Lawrence Berkeley National Laboratory

This study investigates the effect of shale creep on the stress evolution of a generic nuclear waste repository. Thermohydromechanically (THM) coupled simulations were carried out with the TOUGH-FLAC simulator to model temperature/pore pressure/stress changes due to the decay heating from nuclear waste. The Norton-Bailey creep model, which was calibrated against published creep experiments on shale, was used to simulate different creep properties of shale (i.e., no creep vs. creep). Results show that shale without creep generated and maintained high stress concentrations (~40 MPa) near the disposal tunnel for an extremely long period (~10, 000 years), whereas shale with creep dissipated the stress concentrations and developed uniform and isotropic stress field of roughly 12 MPa (in compression), which corresponded with the overburden stress level at the tunnel depth, by as early as 100 years since the emplacement of nuclear waste. Also, it was found that shale with creep increased compressive stresses in the bentonite backfill compared to shale without creep by as much as 130% at 10, 000 years due to creep-induced tunnel volume reduction.