Separation of Americium by Liquid-Liquid Extraction Using Diglycolamides Water-Soluble Complexing Agents

Recycling americium (Am) alone from spent nuclear fuels is an important option studied for the future nuclear cycle (Generation IV systems) since Am belongs to the main contributors of the long-term radiotoxicity and heat power of final waste. Since 2008, a liquid-liquid extraction process called EXAm has been developed by the CEA to allow the recovery of Am alone from a PUREX raffinate (a dissolution solution already cleared from U, Np and Pu). A mixture of DMDOHEMA (N,N’-dimethyl-N,N’-dioctyl-2-(2-(hexyloxy)ethyl)-malonamide) and HDEHP (di-2-ethylhexylphosphoric acid) in TPH is used as the solvent and the Am/Cm selectivity is improved using TEDGA (N,N,N’,N’-tetraethyldiglycolamide) as a selective complexing agent to maintain Cm and heavier lanthanides in the acidic aqueous phase (5 M HNO₃). Americium is then stripped selectively from light lanthanides at low acidity (pH=3) with a polyaminocarboxylic acid. The feasibility of sole Am recovery was already demonstrated during hot tests in ATALANTE facility and the EXAm process was adapted to a concentrated raffinate to optimize the process compactness. The speciation of TEDGA complexes formed in the aqueous phase with Am, Cm and lanthanides was studied to better understand and model the behavior of TEDGA in the process. Some Ln-TEDGA species are extracted into the organic phase and this specific chemistry might play a role in the Am/Cm selectivity improvement. Hence the hydrophilicity-lipophilicity balance of the complexing agent is an important parameter. In this comprehensive study, new analogues of TEDGA were synthesized and tested in the EXAm process conditions to understand the relationship between their structure and selectivity. New derivatives of TEDGA with different N-alkyl chain lengths and ramifications were synthesized. The impact of lipophilicity on ligand partitioning and Am/Cm selectivity was investigated.     

Sensitivity of Americium and Curium Splitting Flowsheet and Running Procedure

As part of the 2006 French Act on sustainable radioactive waste and waste management, CEA has been developing a process, (EXAm), in order to separate americium from curium and fission product downstream of COEXTM or PUREX processes. The goal is to recover Am up to 99%. The first step mainly consists of splitting americium from curium thanks to the diamide in organic phase combined with a complexing agent in high nitric acid. The low separation factor between Am and Cm leads to a very sensitive process flowsheet towards operating conditions. It is then difficult to manage high recovery yields with good purity. A model has been built taking into account complexation equilibria by TEDGA in aqueous phase and extraction equilibria in organic phase for each element. This model was put into the PAREX code to find the correct flowsheet, and then to conduct sensitivity studies regarding several parameters such as feed flow, acidity, temperature, solvent flow and reagent concentration. These studies have pointed out a high correlation between americium yield and decontamination factor and, also, an equivalence between any change of the most sensitive parameters and a change in TEDGA concentration. A running procedure was followed during two hot tests: the main concept was to start with a less efficient process and then to improve it during the test in order to reach required performances by adjusting the TEDGA flow rate.     

Separation of AM(III) by extraction from nitrate solutions using some quaternary benzylammonium salts

Benzyldimethyldodecylammonium nitrate and benzyltrioctylammonium nitrate were used for the extraction of Am(III) from aqueous nitrate solutions. The dependence of the extraction performance for Am(III) on the concentration of nitric acid, the kind and concentration of salting-out agents in the aqueous phase, and the kind of solvent was investigated. Americium is extracted by the above quarternary salts as a R₄NAm(NO₃)₄ associate. The extraction of Am(III) is compared with the extraction of lanthanides. The high differences in the distribution coefficients for lanthanides and americium can be utilized for the separation of lanthanides and americium.     

