Trivalent actinides (An3+) and lanthanides (Ln3+) are present in used nuclear fuel (UNF). Their separation is needed for an effective transmutation process that can increase the capacity of geological repositories. 241Am (t1/2=433 years) is one of the long-lived minor actinide isotopes present in UNF that is responsible for a great part of its radiotoxicity and heat generation. An/Ln separation processes and minor actinide recycling can increase uranium utilization. Challenging separation problems can arise for the separation of Ln3+ from An3+ because of their similar properties. Ligands with soft sites can take advantage of the slight difference in hardness between An3+ and Ln3+ either as organic complexants or as aqueous holdback or stripping agents. In our previous work we have investigated the selectivity for An3+ vs. Ln3+ binding and extraction using dipicolinamide analogs containing the C=O vs. C=S groups in solution, gas-phase, and by DFT calculations. Analogous water-soluble diamides and dithioamide derivatives of dipicolinic acid are used as selective holdback reagents like TALSPEAK for separation of 241Am from 154Eu. We will discuss solvent extraction with HDEHP in n-dodecane and diamides/dithioamide ligands in aqueous media with spikes of 241Am, 154Eu, and 139Ce. The diamide ligand, N’,N’’-bis(pyridin-2-ylmethyl)pyridine-2,6-dicarboxamide has demonstrated excellent separation at pH = 2.0 between Eu/Am (SF = 74) and Ce/Am (SF = 202). Slope analysis of log D vs. log [ligand] suggests a 1:3 metal/ligand complexation ratio, which is consistent with our prior work. Our system and its complexation properties were characterized by several analytical techniques including UV-Vis and NMR titrations, as well as electrochemistry.

Oxoamide and thioamide ligands for solvent extraction applications from used nuclear fuel: Investigating actinide vs lanthanide extraction selectivity

2019 Spring National Meeting

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