Numerical study on pebble beds powder migration and clogging mechanism with purge gas

摘要: Objective: As the primary functional component of a fusion reactor, the fusion blanket pebble bed, composed of numerous particles, is crucial for tritium breeding, neutron multiplication, and radiation shielding. Particles within tritium-breeding pebble beds are subjected to prolonged neutron irradiation, high thermal loads, and strong magnetic fields in fusion environments. Such conditions render them susceptible to pulverization and fragmentation. The resulting fragments and powders migrate and are deposited into the gas channel, driven by the purge gas. The reduction in the effective flow area of the gas increases the flow resistance, resulting in tritium retention, degraded heat transfer, and other adverse effects. These conditions impair the thermodynamic properties of the pebble beds and hinder the self-maintenance of tritium. Limited information exists on powder migration and clogging mechanisms in fusion blanket pebble beds, particularly under diverse physical conditions.
Methods: The aim of this study was to use a computational fluid dynamics model coupled with the discrete element method (CFD-DEM) to numerically explore powder migration and clogging in pebble beds.
Results: We propose two migration and clogging mechanisms. One involves powder with a large particle size, and the other does not. The results indicate that the powder migration velocity progresses through three stages: rapid decay, linear decay, and stability. Pebble-bed clogging manifests in two forms: extensive superficial clogging and uniform internal clogging. Two fitted curves were used to depict the migration and clogging tendencies. The powder size distribution significantly influenced the powder migration. The breeder orientation, powder size, and friction coefficient affected the distribution of the clogging powders. However, the impact of the purge velocity on powder migration and clogging in pebble beds was limited, and this effect varied significantly with different particle size ratios. Based on the analysis, a formula is proposed to characterize the behavior of the powder in the pebble beds.
Limitations: For simplicity, all powders were modeled as spheres in this study, although real-world powder particles are irregularly shaped.