Chloride diffusion in pore water in Olkiluoto veined gneiss and pegmatitic granite from a structural perspective
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Chloride diffusion in pore water in Olkiluoto veined gneiss and pegmatitic granite from a structural perspective J. Sammaljärvi1, J. Ikonen1, M. Voutilainen1, P. Kekäläinen2, A. Lindberg3, M. Siitari-Kauppi1, P. Pitkänen4 and L. Koskinen4 1 Laboratory of Radiochemistry, Department of Chemistry, University of Helsinki, Finland 2 Department of Physics, University of Jyväskylä, Finland 3 Geological Survey of Finland, Finland 4 Posiva oy, Olkiluoto, 27160 Eurajoki, Finland ABSTRACT Spent nuclear fuel from Finnish power plants is planned to be deposited deep in the crystalline bedrock in Olkiluoto, Finland. The bedrock and more specifically the elemental composition of ground water, which is composed of the fracture water and the matrix pore water, needs to be well characterized to assess the risks inherent to the long term safety of the site. To this end, it is valuable to investigate elemental composition of the matrix pore water since it tends to conserve hydrogeological signals for longer time spans compared to open fracture waters. In this study, the chloride concentration of matrix pore water in veined gneiss (VGN) and pegmatitic granite (PGR) samples were investigated. Chloride was out-diffused from the naturally saturated rock cores into deionized water. Chloride pore diffusion coefficients were derived by modelling the chloride breakthrough curves obtained from the out-diffusion experiments. Two component modelling gave best fit to the experimental results. There two diffusion coefficients were (9±2)×10-11 m2/s and (0.5±0.1)×10-11 m2/s for PGR and (2.5±0.5)×1011 m2/s and (0.4±0.1)×10-11 m2/s for VGN. Porosity distribution and total porosities of the rock samples were studied with the C-14-PMMA autoradiography. Porosity for PGR was found to be 0.6 % with large mineral transecting fissures, and porosity for VGN was found to be 0.7 % with highly porous mineral clusters connected to each other via grain boundaries and intragranular pores. The findings here show that heterogeneity has to be taken into account in modelling to find better agreement with the experimental results. C-14-PMMA autoradiography results indicate dual-component behavior for diffusion in PGR and VGN which were used in the modelling. INTRODUCTION In Finland the safety of built repository for spent nuclear fuel is based on using KBS-3 multi-barrier system [1]. This system consists of multiple barriers preventing the migration of radionuclides. The canisters containing the spent nuclear fuel are placed in the tunnels 400-500 meters deep in the bedrock and the tunnels are filled with bentonite buffer. The bedrock is the final barrier preventing the entry of radionuclides into biosphere and therefore, the understanding of the migration of elements in bedrock is essential in the safety assessment of the repository. The elemental composition of the water that resides in the fractures and in the rock matrix pores holds information about the hydrogeological past of the area and the changes in the elemental composition in fracture water versus matrix pore
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