A COMBINED CYANEX-923 AND HEH[EHP] PROCESS FOR PARTITIONING USED NUCLEAR FUELS: CHARACTERIZING COMPLEX INTERACTIONS
Johnson, Aaron Thomas
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The efficient separation of used nuclear fuel into its components remains a significant challenge in the operation of a closed nuclear fuel cycle. Many solvent extraction processes have been either successfully applied or proposed to accomplish this separation. However, most of these processes necessitate several different discrete operations to fully partition fuel, which introduces challenges for industrial implementation. To reduce the cost and complexity of the separations, the concept of combining an acidic and solvating extractant into one process solvent has become the focus of a large amount of research. Initial studies have shown that these processes hold great promise in their ability to partition used nuclear fuel. However, the fundamental chemistry controlling these processes is not well understood. Strong solute-solute complexes (adducts) are known to form between the extractants in the organic phase. Evidence interpreted to indicate the presence of mixed complexes between metals and the extractants also has been reported. These adducts introduce additional equilibria that can complicate industrial application of these processes. To attempt to reduce the strength of the adduct that forms, a new combination of extractants has been proposed, Cyanex-923 (a mixture of C6-C8 trialkyl phosphine oxides) and HEH[EHP] (2-ethyl(hexyl)phosphonic acid, mono-2-ethyl(hexyl)ester). Each is a monofunctional, industrial organophosphorus extractant. Additionally, the underlying chemistry that controls combined extractant processes is still not well understood. Studies have been complicated by the large and multi-functional extractants that have been used in earlier studies. By using simple solvating (Cyanex-923) and cation exchanging (HEH[EHP]) extractants, the fundamental chemistry that defines the performance of this combination is simpler to elucidate. The combination of these extractants has been investigated by solvent extraction to determine their ability to separate the minor actinides from the fission product lanthanides. In addition, the fundamental chemistry controlling the extraction process was investigated through use of FT-IR, UV-Vis, and time resolved fluorescence spectroscopy, and thermodynamic modeling techniques. The work presented here is the results of this study. It is hoped that these findings can be applied to other systems to better understand the nature of mixed extractant systems.