Innovation in the Design of Improved Actinide Selective Extractants Suitable for use in Large Scale Spent Nuclear Fuel Reprocessing

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A key challenge that needs to be overcome in spent nuclear fuel reprocessing is to separate the radiotoxic minor actinides from the non-radiotoxic lanthanides. Extracting and separating these elements from the lanthanides after the 'Plutonium and URanium EXtraction' (PUREX) process will reduce the long lived radiotoxicity of the remaining spent fuel and enable these elements to be rendered safe by transmutation (high energy neutron bombardment) or used as fuel in advanced GenerationIV reactors. This separation is critical because the lanthanides have higher neutron capture radii than the actinides and would thus prevent the neutron chain reaction that would render the actinides safe. Three families of N-donor ligand containing the 1,2,4-triazine moiety have been developed that can extract and separate the minor actinides from the lanthanides with very high selectivity. These are known as bistriazinylpyridines (BTPs), bistriazinylbipyridines (BTBPs) and bistriazinylphenanthrolines (BTPhens). However, whilst these ligands fulfill many of the requirements for use in a 'Selective ActiNide EXtraction' (SANEX) separation process, two main limitations remain to be overcome prior to industrial scale use:1. Their limited solubility range in solvents acceptable to the nuclear industry (typically less than 10 mM). 2. Their relatively slow rates of extraction.To the best of our knowledge, there has been no research on the exploration of other N-donor ligand families containing the crucial 1,2,4-triazine moiety. Furthermore, the application of an innovative approach to improving the above limitations has not previously been carried out. This research proposal aims to investigate new families of N-donor ligand based on the 1,2,4-triazine moiety for the selective extraction of actinides over lanthanides. This will advance the fields of spent nuclear fuel reprocessing and radioactive waste management, with the potential for the ligands to be used by the nuclear industry in future large-scale actinide-lanthanide separation processes that will underpin the current resurgence in nuclear energy. If successful, this proposed research would expand the scope of currently available ligands for use in the processing of spent nuclear fuel and provide one or more lead structures with improved solvent extraction properties suitable for future industrial use. The novelty of the proposed approach lies in (i) Synthesis of new ligand families that have never been explored previously for the separation of actinides from lanthanides. (ii) The application of an innovative approach to ligand design that addresses the two most critical shortcomings of existing 1,2,4-triazine based ligands; low solubilities and slow rates of extraction. This innovative approach is inspired by those aspects of drug discovery that specifically relate to solubility and polarity (which can influence surface concentration at an organic/aqueous interface and hence extraction kinetics). The research is timely because, after several decades of decline in the UK and worldwide, nuclear energy is experiencing a resurgence in fortunes (the 'nuclear renaissance') and several countries (including the UK) have announced plans for new reactor build. This means the volume of spent nuclear fuel will continue to increase in future. However, no process is currently available to render this material safe for geological disposal. Thus it is even more important to develop a feasible selective actinide extraction process in the near future. The project includes two academic collaborators who have extensive expertise in performing solvent extraction measurements on ligands using radionuclides, and developing novel selective actinide extraction processes. The project also includes one industrial collaborator who has expertise in the commercialization and exploitation of new technology and products, and who already markets a range of ligands to the global nuclear industry.