Thermal cycling: impact on bentonite permeability

Abstract

Due to its favourable properties, in particular, low permeability and swelling capacity, bentonite has been favoured as an engineered-barrier and backfill material for the geological storage of radioactive waste. To ensure its safe long-term performance it is important to understand any changes in these properties when the material is subject to heat-emitting waste. As such, this study investigates the hydraulic response of bentonite under multi-step thermal loading subject to a constant-volume boundary condition, to represent a barrier system used in a crystalline or other hard-rock host rock. The experimental set up allows continuous measurement of the hydraulic and mechanical responses during each phase of the thermal cycle. After the initial hydration of the bentonite, the temperature was raised in 20°C increments from 20 to 80°C followed by a final step to reach 120°C. Each temperature was held constant for at least 7-10 days to allow the hydraulic transients to equilibrate. The data suggest that the permeability of bentonite appears to be sensitive to changes in temperature which may extend beyond those explained by simple changes in water viscosity. However, permeability may be boundary-condition dependent and this should be considered when designing experiments or applying these results to other repository host rocks. Either way, the magnitude of the change in permeability observed in this study is minor and its impact on the hydraulic performance of the barrier is negligible.

Bentonite Permeability at Elevated Temperature

“…This has been attributed to the reduction in water viscosity with increasing temperature [25]. However, Zihms and Harrington [26] found that the sensitivity of bentonite permeability to temperature could not entirely be explained by changes in water viscosity. Repository designs will need to address possible variations in the surrounding temperature of the disposal site and ensure that the maximum design temperature is not exceeded [27,28].…”

“…The disadvantage of the isotropic system was that the surface of the clay could not be instrumented. In this paper, we combine new measurements on the sensitivity of permeability to temperature with a reanalysis of previous data (constant volume radial flow (CVRF) experiment CVRF-1) originally presented by Zihms and Harrington [26]. The results of four experiments presented here provide details of the intrinsic permeability, the total stress, the swelling pressure and the porewater pressure during thermal loading of compact bentonite.…”

“…The sample dimensions and weight were recorded and X-ray radiography was used to image the fabric of the bentonite, both pre-and post-test (Table 1). The post-test samples were then dried in an oven at 105 • C for more than 48 h. This allowed the calculation of the geotechnical properties from the wet and dry weights as well as the sample volume, based on a grain density of 2770 kg/m 3 [26].…”

“…The aim of this study is to experimentally investigate the response of compact MX80 bentonite during a thermal loading cycle up to 200 • C, extending the temperature range that has previously been considered (e.g., [25,26]). While repeated thermal cycling is highly unlikely to occur in-situ, it has been examined as part of this study to assess permanent changes to the bentonite thereby gaining a broader understanding of its response to thermal loading.…”

“…Repository designs will need to address possible variations in the surrounding temperature of the disposal site and ensure that the maximum design temperature is not exceeded [27,28]. Studies have found that at temperatures below 100 • C, the sealing performance of clays is not significantly affected [26,29,30]. However above this temperature, illitisation and mechanical degradation of the clay could lead to a reduced performance [12].…”