Continuous extraction and separation of Am(III) and Cm(III) using a highly practical diamide amine extractant

Efficient separation of americium by a mixed solvent of two extractants, a diamideamine and a nitrilotriacetamide

The Japan Atomic Energy Agency (JAEA) has proposed the Solvent Extraction from Liquid waste using Extractants of CHON-type for Transmutation (SELECT) process by solvent extraction as a new separation technology to recover minor actinides (MA) from high-level liquid waste (HLLW) produced by spent fuel reprocessing. The MA separation in the SELECT process comprises the recovery of MA and rare earths (RE) from HLLW, MA/RE separation, and Am/Cm separation. Three highly practical extractants are used in the MA separation. Furthermore, this flow configuration facilitates the preparation of nitric acid concentrations in the aqueous phase. However, the separation factor between Cm and Nd in the MA/RE separation is small (SF_Cm/Nd = 2.5), requiring many extraction stages for continuous extraction in a mixer settler. Therefore, this study investigated the separation of only Am from an aqueous nitric acid solution containing MA (Am and Cm) and RE using an organic phase mixed with two extractants alkyl diamideamine with 2-ethylhexyl alkyl chains (ADAAM(EH)) and hexa-n-octylnitrilotriacetamide (HONTA) used in the SELECT process. Under high-concentration nitric acid conditions, Am and La, Ce, Pr, Nd (light lanthanides) were extracted in the ADAAM(EH) + HONTA mixed solvent, whereas Cm, medium, and heavy lanthanides, and Y were partitioned in the aqueous phase. Subsequently, only light lanthanides could be back extracted from the ADAAM(EH) + HONTA mixture solvent containing Am and light lanthanides in low nitric acid concentrations. Furthermore, Am could be easily stripped with 0.2 M or 5 M nitric acid. This method does not require the mutual separation of Cm and Nd, which have low separation factors. Am can be efficiently separated by one extraction and two back extractions, reducing the number of steps in the SELECT process.

Understanding the Coordination Chemistry of AmIII/CmIII in the DOTA Cavity: Insights from Energetics and Electronic Structure Theory

The minor actinides Am/Cm show multiple possibilities for coordination, providing great opportunities for their extraction and adsorption separation. Herein, we report complexation in an aqueous medium of Am^III/Cm^III in the DOTA (H₄DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) cavity with axial ligands (OH^-, F^-, and H₂O), based on the energetics and electronic structure properties using density functional theory (DFT). The formation and substitution reactions of OH^--capped complexes are more likely to occur due to their enhanced hydration Gibbs free energies, followed by F^-, and then H₂O. Both the longer An-O_DOTA bond lengths and the larger bite angle (∠O-An-O) in the OH^--capped complexes reflect the enhanced coordination provided by the axial ligand, slightly less so for F^-. Energy decomposition analysis based on the electronic structure supports the preference for OH^--capped complexes with a near-perfect balance between attractive and repulsive contributions toward the interaction. Furthermore, molecular orbital analysis revealed that the frontier molecular orbitals of Am and Cm complexes are substantially different; that is, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) compositions of the Am complexes are all contributed by 5f, while the HOMO and LUMO compositions of the Cm complexes are derived from 5f and 6d, respectively. Finally, the metal-exchange reactions demonstrate competitive complexation of DOTA toward Am^III over Cm^III for the OH^--capped system. These results imply the importance of coordination chemistry in actinide chemistry in general and specifically in Am^III/Cm^III solution chemistry.

