nuclear science and technology

(1)

EUROPEAN COMMISSION

nuclear science and technology

Partitioning: New Solvent Extraction Processes for Minor Actinides

(PARTNEW)

Contract N^o FIKW-CT2000-00087

Final report

Work performed as part of the European Atomic Energy Community's R&T specific programme Nuclear Energy 1998-2002, key action Nuclear Fission Safety (Fifth Framework Programme)

Area: Safety of the fuel cycle Directorate-General for Research

2007 Euratom

(2)

Project coordinator (responsible for this final report) Charles Madic, CEA (FR)

Project partners

CEA-DEN Commissariat à l’énergie atomique, Direction de l’Energie Nucléaire (FR) CEA-DSM Commissariat à l’énergie atomique, Direction des Sciences de la Matière (FR) UREAD University of Reading (UK)

CTU Chalmers University of Technology (SE)

ITU Institute for Transuranium Elements, JRC, Karlsruhe (EU) ENEA Ente per le Nuove Tecnologie, l’Energia e l’Ambiente (IT) PoliMi Politecnico di Milano (IT)

FZK-INE Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung (DE)

FZJ-ISR Forschunsgzentrum Jülich, Institut für Sicherheitsforschung und Reaktortechnik (DE) CIEMAT Centro de Investigaciones Energéticas, Medioambientales y Tecnologicas (ES) UAM Universidad Autonoma de Madrid (ES)

Scientists involved (the name of the responsible scientist of each laboratory is underlined) CEA-DEN: J.M. Adnet; B. Amekraz; P. Baron; G. Bernier; C. Berthon; L. Berthon; I. Bisel; G. Borda; B.

Camès; M.-C. Charbonnel; S. Colette; L. Couston; C. den Auwer; M. Dobler; G. Ferlay; N. Frederich;

S. Giroux; P. Guilbaud; D. Guillaneux; D. Guillaumont; F. Gutierrez; X. Héres; C. Hill; J. Laurent; M.

Lecomte; D. Lemaire; J. Leybros; C. Madic; M. Miguirditchian; C. Moulin; C. Nicol; A. Pichon; C.

Rabbe; M. Ranchoux; P. Rivalier; H. Roussel; L. Venault

CEA-DSM: B. Abécassis; C. Mandin; L. Martinet; S. Nave; N. Peineau; F. Testard; T. Zemb

UREAD: C. Boucher; M. G. B. Drew; M. R. StJ. Foreman; P. Giddings; L. M. Harwood; M. J. Hudson;

P. B. Iveson; K. Kennedy; V. Norman; T. G. A. Youngs

CTU: S. Andersson; C. Ekberg; Å. Enarsson; J.-O. Liljenzin; M. Nilsson; G. Skarnemark

ITU: S. Birck; B. Christiansen; J.-P. Glatz; S. Hollas; R. Malmbeck; K. Römer; C. Scheppler; D.

Serrano-Purroy; E. Teixera; W. de Weerd; S. van Winckel ENEA: G. Bilancia; M. Ferrando

PoliMi: A. Facchini; M. Giola; F. Maluta; M. Mazzuccato

FZK-INE: M. A. Denecke; A. Geist; K. Gompper; U. Müllich; M. Weigl FZJ-ISR: G. Modolo; S. Nabet; S. Seekamp; H. Vijgen

CIEMAT: A. G. Espartero; J. L. Gascon; A. G. de la Huebra; J. A. Suárez UAM: J. de Mendoza; P. Prados

(3)

Executive summary

1. Objectives

The final disposal of vitrified high-active wastes issuing the reprocessing of nuclear spent fuels in an underground repository is a complex problem mainly related to the fact that these wastes contain long-lived radionuclides with high radiotoxicity and this for a very long period of time, i.e. thousands to millions of years. A possible solution to this problem is not to incorporate these long-lived radionuclides into the vitrified wastes. After separation (partitioning), before the vitrification step, the long-lived radionuclides can be destroyed (transmutated) into short- or medium-lived or stable nuclides by nuclear means. This is the so- called partitioning-transmutation strategy (P&T) under study in several countries. Among the long-lived radionuclides to remove from the HAW (i.e. the aqueous raffinate or concentrate issuing the PUREX process), those belonging to the so-called minor actinides (MAs), Np, Am and Cm, are the most important to eliminate. The partitioning of neptunium (Np) can certainly be done by the PUREX process. This is not the case for Am and Cm, which exist at the oxidation state +III in the spent-fuel aqueous dissolution liquor and which possess very low affinity for tri-n-butylphosphate (TBP), the extractant of the PUREX process.

The aim of the research carried out within PARTNEW was to study and define efficient extraction processes for the partitioning of the trivalent Am and Cm ions from the HAW and for mutual Am(III)/Cm(III) separation.

The work programme was organised into eight work packages (WP) corresponding to five domains:

• DIAMEX basic and process development studies (WP1 & WP2);

• SANEX basic and process development studies with N-polydentate ligands (WP3 & WP4);

• SANEX basic and process development studies with bis-(substituted-phenyl)- di-thiophosphinic acid + neutral ligand synergistic mixtures (WP5 & WP6);

• SANEX basic studies with new S-bearing ligands (WP7);

• Am(III)/Cm(III) separation: basic and process development studies (WP8).

