Spent nuclear fuel accounts for over 95% of the total radioactivity in the radioactive wastes in the United States that require disposal, disposition or remediation. Uranium is the dominant actinide element in most of these nuclear wastes. The UO(sub 2) in spent nuclear fuel is not stable under oxidizing conditions and may be altered even under reducing conditions. Under oxidizing conditions, uranium has a strong tendency to exist as U(sup 6+) in the uranyl molecule, UO(sub 2)(sup 2+) The uranyl ion reacts with a wide variety of inorganic and organic anions to form complexes that are often highly soluble. The result is a rather rapid dissolution of UO(sub 2) and the formation of a wide variety of uranyl oxide hydrates, uranyl silicates and uranyl phosphates. The kinetics for this transformation is rapid and essentially instantaneous on geologic time scales. Under reducing conditions UO(sub 2) is stable, but may alter to U(sup 4+) compounds, such as coffinite, USiO(sub 4), depending on ground water compositions. Under both oxidizing and reducing conditions, the formation of new uranium phases may lead to the release or retardation of trace elements, such as the fission product elements and actinides in spent nuclear fuel. Over the long term, and depending on the extent to which the secondary uranium phases can incorporate fission products and actinides, these alteration phases become the near-field source term. This research program is providing fundamental data on the geochemistry of uranium and related transuranium elements, as well as the fate and transport of longlived fission products. We provide a basis for testing the models that are now used in performance assessments of nuclear waste repositories. These data and this approach are essential to determining the long-term behavior of spent nuclear fuel in a nuclear waste repository.
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