Modeling and Flowsheet Design of an Am Separation Process Using TODGA and H4TPAEN

Recycling americium from spent fuels is an important consideration for the future nuclear fuel cycle, as americium is the main contributor to the long-term radiotoxicity and heat power of the final waste, after separation of uranium and plutonium using the PUREX process. The separation of americium alone from a PUREX raffinate can be achieved by co-extracting lanthanide (Ln(III)) and actinide (An(III)) cations into an organic phase containing the diglycolamide extractant TODGA, and then stripping Am(III) with selectivity towards Cm(III) and lanthanides. The water soluble ligand H₄TPAEN was tested to selectively strip Am from a loaded organic phase.Based on experimental data obtained by Jülich, NNL and CEA laboratories since 2013, a phenomenological model has been developed to simulate the behavior of americium, curium and lanthanides during their extraction by TODGA and their complexation by H₄TPAEN (complex stoichiometry, extraction and complexation constants, kinetics). The model was gradually implemented in the PAREX code and helped to narrow down the best operating conditions. Thus, the following modifications of initial operating conditions were proposed:

• •An increase in the concentration of TPAEN as much as the solubility limit allows.
• •An improvement of the lanthanide scrubbing from the americium flow by adding nitrates to the aqueous phase.

A qualification of the model was begun by comparing on the one hand constants determined with the model to those measured experimentally, and on the other hand, simulation results and experimental data on new independent batch experiments.A first sensitivity analysis identified which parameter has the most dominant effect on the process. A flowsheet was proposed for a spiked test in centrifugal contactors performed with a simulated PUREX raffinate with trace amounts of Am and Cm. If the feasibility of the process is confirmed, the results of this test will be used to consolidate the model and to design a flowsheet for a test on a genuine PUREX raffinate. This work is the result of collaborations in the framework of the SACSESS European Project.     

Modelling of Americium Stripping in the EXAm Process

The EXAm process aims at recovering americium alone contained in the PUREX raffinate. The americium stripping model has been revised to take into account a change of stripping aqueous phase and up-to-date experimental results conducted within DRCP to improve knowledge about complexes.This work represents a first approximation at modelling americium stripping. The modelling work has led to synthesize the knowledge on chemical phenomenology and adopt assumptions that best reflect experimental results. The modelling has been implemented in PAREX code in order to simulate this step to prepare and understand tests to be carried out in mixer settlers.     

Selective recognition of americium by peptide-based reagents

The separation of lanthanides from minor actinides such as americium and curium is an important step during the recycling process in the treatment of nuclear waste. However, the similar chemistry and ionic size of lanthanide and actinide ions make the separation challenging. Here, we report that a peptide-based reagent can selectively bind trivalent actinides over trivalent lanthanides by means of introducing soft-donor atoms into a peptide known as a lanthanide-binding tag (LBT). Fluorescence spectroscopy has been used to measure the dissociation constant of each metal/peptide complex. A 10-fold selectivity was obtained for Am(3+) over the similarly sized lanthanide cation, Nd(3+), when the asparagine on the fifth position of a LBT was mutated to a cysteine and further functionalized by a pyridine moiety.     

Minor Actinides Can Replace Essential Lanthanides in Bacterial Life**

Certain f-block elements—the lanthanides—have biological relevance in the context of methylotrophic bacteria. The respective strains incorporate these 4 f elements into the active site of one of their key metabolic enzymes, a lanthanide-dependent methanol dehydrogenase. In this study, we investigated whether actinides, the radioactive 5 f elements, can replace the essential 4 f elements in lanthanide-dependent bacterial metabolism. Growth studies with Methylacidiphilum fumariolicum SolV and the Methylobacterium extorquens AM1 ΔmxaF mutant demonstrate that americium and curium support growth in the absence of lanthanides. Moreover, strain SolV favors these actinides over late lanthanides when presented with a mixture of equal amounts of lanthanides together with americium and curium. Our combined in vivo and in vitro results establish that methylotrophic bacteria can utilize actinides instead of lanthanides to sustain their one-carbon metabolism if they possess the correct size and a +III oxidation state.     

Evaluation of the impacts of gamma radiolysis on an ALSEP process solvent

Journal of Radioanalytical and Nuclear Chemistry - Separating the minor actinide elements (americium and curium) from the fission product lanthanides is an important step in closing the nuclear...     