2. Main results

2.1. DIAMEX basic and process development 2.1.1. Basic studies

2.1.1.1. Malonamides

The malonamides, with the following formula: RR’NC(O)CHR”C(O)NR’R, with R, R’

= alkyl groups and R” = alkyl or oxyalkyl group, are efficient extractants of the nitrates of the trivalent 5f (An) and 4f (Ln) elements from aqueous nitric acid solutions, as those of the HAW issuing the PUREX process. Numerous basic studies have been carried out to better understand the interaction of these ligands with the nitrates of the trivalent Ln and An elements. Thermodynamic studies of complexation (with tetraethylmalonamide, TEMA) and extraction (with DMDBTDMA and DMDOHEMA) demonstrated that the driving force for the reactions was entropy for complexation and enthalpy for extraction, even if malonamides

(8)

are bidentate extractants. The main stoichiometry of the extracted complexes is: ML2(NO3)3, with L = malonamide molecule. This stoichiometry value was determined by several techniques such as: ° extraction data slope analysis, ° ESI-MS spectroscopy, ° NMR spectroscopy, ° TRLIFS spectroscopy.

The structures of the extracted complexes were determined by NMR spectroscopy and by TRLIFS, in the case of Eu(III). It was demonstrated that the two malonamide ligands engaged in the extracted complexes are both bidentate and bound to the M(III) ion through their O-carbonyl atoms. These results were confirmed by molecular modelling studies using quantum chemistry and molecular dynamic approaches of the reactions. For the diamide DMDOHEMA possessing an ether-O within its R” central group, it was demonstrated that the high efficiency of this malonamide for M(III) nitrate extraction is connected to the fact that water molecule(s) create links between the M(III) ion and the ether-O of the malonamide molecules in the extracted complex.

The kinetics of extraction of M(III) nitrates were studied with AMADEUS (CEA-DEN) and Nitsch (FZK-INE) experimental cells. It was shown that for the selected experimental conditions, the extraction rate was limited by diffusion phenomena.

The supramolecular aspect of the extraction of polar compounds such as water, nitric acid and metallic nitrates by malonamides was also studied by CEA-DSM scientists. Among the methods used, small angle diffusion spectroscopies, with X-rays (SAXS) or neutrons (SANS), were found highly successful. It was demonstrated that due to their surface active properties, the malonamide extractants were organised into reverse micelles interacting through an attractive potential. These reverse diamide micelles formed by a polar core (water, solute and extractant polar part) of 1.1-1.4 nm in diameter have an aggregation number between 4 and 10. The van der Waals attractions between the cores of the reverse micelles were found to be the key for the understanding and modelling of the formation of the "third phase" (i.e. the splitting of the organic phase into two phases, one essentially made by the malonamide-solute compound and the second by the diluent). This phenomenon was studied:

1) for a certain malonamide in solution in alkane diluents with varying chain lengths, 2) for a certain aliphatic diluent, with malonamides with increasing alkyl chain lengths. The important result obtained from this study is the fact that opposite trends were observed for both variables: decreasing the chain length of the alkane diluent or increasing the length of the alkyl substituents of the malonamide minimised the occurrence of the third phase. Those solutions behave as classical reverse micelles made of surfactant and the generalities of phase transition in classical reverse microemulsion can be used to predict qualitatively the third phase apparition. Moreover, a physical model (sticky hard sphere: Baxter) was found efficient to predict the third phase boundaries of the malonamide-solute organic solutions.

2.1.1.2. Bis-malonamides

As noted above, M(III) nitrates extracted by malonamides form 1/2 complexes. That is why UAM scientists proposed to synthesise bis-malonamide molecules, expecting thus a better affinity of these new extractants in comparison with the corresponding malonamides. A total of 26 bis-malonamides, with: ° different linkers between the two malonamide moieties, ° different substituents on the amidic functions, were prepared at UAM and their extracting properties vs trivalent Ln and An nitrates were studied by CIEMAT and CEA-DEN scientists.

Among the difficulties encountered with these totally new extractants, one can mention their poor solubility in aliphatic diluents. So, chlorinated diluents were used for the extraction studies. Among the large series of bis-malonamides prepared, the so-called UAM-007, was found to possess interesting extraction properties.

(9)

2.1.2. Process development studies 2.1.2.1. Malonamides

The most suitable malonamide for DIAMEX process development was found to be the dimethyldioctylhexylethoxymalonamide (DMDOHEMA). Numerous process works were carried out with this extractant. One can cite the most important results obtained.

• At FZK-INE, it was shown that the DIAMEX process can be implemented successfully on synthetic spiked High-active Raffinate (HAR) using the extraction devices named hollow-fibre-modules (HFM). Several successful tests were carried out.

• At CEA-Marcoule, the degradation of the DMDOHEMA solvent due to hydrolysis and radiolysis was studied. The degradation products were identified and solvent clean-up processes were developped. Tests of the best solvent clean-up process was carried out in the MARCEL test-loop which comprises a γ-irradiator for solvent degradation and a bank of centrifugal contactors for implementing the solvent clean- up process. Good results were obtained for the tests carried out.

• A special flow sheet has been defined for the DIAMEX treatment of High-active Concentrate (HAC). Several partners were involved in this research, including: ° ENEA and PoliMi who worked on cold HAC, FZJ who worked on cold and spiked HACs, °ITU for the hot test on a genuine HAC. The flow sheet of that test was calculated at CEA-DEN. This hot test was carried out at the ITU in the summer of 2003 and the HAC concentration factor compared to the corresponding HAR was equal to 10.

