The disposal of heat-emanating radioactive waste in deep repositories can induce strongly coupled Thermal (T), hydrological (H), mechanical (M) and chemical (C) processes. Adequate coupled THMC models are important for the safety assessment of such a repository. Here we present a series of coupled THMC model simulations for a bentonite barrier. After providing a brief overview of the typical coupled processes encountered in bentonite barriers, we describe the development of a simulator that can simultaneously compute THMC processes, and how the capability of the THMC simulator is demonstrated in simulating an in situ heating experiment. Finally, we describe exploratory coupled THM models for generic geologic disposal cases to evaluate the chemical changes in the bentonite and their effects on mechanical behavior under high temperature. In addition to multiphase phase Darcy’s law for fluid flow, and advection and diffusion for solute transport, poro-elasto-plasticity for clay, key coupled processes include vapor diffusion, porosity and permeability changes due to swelling, thermal osmosis, and stress reduction by illitization.
Geological repositories for disposal of high-level nuclear waste generally rely on a multi-barrier system to isolate radioactive waste from the biosphere. The multi-barrier system typically consists of the natural system, which includes the repository host rock and its surrounding subsurface environment, and the engineered barrier system (EBS), which comprises the waste canister and in many design concepts a bentonite-based buffer or backfill, as shown in Fig. 1 for a typical repository layout. Argillite, crystalline rock, and salt are the major potential host rocks that have been studied extensively. In most repository concepts, bentonite was selected as the buffer material because it has low permeability, high retardation capability for radionuclides, and swelling ability. After emplacement adjacent to heat-generating waste, the buffer will experience heating from the waste package and hydration from host rock (Fig. 2), which result in changes in mechanical processes such as swelling, stress evolution, possibly leading to damage, and chemical changes including solute transport, radionuclide migration, and mineralogical change. These thermal, hydrological, mechanical, and chemical (THMC) processes are coupled and evolve temporally and spatially (Fig. 3): As the temperature in the buffer increases then decreases with time, the buffer, which is initially partially saturated, will experience desaturation followed by resaturation and go through changes in stress due to swelling and thermal expansion. Alteration of minerals with high solubility may occur at early times, followed by alteration of clay minerals at later times. Physical and chemical properties of the buffer evolve in response, potentially affecting fluid flow, solute transport, porewater chemistry, and the stresses experienced by the waste package and at the drift wall.