Phosphate precipitation in molten chlorides is considered as a possible method for removing rare earth and alkaline earth fission product elements from technological melts at a final stage of pyrochemical reprocessing of spent nuclear fuels. Phosphates can be incorporated into a glass matrix in a high level waste vitrification process in preparation for a long term storage or disposal. There are a number of studies considering the formation of rare earth phosphates in the melts of technological interest, i.e. LiCl–KCl, NaCl–KCl, and NaCl–CsCl mixtures, but some uncertainty still remains concerning the conditions when different types of phosphates are formed. While the reaction in LiCl–KCl melts always produces normal orthophosphates (REPO₄), double alkali metal-rare earth phosphates, e.g. M₃RE(PO₄)₂ and M₃RE₂(PO₄)₃, can be formed in the melts not containing LiCl. The aim of the present study was investigating the effect of the initial phosphate-to-rare earth mole ratio on the outcome of the reaction.

The experiments were performed in the melts based on the equimolar mixture of sodium and potassium chlorides at 750 ^oC. The rare earth elements included Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu (heavier lanthanides not representing fission products were included for completeness). Sodium orthophosphate was used as a source of PO₄^3– ions and the initial PO₃^3– : RE³⁺ mole ratio was varied from 0.3–0.5 to 10. Formation of the phosphates was studied in the melts containing individual rare earth elements as well as a mixture of rare earths in the proportion simulating their abundance in the thermal neutron reactor spent fuel.

The experiments were performed under static conditions and the parameters analyzed included degree of RE precipitation, phase composition of the precipitate and size of particles in the solid phase. Analysis of the experimental results showed that at PO₃^3– : RE³⁺ mole ratios below five only normal REPO₄ phosphates were formed, while at the ratios above 8 double Na₃RE(PO₄)₂ and/or K₃RE(PO₄)₂ double phosphates were produced. Particle size of the rare earth phosphates varied from 0.1 to hundredths of microns and increasing the initial PO₃^3– : RE³⁺ mole ratio resulted in increasing particle size. Complete conversion of RECl₃ to phosphate required 2–5 times molar excess of the phosphate to the rare earth element.