Excellent results were obtained, and this certainly corresponds to a major event in the domain of the partitioning of minor actinides.

2.1.2.2. TODGA

FZJ scientists considered it interesting to study the diamide TODGA molecule, first proposed for MA partitioning by Japanese scientists from JAERI. TODGA possesses higher affinity for An(III) and Ln(III) nitrates than the malonamides. After having studied batch extraction of the elements contained within a HAR, a process flow sheet was defined and tests on spiked HAR were performed very successfully using a bank of centrifugal contactors.

2.2. General consideration for SANEX extraction systems

As the product of the implementation of the DIAMEX process is a nitric acid solution containing the mixture of trivalent An (Am and Cm) and Ln (about 30 times more abundant than the An in molar ratio for UOX1 EDF’s spent fuel), it is necessary to define a Selective ActiNide EXtraction (SANEX) process for An(III)/Ln(III) separation. To discriminate between the An(III) and Ln(III) ions, one possibility is to use extracting agents having donor atoms less hard than O. So one can consider N and S as donor extractants.

(10)

2.3. SANEX basic and process development studies with N-polydentate ligands 2.3.1. Basic studies

A very large quantity of work has been done in this field within PARTNEW. The most important results obtained are briefly summarised below.

• Syntheses of nitrogen polydentate ligands and mixed O or S and N polydentate ligands. Numerous new ligands have been synthesised by traditional and also by combinatorial methods. The syntheses were related to: ° new BTPs with high resistance towards hydrolysis and radiolysis, ° hemi-BTPs, ° oxazolo(4,5, b)pyridine and ° thiazolo(4,5,b)pyridine. The ligands prepared at UREAD were used at Reading for basic studies, like the determination of the crystal structures of the complexes formed between these ligands and Ln(III) salts, and samples of the ligands were sent to CTU and CEA-DEN-Marcoule for the determination of their extracting properties for An(III)/Ln(III) separation.

• Computer modelling for ligand design. Numerous calculations have been done on these systems at UREAD, CTU and CEA-DEN using quantum chemistry and molecular dynamic models. The results of these calculations were generally found to be in good agreement with experimental results. This will be helpful for future ligand design.

• Structural studies. Numerous structural methods have been used to study the interaction between M(III) ions and the polydentate N and O+N ligands.

Among the main methods used one can cite: ° X-ray diffraction, ° EXAFS, ° NMR, ° ESI-MS and ° TRLIFS. Most of the work done concerned Ln(III) owing to the fact that most of these methods cannot yet be used for the study of radioactive actinides. The information obtained from these studies demonstrated the tridentate character of all the N-bearing ligands studied. For the BTP ligands, it was shown that the 1/3 complex LnL33+ was formed preferentially even for low L/Ln molar ratio. In this complex, the inner- coordination sphere of the Ln(III) ion is totally occupied with 9 N atoms from 3 BTP ligands. The nitrate counter-ions are located in the outer co-ordination sphere of Ln(III). This property was shown for crystalline compounds (X-ray diffraction study done at UREAD) and also for complexes in MeOH/H2O solutions (ESI-MS study done at CEA-DEN).

• Thermodynamics and kinetics of extraction and complexation. Numerous studies related to these fields were done in several partner laboratories: ° thermodynamics (CEA-DEN ; CTU), ° kinetics (FZK-INE ; CEA-DEN). The thermodynamic studies were related to the following reactions: ° ligand protonation, ° M(III) complexation, ° M(III) extraction with polydentate ligands alone or in synergistic mixture with α-Br-decanoic acid. In the complexation and extraction studies of Ln(III) and An(III) ions, it was shown that the reactions are mostly driven by their enthalpic term, and, for corresponding Ln(III) and An(III), the enthalpic term was found larger for An(III) than for Ln(III). So, this enthalpic difference, possibly connected to more covalent N-M(III) bonds within the complexes for An(III) vs Ln(III), is mostly responsible for the An(III)/Ln(III) selectivity observed for both complexation and extraction reactions. Series of N-bearing ligands prepared at

(11)

UREAD by combinatorial synthesis, were studied at CEA-DEN for the determination of their Am(III)/Eu(III) extraction and separation properties and the results obtained were treated by a QSAR method, which worked well. This type of successful study was the first ever done in this domain of chemistry.

The kinetics of the extraction of Ln(III) and Am(III) by BTP ligands were extensively studied at FZK-INE and CEA-DEN. It was shown that the kinetics of extraction were not so rapid and that the extraction rates were limited chemically. This phenomenon is worst in the case of the so-called BATPs, which possess very high Am(III)/Eu(III) selectivity. But, it was shown that the M(III) extraction rate can be increased by the addition of a phase transfer catalyst (the diamide, DMDOHEMA) into the organic phase. Moreover, it must be noted that the resistance of the BATP molecules vs hydrolysis (in the presence of nitric acid) and radiolysis is strongly reinforced in comparison to the BTPs.

2.3.2. Process studies

• BTP ligands. Process studies using BTP ligands were developed at CEA-DEN, FZK-INE, and ITU. At FZK-INE, it was shown that a SANEX process could be implemented in HFM using n-Pr-BTP extractant. At ITU, batch extraction tests were performed using genuine An(III)+ Ln(III) mixture using the n-Pr- BTP extractant. At CEA-DEN, SANEX hot tests with genuine An(III) + Ln(III) mixtures were performed using first n-Pr-BTP, then i-Pr-BTP. Even if the results for the tests were found very good, a major drawback was identified:

the BTP extractants were found not sufficiently resistant towards hydrolysis and radiolysis to be proposed as possible industrial extractants. That is why new basic research was carried out to design resistant ligands and the BATP extractants were therefore prepared at UREAD.

