The Department of Energy’s Oak Ridge National Laboratory is a world leader in molten salt reactor technology development — and its researchers moreover perform the elemental science crucial to enable a future where nuclear energy becomes more efficient. In a recent paper published within the Journal of the American Chemical Society, researchers have documented for the primary time the unique chemistry dynamics and structure of high-temperature liquid uranium trichloride (UCl3) salt, a possible nuclear fuel source for next-generation reactors.
“It 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 greater ability to predict and calculate the microscopic behaviors is critical to design, and reliable data help develop higher models.”
For many years, molten salt reactors have been expected to own the capability to provide secure and reasonably priced nuclear energy, with ORNL prototyping experiments within the Sixties successfully demonstrating the technology. Recently, as decarbonization has change into an increasing priority all over the world, many countries have re-energized efforts to make such nuclear reactors available for broad use.
Ideal system design for these future reactors relies on an understanding of the behavior of the liquid fuel salts that distinguish them from typical nuclear reactors that use solid uranium dioxide pellets. The chemical, structural and dynamical behavior of those fuel salts on the atomic level are difficult to grasp, especially after they involve radioactive elements corresponding to the actinide series — to which uranium belongs — because these salts only melt at extremely high temperatures and exhibit complex, exotic ion-ion coordination chemistry.
The research, a collaboration amongst ORNL, Argonne National Laboratory and the University of South Carolina, used a mix of computational approaches and an ORNL-based DOE Office of Science user facility, the Spallation Neutron Source, or SNS, to check the chemical bonding and atomic dynamics of UCl3in the molten state.
The SNS is one in every of the brightest neutron sources on this planet, and it allows scientists to perform state-of-the-art neutron scattering studies, which reveal details concerning the positions, motions and magnetic properties of materials. When a beam of neutrons is aimed toward a sample, many neutrons will go through the fabric, but some interact directly with atomic nuclei and “bounce” away at an angle, like colliding balls in a game of pool.
Using special detectors, scientists count scattered neutrons, measure their energies and the angles at which they scatter, and map their final positions. This makes it possible for scientists to glean details concerning the nature of materials starting from liquid crystals to superconducting ceramics, from proteins to plastics, and from metals to metallic glass magnets.
Every year, a whole bunch of scientists use ORNL’s SNS for research that ultimately improves the standard of products from cell phones to pharmaceuticals — but not all of them need to check a radioactive salt at 900 degrees Celsius, which is as hot as volcanic lava. After rigorous safety precautions and special containment developed in coordination with SNS beamline scientists, the team was in a position to do something nobody has done before: measure the chemical bond lengths of molten UCl3and witness its surprising behavior because it reached the molten state.
“I have been studying actinides and uranium since I joined ORNL as a postdoc,” said Alex Ivanov, 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 space of the bonds holding the uranium and chlorine together actually shrunk because the substance became liquid — contrary to the everyday expectation that heat expands and cold contracts, which is commonly true in chemistry and life. More interestingly, amongst the varied bonded atom pairs, the bonds were of inconsistent size, and so they stretched in an oscillating pattern, sometimes achieving bond lengths much larger than in solid UCl3 but additionally tightening to extremely short bond lengths. Different dynamics, occurring at ultra-fast speed, were evident throughout the liquid.
“That is an uncharted a part of chemistry and divulges the elemental atomic structure of actinides under extreme conditions,” said Ivanov.
The bonding data were also surprisingly complex. When the UCl3reached its tightest and shortest bond length, it briefly caused the bond to seem more covalent, as an alternative of its typical ionic nature, again oscillating out and in of this state at extremely fast speeds — lower than one trillionth of a second.
This observed period of an apparent covalent bonding, while transient and cyclical, helps explain some inconsistencies in historical studies describing the behavior of molten UCl3. These findings, together with the broader results of the study, may help improve each experimental and computational approaches to the design of future reactors.
Furthermore, these results improve fundamental understanding of actinide salts, which could also be useful in tackling challenges with nuclear waste, pyroprocessing. and other current or future applications involving this series of elements.
The research was a part of DOE’s Molten Salts in Extreme Environments Energy Frontier Research Center, or MSEE EFRC, led by Brookhaven National Laboratory. The research was primarily conducted on the SNS and likewise used two other DOE Office of Science user facilities: Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing Center and Argonne National Laboratory’s Advanced Photon Source. The research also leveraged resources from ORNL’s Compute and Data Environment for Science, or CADES.
