Executive Summary The overall objective of this project was to provide a scientific foundation for safely ~ processing high organic content nuclear tank wastes for disposal. The harsh chemical and radiation environment of high-level waste causes "aging" of the organic compounds and radionuclides, producing new chemical forms of the radionuclides that resist standard pretreatment methods. Advanced pretreatment techniques are costly to develop and implement, and unplanned behavior in waste

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... nt processes (e.g., the release of benzene during Savannah River's ITP demonstration in 1995) can adversely impact public health, environmental quality, and the credibility of the Department of Energy. Therefore, it is imperative that the chemistry and processing behavior of high-level wastes be fully understood. Key goals of this project were to identify methods to realistically simulate tank waste aging processes in the laboratory, and then to use those methods to predict how this aging impacts the DOE's,ability to process wastes for geologic disposal. Improved understanding of radionuclide chemistry in the wastes also supports modeling of contaminant transport from tanks that have leaked into the subsurface. The most significant and unexpected result from this project is the identification of a new reaction pathway that produces intractable chemical forms of technetium-99, similar to technetium compounds recently discovered in certain Hanford tanks. Although conventional wisdom suggested that radiation in the tanks was responsible for producing these compounds, this project provided direct evidence that the compounds are rapidly formed in simulated tank 2 LA-UR-00-5998 _-: l /+ i wastes, even when no radiationfield is applied. Because the reaction is catalyzed by trace amounts of highly reactive metals that form during uranium fission, we predict that this reaction can occur in any alkaline nuclear waste environment that also contains organic compounds. This research also indicates that catalytic metals may exhibit a broader scope of chemical reactivity in tank waste than previously imagined, and that unplanned behavior could occur during waste processing if these chemical effects are not examined in the early stages of process development. 3 LA-UR-00-5998