Study on Pebble-Fluid Interaction Effect in Pebble Bed Reactors

INTRODUCTION In pebble bed reactors (PBRs), fuel pebbles containing TRISO particles continuously circulate within the core during operation, while the coolant fluid, either helium gas (in Pebble Bed Gas-cooled Reactors) or molten flibe salt (in Pebble Bed-Advanced High Temperature Reactors), continuously passes through the pebbles to transfer the heat generated by fission reactions out of the core. Such a design has many advantages in fuel efficiency and reactor safety. To accurately predict its neutronic and thermal-hydraulic behavior, high fidelity simulations are needed to obtain accurate pebble distributions and coolant fluid porosity distributions [1]. In PBRs, both pebble flow and coolant flow exist. They are not independent from each other but coupled through pebble-fluid interactions such as the fluid drag force and the pressure gradient force. In previous work [2, 3], coupled pebble and coolant flow were simulated using a high fidelity coupled Discrete Element MethodComputational Fluid Dynamics (DEM-CFD) model. However, the significance of the coupling was not addressed. This becomes the motivation for this summary, which is to quantitatively investigate the impact of the pebble-fluid interactions (coupling) on both the pebble flow and the coolant flow. Two scenarios with different fidelities are investigated: 1) Simulation of the pebble flow and the coolant flow without coupling. The spatial distribution of steady-state pebble flow is first calculated by DEM, and then the coolant field is calculated by CFD approach based on this static pebble distribution. 2) Fully coupled pebble flow and fluid flow simulation via DEM-CFD approach, in which the dynamic interactions between both flows are considered at each simulation step. By comparing the pebble/coolant behaviors under these two scenarios, the effects of pebble-fluid interactions on both flows are quantitatively analyzed. For pebble flow, the interaction impact on the average pebble speed and axial distributions is studied. For coolant flow, the influence on the axial/radial profile of velocity and pressure drop is investigated.