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 × 10¹⁶ He²⁺/cm ² at low-temperature (<200 °C). Transmission electron microscopy (TEM) showed homogenous irradiation damage across an 8 μm 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 μm) in the sample subjected to 5 × 10 ¹⁶ He²⁺/cm², the highest fluence reached, while similar features were not detected at 9 × 10¹⁵ He ²⁺/cm². Laser-based thermo-reflectance 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.