Electronic Structure of U (III) and U (IV) Ions in a LiCl-KCl eutectic melt at 450 °C

We have measured the electronic absorption spectra of the U (III) and U (IV) ions in a LiCl-KCl eutectic melt at 450 °C to understand their spectrochemical behavior in the context of pyrochemical process of spent nuclear fuel. The UV-VIS spectra of the U (III) ion consist of two main peaks in the range of 400-600 nm which are attributable to the 5f3-5f26d1 transitions. The 5f-6 d transitions of U (III) ion occurred at lower energy as low as 400-600 nm range while that of Nd (III) and U (IV) ion occur at much higher energy over the normal UV range. For the U (III) ion, the intensities of 5f-5f transitions are about one order of magnitude larger than the 4f-4f transitions in lanthanides. And the 5f-6 d transitions are about one order of magnitude smaller than in the 4f-5 d transitions in the lanthanide ions. The electronic energy levels of the U (III), U (IV) and Nd (III) ions are estimated to understand their electronic transitions. The features of the electronic absorption spectra of U (III), Nd (III), U (IV) ions are reasonably explainable by their electronic structure. In-situ monitoring of the U (III) ↔ U (VI) redox reaction in a high temperature LiCl-KCl showed that the redox reaction is reversible.