• BADPTZ + α-CN-decanoic acid + malonamide tri-synergistic mixture. This tri-synergistic mixture permits to obtain sufficient affinity for the An(III) and Ln(III) nitrates to be extracted without third phase formation. ENEA and PoliMI defined a SANEX process based on the use of BADPTZ + α-CN- decanoic acid + malonamide tri-synergistic mixture. A process flow sheet was defined at PoliMI and a successful test was carried out on synthetic spiked solution at ENEA using a bank of centrifugal contactors.

2.4. SANEX basic and process development studies with bis-(substituted-phenyl)-di- thiophosphinic acid + neutral ligands synergistic mixtures

2.4.1. Basic studies

This work was essentially done by FZJ-ISR and FZK-INE scientists. The ligands studied were: ° the bis-(chlorophenyl)-di-thiophosphinic acid ((ClPh)2PSSH), ° neutral organo-phosphorous extractants, such as TBP, TEHP and TOPO. The basic studies carried out concerned: ° the thermodynamics of extraction of Ln(III) and An(III) ions, ° the kinetics of extraction of Ln(III) and An(III) ions, ° the structures of the extracted complexes. The main results obtained are summarised below.

• The extraction reaction of Ln(III) and An(III) ions was the following:

(12)

← +

+ +3HA+sS⎯⎯→MA S +3H

M³ ₃ _s

with HA = (ClPh)2PSSH, and S = a neutral synergist. The value of x depends on the nature of the extracted M(III) ion and of S. For example, for S = TEHP, x= 2 for La, Ce, Pr, Nd, Am and Cm, and x= 3 for Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu and for Y. It was found that the Am(III)/Ln(III) separation factors (SFAm/Ln) were very important, and for more than 10 Ln(III) ions, SFAm/Ln >

10³. Owing to the synergistic behaviour of the extractant mixture(s), the An(III) distribution ratios were found high, even for acidic aqueous solutions (HNO3 = 0.1 mol/L) and this was considered important for An(III)/Ln(III) separation process design.

• The thermodynamic properties, ∆G^°, ∆H^° and ∆S^° for M(III) synergistic extraction were determined for the whole series of Ln(III) and for Am(III) and Cm(III), in the case of S = TEHP (the solvent was t.butylbenzene). For all the M(III) ions studied, it was found that the driving force for extraction was the enthalpy ∆Η°. Important variations were observed for enthalpy values within the Ln(III) series of ions. For example, the more negative value was found for Gd (-67.03 kJ/mol) and the least was observed for Nd (-22.25 kJ/mol).

Moreover, an interesting and important entropy/enthalpy compensation effect was observed. So, for the two Ln(III) ions noted above, the ∆G^° values were found to be very close: -3.06 kJ/mol (Gd) and -3.88 kJ/mol (Nd). The origin of the Am(III)/Ln(III) selectivity was not found to be the enthalpy but the entropy. For Am(III), ∆H^° and ∆G^°were found equal to -27.92 and -14.47 kJ/mol, respectively, values to be compared with those of Eu(III): ∆H^° (-25.64 kJ/mol) and ∆G^° (-6.04 kJ/mol). Such a result is astonishing and was not expected, because it was thought that the bonding between S atoms of the (ClPh)2PSS^- anions and the M(III) ions within the extracted complexes would be more covalent for An(III) than for Ln(III) and thus, larger enthalpy values were expected for the extraction of An(III). Theoretical explanation is required to interpret these results.

• The kinetics of the extraction of Am(III), Cm(III) and Eu(III) by the synergistic mixture made of (ClPh)2PSSH + TOPO was studied at FZK-INE using a Nitsch cell. It was found that for both extraction and back-extraction of these M(III) ions, the extraction rates were found very close, all together, and limited by the diffusion of the metallic species.

• The structural aspects of the extracted complexes were studied by EXAFS at the APS synchrotron facility in Argonne (USA) by FZK-INE scientists. The systems studied were: ° Cm(III) extracted by (ClPh)2PSSH + TBP or TEHP or TOPO, ° Eu(III) extracted by (ClPh)2PSSH + TBP or TOPO. It was shown that: ° the first co-ordination spheres of Cm(III) and Eu(III) extracted complexes were different, ° (ClPh)2PSS^- anions are bound to Cm(III) in a bidentate fashion, ° (ClPh)2PSS^-and the synergist ligand are both bound to the M(III) cation, ° from the S/O intensity ratios of the spectra, one can conclude that (ClPh)2PSS^-preferentially binds to Cm, compared to Eu, ° a high SFCm/Eu

value is correlated to a high S/O intensity ratio in the spectra.

(13)

2.4.2. Process development studies

Two teams, FZJ-ISR and FZK-INE worked for the development of a SANEX process for An(III)/Ln(III) separation. The main system studied was made of (ClPh)2PSSH + TOPO. This is the so-called SANEX-ALINA process. The main results obtained are summarised below.

