2, Lawrence Livermore National Laboratory, Livermore, California, United States
We performed the material characterizations of electron microscopies (SEM and TEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy to reveal structural evolution of the crystalline α-U3O8 powders during a long-term (>30 years) storage in the ambient humidity and temperature. A rim of amorphous uranyl-hydrate phase was observed forming around the α-U3O8 crystalline core. The nano-scale fcc UO2 phase was also observed, which occurred either in the amorphous uranyl hydrate layer or at the boundary between the crystalline α-U3O8 core and surface amorphous phase. A preferential crystallographic orientation for epitaxial UO2 growth on the U3O8 was verified. Different from the UO2 oxidation process that involves the transition phase of U4O9 or U3O7 before transforming into U3O8, the occurrence of the middle phases during the reducing progress has not been identified. The stability and oxidation mechanism of U3O8 exposed to humidity will be discussed.
The work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Office of Defense Nuclear Nonproliferation Research and Development within the U.S. Department of Energy’s National Nuclear Security Administration under Project Number LL15-U_Surface_Oxidation-NDD3B.