Rethinking Porosity-Based Diffusivity Estimates for Sorptive Gas Transport at Variable Temperatures.

Area of Science:

• Geochemistry
• Environmental Science
• Nuclear Science

Background:

• Detection of noble gas radioisotopes is key for identifying underground nuclear explosions.
• Accurate interpretation of radioisotopic signatures requires understanding subsurface transport processes.
• Diffusive gas transport in the far-field is influenced by temperature and lithology.

Purpose of the Study:

• Investigate the diffusive transport of xenon (Xe), krypton (Kr), and sulfur hexafluoride (SF6) through Bandelier tuff at elevated temperatures.
• Determine diffusion coefficients and assess the impact of temperature and sorption on gas propagation.
• Compare experimental results with empirical porosity-based diffusion estimates.

Main Methods:

• Utilized a newly developed high-temperature diffusion cell.
• Employed Finite Element Heat and Mass transfer code simulations and the Parameter ESTimation tool.
• Conducted experiments on intact Bandelier tuff at temperatures of 20 °C, 40 °C, and 70 °C.

Main Results:

• Diffusion coefficients for Xe, Kr, and SF6 ranged from 2.6-3.1 × 10⁻⁶ m²/s at 20 °C to 4.3-7.0 × 10⁻⁶ m²/s at 70 °C.
• Sorption significantly impacted transport at ambient temperatures (20 °C).
• Empirical porosity-based diffusion estimates did not accurately capture the observed magnitude or trends for these gases in tuff.

Conclusions:

• Experimental transport investigations are critical for understanding subsurface gas propagation.
• Elevated temperatures increase diffusion rates of noble gases and SF6 through tuff.
• Findings will enhance models for proliferation detection and environmental contamination assessment.