• At FZJ-ISR, a process flow sheet, based on the use of the following solvent:

0.5 M (ClPh)2PSSH + 0.15 M TOPO + 2 % TBP in t-butylbenzene + 20 vol. % iso-octane and with a synthetic spiked aqueous feed with a nitric acid concentration of 0.5 mol/L, which is rather high, was tested using a centrifugal contactor test-loop. Good performances were observed: ° Am(III) extraction yield was good, good decontamination of Am(III) vs Ln(III) was obtained, ° experimental and calculated Am(III) concentration profiles were in good agreement. Nevertheless, improvement is still required because 20 % of Cm(III) inventory were left within the aqueous raffinate.

• At FZK-INE, the SANEX-ALINA process was tested with HFMs using the solvent made of: 0.5 M (ClPh)2PSSH + 0.25 M TOPO in t-butylbenzene. Large and small HFM set-ups were used. Very good Am(III)/Ln(III) separation performances were obtained. These separation performances were found to depend importantly on the flow rates of the aqueous solutions within the HFMs.

2.5. SANEX basic separation studies using new S-bearing ligands

Numerous new S-bearing ligands were synthesised by UREAD, UAM and FZJ-ISR scientists and their extracting and An(III)/Ln(III) separation properties were then tested at CIEMAT, CEA-DEN and FZJ-ISR. The main results obtained are summarised below.

• Five S-bearing ligands were prepared by UREAD. They were substituted imino-dithiophosphines. Their extraction and Am(III)/Eu(III) separation properties were found at CEA-DEN to be very poor. This was also the case for thiomalonamides and thioglycolamides prepared at UAM and tested at CIEMAT.

• A series of ten bis-substituted-di-thiophosphinic acids was prepared and studied at FZJ-ISR. Their Am(III) and Eu(III) extraction and separation properties were studied in the following conditions: ° solvent: 0.5 mol/L bis- substituted-di-thiophosphinic acid + 0.25 mol/L TOPO in toluene, ° aqueous solution: HNO3= 0.2 mol/L spiked with trace amounts of ²⁴¹Am and ¹⁵²Eu. The best extraction and separation properties were obtained with the bis-(FPh)-di- thiophosphinic acid: DAm close to 30 and SFAm/Eu close to 26 were observed.

Of course, while this ligand was interesting, its Am(III)/Eu(III) separation property was less interesting than that of (ClPh)2PSSH. So, only this last ligand was studied for the development of an An(III)/Ln(III) SANEX separation process (ALINA process).

(14)

2.6. Am(III)/Cm(III) separation, basic and process development studies

Two extraction systems were studied for the definition of Am(III)/Cm(III) separation, based on: ° malonamide, ° (ClPh)2PSSH + S synergistic mixture. The main results obtained are the following:

• Malonamide. Several malonamides and bis-malonamides (from UAM) were tested at CEA-DEN for their ability to separate Am(III) from Cm(III).

The SFAm/Cm related DMDOHEMA was found to be close to 1.6 and almost independent of the composition of the organic and aqueous phases, while that related to UAM-007 was smaller, 1.2 to 1.3. Although the SFAm/Cm was modest, the results related to the DMDOHEMA extractant were considered interesting for the design of an Am(III)/Cm(III) separation process. A successful active test was carried out at CEA-Marcoule using a bank of mixer-settlers with 56 stages.

• (ClPh)2PSSH + TEHP. The study related to this system was first initiated by FZJ scientists, then FZK scientists collaborated for the process design.

Among the different (ClPh)2PSSH + S synergistic systems studied, that with S = TEHP was found very efficient for Am(III)/Cm(III) separation.

Numerous batch studies were carried out to determine the effect of different parameters, such as: ° the composition of the aqueous phase (nitric acid and sodium nitrate concentrations), ° the nature of the organic diluent, on the selectivity of the Am(III)/Cm(III) separation. Values of SFAm/Cm

close to 10 were obtained, which are certainly among the most effective Am/Cm separation data ever obtained. A process flow sheet was designed, based on the use of the following experimental conditions: ° feed : 0.1 M HNO3 + 0.5 NaNO3, ° solvent: 0.4 M (ClPh)2PSSH + 0.15 M TEHP in t- butylbenzene + isooctane (20 vol %), ° scrubbing solution: 0.2 M HNO3 + 0.5 NaNO3, ° stripping solution: 0.5 M HNO3. The results obtained were quite interesting: ° 99.8 % of Am(III) inventory was extracted and then stripped, ° the Am(III) product contained 4.75 % of the Cm(III) inventory (to be compared with 6.4 % calculated), ° the raffinate contained 35 % of Cm(III) but only 0.05 % of Am(III) inventory. So, the results obtained are quite interesting and after some adjustments of the experimental conditions it is thought that this system will be promising for the design of an efficient Am(III)/Cm(III) separation process.

3. Conclusions

The research carried out within the PARTNEW programme was very successful.

Significant progress was obtained for the basic understanding of the solvent extraction chemistry of An(III) and Ln(III), based on experimental results but also on theoretical ones obtained by computer calculations. Concerning process developments, one can mention that:

° the DIAMEX process, based on the use of the malonamide DMDOHEMA, is mature, even for the treatment of HAC,

(15)

° the SANEX process based on BTP, while giving good An(III)/Ln(III) separation performances cannot be proposed for an industrial development owing to the insufficient stability of the BTP extractant,

° the An(III)/Ln(III) SANEX- ALINA separation process seems good,

° the DMDOHEMA and (ClPh)2PSSH + TEHP extraction systems are also promising for the development of Am(III)/Cm(III) separation processes.

