In thorium-based molten salt reactors (TMSRs), ²³³Pa is an important intermediate nuclide in the conversion chain of ²³²Th to ²³³U, and ²³³PaF₅ can be effectively separated from carrier salt by low-pressure distillation. However, some of the metal fluoride is vaporized along with ²³³PaF₅ simultaneously, and the evaporated fluoride may condense at different temperatures.

This study aims to investigate the condensation behavior of ²³³PaF₅ and other key metal fluorides in the FLiBeZr molten salt during the low-pressure distillation process.

The FLiBeZr molten salt containing PaF₅ and various metal fluorides was low-pressure distillated to examine the condensation behavior of ²³³PaF₅ and key metal fluorides at different temperatures. Then, the optimal temperature of key metal fluorides was evaluated and the separation characteristics between them and ²³³PaF₅ were figured out. Finally, the separation factors of ²³³PaF₅ and the other metal fluorides were calculated and compared with those of the entire low-pressure distillation process at the optimum condensation temperature of ²³³PaF₅.

The results show that the optimal condensation temperature for ²³³PaF₅ and ⁹⁵NbF₅ is within the range of 600~700 ℃, whilst it is within the range of 400~500 ℃ for ²³⁷UF₄ and ⁹⁵ZrF₄. The comparison results show that there is no significant difference in the separation factors of ⁹⁵NbF₅ and ²³³PaF₅, but the separation factors of ²³³UF₄ and ⁹⁵ZrF₄ are increased by a factor of two to 20 times.

It is confirmed that the separation coefficient is determined primarily by volatilization, but can be further improved by varying the condensation temperature.