For well-contained subsurface nuclear tests, isotopic ratios of volatile fission products detected at the surface are greatly affected by their subsurface transport. The impact of interactions between gaseous fission products, such as Xe, and porous rock, which can vary in saturation state and mineralogical composition, on gas migration is not well understood. To improve prediction of breakthrough times and elemental fractionation, we conducted a series of diffusive transport experiments to monitor Xe, Kr, and SF6 transport through variably saturated, zeolitic tuff. Transport was measured for dry, 17%, 45%, 85%, and 100% saturated rock cores. We observed significant gas uptake for the dry system, particularly for Xe. As a result of its enhanced sorption, Xe breakthrough was significantly slower than the other gases. Notably, the differences in SF6 and Xe behavior at low saturations suggest that, contrary to its common application as such, SF6 may not be good surrogate for Xe in sub-surface transport studies. When core saturation increased to 17-45%, less Xe uptake was measured, resulting in faster gas breakthrough. Once saturation reached 85%, breakthrough slowed due to limited diffusion pathways. Neutron scattering characterization of the core revealed that water is preferentially taken up by smaller zeolite pores upon saturation, suggesting that these zeolites play an important role in Xe sorption and cause the observed gas transport differences at low saturations (i.e., 17% saturated). Results demonstrate the important role of saturation in predicting breakthrough times and fractionation of gas signatures of underground nuclear detonations. These new insights will be used to improve transport models for subsurface fission product releases, which have implications for treaty monitoring, repository science, and radioactive contaminant transport.

Effects of water saturation on noble gas diffusion through zeolitic tuff

ACS Spring Meeting 2021

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