Technetium and molybdenum in oxide spent nuclear fuel: Impact on release estimates

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.