Fluoride-salt cooled High-temperature Reactors (FHR) use a 2LiF-BeF₂ salt mixture (FLiBe) as a primary coolant for a graphite pebble-bed-fueled core. Tritium management is a challenge for the design of FHR reactors: tritium is produced through the transmutation of Lithium-6 in a neutron environment. The amount of tritium produced in FHR is about 1000 times larger than that for current commercial pressurize water reactors and it can readily permeate through the heat exchanger walls due to its high diffusivity in metals. Understanding the tritium transport properties from FLiBe solution to the reactor materials as a function of tritium oxidation state is essential for the design of FHR reactors.

Hydrogen isotope solubilities in FLiBe were studied with saturation and stripping experiments. The solubilities values for hydrogen isotopes in molecular and fluoride form were obtained from those experiments [1][2]. Hydrogen isotope diffusivities in FLiBe were studied with permeation experiments. It was observed that hydrogen oxidation state in the salt affects the diffusivity values. The oxidation state is known to be dependent on the salt chemistry. Electrochemistry techniques could allow the study of hydrogen isotope transport properties in FLiBe solution. Hydrogen evolution in high-temperature molten salt has been studied electrochemically for tritium recovery in fusion applications [3][4], for fluorine gas production [5] and for fundamental chemistry studies [6]. However, no electrochemical studies are available in the literature on hydrogen isotopes in FLiBe.

This paper reviews hydrogen studies in high-temperature fluoride salts and introduces the ongoing experimental research at UW Madison. The experimental work focuses on understanding the behavior of tritium in molten FLiBe using hydrogen as a surrogate for tritium. The paper puts emphasis on the experimental choices made to design the electrochemical cell to study hydrogen in FLiBe. Thermodynamic predictions of the hydrogen reduction potential are provided, and preliminary experimental results on hydrogen evolution are discussed.

References

[1] E. Field, H. Shaffer, et al., ‘The Solubilities of Hydrogen Fluoride and Deuterium Fluoride in Molten Fluorides ’.’, vol. 71, no. 10, pp. 3218–3222, (1967).

[2] A. P. Malinauskas and D. M. Richardson, ‘The Solubilities of Hydrogen, Deuterium, and Helium in Molten Li2BeF4’, Ind. Eng. Chem. Fundam., vol. 13, no. 3, (1974).

[3] V. A. Maroni, ‘Process for Recovering Tritium from Molten Lithium Metal’, (1976).

[4] H. Qiao, T. Nohira, et al., ‘Electrochemical Behavior of Hydride Ion in a LiF-NaF-KF Eutectic Melt’, Electrochemistry, vol. 6, no. 67, pp. 643–648, (1999).

[5] H. Groult, C. Simon, et al., ‘Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF – 2HF’, Fluorinated Mater. Energy Convers., vol. 6, no. 3, pp. 1–29, (2005).

[6] S. Pizzini, G. Sternheim, et al., ‘Hydrogen Evolution from KHF2 Melts at Platinum Electrodes’, Electrochem. Acta, vol. 8, no. July 1962, pp. 227–232, (1963).