New findings are shedding light on the essential science that can enhance the efficiency of nuclear energy. Researchers have now explored the unique chemical dynamics and structure of high-temperature liquid uranium trichloride salt, which holds promise as a nuclear fuel source for future reactors.
At the forefront of molten salt reactor technology is the Department of Energy’s Oak Ridge National Laboratory (ORNL), where researchers are also engaged in the fundamental science needed to pave the way for more efficient nuclear energy solutions. Recently, in a paper published in the Journal of the American Chemical Society, scientists documented for the first time the distinctive chemical dynamics and structural properties of high-temperature liquid uranium trichloride (UCl3) salt, a potential candidate for next-generation nuclear fuels.
“This marks an important first step towards creating reliable predictive models for future reactor designs,” stated Santanu Roy from ORNL, who co-led the study. “Enhancing our capability to anticipate and calculate microscopic behaviors is essential for effective design, and having trustworthy data is crucial for developing improved models.”
Molten salt reactors have long been considered capable of producing safe and cost-effective nuclear energy, as demonstrated by successful prototyping experiments at ORNL during the 1960s. More recently, as decarbonization efforts gain momentum globally, numerous countries are revitalizing initiatives to make these nuclear reactors widely accessible.
The optimal design of these upcoming reactors hinges on a deep understanding of how liquid fuel salts behave, setting them apart from conventional reactors that use solid uranium dioxide pellets. Analyzing the chemical, structural, and dynamic behaviors of these salts at the atomic level presents challenges, particularly with radioactive elements in the actinide series, such as uranium, since these salts only melt at extraordinarily high temperatures and display intricate ion coordination chemistry.
This study, a collaborative effort between ORNL, Argonne National Laboratory, and the University of South Carolina, employed a mix of computational methods and the Spallation Neutron Source (SNS)—a DOE Office of Science facility at ORNL—to investigate the chemical bonding and atomic dynamics of UCl3 while in a molten state.
The SNS is recognized as one of the world’s most powerful neutron sources, facilitating advanced neutron scattering studies that provide insights into the positions, movements, and magnetic attributes of materials. When a neutron beam is directed at a sample, many neutrons pass through the material, while some collide with atomic nuclei and “scatter” at an angle, similar to billiard balls colliding on a pool table.
By using specialized detectors, scientists can tally scattered neutrons, evaluate their energies, and determine the angles at which they scatter, allowing them to map out their final locations. This process grants scientists a deeper understanding of various materials, including liquid crystals, superconducting ceramics, proteins, plastics, metals, and metallic glass magnets.
Every year, numerous researchers utilize the SNS at ORNL to enhance the quality of products ranging from smartphones to pharmaceuticals—yet not everyone studies a radioactive salt at temperatures reaching 900 degrees Celsius, equivalent to that of molten lava. After implementing stringent safety measures and specialized containment protocols in collaboration with SNS beamline scientists, the team accomplished something unprecedented: measuring the chemical bond lengths of molten UCl3 and observing its intriguing behavior as it transformed into a liquid state.
“Since I joined ORNL as a postdoc, I have been researching actinides and uranium,” remarked Alex Ivanov, another co-leader of the study, “but I never anticipated that we could explore the molten state and uncover such fascinating chemistry.”
The team discovered that, on average, as the substance transitioned to a liquid form, the average distances of the bonds connecting uranium and chlorine actually decreased—opposite to the general expectation that heat causes expansion while cold leads to contraction, a principle often valid in chemistry and everyday life. Even more intriguingly, among the bonded atom pairs, bond lengths varied irregularly, stretching in an oscillating manner, sometimes elongating far beyond those in solid UCl3, while at other times tightening to remarkably short lengths. Distinct dynamics unfolded at incredibly rapid speeds within the liquid state.
“This is an unexplored domain of chemistry that sheds light on the fundamental atomic structure of actinides under extreme conditions,” Ivanov stated.
The bonding data showed unexpected complexity. When UCl3 achieved its nearest bond lengths, it momentarily exhibited characteristics of more covalent bonding as opposed to its usual ionic nature, oscillating in and out of this state at speeds surpassing one trillionth of a second.
This fleeting covalent bonding observation, while brief and cyclical, aids in clarifying some discrepancies in earlier research regarding the behavior of molten UCl3. These discoveries, alongside the broader implications of the study, may advance both experimental and computational efforts in designing future reactors.
Furthermore, these findings enhance our fundamental knowledge of actinide salts, potentially offering solutions to issues related to nuclear waste and pyroprocessing, as well as other current or prospective applications involving these elements.
The research was part of the DOE’s Molten Salts in Extreme Environments Energy Frontier Research Center (MSEE EFRC), led by Brookhaven National Laboratory. It was primarily conducted at the SNS and also utilized two other DOE Office of Science user facilities: the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory, and the Advanced Photon Source at Argonne National Laboratory. Additionally, it utilized resources from ORNL’s Compute and Data Environment for Science (CADES).
