Dynamic and Solvation Behaviors of ALSEP Organic Ligands

The Actinide-Lanthanide Separation Process (ALSEP) is a solvent extraction approach for separating relevant trivalent minor actinides (e.g., americium and curium) from used nuclear fuel. However, relatively slow kinetics in the stripping step of the process restricts process throughput when scaled for industrial implementation. To assist in identifying specific kinetic barriers associated with the separation, the solvation and dynamic behaviors of the two organic extractants in the current ALSEP implementation, N,N,N',N'-tetra(2-ethylhexyl)diglycolamide (T2EHDGA) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP), were probed through molecular dynamics (MD) simulations. The simulations examined the effects of extractant and nitric acid concentration on the interfacial behavior of the extractants in three solvent systems (n-dodecane, water, and n-dodecane + water). Solvation analyses of T2EHDGA revealed expected amphiphilic behavior in pure solvent systems. In a nitric-acid-free biphasic solvent, it was found that T2EHDGA expressed similar interfacial conformations as HEHEHP, suggesting that a parallel-like configuration, relative to the interface, is adopted at low concentrations. When HNO₃ was introduced to biphasic systems containing a single molecule of extractant, HEHEHP was observed to retain a relatively parallel orientation while the T2EHDGA orientation was no longer affected by the presence of the interface. At bulk extractant concentrations, representative of the ALSEP process, the presence of nitric acid had minimal impact on the ligand orientation. Calculated diffusion constants showed that only some systems involving T2EHDGA were affected by the presence of acid.