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

Abstract

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.