Effects of Grain Size and Dopants on the Irradiation Response of Actinide Oxides

High energy irradiation can induce physical and chemical changes in nuclear materials, impacting their properties and performance in reactor systems. Of particular interest is the radiation response of actinide oxides, such as UO₂ [Uranium Dioxide] and ThO₂ [Thorium Dioxide], as well as analogue materials such as CeO₂ [Cerium Dioxide]. During the course of reactor operations, these nuclear materials are exposed to high energy ionizing radiation in the form of nuclear fission fragments. This study simulates the extreme conditions found in a nuclear reactor by utilizing accelerated heavy ions with mass and kinetic energy comparable to fission fragments in order to examine the effects of microstructure and rare earth doping on the irradiation response of nuclear-fuel materials. Synchrotron X-ray diffraction experiments performed at the Advanced Photon Source and transmission electron microscopy were used to characterize the samples before and after ion irradiation.

The effect of grain subdivision on radiation response at the outer rim of fuel pellets is simulated through the irradiation of oxide powders of ~20 nm grain size. Structural modifications were compared to the effect of the same irradiation of oxide powders of ~1 μm [micrometer] grain size. Samples of each grain size for three materials (UO₂, ThO₂, and CeO₂) were irradiated with 945.6 MeV Au ions to fluence values ranging from 1×10¹¹ [one times ten to the eleventh] – 3×10¹³ [three times ten to the thirteenth] ions/cm² [ions per square centimeter]. The grain size was shown to have a considerable effect on the defect-induced unit-cell expansion with an increased radiation resistance of microcrystalline samples. The highly ionizing irradiation caused additional redox effects in CeO₂ resulting in significant structural changes.

Compositional changes which occur during the course of reactor operation, due to the accumulation of heavy fission products, were simulated via swift heavy ion irradiation of UO₂ samples doped with an increasing amount of rare earth elements (La, Y, and Nd). These samples were irradiated along with undoped reference samples using 167 MeV Xe ions at fluences ranging from 1×10¹¹– 5×10¹⁴ [five times ten to the fourteenth] ions/cm2. Initial results show that doping of rare earth elements up to 32.87 weight % does not significantly affect the radiation response as compared to undoped UO₂.