Review: Solvent Systems Combining Neutral and Acidic Extractants for Separating Trivalent Lanthanides from the Transuranic Elements

Abstract

This paper is a review of recent publications that have focused on combined extractant systems for separating trivalent actinides from the lanthanides. These mixed solvent systems combine an acidic extractant with a neutral extractant to achieve the actinide/lanthanide separation. Depending on the neutral extractant used, three categorizations of systems can be considered, including combinations of acidic extractants with 1) diamides, 2) carbamoylmethylphosphine oxides, and 3) polydentate nitrogen-donor ligands. This review of relevant publications indicates that, although there is significant potential for practical exploitation of mixed neutral/acidic extractant systems to achieve a single-step separation of trivalent actinides from acidic high-level waste solutions, the fundamental chemistry underlying these combined systems is not yet well understood. For example, although there is strong evidence suggesting that adducts form between the neutral and acidic extractants, the nature of these adducts generally is not known. Likewise, the structures of the mixed complexes formed between the metal ions and the two different extractants are not fully understood. Research into these basic phenomena likely will provide clues about how to design practical mixed-extractant systems that can be used to efficiently separate the transuranic elements from the lanthanides and other components of irradiated fuel.

Keywords:

• Lanthanide separation
• actinide separation
• lanthanide/actinide separation
• mixed solvent

ACKNOWLEDGMENT

This work was funded by the U.S. Department of Energy, Office of Nuclear Energy, through the Advanced Fuel Cycle Initiative. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under contract DE-AC05-76RL01830.

Notes

^(a)It should be noted that there is some uncertainty in the K_m value reported in reference 20 for reaction 4. In the paper, the expression for K_m is errantly given as K_m = [(HA)₂B₅]/[(HA)₂]²[B]⁵; it is unclear whether that form of the equilibrium expression was used to calculate K_m .