The molecular structure and phase equilibria of molten fluoride salt with and without dissolved cesium: FLiNaK-CsF (5 mol%)

Abstract

We demonstrate effects of Cs ions on the melting transition and molecular structure of molten FLiNaK (a eutectic mixture of LiF-NaF-KF). FLiNaK is a commonly studied multi-component model system, which represents the physical and chemical behavior of fluoride salts for nuclear energy applications. Dissolution of nuclear fuels leads to the formation of fission products directly in the molten salt. Cs is one of the most important fission products, due to its relative abundance, long half life, and potential environmental and health effects. Here, we determine the molecular structure and phase equilibria of dissolved Cs in FLiNaK by a combination of X-ray diffraction, X-ray total scattering, ab initio molecular dynamics calculations, and computational thermodynamics. Although Cs ions have a relatively large size, we did not find significant evidence that they disrupt the existing molecular structure of the liquid. We found good agreement between our simulated and measured structure factors, and calculated that the coordination number of Cs is close to 10. X-ray diffraction in combination with computational thermodynamics demonstrates that upon freezing Cs ions are captured into a CsLiF2 compound, with a lower melting temperature than the FLiNaK mixture and much higher than that predicted for CsLiF2 by computational thermodynamics. We also demonstrated a novel sample environment that we developed to X-ray measurements of molten fluoride or fuel salts.

The effect of interfacial phenomena on gas solubility measurements in molten salts

“…Noble metal fission products have unstable fluorides and chlorides and will form dispersed solids in the salt, perhaps serving as nucleation loci for noble gas bubble generation. Corrosion products, such as chromium, generated from the reaction of stainless steel with salts, can accumulate in solution and form ternary compounds (Sprouster et al, 2022). Certain fission products, such as tellurium, have complicated chemistry.…”

Progress on Near-Term Tasks for the Development of Advanced Reactor Mechanistic Source Term Modeling and Simulation Tools

“…The relationship between the structure of actinides in molten salt mixtures and thermal properties was recently reviewed, with considerable discussion that may be useful in future MSTDB-TC database development for actinides not yet included [57]. The effect of adding 5 mol% CsF to FLiNaK salt was also recently investigated with X-ray diffraction, ab initio and neural network molecular dynamics, and CALPHAD modeling [58]. The thermodynamic properties of Ce in LiCl-KCl-CsCl eutectic melt was recently investigated experimentally, providing measured values for the Gibbs free energy of formation of CeCl3 in the eutectic melt [59].…”