Structural investigation of thorium in molten lithium-calcium fluoride mixtures for salt treatment process in molten salt reactor

摘要

X-ray absorption fine structure (XAFS) measurements on thorium fluoride in molten lithium-calcium fluoride mixtures and molecular dynamics (MD) simulation of zirconium and yttrium fluoride in molten lithium-calcium fluoride mixtures have been carried out. In the molten state, coordination number of thorium (N_i) and inter ionic distances between thorium and fluorine in the first neighbor (r_i) are nearly constant in all mixtures. However the fluctuation factors (Debye-Waller factor (σ~2) and C_3 cumulant) increase until xCaF_2 = 0.17 and decrease by addition of excess CaF_2. It means that the local structure around Th~(4+) is disordered until xCaF_2 = 0.17 and stabilized over xCaF_2 = 0.17. The variation of fluctuation factors is related to the number density of F~ in ThF_4 mixtures and the stability of local structure around Th~(4+) increases with decreasing the number density of F~ in ThF_4 mixtures. This tendency is common to those in the ZrF_4 and YF_3 mixtures. However, in the case of YF3 mixtures, the local structure around Y~(3) becomes disordered until xCaF_2 = 0.40 and it becomes stabilized by addition of excess CaF_2. The difference between ThF_4 mixtures and YF3 mixtures is related to the difference of Coulumbic interaction between Th_(4+)-F~- and Y~(3+)--F~-. Therefore, the variation of local structure around cation is related to not only number density of F~ in molten salts but also the Coulumbic interaction between cation and anion.