Extraction of Am(III) in the presence of lanthanides and yttrium by benzyldimethyldodecylammonium nitrate from acidic nitrate solutions

We have investigated the effect of coextraction of lanthanides and yttrium on the distribution coefficients D_Am in the extraction of americium by benzyldimethyldodecylammonium nitrate (BDMLNNO₃) from nitrate solutions. In the coextraction of lanthanides, the extraction of Am(NO₃)₃ is suppressed, which is markedly manifested in the extraction of light lanthanides (La, Ce, Pr); of the series of lanthanides their extraction is the highest. The effect of nitric acid and the possibility of separation of lanthanides and americium by the application of three-stage multiple extraction is discussed.     

Efficient separation between trivalent americium and lanthanides enabled by a phenanthroline-based polymeric organic framework

Separation of the minor actinides (Am and Cm) from lanthanides in high-level liquid wastes (HLLW) is one of the most challenging chemical separation tasks known owing to their chemical similarities and is highly significant in nuclear fuel reprocessing plants because it could practically lead to sustainable nuclear energy by closing the nuclear fuel cycle. The solid phase extraction is proposed to be a possible strategy but all reported sorbent materials severely suffer from limited stability and/or efficiency caused by the harsh conditions of high acidity coupled with intense irradiation. Herein, a phenanthroline-based polymeric organic framework (PhenTAPB-POF) was designed and tested for the separation of trivalent americium from lanthanides for the first time. Due to its fully conjugated structure, PhenTAPB-POF exhibits previously unachieved stability under the combined extreme conditions of strong acids and high irradiation field. The americium partitioning experiment indicates that PhenTAPB-POF possesses an ultrahigh adsorption selectivity towards Am(III) over lanthanides (e.g., SF_{Am(III)/Eu(III)} = 3326) in highly acidic simulated HLLW and relatively fast adsorption kinetics in both static and dynamic experiments. Am(III) can be almost quantitatively eluted from the PhenTAPB-POF packed-column using a concentrated nitric acid elution. The high stability and superior separation performance endow PhenTAPB-POF with the promising alternative for separating minor actinides over lanthanides from highly acidic HLLW streams.     

Use of actinides in uncommon oxidation states for their extraction and separation from alkaline solutions

The extraction behavior of Am(IV–VI) from high pH solutions in the presence of carbonates, pyrophosphates or polyphosphates of alkali metals and of Np(VI–VII) from alkaline solutions with acylpyrazolones (1-phenyl-3-methyl-4-benzoylpyrazolone-5, PMBP) and extractants of the phenol type [bis(2-oxy-4-alkyl-benzoil)amin, CAAF] has been studied. The extraction ability of phenolic extractants with respects to Np(VII) is determined generally by its state in the alkaline solution. Maximum extraction is observed when Np(VII) is present as hydroxo complex and minimum extraction, when the solution contains oxo-ions. During the extraction the reduction of Np(VII) to Np(VI) is possible. Hexavalent neptunium can be extracted by phenol extractants too, but more slowly and with smaller distribution coefficients in comparison with Np(VII). The stabilization of transplutonium elements (TPE) in the highest oxidation states in alkaline solutions contaning carbonate and pyrophosphate ions, in combination with extraction by PMBP and CAAF, allows to realize the separation of transplutonium elements which are very similar in their properties. Methods of separation for americium and curium have been developed. They are based on the ability of trivalent curium to be extracted quantitatively from 0.1M sodium pyrophosphate solution (pH 10) and 1.0M potassium carbonate solution (ph 13.4) by PMBP in chloroform and by CAAF in carbon tetrachloride, respectively, with high distribution coefficients, whereas americium which is electrochemically oxidized to Am(VI) in these media, remains in the aqueous phase, since it reduces only to Am(V) when contacting the extractant. The separation factor of the couple Cm(III) Am(VI) is about 10³.     

Extraction studies of trivalent ions from TALSPEAK medium using phsophonic acid-TBP solvent mixture

Indigenously synthesized extractant, phenyl (octyl) phosphonic acid (POPA) in tri-n-butylphosphate (TBP) and dodecane, has been investigated for the separation of americium from trivalent lanthanides in nitric acid medium as well as diethylene triaminepentaacetic acid (DTPA) and lactic acid mixture (TALSPEAK medium). Various experimental parameters like concentration of DTPA, lactic acid, TBP, nitrate ions and pH of the aqueous feed solution have been optimized to obtain the highest separation factor between americium and europium. Bulk actinide–lanthanide separation reagent, tetra (ethylhexyl) diglycolamide (TEHDGA), was equilibrated with simulated solution of americium and lanthanides, equivalent in concentration to the reprocessing waste originating from PHWR spent fuel. DTPA/lactic acid mixture was used to strip the metal ions from the loaded organic phase and re-extracted into POPA in TBP/dodecane to evaluate the separation factor of individual lanthanides with respect to americium. Very good separation factors between americium and trivalent lanthanides were obtained.     

