Abstract
A molten LiF–NaF–KF alkali metal fluoride mixture having the eutectic (46.5–11.5–42 mol %)–FLiNaK composition is considered to be the most promising candidate for the use in molten salt nuclear reactors. The presence of oxygen in the melt promotes the formation of poorly soluble oxygen-containing impurities. This can lead to a local increase in the radioactivity and radiation damages, increased corrosion rates, and degradation and destruction of the structural materials of nuclear reactor. Differential scanning calorimetry is used to obtain data on the phase equilibria in the FLiNaK–La2O3 system containing up to 6.5 mol % lanthanum oxide for a temperature range of 30–700°С. For all studied compositions, caloric effects are recorded near 452 and 506°C. A temperature of 452°C corresponds to the solidus of the system. The thermal effect corresponding to 506°С is associated with the polymorphous transformation of the lanthanum oxyfluoride from the rhombohedral (R3m) crystalline modification to the cubic (Fm3m) one, namely, β-LaOF \( \to \) α-LaOF. The eutectic point of the system corresponds to a lanthanum oxide concentration of less than 0.4 mol %. The Raman spectra of the FLiNaK–La2O3 oxide–fluoride mixture with an oxide content of 0.5, 1.5, 2, 3, 3.9, and 6 mol %, which are measured at room temperature, contain two vibrational bands at 193 and 409 cm–1, which correspond to the A1g and (A1g + Eg) phonon modes of lanthanum oxide with the \({\text{D}}_{3{\text{d}}}^3\) hexagonal crystal structure. All compositions show similar temperature transformations of their spectra. No interaction between FLiNaK and La2O3 in the solid state is detected at elevated temperatures. At temperatures above the melting temperature of the FLiNaK fluoride system, vibrational bands at 187, 252, and 383 cm–1, which belong to the LaOF lanthanum oxyfluoride being in the rhombohedral (R3m) crystalline modification, are recorded in the spectrum. No vibrational bands of La2O3 are detected. During further heating to 600°C, the vibrational bands broaden significantly and overlap, and a new band appears at 486 cm–1. Such changes in the spectrum can be associated with the β-LaOF \( \to \) α-LaOF phase transition and the dissolution of LaOF in the melt. For the solidified melts, the vibrational bands of LaOF having the stoichiometric composition (space group R3m) are observed at 173, 262, and 392 cm–1 and the bands of lanthanum oxyfluoride, which has a nonstoichiometric LaO1 – xF1 + 2x composition (tetragonal structure, space group P4/nmm) in which some fluorine ions substitute for oxygen ions, exist at 226, 370, and 448 cm–1. The formation of two modifications of LaOF during melt solidification is confirmed by X-ray diffraction data. The La2O3 phase has not been found.
