Phase Equilibria in the Alkali Fluoride-Uranium Tetrafluoride Fused Salt Systems: I, The Systems LiF-UF4 and NaF-UF4

Detailed phase diagrams, and a description of the techniques which were employed, are presented for the systems LiF‐UF4 and NaF‐UF4. The phase diagrams are complete with respect to the stable phase boundaries from the liquidus to 300°C. and are accurate to within 5°C. with respect to temperatures. The data were obtained from thermal analysis, equilibration followed by quenching, differential thermal analysis, and visual observation methods closely coupled with examination by X‐ray diffraction techniques and optical identification of the cooled melts from all these procedures. The system LiF‐UF4 contains only incongruently melting compounds with the following peritectic liquid compositions and temperatures: 4LiF. UF4, 26 mole % UF4, 500° C.; 7LiF.6UF4, 40 mole % UF4, 610°C.; and LiF‐4UF4, 57 mole % UF4, 775° C. A single eutectic occurs at 27 mole % UF4 and 490°C. The compound 4LiF.UF4 has a lower limit of stability at 470° C., whereas 3LiF.UF4 is metastable. Five compounds occur in the system NaF‐UF4 Congruent melting points were found for 3NaF.UF4 at 629°C. and 7NaF.6UF4 at 718°C. Peritectic liquids occur at 32.5 mole % UF4 and 648° C. for 2NaF UF4 and at 37 mole % UF4 and 673° C. for 5NaF.3UF4. NaF‐2UF4 has a subsolidus upper limit of stability at 660° C. Eutectic liquids appear at 21.5 mole % UF4 and 618°C., 28 mole % UF4 and 623°C., and 56 mole % UF4 and 680°C. The compound 3NaF.UF4 has a transition at 528°C. and is unstable below 497°C.; the compound 5NaF‐3UF4 is not stable below 63O° C. The only stable modification of 2NaF.UF4 above 273°C. is a hexagonal form not previously reported. The “α‐2NaF.UF4” phase reported by Zachariasen apparently is 5NaF.3UF4, and the γ‐2NaF. UF4 which he described is metastable.