Transport and sorption of noble gases in porous geological media




Paul, Matthew John

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The transport of noble gas radionuclides in porous media is critical to the detection of underground nuclear explosions as well as the sequestration of reprocessing off-gases. However, in field tests releasing radioxenon underground, the quantity of radioxenon observed at the surface has fallen well below expectations. This research has examined the diffusivity and sorption of noble gases (Ar, Kr, and Xe) and the inert molecular gas sulfur hexafluoride (SF₆) in both dry and wet porous media seeking a plausible mechanism for this discrepancy. In support of this, the two-bulb method for estimating diffusivity has been modified for experimentation in porous media. To replicate underground transport conditions, low-level concentrations of the tracer gas species were required. Detecting trace quantities of these inert species necessitated the development of precision gas chromatography-mass spectrometry (GC-MS) capabilities for these permanent gases. This was accomplished through the use of internal standards; atmospheric nitrogen, which contains the scarce isotope ¹⁵N, and synthetic carbon tetrafluoride, CF₄, were both utilized. Both internal standards were shown to be capable of producing acceptable results under ideal conditions, but the CF₄ method showed more resiliency. The results of the diffusivity trials demonstrated the adequacy of the porosity-tortuosity factor model for both dry and wet macroporous media. However, in both the dry and wet trials, the equilibrium concentration of Xe converged at a lower fraction of initial concentration than the other tracer gases considered. The deviation in equilibrium fraction is expected in the wet trial due to the increased solubility of Xe versus the other tracer species. However, deviation in the dry trial necessitates consideration of adsorption effects. While the physical adsorption of Xe on shale formations has been considered as a potential mechanism for the scarcity of primordial Xe isotopes in the atmosphere, adsorption measurements were conducted on a range of materials demonstrating not only the relative strength of xenon adsorption over other noble gases, but also that the magnitude of this effect readily exceeds that of solubility. Consequently, with the observation of significant gas adsorption, consideration of adsorbed phase accumulation is necessary when scaling to larger geological systems.


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