Phase Equilibria in the Alkali Fluoride‐Uranium Tetrafluoride Fused Salt Systems: III, The System NaF–LiF–UF₄

Abstract

Oak Ridge National laboratory, Oak Ridge, TennesseeThe phase equilibrium diagram of the ternary system NaF-LiF-UF4 described in this paper is based on evidence from thermal analysis and from microscopic and X-ray analyses of samples obtained from thermal gradient quenching experiments. Temperature-composition relations were established through the temperature interval between the liquidus and a lower limit of about 300OC. All the stable compounds in the binary systems LiF-UF4 and NaF-UF, (including the subsolidus compound NaF .2UF4) display primary phase fields in the ternary system. No ternary compounds of NaF-LiF-UF4 occur. The following eutectics are to be found in the ternary system: 60 NaF, 21 LiF, 19 mole yo UF,, 48OOC. ; 35 NaF, 37 LiF, 28 mole % UFI, 48OOC. ; 24.3 NaF, 43.5 LiF, 32.2 mole % UFI, 445OC.;and 24.5 NaF, 29 LiF, 46.5 mole yo UP,, 602OC.The compounds 7NaF.6UF4 and 7LiF.6UF4 display limited solubility in each other. The exact limits of the miscibility gap in the 7NaF.6UFr7LiF.6UF4 join have not been determined.

Thermodynamic assessment of the LiF–NaF–ThF4–UF4 system

“…The temperatures and the exact compositions of all calculated equilibria are reported in Table 8. The phase diagram obtained in this study is in good correlation to the one suggested by Thoma et al [14] who used thermal analysis and quenching experiments for their phase equilibrium studies. The only slight disagreement appears in the middle part of the phase diagram where Thoma et al found a narrow field of primary crystallization of the NaU 2 F 9 compound whereas in our calculation this field was not reproduced.…”

“…Compound parameters had to be introduced in order to better reproduce the liquidus temperatures measured by Thoma et al [14] and the ternary invariant equilibria experimentally determined by Thoma et al [15] respectively. The obtained parameters are given below:…”