Molecular Structure and Phase Equilibria of Molten Fluoride Salt with and without Dissolved Cesium: FLiNaK-CsF (5 mol %)

David Sprouster, Guiqiu Zheng, Shao Chun Lee, Daniel Olds, Can Agca, Joanna McFarlane, Y Z, Boris Khaykovich

Research output: Contribution to journalArticlepeer-review


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 multicomponent 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 CsLiF2compound, with a lower melting temperature than that of the FLiNaK mixture and a much higher temperature than that predicted for CsLiF2by computational thermodynamics. We also demonstrated a novel sample environment that we developed to X-ray measurements of molten fluoride or fuel salts.

Original languageEnglish (US)
Pages (from-to)8067-8074
Number of pages8
JournalACS Applied Energy Materials
Issue number7
StatePublished - Jul 25 2022


  • Cs
  • fission products
  • FLiNaK
  • melting
  • molten fluoride
  • molten salt
  • phase diagram
  • structure function
  • total diffraction

ASJC Scopus subject areas

  • Chemical Engineering (miscellaneous)
  • Energy Engineering and Power Technology
  • Electrochemistry
  • Materials Chemistry
  • Electrical and Electronic Engineering


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