Nuclear energy is a clean, carbon neutral energy source that can mitigate the effects of climate change in the coming decades by replacing fossil fuel energy sources. Political and societal acceptance of nuclear power hinges on tractable solutions for disposal of used nuclear fuel. Advanced reprocessing, where minor actinides (MA) are separated from used nuclear fuel, is one option to close the nuclear fuel cycle and to optimize permanent repository storage, as it provides the greatest reduction in radioactive waste inventory and long-term hazard. The key technical challenge in advanced reprocessing is separation of the MA from the trivalent lanthanides (Ln). The Actinide Lanthanide SEParation (ALSEP) solvent extraction method has been proposed for MA/Ln separation and is currently studied by DOE to advance the current state of knowledge of fuel reprocessing to support waste reduction goals.
The ALSEP solvent extraction process combines two metal extracting ligands T2EHDGA and HEH[EHP], each with unique extraction properties and coordination behavior, in a single alkane diluent. The feed for the process is a nitric acid-based post-PUREX raffinate with uranium, plutonium, and neptunium removed. Trivalent MA and Ln are co-extracted from the feed into the ALSEP solvent, and Ln/MA separation is achieved by subsequent stripping stages using buffered polyaminocarboxylic acid solutions. Little knowledge exists regarding the functionality of HEH[EHP] during metal extraction in the combined T2EHDGA -HEH[EHP] solvent system. The specific aims of the project are to (1) investigate the organic phase metal speciation and coordination of MA and Ln ions in the simplified extraction systems, containing each ligand independently, and (2) evaluate the metal speciation and coordination in the extraction system containing both ligands together. The results of this work contribute to the further development of the ALSEP process, in particular correlating mechanism of transport of MA and Ln across liquid-liquid interface with the organic phase speciation effects. This directly aligns with EMSL's Molecular Transformations science theme.
The proposed project will utilize EMSL and RadEMSL's unique instrumentation and capabilities in nuclear magnetic resonance spectroscopy , time-resolved fluorescence spectroscopy , and density functional theory simulations to conduct a study of the of the speciation and structure of metal coordination complexes in the solvent extraction system after metal transport across the liquid-liquid interface. RadEMSL's instrumentation will allow direct investigation of the MA complexes (instead of non-radioactive surrogate complexes) to compare them with Ln speciation in ALSEP organic phases. Understanding the coordination and speciation of Ln and MA complexes formed in the ALSEP system will help to illuminate the fundamental mechanism responsible for differences in selectivity and efficiency in this Ln/MA separation process, guiding the development and refinement of the fuel reprocessing process.
The proposed studies will continue on-going work. Established collaboration between the PNNL PI and Oregon State University participants will facilitate successful completion of this project. This work will contribute to student doctoral dissertations and to peer-reviewed journal publications.