Spectrochemical analysis of curium and americium samples

Spectrochemical procedures have been developed to determine impurities in americium and curium samples. The simultaneous separation of many impurity elements from the base material (americium and curium) is carried out with extraction and extraction-chromatographic methods using di(2-ethyl hexyl phosphoric acid (D2EHPA).

It is shown that part of the elements (alkalis, alkaline earths, silicon, tungsten, tantalum and other elements) are separated with extraction or sorption of americium and curium; the other part (rare earths, titanium, zirconium, niobium, molybdenum) with the Talspeak process.

Two fractions in the extraction chromatography and three fractions in the extraction separation of americium and curium, containing impurities, are analyzed separately by a.c. or d.c. arc spectrography. To increase the sensitivity of the spectrographic analysis and accelerate the burn-up of impurities from the crater of the carbon electrode bismuth fluoride and sodium chloride were used as chemically active substances. The extraction of impurities from weighed quantities of americium and curium samples of 5–10 mg permits the lower limit of determined impurity concentrations to be extended to 1 × 10⁻⁴–5 × 10⁻³% m/m.     

Separation of curium from americium using composite sorbents and complexing agent solutions: part 2

TODGA–PAN composite sorbent and (PhSO₃H)₂–BTPhen in nitric acid solution were employed as a system for separation of curium from americium. The influence of aqueous phase composition (complexing agent and nitric acid concentrations) on weight distribution coefficients and Cm/Am separation factor was studied in batch experiments with trace amounts of ²⁴¹Am and ²⁴⁴Cm. Based on the results obtained, column experiment was designed and conducted. The Cm/Am separation factor of 3.8 ± 0.1 found in batch experiments with TODGA–PAN could be reproduced also in column experiment resulting in good separation of Cm from Am. The efficiency of Cm separation from Am in the TODGA–PAN system was compared with the analogous system with DGA resin (Triskem International). After separation on a 0.5 mL column (φ4.7 × 29 mm) the Cm fraction containing 93% of Cm(III) contained only 3% of Am(III) in optimum conditions.

     

Extraction chromatographic behaviour of Am/IV/ in the system: H2SO4-K10P2W17O61-Primene JMT

The extraction chromatographic separation of Am/IV/ and Cm/III/ in the H₂SO₄-K₁₀P₂W₁₇O₆₁-Primene JMT system was studied. The elaborated method permitted to purify effectively americium. The yield of americium at an initial concentration of 10^–3M is about 93–95%, the curium content does not exceed 0.1% of the initial concentration.     

Relationship between the solubilities and separation factors for the co-crystallization of lanthanide ethylsulphates

Co-crystallization coefficients of lanthanide ethylsulphates [Ln(H₂O)₉] (C₂H₅SO₄)₃ were determined and compared with the solubility of these salts. In contrast to the common opinion denying any simple correlation between co-crystallization coefficients and solubilities it was established that a simple relationship does exist for ethylsulphates of light lanthanides. It was concluded that in the case of light lanthanides an almost complete compesation of activity coefficients in the RATNER equation is due to the similar structure of the hydrated cations [Ln(H₂O)₉]³⁺ in both the solid and the aqueous phase. For ethylsulphates of heavy lanthanides such a compensation was not observed, since in this case the cations in the aqueous phase are probably hydrated by 8 water molecules, while in the solid phase the coordination number is still 9. The solubility of Pm, Pu, Am and Cm ethylsulphates was determined by the method “from matrix”. Presented in part at the Italian-Polish Meeting on the Properties and the Development of Methods for Separating Lanthanide Fission Products and Transuranian Elements in Post-Irradiation Analysis of Nuclear Fuels, held in Rome, 19–26 November 1974.     