(16)

(17)

SECTION 1: Objectives

1. Overview

In Europe, about 35 % of the electricity consumed comes from nuclear reactors. The spent fuels discharged annually from the reactors are considered by some countries as wastes and by others as sources of energetic valuable materials, U and Pu, for further electricity production. In this last case, U and Pu are recovered from the spent fuels by reprocessing using the PUREX solvent extraction process. The nuclear wastes, composed mainly of fission products (FPs) and of minor actinides (MAs = Np, Am and Cm), issuing the PUREX process into the aqueous raffinate, are then incorporated into a solid glass matrix. The vitrified wastes or the spent fuels will in the future be disposed of in an underground repository. If we consider the radiotoxicity (in Sv/TWhe) of UOx1 spent fuel (with a burn-up equal to 33 GWd/t) as a function of time, as shown in Figure 1, one can observe that after three centuries, the radiotoxicity is essentially related to Pu and minor actinide nuclides. In case of reprocessing, the radiotoxicity of the vitrified wastes is essentially limited to the MAs.

10²

10 10² 10³ 10⁴ 10⁵ 10⁶

time (years)

radiotoxicity(Sv/TWhe)

Plutonium Total

Fission products Minor actinides

Uranium 10⁴

10⁶ 10⁸ 10¹⁰

10²

10 10² 10³ 10⁴ 10⁵ 10⁶

time (years)

radiotoxicity(Sv/TWhe)

Plutonium Total

Fission products Minor actinides

Uranium 10⁴

10⁶ 10⁸ 10¹⁰

Figure 1: Evolution of the radiotoxicity of UOx1 spent fuel (burn-up = 33 GWd/tinitial U)

So a solution to simplify the definition of an underground repository for the vitrified waste is not to incorporate these MAs into the glass waste. This means that the MAs should be removed from the high-active raffinate (HAR) or high-active concentrate (HAC) issuing the reprocessing of the spent fuels by the PUREX process prior to the vitrification of these high-active wastes (HAW). After separation of the MAs (partitioning) from the HAW, it can be considered to destroy them into short- or medium-lived or stable fission products by nuclear means (i.e. transmutation). This strategy for the management of the HAW is called partitioning and transmutation (P&T). Numerous countries in the world [1] are performing research in this domain. In Europe, two main directions have been chosen for partitioning MAs and research programmes have been financed by European Commission for a long period of time [2, 3]:

° Hydrometallurgy: Solvent extraction processes are developed for MA removal from the aqueous HAR issuing the implementation of the PUREX process

(18)

° Pyrometallurgy: The FPs and MAs contained within the HAR or HAC issuing the PUREX process implementation are transferred into a molten salt liquid phase (halide:

chloride or fluoride); then MAs are separated.

The partitioning of neptunium (Np) can certainly be done by the PUREX process after slight modifications of its implementing conditions in order to force the Np to be oxidised as Np(VI) which possesses, as U(VI), a rather high affinity for tri-n-butyl-phosphate (TBP), the extractant of the PUREX process. The partitioning of Am and Cm cannot be performed by the PUREX process because they exist in the acidic HAR or HAC effluents at the oxidation state +III, for which TBP presents almost zero affinity. The objectives of the research carried out within the PARTNEW programme were then to define solvent extraction processes

° of the trivalent Am and Cm, for their separation from the FPs,

° of Am(III) vs Cm(III) for their mutual separation, as both elements will follow different routes for nuclide transmutation.

2. Report framework

The work programme of PARTNEW was organised into eight work packages (WPs) corresponding to five domains:

• DIAMEX basic and process development studies (WP1 & WP2),

• SANEX basic and process development studies with N-polydentate ligands (WP3 & WP4),

• SANEX basic and process development studies with bis-(substituted-phenyl)- di-thiophosphinic acid + neutral ligand synergistic mixtures (WP5 & WP6),

• SANEX basic studies with new S-bearing ligands (WP7),

• Am(III)/Cm(III) separation: basic and process development studies (WP8).

The list of the molecules prepared and studied within the PARTNEW programme is presented in Annex I. Annex II gives a list of PARTNEW publications.

3. Partnership

Eleven European research teams, belonging to national nuclear research centres and to universities, participated in PARTNEW:

(1) CEA-DEN – Commissariat à l’énergie atomique, Direction de l’énergie nucléaire, Marcoule (France)

(2) CEA-DSM – Commissariat à l’énergie atomique, Direction des sciences de la matière, Saclay (France)

(3) UREAD – University of Reading, Reading (United Kingdom) (4) CTU – Chalmers University of Technology, Göteborg (Sweden) (5) ITU – Institute for Transuranium Elements, JRC, Karlsruhe (EU)

(6) ENEA – Ente per le Nuove Tecnologie, l’Energia e l’Ambiente, Saluggia (Italy) (7) PoliMi – Politecnico di Milano, Milano (Italy)

(8) FZK-INE – Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, Karlsruhe (Germany)

(9) FZJ-ISR – Forschungszentrum Jülich, Institut für Sicherheitsforschung und Reaktortechnik, Jülich (Germany)

(19)

(10) CIEMAT – Centro de Investigaciones Energéticas, Medioambientales y Tecnologicas, Madrid (Spain)

(11) UAM – Universidad Autonoma de Madrid (Spain).

