Local atomic structure of chromium bearing precipitates in chromia doped uranium dioxide investigated by combined micro-beam X-ray diffraction and absorption spectroscopy

摘要

Experimental studies have evidenced the presence of some chromium oxide precipitates in chromia (Cr_2O_3) doped UO_2 fuels, currently being used in many light water reactors. The main objective of this study was to analyze the atomic scale microstructure of these precipitates in a non-destructive way employing modern synchrotron-based X-ray radiation. To investigate UO_2 microstructures and chemical compositions of the precipitates an industrial grade fresh Cr_2O_3-doped UO_2 pellet (produced with an initial amount of 0.16 wt.% Cr_2O_3 powder) was examined using scanning electron microscopy (SEM) and electron probe micro analysis (EPMA). Average grain diameter of the UO_2 matrix was measured by SEM image analysis method. Precipitates were detected by EPMA. For all precipitates of micrometer scale analyzed by EPMA the composition was determined to be very close to Cr_2O_3. The structural property and the next neighbor Cr atomic environment in these precipitates were studied by a combination of more sensitive tools such as micro-focused X-ray diffraction ((μ-XRD) and absorption spectroscopy (μ-XAS). According to the XRD data, the chromia precipitates contain structural disorder and have a somewhat distorted lattice structure as compared to that of standard and crystalline α-Cr_2O_3 hexagonal crystal unit cell. The l-XAS results provide insight into the local atomic environment of chromium including the oxidation state assignment.