Contribution of the synchrotron diffraction study of the oxidation of uranium dioxide at 250°C

The structural evolution of UO2 during its oxidation into U3O8 at 250degreesC in air was studied by in-situ synchrotron X-ray diffraction on the D2AM-CRG beamline at ESRF. The aim of this study is to determine the phases which are likely to appear during a long term storage of used nuclear fuel. Our results are in disagreement with the literature where the existence of the secondary cubic phase is not reported, and an alpha-U3O7 tetragonal phase (c/a < 1) is also mentioned but definitely not observed. These previous interpretations are possibly due to a poor instrumental resolution, inducing a sensible broadening of the diffraction peaks. Particularly, the fact that the instrumental resolution was unable to resolve the splitting of the 111 diffraction peak, when UO2 and U4O9 exist simultaneously, confirms the wrong phase attribution in the literature.