XENON ISOTOPES IN SOIL GAS AS INDICATORS OF BURIED RADIOACTIVE WASTE

Levels of short-lived and stable xenon isotopes were measured in soil gas samples collected from three different waste disposal sites at the U.S. Department of Energy?s Hanford Site, Washington State. Short-lived isotopes Xe-133 and Xe-135 were measured using a modification of the Automated Radioxenon Sample and Analysis (ARSA) system developed for Comprehensive Test Ban Treaty Verification. Stable isotopes were measured by rare gas mass spectrometry. A site that received large amounts of plutonium waste after separation from fission products showed detectable levels of radioxenon from the spontaneous fission of Pu-240 but no detectable alteration of stable xenon isotopes from the atmospheric background. Altered stable xenon isotope ratios were detected in soil gas samples collected near a radioactive waste landfill. The altered stable xenon isotope ratios are consistent with the probable presence of irradiated fuel elements in the burial ground.

The measurements show the usefulness of xenon isotopic measurements for location and characterization of waste sources using minimally invasive techniques. Column transport studies indicate xenon transport through moist Hanford soil will be retarded relative to helium transport. This retardation may limit the distance short-lived isotopes can be detected from the source.