Evaporation behavior of 233Pa in FLiBeZr molten salt

Abstract

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.