Several barrier types are envisaged to minimize the release of radionuclides from waste matrices into groundwater. In a number of countries argillaceous rocks make up the natural barrier that will isolate radioactive substances from the aquifer. The present study addresses the influence of pore geometry as a limiting factor for anion diffusion in argillaceous rocks. Irrespective of the pore core size, anion diffusion can be limited by the pore-size opening, i.e. if the pore opening is so narrow that the electric double layers overlap and form a barrier to anions irrespective of the pore size. This so-called ‘bottleneck effect’ limits the anion diffusion. The present study extends previous investigations that focused on other factors which limit anion diffusion, e.g. mineralogy or interlayer equivalent pores. The existence of bottleneck pores was confirmed by effective tortuosity calculations and retention-potential measurements using mercury intrusion porosimetry. On the basis of two different core samples from argillaceous rocks from Switzerland, Opalinus Clay and Helvetic Marl, this work shows evidence of the existence of bottleneck pores. The larger permanent anion exclusion in the Helvetic Marl sample compared to the Opalinus Clay sample can be explained by the larger retention potential and larger effective tortuosity of the Helvetic Marl rock, which indicates more pores with bottleneck effects than is the case for the Opalinus Clay rock.

Most investigations into clay-mineral stability and new mineral formation within engineered barrier system (EBS) materials for geologic repositories of nuclear waste have focused on temperatures <100°C. In response to the United States Department of Energy’s interest in disposing of waste packages with higher thermal loads, higher temperature (200–300°C) and pressure (~150 bar), long-term (6-week to 6-month), hydrothermal experiments were conducted to evaluate the interaction of Opalinus Clay (wall rock) and Wyoming bentonite (clay buffer) with synthetic Opalinus Clay groundwater. Experiments were conducted in autoclaves using a flexible gold reaction cell with water:rock ratios between 6:1 and 9:1. Run products were characterized in terms of mineralogy and geochemistry. Montmorillonite remained stable at 200 and 300°C; traces of illite-smectite interstratified minerals were observed. Clay minerals in Opalinus Clay experienced significant changes at 300°C, including the formation of illite, illite-smectite, and chlorite-smectite. Montmorillonite illitization within the Wyoming bentonite EBS material was likely limited by the bulk chemistry of the system (i.e. low potassium) and newly formed illite was likely limited to the Opalinus Clay fragments, nucleating on pre-existing illite in the clay rock. Zeolite minerals with compositions between analcime and wairakite formed at 300°C along edges of Opalinus Clay fragments and within the bentonite matrix, but not at 200°C. Aqueous fluids remained undersaturated with respect to quartz in Opalinus Clay ± Wyoming bentonite 300°C experiments, and dissolution and re-precipitation of phases such as kaolinite, calcite, and smectite likely contributed to zeolite formation. These results can be applied to understanding zeolite formation, clay-mineral phase stability, and silica saturation within EBS materials of a high-temperature repository.

The radiogenic isotope systematics of clay minerals are complex because of the intimate mixture of minerals from different origins such as detrital and authigenic sources. An important aspect of dating clays is the primary sample preparation and disintegration method. In the present study, a sample of weakly deformed Opalinus claystone from the Mont Terri underground site (Switzerland) was investigated after disintegration by three different methods. The Opalinus Clay was sedimented in the late Toarcian and early Aalenian and reached maximum temperatures of ~85°C during burial in the Cretaceous. The present study reports data from a comprehensive investigation comparing the effects of disintegration by: (1) disc milling; (2) repeated freezing and thawing; and (3) high-voltage discharges. K-Ar age values of the finest clay (<0.1 µm) released by the different disintegration methods are indistinguishable, indicating that the high-voltage liberation method does not influence grains as small as 100 nm. The K-Ar age values of particle-size separates decreased with decreasing particle size. The age values of the 2–6 µm separates correspond to the Carboniferous Period, which reflects the dominance of Paleozoic detritus in that size range. The age values of the smallest separates (<0.1 µm), on average 213 ± 4 Ma, exceed the numerical age of the formation (~177−172 Ma), which show predominance of detrital grains over authigenic grains even in the finest illite. In summary, isotope geochronology data suggest that the high-voltage method can be applied reliably for disintegrating claystones.

The concept of effective stress is one of the basic tenets of rock mechanics where the stress acting on a rock can be viewed as the total stress minus the pore water pressure. In many materials, including clay-rich rocks, this relationship has been seen to be imperfect and a coefficient (χ) is added to account for the mechanical properties of the clay matrix. Recent experimental results during the flow testing (both gas and water) of several rocks (Callovo-Oxfordian claystone, Opalinus Clay, Boom Clay) and geomaterials (bentonite, kaolinite) has given evidence for stable high pressure differentials. The design of the experiments allows multiple measurements of pore pressure, which commonly shows a complex distribution for several different experimental geometries. The observed stable high pressure differentials and heterogeneous pore pressure distribution makes the describing of stress states in terms of effective stress complex. Highly localized pore pressures can be sustained by argillaceous materials and concepts of evenly distributed pore pressures throughout the sample (i.e. conventional effective stress) do not fit many clay-rich rocks if the complexities observed on the micro-scale are not incorporated, especially when considering the case of gas flow.

Anisotropy and compositional and structural heterogeneity in clays are causes of considerable deviations from homogeneous diffusion, in particular in terms of direction-dependent transport rates and preferred transport zones. Conventional diffusion experiments, in which the sample is treated as a homogeneous black box in a concentration gradient, are interminable and insensitive to spatial effects. In contrast, tomographic imaging methods are capable of both reducing the amount of observation time required and revealing space-dependent features of the diffusion process.

In the present study, positron-emission-tomography (PET) was applied as the most sensitive quantitative spatiotemporal tomographic modality for direct observation of positron-emitting radiotracers in opaque media at reasonable resolution (1 mm) on a laboratory scale (100 mm).

Geoscientific applications of PET, or GeoPET, have revealed anisotropic and heterogeneous effects in diffusion experiments that have been conducted on Opalinus clay samples of different sizes, as well as on other rock types. Applying the Comsol Optimization Module to 2D-image sections of the PET tomograms, effective parameter values were derived, thereby quantifying the anisotropic diffusion.

Opalinus clay has a high sealing capacity and is therefore considered as a viable candidate for hosting high-level radioactive waste. Assessment of the long-term containment function of clays requires understanding and modelling of mass transport through evolving pore systems. Development of pore network models for diffusion, which can be coupled to models for deformation and micro-cracking, is reported. Effects of clay texture and solid phase constitution are calculated and analysed. The results are in the range of, but slightly over-predicting, experimentally measured coefficients of diffusion in different clay directions. Further model improvements require better knowledge of micro-pore tortuosity, which awaits higher resolution experimental techniques.