Phase Equilibria in the Alkali Fluoride‐Uranium Tetrafluoride Fused Salt Systems: II, The Systems KF‐UF₄ and RbF‐UF₄

Abstract

Mixtures of alkali fluorides with uranium tetrafluoride are of potential importance in moltensalt reactors and in the reprocessing of nuclear fuels. In this paper detailed phase diagrams are presented for the binary systems KF-UF4 and RbF-UF4. Data for the determination of phase boundaries were obtained in large part by quenching after equilibration. Thermal analysis and visual observation were used as supplementary methods. Phase identifications were made by X-ray powder diffraction and by optical microscopy. In the system KF-UF4 four compounds have been identified. Two of these, 3KF.UF4 and 7KF .6UF4, melt congruently at 957°C. and at 78Q°C., respectively. The compound 2KF .UF4 melts incongruently to 3KF.UF4 and liquid at 755"C., and the compound KF.2UF4 melts incongruently to UF4 and liquid at 765°C. There are three eutectic points in the system: 15.0 mole yo UP4, 735OC.; 38.5 mole ' % UF4, 740°C.; and 54.5 mole % UF4, 735°C. The compound 2KF.UF4 decomposes during cooling to 3KF.UF4 and 7KF. 6UF4 at 608OC. Two other compounds, KF.3UF4 and KF.6UF4, have been described by another investigator. One of these, KF.3UF4, was not obtained at all in the present investigation. The other, KF.6UF4, was not obtained under equilibrium conditions and was found only in KF-UF4 mixtures that had been exposed to the atmosphere while molten. Seven solid compounds are present in the system RbF-UF4. All of these melt incongruently with the exception of 3RbF.UF4 and RbF-UF4, which melt at 995°C. and at 735"C., respectively. The incongruently melting compounds and their melting points, peritectic compositions, and melting products are: ZRbF.UF4, 81S0C., 38 mole ' % UF4,3RbF-UF4 + liquid; 7RbF.6UF4, 693"C., 44 mole % UF4, RbF.UF4 + liquid; 2RbF.3UF4, 722"C., 56.5 mole % UF4, RbF.3UF4 + liquid; RbF.-3UF4, 73OoC., 57 mole yo U F 4 , RbF.6UF4 + liquid; and RbF.6UF4, 832"C., 70.5 mole yo UF,, UF4 + liquid. The three eutectic temperatures and compositions in the system are: 10.0 mole % UF4, 71OOC.; 43.5 mole ' % UF4, 675°C.; and 55.0 mole % UF2, 714°C.

Thermodynamic Assessment of the NaF-KF-UF4 System

“…The assessed pseudo-ternary parameters (in J/mol) are: The phase equilibria temperatures experimentally obtained in this study by DSC for the NaF-UF 4 and KF-UF 4 system are presented in Tables 3 and 4 and in Figures 2 and 3, respectively. The type of equilibria were identified by the phase diagram assessment and comparison to earlier studies [15,16]. For comparison, the equilibrium temperatures calculated from the thermodynamic assessment performed in this study are shown in Tables 3 and 4 as well.…”

“…Using the experimental equilibrium data obtained in this study and the data from the literature [15,16], the pseudo-binary NaF-UF 4 and KF-UF 4 phase diagrams were thermodynamically assessed. They are presented in Figures 2 and 3, respectively.…”

The crystal structure of α-KTh6F25, a polymorph of CsU6F25

“…Zachariasen (1948) published lattice parameters and the space group (P63/mmc) for KTh6F2s and KU6Fzs, although Asker, Segnit & Wylie (1952) in their work on molten salt system KF-ThF4 and Thoma, Insley, Landau, Friedman & Grimes (1958) in their work on system KF-UF4 did not find an equilibrium compound at KF.6ThF4 or at KF. 6UF4.…”

“…Equilibrium relationships of MF. 6AF4 compounds were first established by Thoma et al (1958) in their investigation of the RbF-UF4 system and later by Thoma & Carleton (1961) in their study of system CsF-ThF4. Unpublished work by Thoma and co-workers has established the existence of the remaining complex fluorides (Table 1).…”