Lawrence Berkeley National Laboratory

We present coupled Thermo–Hydro–Mechanical (THM) modeling of geologic nuclear waste disposal in argillaceous claystone, focusing on thermally-induced pressure changes and the potential for such pressure changes to induce hydro-fracturing. To investigate this possibility, we first conduct a three-dimensional repository scale model, with host rock properties, repository design, and nuclear waste decay heat functions derived from the French concept of geologic disposal in argillaceous claystone. The model simulations show that the highest potential for hydro-fracturing occurs between emplacement micro-tunnels (cells) at the center rather than at the edge of the repository. We further investigate the use of a two-dimensional single cell model as a simpler surrogate for a full three-dimensional model. Our results reveal that such geometric simplification is reasonably accurate for modeling coupled THM processes at the center of the repository, though it overestimates the likelihood for hydro-fracturing and substantially overpredicts ground surface uplift. A parameter study shows the importance of adjacent higher permeability geological layers that play a significant role in dissipating overpressure in the host rock layer. Finally, the study shows the importance of the spacing between emplacement tunnels, which if too short results in a higher fluid pressure and a strongly increased potential for hydro-fracturing. Overall, the study suggests, the limiting factor in the thermal management and design of a repository is not necessarily the maximum temperature in the engineered barrier system near the waste packages, but rather the more modest host rock temperature between emplacement tunnels, due to the potential for thermal damage.