The effect of spatial variability on uranium diffusion in Opalinus Clay over a million years was assessed by 2D reactive transport simulations. Different rock types and porosity impacted results, with variability's influence growing with longer correlation lengths of geostatistical simulations (up to 12.9 %). This highlights that 1D models may underestimate uncertainty in long-term radionuclide transport at repository sites, and spatial variability must be considered.