Variation of k_eff with Perlin noise scale factor S and chord length Λ in a randomized PWR assembly containing UOX fuel (left) and MOX fuel (right). Each point represents the mean of 15 random realizations. The error bars represent the standard error on the mean of the ensemble, while the shading displays the standard deviation of the ensemble. The horizontal black line represents k_eff for the corresponding intact benchmark fuel assembly, before randomization.