(20)

SECTION 2: Presentation of the scientific results

1. DIAMEX basic and process development studies 1.1. Introduction

The malonamide molecules: RR’NC(O)CHR”C(O)NR’R, with R and R’ = alkyl groups and R” = alkyl or oxy-alkyl group, are interesting chelating extractants which possess good affinity for the nitrates of actinides(III) (An(III) = Am(III) and Cm(III)) and of lanthanides(III) (Ln(III)), from nitric acid aqueous solutions. These molecules are made only by C, H, O and N atoms (they follow the so-called CHON principle), so at the end of their use within a partitioning process, named DIAMEX for DIAMide EXtraction, they can be destroyed into gases (CO2, N2 and H2O), which can easily be released into the atmosphere.

So, no solid secondary wastes will be generated from the solvent when implementing the DIAMEX process. The main mechanism of the extraction reaction of the nitrates of An(III) of Ln(III) by a malonamide extractant (L) is the following (the species present in the organic phase are underlined):

(

₃

)

2 -

3

3 3NO 2L ML NO

M ⁺ + + ⎯⎯→^← (1)

In the ML2(NO3)3 extracted complexes, the malonamide molecules are bound to the M(III) ion in a bidentate fashion through the 2 carbonyl oxygen atoms. Moreover, the extracted solutes ML2(NO3)3 can interact and this can lead to the splitting of the organic phase into two layers, one made essentially by the diluent and the second by the M(III) extracted solute. This phenomenon, known as “third phase formation”, is of course a major drawback for a solvent extraction process. The research carried out within the DIAMEX domain concerned two fields:

° basic research (WP1). Numerous research works were done, including: 1/ the synthesis of bis-malonamides, 2/ the study of the thermodynamics of the complexation and extraction of An(III) and Ln(III) ions by malonamides and bis-malonamides, 3/ the determination of the structures of the complexes formed between M(III) nitrates and the malonamides, using numerous experimental methods, 4/ the study of the kinetics of An(III) and Ln(III) nitrate extraction by malonamides, 5/ the calculation of the structures of the M(III) complexes formed with malonamides using quantum and molecular dynamic methods, 6/ the study of the supramolecular organization of the organic solutions loaded after extraction of nitric acid and M(III) nitrates,

° process development (WP2). DIAMEX processes were developed and tested with: 1/

synthetic spiked HARs, 2/ genuine HAR and HAC. DIAMEX processes were based mostly on the use of the dimethyldioctylhexylethoxymalonamide (DMDOHEMA) and tetraoctyldiglycolamide (TODGA) extractants. Several extractor types were used for the implementation of the DIAMEX processes, including: 1/ centrifugal contactors (CCs) and 2/

hollow-fibre-modules (HFMs). The degradation of the DIAMEX solvent (with DMDOHEMA) by hydrolysis and radiolysis was intensively studied, as well as the methods for cleaning-up the degraded solvent.

(21)

1.2. DIAMEX basic studies

1.2.1. Synthesis of bis-malonamides

As shown in equation 1, two malonamide molecules are involved in the M(III) extracted complex ML2(NO3)3. So, UAM’s scientists proposed to prepare bis-malonamide molecules expecting for these new extractants better affinity for M(III) nitrates than for malonamides.

Several methods were used to link two malonamide moieties onto the same moleculs: 1/ using an aromatic spacer through the NH-, 2/ using aromatic or aliphatic spacer through the central carbons. An example of synthesis of a bis-malonamide corresponding to the first case is given in Figure 2, and the general structure of bis-malonamides corresponding to the second case is shown in Figure 3.

OH OC₁₄H₂₉

BrC₁₄H₂₉

OC14H29

Br Br

NBS

OC14H29

NH HN

O OMe O O

MeO O

OC14H29

NH HN

O NHBu O O

BuHN O K2CO3

CH3CN AIBN

Cl₄C

NK O

O 18-Crown-6 Toluene

Cl O

OMe O

Et₃N/CH₂Cl₂

OC₁₄H₂₉ N O

O N

O

O OC₁₄H₂₉

NH2 NH2 BuNH₂ MeOH

BuNH₂

(54%) (35%)

(89%) (74%)

UAM-059

OH OC₁₄H₂₉

BrC₁₄H₂₉

OC14H29

Br Br

NBS

OC14H29

NH HN

O OMe O O

MeO O

OC14H29

NH HN

O NHBu O O

BuHN O K2CO3

CH3CN AIBN

Cl₄C

NK O

O 18-Crown-6 Toluene

Cl O

OMe O

Et₃N/CH₂Cl₂

OC₁₄H₂₉ N O

O N

O

O OC₁₄H₂₉

NH2 NH2 BuNH₂ MeOH

BuNH₂

(54%) (35%)

(89%) (74%)

UAM-059

Figure 2: Synthesis of the bis-malonamide UAM-059.

N

N N

N O O

O O SPACER R

R'

R R'

R, R'= H, alkyl

Figure 3: General structure of bis-malonamide with spacer connected to central carbons.

Bis-glycolamides and tris-malonamides, were also synthesized. Figure 4 describes the synthesis of a tris-malonamide.

Cl

Cl Cl

N₃

N₃ N₃

NH₂

NH₂ H₂N MOM-Cl, SnCl4

CS₂, rt.

NaN₃, DMF, rt.

PPh₃ THF/H₂O rt.

(84%) (80%) (44%)

HN

NH NH

O O OCH₃

O OCH₃ O O H₃CO HN O

NH NH

O O NH

O NH O O HN

O

C₄H₉

C₄H₉ C₄H₉

C4H9NH2, rt

Cl OCH3

O O

NEt₃, CH₂Cl_2, rt

(40%) UAM-062

Figure 4: Synthesis of the tris-malonamide UAM-062.

