Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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In Situ Study of Particle Precipitation in Metal-Doped CeO2 during Thermal Treatment and Ion Irradiation for Emulation of Irradiating Fuels Weilin Jiang,*,† Michele A. Conroy,†,§ Karen Kruska,† Matthew J. Olszta,† Timothy C. Droubay,† Jon M. Schwantes,† Caitlin A. Taylor,‡ Patrick M. Price,‡ Khalid Hattar,‡ and Ram Devanathan† †
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185, United States
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‡
ABSTRACT: 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.
1. INTRODUCTION Metallic particles precipitated from fission products in an oxide fuel1,2 (e.g., UO2), along with other oxide precipitates, gas bubbles, and irradiation-induced structural defects can significantly degrade the physical properties of the fuel, including thermal conductivity,3 swelling, and melting point.4 A burn-up of 40 MW d/kg U results in the conversion of 4% of the uranium to approximately 3% fission products and 1% transuranium elements.5 Larger metallic particles are often found near the central part of the fuel pellet that has a higher temperature. Most of the particles possess a hexagonal closepacked structure of ε phase Ru(Mo, Tc, Rh, Pd) with a broad variation in the component concentrations.2,4 A smaller fraction of body-centered cubic (bcc) β phase Mo(Tc, Ru) and face-centered cubic α phase Pd(Ru, Rh) also exist in the spent fuel. In addition, simulated compositions with Re as a surrogate for Tc were investigated6 as a waste form for 99Tc, an isotope with a long half-life of 2.13 × 105 years and a high solubility under oxidizing conditions in a corrosion process.7 There is a pressing need to understand the factors influencing the nucleation and growth of the metallic phases in fuels. While © XXXX American Chemical Society
characterization of the irradiated structures is needed to provide vital information about what compositions are present in the irradiated fuel, such as metallic precipitates, it is of fundamental interest to investigate critical processing parameters, including temperature and dose, under which certain phase of precipitates can form. Ion irradiation combined with thermal annealing can play a significant role in this effort. It allows for simulation of different stages of particle precipitation at different doses as well as different locations in the pellets at different temperatures. As in our previous report,8 cubic phase CeO2 is used in this study as a surrogate for UO2 to emulate precipitation of nanoparticles in spent fuels. Ceria and urania have the same cubic crystal structure (space group 225, Laue grope 11) with nearly identical lattice parameters (0.5411 and 0.5471 nm for CeO2 and UO2, respectively). They also have similar physical properties, such as melting temperature (2873 K for CeO2 and 3143 K for UO2) and thermal diffusivity.9 A Received: November 13, 2018 Revised: December 27, 2018 Published: January 8, 2019 A
DOI: 10.1021/acs.jpcc.8b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Table 1. Precipitates in Doped CeO2 as a Function of Annealing and Irradiation Conditions irradiation/annealing in in in in
situ situ situ situ
heating STEM (isochronal) 2 MeV Al2+ irradiation TEM 1.7 MeV Au3+ irradiation TEM 1.7 MeV Au3+ irradiation TEM
temperature (K)
fluence (ions/cm2)
dose (dpa)
873−1373 step: 100 300 973 1173
0 up to 1 × 1017 up to 1 × 1015 up to 1 × 1015
0 up to ∼20 up to ∼10 up to ∼10
duration (min) 20 up up up
each to 565 to 90 to 21.3
precipitate 1373 K: Mo, Ru, Re and Pd, up to 75 nm not visible Pd, 2 nm Pd (?), 6 nm
Figure 1. SRIM quick Kinchin−Pease simulations of (a) atomic displacements and (b) atomic percentages of implanted species in ceria irradiated with 90 keV He+ ions to 4 × 1017 ions/cm2, 2 MeV Al+ ions to 1 × 1017 ions/cm2, and 1.7 MeV Au+ ions to 1 × 1015 ions/cm2.
The as-grown film was ozone (O3) plasma-cleaned prior to XPS performed using a Scienta Omicron photoelectron spectrometer with a monochromatic Al Kα (1486.7 eV) Xray source.11 The angle between the X-ray beam and the axis of the analyzer was fixed at the magic angle of ∼54°, while the photoelectron emission angle could be varied between normal emission (bulk sensitive) and grazing emission (surface sensitive). In situ heating XPS under vacuum (∼5 × 10−9 Torr) was performed for the as-grown film at 673, 1073, and 1273 K. The data were collected using a pass energy of up to 1000 eV with a step size of typically 0.05 eV. XPS measurements were carried out at room temperature with the use of an electron flood gun for charge compensation of the as-grown and as-irradiated samples and those after annealing at 673 and 1073 K. Following the anneal at 1273 K, however, there were no charging issues and the flood gun was not needed. In addition, in situ ion irradiation TEM was performed at three different temperatures (300, 973, and 1173 K) using the I3TEM facility12 at Sandia National Laboratories. Samples were irradiated at 60° off normal. For the 300 K experiment, thin TEM foils (
