Prediction of Pebble Motion in Pebble-Bed Reactors Using Monte Carlo Molecular Dynamics Simulation

Pebble motions in pebble-bed reactors (PBRs) have been investigated by generating pebble motion histories with Monte Carlo molecular dynamics simulations. This extension of molecular dynamics to PBR-sized pebble motion is accomplished by splitting the simulation into two parts. The first part simulates the dropping of pebbles into the PBR with a closed exit that allows one to obtain the correct initial placement of all pebbles within the pebble bed. The second part simulates what happens when the PBR exit is opened and normal pebble flow begins. Using this combined approach the pebble piling up and subsequent discharge are predicted. Simulations have been conducted with this approach by monitoring the mass flow rate, the pebble piling up, and the subsequent discharge for a range of pertinent parameters using the Hertz-Mindlin force for pebble interactions. The simulation output data include the force, velocity, and position of the pebbles as a function of time. Note that arching or locked flow, a very important phenomenon, is predicted by this approach under certain operating conditions. Using this approach, PBR results (including arching) for a range of the parameters of interest are reported and are discussed herein.