Using new tools developed to examine nuclear materials at very high spatial and chemical resolutions, researchers have discovered that the nuclear fuel UO₂ can accommodate a significantly greater amount of interstitial oxygen at ambient conditions than was previously thought possible—without a phase change or disruption of the material’s cubic structure. This first-known study of any actinide using atomic resolution spectroscopy and aberration-corrected scanning transmission electron microscopy (STEM) has produced new insights into how oxidation occurs at the nanoscale. Fundamental research of this nature could potentially lead to a better understanding of the useful lifetime of nuclear materials and to safer storage of nuclear waste materials over longer periods in the future.

“Understanding how the oxidation state of uranium affects transport through groundwater is really a central question,” says Steven Spurgeon of the Pacific Northwest National Laboratory (PNNL), the lead author of the research recently reported in the Proceedings of the National Academy of Sciences. “It’s been a question for a very long time in this field. Developing new techniques to understand how the oxidation state changes, which in turn can make the materials more or less prone to migrate through soil, is a big motivator for this work.” 

PNNL’s recent acquisition of a high-resolution, aberration-corrected JEOL GrandARM-300F STEM with a 63 picometers point-to-point resolution, which is one of only three of its kind in North America and the only one in a category 2 nuclear facility, made the current research possible. PNNL collaborated with researchers from the Center for Electron Microscopy at Lawrence Berkeley National Laboratory and the Center for Advanced Radiation Sources at the University of Chicago, who provided image simulations and samples, respectively. The decision to study single crystals of UO₂ subjected to controlled oxidation conditions—instead of the more complex, less well-controlled actual nuclear fuels taken from a reactor—was essential to this research.  

Spurgeon and colleagues prepared two types of UO₂ single crystals for analysis: An unoxidized (111)-oriented control sample stored in an inert gas environment and a heavily oxidized (001) sample exposed to pure O₂ for 21 days, followed by several months in ambient conditions.

To ensure a thorough analysis, the researchers performed simultaneous aberration-corrected STEM and atomic-resolution electron energy loss spectroscopy analysis (STEM-EELS) to determine how much oxygen is in the lattice, where that oxygen is positioned, and how the uranium atoms are bonded to oxygens around them. The researchers then performed density functional theory (DFT) calculations to simulate what the experimental data might look like under a variety of conditions to verify and interpret the experiments.

Incoherent STEM high-angle annular dark field images revealed a uniform image contrast for the (111) unoxidized surface, while the (001) orientation showed increased intensity in an approximately 15-nm-thick layer near the surface. Atomic-scale STEM-EELS mapping of the U M_4,5 and O K ionization edges for the two samples showed fine-structure changes in several peaks of the EELS spectrum for the (001) sample. DFT calculations reveal that these spectral features and the associated increase in the STEM image intensity are created not by O lattice expansion or O vacancies, but rather by interstitial oxygen defects that affect the U coordination environment. The analysis shows that a stoichiometry of nearly UO_2.67 is attained in the cubic structure of UO₂, comparable to the stoichiometry obtained during high-pressure synthesis.

“It has been predicted that if you expose uranium oxides to oxygen for a long period of time, you're going to get a large-scale phase transformation, which will alter many properties of the material,” Spurgeon says. “In our case, we found that you could incorporate a substantial amount of oxygen into a very thin layer of the surface of UO₂ without the phase change occurring. We think that, essentially, the underlying materials constrain the phase change.”

“Nuclear energy is an important component in the provision of electrical power, and uranium dioxide is the most widely used nuclear fuel,” says J.G. Tobin of the University of Wisconsin-Oshkosh, an expert in the nuclear field who was not involved in this research. “Regardless of whether one supports the continued use of nuclear power ….., it is essential to have a firm scientific foundation for the understanding of actinide materials, if only to optimize safety, environmental remediation, and long-term storage and disposal. This [work] is a splendid example of the type of cutting-edge technical investigations that can provide such a scientific foundation.”

Future work on the oxidation of other actinides and on the dynamics and kinetics of actinide oxidation is planned.

Read the article in Proceedings of the National Academy of Sciences.