Evolution of extended defects in polycrystalline UO2 under heavy ion irradiation: combined TEM, XRD and Raman study

A well-suited way to understand the behavior under irradiation of the UO2 nuclear fuel is to use ion implantations and further analyze the microstructure. The present work is focused on the characteristics of extended defects created by Kr implantations in UO2 polycrystals. The influence of many parameters on these extended defects has been studied separately: irradiation temperature (room temperature, 500 and 600 °C), ion fluence (5 × 1015 and 1.26 × 1017 i/cm2), ion energy (4 and 27 MeV) and conditions of subsequent thermal annealing. The evolution of extended defects characteristics and density was determined using transmission electron microscopy and additional damage characterizations were performed by X-ray diffraction and Raman spectroscopy. Results obtained by the different analytical techniques are shown to be in excellent agreement and to complement literature data. In addition, this work presents the extended defect transformation induced by fluence (dislocation loops and lines → tangled dislocation network) and shows that temperature influences directly their evolution kinetics. Moreover we demonstrate that the temperature to allow extended defects mobility is different during irradiation and under thermal annealing conditions. Finally dislocation line density and swelling obtained from this study are found in the same order of magnitude as the ones obtained from in pile irradiations.