Geophysical Monitoring of Bentonite Response to Hydration and Heating

Abstract:

Engineered barriers systems (EBS) based on Bentonite are a key component in geological disposal of high-level nuclear waste. A combination of thermal, hydraulic, mechanical, and (bio)chemical processes affects the behavior of bentonite barriers at different temporal and spatial scales, challenging the characterization and monitoring of its safety performance. At relatively high temperatures (above 100 °C), only limited data are available on how bentonite response to temperature and saturation changes, for example, in terms of how swelling (self-sealing) impact the spatiotemporal dynamics of the hydraulic properties. We performed long-term laboratory heating and hydration column experiments to investigate the impacts of temperature, pressure, and saturation on bentonite in the 20-200 °C temperature range.

We addressed the spatial variability with the combined use of electrical resistivity tomography (ERT) and X-ray CT. The monitoring period (> 1 year) captured the highly dynamic initial phases and the subsequent stable thermal and hydraulic conditions. Because of their different sensitivities to density and water content, the joint use of ERT and CT allowed the imaging and differentiation of swelling and brine-flow dynamics. The results from this laboratory experiments validated the feasibility of electrical geophysical methods as a long term monitoring strategy to evaluate EBS performances. In this context, we performed simulations to evaluate the sensitivity of ERT monitoring to EBS performance at the field scale based on the planned long term HotBENT experiment at the Grimsel site in Switzerland. The simulations addressed the optimization of the ERT coverage for the expected highly-dynamic system with significant resistivity changes from the thermohydrological processes and geological context, as well as the presence of metallic infrastructures.