Numerical study of the effect of particle size on pebble flow in the HTR-PM

The core of pebble-bed high-temperature gas-cooled reactors faces complex issues concerning particle flow. Previous studies indicate that the size, shape, and physical properties of the particles can influence the flow pattern of the pebble bed, thus affecting the temperature distribution of the reactor and further affecting its economic efficiency and safety. Moreover, the optimal diameter for spherical fuel elements remains a controversial topic. Based on a GPU-DEM numerical model, this study simulates a full-core-scale pebble flow of the HTR-PM, comparing the effects of different particle sizes under the current geometry of the pebble bed. Specifically, the numerical simulations consider five particle diameters, involving many particles ranging from 178,047 to 1,420,452. The discharging speed, particle retention, porosity, and velocity distribution are compared for different conditions. The results indicate that the fluidity of the pebble bed increases progressively with the reduction in particle size. However, particle retention in the pebble bed becomes more severe when the particle size becomes too small. Considering multiple factors, the current 60 mm diameter of the fuel element is indeed an excellent choice, although particles with a 50 mm diameter seem to exhibit better flow characteristics. It is speculated that there might be a "resonance" relationship between particle diameter and the size of the discharge port, which warrants further investigation in the future.