Numerical Simulation of Particle Flow Motion in a Two-Dimensional Modular Pebble-Bed Reactor with Discrete Element Method

Modular pebble-bed nuclear reactor (MPBNR) technology is promising due to its attractive features such as high fuel performance and inherent safety. Particle motion of fuel and graphite pebbles is highly associated with the performance of pebbled-bed modular nuclear reactor. To understand the mechanism of pebble’s motion in the reactor, we numerically studied the influence of number ratio of fuel and graphite pebbles, funnel angle of the reactor, height of guide ring on the distribution of pebble position, and velocity by means of discrete element method (DEM) in a two-dimensional MPBNR. Velocity distributions at different areas of the reactor as well as mixing characteristics of fuel and graphite pebbles were investigated. Both fuel and graphite pebbles moved downward, and a uniform motion was formed in the column zone, while pebbles motion in the cone zone was accelerated due to the decrease of the cross sectional flow area. The number ratio of fuel and graphite pebbles and the height of guide ring had a minor influence on the velocity distribution of pebbles, while the variation of funnel angle had an obvious impact on the velocity distribution. Simulated results agreed well with the work in the literature. 1. Introduction MPBNR technology is currently being focused on around the world, due to its preferable features such as high performance and safety. In the core of MPBNR, fuel and graphite pebbles drain slowly in a continuous process of refuel due to the gravity force on both fuel and graphite pebbles. Such a system has high temperature. Helium is usually used as the coolant in the way of flowing across the aperture of pebbles, raising fundamental questions about dense granular flow characteristics in the reactor. Up to now, dense granular flow of pebbles in MPBNR has been poorly understood and has not been easily accessible to experiments, and yet it has a major impact on reactor physics. To address this problem, some researchers tried to use either experimental or numerical simulation methods to investigate the flow characteristics of dense granular flow. In the literature, Kadak and Bazant [1] conducted a series of one-to-ten-scale three dimensional experiments to assess the flow lines and the relative velocities of the pebbles with various distances from the center of the core. It was found that the mixing zone of pebbles could be effectively eliminated while maintaining the annular column during the recirculation process. Hassan and Dominguez-Ontiveros [2] presented the full velocity fields of pebbles and gas using particle image velocimetry