Comparison of excess reactivity in a UOX assembly due to fuel degradation, as described by different random media models (Poisson tessellations in blue squares, Poisson-Box tessellations in red triangles, and Voronoi tessellations in green circles). The average multiplication factor is shown as a function of the typical length scale of the fuel randomization. Figure taken from reference [112]. (This figure is subject to copyright protection and is not covered by Creatice Commons License. Elsevier, 2018).