Versatile Cell Design for Molten Fluoride Salt Spectroscopy: Investigating Metal-Ion Speciation in High-Temperature eutectic LiF-NaF-KF

Abstract Fluoride-based molten salts are candidate coolants and fuel salts in certain Generation IV molten salt reactors (MSRs). Effective monitoring of these salts is crucial, because salt composition plays a key role in reactor safety and efficiency. Optical probes offer a potentially powerful method of remotely monitoring fission and corrosion products within these salts, providing valuable data for reactor process control. However, performing spectroscopic measurements of fluoride-based salts is particularly challenging due to their highly corrosive nature, which can degrade many common optical materials. In this work, we present a novel optical cell design that enables spectroscopic measurements of transition-metal analytes in LiF-NaF-KF eutectic salt (i.e., FLiNaK). This innovative cell design overcomes the challenges posed by the corrosive nature of the salts, allowing for accurate and consistent spectroscopic analysis. This work reports temperature dependent absorption measurements for Co2+, Ni2+, and Cr3+ analytes, which are common corrosion products originating from structural alloys in MSR systems. Absorption spectra were used to understand interactions of these analytes with FLiNaK, particularly ligand field coordination. The analysis of absorption spectra was complemented by structural analyses using ab initio molecular dynamics (AIMD) simulations, providing deeper insights into the behavior of the analytes in FLiNaK. Our findings indicate that the analytes studied in this work exist in octahedral or near-octahedral coordination states that remain stable across the temperature range of 500-600°C.