Assessment of measurement techniques to monitor engineered barrier systems for nuclear waste repository

A major challenge in the implementation of Engineered Barrier Systems (EBS) for the geological disposal of nuclear waste is monitoring their response to thermal and hydraulic loading. This applies to mock-up scale tests designed to validate the EBS concept via ‘wired’ local sensors, but also to future full-scale EBSs to be monitored ‘wireless’ via electrical geophysical techniques for EBS validation during the operational stage. To underpin the monitoring of EBS, this study investigates experimental techniques for measuring the volumetric water content, suction, and electrical conductivity of bentonite-based materials. It is demonstrated that Time Domain Reflectometry (TDR) probes can be successfully used to monitor volumetric water content provided that i) the measured apparent relative dielectric permittivity is corrected for the effect of clay electrical conductivity and ii) the calibration equation accounts for the effect of bulk density and adsorbed water. It was also shown that the electrical conductivity inferred from TDR and direct current four-point method measurements may differ substantially, and such a difference can be associated with the frequency-dependent contribution of free and adsorbed water to bulk electrical conductivity. Finally, encapsulated Micro-Electro-Mechanical Systems (MEMS) sensors were demonstrated to measure total suction with satisfactory accuracy over the full relative humidity range.