Dynamic and Solvation Behaviors of ALSEP Organic Ligands

Abstract

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(2ethylhexyl)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 HNO 3 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.

Evolution of Acid-Dependent Am3+ and Eu3+ Organic Coordination Environment: Effects on the Extraction Efficiency

“…Alternatively, HEH­[EHP] has been predominantly evaluated under the advanced TALSPEAK extraction − and ALSEP stripping conditions, − and its role during the extraction step remains unclear. The esters of phosphonic and phosphoric acids have been the focus of a number of structural studies with trivalent metals, which generally conclude a pseudooctahedral structure involving six phosphoric/phosphonic acid ligands binding as dimers and sharing a single acidic proton between each dimer. , However, this leaves the question, in a system with two potentially competing ligands, do the coordination environments of the lanthanides and minor actinides change and how does this manifest as a change in the extraction efficiency?…”

Effect off-element complexation on the radiolysis of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP])

“…[9][10][11] That said, there are still significant knowledge gaps in the radiolytic behaviour of ALSEP formulations that must be addressed to fully optimize this technology. 12 All nuclear waste reprocessing systems occur in an intense radioactive environment. The presence of these intense fields (up to kGy h −1 ) 13 of alpha, beta, and gamma radiation can significantly affect the performance of solvent system separations.…”