Need to know: Studying the atomic-scale behavior of molten fuel salts is challenging. Not only does it require precautions to handle radioactive elements, it also requires extremely high temperatures—“as hot as volcanic lava”—to melt the salt, according to ORNL. Once melted, the salt exhibits exotic ion-ion coordination chemistry that is challenging to model.
“This is a first critical step in enabling good predictive models for the design of future reactors,” said ORNL’s Santanu Roy, who co-led the study. “A better ability to predict and calculate the microscopic behaviors is critical to design, and reliable data help develop better models.”
After a special containment was developed with SNS beamline scientists, the team was able to measure the chemical bond lengths of molten UCl3 and witness its behavior as it reached the molten state.
Fascinating chemistry: “I’ve been studying actinides and uranium since I joined ORNL as a postdoc,” said Alex Ivanov, an R&D scientist at ORNL, who also co-led the study, “but I never expected that we could go to the molten state and find fascinating chemistry.”
What they found was that, on average, the distance of the bonds between uranium and chlorine atoms shrank as the substance became liquid, in contrast to many common substances—water, for one—in which those bonds expand with heat and contract with cold. “More interestingly, among the various bonded atom pairs, the bonds were of inconsistent size, and they stretched in an oscillating pattern, sometimes achieving bond lengths much larger than in solid UCl3 but also tightening to extremely short bond lengths,” according to ORNL.
“This is an uncharted part of chemistry and reveals the fundamental atomic structure of actinides under extreme conditions,” said Ivanov.
What’s more, the researchers found that when the bonds within UCl3 molecules reached their tightest and shortest lengths, they briefly appeared to be covalent rather than ionic and oscillated between those states at “extremely fast speeds” of less than one trillionth of a second.
That apparent covalent bonding, while brief and cyclical, “helps explain some inconsistencies in historical studies describing the behavior of molten UCl3,” according to ORNL.
The findings may help improve both experimental and computational approaches to the design of future reactors, ORNL said, and could be useful in other applications of actinide salt chemistry, including spent nuclear fuel processing.
Making use of facilities: The SNS is a Department of Energy Office of Science user facility and one of the brightest neutron sources in the world, allowing scientists to perform state-of-the-art neutron scattering studies to detect the positions, motions, and magnetic properties of materials by measuring the energies, angles, and positions of neutrons scattered off the atomic nuclei of the material being studied.
The research was part of the DOE’s Molten Salts in Extreme Environments Energy Frontier Research Center, or MSEE EFRC, led by Brookhaven National Laboratory. The research was primarily conducted at the SNS and used two other DOE-SC user facilities: Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing Center and Argonne’s Advanced Photon Source. The research also leveraged resources from ORNL’s Compute and Data Environment for Science, or CADES.
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