(146e) Chemistry Control and Hydrogen Speciation in Molten Fluoride Salts

Molten fluoride salt may be used as a fuel solvent or coolant in advanced fission reactors or may serve as a tritium breeding blanket in fusion reactors. Characterizing the salt’s chemistry involves measurement of its redox potential (the oxidation state in which its species exist dictates which elements are thermodynamically favored as fluorides or as reduced species) and its fluoroacidity (the degree of association/dissociation of the ions describes how solutes are incorporated into the salt). Evaluating how the salt chemistry evolves over the course of a reactor’s lifetime is of engineering concern in order to avoid corrosion, detect leaks, and ensure thermophysical properties are appropriately maintained for sufficient cooling and safe operation.

In both fission and fusion reactors which use lithium fluoride, tritium (hydrogen-3) may be generated via neutron activation of lithium-6. (It may also exist as a fission product in fission reactors.) Management of tritium inventories is a key engineering concern due to its radioactivity and ability to diffuse through metals. The species in which tritium exists in a molten fluoride system is dependent on its redox potential. Hypothetically, speciation would also be dependent on the fluoroacidity of the system, but the relationship between oxidation state, species, and solubility of tritium in molten fluorides is still unknown. In this presentation, experimental investigations of hydrogen, serving as a surrogate for tritium, in molten 2LiF-BeF₂ will be shared within the context of chemistry control and tritium management in advanced nuclear reactors. Hydrogen species have been introduced to 2LiF-BeF₂ via additions of LiH and via electrochemical desorption of H₂ gas from graphite. Cyclic and square wave voltammograms have been collected and will be analyzed to hypothesize the electroactive hydrogen species dissolved in the melt. Possible hydrogen species and their relationship to the redox and acidity conditions in the melt will be discussed.

Overall, these fundamental studies may inform electrochemical sensor design for chemistry control systems. Additionally, a more complete understanding of speciation is relevant to techniques for the separations or removal of tritium. A deeper understanding of the relationship and utility of speciation, structure, and electrochemistry in molten salts is also relevant to the development of pyroprocessing techniques and other technologies which employ molten salts. This talk will include a presentation of some cross-cutting studies of molten salt speciation across various technologies.