Evaporation behavior of ²³³Pa in FLiBeZr molten salt

In thorium molten salt reactors (TMSR), ²³³Pa is an important intermediate nuclide in the conversion chain of ²³²Th to ²³³U, its timely separation from the fuel salt is critically important for both the thorium-uranium (Th-U) fuel cycle and the neutron economy of the reactor. In this study, the evaporation behavior of ²³³Pa in the FLiBeZr molten salt was investigated during a vacuum distillation process. The separation characteristics between ²³³Pa and the major components of the fuel (salt and fission products) were evaluated in a calculation of the separation factors between these components. It was found that ²³³Pa⁵⁺ evaporated more readily than ²³³Pa⁴⁺ and the other components of the fuel, the relatively low temperature and medium pressure were much more beneficial to the separation of ²³³Pa⁵⁺ from FLiBeZr salt in the evaporation process, with the maximum value of the separation factor achieving more than 10². Results of distillation experiments also show that increasing the temperature and decreasing the ambient pressure enhances the separation between ²³³Pa⁵⁺ and most of the fission product nuclides due to the ²³³Pa⁵⁺ volatility more strongly depending on the process conditions. These results will be utilized to design a concept for a process for ²³³Pa separation from the fuel of a molten salt reactor.