Corrosion of commercial spent nuclear fuel. 1. formation of studtite and metastudtite

The contact of commercial spent nuclear fuel (CSNF) with water over a 2-year period led to an unexpected corrosion phase and morphology. At short hydration times, crystallites of metaschoepite [(UO2)8O2(OH)12](H2O)10 were observed on the hydrated CSNF particles. Over the 2-year contact period, all evidence of metaschoepite disappeared, and the fuel particles were coated by a new alteration phase. Additionally, films of the reacted fuel were observed at the sample air-water interface of each sample. The corrosion phases on fuel powders and on the suspended films were examined by scanning electron microscopy, energy-dispersive X-ray fluorescence, and X-ray diffraction and were identified as studtite [(UO2)(O2)(H2O)2](H2O)2 and metastudtite (UO4.2H2O), respectively. The reason for the partitioning of the latter phase to the sample air-water interface is unclear at this time but may be due to structural differences between the two phases. Scanning electron micrographs of the CSNF powders indicated surface corrosion along grain boundaries and fragmentation of the primary solid. The occurrence of studtite and metastudtite on CSNF could have implications for the potential attenuation of released radionuclides during oxidative corrosion of CSNF in a geologic repository. (orig.)