The Effect of the Excavation Damaged Zone Adjacent to the Walls of Deposition Holes on the Migration of Radionuclides
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wnloaded from https://www.cambridge.org/core. Squire Law Library, on 20 Aug 2019 at 04:12:14, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-663-645
is low, the EDZ at that site is primarily the result of the boring of the deposition holes. The technique used in the boring process, including a description of the cutting tools, has been presented in [1]. The characterization of the experimental full-scale deposition holes and the rock in the EDZ has been presented in [2]. The permeability and diffusivity in the rock in the EDZ has been studied further by using the He-gas method in [3,4,5] and by studying microfracturing and the spatial distribution of porosity in [6,7]. The hydraulic conductivity of compacted bentonite is low and the predominant method by which radionuclides will migrate through this material will therefore be diffusion. Transport along the EDZ depends on the volume of the water flow in the EDZ and on diffusion between the EDZ and the bentonite. Transport between the EDZ and the host rock via fractures and via the rock matrix will also take place. The effect of the EDZ on transport is analyzed in this paper by comparing two different cases: 1) diffusion through the compacted bentonite and 2) transport through the EDZ by advection and diffusion. In the first case, the radionuclides which escape from a canister are considered to primarily diffuse through the compacted bentonite to the tunnel floor. In the second case, an alternative situation is assumed in which a direct connection exists between the waste canister and the EDZ. The EDZ actually forms a heterogeneous conductive cylindrical fractured layer on the interface between the compacted bentonite barrier and the intact host rock, the most highly-fractured part of the EDZ being similar in its structure to a dense but permeable layer of gravel (Figure 1). The cause of flow in the EDZ is considered to be the hydraulic gradient between a fracture plane which intersects the deposition hole and the tunnel floor. All the radionuclides escaping from the waste canister are considered to move directly to the EDZ as a layer of constant concentration, they are then captured by the flow in the EDZ and taken directly to the tunnel floor.
Figure 1. A scanning electron microscope image of the disturbed surface (left) and a thin section fluoroscene microscopic image (right). Both sections are taken perpendicular to the surface of the full-scale deposition hole. This study concerns the processes of advection and diffusion, other processes such as sorption and retardation which will affect the transport of radionuclides are not considered. The compacted bentonite barrier which surrounds the waste canister in the deposition hole will start to swell once it is in contact with water, and will reach its maximum swelling pressure of some
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