XRD Study On The Surface Layer Structure Of Uranium And Triuranium Octaoxied

The structure of triuranuim octaoxide (U3O8) and the oxidation of uranium metal were studied by X-ray diffraction (XRD) and Rietveld method in different partial pressure of oxygen and temperatures. The conclusions are reported as following:The structure change of triuranium octaoxide (U3O8) was studied by XRD at ambient atmosphere at temperature form 25 ℃ to 850℃. There are two kind of phases of U3O8 in the temperature range studied, and the transformation temperature is about 300℃. The low temperature phase is orthorhombic with space group Amm2, and it change to another phase with better symmetry around 300℃. Another phase, the high temperature phase, is hexagonal with space group P-62m, which is stable from 300℃ to 800℃. The lattice parameter and the full width of half maximum (FWHM) of XRD peaks of U3O8 were also investigated at different temperature, and it were found that they change with the temperature under rules. The mean crystallite size and microstrain of U3O8 in different temperature were also discussed in the paper.The specimens of uranium flat were heated in air, oxygen or low pressure, to investigate the formation of UO2+x and U3O8 on the surface layer of specimens. The processes of oxidation to UO2+x and U3O8 were analyzed by XRD to determine the extent of surface layer oxidation and the structure of oxidations. The XRD patterns were fitted by Rietveld method to refine the structure of different phases and to calculate quantificationally. The results indicate that the kinetic expression for the formation of UO2+x in 25-150℃ was initially nonlinear, but switched to linear at long reaction times, that is, there exist two stages in the UO2 growth kinetics: nonlinear portion and linear portion. The activation energy is about 52.4kJ/mol. The lattice parameter, a0, and FWHM of the UO2+x (fluorite type) decrease initially and stabilize later as the increasing of reaction times at 200℃. The UO2+X is progressively replaced by U3O8 as the oxidation proceeds at 300℃. The formation rate of U3O8 is insensitive to the partial pressure of oxygen, and decreased with the pressure of oxygen drop.