Molten Salt Chemistry

We all know the secret to good cooking is to use good ingredients. The same goes for preparation and maintenance of a molten salt reactor coolant. The salts in their purest form are the most chemically stable and least likely to cause corrosion of high-temperature systems. In general, halide salts such as fluorides and chlorides are hygroscopic and tricky to purify. The introduction of salt impurities such as moisture or corrosion products has the potential to make salts more corrosive. UW-Madison has the technology to make and purify fluoride and chlorides salts for its corrosion research and has developed analytical chemistry and electrochemical methods to characterize its salts.

A comparison of LiF-BeF2 (66-34 wt.%) made at the University of Wisconsin-Madison. From left to right: re-purified MSRE coolant salt, salt melted in open atmosphere from raw components, salt melted in a glove box from raw components [1].

Salt characterization is an essential part of molten salt corrosion research. The group aims to develop in situ electrochemical tools that can be utilized in both the laboratory and plant setting. Techniques for measuring the oxidizing nature of the molten salt help predict the tendency of a salt chemistry to corrode various materials [2]. Voltammetry techniques, such as cyclic voltammetry, identify and measure the levels of various corrosion products in molten salt[1], [3]. These techniques enable more responsive salt chemistry monitoring and control for MSRs.

Cyclic voltammograms of molten FLiBe salt samples with various impurity additions at 650˚C. Scan rate was 80 mV/sec. Peak 2a and 2c correspond to the oxidation and reduction of chromium and chromium fluoride. Peak 4a and 4c correspond to the oxidation and reduction of iron and iron fluoride [3].

Future Work

Purification and characterization of FLiNaK and FLiBe.
Evaluation of new experimental alloys using potentiodynamic polarization techniques.
Pre and post characterization of salts used in corrosion experimentation.
In situ electrochemical salt characterization during corrosion.

References

[1] C. W. Forsberg, P. F. Peterson, K. Sridharan, L. Hu, M. Fratoni, and A. K. Prinja, “Integrated FHR technology development: Tritium management, materials testing, salt chemistry control, thermal hydraulics and neutronics, associated benchmarking and commercial basis,” Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Center for Advanced Nuclear Energy Systems (CANES); Univ. of California, Berkeley, CA (United States); Univ. of Wisconsin, Madison, WI (United States); Univ. of New Mexico, Albuquerque, NM (United States), DOE-MIT-0008285, Oct. 2018.

[2] B. Kelleher, K. Dolan, M. Anderson, and K. Sridharan, “Observed Redox Potential Range of Li 2 BeF 4 Using a Dynamic Reference Electrode,” Nucl. Technol., vol. 195, no. 3, pp. 239–252, 2016.

[3] W. H. Doniger and K. Sridharan, “Application of voltammetry for quantitative analysis of chromium in molten 2LiF-BeF2 (FLiBe) salt,” J. Electroanal. Chem., vol. 838, pp. 73–81, Apr. 2019.