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Abstract
G.J. Auwerda, J.L. Kloosterman, A.J.M. Winkelman, J. Groen and V. van Dijk,
Comparison of Experiments and Calculations of Void Fraction Distributions in Randomly Stacked Pebble Beds,
International Conference on the Physics of Reactors, "Advances in Reactor Physics to power the Nuclear Renaissance" (PHYSOR-2010), Pittsburgh, USA (2010).
In pebble bed reactors the fuel forms a randomly stacked pebble bed with non-uniform fuel densities, affecting
neutronics (streaming) and thermodynamics (wall channeling). To investigate these effects, computational tools
are needed capable of generating realistic pebble beds, and experimental results to validate these tools.
Using gamma-ray scanning the absolute and radial void fraction profiles of a randomly
stacked pebble bed was measured. Results were used to validate three different methods: Discrete Elements Method
(DEM), Monte Carlo (MC) rejection method, and expanding system method.
The bed consisted of 5457 acrylic pebbles with a diameter d=12.7 mm in an acrylic cylinder with diameter
D=229 mm (D/d=18.0), and had an average void fraction 0.395. The radial void fraction profile
showed large, dampened oscillations near the wall extending up to five pebble diameters into the pebble bed,
with a minimum void fraction of 0.22 half a pebble diameter away from the wall.
The MC rejection method resulted in an average void fraction much higher than measured, and could not reproduce well
the oscillations in radial void fraction observed in the experiment. Both the DEM and expanding system method showed
excellent agreement with the experiment for both average and radial void fractions, with the expanding system
method having the benefit of creating pebble beds with no overlapping pebbles, suitable for exact pebble bed
models in other codes.
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