A team of University of Wisconsin-Madison engineers has developed a new technique that allows them, for the first time, to see how materials corrode directly within a high-temperature flowing molten salt system.
Researchers can use the technique to acquire crucial data that can inform the design of sustainable energy systems, including next-generation nuclear reactors, thermal energy storage, and concentrated solar power plants. The team detailed its method in a paper published April 10, 2024, in the journal Nature Communications.
Adrien Couet
Molten, or liquid, salt is attractive for solar energy because it can store the sun’s heat for later use. Advanced nuclear reactors could use molten salt as a coolant, making them potentially smaller, safer and more economical than current nuclear power plants.
However, there’s a major challenge with molten salt: It’s highly corrosive, so metals in contact with the salt will wear away.
“The metal will dissolve into the salt, and a big question is how fast the material will dissolve,” says Adrien Couet, an associate professor of nuclear engineering and engineering physics at the University of Wisconsin-Madison, who led the research. “Will the amount of material lost be about the thickness of a human hair in one year, or more like the thickness of a finger? There hasn’t been a way to measure the rate at which the material dissolves in a complex molten salt system with much precision.”
In addition, researchers don’t know where that dissolved material will end up in the system. That’s important information, because the material could end up traveling into the heat exchanger and clogging it, affecting the system’s performance.
Couet says it has been very difficult to answer these questions due to challenges with monitoring a dynamic, high-temperature environment with flowing molten salt. And because exposure to air can make the salt sticky, similar to what happens with table salt, it needs to be studied in an oxygen-free environment.
To overcome these challenges, the researchers’ technique harnesses gamma ray spectrometry. First, they irradiated a piece of stainless steel by blasting it with high-energy particles, which produces radionuclides in the material. Then, the researchers welded the steel piece to their molten salt test loop. As the radionuclides decay, they emit gamma rays, which are detected by spectrometers positioned in multiple locations around the test loop. Since gamma rays have specific energy signatures, the researchers can use the spectrometers to precisely track the material corrosion and transport throughout the system.
Using their technique, the researchers measured the recession rate of the stainless steel and also discovered that the material mostly corroded in a manner that wasn’t expected from more classic corrosion studies without in-situ trackers.
“Applying this technique to such a high-temperature system had never been done before, and we didn’t know if it would work,” Couet says. “It was very exciting to show that it works and that researchers can use it to acquire important data that we couldn’t get before. This data is really useful for helping designers plan energy systems using molten salt that can accommodate the material corrosion, as well as for designing and testing materials that corrode less.”
Couet says strong teamwork among the collaborators was key to the project’s success, since the project posed big logistical challenges. The researchers needed to complete various steps of the process at several different facilities on the UW-Madison campus, including in labs equipped with the necessary radiation shielding. And everything, including the experimental testing, needed to be completed in less than a week to ensure that the team could detect the gamma rays for a useful amount of time. “This was extremely difficult to pull off, and I’m very proud of how the team was able to work together to overcome these logistical hurdles,” Couet says.
This research was funded by the U.S. Department of Energy NEUP award number DE-NE0008904.
UW-Madison co-authors on the Nature Communications paper include Yafei Wang, Hongliang Zhang; Jonathan Engle, an associate professor of medical physics and radiology; Aeli Olson; and Kumar Sridharan, Grainger Professor of nuclear engineering and engineering physics and materials science and engineering. Additional co-authors include Cody Falconer (MSMSE ’20, PhDMSE ’22), Brian Kelleher (MSNE ’12, PhDNE ’15) and Ivan Mitchell of TerraPower.
Featured image caption: Paper co-author Cody Falconer (MSMSE ’20, PhDMSE ’22), who now works at TerraPower, an industry collaborator on this project, is pictured with the molten salt loop in the laboratory with the gamma ray detectors. Photo courtesy of Adrien Couet.