Oxygen atoms create detailed architectures in uranium dioxide, altering our understanding of corrosion

July 28, 2015, Pacific Northwest National Laboratory
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. Credit: Cortland Johnson, PNNL

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.

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 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.

When 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."

Explore further: Finding that nitrogen can combine with oxygen in zirconia to form NO may lead to safer materials for nuclear reactors

More information: "UO2 Oxidative Corrosion by Nonclassical Diffusion." Physical Review Letters 114:246103. DOI: 10.1103/PhysRevLett.114.246103

Related Stories

The search for molecular oxygen among cosmic oxygen atoms

July 27, 2015

Oxygen is the third most abundant element in the universe (after hydrogen and helium) and of course it is important: all known life forms require liquid water and its oxygen content. For over thirty years, astronomers have ...

Study could change nuclear fuel

March 4, 2015

The adverse effects of radiation on nuclear fuel could soon be better controlled thanks to research involving UT's College of Engineering.

The finer details of rust

December 4, 2014

Scientists at the Vienna University of Technology have been studying the behavior of iron oxide surfaces. The atomic structure of iron oxide, which had been assumed to be well-established, turned out to be wrong. The behavior ...

Recommended for you

New insights into magnetic quantum effects in solids

January 23, 2019

Using a new computational method, an international collaboration has succeeded for the first time in systematically investigating magnetic quantum effects in the well-known 3-D pyrochlore Heisenberg model. The surprising ...

Rapid and continuous 3-D printing with light

January 22, 2019

Three-dimensional (3-D) printing, also known as additive manufacturing (AM), can transform a material layer by layer to build an object of interest. 3-D printing is not a new concept, since stereolithography printers have ...

Scientists discover new quantum spin liquid

January 22, 2019

An international research team led by the University of Liverpool and McMaster University has made a significant breakthrough in the search for new states of matter.

Researchers capture an image of negative capacitance in action

January 21, 2019

For the first time ever, an international team of researchers imaged the microscopic state of negative capacitance. This novel result provides researchers with fundamental, atomistic insight into the physics of negative capacitance, ...


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (1) Jul 28, 2015
Deaerators take out much radioactive gases continuously from nuclear powerplants. See the really high stacks? What do you think those are for?
Uncle Ira
not rated yet Jul 28, 2015
Deaerators take out much radioactive gases continuously from nuclear powerplants. See the really high stacks? What do you think those are for?

Okay, I'll take a stab at him. Are they for cooling? I thought you were the Skippy who was so picky about the terminologies, like windmills and wind turbines? Why you want to think we going to turn the other cheek when you call the cooling towers "really high stacks"?
1 / 5 (1) Jul 28, 2015
No, Ira, the hyperbolic towers are for cooling. These are the usually-slim, but high stacks you can see at many nuke powerplants. At Diablo Canyon, we had them put through a "nipple" on top of the containment, but at Fukushima, they are horrendously tall.

Look up deaerators. They are necessary for thermal plants, boilers, and other systems.

But thanks for doubting me.

1 / 5 (1) Jul 28, 2015
I do not know why you folk assume all other people lie. Why would I give you my real name, references, and everything else and lie? Is that what you would do?

Perhaps you folk have spent too much time in this fantasy land where you are all hiding behind pseudonyms, . . for some reason.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.