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​By Tiffany Adams

Picture a single grain of sand. Now split that into 50 pieces. Something that small is difficult to imagine and impossible to see with the naked eye. But that fragment, roughly 10 microns or one one-hundredth of a millimeter, is the width of the sample Lingfeng He, an instrument scientist and senior research scientist at Idaho National Laboratory (INL), is analyzing in the hopes that it will aid in the advancement of generation IV molten salt reactors.

Molten salt reactors were developed at Oak Ridge National Laboratory (ORNL) in the 1950s; however, molten salt reactor research was not restarted nationally until the early 2000s. The microscopic sample He received from the Massachusetts Institute of Technology (MIT) Nuclear Reactor Laboratory is made of Hastelloy-N, a nickel-based alloy, also developed at ORNL in the midcentury. This material appears to be a good candidate for structural materials for the current prototype of molten salt reactors; however, little research has been done on the corrosion resistance of Hastelloy-N while in a reactor.

That's where He steps in.

Receiving the sample from MIT after it had been exposed to molten FLiBe salt for 1000 hours in the test reactor, He analyzed the sample using a transmission electron microscope (TEM) located in the Irradiated Materials Characterization Laboratory at INL's Materials and Fuels Complex (MFC). TEMs work by beaming electrons through a specimen to determine the material's structure and elemental makeup and thereby displaying how the material withstood exposure to extreme environments. Other TEMs with only a single energy-dispersive X-ray spectroscopy detector, the mechanism that collects the number of X-rays emitted from a specimen after electrons have been beamed through the sample, can take up to an hour to determine the elemental makeup of the material and don't always produce accurate results, He said. But MFC's TEM with four detectors delivers results in minutes. "The efficiency of this microscope is 10 times greater than others," He said. "You can measure for a lot of elements even if the concentration is very low."

In the upper left quadrant, molybdenum-rich precipitates are shown near the surface and at the grain boundaries of the sample. The other sections of the image display the concentrations of other elements such as nickel, chromium, iron and platinum. 

More specifically for this initial research, He was looking at how grain boundaries affect the corrosion behavior of Hastelloy-N. A grain boundary is the interface between two grains or crystallites. If the grain boundary is weakened in structure materials, this could affect the material's performance, and therefore the safety, efficiency and life span of a reactor. By analyzing the chemical structure of the sample, researchers have a better idea of where weaknesses may arise and can determine solutions to prevent them. During his examination of the material, He was able to see microstructural changes that may degrade the efficacy of the material; however, more research is needed to determine if this affects the ability of Hastelloy-N to be used in molten salt reactors.

Because of this, He hopes this research is only the beginning. He, along with MIT collaborators Guiqiu Zheng and David Carpenter, plan to submit a proposal to continue this work at the INL Nuclear Science User Facilities (NSUF), enabling further analysis of the entire sample, which comes in at a whopping two millimeters wide.

Through a peer-reviewed proposal process, NSUF provides external research teams cost-free access to reactor, post-irradiation examination and beamline capabilities at INL and a diverse mix of affiliated partner institutions at universities, national laboratories and industry facilities located across the country.