Levitating nuclear fuel offers key to understanding its structure

Levitating nuclear fuel offers key to understanding its structure

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It took a year to plan for something that would last about ten seconds. All the safeguards were in place, which meant no one could touch any of the seven spheres that were about 1/8 the size of a Ping-Pong ball.

The first five balls didn’t make it into the right spot, leaving the team with only two more attempts to make it work.

“It was the equivalent of the bottom of the ninth inning and there were two pitches left,” said John Parise, a distinguished professor in the department of geosciences at Stony Brook. “If you don’t hit a home run, you’re going away empty-handed.”

Instead of baseballs, the group was working with forms of uranium dioxide, the major nuclear fuel component of fission reactors, which produce nuclear power. Scientists at Argonne National Laboratory in Lemont, Illinois, and Stony Brook were trying to get a clearer picture of the structure of this compound at extreme temperatures. In nuclear reactor accidents, like Chernobyl in 1986 and Fukushima in 2011, uranium dioxide can reach temperatures of over 3,000 degrees Celsius. At that level, uranium dioxide can melt many of the containers designed to hold the radioactive liquid.

Scientists had come up with several theories about what the structure of this compound is at these extreme temperatures, but no experiments had provided direct evidence.

Fortunately, the team, led by Lawrie Skinner, a research assistant professor at Stony Brook, was able to get the final pellets in the correct spot, giving the researchers a chance to heat it to these extreme temperatures and then study its structure with specialized x-rays produced by the synchrotron at the Advanced Photon Source at Argonne.

The researchers discovered that each atom of uranium starts with 8 oxygen atoms nearby and, at extreme temperatures, has that number reduced to 6.7 oxygen neighbors. This, Skinner explained, affects the physical properties of the liquid, like its viscosity.

Parise, Skinner, scientists at Argonne National Laboratory and colleagues including Richard Weber, the founder of Materials Development Inc. in Evanston, Illinois, recently published their findings in Science.

Levitating the uranium dioxide pellets was critical because it prevented the compound from coming into contact with anything else. Skinner likened the process to keeping a ping pong ball afloat by using a hair dryer. To heat the compound in the experiment, which was not radioactive, the scientists hit it with a carbon dioxide laser that is about 100,000 times more powerful than a bright laser pointer, Skinner offered.

Scientists and those involved with nuclear reactors need to understand the viscosity of uranium dioxide so they “know how it will behave in a reactor meltdown,” said Parise. “The chief motivation behind studying the structure of uranium dioxide is to provide theoreticians with an accurate set of data that they can use to derive the atomic interactions that’ll allow them to predict behavior,” Parise said.

Skinner and Parise started working together over four years ago, when Skinner was a postdoctoral researcher in Parise’s lab. Since then, Skinner has gone on to conduct his own research.

Parise explained that he and his colleague will continue to try to understand how atomic interactions give rise to physical properties and behaviors. They would like to understand how liquids evolve as a function of pressure and temperature.

The next material Parise is planning to study is iron-containing liquids, including those involved in steelmaking.

Parise sees considerable potential for work with lava. The hot magma that comes from the center of the Earth behaves differently depending on the conditions and location where it erupts. In Hawaii, for example, lavas are much more liquid, while those in the Pacific Northwest are more explosive.

The next challenge, Parise said, is to understand how the composition of gas that’s over the liquids affects the viscosity and internal structure of those liquids.

Parise and Skinner each grew up far from Long Island. A native of Far North Queensland, Australia, Parise now lives in Poquott with his wife Alyse, who is the owner of a business- coaching company called Power Outcomes.

Skinner, who spends much of his time in Illinois at Argonne National Laboratory, grew up in the United Kingdom. He is married to Sonia, who works as an engineer for S&C Electric Company, which makes parts for an electric power grid.

As for the experiment, Skinner said he was “a little tense,” when the first few uranium dioxide balls didn’t make it into the levitator, but he “had faith.” It’s important, he urged, to stay positive in the face of failure. “If we just did things that were easy, we would not progress [in] our knowledge as much.”