Rethinking Porosity-Based\nDiffusivity Estimates for\nSorptive Gas Transport at Variable Temperatures

The detection of noble gas radioisotopes following a\nsuspected\nunderground nuclear explosion is the surest indicator that nuclear\ndetonation has occurred. However, the accurate interpretation and\nattribution of radioisotopic signatures is only possible with a complete\nunderstanding of transport processes occurring between the nuclear\ncavity and surface. In the far-field, diffusive forces contributing\nto gas transport are impacted by temperature gradients and subsurface\nlithology. In the current study, we investigate diffusive transport\nof xenon (Xe), krypton (Kr), and sulfur hexafluoride (SF<sub>6</sub>) through intact Bandelier tuff at elevated temperatures using a\nnewly developed high temperature diffusion cell. Diffusion coefficients\ndetermined using Finite Element Heat and Mass transfer code simulations\nand the Parameter ESTimation tool range from 2.6–3.1 ×\n10<sup>–6</sup> m<sup>2</sup>/s at 20 °C, 3.4–5.1\n× 10<sup>–6</sup> m<sup>2</sup>/s at 40 °C, and 4.3–7.0\n× 10<sup>–6</sup> m<sup>2</sup>/s at 70 °C. Sorption\nwas found to be an important transport mechanism at ambient temperatures\n(20 °C). Most critically, our study shows that empirical porosity-based\ndiffusion estimates for these gases through tuff captured neither\nthe magnitude nor trends relative to a nonsorbing sandstone. These\nnew insights highlight the importance of experimental transport investigations\nand will be used to improve models for subsurface gas propagation\nrelevant to proliferation detection and environmental contamination.