Michal Sypula

Direct Selective Extraction of Actinides (III) from PUREX Raffinate using a Mixture of CyMe4BTBP and TODGA as 1-cycle SANEX Solvent

Abstract Within the framework of our research activities related to the partitioning of spent nuclear-fuel solutions, the direct selective extraction of trivalent actinides from a simulated PUREX raffinate was studied using a mixture of CyMe4BTBP and TODGA (1-cycle SANEX). The solvent showed a high selectivity for trivalent actinides with a high lanthanide separation factor. However, the coextraction of some fission product elements (Cu, Ni, Zr, Mo, Pd, Ag, and Cd) from a simulated PUREX raffinate was observed, with distribution ratios up to 30 (Cu). The extraction of Zr and Mo could be suppressed using oxalic acid but the use of the well-known Pd complexant N-(2-Hydroxyethyl)-ethylendiamin-N,N′,N′-triacetic acid (HEDTA) was unsuccessful. During screening experiments with different amino acids and derivatives, the sulfur-bearing amino acid L-Cysteine showed good complexation of Pd and prevented its extraction into the organic phase without influencing the extraction of the trivalent actinides Am (III) and Cm (III). The optimization studies included the influence of the L-Cysteine and HNO3 concentration and the kinetics of the extraction. The development of a process-like extraction series showed very promising results in view of further optimizing the process. A strategy for a single-cycle process is proposed within this article.

Direct Selective Extraction of Actinides (III) from PUREX Raffinate using a Mixture of CyMe4BTBP and TODGA as 1-cycle SANEX Solvent Part III: Demonstration of a Laboratory-Scale Counter-Current Centrifugal Contactor Process

The direct selective separation of the trivalent actinides americium and curium from a simulated Plutonium Uranium Refining by EXtraction (PUREX) raffinate solution by a continuous counter-current solvent extraction process using miniature annular centrifugal contactors was demonstrated on a laboratory scale. In a 32-stage spiked test (12 stages for extraction, 16 stages for scrubbing, and 4 stages for Am/Cm stripping), an extractant mixture of CyMe4BTBP and TODGA in a TPH/1-octanol mixture was used. The co-extraction of some fission and corrosion product elements, such as zirconium and molybdenum, was prevented by using oxalic acid. Co-extracted palladium was selectively stripped using an L-cysteine scrubbing solution and the trivalent actinides were selectively stripped using a glycolic acid-based stripping solution. It was demonstrated that a selective extraction and high recovery of > 99.4% of the trivalent minor actinides was achieved with low contamination by fission and corrosion products. The product contained 99.8% of the initial americium and 99.4% of the initial curium content. The spent solvent still contained high concentrations of Cu, Cd, and Ni. The experimental steady-state concentration profiles of important solutes were determined and compared with those from computer-code calculations.

Laboratory-Scale Counter-Current Centrifugal Contactor Demonstration of an Innovative-SANEX Process Using a Water Soluble BTP

In this paper the development and laboratory-scale demonstration of a novel “innovative-SANEX” (Selective Actinide Extraction) process using annular centrifugal contactors is presented. In this strategy, a solvent comprising the N,N,N’,N’-tetraoctyldiglycolamide (TODGA) extractant with addition of 5 vol.-% 1-octanol showed very good extraction efficiency of Am(III) and Cm(III) together with the trivalent lanthanides (Ln(III)) from simulated Plutonium Uranium Refining by Extraction (PUREX) raffinate solution without 3rd phase formation. Cyclohexanediaminetetraacetic acid (CDTA) was used as masking agent to prevent the co-extraction of Zr and Pd. An(III) and Ln(III) were co-extracted from simulated PUREX raffinate, and the loaded solvent was subjected to several stripping steps. The An(III) were selectively stripped using the hydrophilic complexing agent SO3-Ph-BTP (2,6-bis(5,6-di(sulfophenyl)-1,2,4-triazin-3-yl)pyridine). For the subsequent stripping of the Ln(III), a citric acid based solution was used. A 32-stage process flow-sheet was designed using computer-code calculations and tested in annular miniature centrifugal contactors in counter-current mode. The innovative SANEX process showed excellent performance for the recovery of An(III) from simulated High Active Raffinate (HAR) solution and separation from the fission and activation products. ≥ 99.8% An(III) were recovered with only low impurities (0.4% Ru, 0.3% Sr, 0.1% Ln(III)). The separation from the Ln(III) was excellent and the Ln(III) were efficiently stripped by the citrate-based stripping solution. The only major contaminant in the spent solvent was Ru, with 14.7% of the initial amount being found in the spent solvent. Solvent cleaning and recycling therefore has to be further investigated. This successful spiked test demonstrated the possibility of separating An(III) directly from HAR solution in a single cycle which is a great improvement over the former multi-cycle strategy. The results of this test are presented and discussed.

