Evaporation of LiCl–KCl–LaCl₃–CeCl₃–NdCl₃–UCl₃ molten mixtures components at reduced pressures

The present paper provides a brief review of the available data on the saturated vapor pressure and relative volatility of various individual chlorides (LiCl, KCl, NdCl₃, CeCl₃, LaCl₃, UCl₃) being present in the processes of pyrochemical reprocessing of spent nuclear fuel (SNF). It is shown that alkali metal chlorides are the most volatile. The volatility of rare earth metal and uranium trichlorides in the temperature range of 500–1000°C is 2–5 orders of magnitude lower. High-temperature vacuum distillation of components of molten chloride electrolytes based on the LiCl–KCl eutectic, placed in nickel boats containing uranium and rare earth metal trichlorides, was carried out under various conditions: temperature range 700–1000°C, exposure time 0.4–4 h, vacuum degree 2·10^-3–2 Pa, UCl₃ and REE trichlorides concentrations 0.25–1.7 mol. % and 0.13–0.7 mol. % (in total), respectively. The redistribution of salt components between the melt and vapor condensates was determined. It follows from the experimental data obtained in this study that alkali metal chlorides (LiCl, KCl) and REE chlorides (NdCl₃, CeCl₃, LaCl₃) can be fairly quickly (in 2–4 h) and completely distilled from a multicomponent salt electrolyte at the temperatures up to 850–900°С; their concentrations in the electrolyte by the end of distillation decrease by 2.5–4 orders of magnitude (for more volatile alkali chlorides – to a greater extent). Under the same conditions, the content of uranium compounds (in the form of UCl₃) can be reduced by no more than an order of magnitude, apparently due to incongruent (occurring with decomposition) evaporation of trichloride. Increasing the temperature above 900°С has little effect on the completeness of distillation for all components of molten mixtures. Conclusions have been made about the relative volatility of the components of molten salt mixtures (chlorides of alkali metals, REE and uranium). Optimal distillation modes have been selected. The dependences found may be useful for developing promising SNF processing schemes using salt distillation.