Effective separation of lanthanides (Ln) from the minor actinides (MA) is a crucial technical challenge to closing the nuclear fuel cycle. This separation is a necessary prerequisite to transmute long-lived isotopes of Am and Cm, which will allow a reduction of the repository volume, thermal load, and radiological toxicity of nuclear wastes. The US Department of Energy (USDOE) Fuel Cycle Research and Development initiative is investigating the Actinide Lanthanide Separation (ALSEP) solvent extraction process to perform the Ln/MA separation from dissolved used nuclear fuel. ALSEP has achieved substantial improvements upon currently available separations, but further development of ALSEP requires an enhanced understanding of the fundamental aspects of this complicated multicomponent system. The focus of this research has been to determine the coordination environment in the organic phase, particularly, of the ligands and of the extracted lanthanides and minor actinides.
The ALSEP process combines the neutral extractant N,N,N',N'-tetra-2-ethylhexyl diglycolamide (T2EHDGA) with HEH[EHP] in an aliphatic diluent. The ALSEP feed is a nitric acid-based post-PUREX raffinate with uranium, plutonium, and neptunium removed. Trivalent actinides and lanthanides are co-extracted by the ALSEP solvent, and Ln/An separation is achieved by subsequent selective 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.
In this work, the role of HEH[EHP] in the metal extraction step is investigated as a function of aqueous phase acidity. The ALSEP system is found to exhibit synergistic metal extraction toward trivalent Eu and Am, and this synergism is found to be dependent on aqueous phase acid concentration. Spectroscopic (IR and UV-vis) evidence is consistent with the participation of HEH[EHP] in the extracted organic phase metal complex. NMR spectroscopy indicates adduct formation between the ligands T2EHDGA and HEH[EHP] in organic phases before contact with any aqueous phase. Adduct formation is substantiated by diffusion ordered spectroscopy (DOSY) NMR, which further indicates the presence of HEH[EHP] in the extracted metal complexes, consistent with the UV-vis and IR spectroscopic results.