FLiBe

Molten FLiBe flowing; this sample's green tint is from dissolved uranium tetrafluoride.

FLiBe is the name of a molten salt made from a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2). It is both a nuclear reactor coolant and solvent for fertile or fissile material. It served both purposes in the Molten-Salt Reactor Experiment (MSRE) at the Oak Ridge National Laboratory.

The 2:1 molar mixture forms a stoichiometric compound, Li2[BeF4] (lithium tetrafluoroberyllate), which has a melting point of 459 °C (858 °F), a boiling point of 1,430 °C (2,610 °F), and a density of 1.94 g/cm3 (0.070 lb/cu in).

Its volumetric heat capacity, 4540 kJ/(m3·K), is similar to that of water, more than four times that of sodium, and more than 200 times that of helium at typical reactor conditions.[1] Its specific heat capacity is 2414.17 J/(kg·K), or about 60% that of water.[2] Its appearance is white to transparent, with crystalline grains in a solid state, morphing into a completely clear liquid upon melting. However, soluble fluorides such as UF4 and NiF2, can dramatically change the salt's color in both solid and liquid state. This made spectrophotometry a viable analysis tool, and it was employed extensively during the MSRE operations.[3][4][5]

The eutectic mixture is slightly greater than 50% BeF2 and has a melting point of 360 °C (680 °F).[6] This mixture was never used in practice due to the overwhelming increase in viscosity caused by the BeF2 addition in the eutectic mixture. BeF2, which behaves as a glass, is only fluid in salt mixtures containing enough molar percent of Lewis base. Lewis bases, such as the alkali fluorides, will donate fluoride ions to the beryllium, breaking the glassy bonds which increase viscosity. In FLiBe, beryllium fluoride is able to sequester two fluoride ions from two lithium fluorides in a liquid state, converting it into the tetrafluoroberyllate ion [BeF4]2−.[7]

  1. ^ http://www.ornl.gov/~webworks/cppr/y2001/pres/122842.pdf Archived 2010-01-13 at the Wayback Machine CORE PHYSICS CHARACTERISTICS AND ISSUES FOR THE ADVANCED HIGH-TEMPERATURE REACTOR (AHTR), Ingersoll, Parma, Forsberg, and Renier, ORNL and Sandia National Laboratory
  2. ^ https://inldigitallibrary.inl.gov/sites/STI/STI/5698704.pdf Engineering Database of Liquid Salt Thermophysical and Thermochemical Properties
  3. ^ Toth, L. M. (1967). Containers for Molten Fluoride Spectroscopy.
  4. ^ Phillip Young, Jack; Mamantov, Gleb; Whiting, F. L. (1967). "Simultaneous voltammetric generation of uranium(III) and spectrophotometric observation of the uranium(III)-uranium(IV) system in molten lithium fluoride-beryllum fluoride-zirconium fluoride". The Journal of Physical Chemistry. 71 (3): 782–783. doi:10.1021/j100862a055.
  5. ^ Young, J. P.; White, J. C. (1960). "Absorption Spectra of Molten Fluoride Salts. Solutions of Several Metal Ions in Molten Lithium Fluoride-Sodium Fluoride-Potassium Fluoride". Analytical Chemistry. 32 (7): 799–802. doi:10.1021/ac60163a020.
  6. ^ Williams, D. F., Toth, L. M., & Clarno, K. T. (2006). Assessment of Candidate Molten Salt Coolants for the Advanced High-Temperature Reactor (AHTR). Tech. Rep. ORNL/TM-2006/12, Oak Ridge National Laboratory.
  7. ^ Toth, L. M.; Bates, J. B.; Boyd, G. E. (1973). "Raman spectra of Be2F73- and higher polymers of beryllium fluorides in the crystalline and molten state". The Journal of Physical Chemistry. 77 (2): 216–221. doi:10.1021/j100621a014.