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Physical Sciences
Research Highlights

July 2015

Oxygen: Not at All Random

Rejecting random diffusion, oxygen atoms create detailed architectures in uranium dioxide, radically altering our understanding of corrosion

Oxygen diffuses into uranium dioxide
Oxygen atoms follow a set pattern in corroding uranium dioxide, the primary component of fuel rods in nuclear reactors, not random diffusion. Understanding this pattern opens new doors for controlling corrosion. Image by Cortland Johnson, PNNL. zoom Enlarge Image.

Results: Corrosion follows a different path when it comes to uranium dioxide, the primary component of the rods that power nuclear reactors, according to a new study by scientists at the Pacific Northwest National Laboratory, University of Chicago, and the Stanford Synchrotron Radiation Lightsource. In uranium dioxide, the oxygen atoms-key corrosion creators-do not diffuse randomly through the material. Rather, the oxygen atoms settle into the third, sixth, ninth, etc., layers. They space themselves within the layers and alter the structure by causing the layers of uranium atoms above and below to draw closer to the oxygen. The oxygen atoms essentially self-assemble into a highly structured array.

Why It Matters: Oxygen's interactions can extensively corrode materials, whether it is a car in a field or a fuel canister in a nuclear reactor. Under certain conditions, oxygen corrodes fuel rods and causes them to swell by more than 30 percent, creating problems during both routine operations and emergency situations. Also, this swelling can be a problem for long-term storage of nuclear waste. The study shows atomic-level changes counter to those shown by the classical diffusion model that states most of the oxygen atoms are near the surface. The new study gives scientists accurate information to understand the start of corrosion, possibly leading to new ways to avoid corrosion-related failures.

Oxygen atoms self-assemble in uranium dioxide
Oxygen atoms (red) move into the third, sixth, ninth, etc., layers of uranium dioxide. Play the oxygen diffusion video (Shockwave).

Methods: When uranium dioxide is exposed to oxygen, the classical diffusion model shows the oxygen randomly moving into the uranium. Specifically, the oxygen atoms settle on the surface and grab two electrons from one uranium atom or one electron from each of two on the way in.

That's not the case.

Instead, oxygen atoms pull in a bit of the negative charge from the nearest uranium atoms, from the next nearest, and from the next-next nearest. This shell of slightly oxidized (more positively charged) uranium forms a protective sphere around the oxygen. The next oxygen atom in moves just far enough away to get electrons from untouched atoms. As more oxygen enters, the atoms form an organized structure.

The team from PNNL, University of Chicago, and SSRL made their discovery by combining X-ray scattering and spectroscopy experiments to determine the positions of the uranium atoms. The results showed the uranium atoms contracting every third layer. As no model explained why and the X-ray scattering was unable to "see" the oxygens, the team calculated the positions of the atoms using two supercomputers. They found that the oxygen atoms went into every third layer, spaced themselves out within the layer (occupying about a quarter of the layer) and caused the nearby uranium layers to move.

Why the third layer? This layer is most thermodynamically favorable.

Why is the arrangement of quarter occupancy every three layers the most thermodynamically favorable? Because of the uranium atoms' ability to donate a small fraction of electronic charge to the corroding oxygen atoms, creating the oxidized sphere of three shells of uranium atoms around each oxygen.

"Nobody had ever seen something like this before - where the oxygen comes in and self organizes every three layers," said Dr. Anne Chaka, a geochemist at PNNL who conducted the quantum mechanical modeling. "It took quantum mechanical modeling on large supercomputers to understand what the electrons were doing and how that drove the spacing of the oxygen atoms."

What's Next? Now, the scientists are seeing if this result is unique to uranium dioxide or it applies more broadly to other metal oxides, such as ceramics and semiconductors important in the energy landscape.

Acknowledgments

Sponsors: A. M.C. and E. S. I. were funded by the Geosciences Research Program at Pacific Northwest National Laboratory (PNNL); U.S. Department of Energy (DOE), Office of Science (SC), Office of Basic Energy Sciences (BES), Chemical Sciences, Geosciences, and Biosciences Division. Support was provided by DOE SC Office of Biological and Environmental Research (BER), Subsurface Biogeochemical Research, through the SLAC Subsurface Focus Area program.

Facility Use: Crystal truncation rod data were collected at GeoSoilEnviroCARS, using resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE SC by Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation Earth Sciences BES GeoSciences. X-ray photoelectron spectroscopy data were collected in the Radiochemistry Annex at EMSL, Environmental Molecular Sciences Laboratory, and a portion of the density functional theory study was performed using the computational resources of EMSL, a national scientific user facility sponsored by BER and located at PNNL. Resources at the PNNL Institutional Computing were also used. Early sample preparation benefitted from access to the Berkeley Nanogeoscience Center.

Research Area: Chemical Sciences

Research Team: Joanne E Stubbs, Craig A Biwer, and Peter J Eng, University of Chicago; John R Bargar, SSRL; Anne M Chaka, Eugene S Ilton, and Mark H Engelhard, Pacific Northwest National Laboratory

Reference: Stubbs JE, AM Chaka, ES Ilton, CA Biwer, MH Engelhard, JR Bargar, and PJ Eng. 2015. "UO2 Oxidative Corrosion by Nonclassical Diffusion." Physical Review Letters 114:246103. DOI: 10.1103/PhysRevLett.114.246103


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