This paper presents a new FEM approach and computer code for modeling fully coupled thermo-hydro- mechanical processes associated with underground nuclear waste repositories. The governing equations are based on the theory of mixtures applied to the multiphysics of porous media, considering solid-phase deformation, liquid-phase flow, gas flow, heat transport, thermally-induced water flow, phase change of water, and swelling deformation in buffer materials. For three-dimensional problems, three displacement components, water pressure, gas pressure, vapor pressure and porosity are chosen as the eight primary variables. The code was tested against a benchmark test that was performed in laboratory conditions on vertical cylindrical columns of compacted MX-80 bentonite by the French Commission of Atomic Energy from 2003 to 2004. The comparison with the benchmark tests shows good agreement between the numerical predictions and the measured data, thus providing a partial validation of our new code. Discussion of outstanding issues and conclusions are presented at the end of the paper.
In some countries, a buffer of compacted bentonite is planned to be placed around the waste canisters in underground geological repositories for radioactive wastes. Fluid flow, phase changes, mechanical behaviour of buffer and rock, and the heat produced by the waste constitute a coupled thermo-hydro-mechanical (THM) system. Coupled THM mathematical models and computer codes need to be developed, verified and improved for performance assessment of long-term safety of nuclear waste repositories.
In the field of geological disposal of radioactive wastes, many coupled THM numerical models have been developed (Olivella, 1994; Börgesson, 1996; Nguyen, 1996; Noorishad, 1996; Ohnishi, 1996; Thomas, 1996). Some of them have the capability to model liquid flow in unsaturated media through the use of the Richards equation (Richards, 1931), but the gas phase movement is usually ignored using the assumption of a constant and small gas pressure. Although some THM numerical models and codes can simulate two-phase (gas and liquid) fluid flow with two components (air and water) in partially saturated soil, coupled with heat transport and mechanical responses (Rutqvist, 2001), they usually neglect the advective flow of vapor and cannot consider the transfer of heat between the phases. By making the assumption of spontaneous thermo-dynamic equilibrium between the soil liquid and the water vapor, the vapor pressure becomes a variable that depends on suction and temperature, and the flux of vapor and liquid water can be modelled using a single equation (Khalili, 2001).
In order to increase the capability of modeling coupled THM processes in buffer and rock-buffer interfaces, a fully coupled THM numerical model, along with a computational FEM code ROLG, are developed and presented in this paper. In this model, the gas flow and the vapor flow are described by their mass conservation equations, and moisture movement and phase changes are considered. The developed numerical model and code are verified through a laboratory benchmark test as part of Work Package 4 of the EC sponsored project THERESA.