Abstract: Nuclear energy is a sustainable baseload power source with low life cycle carbon emissions, and no emissions during power plant operations. Research into new separatory schemes and technology to reprocess used nuclear fuel (UNF) can further improve carbon emissions and the efficiency of the nuclear fuel cycle. Improved aqueous separatory and extraction agents have the additional benefit of making the use of radionuclides safer with their ability to carefully segregate selected materials from environments such as natural bodies of water or waste streams. Prior research has focused on liquid-liquid extraction (LLE)/solvent extraction (SX) and solid-phase extraction (SPE)/ion exchange (IE) to reprocess and reclaim nuclear material, particularly the mid- and minor-actinides which contain most of the radiotoxicity of UNF. However, these types of schemes face several challenges, such as generation of secondary waste, selection or design of a ligand to separate the similar lanthanides and actinides, low adsorption and retention capacity of extracting agents, and fouling, degradation or delamination of solid support systems.
This work focuses on utilizing poly(amido amine) (PAMAM) dendrimers for improved aqueous sequestration and solid phase separation of the actinyl ions, which take the form AnO2+/AnO22+. Dendrimers are hyperbranched polymers that grow outward in numerous branching arms from a single core rather than linearly. This introduces dendritic effects, which are steric, size and chemical effects that differ from those that would be expected from a monomeric or linear form, and can positively or negatively influence metal ion binding and coordination chemistry of dendrimers. These effects also include solubility and phase variation due to the colloidal nature of PAMAM dendrimers. It is observed that PAMAM dendrimers have an enhanced affinity and capacity for precipitation/solid-phase separation when binding with actinyl ions through a combination of dendritic effects and unique coordination to the actinyl ions due to their open equatorial coordination plane available for multidentate binding. Spectroscopic and analytical techniques such as fluorescence spectroscopy (steady-state and time-resolved), UV-Vis-NIR spectroscopy, neutron activation analysis, dynamic light scattering and x-ray absorption spectroscopy are utilized to examine the coordination chemistry and the separatory efficiency of these dendrimers. Factors such as metal ion:ligand ratio, pH, oxidation state, and absolution concentration of the actinyl ion are explored to improve separatory efficiency if the process were to be scaled up. Efficient aqueous coextraction of the actinyl ions is expected to make the use of radionuclides safer, and help close the nuclear fuel cycle by reusing these actinyl ions for materials such as mixed oxide fuel (MOX).