Abstract
Three cationic tracers, Sr2+, Co2+ and Cs+ were tested with a modified electromigration device by applying 2V, 3V and 4V voltage gradients over an intact Grimsel granodiorite rock sample. An ideal plug-flow model and an advection-dispersion model were applied to analyze the breakthrough curves. Matrix characterization by C-14-PMMA autoradiography and scanning electron microscopy showed that in the centimeter scale of Grimsel granodiorite rock, the interconnected matrix porosity forms a well-connected network for diffusion. Micrometer-scale fissures are transecting biotite and chlorite minerals, indicating sorption of the studied cations. The ideal plug-flow model indicated that the effective diffusion coefficients (De values) for Sr2+, Co2+ and Cs+ tracer ions within the Grimsel granodiorite rock were 3.20 × 10−13 m2/s, 1.23 × 10−13 m2/s and 2.25 × 10−12 m2/s, respectively. De values were also derived from the advection-dispersion model, from which 2.86 × 10−13 m2/s, 1.35 × 10−13 m2/s and 2.26 × 10−12 m2/s were calculated. The diffusion speed for the tracers was in the sequence of Cs+ > Sr2+ > Co2+ that is in the same sequence as their diffusion in diluted water. The distribution coefficients (Kd values) calculated from the models covered the range of two magnitudes (from 10−7 m3/kg to 10−5 m3/kg). The result indicated that the sorption process of the studied elements did not reach equilibrium during the electromigration process, mainly due to the too much acceleration of the migration speed by the voltage gradients applied over the rock sample.
Highlights
Geological disposal has been widely accepted as a realistic option for nuclear waste management in many countries, such as Finland, Sweden, France and Switzerland
Three voltages (2V, 3V and 4V) over the rock sample were applied in the electromigration experiments with SrCl2 using the modified electromigration device
The results show that the effective diffusion coefficients evaluated from the ideal plug-flow model (Equation (4)) for Sr2+ migrating through the rock sample is around 3.2 × 10−13 m2/s
Summary
Geological disposal has been widely accepted as a realistic option for nuclear waste management in many countries, such as Finland, Sweden, France and Switzerland Experiments, both in-situ and laboratory scale, have been performed to provide solid parameters for concept development and safety assessment of a nuclear waste repository [1,2]. The Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) has been conducting in-situ field experiments in its underground rock laboratory (URL), named the Grimsel Test Site (GTS), to assess the relevance of laboratory data to repository scales and to evaluate the diffusion and sorption properties of radionuclides in in-situ conditions [3,4,5]. The electrolytes in the source and recipient chambers were stirred all the time by a magnetic stirrer. The tracer concentrations in the recipient chamber were followed by taking out 2 mL of electrolytes from time to time and adding the same amount of background electrolytes
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