Bentonite Permeability at Elevated Temperature

Abstract

Repository designs frequently favour geological disposal of radioactive waste with a backfill material occupying void space around the waste. The backfill material must tolerate the high temperatures produced by decaying radioactive waste to prevent its failure or degradation, leading to increased hydraulic conductivity and reduced sealing performance. The results of four experiments investigating the effect of temperature on the permeability of a bentonite backfill are presented. Bentonite is a clay commonly proposed as the backfill in repository designs because of its high swelling capacity and very low permeability. The experiments were conducted in two sets of purpose-built, temperature controlled apparatus, designed to simulate isotropic pressure and constant volume conditions within the testing range of 4-6 MPa average effective stress. The response of bentonite during thermal loading at temperatures up to 200 • C was investigated, extending the previously considered temperature range. The results provide details of bentonite's intrinsic permeability, total stress, swelling pressure and porewater pressure during thermal cycles. We find that bentonite's hydraulic properties are sensitive to thermal loading and the type of imposed boundary condition. However, the permeability change is not large and can mostly be accounted for by water viscosity changes. Thus, under 150 • C, temperature has a minimal impact on bentonite's hydraulic permeability.

Geochemical, Geotechnical, and Microbiological Changes in Mg/Ca Bentonite after Thermal Loading at 150 °C

“…The first approach examined HM properties at different temperatures. Geotechnical parameters of water-saturated compacted bentonites (FEBEX, MX-80, and Korean Ca-bentonite) were determined at temperatures ranging from 20 to 150 • C [8][9][10][11][12]. Decreasing swelling pressure and suction pressure and increasing hydraulic conductivity with temperature were assigned to the temperature-dependent water properties and physicochemical interactions of water at the microscopic level [9].…”

“…Decreasing swelling pressure and suction pressure and increasing hydraulic conductivity with temperature were assigned to the temperature-dependent water properties and physicochemical interactions of water at the microscopic level [9]. However, once corrected for temperature-dependent water density and viscosity, the intrinsic permeability was not significantly temperature dependent [11,12]. This approach demonstrates buffer safety functions (namely swelling pressure and hydraulic conductivity) at all thermal stages within the DGR evolution.…”

Gel Formation at the Front of Expanding Calcium Bentonites

“…The repository designs often include a number of different natural and engineered barriers to prevent the waste from contaminating the environment (e.g., [1,4,5]). Bentonite is commonly included as the clay backfill or engineered barrier in these designs [6][7][8] because of its low permeability, high swelling capacity and self-sealing properties [9][10][11][12]. Many different compositions of bentonite exist, and much research has been focussed on the properties and behaviour of sodium (Na) and calcium (Ca) bentonites [13][14][15][16][17][18][19].…”

Pore Network Modeling Study of Gas Transport Temperature Dependency in Tight Formations

“…10,11 Recent studies have shown that such mechanisms can be augmented at elevated temperatures. 12 Enhanced diffusion can counteract thermal loading, indicating that the permeability of tight formations may either positively or negatively correlate with temperature. The objective of the present research is to couple the stress-related aspects of rocks with non-Darcian transport mechanisms to better understand the temperature's effects on the permeability of tight formations through pore network modeling.…”