Discriminating Underground Nuclear Explosions Leading To Late‐Time Radionuclide Gas Seeps

Utilizing historical data from the U.S. nuclear test program and freely available barometric pressure data, we performed an analytical barometric‐pumping efficiency analysis to determine factors resulting in late‐time radionuclide gas seeps from underground nuclear explosions. We considered 16 underground nuclear explosions with similar geology and test setup, of which five resulted in the measurement of late‐time radionuclide gas concentrations at the ground surface. The factors we considered include barometric frequency and amplitude, depth of burial, air‐filled porosity, intact‐rock permeability, fracture aperture, and fracture spacing. The analysis indicates that the best discriminators of late‐time radionuclide gas seeps for these explosions are barometric frequency and amplitude and air‐filled porosity. While geologic information on fracture aperture and spacing is not available for these explosions, the sensitivity of barometric‐pumping efficiency to fracture aperture indicates that it would likely also be a good discriminator. Plain Language Summary Variations in air pressures (barometric variations) can drive gases created during underground nuclear explosions to the ground surface. The changes in air pressure above the ground push and pull the air in connected spaces between rocks (fractures) allowing these pressure changes to access hundreds of meters into the subsurface. Some barometric variations and geologies are more conducive to driving gases to the ground surface than others. Using a model that combines the effect of the barometric variations and geology, we are able to identify scenarios that are more likely to lead to gases arriving at the ground surface after an underground nuclear explosion. Identifying underground nuclear explosions that will likely result in gases arriving at the ground surface and entering the atmosphere provides information for investigators trying to verify nuclear test ban treaties. Key Points Barometric‐pumping efficiency facilitates discrimination of underground nuclear explosions leading to late‐time seeps Longer high‐efficiency barometric periods and higher high‐efficiency amplitudes indicate a greater chance of late‐time seeps Low air‐filled porosity indicates a greater chance of late‐time seeps