1/24 Title: Acceleration of Coupled Granular Flow and Fluid Flow Simulations in Pebble Bed Energy Systems Author names and affiliations: Yanheng Li Department of Mechanical, Aerospace, and Nuclear Engineering Rensselaer Polytechnic Institute 110 8 th street Troy, New York, USA liy19@rpi.edu Wei Ji* (Corresponding author) Department of Mechanical, Aerospace, and Nuclear Engineering Rensselaer Polytechnic Institute 110 8 th street Troy, New York, USA Tel: 1-(518)2766602 Fax: 1-(518)2766025 jiw2@rpi.edu ABSTRACT Fast and accurate approaches to simulating the coupled particle flow and fluid flow are of importance to the analysis of large particle-fluid systems. This is especially needed when one tries to simulate pebble flow and coolant flow in Pebble Bed Reactor (PBR) energy systems on a routine basis. As one of the Generation IV designs, the PBR design is a promising nuclear energy system with high fuel performance and inherent safety. A typical PBR core can be modeled as a particle-fluid system with strong interactions among pebbles, coolants and reactor walls. In previous works, the coupled Discrete Element Method (DEM)-Computational Fluid Dynamics (CFD) approach has been investigated and applied to modeling PBR systems. However, the DEM-CFD approach is computationally expensive due to large amounts of pebbles in PBR systems. This greatly restricts the PBR analysis for the real time prediction and inclusion of more physics. In this work, based on the symmetry of the PBR geometry and the slow motion characteristics of the pebble flow, two acceleration strategies are proposed. First, a simplified 3D-DEM/2D-CFD approach is proposed to speed up the DEM-CFD simulation without loss of accuracy. Pebble flow is simulated by a full 3-D DEM, while the coolant flow field is calculated with a 2-D CFD simulation by averaging variables along the annular direction in the cylindrical