'Better Than PUREX': Towards the sustainable recovery of uranium from spent nuclear fuel

A sustainable fully-closed nuclear fuel cycle is required in order to fully utilise the limited amount of uranium fuel on Earth. The current process used for the recycling of nuclear fuel, PUREX, can suffer from undesired extraction, poor phase-control, and secondary waste issues due to the use of the solvating extractant tri-n-butyl phosphate (TBP) rendering reprocessing economically unfavourable. The work in this thesis looks to replace the TBP solvent with novel monoamide-based alternatives using N,N-dihexyl octanamide (DHOA) to reduce or eliminate phosphorous in reprocessing flowsheets to reduce waste and required process steps. Novel ligand mixtures of monoamide with a ‘secondary’ ligand are studied to offset the reported low uranium loading of standard monoamide solvents by either a phase modifying effect or through the production of a more hydrophobic dual-ligand complex. Tested secondary ligands are diamides, diglycolamides, phosphates and phosphine oxides. Initial spectroscopic tests indicate that monoamide-diamide mixtures do not form a sufficient amount of a dual-ligand complex, but monoamide-diglycolamide solvents likely form two types of dual-ligand complexes. Monoamide-phosphate and monoamide-phosphine oxide solvents were largely inconclusive. Solvent
extraction tests showed that monoamide-phosphine oxide solvents are unlikely to form mixed-ligand complexes, but Jobs plots indicate that DHOA-TBP and DHOA-TODGA (N,N,N’,N’-tetraoctyl diglycolamide) solvents do form enhanced extraction systems over standard DHOA or TBP solvents. Case study solvents of 20 mol% DHOA – 80 mol% and 80 mol% DHOA – 20 mol% TODGA were further studied. These case study solvents were found to be comparable with PUREX solvents within the operating acidity range of PUREX (4-6 M nitric acid) and were more stable solvents than their single-ligand counterparts. Experimental extraction mechanism analysis and scenario modelling with slope analysis revealed that DHOA-TBP solvents are likely enhanced via a solvent phase modifying effect by the presence of both ligands, whereas the DHOA-TODGA solvents are likely enhanced through the formation of a ternary complex as predicted from earlier fundamental spectroscopic studies. Metal loading models used to predict solvent performance and contact stages for a specified mass transfer fit well except at high metal loadings where nitric acid extraction effects interfere with the model assumptions. Main future work for this thesis includes an assessment of competitive extraction (which have been completed pending analysis) and more robust metal uptake models.