No abstract

[1] The ratios of noble gas radioisotopes can provide critical information with which to verify that a belowground nuclear test has taken place. The relative abundance of anthropogenic isotopes is typically assumed to rely solely on their fission yield and decay rate. The xenon signature of a nuclear test is then bounded by the signal from directly produced fission xenon, and by the signal that would come from the addition of xenon from iodine precursors. Here we show that this signal range is too narrowly defined. Transport simulations were done to span the range of geological conditions within the Nevada Test Site. The simulations assume a 1 kt test and the barometric history following the nuclear test at Pahute Mesa in March 1992. Predicted xenon ratios fall outside of the typically assumed range 20% of the time and situations can arise where the ground level signal comes entirely from the decay of iodine precursors. Citation: Lowrey, J. D., S. R. Biegalski, A. G. Osborne, and M. R. Deinert (2013), Subsurface mass transport affects the radioxenon signatures that are used to identify clandestine nuclear tests, Geophys.

A Noble Gas Migration Experiment injected 127 Xe, 37 Ar, and sulfur hexafluoride into a former underground nuclear explosion shot cavity. These tracer gases were allowed to migrate from the cavity to near-surface and surface sampling locations and were detected in soil gas samples collected using various on-site inspection sampling approaches. Based on this experiment we came to the following conclusions: (1) SF 6 was enriched in all of the samples relative to both 37 Ar and 127 Xe. (2) There were no significant differences in the 127 Xe to 37 Ar ratio in the samples relative to the ratio injected into the cavity. (3) The migratory behavior of the chemical and radiotracers did not fit typical diffusion modeling scenarios.

The detection of radioactive noble gases is a primary technology for verifying compliance with the pending Comprehensive Nuclear-Test-Ban Treaty. A fundamental challenge in applying this technology for detecting underground nuclear explosions is estimating the timing and magnitude of the radionuclide signatures. While the primary mechanism for transport is advective transport, either through barometric pumping or thermally driven advection, diffusive transport in the surrounding matrix also plays a secondary role. From the study of primordial noble gas signatures, it is known that xenon has a strong physical adsorption affinity in shale formations. Given the unselective nature of physical adsorption, isotherm measurements reported here show that non-trivial amounts of xenon adsorb on a variety of media, in addition to shale. A dual-porosity model is then discussed demonstrating that sorption amplifies the diffusive uptake of an adsorbing matrix from a fracture. This effect may reduce the radioxenon signature down to approximately one-tenth, similar to primordial xenon isotopic signatures.