Hydrogen Diffusion and Trapping in Fluoride Salt Melts and Graphite Electrodes
Authors: Scarlat Raluca , Carotti Francesco , Wu Huali , Macdonald Digby D. 
Published: February 2020

Molten salt processes were developed decades ago as a promising technology for material processing and industrial process heat applications. This technology is still considered applicable for thermal energy storage and power production. Liquid salts can indeed be considered legitimate candidates for several heat transfer applications, thanks to favorable physical properties such as high boiling point, low vapor pressures, large specific heat, and large thermal conductivity. In particular, fluoride-based molten salts are considered as a coolant for several nuclear reactor advanced design owing to favorable neutronic cross sections and reduced corrosion of structural materials.

FHR, Fluoride-salt cooled High-temperature Reactors, use flibe salt (2LiF-BeF2) to cool a graphite pebble-bed-fueled core. A detailed knowledge of the chemistry of fluoride melts is important for an efficient corrosion control and reliable FHR reactor design. The corrosion mechanisms in a molten salt system are different from aqueous systems; molten salt chemistry prevents the formation of a thermodynamically stable passivation layer on structural metals, therefore the corrosion control has to be achieved through the control of the redox potential of the fluoride salt. Solubilities and diffusivities of metals, rare earths, fission products and tritium in the fluoride melt and within graphite framework, are of direct interest for the FHR design community. Ionic coordination in the liquid and gas phase also needs to be fundamentally understood in order to address the above mentioned problems. This paper will provide a review of the electrochemical techniques and spectroscopy methods used in the past to study fluoride salt systems. Furthermore, state of the art of techniques used in other high temperature molten salt environments and low temperature ionic liquids will be reviewed.

Tritium management is the main design challenge for the FHR reactors. Tritium is produced through transmutation of Lithium-6 in a neutron environment and might be released into the atmosphere through the heat exchanger walls. However, in FHR reactors, the graphite pebble bed and graphite reflector may be the ultimate sink for tritium. Goal of the ongoing research at UW Madison is to define tritium diffusion and absorption into graphite, using hydrogen as a surrogate. This paper will address theoretical electrochemical techniques used in aqueous system to study absorption of hydrogen into metals (Potentiostatic Double-Step [1]) and also experimental techniques (Electrochemical Impedance Spectroscopy [2]) applicable to the same issue. These techniques will be applied initially to graphite/water systems and later to the graphite/flibe system. The paper will present preliminary results and report the challenges pertaining to experimental setup with fluoride salt systems, due to the high melting temperatures, the highly corrosive nature of the salt, in addition tothe toxicity of Beryllium, main component of flibe salt.

REFERENCES:

[1] B. Pound, G. Wright, et al., ‘A potentiostatic double-step method for measuring hydrogen atom diffusion and trapping in metal electrodes—II. Experimental’, Acta Metall., vol. 35, no. I, pp. 263–270, (1987).

[2] D. D. Macdonald and M.C.H. McKubre. “Impedance Measurements in Electrochemical Systems”. Chapter 2 in Modern Aspects of Electrochemistry. 14, 61(1982).