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

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 (Ni) and inter ionic distances between thorium and fluorine in the first neighbor (ri) are nearly constant in all mixtures. However the fluctuation factors (Debye–Waller factor (σ2) and C3 cumulant) increase until xCaF2 = 0.17 and decrease by addition of excess CaF2. It means that the local structure around Th4+ is disordered until xCaF2 = 0.17 and stabilized over xCaF2 = 0.17. The variation of fluctuation factors is related to the number density of F− in ThF4 mixtures and the stability of local structure around Th4+ increases with decreasing the number density of F− in ThF4 mixtures. This tendency is common to those in the ZrF4 and YF3 mixtures. However, in the case of YF3 mixtures, the local structure around Y3+ becomes disordered until xCaF2 = 0.40 and it becomes stabilized by addition of excess CaF2. The difference between ThF4 mixtures and YF3 mixtures is related to the difference of Coulumbic interaction between Th4+–F− and Y3+–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.

► Coordination number of ThF4 mixtures decreased from c.a 9 to 7 and inter ionic distances between thorium and fluorine in the first neighbor also decreased from c.a. 2.34 to 2.30 Å by melting.
► The local structure around Th4+ is disordered until xCaF2 = 0.17 and stabilized over xCaF2.= 0.17. However, in the case of YF3 mixtures, the local structure around Y3+ becomes disordered until xCaF2 = 0.40 and it becomes stabilized by addition of excess CaF2.
► The variation of local structure around cation is related number density of F in molten salts and the Coulumbic interaction between cation and anion.