GEOLOGIC CONTROLS OF RADIONUCLIDE GAS SIGNATURES FROM UNDERGROUND NUCLEAR EXPLOSIONS

Radioactive gases from an underground nuclear explosion (UNE) can be transported through fractures to the ground surface over the seconds to months following the explosion. The presence of particular short-lived radioactive noble gases can provide strong evidence of underground testing. Using historical UNE data from late 1980s, paired with tracer injection experiments performed in the 2010s, we have identified geologic controls on gas migration. Controls include the likelihood and timing of UNE-gas seepage and the fractionation of different noble gases based on differences in transport properties in the subsurface. We draw comparisons between sixteen historical U.S. UNEs where radioactive gas was or was not detected; the UNEs all had similar configurations including testing location at Pahute Mesa on the Nevada Test Site (now the Nevada National Security Site), announced yield range (20-150 kt), and relatively deep burial depths in the vadose zone. We show that air-filled porosity, surface layer fracturing, and post-test atmospheric conditions control the occurrence and timing of breakthrough at the ground surface at these UNE sites. Numerical models with simple geology of each of the 16 historical UNEs that include these factors successfully predict the occurrence (5 of the UNEs) or lack of occurrence (remaining 11 UNEs) of post-UNE gas seepage to the ground surface. Additionally, we have identified that transport processes of noble gases in the subsurface, including Henry’s Law partitioning and dispersion, cause fractionation of the noble gas species that may be important for isotope ratios measured at the ground surface. The comparison of UNE cavity evolution models and historical post-UNE noble gas observations has shown that saturation and pore space in the host rock for UNEs may provide storage for detectable radioactive noble gases and thus change the radionuclide signatures of underground testing.