Ruthenium nitrosyl structure in solvent extraction systems: A comparison of tributyl phosphate (TBP), tetrabutyl urea (TBU), N-methyl, N-octyl ethylhexanamide (MOEHA) and N, N, N’, N’ tetraoctyl diglycolamide (TODGA)

In the course of nuclear fuel treatment, solvent extraction cycles separate uranium and plutonium from fission products and minor actinides. During these steps, small amounts of the fission product ruthenium may follow the uranium and plutonium products. Herein, we show by Raman spectroscopy how ruthenium complexes in the aqueous phase differ as a function of the nitric acid concentration. Furthermore, we compare ruthenium extraction by the industrially used solvent tri-n-butylphosphate to other extractants such as tetrabutyl urea, N-methyl, N-octyl ethylhexanamide, and N,N,N',N'-tetraoctyl diglycolamide. Analysis by inductively coupled plasma atomic emission spectroscopy demonstrates that all of these solvents have different ruthenium distribution ratios. In contrast, similar ruthenium complexes were identified by Fourier transform infrared and extended X-ray absorption fine structure spectroscopy in all extractions from 4 M nitric acid. Hence, different distribution ratios of ruthenium are not due to different ruthenium complexes. By comparing ruthenium extraction with nitric acid and water extraction, we show a linear correlation between ruthenium and water concentration in the organic phase. This suggests an interaction between the solvent and water ligands during ruthenium extraction. To limit the ruthenium coextraction in nuclear fuel treatments, we suggest further investigation of solvents with low water coextraction.