Evolution of extended defects in polycrystalline Au-irradiated UO2 using in situ TEM: Temperature and fluence effects

In situ Transmission Electron Microscopy irradiations were performed on polycrystalline UO2 thin foils with 4 MeV gold ions at three different temperatures: 600 degrees C, room and liquid nitrogen temperature. In order to study the dislocation evolution and to determine the growth mechanisms, the dislocation loop and line densities and the loop size repartition were monitored as a function of fluence, and irradiation temperature. We show that dislocation loops, with Burgers vectors along the <110> directions, evolve into dislocation lines with increasing fluence by a loop overlapping mechanism. Furthermore, a fluence offset is highlighted between the irradiations performed at high and low temperature due to an increase of the defect mobility. Indeed, a growth by Oswald ripening is probably activated at room temperature and 600 degrees C and changes the kinetic evolution of loops into lines. After this transformation, and for all the irradiation temperatures, a steady state equilibrium is reached where both extended defects (dislocation lines and small dislocations loops -around 5 nm in size-) are observed simultaneously. A continuous nucleation of small dislocation loops and of nanometer-sized cavities formed directly by irradiation is also highlighted. (C) 2016 Elsevier B.V. All rights reserved.