Concept of a fast breeder reactor to transmute MAs and LLFPs

The long-term issues of nuclear power systems are the effective use of uranium resources and the reduction of radioactive waste. Important radioactive wastes are minor actinides (MAs: ²³⁷Np, ²⁴¹Am, ²⁴³Am, etc.) and long-lived fission products (LLFPs: ¹²⁹I, ⁹⁹Tc, ⁷⁹Se, etc.). The purpose of this study was to show a concept that can simultaneously achieve the breeding of fissile materials and the transmutation of MAs and LLFPs in one fast reactor. Transmutation was carried out by loading innovative Duplex-type MA fuel in the core region and LLFP-containing moderator in the first layer of the radial blanket. Breeding was achieved in the core and axial blanket. As a result, it was clarified that in this fast breeder reactor, a breeding ratio of approximately 1.1 was obtained, and MAs and LLFPs achieved a support ratio of 1 or more. The transmutation rate was 10.3%/y for ²³⁷Np, 14.1%/y for ²⁴¹Am, 9.9%/y for ²⁴³Am, 1.6%/y for ¹²⁹I, 0.75%/y for ⁹⁹Tc, and 4%/y for ⁷⁹Se. By simultaneously breeding fissile materials and transmuting MAs and LLFPs in one fast reactor, it will be possible to solve the long-term issues of the nuclear power reactor system, such as securing nuclear fuel resources and reducing radioactive waste.

Enhancing the Am3+/Cm3+ separation ability by weakening the binding affinity of N donor atoms: a comparative theoretical study of N, O combined extractants

Mutual separation of trivalent americium (Am3+) and curium (Cm3+) ions through liquid-liquid extraction is challenging due to the similarity in their chemical properties. Three N, O combined extractants 2,6-pyridinedicarboxylic acid di(N-ethyl-4-fluoroanilide) (Et(pFPh)DPA), diphenyl(2-pyridyl)phosphine oxide (Ph2PyPO), and alkyldiamide amine with 2-ethylhexylalkyl chains (ADAAM(EH)) have been identified to exhibit selectivity for Am3+ over Cm3+. In this work, the structures, bonding nature, and thermodynamic behaviors of a series of representative Am- and Cm-complexes with these ligands have been systematically investigated using density functional theory (DFT) calculations. Based on our calculations, the ONO angle formed by three donor atoms of the ligand in the Am-complex is slightly larger than that in its Cm-analogue. The studied ligands show their preference toward Am3+ by opening their "mouths" slightly wider. According to the Mayer bond order and the quantum theory of atoms in molecules (QTAIM) analyses, the interactions between the O donor atoms of these ligands and Am3+ and Cm3+ ions show some weak partial covalent character, and compared to the Am-O bond, there is relatively more covalency in the Cm-O bond in the corresponding complex. However, opposite results can be found in the Am-N and Cm-N bonding for the first two ligands. Particularly, for the better separation ligand ADAAM(EH), the Am-N and Cm-N interactions are extremely weak and no covalent character exists in the bonding. Nevertheless, the difference between the very weak Am-N and Cm-N interactions still leads to a better performance of ADAAM(EH). Based on the comparison of these ligands, we can find that weakening the binding ability of N atoms in the ligand may increase the difference between the Am-N and Cm-N interactions, thus enhancing the Am3+/Cm3+ separation ability of the ligand. Our study might provide new insights into understanding the selectivity of these three N, O combined ligands toward minor actinides and pave the way for designing efficient Am3+/Cm3+ extraction and separation ligands.

Direct Temperature-Swing Extraction of Rare-Earth Elements from Acidic Solution Using the Hydrophobic Interactions of Poly(N-isopropylacrylamide) with Diglycolamide-typed Ligands

The phase transition-based gelification phenomenon of poly(N-isopropylacrylamide) [poly(NIPAAm)] has great potential in developing new waste-free extraction processes. In this study, we realized the direct and complete temperature-swing extraction of all trivalent rare-earth (RE) ions from a multi-component nitric acid solution onto a poly(NIPAAm) gel as chelate complexes with hydrophobic diglycolamide-typed ligands. Moreover, we elucidated that the extractabilities are affected by not only the coordination ability of the ligands with RE ions but also by the hydrophobic interaction between the poly(NIPAAm) and the ligands.

A first phosphine oxide-based extractant with high Am/Cm selectivity

A new phosphine oxide ligand demonstrates high selectivity for the Am–Cm pair with SF = 2.9–3.5 and the Am–Eu pair with SF = 7.3–8.5 in the range of 0.1–3 M nitric acid.

Understanding Am3+/Cm3+ separation with H4TPAEN and its hydrophilic derivatives: a quantum chemical study

Quantum chemical calculations have been used to help understand the back-extraction and separation of Am³⁺/Cm³⁺ with H₄TPAEN and its two hydrophilic derivatives.