A Science-Based Approach to Understanding Waste Form Durability in Open and Closed Nuclear Fuel Cycles
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0985-NN07-03
A Science-Based Approach to Understanding Waste Form Durability in Open and Closed Nuclear Fuel Cycles M. T. Peters1, and R. C. Ewing2 1 Applied Science & Technology, Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL, 60439 2 Department of Geological Sciences, The University of Michigan, 2534 C.C. Little Bldg., 1100 N. University, Ann Arbor, MI, 48109-1005 ABSTRACT There are two compelling reasons for understanding source term and near-field processes in a radioactive waste geologic repository. First, almost all of the radioactivity is initially in the waste form, mainly in the spent nuclear fuel (SNF) or nuclear waste glass. Second, over long periods, after the engineered barriers are degraded, the waste form is a primary control on the release of radioactivity. Thus, it is essential to know the physical and chemical state of the waste form after hundreds of thousands of years. The United States Department of Energyís Yucca Mountain Repository Program has initiated a long-term program to develop a basic understanding of the fundamental mechanisms of radionuclide release and a quantification of the release as repository conditions evolve over time. Specifically, the research program addresses four critical areas: a) SNF dissolution mechanisms and rates; b) formation and properties of U6+secondary phases; c) waste formñwaste package interactions in the near-field; and d) integration of in-package chemical and physical processes. The ultimate goal is to integrate the scientific results into a larger scale model of source term and near-field processes. This integrated model will be used to provide a basis for understanding the behavior of the source term over long time periods (greater than 105 years). Such a fundamental and integrated experimental and modeling approach to source term processes can also be readily applied to development of advanced waste forms as part of a closed nuclear fuel cycle. Specifically, a fundamental understanding of candidate waste form materials stability in high temperature/high radiation environments and near-field geochemical/hydrologic processes could enable development of advanced waste forms ìtailoredî to specific geologic settings. INTRODUCTION There are two important reasons for understanding source term and near-field behavior. First, almost all of the radioactivity is initially in the waste form, mainly in the SNF and vitrified waste. Therefore, the waste form places the initial limits, as well as the long-term limits, on radionuclide release. Interactions of the source term with the near-field environment, such as the corroded waste packages, place additional constraints on the long-term behavior and mobility of radionuclides (cf. [1] and references therein; [2] and references therein). An enhanced understanding and realistic estimates of the extent to which radionuclides will be retained in the waste form or near-field environment reduce the demands on the performance of subsequent, farfield barriers. Realistic estimates of radionuclide release will al
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