Determination of the Flow-Wetted Surface in Fractured Media
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Determination of the Flow-Wetted Surface in Fractured Media Luis Moreno, Erik Svensson and Ivars Neretnieks Chemical Engineering and Technology, Royal Institute of Technology, S-100 44 Stockholm, Sweden Email: [email protected] ABSTRACT Diffusion and sorption in the rock matrix are important retardation mechanisms in radionuclide transport in fractured media. The surface area in contact with the flowing water so called flow-wetted surface controls the interaction with the rock matrix. The flow-wetted surface, FWS, may be determined from the frequency of open fractures intersecting a borehole. The choice of the packer distance used in these hydraulic measurements is however crucial. If the packer distance is large, several open fractures may be found in the same packer interval. Analytical solutions and numerical calculations are used in order to address the determination of the flow-wetted surface from borehole data. The study is focused in two important aspects of the fieldwork, namely the distance used between the packers and b) the number of measurements that are required in order to obtain a accurate determination of the flow-wetted surface. A large volume of hydraulic data in Sweden obtained by using quite small packer distance is revised. The most usual packer distance was 3 meters. However, for some boreholes or sections of them even smaller packer distances were used. From these data, the flow-wetted surface was determined at several sites in Sweden. In some sites, the used packer distance was too large that no accurate determination of the flow-wetted surface could be done. INTRODUCTION In Sweden and other countries, the canister containing the spent fuel will be deposited in repositories excavated in crystalline rock. The radionuclides escaping from a damaged canister will be transported from the repository into the biosphere by the water flowing in the fractures in the rock. From the fracture, they may diffuse into the rock matrix and be sorbed onto the microfractures within the rock matrix. Diffusion into the rock matrix and sorption onto the microsurfaces of the rock are important retardation mechanisms for radionuclide transport. The larger the surface the rock mass in contact with the flowing water (the flow-wetted surface), the greater is the interaction between the radionuclides and the rock matrix [1]. The determination of the flow-wetted surface, FWS, is therefore of vital importance in the migration of radionuclides from the repository to the biosphere. The flow-wetted surface may be determined from borehole measurements [2]. The number of open fractures intersected by a borehole drilled in the rock mass gives information about the specific flow-wetted surface. If the length and width of the fractures are large compared with the borehole diameter the flow-wetted surface may be determined as 4/H where H is the average distance between open fractures intersected by the borehole.
The aims of this paper is to determine the packer distance to be used in the borehole measurements and to estimate the numb
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