Use of Polyaminocarboxylic Acids as Hydrophilic Masking Agents for Fission Products in Actinide Partitioning Processes

During the partitioning of trivalent actinides from High Active Raffinate (HAR) solutions, most processes have to cope with an undesirable co-extraction of some of the fission products. Four hydrophilic complexing agents of the group of polyaminocarboxylic acids, namely EDTA, HEDTA, DTPA, and CTDA were tested and compared for their ability to complex fission products in a simulated PUREX raffinate solution, thereby preventing their extraction into an organic solvent. Several solvents, based on TODGA and the DIAMEX reference molecule DMDOHEMA, that are commonly known to show quite high Zr and Pd co-extraction, were studied. Our investigations ultimately resulted in a substitution of oxalic acid and HEDTA by cyclohexanediaminetetraacetic acid (CDTA). A small addition of this hydrophilic complexing agent to the feed decreased the distribution ratios of Zr from 100 to <0.01. The suppression of Pd was also very effective, resulting in >90% of the metal retained in the feed solution. The extraction of trivalent actinides and lanthanides was not negatively affected by the presence of CDTA. Furthermore, experiments with high concentrations of Zr proved the applicability of this new masking agent. The suppression of Zr and Pd extraction was also verified at a high Pu loading which makes CDTA as a masking agent attractive for grouped actinide extraction processes (GANEX) as well as DIAMEX-SANEX type separations.

Synthesis and Am/Eu extraction of novel TODGA derivatives

Various ligands with structural modifications of the N,N,N′,N′-tetraoctyl-3-oxapentanediamide (TODGA) skeleton were synthesised in good yields. These modifications include (1) the increase in chain length from one carbon to two carbons between the central ether oxygen atom and the amide moieties, (2) the addition of substituents on the carbon between the central oxygen atom and the amide moieties on one and both sides of the central oxygen, (3) the replacement of the central oxygen by a (substituted) nitrogen atom and (4) synthesis of a rigidified glycolamide. The effect of the structural modifications on their extraction behaviours toward Am(III) and Eu(III) at various nitric acid concentrations was studied. In most of the cases, the extraction does not exceed that of TODGA in the entire acidity range of 0.001–4 mol/l HNO3. The extraction behaviour of monomethyl–TODGA derivative 10a resembles that of TODGA at high nitric acid concentrations. However, at lower acidities, its D values are much lower, which is beneficial for possible back-extraction steps. The aza-tripodal ligands 18a,b show reverse extraction properties compared to TODGA as far as the pH influence is concerned: at pH 2, the D Am values are 49.9 and 3.1, the D Eu values are 5.9 and 0.2, and the S Am/Eu values are 8 and 11, respectively.

Synthesis and evaluation of novel water-soluble ligands for the complexation of metals during the partitioning of actinides

Different types of water-soluble ligands were synthesized and their capability was evaluated by solvent extraction studies to complex trivalent actinides and suppress their extraction by a strong lipophilic ligand, such as TODGA. The back extraction efficiency of hydrophilic diglycolamide (DGA) derivatives with a varying number of ethylene glycol groups, or containing sodium acetate moieties on the amidic nitrogen shows a decrease in back-extraction efficiency with increasing number of ethylene glycol units on the amidic nitrogen at various pH values of the aqueous phase. Among the PS donating ligands only the ligand with a malonamide backbone exhibits a high reverse extraction efficiency, although, with no selectivity for americium. Within the water-soluble tripodal ligands, i.e. the amide derivatives of nitrilotriacid with N,N-dimethyl and N,N-bis(hydroxyethyl) moieties, the first one shows a pronounced selectivity for Am(III) over Eu(III), with a maximum separation factor of 11.1, while the latter one more efficiently complexes the radionuclides in the aqueous phase with a maximum separation factor of 5. Isothermal microcalorimetry experiments of the complexation of Eu(III) by a selected series of ligands confirm the observed trend in the back extraction properties.

Synthesis and evaluation of ligands with mixed amide and phosphonate, phosphinoxide, and phosphonothioate sites for An(III)/Ln(III) extraction

Various organophosphorus ligands with a combination of different donor sites were synthesized and evaluated by solvent extraction studies for the complexation of Am(III)/Eu(III). Among the ligands with a glycolamide backbone, those with mixed amide and PO donor sites and a central oxygen or nitrogen atom showed a reasonable extraction of Am(III) and Eu(III). Ligands with a central oxygen atom exhibited selectivity towards Eu(III), and those with a central nitrogen atom towards Am(III). Ligands with PS donor sites and a glycolamide backbone did not show any reasonable extraction. Amongst the ligands with a malonamide backbone, high extraction efficiency was observed for the ligand with electron-rich substituents on phosphorus, however, with almost no discrimination between Am(III) and Eu(III). The extraction efficiency of different ligands towards Eu(III) was confirmed by microcalorimetry.