Development of new ligands for selective separation of americium

Safe long-term management of spent nuclear fuel is a societal issue and its necessity is independent of the operation of future nuclear power plants. The design footprint (and construction cost) of geological repositories is highly dependent on the residual heat emission of the conditioned waste, causing the need for partitioning and transmutation of highly active waste. Together with plutonium, the small percentage of minor actinides, in particular americium, are the main cause of the residual heat load and long-term radiotoxicity of nuclear waste. The separation of uranium and plutonium from spent nuclear fuel is already being carried out on an industrial scale via a mature technology called the Plutonium Uranium Reduction Extraction (PUREX) process, which enables the recycling of those elements into new nuclear fuel. In a next step, it would be highly desired to convert the minor actinides into shorter lived isotopes by means of high flux neutron irradiation. Unfortunately, in the presence of the neutron absorbing lanthanides transmutation would be inefficient. In addition, the fabrication of transmutation targets, fuel transport and reactor operation would be technologically very challenging in the presence of curium isotopes due to their high neutron emission. As the transmutation of curium would have only negligible impact on the repository footprint, only americium should be separated for transmutation. Therefore, An(III)/Ln(III) and Am(III)/Cm(III) partitioning are an essential link in the management of spent nuclear fuel. The separation of trivalent minor actinides (Am(III) and Cm(III)) from trivalent lanthanides is challenging due to their similar chemical behavior. Whereas common O-donor extracting agents are not capable of separating these two groups of elements, sulfur and soft nitrogen donors are more promising. Sulfur and phosphorus-containing ligands, however, are less preferred as they leave behind corrosive residues after incineration, generating additional radioactive waste streams. The ‘CHON’ principle proposes the use of chemicals that are only composed of C, H, O and N atoms, which only lead to gaseous products upon incineration. During the past two decades, bis-1,2,4-triazinyl-2,2’-bipyridine or -1,10-phenanthroline ligands (CyMe4-BTBP and CyMe4-BTPhen) were identified as superior extractants with high selectivities for the actinides, and became the reference ligand for the European SANEX (Selective ActiNide EXtraction) process. It was postulated that the α-effect of the adjacent nitrogens of the triazine ring causes an increased interaction with the more diffuse 5f orbitals of the actinides compared to the 4f orbitals of the lanthanides. In the more recent innovative SANEX (i-SANEX) approach, both actinides and lanthanides are first co-extracted from the PUREX raffinate to an organic phase using a hard-donor TODGA (N,N,N’,N’-tetra-n-octyl diglycolamide), after which the aim is to selectively back-extract the actinides to the aqueous phase. The selective back-extraction of americium was exemplified by the AmSel (Americium Selective Extraction) process. An effective sulfonated bistriazinylbipyridine or phenanthroline (TS-BTBP and TS-BTPhen) was used for this, but this extractant violates the CHON principle. Despite the favorable properties of triazine-based ligands, limitations such as the large number of extraction stages required, poor solubility and non-CHON compliance have still not all been addressed. A hydrophilic CHON bis-1,2,3-triazolylphenanthroline (BTrzPhen) with promising selectivity for Am(III) over Cm(III) was reported. Unfortunately, parameters such as the kinetics, radiolytic stability or the influence of other lanthanides have not been studied to date. It is clear that opportunities remain to vary the heterocyclic moiety (and its substituents) at positions 2 and 9 (or 6 and 6’) of the phenanthroline (or bipyridine) core, which might lead to the development of new potent CHON soft N-donor ligands. Am(III) selective hydrophilic CHON extractants in particular have only been poorly studied, and such ligands with process-suitable properties can be regarded as a current ‘terra incognita’ in the field of spent nuclear waste management. A process-suitable ligand should combine a high selectivity for Am(III) in the presence a representative mixture of fission products with favorable extraction kinetics, radiolytic and hydrolytic stability, while the solubility in acidic medium needs to be sufficiently high. Aiming to find a breakthrough in the selective extraction of Am(III) from spent nuclear fuel, the objective of this project is to develop novel tetradentate nitrogen soft-donor ligands. The core structure of these ligands will comprise the well-known 1,10-phenanthroline or 2,2’-bipyridine, which have already proven their potency in previously studied extraction systems. The 1,10-phenanthroline is preferred due to its cis-locked conformation, improving extraction kinetics. However, its rigid structure also results in a decreased solubility compared to bipyridines, and will have to be addressed by decorating the heterocyclic moieties with the appropriate substituents. Importantly, synthetic approaches to functionalize phenanthroline and bipyridine with heterocycles are underexplored. This is also the case for the incorporation of hydrophilic substituents, which are hypothesized to give rise to water-soluble extractants for use in an i-SANEX and/or AmSel-type of process. The Dehaen group at KU Leuven is specialized in synthetic organic and heterocyclic chemistry, and the combined expertise of SCK CEN and KU Leuven will help us to develop new ligands, of which the heterocyclic (triazole, tetrazole or triazine) substituents will be tailored to obtain the desired soft-donor character. Although the focus of this project will be on water-soluble extractants, both lipophilic and hydrophilic substituents will be incorporated into the heterocyclic moieties in order to obtain ligands that can be used in various process designs. If a class of ligands appears to be problematic with respect to the solubility, the established methods can also be applied to 2,2'-bipyridine.