Diffusion Measurements of HTO, ³⁶Cl^- and ²²Na⁺ on Rock Samples of Opalinus Clay and Confining Geological Units from Deep Boreholes of Northern Switzerland: Jura Ost, Nördlich Lägern and Zürich Nordost

pdf NAB 23-26 Diffusion Measurements of HTO, ³⁶Cl^- and ²²Na⁺ on Rock Samples of Opalinus Clay and Confining Geological Units from Deep Boreholes of Northern Switzerland: Jura Ost, Nördlich Lägern and Zürich Nordost(86.49 MB)

This summary report focuses on through-diffusion experiments of rock samples with different mineralogical compositions taken from several new deep boreholes in the Mesozoic rock sequence of Northern Switzerland. The borehole campaign was organised and conducted by Nagra (National Cooperative for the Disposal of Radioactive Waste). The Opalinus Clay, a Jurassic-age argillaceous shale with > 100 m thickness, is the primary target rock for hosting a planned deep geological repository for radioactive waste in Switzerland. Argillaceous rocks are currently being examined worldwide as potential host rocks for repositories for radioactive waste and spent fuel due to their favourable characteristics, such as the self-sealing capacity and limited diffusive transport of fluids, solutes or gases, for safe, long-term disposal.

This study summarises the results of the diffusion measurements, during which diffusion parameters of tritiated water (HTO), ³⁶Cl^- and ²²Na⁺ tracers perpendicular to the bedding plane of the rock samples were determined and both effective diffusion coefficients and rock capacity factors were calculated from the data obtained. Accessible porosities were derived from the rock capacity factors in the case of HTO and ³⁶Cl^-, and sorption distribution factors in the case of ²²Na⁺. Rock samples from seven different boreholes distributed across the three siting regions defined by Nagra were used, involving four different types of synthetic porewaters. Their composition was considered to be representative with respect to in-situ measurements on similar rock samples. For the Opalinus Clay, the effective diffusion coefficients (D_e) and accessible porosity values (ε) of the neutral HTO within and across all three siting regions are very consistent (D_e = 8.8 ± 1.9 × 10-12 m2 s-1; ε = 0.12 ± 0.02), indicating that the material can be considered homogeneous in terms of diffusion properties. The rocks above and below the Opalinus Clay, up to the next aquifer (also termed confining geological units), consist of tight sedimentary units (carbonates, siliciclastics including argillaceous rocks) with a wider range of effective diffusion coefficients, indicating a higher level of heterogeneity. A direct comparison of the effective diffusion coefficients of the Opalinus Clay with the different heterogenous rock units across the three siting regions is based on HTO data for which the diffusion behaviour is unaffected by the chemical composition of the porewater.

The effective diffusion coefficients of anions such as ³⁶Cl^- tend to be smaller than those of HTO due to anion exclusion effects, while those of cations such as ²²Na⁺ tend to be larger due to surfaceenhanced diffusion. The extent of these effects is largely determined by the composition of the porewater and, in the case of cations, by the extent of sorption which depends also on the sorption capacity – indicated by the cation exchange capacity – of the solid. As a result, variations in the effective diffusion coefficient of ³⁶Cl^- and ²²Na⁺ are observed between different rock types (i.e. composition, fabric) as well as between the different siting regions where differences in the chemical composition of the porewaters in contact with the solid phase are present.

It is widely recognised that diffusive transport of charged species in charged clay media must be consistently described for ions with different charge signs and numbers, as well as with pore diffusion schemes for inert solutes that do not undergo interactions with the clay surface. A modelling approach that meets these requirements for the diffusion of HTO, ²²Na⁺ and ³⁶Cl^- tracers in heterogeneous Mesozoic sediment sequences of multiple deep boreholes in Northern Switzerland is also presented here. The model uses an electrical double layer (EDL) approach and a discrete Donnan potential description of cation enrichment and concomitant anion depletion in the Donnan layer close to the basal (planar) clay surfaces to predict effective diffusion coefficients and rock capacity factors of the charged tracers. It incorporates the diffusion data of HTO to determine the parameters related to the functional relationships between the accessible porosity and geometry factors with the total clay content as the main input variable. It also calculates concentration enrichment or depletion factors for mobile cationic and anionic species using relationships with the total clay content, such as the average density of surface sites or the thickness of the Donnan layer and the related volume fraction of free porewater. These factors are subsequently used to derive effective diffusion coefficients that include surface diffusion of cations and anion exclusion effects and the related rock capacity factors. Despite the different lithologies of the rock samples, where significant variability between carbonates and siliciclastic contents are observed, the model shows good agreement with experimental data. Discrepancies between experimental and modelled data are mainly due to inadequacies in the geometric description of diffusion pathways, rather than by potential bias in the calculation of concentration enhancement or depletion factors. However, the model approach assumes that diffusion in clay rock occurs via the pore space of clay minerals and treats all related parameters as simple average values, neglecting heterogeneity of mineral constituents and the unknown statistical distribution of the parameter values.

The analysis of 130 samples shows that the effective diffusion coefficients of the Mesozoic units in Northern Switzerland exhibit a significant correlation with the total clay content of the rocks. These findings resulted in a novel empirical relationship that paved the way for the establishment of a diffusion database (NTB 23-08) (Glaus et al. 2024a) for the relevant elements under consideration in the Swiss radioactive waste disposal programme and the safety case.

The model has identified the chemical composition of the equilibrium porewater as a significant influencing variable for predicting concentration enhancement or depletion factors and related effective diffusion coefficients. Experimental data obtained for different synthetic porewater compositions support this finding. The robustness of the model predictions across various types of lithologies and formations, all of which exhibit diffusion as the dominant transport mode, is important for the generic application of the EDL diffusion model to predict diffusion parameters for elements with no or limited experimental data.

This report is a compilation of two peer-reviewed publications (Van Loon et al. 2023, Glaus et al. 2024b), extended with appendices containing detailed additional information that has not been published so far.