THMC Behavior of Bentonite under hydration and high temperature at laboratory scale

Bentonite backfill (part of engineered barrier system) surrounding nuclear waste containers is a key component to ensure long term safety of high level nuclear waste geological repository. While behavior of bentonite at <100°C temperature have been well documented base on a range of laboratory and field experiments, existing data on characteristics and dynamics of bentonite at temperature above 100°C is relatively limited. A high temperature (175°C ̶ 200°C) experiment in a crystalline rock environment, called HotBENT, is planned at the Grimsel site in Switzerland to understand impacts of heating and hydration effects on bentonite, and a parallel laboratory experiment with more comprehensive characterization and monitoring is carried out at Lawrence Berkeley National Lab (LBNL). The laboratory experiment is conducted in cylindrical Aluminum pressure vessels for its transparency to X-ray and mechanical strength that meet the pressure design at 1000 psi (69 bars) at 200°C. The vessel was packed with MX-80 bentonite with a cartridge heater installed in the center of the column. To simulate homogeneous hydration along the perimeter of the clay, a thin sand layer was packed between the clay and vessel wall which stays hydrated at elevated pore pressure during the experiment. The saturating fluid is a synthetic brine based on the characteristics of groundwater at the FEBEX site. Time lapse X-ray CT images and ERT monitoring provide 3D visualization of the clay swelling and moisture content change due to water inhibition, ionic exchange and possible phase transformation. Saturating fluid was sampled and analyzed during the heating to assess geochemical changes in the systems. Clay samples from pre- and post- experiments will be analyzed to understand changes in its physical, chemical and hydrological characteristics due to hydration and heating. Preliminary results from this ongoing experiment has indicated the strong impact of heating and hydration on inhibition, clay swelling as well as geochemical changes due to ion exchanges. Numerical simulations and parallel petrophysical experiments will provide additional datasets to further understand the thermal, hydrological, mechanical and chemical (THMC) processes during clay heating under high temperatures.