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Anisotropic diffusion in layered argillaceous rocks: a case study with Opalinus Clay.
Luc R Van Loon1, Josep M Soler, Werner Müller
1Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland. luc.vanloon@psi.ch
Environmental Science & Technology
|December 4, 2004
Summary
Opalinus Clay exhibits anisotropic diffusion, with radionuclide movement parallel to bedding being 4-6 times faster than perpendicular. This property is crucial for nuclear waste disposal site selection in Switzerland.
Area of Science:
- Geosciences
- Nuclear Engineering
- Materials Science
Background:
- Opalinus Clay is a key Swiss host rock candidate for radioactive waste disposal.
- Understanding solute transport in Opalinus Clay is vital for repository safety assessments.
Purpose of the Study:
- To investigate anisotropic diffusion in Opalinus Clay.
- To quantify diffusion rates parallel and perpendicular to the clay bedding.
- To assess the influence of geological site conditions on diffusion anisotropy.
Main Methods:
- Utilized radial and planar through-diffusion techniques to measure diffusion coefficients.
- Studied diffusion of HTO, 36Cl-, and 22Na+ under varying confining pressures.
- Compared diffusion characteristics in samples from Mont Terri and Benken sites.
Main Results:
- Diffusion parallel to bedding is significantly faster (4-6x) than perpendicular diffusion for all tested radionuclides.
- Opalinus Clay demonstrates marked anisotropic diffusion behavior.
- Diffusion anisotropy is less pronounced in Mont Terri clay compared to Benken clay, linked to lower overburden pressure and platelet orientation.
Conclusions:
- Opalinus Clay's anisotropic diffusion is confirmed, impacting radionuclide migration pathways.
- The degree of anisotropy is influenced by geological factors like overburden pressure and clay fabric.
- Findings are critical for accurate modeling of contaminant transport in potential nuclear waste repositories.