ADVANCING MODELING OF SUBSURFACE GAS TRANSPORT AT THE SITE OF AN UNDERGROUND NUCLEAR TEST USING TRACER EXPERIMENTS AND HISTORICAL OBSERVATIONS

An underground nuclear explosion (UNE) produces radioactive gases that can be transported to the ground surface though fractures over hours to months following detonation. If detected, the presence of short-lived radionuclides in gas is evidence of a recent UNE. Numerical modeling can provide estimates of surface arrival times that can help inform gas monitoring strategies at suspected foreign test sites. To improve our conceptual and numerical models of subsurface gas-transport processes following an UNE, field-scale tracer experiments, geologic characterization of the near-field damage structures, and subsurface air pressure monitoring has been performed at historic U.S. UNE sites, including the U20az site at the Nevada National Security Site where the BARNWELL device was expended in December, 1989. A two-phase (water and air), dual-permeability flow and transport model of the U20az site was built to investigate gas transport processes under experimental conditions and following the BARNWELL nuclear event. The model predicts the arrival and relative concentrations of three gas tracers (¹²⁷Xe, ³⁷Ar, and SF­₆) in a 2013 field experiment, as well as the observed arrival of radioactive gases following the 1989 BARNWELL event using barometric pressure records from the respective periods. The results demonstrate that the character of the barometric records may be a key factor in explaining the differences in transport behavior.