Analysis of granular flow in a pebble-bed nuclear reactor

TLDR

Pebble‑bed nuclear reactors, now being revived worldwide, rely on slow draining graphite fuel pebbles whose dense granular flow is poorly understood yet crucial for reactor physics. To address this problem, we perform full‑scale discrete‑element simulations of up to 440,000 frictional, viscoelastic 6‑cm pebbles draining in realistic reactor geometries. The simulations model a cylindrical vessel (3.5 m × 10 m) with angled bottom funnels (30° or 60°) and include a bidisperse core with a dynamic central column of smaller moderator pebbles, allowing analysis of velocity, diffusion, mixing, ordering, porosity, residence‑time distribution, and wall‑friction effects. The results show little mixing down to a 1:2 diameter ratio, and the detailed flow statistics provide insights into reactor design and the fundamental physics of granular flow.

Abstract

Pebble-bed nuclear reactor technology, which is currently being revived around the world, raises fundamental questions about dense granular flow in silos. A typical reactor core is composed of graphite fuel pebbles, which drain very slowly in a continuous refueling process. Pebble flow is poorly understood and not easily accessible to experiments, and yet it has a major impact on reactor physics. To address this problem, we perform full-scale, discrete-element simulations in realistic geometries, with up to 440,000 frictional, viscoelastic 6-cm-diam spheres draining in a cylindrical vessel of diameter 3.5m and height 10 m with bottom funnels angled at 30 degrees or 60 degrees. We also simulate a bidisperse core with a dynamic central column of smaller graphite moderator pebbles and show that little mixing occurs down to a 1:2 diameter ratio. We analyze the mean velocity, diffusion and mixing, local ordering and porosity (from Voronoi volumes), the residence-time distribution, and the effects of wall friction and discuss implications for reactor design and the basic physics of granular flow.