Characterization of the ALSEP Process at Equilibrium: Speciation and Stoichiometry of the Extracted Complex

Abstract

We have determined the identity of the complexes extracted into the ALSEP process solvent from solutions of nitric acid. The ALSEP process is a new solvent extraction separation designed to separate americium and curium from trivalent lanthanides in irradiated nuclear fuel. ALSEP employs a mixture of two extractants, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N,N,N′,N′-tetra(2-ethylhexyl)diglycolamide (TEHDGA) in n-dodecane, which makes it difficult to ascertain the nature of the extracted metal complexes. It is often asserted that the weak acid extractant HEH [EHP] does not participate in the extracted complex under ALSEP extraction conditions (2−4 M HNO 3 ). However, the analysis of the Am extraction equilibria, Nd absorption spectra, and Eu fluorescence emission spectra of metal-loaded organic phases argues for the participation of HEH[EHP] in the extracted complex despite the high acidity of the aqueous phases. The extracted complex was determined to contain fully protonated molecules of HEH [EHP] with an overall stoichiometry of M(TEHDGA) 2 (HEH[EHP]) 2 •3NO 3 . Computations also demonstrate that replacing one TEHDGA molecule with one (HEH[EHP]) 2 dimer is likely energetically favorable compared to Eu(TEHDGA) 3 •3NO 3 , whether the HEH[EHP] dimer is monodentate or bidentate.

Characterization of a Ternary Neodymium-HEDTA-Citrate Complex in the Actinide Lanthanide Separation Process

“…The organic extractant HEHEHP (Carbosynth) was purified to 98% HEHEHP/2% bis(2-ethylhexyl)phosphoric acid (HDEHP) using the third phase purification method as previously described. 49 The secondary organic extractant TEHDGA (99%, Eichrom) was used as received. HEDTA (99%, Sigma Aldrich), citric acid (99.5%, Sigma Aldrich), and sodium nitrate (99% Sigma Aldrich) were used as received to prepare the aqueous phases.…”

“…Neodymium solutions were prepared from a standardized stock solution of Nd(NO 3 ) 3 as described previously. 49 Americium solutions were prepared from a radiochemically pure Am-241 stock solution at 2 μCi/μL in 1 M HNO 3 (Am was sourced from Eckert & Zeigler). Except for the liquid–liquid extraction experiments used to verify the citrate and HEDTA stoichiometry of the aqueous complexes, the ionic strength of the aqueous solutions was controlled by adding 1 M NaNO 3 to minimize changes in activity coefficients.…”

Characterization of the ALSEP Process: Investigating Equilibrium and Intermediate Complexes of the Scrub Stage

“…A model of trivalent f-element cation extraction by the ALSEP organic solvent under acidic conditions was previously developed to determine the effects of nitric acid on metal distribution. 2 The model considers variations in the activity coefficients of aqueous solutes and the activity of water and accounts for the corresponding changes in the degree of metal-nitrate complex formation and nitric acid extraction equilibria. The activity coefficients of organic phase species were considered to be independent of the aqueous phase composition and thus constant for our nitric acid dependence experiments.…”

“…(HEH[EHP])2 were previously described by Picayo et al 2 and the values used are summarized in Table S2. log 1 = 1.33…”

Countercurrent Actinide Lanthanide Separation Process (ALSEP) Demonstration Test with a Simulated PUREX Raffinate in Centrifugal Contactors on the Laboratory Scale

“…Furthermore, to be applied under the harsh conditions of used nuclear fuel recycling, innovative processes should be robust, use conventional and commercially available equipment and chemicals, and show good hydrolytic and radiolytic stability. For this purpose, the Actinide Lanthanide Separation Process (ALSEP) process was developed and proved to fulfill these requirements [18,[23][24][25][26][27][28][29]. The optimized ALSEP solvent is composed of 0.5 mol L −1 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP], Figure 1) and 0.05 mol L −1 N,N,N ,N -tetra-(2-ethylhexyl)-diglycolamide (T2EHDGA, Figure 1) in n-dodecane [27].…”