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Analysis of Granular Flow in a Pebble-Bed Nuclear Reactor

Chris H. Rycroft·Gary S. Grest·James W. Landry·Martin Z. Bazant
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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 6cm-diameter spheres draining in a cylindrical vessel of diameter 3.5m and height 10m 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.

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Chris H. Rycroft, Gary S. Grest, James W. Landry, Martin Z. Bazant. 2006-02-16. Analysis of Granular Flow in a Pebble-Bed Nuclear Reactor. https://doi.org/10.1103/physreve.74.021306

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