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Under the microscope: new research on molten uranium dioxide

A laser-molten drop of uranium dioxide (UO2) floats on a gas stream inside a water-cooled nuzzle. (PHOTO CREDIT: STONYBROOK.EDU)
A laser-molten drop of uranium dioxide (UO2) floats on a gas stream inside a water-cooled nuzzle. (PHOTO CREDIT: STONYBROOK.EDU)

Nuclear power has long been touted as one of the cleanest and greenest energy sources. Even so, the 2011 Fukushima meltdown demonstrated more could be done in the way of safety, including the ability to better predict the flow of melted reactor cores.

Now, thanks to research led by a Stony Brook professor, the structure of molten uranium dioxide, the chief component of nuclear fuel rods, has been discovered—and that is just for starters.

In a study published in “Science,” Stony Brook’s John Parise and his colleagues used a special levitator to determine the atomic structure of uranium dioxide, or UO2, at extremely high temperatures—over 3000 Kelvin. They found that UO2 undergoes oxygen disorder near its melting temperature of 3140 Kelvin, above which the average number of bonds to oxygen drops from eight to 6.7.

“Materials scientists use models, such as in the design of aircraft wings, since you cannot just keep testing materials,” Parise said. “Previously these models had been really difficult to develop for UO2, because no one had been able to melt and measure a sample.”

“Up until we did the experiment there had been several models predicting the properties of molten UO2,” he continued. “Our findings helped us distinguish between the different models. It basically set an experimental benchmark against which future models must fit.”

To study the structure of molten uranium dioxide, Parise and his colleagues had to figure out how to achieve such high temperatures without melting their testing apparatus. To do this, the scientists aerodynamically levitated a sample of UO2 in argon gas and heated the sample with a laser. They then used synchrotron x-ray diffraction to analyze its structure.

“When you shine x-rays at material, the waves interact with atoms and give rise to a diffraction pattern,” Parise said. “That is, the beam deviates from the beam path and scatters in one particular direction or another. We measure that pattern to learn more about the structure of the atoms in the material.”

“By analyzing the patterns and intensities produced by x-ray diffraction, you can draw inferences about which atoms are where,” Parise continued. “We used this in the context of molten UO2 to test previous models that look at properties
like viscosity.”

One of the most pressing applications for the structural data will be in modeling what occurs during the meltdown of a nuclear fission plant.

“While designing nuclear plants, engineers want to be able to predict how quickly the molten uranium will flow, for instance, during a meltdown,” Parise said. “Of course, in a meltdown it will not be pure molten UO2, so this is a first step.”

Parise and his colleagues plan to use a similar apparatus to test the structural properties of other molten compounds.

“Our current work was done using argon gas,” Parise said. “However, many materials, like iron-containing compounds, exhibit different properties depending on the atmosphere. Our next step will be to look at compounds suspended in gases other than argon.”

With nuclear fission becoming a widespread energy source in the developed world, structural data like this will contribute to safer and better-designed nuclear power plants. Nonetheless, the techniques used by Parise and his colleagues can be used to study a host of other compounds with properties still unknown due to their high melting points.

In the meantime, Parise had a few words of advice for budding Stony Brook scientists and engineers.

“If you are interested in a topic try to find a mentor who can introduce you to their research or lab,” he said. “Even if you start with very basic experiments, so long as it is an area that interests and excites you. There’s nothing like excitement to really
spur you on.”

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