Levitation allows researchers to better study melted uranium

Chuck Bednar for redOrbit.com – Your Universe Online

Researchers from Stony Brook University in New York have discovered a unique way to test whether or not the most commonly used fuel in nuclear reactors is safe: levitating it.

Uranium dioxide (UO2), which occurs naturally as the mineral uraninite, is the primary nuclear fuel component of fission reactors, the researchers explained. However, its high melting point (near 3140 Kelvin) makes studying its properties at, near or above this temperature very difficult.

Understanding molten UO2 is essential for understanding how the melt would interact with a nuclear reactor’s container materials, corresponding author Lawrie Skinner, a research scientist in the Mineral Physics Institute at Stony Brook University, and his colleagues explain in a paper published in a recent edition of the journal Science.

The concern during severe accidents, the researchers added, is the melting and the leakage of radioactive uranium dioxide as it corrodes through protective containment systems. Learning how the substance behaves at extreme temperatures in order to better predict its behavior in these types of events is crucial to improved safety and efficiency of this power source.

In the new study, which provides the first structure measurements of molten UO2, the researcher team “melted uranium dioxide and studied its structure using X-rays,” Skinner said.

“We also studied structural changes in hot, solid UO2 before melting,” he added. They discovered that, upon melting, the structure of uranium dioxide “goes from 8 oxygen atoms surrounding each uranium atom down to an average of 6.7 oxygen neighbors. This affects the predicted physical properties of the liquid, like its viscosity.”

Despite the fact that the behavior of UO2 upon melting is vital in extreme nuclear reactor incidence such as the 2011 disaster at Japan’s Fukushima Daiichi power plant, the extreme temperatures required for testing had limited the investigation into this phenomenon. This prevented structural studies and an accurate understanding of inter-atomic interactions.

“In fact, before these findings, no experimental structure measurement of molten UO2 had been reported,” the university said in a statement. “While physical property measurements and molecular dynamics models did exist for molten UO2, they were often parameterized from solid-state properties and exhibited large differences in their melt structures.”

This uncertainty led to molten uranium dioxide models which exhibited different characterizations of viscosity, compressibility and other physical properties vital to determining the safety of a nuclear reactor. Skinner and his colleagues found that, by combining laser heating, sample levitation, and synchrotron X-rays, they were able to obtain the required measurements.

“Levitation of the sample was crucial,” the university explained. Since UO2 has such a high melting point, it can post “serious problems for traditional furnace heating methods.” Most types of container materials (including  magnesium oxide and platinum) melt or become chemically reactive at those temperatures.

To prevent that from happening, the researchers levitated the uranium dioxide sample on a stream of Argon gas while heating it with a 400W continuous CO2 laser. Using this method prevented any solid contact with the sample, allowing it to maintain high chemical purity.

“In the future, we would like to investigate the atomic structure and properties of important U-containing compounds. This includes eutectic U-Zr-O, which forms in extreme accidents as the UO2 melts and reacts with its zirconium cladding,” said Skinner, whose research was supported by the US Department of Energy and the Argonne National Laboratory.

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