Determination of the Porosity, Permeability and Diffusivity of Rock in the Excavation-Disturbed Zone Around Full-Scale D
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ABSTRACT In a nuclear waste repository, rock in the excavation-disturbed zone adjacent to the walls of deposition holes for waste canisters is a potential pathway for the transport of corrosive agents and radionuclides. Three experimental holes the size of deposition holes in a KBS-3 type repository (depth 7.5 m and diameter 1.5 m) were bored in hard granitic rock in the Research Tunnel at Olkiluoto and the porosities, effective diffusivities and permeabilities of rock in the excavation-disturbed zone were determined in a direction parallel to the disturbed surface using He-gas methods. Permeability and diffusivity in a direction parallel to the rock schistosity was found to be clearly larger than in a direction perpendicular to it. INTRODUCTION In a nuclear waste repository, rock in the excavation-disturbed zone adjacent to the walls of deposition holes for waste canisters is a potential pathway for the transport of corrosive agents and radionuclides. Rock in this zone may also act as a mixing tank, or, in the case of low axial conductivity and dead-end type radial fissures, efficiently sorb and retard the flow of radionuclides which diffuse through the bentonite buffer. Rock characteristics in the excavation-disturbed zone may also play an important role in saturation of the bentonite buffer and in gas release. To assess the characteristics of rock in the disturbed zone, three experimental holes of the size of deposition holes (depth 7.5 m and diameter 1.5 m) in a KBS3 type repository were bored in hard granitic rock in the Research Tunnel at Olkiluoto. Details of the boring technique [1], previous characterisation of the excavation disturbance caused by boring [2,3], and the He-gas method [4,5,6,7] have been reported earlier. He-gas methods were used [8] to establish the degree of rock disturbance in terms of porosity, effective diffusion coefficient and permeability. Measurements were based on either the diffusion or flow of helium through a rock sample saturated with nitrogen gas, and included porosity determinations using He-gas pycnometry. EXPERIMENTAL METHODS Two types of sample geometries were used for He-gas measurements. Disc samples with a central hole (i.e. rings) were used to establish the total permeability and diffusion coefficient. Cubic samples were used to measure the same properties in two different perpendicular directions. The porosities of both types of samples were measured, and after halving the cubic samples, porosity measurements of each half were made to provide porosity values for the disturbed zone (Samples A) and intact rock (Samples B). In the case of both types of sample, the surface of the disturbed rock was sealed with fluorescent epoxy. This 759 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
sealant penetrates the open, connected, large-aperture fractures close to the disturbed rock surface and prevents rapid gas flow along these fractures. It is assumed that a similar sealing effect will be achieved in the long term by compacted bentonite under swelling
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