Effects of restitution coefficient on pebble motion in a thin pebble bed – Lagrangian analysis

The core of pebble bed type reactor (HTGR) is a packed bed composed of spherical pebbles (fuel element and graphite moderator). The restitution coefficient is an important parameter which is directly related to the flow of the core pebbles and affects the motion trajectory and stacking state of the fuel pebbles. Herein, Discrete Element Method (DEM) is used to simulate pebble flows within a thin pebble bed. The packing peaks, apex angles, trajectory and velocity deviations, residence time and residence ratios are analyzed in details. The influence of restitution coefficient on the motion characteristics of pebble flow are studied based on the trajectory of fuel pebble. A new evaluation criterion for the uniformity of pebble flows is put forward, and its influencing mechanisms are explored. The relationship between the restitution coefficient and pebble motion is proposed, which can help understand the flow uniformity of fuel pebbles in nuclear reactor core.