Anisotropic Diffusion in Layered Argillaceous Rocks:  A Case Study with Opalinus Clay

Abstract

Anisotropic diffusion was studied in Opalinus Clay, a potential host rock for disposal of spent fuel, vitrified high-level waste, and long-lived intermediate-level waste in Switzerland. Diffusion parallel to the bedding was measured using a radial through-diffusion technique and diffusion perpendicular to the bedding bythe classical (planar) through-diffusion technique. The samples used were from Mont Terri (MT) and from Benken (BE). Diffusion of HTO, 36Cl-, and 22Na+ parallel and perpendicular to the bedding was studied under confining pressures of 7 MPa (MT) and 14 MPa (BE). The results indicate that diffusion parallel to the bedding is faster than diffusion perpendicular to the bedding by a factor of 4-6 for the three radionuclides, indicating that the Opalinus Clay is anisotropic. This might be explained by smaller path lengths (tortuosity) for species diffusing parallel to the fabric. The degree of anisotropy is slightly smaller for Opalinus Clay from Mont Terri than from Benken. This is due to the lower overburden pressure in Mont Terri resulting in a lower preferential orientation of the clay platelets.

An axisymmetric diffusion experiment for the determination of diffusion and sorption coefficients of rock samples

“…The diffusion anisotropy estimated from HTO and 36 Cl -were consistent with those evaluated by Van Loon et al (2004a) from through-diffusion experiments. García-Gutiérrez et al (2008) applied the same experimental configuration to the Callovo-Oxfordian clay, and Samper et al (2008a) examined the reliability of estimating parameters in these experiments.…”

“…Extensive studies on the Opalinus clay (Switzerland) and Callovo-Oxfordian clay (France) have shown that the diffusion in these media is prominent in the direction parallel to the bedding plane (e.g., Van Loon et al, 2004a, 2004bGarcía-Gutiérrez et al, 2006Samper et al, 2008a). Numerical models of in-situ experiments performed in these areas have treated directional diffusivities as a diffusion tensor of rank two (e.g., Wersin et al, 2004;Yllera et al, 2004).…”

“…Van Loon et al (2004a) evaluated the diffusion anisotropy of Opalinus clay under 1D mechanical stresses by 1D and radial through-diffusion experiments using HTO, 36 Cl -, and 22 Na + . The derived properties have been successively applied to the interpretation of in-situ experiments and the performance confirmation of newly developed experiments measuring diffusion anisotropy (Soler et al, 2008;García-Gutiérrez et al, 2006).…”

Biogeochemical processes in a clay formation in situ experiment: Part B – Results from overcoring and evidence of strong buffering by the rock formation

“…Measurements of diffusion of both water (measured using HTO) and chloride have also been made on several samples from Mont Terri. The ratio of the diffusion porosity for chloride to that of HTO is also between 0.5 and 0.7 (Pearson et al, 2003, Table A10.8;Van Loon et al, 2003, 2004. As a consequence, the value obtained from the overcore samples is the highest reliable value measured on…”

Generic Argillite/Shale Disposal Reference Case

“…Previous studies collected and reported datasets of diffusive parameters from argillaceous formations (e.g., Aertsens et al 2004;van Loon et al 2004a and2004b;Descostes et al 2008;Appelo et al 2007). A summary of the diffusion parameters collected in clay-rock formations available in the literature was presented by Zheng et al (2011b For the reference case described in this report, the water diffusion coefficient of generic radionuclide is assumed to be equal to 1.08×10 -9 m 2 /s (Bianchi et al 2013;, and with radionuclide-specific effects (e.g., due to size and charge) not considered at this stage.…”