In Pebble-Bed Modular Reactor (PBMR) systems, typical representatives of large-scale fluid-particle energy systems, the gaseous coolant has strong interactions with the fuel pebbles that slowly move downward. To accurately predict the coolant dynamics and fuel bed mechanical properties, the fluid-pebble interactions should be taken into consideration in the mechanical modeling of the core. However, previous research usually neglected these interactions in the pebble flow analysis, assuming a fixed bed that is not affected by the coolant. The effects of these interactions on the core behavior remain unknown and have never been fully addressed. To investigate the interaction effect on the pebble flow mechanics, a coupled multi-physics model is employed which accounts for the fluid-pebble interactions in the dynamics simulations of a large annular nuclear reactor system. Discrete Element Method is used to simulate the pebble motion and predict important granular properties such as pebble packing density, coordination number and contact stress distributions. Computational Fluid Dynamics is employed to predict the locally averaged coolant properties. The simulations of pebble flow and coolant flow are coupled through the calculation and exchange of fluid-pebble interaction forces between two simulation solvers at a fixed time interval. To observe the interaction effect, decoupled simulations are also performed as references for comparison, i.e. a single pebble flow simulation without interactions with coolant, followed by a single coolant flow simulation in a static fixed packed bed. The comparison indicates that the interactions have significant effects on the local mean pebble contact force and observable effects on the axial and radial distributions of pebble volume packing fraction, as well as the pebble flow velocity/pressure profiles. The interaction effects are especially noticeable in the regions near the wall and close to the core inlet. The change in the pebble packing distribution will consequently bring in discrepancies in the distributions of axial and radial fluid velocity and pressure. Therefore, the interactions between coolant and pebble cannot be negligible in the study of the thermal-hydraulic properties of large-scale fluid-particle energy systems, although some macroscopic properties of the pebble flow (such as the coordination number distribution) are barely changed by the fluid-pebble interactions. (C) 2015 Elsevier Ltd. All rights reserved.