Irradiation may slow corrosion of alloys in molten salt, a team of Oak Ridge National Laboratory (ORNL) (Oak Ridge, Tennessee, USA) scientists has found in preliminary tests.1
The researchers put samples of stainless steel (SS) and a nickel-based alloy in capsules in the Ohio State University Research Reactor in Columbus and exposed them to a molten chloride salt with simultaneous neutron irradiation.
Initial Research Findings
In their experiment, steel that was irradiated in molten salt showed less corrosion than samples that went through the same environmental treatment but without irradiation.
Though a small sample size was used in the study, they believe further research could support their findings and inform future molten salt reactor designs.
“These types of irradiation experiments help ORNL generate data directly impacting material selections for reactor designers,” says Dianne Ezell, group leader for the nuclear and extreme environment measurement group at ORNL. “This could be a significant step in overcoming one of the largest hurdles for molten salt reactors.”
How Irradiation Affects Corrosion
In their study, the irradiation appeared to have a greater effect on the SS sample exposed in wet salt. When compared to the unirradiated SS samples, the sample exposed in wet salt exhibited a greater degree of attack, since wet salt is known to be more aggressive. The sample irradiated in wet salt, however, had approximately the same degree of attack.
If this effect is real, according to the researchers, it suggests a radiolysis effect in the salt that lessens the effect of oxidizing impurities. According to the ORNL team, that is among several potential methods by which irradiation could lead to corrosion.
“First, radiation-enhanced diffusion may affect the diffusion of species to the salt–material interface,” the research team explains in a paper in Nuclear Engineering and Technology on its findings. “This is especially important if diffusion of Cr [chromium] is the rate-controlling mechanism. In salt-facing materials, Cr depletion is the primary mode of corrosion.”
Second, radiation may create oxidizing or reducing species in the salt melt through radiolysis, according to the researchers. Finally, they say that electronic excitation of material surfaces may affect the rate of chemical reactions between the salt melt and the material.
Additional Research Needed
While the results from this irradiation demonstrated a slowing effect of corrosion, the team cautioned that the sample size is small and more studies are needed to confirm its validity.
“Molten salts are very hygroscopic, and corrosion rates are highly sensitive to salt purity, so larger data sets are necessary to convincingly attribute variations in corrosion rate to anything other than salt purity variation,” the research team writes. “For this work, all capsules were prepared together, and with the same batch of salt, to minimize variations. Still, unpredictable errors during handling still occur, and cannot be ruled out in this experiment. Second, while great effort was made to use the same temperature profile for the irradiated and unirradiated experiments [including using an identical irradiation vessel and heater for the unirradiated exposure], it remains a possibility that a variance in temperature profile accounts for the observed differences in corrosion.
Moving forward, ORNL is proposing further irradiations with increased sample quantities, multi-salt studies, and integrated in situ monitoring of the salts. The research team notes that future irradiation experiments could be performed at other research reactors, which may enable longer irradiation periods or higher flux fields.
Source: ORNL, www.ornl.gov.
1 “Nuclear–Slowing Corrosion,” ORNL News Desk, Oct. 6, 2020, https://www.ornl.gov/news/nuclear-slowing-corrosion (Dec. 14, 2020).