Discrete element modeling of pebble bed packing structures for HCCB TBM

In solid tritium breeder blanket, the tritium breeder and neutron multiplier are used in formed of pebbles. To reveal the inner packing structures of the pebble bed, in this paper, the discrete element method (DEM) was applied to simulate the pebble bed packing structures by pouring pebbles from the top of container. The current numerical results obviously show that with the increase of aspect ratios the packing factor can be significantly increased in both cylinder pebble bed and cubic pebble bed. Namely, the larger aspect ratios in cylinder and cubic pebble bed, the smaller the proportion of the wall affected region, the greater the average packing factor of the cylinder and cubic pebble bed. Furthermore, the pebble packing structures in a U-shaped container were also simulated and analyzed. In the bend pebble bed, the average packing factor of 0.6278 was obtained. Close to the convex and concave container wall, the wall effects of pebble distribution and packing factor were observed. With the increase of the distance to the container wall, the layered arrangements disappear gradually and the oscillating characteristics of local packing factor are also damped little by little. The oscillations of local packing factors are limited in about 5 diameters. A transition region is formed between the layered packing in the near wall region and the uniformly random packing in the inner bulk region. In addition, the contact force distribution and coordination number distribution were also detailed analyzed.