Effects of irradiation temperature on the response of CeO₂, ThO₂, and UO₂ to highly ionizing radiation

Microcrystalline CeO₂, ThO₂, and UO₂ were irradiated with 198 MeV ¹³²Xe ions to the same fluence at temperatures ranging from 25 °C to 700 °C then characterized by synchrotron X-ray diffraction and X-ray absorption spectroscopy. All samples retain crystallinity and their nominal fluorite-type phase at a fluence of 1.5 × 10¹³ ions/cm². Both CeO₂ and ThO₂ display defect-induced unit cell expansion after irradiation at room temperature (∼0.15% and ∼0.10%, respectively), yet as irradiation temperature increases, the maximum swelling produced decreases to ∼0.02%. Alternatively, UO₂ shows an initial contraction in unit cell parameter (approximately −0.05%) for room temperature irradiation, most likely related to irradiation-enhanced annealing or irradiation-induced oxidation. At higher temperatures (above 200 °C) UO₂ begins to swell, surpassing its unit cell parameter prior to irradiation (∼0.05%), an effect which could be attributed to minor reduction in uranium oxidation state in vacuum. However, while CeO₂ irradiated at room temperature undergoes partial reduction, both UO₂ and ThO₂ exhibit no measurable change in cation oxidation state as evidenced by X-ray absorption spectroscopy. All samples display a decrease in irradiation-induced heterogeneous microstrain as a function of increasing irradiation temperature.