Subsurface Migration of Signature Noble Gases for Underground Nuclear Explosion Detection
Authors: K C B. , Meng M. , Li W. , Frash L. P. , Neil C. W. , Stauffer P. H. 
Published: August 2024

ABSTRACT:
Surface detection of noble gas radionuclides (RN) is critical to identify underground nuclear explosions (UNEs). However, predicting RN gas migration in the subsurface following a UNE involves complex processes of gas migration through the rock matrix and fracture network, rock damage, and the opening and closing of natural faults and fractures, which are not well understood. In this study, we conducted a triaxial direct shear experiments to investigate the influence of hydro-mechanical coupling and rock damage on the arrival time of different signature noble gases through a tight granite rock. The triaxial direct shear system features an in-line mass spectrometer connected to the downstream side of the sample, allowing the accurate detection of arrival time of different signature noble gases. We measured gas permeability and arrival time of different noble gases through both intact and fractured granite at a variety of stress conditions and fracture displacements. Our results indicate that Klinkenberg's effect on the gas permeability in an intact granite prevails up to the maximum differential pore pressure of 3.0 MPa tested in this study. We observed a shift in the pattern of arrival time of lighter helium (He) gas and heavier sulfur hexafluoride (SF6) in an intact granite at larger confining stress, where heavier SF6 arrived faster than the lighter He gas. However, when the rock is fractured and permeability is enhanced, lighter gases consistently arrived faster than heavier gases in the pressure dominated flow regime. The stress dependency of permeability was pronounced on fractured granite at all levels of shear displacements. This study sheds light on the dynamics of RN gas migration in the subsurface post-UNE, paving the way for more accurate predictions and monitoring techniques.

1. INTRODUCTION
The United States maintains interest in detecting subsurface nuclear explosions (NNSA, 2024). Seismic, infrasound, atmospheric, and satellite data cannot discriminate chemical explosions from nuclear explosions. Nobel gas radionuclides (RN) such as helium (He), argon (Ar), krypton (Kr), and xenon (Xe) are produced during UNEs, which are hard to contain and tend to migrate to the surface (Carrigan et al, 1996; CTBTO, 2024). Surface detection of these signature gases serve as a confirmative tool for UNE detection (Carrigan et al, 1996; Jordan et al., 2015; Carrigan et al 2016). Thus, it is critical to understand the transport and fractionation behavior of the RN gases as they migrate through intact and fractured rocks for effective detection of a UNE. Gas fractionation can occur through isotopic fractionation and physical fractionation (Bourret et al., 2021). In an isotopic fractionation process, differences in the rate of radioactive decay of various isotopes of a single radionuclide species affect the relative concentrations and their arrival time at the land surface. Whereas, in the case of stable species, such as the ones being considered in this study, the physical fractionation is caused by the variability in diffusivity related to the molecular size and weight of a gas species will lead to differences in the surface arrival time and concentration as the mixture migrates through subsurface.