Safety-relevant hydrogeological properties of the claystone barrier of a Swiss radioactive waste repository: An evaluation using multiple lines of evidence

Abstract

In Switzerland, the Opalinus Clay -a Jurassic (Aalenian) claystone formation -has been proposed as the first-priority host rock for a deep geological repository for both low-and intermediate-level and high-level radioactive wastes. An extensive site and host rock investigation programme has been carried out during the past 30 years in Northern Switzerland, comprising extensive 2D and 3D seismic surveys, a series of deep boreholes within and around potential geological siting regions, experiments in the international Mont Terri Rock Laboratory, compilations of data from Opalinus Clay in railway and motorway tunnels and comparisons with similar rocks.The hydrogeological properties of the Opalinus Clay that are relevant from the viewpoint of long-term safety are described and illustrated. The main conclusions are supported by multiple lines of evidence, demonstrating consistency of conclusions based on hydraulic properties, porewater chemistry, distribution of natural tracers across the Opalinus Clay as well as small-and large-scale diffusion models and the derived conceptual understanding of solute transport.

Capturing the structural and compositional variability of Opalinus Clay: constraints from multidisciplinary investigations of Mont Terri drill cores (Switzerland)

“…It was found that most faults occur in the clay-rich facies types and intervals. This probably reflects the low shear strength of the investigated clay-rich sections which can be attributed to the anisotropy of the mechanical properties (Gautschi 2017), favoring the development of fault planes aligned subparallel to bedding (Orellana et al 2018a;. Brittle fault planes and lithological heterogeneities represent planes of mechanical weakness (Marschall et al 2017), which are prone for reactivation (e.g.…”

“…The preferred alignment of phyllosilicates is related to bedding, which causes anisotropic hydraulic and mechanical properties (e.g. Gautschi 2017).…”

Permeability Properties of Jointed Rock with Periodic Partially Filled Fractures

“…Rock fractures can greatly influence the hydrological properties and control the mechanical behaviour of a rock mass. The hydraulic and mechanical properties of rock fractures are of considerable interest in several areas of rock engineering, such as near-field modelling of hydraulic processes around radioactive waste repositories [1,2], underground tunnels [3,4] and storage caverns [5,6], rock slope stability [7,8], petroleum and geothermal reservoirs [9], and solute transport [10]. Single fractures, the elementary unit of a fractured rock mass, have become a research focus.…”

Radioactive selenium: origin and environmental dispersion scenarios

“…For instance, existing models assume releases of Se as Se(IV) and Se(VI) oxyanions. However, thermodynamic calculations suggest that Se should be in lower oxidation states (-II and 0) due to interactions with the iron from the canister materials or due to developed reducing conditions within the host rock (e.g., diffusion through clays; Basu et al, 2007;Beauwens et al, 2005;Descostes et al, 2008;Gautschi, 2017). In fact, solid samples containing Se(0) and Se(IV) next to each other originating from reducing environments in Boom Clay have been reported (e.g., Breynaert et al, 2010;Hoving, 2018).…”