Layer charge characterization of smectites by different methods in the FEBEX in situ test

The FEBEX Project is an "in-situ" real-scale test performed throughout more than 18 years in Grimsel, Switzerland. It consists of compacted bentonite, mainly composed of smectite (> 92 wt.% montmorillonite), isolating the canisters (simulated by heaters) from the hosting rock. Its main objective was to demonstrate the viability of the engineered barrier system (EBS) as a high level nuclear waste (HLW) disposal in a crystalline host rock. Therefore, one of the goals of FEBEX was the verification of the long term stability of the clay barrier after years of natural hydration and heating (up to 100 C). In this work, in order to test the behavior of the clay barrier, we have focused our studies in the evolution of the layer charge of the smectites (ξ), as one of the main parameters which controls smectite properties, such as swelling, cation exchange capacity and ion exchange between the clay and fluids in the EBS.