Physical Rock Matrix Characterization: Structural and Mineralogical Heterogeneities in Granite
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1124-Q07-03
Physical Rock Matrix Characterization: Structural and Mineralogical Heterogeneities in Granite
M. Voutilainen*, S. Lamminmäki**, J. Timonen*, M. Siitari-Kauppi** and D Breitner*** *Department of Physics, University of Jyväskylä, P.O.Box 35, FIN-40351 Jyväskylä, Finland **Laboratory of Radiochemistry, University of Helsinki, P.O. Box 55, FIN-00014 University of Helsinki, Finland ***Lithosphere Fluid Research Lab, Institute of Geography and Earth Sciences, Eötvös University, Budapest, 1117, Hungary ABSTRACT Evaluation of the transport and retardation properties of rock matrices that serve as host rock for nuclear waste repositories necessitates their thorough pore-space characterization. Relevant properties to be quantified include the diffusion depth and volume adjacent to water conducting features. The bulk values of these quantities are not sufficient due to the heterogeneity of mineral structure on the scale of the expected transport/interaction distances. In this work the 3D pore structure of altered granite samples with porosities of 5 to 15%, taken next to water conducting fractures at 180–200 m depth in Sievi, Finland, was studied. Characterization of diffusion pathways and porosity were based on quantitative autoradiography of rock sections impregnated with 14C-labelled polymethylmethacrylate (PMMA). Construction of 3D structure from PMMA autoradiographs was tested. The PMMA method was augmented by field emission scanning electron microscopy and energy-dispersive X-ray analyses (FESEM/EDAX) in order to study small pore-aperture regions in more detail and to identify the corresponding minerals. The 3D distribution of minerals and their abundances were determined by X-ray microtomography. Combining the mineral specific porosity found by the PMMA method with these distributions provided us with a 3D porosity distribution in the rock matrix. INTRODUCTION Over extended periods, long-lived radionuclides within geologic disposal sites may be released from the spent fuel and migrate to the geo/biosphere. In the bedrock, contaminants will be transported along fractures by advection and retarded by sorption on mineral surfaces and by molecular diffusion into stagnant pore water in the matrix along a connected system of pores and micro-fissures. Because chemical interactions of groundwater and transported components with inner mineral surfaces play a major role in the retardation process, the effectiveness of the rock matrix as a natural barrier is influenced by the size, shape and spatial arrangement of the effective rock porosity network. A reliable picture of the pore-space structure in different mineral phases of rock can be determined using different complementary methods of characterization. For many years the 14C-polymethylmethacrylate method (PMMA) [1, 2] has been widely used as a pore-structure analysis tool for low-porosity granitic rock. PMMA porosity results have been compared with those of confocal laser scanning microscopy [3,4] and X-ray micro computed tomography (µCT) [5]. These non-destruc
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