Nitric Acid Extraction into TODGA

Advanced solvent extraction processes, namely DIAMEX, SANEX or GANEX, for the separation of the minor actinides (americium and curium) are under development within Europe. The tridentate diglycolamide ligand, TODGA, shows many interesting properties and is under investigation in conjunction with a variety of other extractants for the DIAMEX and SANEX processes as well as the GANEX process. In order to successfully demonstrate these processes, understanding the acid extraction into the organic phases is critical to process flowsheet design and modelling. Here nitric acid extractions into TODGA have been measured and models produced using an equilibrium based approach accounting for nitric acid activities in the aqueous phase.     

Hexanoic acid as an alternative diluent in a GANEX process: feasibility study

Used nuclear fuel is radiotoxic for mankind and its environment for a long time. However, if it can be transmuted, the radiotoxicity as well as its heat load are reduced. Before a transmutation the actinides within the used fuel need to be separated from the fission, corrosion and activation products. This separation can be achieved by using the liquid–liquid extraction technique. One extraction process that can be used for such a separation is the Group ActiNide EXtraction (GANEX) process. One GANEX process that can successfully accomplish the separation utilizes the diluent cyclohexanone in combination with the extractant tributylphosphate (TBP) (30 % vol) and a second extractant, CyMe₄-BTBP (10 mM). However, there are some issues when using cyclohexanone as diluent. In this work an alternative diluent has therefore been tried in order to determine if it can replace cyclohexanone. The diluent used was hexanoic acid. In a system containing 10–12 mM CyMe₄-BTBP and 30 % vol TBP in hexanoic acid with the aqueous phase 4 M HNO₃, the distribution ratios for americium and curium are unfortunately low (D _Am = 1.1 ± 0.27, D _Cm = 1.6 ± 1.81). The concentration of CyMe₄-BTBP ligand, the extractant of curium and americium, could unfortunately not be increased, because of limited solubility in hexanoic acid. The distribution ratios for fission, corrosion and activation products were low for most metals; however, cadmium, palladium and molybdenum all unfortunately have distributions ratios above 1. To conclude, low americium and curium extractions indicate that hexanoic acid is not a suitable diluent which could replace cyclohexanone in a GANEX process.     

Picomolar Traces of Americium(III) Introduce Drastic Changes in the Structural Chemistry of Terbium(III): A Break in the “Gadolinium Break”

The crystallization of terbium 5,5′‐azobis[1H‐tetrazol‐1‐ide] (ZT) in the presence of trace amounts (ca. 50 Bq, ca. 1.6 pmol) of americium results in 1) the accumulation of the americium tracer in the crystalline solid and 2) a material that adopts a different crystal structure to that formed in the absence of americium. Americium‐doped [Tb(Am)(H₂O)₇ZT]₂ ZT⋅10 H₂O is isostructural to light lanthanide (Ce–Gd) 5,5′‐azobis[1H‐tetrazol‐1‐ide] compounds, rather than to the heavy lanthanide (Tb–Lu) 5,5′‐azobis[1H ‐tetrazol‐1‐ide] (e.g., [Tb(H₂O)₈]₂ZT₃⋅6 H₂O) derivatives. Traces of Am seem to force the Tb compound into a structure normally preferred by the lighter lanthanides, despite a 10⁸‐fold Tb excess. The americium‐doped material was studied by single‐crystal X‐ray diffraction, vibrational spectroscopy, radiochemical neutron activation analysis, and scanning electron microcopy. In addition, the inclusion properties of terbium 5,5′‐azobis[1H‐tetrazol‐1‐ide] towards americium were quantified, and a model for the crystallization process is proposed.     

Implementation of Americium Separation from a PUREX Raffinate

Recovering of minor actinides from spent nuclear fuel is investigated for heterogeneous recycling in Generation-IV reactors. After plutonium, americium is the main contributor to residual heat of long term radioactive waste which determines waste density within geological repository. Selective americium separation (EXAm process) by liquid-liquid extraction from a PUREX raffinate was studied. Two experiments were performed in ATALANTE facilities. The first test, on surrogate solution, validated the americium extraction performance. The second trial was carried out in the high-level shielded process line from a genuine PUREX raffinate. Tools used to manage the process are introduced to show performance process achievement.