In a groundbreaking study published in the Journal of the American Chemical Society, researchers have detailed, for the first time, the unique chemical dynamics and structure of high-temperature liquid uranium trichloride (UCl3) salt, a material considered a potential nuclear fuel for next-generation reactors. This research marks a critical advancement in understanding the fundamental properties of materials crucial for the future of nuclear energy.
“This is a first critical step in enabling good predictive models for the design of future reactors,” explained Santanu Roy, a researcher at Oak Ridge National Laboratory (ORNL) and co-leader of the study. “A better ability to predict and calculate the microscopic behaviors is critical to design, and reliable data helps develop better models.”
Molten salt reactors, a technology anticipated to deliver safe and affordable nuclear energy, have been the focus of research for decades. ORNL’s early experiments in the 1960s demonstrated the potential of this technology. However, the global push for decarbonization has renewed interest in these reactors, with many countries investing in research and development to bring them to the forefront of nuclear energy solutions.
The successful design of these reactors hinges on a deep understanding of the behavior of liquid fuel salts, which sets them apart from conventional nuclear reactors that use solid uranium dioxide pellets. The atomic-level chemical, structural, and dynamical properties of these salts, particularly when involving radioactive elements like uranium, are challenging to study due to their complex ion-ion coordination chemistry and the extreme temperatures required to melt these salts.
The research, conducted through a collaboration between ORNL, Argonne National Laboratory, and the University of South Carolina, combined advanced computational methods with experimental data obtained from ORNL’s Spallation Neutron Source (SNS). The SNS, a Department of Energy Office of Science user facility, is one of the most powerful neutron sources globally, enabling state-of-the-art neutron scattering studies that reveal detailed information about the atomic structure and behavior of materials.
Neutron scattering works by directing a beam of neutrons at a sample material. While many neutrons pass through the sample, some collide with atomic nuclei, scattering in different directions. By analyzing the scattered neutrons—measuring their energies, scattering angles, and final positions—scientists can deduce intricate details about the material’s structure and dynamics.
Although the SNS is frequently used for research on various materials, studying a radioactive salt like UCl3 at temperatures approaching 900 degrees Celsius presented unique challenges. After implementing rigorous safety measures and developing special containment strategies, the research team successfully measured the chemical bond lengths in molten UCl3, observing its behavior as it transitioned to the liquid state—a feat never before accomplished.
“I’ve been studying actinides and uranium since I joined ORNL as a postdoc,” said Alex Ivanov, co-leader of the study, “but I never expected that we could go to the molten state and find fascinating chemistry.”
One of the most surprising findings was that, contrary to conventional expectations, the average bond length between uranium and chlorine atoms in UCl3 actually decreased as the material melted. Typically, heating causes materials to expand, not contract, but this study revealed a more complex behavior. The bond lengths within the molten UCl3 oscillated, stretching and contracting rapidly, with some bonds extending to lengths much larger than in the solid state, while others contracted to extremely short distances.
“This is an uncharted part of chemistry and reveals the fundamental atomic structure of actinides under extreme conditions,” Ivanov noted.
The study also uncovered unexpected complexity in the bonding behavior of molten UCl3. At its shortest bond length, the material displayed characteristics more typical of covalent bonding, in contrast to its usual ionic nature. This oscillation between covalent and ionic states occurred at speeds faster than one-trillionth of a second, providing new insights into the chemical behavior of this material under extreme conditions.
These findings help explain previous inconsistencies in studies of molten UCl3 and offer valuable data that could improve both experimental and computational approaches in nuclear reactor design. Beyond reactor development, the research enhances the fundamental understanding of actinide salts, which could have broader applications in areas such as nuclear waste management and pyroprocessing.
Source: Oak Ridge National Laboratory
