Characterization of UO 2 irradiated in the BR-3 reactor

Abstract

In order to better understand the behavior of irradiated UO 2, six fuel rods containing fuel pellets fabricated by wet and dry processes were irradiated under various power histories in the BR-3 PWR (Mol, Belgium). Subsequent to the irradiation, extensive hotcell measurements were performed on complete rods, whole pellets, pellet cross sections or thin slices, and pellet fragments. The results of these measurements collectively underscore several key characteristics of high burnup (37–65 GWd/t) fuel. Although a clear separation of burnup effects and fuel temperature history effects was not possible, the pellet microstructure evolution can be defined in terms of four distinct radial zones. Further, fission gas release increased with burnup but grain growth was not found to be an integral part of the release mechanism. The onset of accelerated Xe release, as indicated by radial profiles by EPMA, corresponded well to a sudden increase in intragranular pore density within a narrow radial zone. Acicular precipitates having apparent similarity to U 4O 9 phase were found in etched as well as fractured surfaces in high burnup specimens. However, the phase structure was not confirmed and the precipitates might have originated due to changes in the fuel chemistry rather than stoichiometry. Finally, extensive fuel-cladding interaction with the formation of U–Cs–Zr compounds was observed for burnup >40 GWd/t.