Metallic particles formed in oxide fuels (e.g., UO2) during neutron irradiation have an adverse impact on fuel performance. A fundamental investigation of particle precipitation is needed to predict the fuel performance and potentially improve fuel designs and operations. This study reports on the precipitation of Mo-dominant β-phase particles in polycrystalline CeO2 (surrogate for UO2) films doped with Mo, Pd, Rh, Ru, and Re (surrogate for Tc). In situ heating scanning transmission electron microscopy indicates that particle precipitation starts at ∼1073 K with a limited particle growth to ∼10 nm. While particle concentration increases with increasing temperature, particle size remains largely unchanged up to 1273 K. There is a dramatic change in the microstructure following vacuum annealing at 1373 K, probably due to phase transition of reduced cerium oxide. At the high temperature, particles grow up to 75 nm or larger with distinctive facets. The particles are predominantly composed of Mo with a body-centered cubic structure (β phase). An oxide layer was observed after storage at ambient conditions. In situ heating X-ray photoelectron spectroscopy reveals an increasing reduction of Ce charge state from 4+ to 3+ in the doped CeO2 film at temperatures from 673 to 1273 K. In situ ion irradiation transmission electron microscopy with 2 MeV Al2+ ions up to a dose of ∼20 displacements per atom at nominally room temperature does not lead to precipitation of visible particles. However, irradiation with 1.7 MeV Au3+ ions to ∼10 dpa at 973 K produces ∼2 nm sized pure Pd particles; Au3+ irradiation at 1173 K appears to result in precipitates of ∼6 nm in size. Some of the defects produced by ion irradiation could be nucleation sites for precipitation, leading to generation of smaller particles with a higher concentration.
The results from this study provide useful information to assess the temperature gradient effects on the metal particle precipitation and microstructural evolution in irradiating UO2 fuels.