In Situ Study of Particle Precipitation in Metal-Doped CeO₂ during Thermal Treatment and Ion Irradiation for Emulation of Irradiating Fuels

Abstract

Metallic particles formed in oxide fuels (e.g., UO 2 ) 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 CeO 2 (surrogate for UO 2 ) 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 CeO 2 film at temperatures from 673 to 1273 K. In situ ion irradiation transmission electron microscopy with 2 MeV Al 2+ 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 Au 3+ ions to ∼10 dpa at 973 K produces ∼2 nm sized pure Pd particles; Au 3+ 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.

Fabrication, thermal analysis, and heavy ion irradiation resistance of epoxy matrix nanocomposites loaded with silane-functionalized ceria nanoparticles

“…58 Threshold displacement energies ( E d ) of 10, 28, 27, and 56 eV for H, C, O, and Ce (respectively) were adopted from molecular dynamics simulations. 59,60 Surface binding energies were taken from the SRIM database and lattice binding energies were set to zero. 61,62 SRIM simulations were run for the neat epoxy and nanocomposites with 20 and 60 wt% SCA-NP loadings based on stoichiometric atomic percentages of H, C, O, and Ce for each sample (Si was not included due to its very small value relative to the other elements).…”

The Effects of Irradiation on Structure and Leaching of Pure and Doped Thin-Film Ceria SIMFUEL Models Prepared via Polymer-Templated Deposition

“…), non-radioactive surrogate compounds can provide an alternative in situations where large amounts of active materials cannot be easily handled. Ceria (CeO 2 ) is the most common surrogate [22][23][24] used due to the similarity between the MO 2 crystal phases (both fluorite, FCC, Fm-3m) [22,25] and similar ionic radii (effective radii, 6-coordinate, Ce 4+ = 87 pm, U 4+ = 89 pm), though ThO 2 (Th 4+ = 94 pm) [26] has found occasional use [27] as a loweractive substituent. Doped SIMFUELs have been far more widely explored for UO 2 compared to CeO 2 and ThO 2 [25,28] (.…”

“…Ceria (CeO 2 ) is the most common surrogate [22][23][24] used due to the similarity between the MO 2 crystal phases (both fluorite, FCC, Fm-3m) [22,25] and similar ionic radii (effective radii, 6-coordinate, Ce 4+ = 87 pm, U 4+ = 89 pm), though ThO 2 (Th 4+ = 94 pm) [26] has found occasional use [27] as a loweractive substituent. Doped SIMFUELs have been far more widely explored for UO 2 compared to CeO 2 and ThO 2 [25,28] (. Where the use of inactive materials is not suitable or possible, the ALARP principle (as low as reasonably possible/practical) [29] is often used to reduce the volume of hazardous (i.e., radioactive) substances to be handled and analysed.…”

“…Factors such as thermal conductivity are dependent on film thickness [32], while others such as dissolution, especially within the UO 2 system, are surface effects requiring films of sufficient thickness to adequately model the bulk system [12]. Similarly, mimicking the effects of SNF such as the formation of metallic ε-particles requires a shift in approach to the preparation of SIMFUELs [25]. These metallic particles form from the agglomeration of inert metal (Ru, Mo, Tc, Rh, Pd, etc.)…”

“…These metallic particles form from the agglomeration of inert metal (Ru, Mo, Tc, Rh, Pd, etc.) fission products (FPs) produced during nuclear reactions, are typically sub-micron to a few microns in diameter, and can induce swelling of the fuel in addition to altering the thermal [25] and dissolution properties of SNF [33,34]. The desired analyses to be undertaken will determine the required thickness of film and thus the required preparative methods which can be employed [35], varying from highvacuum deposition methods such as PVD [36] and atomic layer deposition (ALD) [37] to chemical approaches such as polymer-assisted deposition (PAD) [38] and layer-by-layer (LbL) approaches [39].…”