Technetium and Molybdenum in Oxide Spent Nuclear Fuel: Impact on Release Estimates
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Technetium and Molybdenum in Oxide Spent Nuclear Fuel: Impact on Release Estimates Jeffrey A. Fortner, * A. Jeremy Kropf, Robert J. Finch, and James C. Cunnane Argonne National Laboratory, Chemical Engineering Division, Argonne, IL 60439 *email: [email protected] ABSTRACT Technetium-99 (99Tc) is an important radionuclide in repository models owing to its relatively long half-life and the high aqueous solubility of compounds where it is in the heptavalent state. The vast majority of the 99Tc inventory presently slated for disposal is contained in oxide commercial spent nuclear fuel (CSNF), where it is divided (along with its cohort, molybdenum) between exsolved, intermetallic “epsilon” particles and isolated, likely oxidized, atoms distributed in the uranium dioxide matrix. We present recent evidence from synchrotron x-ray fluorescence spectroscopy, electron microscopy, and fuel dissolution testing on the likely oxidation state, coordination environment, and physical disposition of technetium and molybdenum in CSNF. Effects of the relative proportioning of technetium and molybdenum among the metallic and oxidized states in CSNF, and their distribution in or near grain boundaries and gaps on release during CSNF corrosion testing are discussed. INTRODUCTION Future nuclear fuel cycles will rely upon understanding the complex chemistry of trace fission products and transuranium actinides. Knowledge of the chemical states of such radionuclides in spent nuclear fuel and related materials provides a first line of insight that may benefit waste disposal in a geologic repository, fuel reprocessing, transmutation/recovery, development of advanced nuclear fuels, and remediation of sites from weapons-related activities. This type of information was unavailable for development of a model used by the Yucca Mountain Repository Development Project to assess the long-term radionuclide release rate from commercial spent nuclear fuel in support of the repository license application. This model is based on the conservative assumption that radionuclides embedded in the fuel lattice will dissolve when the fuel’s UO2 matrix is oxidized and dissolved. Because the chemical state and distribution of these radionuclides within the fuel and its alteration products have not been understood in detail, corrosion rate measurements to date are likely obfuscated by release of soluble radionuclides from gap and grain boundary regions, as well as other possible transient effects [1-3]. Technetium serves as a prime example of the impact that conservatism has had on modeling. As oxide nuclear fuel undergoes fission in a reactor, a portion of the technetium is incorporated into intermetallic, hexagonal close-packed (hcp) “epsilon particle” precipitates that form in the fuel from the 5th-period noble metal fission products technetium, ruthenium, rhodium, and palladium, plus molybdenum [3-6]. Corrosion of these epsilon particles has been implicitly assumed to progress at the rate of general fuel corrosion and to contribute as a source of technet
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