Verfication of matrix diffusion by means of natural decay series disequilibria in a profile across a water-conducting fr
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Verfication of matrix diffusion by means of natural decay series disequilibria in a profile across a water-conducting fracture in granitic rock 3 3 W.R. Alexander', I.G. McKinleyf, A.B. MacKenzie and R.D. Scott 1 2
Paul Scherrer Institute (formerly EIR), 5232 Villigen, Switzerland Nagra, 5401 Baden, Switzerland 3 Scottish Universities Research and Reactor Centre, East Kilbride G75 OQU, Scotland
ABSTRACT
Data from a rock drillcore, taken perpendicular to a water-bearing fracture in crystalline rock, clearly indicate water-rock interactions in and around the fracture. Although there is evidence of microfracturing at some distance from the main fracture, and therefore potential advective flow across the entire shear zone, simple calculations indicate that transport of Ra-226 in the vicinity of the main fracture can be described simply by matrix diffusion. This is a useful simplification for safety assessment studies and the consequences to the estimated radionuclide retardation in the vicinity of the fracture are discussed with respect to the probably over-conservative assumptions made in Nagra's safety assessment model base case.
INTRODUCTION According to current Swiss concepts for the disposal of radioactive waste, emplacement in deep rock formations will ensure that only small (or negligible) concentrations of radionuclides will ever reach the surface and so enter the biosphere [1]. It is considered likely that, in the formations of interest to the Swiss programme (e.g. granitic rocks, argillaceous sediments), the rock will be fractured (even at repository depth) and that advection along the fractures will be the dominant mode of groundwater flow, and hence transport of any radionuclides released from the repository. However, it has been suggested [2,3] that, even in systems where the advective flow is dominantly in distinct fractures, there may be penetration of the surrounding rock by diffusion in a connected system of pores or microfractures. This "matrix diffusion" is of importance in a repository safety assessment in that it provides a mechanism for enlarging the rock surface in contact with the advecting solute from solely fracture surfaces (including infill material) to a portion (or even the majority
Mat. Res. Soc. Symp. Proc. Vol. 176. ©1990 Materials Research Society
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[4]) of the bulk rock. Such a process also acts as a retarding mechanism for non-sorbing radionuclides. Most models of matrix diffusion to date [e.g. 3,5] visualise the water conducting fractures as simple, parallel sided channels, with diffusion from the water into adjacent porous rock. In this study, extensive characterisation on a microscopic scale of a profile perpendicular to a distinct water carrying fracture indicates that the transport of radionuclides into a comparatively deep zone of sheared and relatively accessible rock matrix may still be described by matrix diffusion, a useful simplification for safety assessment studies. METHODS
50 g sub-samples from the rock drillcore were crushed and powdered prior to analyses of
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