Abstract
The ruthenium fission product can challenge the PUREX process for recycling of spent nuclear fuel for several reasons. One is due to diverse ruthenium(III) nitrosyl complexes, which contaminate solvent extraction cycles and impede a simple and clean separation of reusable uranium and plutonium products. This thesis presents a speciation analysis of ruthenium in nitric acid solution and organic solvents composed of TBP, MOEHA, TBU or TODGA extractants using coupled techniques as Raman, ESI-MS, FTIR, EXAFS and 15N-NMR spectroscopy. Identified ruthenium species in the organic phase were linked to measured distribution ratios in order to better explain the behavior of ruthenium in solvent extraction processes.The results show a formation of ruthenium nitrite complexes in presence of enlarged nitrous acid concentrations, which change the extraction characteristics of ruthenium in TBP/nitric acid extraction systems. TBP can coordinate in the inner axial ligand sphere of ruthenium nitrite complexes, which cannot be observed for complexes without nitrite ligands. In absence of nitrous acid, TBP and TODGA solvents mainly extract ruthenium in form of a mer-trinitrate complex from 3 M nitric acid, whereas MOEHA and TBU solvents mainly extract a protonated tetranitrato complex. All applied solvents show abnormal high ruthenium distribution ratios during scrubs with 2 M nitric acid, known as the ruthenium retention. The phenomenon of the ruthenium retention during scrubs was assigned to an equatorial inner sphere coordination of the extractant. In order to better decontaminate PUREX extraction phases from ruthenium, nitrate ligands can be either substituted by nitrites, or scrubs can be performed at temperature >40°C. Both options, also combined, can help to improve the solvent purification in current and future nuclear solvent extraction processes.