Corrosion of Spent Nuclear Fuel: The Long-Term Assessment

Abstract

The successful disposal of spent nuclear fuel (SNF) is one of the most serious challenges to the completion of the nuclear fuel cycle and the future of nuclear power generation. Spent nuclear fuel is essentially UO2 with approximately 4-5 atomic percent actinides and fission product elements. A number of these elements have long half-lives (239Pu: 24,100 years; 237Np: 2 million years; 129I: 16 million years; 79Se: 1.1 million years; 99Tc 200,000 years); hence, the long-term behavior of the UO2 is an essential concern in the evaluation of the safety of a repository for spent nuclear fuel. One of the unique and scientifically most difficult aspects of the successful disposal of spent nuclear fuel is the extrapolation of short-term laboratory data (hours to years) to the long time periods (103 to 105 years). The direct verification of these extrapolations or interpolations is not possible, but methods must be developed to demonstrate compliance with government regulations and to satisfy the public that there is a reasonable basis for accepting the long-term extrapolations of spent fuel behavior. In recent years ''natural analogues'' for both the repository environment (e.g., the Oklo natural reactors) and nuclear waste form behavior (e.g., corrosion and alteration of uraninite, UO2+x) have been cited as a fundamental means of achieving confirmation of long-term extrapolations. In particular, considerable effort has already been made to establish that uraninite, UO2+x, with its impurities, is a good structural and chemical analogue for the analysis of the long-term behavior of the UO2 in spent nuclear fuel. This proposal is based on the study of uraninite and the naturally occurring alteration products of UO2+x under oxidizing and reducing conditions. This scientific program will address the following issues: (1) What are the long-term corrosion products of natural UO2+x under reducing and oxidizing conditions? (2) What is the paragenesis or the reaction path of the phases that form during alteration? How is the paragenetic sequence of formation related to the structures and compositions of the phases? (3) What is the trace element content in the corrosion products (as compared with the original UO2), and does the trace element content substantiate models developed to predict radionuclide incorporation? (4) Are the corrosion products the phases that are predicted from reaction path models (e.g., EQ3/6) that are 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?