Solubility of Beryllium Metal in Molten 2LiF-BeF2
Authors: Ratso Sander , Williams Haley , Gardner Nathanael , Scarlat Raluca O. 
Published: December 2024

Fluoride salt-cooled High-temperature Reactors (FHRs) are a subtype of molten salt reactors (MSRs) in which a molten salt fills the role of a low pressure coolant to coated particle fuel. Another use for molten salt is in fusion reactors as a fusion blanket/coolant. The use of a molten salt as the coolant or blanket offers several safety advantages when compared to the current water-based designs due to the low vapor pressure of molten salts and their boiling point being far above the maximum coolant temperature. It also allows to integrate passive safety systems and to use a high-temperature power cycle. Careful control over the chemistry of the molten salt coolant must be kept in FHRs to reduce the corrosion of structural materials. The primary coolant candidate for FHRs is FLiBe (66.7%LiF-33.3%BeF2), which is not corrosive in its pure form, but impurities in the salt (commonly oxygen or water) and tritium fluoride generated from neutron irradiation of the salt can create an oxidizing environment.1 This in turn leads to structural corrosion, which degrades the system and and further modifies the properties of the salt. Because of this, redox control of the coolant salt is a key issue in FHRs.2 One of the methods for redox control is to introduce excess Be as a redox control agent to the salt, which reacts with tritium fluoride and decreases the redox potential. However, it has been found that when Be metal is contacted with FLiBe containing HF, it does not simply react with the HF, but is also dissolved in the salt. Another method is to add LiH to the FLiBe mixture as a hydrogen donor, which will induce the formation of dissolved metallic Be. This dissolution is not well understood, but could prove to be an elegant way for introducing a reducing agent to the system as the salt could be left in contact with Be metal in one part of the reactor to saturate the salt everywhere or be periodically saturated using LiH. The solubility of Be plays an important role here as it sets the limits for this saturation. Phases with visually different colours at different contents of Be in FLiBe have been reported in the literature, but no comprehensive analysis has been conducted to understand the solubility of metallic Be in FLiBe.3

In this study, we present data on these different phases of dissolved Be in FLiBe using LiH additions to understand the solubility of metallic Be. The crystal structure of different phases will be characterized using powder X-ray diffraction, the elemental composition via inductively coupled plasma mass spectrometry and melting points and latent heats via differential scanning calorimetry. Cyclic voltammetry will be used to determine activity coefficient of BeF2 in with different amounts of Be dissolved in it and electrical conductivities of different phases of the melt will be measured via electrochemical impedance spectroscopy. Additionally, neutron diffraction PDF data of Li2BeF4 with and without 1 mol% of metallic Be dissolved inside will be shown. A better understanding of the characteristics and speciation of dissolved Be in FLiBe will allow for closer redox control of the coolant melt and a longer lifetime for structural elements in FHR coolant loops.

References

(1) Vergari, L.; Scarlat, R. O.; Hayes, R. D.; Fratoni, M. The Corrosion Effects of Neutron Activation of 2LiF-BeF2 (FLiBe). Nuclear Materials and Energy
2023, 34, 101289. https://doi.org/10.1016/j.nme.2022.101289.

(2) Zhang, J.; Forsberg, C. W.; Simpson, M. F.; Guo, S.; Lam, S. T.; Scarlat, R. O.; Carotti, F.; Chan, K. J.; Singh, P. M.; Doniger, W.; Sridharan, K.; Keiser, J. R. Redox Potential Control in Molten Salt Systems for Corrosion Mitigation. Corrosion Science
2018, 144, 44–53. https://doi.org/10.1016/j.corsci.2018.08.035.

(3) Hara, M.; Hatano, Y.; Simpson, M. F.; Smolik, G. R.; Sharp, J. P.; Oya, Y.; Okuno, K.; Nishikawa, M.; Terai, T.; Tanaka, S.; Anderl, R. A.; Petti, D. A.; Sze, D.-K. Interactions between Molten Flibe and Metallic Be. Fusion Engineering and Design
2006, 81 (1), 561–566. https://doi.org/10.1016/j.fusengdes.2005.06.372.