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 sixteen 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 fracture aperture would likely also be a good
discriminator.