Microstructure changes and thermal conductivity reduction in UO2 following 3.9 MeV He2+ ion irradiation

The microstructural changes and associated effects on thermal conductivity were examined in UO2 after irradiation using 3.9 MeV He2+ ions. Lattice expansion of UO2 was observed in X-ray diffraction after ion irradiation up to 5 x 1016 He2+/cm(2) at low-temperature (<200 degrees C). Transmission electron microscopy (TEM) showed homogenous irradiation damage across an 8 mu m thick plateau region, which consisted of small dislocation loops accompanied by dislocation segments. Dome-shaped blisters were observed at the peak damage region (depth around 8.5 mu m) in the sample subjected to 5 x 10(16) He2+/cm(2), the highest fluence reached, while similar features were not detected at 9 x 10(15) He2+/cm(2). Laser-based thermoreflectance measurements showed that the thermal conductivity for the irradiated layer decreased about 55% for the high fluence sample and 35% for the low fluence sample as compared to an un-irradiated reference sample. Detailed analysis for the thermal conductivity indicated that the conductivity reduction was caused by the irradiation induced point defects. (C) 2014 Elsevier B.V. All rights reserved.