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

Abstract

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/tU, 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/tU, 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.

Advanced Multiscale Microscopy Characterization of High Burnup LWR UO₂ Before and After LOCA Testing

“…This phenomenon, designated as high burnup fuel fragmentation (HBFF), raised concerns of fuel fragments relocating into the ballooned region of the cladding and possible dispersal from the cladding through the rupture opening, collectively known as fuel fragmentation relocation and dispersal (FFRD). Although the burnup for the fuel from the Halden tests was above 80 GWd/tU, subsequent high burnup LOCA experimental programs have found the burnup threshold for HBFF to be closer to 62 GWd/tU pellet average [6]. Considering the current discharge limit for commercial fuel is 62 GWd/tU (rod average) and the industries desire to extend the burnup beyond 62 GWd/tU (rod average), the mechanisms responsible for HBFF must be understood so that possible mitigating solutions can be derived.…”