Environmental Management Science Program Research in Michigan

Project participated by Donggao Zhao at the University of Michigan

59849 - Radionuclide Immobilization in the Phases Formed by Corrosion of Spent Nuclear Fuel: The Long-Term Assessment

Description Provided by Investigator:

The UO₂ in spent nuclear fuel is not stable under oxidizing conditions. Under oxic conditions, the U(IV) has a strong tendency to exist as U(VI) in the uranyl molecule, UO₂²⁺. The uranyl ions react with a wide variety of inorganic and organic anions to form complexes which are often highly soluble. The result is rather rapid dissolution of UO₂ and the formation of a wide variety of uranyl oxide hydrates, uranyl silicates and uranyl phosphates. The reaction rates for this transformation are rapid, essentially instantaneous on geologic time scales. Over the long term, and depending on the extent to which these phases can incorporate fission products and actinides, these alteration phases become the near-field source term.

Fortunately, previous investigations (experimental studies and field studies) have established that natural uraninites and their alteration products can be used as natural analogues to study the corrosion of UO₂ in spent nuclear fuel. We propose in this research program to address the following issues:

1. What are the long-term corrosion products of natural UO₂+x, uraninite, under oxidizing conditions?
2. What is the paragenesis or the reaction path of the phases that form during alteration? How is the paragenetic sequence formation related to the structures and compositions of these uranyl phases?
3. What is the trace element content (as compared to the original UO₂+x), and does the trace element content substantiate models developed to predict fission product and actinide incorporation into these phases?
4. Are these the phases that are predicted from reaction path models (e.g., EQ3/6) which will be used in performance assessments?
5. How persistent over time are the metastable phase assemblages that form? Will these phases serve as barriers to radionuclide release?
6. Based on the structures of these phases (mostly sheet structures) can the thermodynamic stabilities of these phases be estimated, or at least bounded, in such a way as to provide for a convincing and substantive performance assessment?

This research is based on over a decade of previous work on uranium mineralogy, paragenesis, and the corrosion of UO₂ by the principal investigator, R. C. Ewing.