Radiolytic stability of BTBP-, BTPhen- and DGA-based ligands for the selective actinide separation by solvent extraction

In the search for a deep geological repository for high-level radioactive waste, in particular for irradiated fuel from the operation of conventional power reactors, the thermal power of the waste to be emplaced and the long half-life of some nuclides (actinides and fission products) play a decisive role in addition to the waste quantity. In Germany, the direct disposal of nuclear waste in deep geological rock formations is being pursued. Internationally, however, the concept of partitioning and transmutation (P&T), i.e., the separation and irradiation and thus conversion of long-lived into short-lived radionuclides using fast reactor systems, is being researched as a supplement to and preparatory step for final disposal. Here, in addition to the PUREX (Plutonium-Uranium Reduction Extraction) process already used industrially for the elements uranium and plutonium, the separation of transplutonium and other repository-relevant elements (e.g., ${}^{129}$I, ${}^{99}$Tc) is of high interest in research activities. In the long term (after decay of the short-lived fission products), these elements dominate in particular the thermal power of the waste, and thus the required size of the repository to be established. In the present work, the two substance classes of diglycolamides (DGAs) and N-donor ligands of the BTBP/BTPhen type are investigated with respect to their stability against radiolytic degradation. DGA ligands cannot distinguish between trivalent actinides (An(III)) and lanthanides (Ln(III)), while selective separation of the trivalent actinides can be achieved by using N-donor ligands. The investigated compounds of the N-donor ligands differ by a modification of the molecular backbone, in which the bipyridine functionality of CyMe${}_4$BTBP was replaced by a phenanthroline functionality in the CyMe${}_4$BTPhen molecule. For DGA-type ligands, either the addition of one or two methyl-groups to the molecular backbone or modifications in the side chains of the molecule while retaining the molecular backbone were of interest. The influence of these modifications on radiolytic stability as well as the resulting radiolysis products were investigated. Since the studied solvent extraction processes are carried out in highly radioactive media with high acidity, radiolysis stability in addition to hydrolysis stability plays a decisive role for the feasibility of such separation processes. Radiolytic decomposition of the used extraction agents (ligand together with diluent) can lead to undesirable effects and reduced performance of the extraction systems. In this work, the decrease in ligand concentration of DGA and N-donor ligands and the formation of radiolysis products as a function of absorbed dose is measured via mass spectrometric methods. The ability and performance of the irradiated ligand solutions to separate An(III) is elucidated by solvent extraction experiments. The radiolysis products were identified and, in the case of the N-donor ligands, addition products of the alpha-hydroxyoctyl radical, originating from diluent radiolysis, were observed in particular. This leads to a remarkable preservation of the extraction properties of the irradiated ligand solutions, since the radiolysis products formed are similarly good extractants. The degradation mechanism was postulated for both classes of compounds, DGA-type as well as N-donor ligands, which was done from the interpretation of the obtained mass spectra.