[en] In this work, we quantify the impact of the randomness of the traversed medium for a simple benchmark problem of neutron transport in a stochastic mixture composed of low-enrichment (3.7%) UO₂ fuel fragments dispersed in water. A tool that allows sampling a large set of stochastic material configurations by Monte Carlo methods has been recently developed. By using this tool, we assess the effects of the stochastic geometries as opposed to lattice dispersions of fuel in a moderator material. The findings of our preliminary investigation show that lattice models are not always conservative with respect to the reactivity. (authors)