebble beds using DEM–CFD simulations

The thermal performance of the Helium Cooled Solid Breeder (HCSB) blanket concept strongly depends on the effective thermal conductivity ( k eff ) of mixed pebble beds composed of titanium beryllide (Be 12 Ti) and lithium meta titanate (Li 2 TiO 3 ). The newly proposed HCSB blanket concept for ITER (International Thermonuclear Experimental Reactor), fusion pilot plants and DEMO adopts a mixed pebble bed configuration with Be 12 Ti as neutron multiplier and Li 2 TiO 3 as tritium breeder material. In this study, Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) simulations for the packed pebble bed mixture of Be 12 Ti and Li 2 TiO 3 have been performed to validate and estimate the k eff , which is essential for evaluating blanket thermal performance. Beyond pure conduction, the effects of convective and radiative heat transfer on the k eff have also been analyzed, for which only limited studies exist in fusion blanket literature. Incorporating radiation in such densely packed pebble beds is particularly challenging due to the very large number of elements involved, requiring significant computational resources. The numerical simulations were performed using one-way coupling of the DEM and CFD. The paper discusses the methodology, inputs, assumptions, and results of the DEM–CFD analysis, highlighting the cumulative effect of conduction, convection and radiation on the k eff of the mixed pebble bed. The k eff ranges from 2.07 W/m-K to 3.7 W/m-K at bed temperatures between 100 ° C and 900 ° C for the pure conduction case, with around a 6.5% rise at higher temperatures when radiation and convective heat transfer are included. Although the quantitative increase is modest, the combined heat transfer modes represent realistic blanket conditions and provide additional temperature margins for design. • DEM–CFD–radiation framework applied to mixed Be 12 Ti–Li 2 TiO 3 breeder pebble beds. • Model validated against published k eff data for conduction. • Inter-particle radiation increases k eff up to ̃6.5% at elevated temperatures. • Convection contributes minimally under low helium purge-flow conditions.