A systematic investigation was performed on morphology change versus oxidation of stoichiometric UO2 powders following aging for periods up to one year under controlled conditions with relative humidity ranging from 34% to 98%. The stoichiometric UO2 powders were prepared or converted from the initial uranium oxides that are different in speciation, isotope and/or configuration, as well as origination and history, respectively. The materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), high-resolution X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. For the cases of low and middle relative humidity, morphology change manifested to be surface texture modification and reconstruction due to oxidation and/or hydroxylation. Oxidized amorphous rim occurred in the sample aged for a long period. For the case of high relative humidity, however, uranyl hydrate phase formed as well. The changes in morphology and structure provide useful evidences for investigation of nuclear forensics.
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.