Restructuring in high burn-up UO₂ fuels: Experimental characterization by electron backscattered diffraction

This paper discusses the use of electron backscattered diffraction to characterize restructuring in a set of UO₂ samples, irradiated in a pressurized water reactor at a burn-up between 35 and 73 GWd/t_U, including standard UO₂ samples and Cr-doped UO₂ samples, to provide a better understanding of restructuring occurring both on the periphery and in the center of high-burn-up pellets. The formation of a high burn-up structure on the periphery of high burn-up UO₂ was confirmed in our experiment. We found restructuring associated with bubble formation of all the samples in the central area, with higher irradiation temperatures when the burn-up exceeded 61 GWd/t_U, regardless of their initial microstructure. This restructuring tended to progress with the increasing burn-up and to sub-divide the initial grains into sub-grains, with orientations close to that of the parent grains. Radial changes and differences between these samples showed that the burn-up and the temperature were not the only relevant parameters involved in restructuring.