Characterizing Low-Permeable Granitic Rock from Micrometer to Centimeter Scale: X-ray Microcomputed Tomography, Confocal
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Characterizing Low-Permeable Granitic Rock from Micrometer to Centimeter Scale: Xray Microcomputed Tomography, Confocal Laser Scanning Microscopy and 14C-PMMA Method T. Lähdemäki1, M. Kelokaski1, M. Siitari-Kauppi1, M. Voutilainen2, M. Myllys2, T. Turpeinen2, J. Timonen2, F. Mateos3, and M. Montoto3 1 Laboratory of Radiochemistry, University of Helsinki, P.O.Box 55, University of Helsinki, FIN00014, Finland 2 Department of Physics, University of Jyväskylä, P.O.Box 35, Jyväskylä, FIN-40351, Finland 3 Department of Geology, University of Oviedo, Oviedo, 33005, Spain
ABSTRACT First results of combining X-ray microcomputed tomography (µCT), confocal laserscanning microscopy (CLSM) and 14C-polymethylmethacrylate (14C-PMMA) impregnation techniques in the study of granitic rock samples are reported. Combining results of µCT and CLSM with those of the 14C-PMMA technique, the mineral-specific porosity and morphology of the open pore space, as well as its connectivity, could be analyzed from a micrometer up to a decimeter scale. Three different types of granite were studied. In two cases part of the micro-fissure and pore apertures were found to be in a micrometer scale, but in one case all grain-boundary openings were below the detection limit. Micrometer-scale apertures could be analyzed by CLSM and µCT. The benefit of µCT is that it can also provide the heterogeneous distribution of minerals in 3D. The 2D porosity distributions in the mineral phases, consisting of nanometer-scale pores, could be measured by the 14C-PMMA method together with the micro-fissures. This method does not, however, give the exact pore apertures. The limitations and applicability of the methods are discussed. INTRODUCTION In the framework of geological disposal of spent nuclear fuel, it is important to consider scenarios in which long-lived radionuclides or activation products are released from the fuel to the geosphere, and consequently to the biosphere. Lack of knowledge of the influence the microscopic structure of rock has on the diffusion of radioactive tracers makes it difficult to extract relevant parameters for the matrix-diffusion concepts used in performance-assessment calculations. To this end we need to understand better the heterogeneous structure of the rock matrix at different length scales. Since chemical interactions of groundwater and transported radionuclides with the inner mineral surfaces of the rock matrix play a major role in the retardation processes, the effectiveness of the rock matrix as a natural barrier is significantly influenced by the size, shape, and spatial arrangement of its connected porosity network. However, such a detailed knowledge about porosity in the rock matrix is difficult to find. In addition, when upscaling matrix-diffusion and sorption processes from laboratory-scale investigations into real in situ conditions, we need realistic data on the physical rock-matrix
properties from a nanometer to a meter scale. In order to get such data, including the association of pore-space structure wi
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