Researchers at the Department of Energy's Oak Ridge National Laboratory (ORNL), Pacific Northwest National Laboratory (PNNL) and Washington State University (WSU) joined forces to study the complex dynamics of low-water liquids that are a problem for nuclear waste processing at federal nuclear waste clean up sites. They were working on the chemical processes in legacy tank waste such as that found in underground tanks at the Hanford nuclear reservation. The presence of unpredictable low-water or “water-in-salt” solutions make such wastes difficult to process.
Hsiu-Wen Wang is a geochemist at the ORNL who led the study. He said “Remarkably, these electrolyte solutions are able to maintain a liquid state at very high salt concentrations; but as a result, they do not move freely like normal, more dilute liquids.”
Water-in-salt solutions like the ones in legacy tanks have very high viscosities that can vary between liquid and almost solid, glass-like states. This makes these substances very difficult to control and process. Inside nuclear waste tanks, these caustic solutions can clog up pipes and pumps making it difficult to remove them for processing.
Improving understanding of the fundamental chemistry of this unusual type of liquids could have broad applications for stabilizing these solutions. This could be of great benefit in improving cleanup strategies for legacy tank waste that were accumulated between the 1940 and 1980s. At the DoE Hanford nuclear reservation, billions of gallons of contaminated liquids were produced during forty years of nuclear weapons development.
Andrew Stack works at the ORNL’s Chemical Sciences Division. He says that "Remediation of the waste is complicated by the unique chemical properties in this type of complex, highly concentrated environment, with radioactivity creating additional challenges. By working to understand what is happening on an atomic level in complex solutions, we can better predict their properties and their reactivity, and that may lead to improved strategies to process nuclear waste.”
The team of researches that carried out this study were supported by IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), a DOE Energy Frontier Research Center. During the study, they worked with a nonradioactive synthetic brine of sodium-hydroxide-aluminate (Na+OH–/Al(OH)4–). The team worked with a higher concentration of this substance than the relatively lower concentration of the same chemical found in Hanford waste tanks. The tanks also contain other electrolyte that behave in a similar way.
In a glass of water at room temperature, the molecules of water move around in picoseconds. (A picosecond is one trillionth of a second.) In water-in-salt solutions, the motion of molecules can be take ten to one hundred picoseconds depending on the concentration of the salts. Basically, the water molecules are embedded in ions undergoing a complex soup of interconnected motions. Wang says, “For one ion to move, a lot of other molecules and ions have to move, which makes the dynamics interesting. many different types of simultaneous motions—some fast and some slow—are taking place at the atomic level.”
In order to study these different speeds of atomic motion, the researchers made use of two DoE Office of Science User Facilities, the Spallation Neutron Source at ORNL and the Environmental Molecular Sciences Laboratory at PNNL. At the ORNL, the researchers studied quasi-elastic neutron scattering (QENS) and, at the PNNL, they conducted nuclear magnetic resonance (NMR) spectroscopy.
Trent Graham carried out the NMR spectroscopy. He said, “NMR spectroscopy reveals the movement of atoms over many milliseconds, while QENS captures atomic motion over picoseconds. In combination, these two techniques provide complementary data at multiple time scales, which is critical to understanding the complex motions of ions in the solutions we are studying.”
The researchers used neutrons to collect unique information that could not be provided by other techniques. Eugene Mamontov is a BASIS instrument scientist who carried out the QENS part of the study. He said, “Neutrons are well suited to water-based systems analysis, since they provide a favorable contrast for weak atoms, like hydrogen, not easily seen by X-rays; and QENS is a specific technique involving the use of neutrons to correlate spatial and temporal information about atoms. Atoms change positions as water moves, and QENS can tell you not only the rate or how quickly the jumps occur but also at what distance and how these details correspond to the chemical structure.”