Comparison of Experiments and Calculations of Void Fraction Distributions in Randomly Stacked Pebble Beds

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 0 and radial void fraction profile r(r) 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 = 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 a 0 much higher than measured, and could not reproduce well the oscillations in r observed in the experiment. Both the DEM and expanding system method showed excellent agreement with the experiment for both 0 and r, 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.