Structural determination of neptunium species in aqueous solutions by EXAFS and quantum chemical calculations

Ikeda, Atsushi  ; Hennig, C.*; Tsushima, Satoru*; Rossberg, A.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

Neptunium (93Np) is one of the most problematic nuclides in the nuclear fuel reprocessing process and the following radioactive waste disposal because of its chemical similarity to the fissile nuclides of uranium (U) and plutonium (Pu). Proper understanding of the behavior of Np in the reprocessing process or in the migration process on the geological disposal of radioactive wastes requires vast fundamental information about the chemical properties of Np in solutions. In the present study, Np solution samples with different oxidation states are electrochemically prepared in aqueous perchlorate, nitrate, and carbonate solutions, and the complex structure of Np species in the sample solutions are determined by EXAFS spectroscopy and DFT calculations. The obtained results reveal the structural difference between different Np oxidation states, as well as the different coordination behavior in each solution system.