PNNL

Abstract

Metallic particles formed in oxide fuels (e.g., UO2) during neutron irradiation have an adverse impact on fuel performance. A fundamental investigation of the 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 beta-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 (STEM) data indicate that particle precipitation starts at ~1073 K with a limited particle growth to ~10 nm. While particle density 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 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 (beta phase). An oxide layer was observed after storage at ambient conditions. In situ heating x-ray photoelectron spectroscopy (XPS) reveals an increasing reduction of Ce charge state from 4+ to 3+ in the doped CeO2 film from 673 K to 1273 K. In situ ion irradiation TEM with 2 MeV Al2+ ions up to a dose of ~20 displacements per atom (dpa) 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.