Radionuclide Retardation in Granitic Rocks by Matrix Diffusion and Sorption
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Radionuclide Retardation in Granitic Rocks by Matrix Diffusion and Sorption P. Hölttä1, M. Siitari-Kauppi1, M. Kelokaski1 and V. Tukiainen2 1 Laboratory of Radiochemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 University of Helsinki, Finland 2 VTT Processes, Nuclear Energy, P.O.Box 1608, FIN-02044 VTT, Finland ABSTRACT Radionuclide retardation in mica gneiss, unaltered, moderately and strongly altered tonalite was studied by a thin section, batch, in-diffusion and column methods. Objectives were to examine retention processes in different scales and understand the influence of the rock matrix heterogeneity. Attempts were made for a more detailed interpretation of experiments using migration models used in performance assessments adapted for interpreting the laboratory scale experiments. Batch experiments were explained adequately using matrix diffusion-sorption model, instantaneous kinetic sorption model or model in which both mechanisms were taken into account. A numerical code FTRANS was able to interpret in-diffusion of calcium into the saturated porous matrix. Elution curves of calcium for the moderately and strongly altered tonalite fracture columns were explained adequately using FTRANS code and parameters obtained from in-diffusion calculations. Kd-values for intact rock obtained from fracture column experiments were lower than Kd-values for crushed rock indicating that batch experiments overestimate the retardation of radionuclides. Higher sorption and fair dependence on fraction size was obtained for altered tonalites due to the composition of alteration minerals and large specific surface areas. INTRODUCTION Granitic rock would act as the ultimate barrier, retarding the migration of radionuclides to the biosphere if radionuclides are released through engineered barriers in a repository of highly radioactive spent nuclear fuel. In transport models, radionuclide retardation has been usually taken into account by the Kd-concept, in which a retardation factor is used to apply the distribution ratio to radionuclide transport [1, 2]. Determination of the retardation factor by fracture flow experiments is a direct approach to application of the effects of sorption in radionuclide transport models. Kd values conventionally used in transport models have been determined for crushed rock with different specific surface areas and values may not be valid for modelling radionuclide transport in fractures. The effect of mineral composition and specific surface area on radionuclide sorption has been discussed earlier [3-5]. In this work retardation of radionuclides in granitic rock was studied by means of the different methods. Objective was to examine the processes causing retention in solute transport through rock fractures, especially focused on the matrix diffusion and sorption. ROCK MATRIX CHARACTERIZATION The rock samples representing different rock features and porosities were obtained from the hole SY-KR7 drilled in the Syyry area in Sievi, Western Finland. Drill hole SY-KR7 [6] i
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