Pebble bed reactor is a nuclear reactor type where the fuel material is in the form of coated particles dispersed inside graphite spheres of approximately 60 mm in diameter. Typical reactor contains about half a million of these pebbles that form a heat-producing column. Pebbles are fed on top and removed from the bottom of the column so that they flow through the core slowly. The column is cooled by helium gas flowing through the pebble bed.

Knowledge of the packing characteristics and behaviour of the fuel pebbles is important for resolving the core neutronics, burnup and thermal-hydraulics accurately. DEM can be used to model the mechanical interactions between pebbles accurately. The results can then be used in further analyses, e.g. as detailed pebble positions or packing density profiles.

In this work, an in-house DEM code was used to compact 450 000 pebbles inside an annular cylinder representing the core of a pebble bed reactor. Three pebble beds with different average packing densities were obtained by varying input parameters. Pebble-scale packing fraction data was extracted from the packed beds using Voronoi tessellation. The data was used to plot packing density profiles for different parts of the columns. The local packing fractions were also used for analyses with statistical and 3D visualization methods to reveal packing structure details. Local dense regions with crystallization were observed especially when the average packing density was high. The results are useful in further analyses, e.g. to identify hot spots in the reactor core.