Effect of spatial variability on uranium diffusion in the three facies of the Opalinus Clay at Mont Terri

This study, conducted in the framework of an Erasmus exchange at GFZ Helmholtz Centre for Geosciences (Potsdam, Germany), investigates the impact of spatial variability on uranium diffusion in the Opalinus Clay formation at Switzerland's Mont Terri underground laboratory; a facility built to research the geological storage of high-level radioactive waste. Using 2D reactive transport simulations with the open-source research code POET developed at GFZ, the research examined three distinct facies: sandy, carbonate-rich, and shaly, each with different porosity ranges and chemical retention capacities. Geostatistical simulations of porosity with varying correlation lengths (5-20m) and anisotropy ratios were used to derive spatially variable effective diffusion coefficients. Two distinct boundary conditions were tested: uranium diffusion from a linear source and from a punctual source, both simulated over one million years. Results indicate that the carbonate-rich facies allows significantly further uranium migration due to reduced chemical buffering. Larger correlation lengths produced more extensive uranium spreading, with a maximum migration length of 33m for linear sources and coverage of 975m² for punctual sources under isotropic conditions. When incorporating more realistic hydro-geochemical gradients into the initial state for the simulations, uranium diffusion increased significantly, reaching 48.25m distance and 2066.3m² coverage in the carbonate-rich facies when diffusing into increasing background concentrations. These findings highlight the fact that, while geochemistry of the formation has the largest impact on uranium migration, spatial variability can not be neglected as source of uncertainty on long-term radionuclide transport in potential repository sites.