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Thursday, 28 October 2004

XAS Provides Scientific Basis for Technetium Nuclear Waste Remediation in the Hanford Tanks

Wayne W. Lukens, David K. Shuh, Norman C. Shroeder and Kenneth R. Ashley

Outline for vitrification of Hanford high-level waste. Separations are illustrated in red.

The Hanford nuclear waste site in southeastern Washington State is one of the most contaminated sites in the DOE complex. It stores millions of gallons of radioactive waste from the nation's nuclear weapons programs. High-level radioactive waste is leaking from about a third of Hanford's underground tanks.

The waste includes the element technetium in the radioactive form of ⁹⁹Tc, which poses a grave threat because of its long half-life (213,000 years) and its potential to contaminate ground water and migrate towards the Columbia River. Radioactive chemicals infiltrating the environment are a cancer hazard to humans.

The proposed solution for remediating Tc is to chemically separate it from the tank wastes and store it in solid glass, a process called vitrification. But separating ⁹⁹Tc into the proper phase before vitrification is proving difficult in some of the tanks because it forms unknown compounds in some storage conditions. To determine the identity of these unknown ⁹⁹Tc compounds in the waste, Wayne Lukens of Lawrence Berkeley National Laboratory and his collaborators performed experiments at SSRL using a technique called XAS, X-ray Absorption Spectroscopy, which uses x-rays to boost electrons to higher-energy states, and provides electronic and geometric structural information. Comparison of the experimental results to spectra from known compounds indicates that the unknown species is a Tc(I)-carbonyl species. These results are extremely valuable because they indicate that technetium separations technologies must be formulated to be effective for Tc(I)-carbonyl species.