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(2020) Diffusion and Ion-Exchange Properties for Conservative and Non-Conservative Ions in Granite: An X-Ray Radiography Method
Cadieux C, Morfin S, Maldonado Sanchez G & Al T
The author has not provided any additional details.
08b: Room 3, View in program
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Impressive improvement in the profile resolution with the addition of sample rotation. In the context of sensitivity of the method, how much tracer needs to diffusive into the sample in order to achieve a reliable signal via radiography?
Given that it's our first attempt with the modified procedure of rotating the sample, we haven't done extensive testing with different tracer concentrations yet. The sensitivity depends on the effective atomic number and transport behaviour of the tracer, hence why we could detect a stronger signal from cesium, which is non-conservative, when using a concentration that was 50% that of our iodide tracer. For now, we know that for the high-atomic number tracers, iodide and cesium, 2 and 1 mole / kg H20, respectively, are sufficient concentrations for signal detection but more testing is required before we can put a number on the lower limit of detection.
Could your experimental setup be combined with a "traditional" through-diffusion experiment, e.g. when involving surface reactive radionuclides?
It might be possible to use the same sample to study diffusion with both through-diffusion and x-ray radiography. This would require several changes in our diffusion cell's current design. However, because the time-scale for these two methods is very different, we are instead running through-diffusion experiments on different samples of the same granite in parallel with our x-ray radiography experiments. Furthermore, we are not currently using reactive tracers in through-diffusion experiments because the time required to reach steady-state would be very long with sorptive solutes such as cesium.
Interesting results using this approach with low porosity rock. It seems the granite sample was not fractured or had microcracks. Is this the case? Can this approach be extended to other low porosity rock types like (argillaceous) clay rock?
The granite is not fractured, these samples were carefully cored from the centre of massive blocks in order to avoid fractures. We cannot comment on micro-fractures because we have not yet completed the petrographic work on the samples. The method that we used here is actually derived from previous successful experiments with other low porosity sedimentary rocks from Canada and Switzerland (shales and argillaceous limestones). That work is published in various forms: Cavé et al., 2009: https://doi.org/10.1016/j.jconhyd.2008.08.001 ; Xiang et al., 2013: https://doi.org/10.1016/j.jconhyd.2013.09.002 and Loomer et al 2013: http://dx.doi.org/10.1016/j.apgeochem.2013.09.019. The argillaceous limestones have porosities between 1 and 2% and the porosity of the shales ranges from 8 to 17% - quite a bit higher than the ~ 0.4% measured in the granite that we used here. Another difference is that, at the mm scale, the shales and limestones are more homogeneous than granite which means there is less "noise" contributed to the data by the image registration process that is required when using a difference-imaging technique such as x-ray radiography. This is the reason we had to try sample rotation during acquisition when using this method with granite.
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