Nanoparticle Precipitation in Irradiated and Annealed Ceria Doped with Metals for Emulation of Spent Fuels

Epsilon-phase alloy precipitates have been observed with varied compositions and sizes in spent nuclear fuels, such as UO₂. The presence of the inclusions, along with other oxide precipitates, gas bubbles, and irradiation-induced structural defects, can significantly degrade the physical properties of the fuel. To predict fuel performance, a fundamental study of the precipitation processes is needed. This study uses ceria (CeO₂) as a surrogate for UO₂. Polycrystalline CeO₂ films doped with Mo, Ru, Rh, Pd, and Re (surrogate for Tc) were grown at 823 K using pulsed laser deposition, irradiated at 673 K with He⁺ ions, and subsequently annealed at higher temperatures. A number of methods, including transmission electron microscopy and atom probe tomography, were applied to characterize the samples. The results indicate that there is a uniform distribution of the doped metals in the as-grown CeO₂ film. Pd particles of ∼3 nm in size appear near the dislocation edges after He⁺ ion irradiation to ∼13 dpa. Thermal annealing at 1073 K in air leads to formation of precipitates of Mo and Pd near the grain boundaries. Further annealing at 1373 K produces 70 nm sized precipitates consisting of nanograins at cavities.