Electronic absorption spectra of U3+ and U4+ in molten LiCl-RbCl eutectic

In the non-aqueous reprocessing process of spent nuclear fuels by the pyroelectrochemical method, a spent fuel is dissolved into molten LiCl-KCl and NaCl-CsCl eutectics and dissolved uranium and plutonium are collected as either metal or oxide. However, the binary alkali chloride mixture with the lowest melting point is the LiCl-RbCl eutectic. In this study, electronic absorption spectra of U3+ and U4+ in molten LiCl-RbCl eutectic at various temperatures between 673 and 973 K were measured by the UV/Vis/NIR spectrophotometry. We confirmed that these spectra were similar to those in molten LiCl-KCl and NaCl-CsCl eutectics. The sensitive absorption bands of U4+ in LiCl-RbCl eutectic were found at 22000, 16500, 14900, 8600, and 4950 cm(-1). The large absorption bands of U4+ over 25000 cm(-1) increased with increasing melt temperature, while absorption peaks at 15500-4000 cm(-1) decreased. The large absorption bands of U3+ in LiCl-RbCl eutectic were observed over 14000 cm(-1). The sensitive absorption bands of U3+ at Vis/NIR region were found at 13300, 11500-11200, 9800-9400, and 8250 cm(-1), and these peaks decreased with increasing temperature.