Chemo-hydro-mechanical coupled consolidation for a poroelastic clay buffer in a radioactive waste repository

Abstract

Salt rock or rock with salty ground water are often encountered as host media for the underground disposal of radioactive waste. The nuclear waste, contained in a metallic canister, is usually placed inside a tunnel or a shaft excavated in the rock deposit together with a buffer of compacted bentonite inserted between the host rock and the canister to provide hydro-mechanical sealing. Due to the very low permeability and rich clay content, the bentonite acts as an osmotic semi-permeable membrane under a gradient of concentration of salt dissolved in the ground water. In addition, chemically induced expansion or shrinkage of the bentonite is generated by changes in the concentration of dissolved salt. By including such important chemical aspects, the hydro-mechanical governing equations are derived for this particular boundary value problem within the framework of a linear Biot-like isotropic poroelastic consolidation. The equations are solved analytically and a parametric study is undertaken to highlight the influence of chemical osmosis and chemical deformation on the flow and mechanical response of the bentonite buffer.

Multi-dimensional chemo-osmotic consolidation of clays

“…This paper extends mechanically/chemically induced porosity change theory [1][2][3]12] to multi-dimensional domains. Following Thomas and Cleall [17], the total strain increment is defined as the sum of the strain increments due to changes in effective stress and chemical concentration, that is…”

“…Following the approach of Barbour and Fredlund [12], also adopted by Chen et al [3] volumetric strains due to changes of chemical concentration are expressed as:…”

“…Whilst the theoretical representation of the various processes considered by the developed model have been previously validated by Kaczmarek and Hueckel [1], Peters and Smith [2]) and for example by consideration of the chemical-related consolidation of clay buffer in a radioactive waste repository [3] the coupling of three governing equations considering force equilibrium, pore water transport and chemical transport for such chemical/osmotic/chemo-osmotic consolidation problems in more than one dimension is novel. As such the presented numerical model requires verification and this is dealt with in the following section.…”

“…[1,2]), buffer systems of nuclear waste disposal (e.g. [3]) and electrokinetic remediation (e.g. [4]).…”

Unsaturated hydro-mechanical–chemical constitutive coupled model based on mixture coupling theory: Hydration swelling and chemical osmosis

“… m/s, often proposed for this purpose, may lead to chemical osmosis, in which the host rock and buffer clay may act as semi-permeable membranes for solute transport [3]. During groundwater resaturation through the host rock or buffer clay containing minerals, complex chemical reactions may occur which change the groundwater chemistry [4].…”