Stress Evolution of a Geological Nuclear Waste Repository in a Creep-Prone Shale Formation

Abstract

ABSTRACT: This study investigates the effect of shale creep on the stress evolution of a generic nuclear waste repository. Thermo-hydromechanically (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. 1. INTRODUCTION A permanent solution to accumulating nuclear waste will be geological nuclear waste repositories (IAEA, 2003). Geological repositories confine nuclear waste deep underground to protect the biosphere with a multi-barrier system (Apted & Ahn, 2010). The barrier system consists of natural barriers (i.e., formations) and an engineered barrier system (EBS) (e.g., bentonite backfill), which is placed around nuclear waste canisters in the disposal tunnel. The EBS not only provides mechanical support to the nuclear waste canister but also enhances the stability of the formation around the tunnel, which could be damaged during tunnel excavation (Rutqvist, 2015). The excavation damaged zone (EDZ) could provide flow paths for radionuclides if they are released (Tsang et al., 2005). Hence, it is crucial to evaluate the stability of the EBS, EDZ, and surrounding formation for the long term in order to assess the safety of geological nuclear waste repositories.