Significance of rock THM parameters in geological nuclear waste storage simulation

A deep geological repository involving a multi-barrier system constitutes one of the most promising options to isolate high-level radioactive waste from the human environment. In order to certify the efficiency of waste isolation, it is essential to understand the behavior of the confining geomaterials under a variety of environmental conditions. The efficiency of an Engineered Barrier System (EBS) is largely based on a combination of bentonite and host rock characteristics. To contribute to a better understanding of the processes involved in the EBS, a case study for sensitivity analysis has been defined and is studied using a thermo-hydro-mechanical (THM) finite element approach including a consistent thermo-plastic constitutive model for unsaturated soils. The model also features a coupled THM approach of the water retention curve for bentonite, using the ACMEG-TS constitutive model. Regarding rock parameters, intrinsic permeability and relative permeability effects are evaluated. Two regimes are found regarding the importance of the estimation of rock permeability: in the first one, precise assessment is unnecessary due to water inflow control by bentonite, while in the second one, a precise assessment is necessary to correctly estimate resaturation time. This study highlights the effects that need to be taken into consideration for a correct assessment of EBS behavior, from bentonite characteristics to the correct quantification of the thermo-hydro-mechanical couplings in host rock.