Safety is a huge issue with nuclear power, which accounts for about 10 percent of the global electricity supply.
Uranium dioxide (UO2) is the major nuclear fuel component of fission reactors, and the concern during severe accidents is the melting and leakage of radioactive UO2 as it corrodes through its protective containment systems.
Understanding—in order to predict—the behavior of UO2 at extreme temperatures is crucial to improved safety and optimization of this electricity source.
A new paper in Science offers first structure measurements of molten uranium dioxide.
Above, the aerodynamic levitation setup at the advanced photon source beamline 6-ID-D. The sample chamber in the foreground is lit from inside by the 3000K laser-heated sample inside. (Credit: Stony Brook University)
“We melted uranium dioxide and studied its structure using X-rays; we also studied structural changes in hot, solid UO2 before melting,” says coauthor Lawrie Skinner, a research scientist in the Mineral Physics Institute at Stony Brook University.
“We found that, upon melting, the UO2 structure 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.”
Even though the behavior of UO2 upon melting is of critical importance in extreme nuclear reactor accidents—such as Chernobyl in 1986 and Fukushima in 2011—the very high temperatures needed for testing had severely limited investigation of this melt. This prevented structural studies and an accurate understanding of the inter-atomic interactions.
In fact, before these findings, no experimental structure measurement of molten UO2 had been reported.
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 structural uncertainty resulted in molten UO2 models that exhibited differing characterizations of physical properties such as viscosity and compressibility—properties that are significant in determining nuclear reactor safety.
“Our synchrotron X-ray diffraction measurements, by contrast, find a precise value r UO of 2.22±0.01 Å at 3270K, providing a new tool to test the validity of liquid UO2 models,” write Skinner and coauthor John P. Parise, professor in the department of geosciences, in the paper.
“This work finds relatively high U-U mobility in the melt, a key reference point against which structural models may now be tested,” adds Skinner.
The scientists combined laser heating, sample levitation, and synchrotron X-rays to obtain pair distribution function measurements—describing the probability of finding atom pairs with a given separation r—of hot solid and molten UO2.
Levitation of the sample was crucial. Uranium dioxide melts around 3140K, posing serious problems for traditional furnace heating methods. Most container materials (such as magnesium oxide, platinum, and zirconium dioxide, among others) melt or become chemically reactive at these extreme temperatures.
To avoid chemical reactions with the sample container, the researchers aerodynamically levitated the sample on a stream of Argon gas while it was heated with a 400W continuous CO2 laser. The levitation negated any solid contact with the sample, maintaining high chemical purity.
“In the future, we would like to investigate the atomic structure and properties of important U-containing compounds,” Skinner says. “This includes eutectic U-Zr-O, which forms in extreme accidents as the UO2 melts and reacts with its zirconium cladding.”
Collaborators contributed from the Argonne National Laboratory, the Carnegie Institute of Washington, and the private company Materials Development, Inc. in Evanston, Illinois.
The US Department of Energy and the Argonne National Laboratory supported the work.
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