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

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.

(1) Economic Impact
The results of this project would be of great help to chemical engineers tasked with designing safe and efficient actinide separation processes for future spent nuclear fuel treatment, thus helping to close the nuclear fuel cycle, reduce stockpile inventory build-up and increase the safety and thus public acceptance of nuclear energy. It should also have a positive economic impact by enabling UK industrial companies and research bodies (eg: National Nuclear Laboratory, Dr. Richard Stainsby) to develop internationally competitive solvent extraction processes and actinide separation methods using the ligands produced. See letter of support.
Technocomm Ltd. (Dr. David Moody) is an SME specialising in the development and commercialisation of new technology and products (www.technocomm.co.uk). They currently market a range of ligands to the nuclear industry, and acquired the license to IP developed by the PI ("Synthesis of Ligands for Use in Actinide Extraction", WO2011077081A1) for an improved method of synthesizing ligands for selective actinide extraction. A Non-Disclosure Agreement is already in place between Northumbria and Technocomm Ltd., and a Collaborative Research Agreement will be set-up at the beginning of the project which will provide a route to commercial exploitation. After appropriate patent filing by Northumbria University, it is expected that promising ligands arising from this project will be added to Technocomm's existing portfolio that it markets to the global nuclear industry. See letter of support.

(2) Academic Impact
The results of this research project will be disseminated to the research community through publications at key international conferences and in high impact, peer-reviewed journals. The project is naturally cross-disciplinary and encompasses the diverse disciplines of organic synthesis, solvent extraction, hydrometallurgy, nuclear fission, coordination chemistry and materials science. To leverage benefit across these diverse communities, the PI will publish in highly regarded multidisciplinary journals such as Journal of the American Chemical Society, Angewandte Chemie, Chemical Science and Chemical Communications. The more application oriented work will be targeted at eg: Inorganic Chemistry and Dalton Transactions. The PI has a publication track record in many of these journals. In addition to original research articles, the PI has previously written an Invited Review (Synlett, 2011), a Forum Paper in themed issue (Inorganic Chemistry, 2013) and an Invited Bookchapter (Strategies and Tactics in Organic Synthesis, 2013). Research publications arising from this project will also be deposited in Northumbria Research Link; Northumbria's green open access repository, to maximise impact and make the results freely available to all. Two public lectures will also be presented to members of the local community to highlight the benefits of nuclear fuel recycling and sustainable nuclear energy to a general audience. The PI will also transfer the knowledge, skills and techniques gained during this project to other academics in the Department of Applied Sciences at Northumbria University, and to our project collaborators and beneficiaries.

(3) People Pipeline
The project is expected to provide excellent training opportunities to the Postdoctoral Research Assistant (PDRA) who will gain valuable multidisciplinary experience in the fields of synthetic and materials chemistry, solvent extraction chemistry and actinide science. It is expected this will ensure that the PDRA is well positioned for a future research career either within academia, or within the UK nuclear industry which is presently experiencing a resurgence in fortunes. Undergraduate research student involvement in the project (final year BSc and MChem projects) at Northumbria will promote interest in this research thereby helping to ensure a continuing pipeline of future PhD students.