Characterization of UO2 irradiated in the BR-3 reactor

In order to better understand the behavior of irradiated UO2, six fuel rods containing fuel pellets fabricated by wet and dry processes were irradiate d under various power histories in the BR-3 PWR (Mol, Belgium). Subsequent to the irradiation, extensive hotcell measurements were performed on comple te rods, whole pellets, pellet cross sections or thin slices, and pellet fr agments. The results of these measurements collectively underscore several key characteristics of high burnup (37-65 GWd/t) fuel. Although a clear sep aration of burnup effects and fuel temperature history effects was not poss ible, the pellet microstructure evolution can be defined in terms of four d istinct radial zones. Further, fission gas release increased with burnup bu t grain growth was not found to be an integral part of the release mechanis m. 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 similari ty to U4O9 phase were found in etched as well as fractured surfaces in high burnup specimens. However, the phase structure was not confirmed and the p recipitates might have originated due to changes in the fuel chemistry rath er than stoichiometry. Finally, extensive fuel-cladding interaction with th e formation of U-Cs-Zr compounds was observed for burnup >40 GWd/t. (C) 199 9 Elsevier Science E.V. All rights reserved.