(22)

A total of 26 bis-malonamides, 4 bis-glycolamides and 1 tris-malonamide were prepared and their extracting properties were studied at CIEMAT and for some of them at CEA-Marcoule. Some of the molecules prepared are presented in Table 1.

Structure Name Provider Acronym

N O

N O N

O N O H

Bu Bu H H

H

N-Butyl-N’-{3-[(2-butylcarbamoyl acetylamino)methyl]

benzyl}malonamide

University Autónoma of

Madrid

UAM- 007

HN O

NHBu O HN

O

NHBu O Et Et

N-Butyl-N’-[4-(2-butylcarbamoyl butyrylamino)phenyl]-2-ethyl malonamide

Madrid

UAM- 024

N O

H H

N O N

O H H

2,9-Bis(butyl carbamoyl)-N,N’- dibutyl decanediamide

Madrid

UAM- 049

N O

H H

N O N

O H H

2-[3-(2,2-Bisbutyl carbamoyl ethyl)benzyl}-N,N’-dibutyl malonamide

Madrid

UAM- 051

N O N

O

H H

O N

H O O Bu

HN

O Bu 2-Butylcarbamoyl methoxy-N-{3- [2-butyl carbamoylmethoxy acetylamino)methyl]

benzyl}acetamide

Madrid

UAM- 055

N O NH

O O

Bu

Bu (H₂C)₆

HN O N

O O

Bu Bu

N,N-Dibutyl-2-{[6-(2- dibutylcarbamoylmethoxy acetylamino) hexylcarbamoyl]

methoxy} acetamide

Madrid

UAM- 065

Table 1: Some of the bis-malonamides and bis-glycolamides prepared at UAM.

1.2.2. Thermodynamics of complexation and extraction of Ln(III) and An(III) by malonamides and bis-malonamides

The study of the complexation of Ln(III) in aqueous solution by the tetraethylmalonamide (TEMA) has been done by microcalorimetry, as shown in Figure 5. The solvent used was water. The interest of such a study is to obtain basic data on TEMA

(23)

complexation, which then will be helpful to better understand the extraction properties of malonamide extractants.

0 20 40 60

Time(min)

60 80 100

Power measured (µW) Dilution of TEMA in H₂O

Complexation of Nd(III) by TEMA + dilution of TEMA and Nd(III) in H₂O

0 20 40 60

Time(min)

60 80 100

Power measured (µW) Dilution of TEMA in H₂O

Complexation of Nd(III) by TEMA + dilution of TEMA and Nd(III) in H₂O

25°C - V=0.9 mL of Nd(NO3)3 0.05 M in H2O – addition of a 0.512 M TEMA solution in H2O by increments of 10 µL.

Figure 5: Heat associated to the addition of the first increments of TEMA solution, during the titration of Nd(III).

TEMA is a weak ligand for Ln(III), similar to the nitrate ligand, as shown by the thermodynamic data for the 1/1 complexes of Nd(III), presented in Table 2.

Ligand K’ (L.mol^–1) ∆G’ (kJ.mol^–1) ∆H’ (kJ.mol^–1) ∆S’ (J.mol^–1.K^-1)

NO3- 1.0 ± 0.3 0 ± 0.3 2.2 ± 1 7.5 ± 3

TEMA 0.8 ± 0.3 0.2 ± 0.4 12 ± 2 39 ± 8

Table 2: Thermodynamic data related to the complexation in water of Nd(III) by the ligands NO3- and TEMA (25°C – I = 0.3 M).

° The thermodynamics of the extraction of Ln(III) and Am(III) nitrates by malonamides were studied at different temperatures. Using the van’t Hoff equation it was then possible to determine the enthalpy and entropy values associated with the extraction of M(III) nitrates.

Examples of experimental results are presented in Figure 6.

0.1 1 10

0.0031 0.0032 0.0033 0.0034 0.0035 1/T (K^-1)

DLnor DAn

D_La D^Nd D_Eu D^Er D^Yb D_Am

L

0.1 1 10

0.0031 0.0032 0.0033 0.0034 0.0035 1/T (K^-1)

DLnor DAn

D_La D^Nd D_Eu D^Er D^Yb D_Am

L

[DMDBTDMA] = 0.5 M in TPH - [HNO3]aq= 3 M - M(III)]init= 0.01 M, except for Am(III) and Eu(III) in traces.

Figure 6: Variations of Ln(III) and Am(III) distribution ratios with the reciprocal temperature.

(24)

In this case, for Nd(III), the enthalpy of extraction was found equal to: ∆H^’= - (70 ± 10) kJ/mol. Exothermic extraction by DMDBTDMA (and other malonamide extractants) was observed compared to endothermic complexation by TEMA. More complete studies were then carried out for different malonamide concentrations and it was shown that the thermodynamics of M(III) nitrate extraction is certainly more complex than noted above, possibly owing to the fact that the organic solutions of malonamides can be the subject of the formation of supramolecular objects (see below). For example, Figure 7 shows that the number of DMDOHEMA ligands involved in the extraction of Nd(III) nitrate is varying with the temperature.

slope 4.2 slope 4.1 slope 3.8 slope 3.3 slope 2.1

-2 - 1.6 - 1.2 - 0.8 - 0.4 0

-0.5 -0.4 -0.3 -0.2 -0.1 0 log [ DMDOHEMA ^free]

log DNd/(1+β[NO3- ])

10°C 20°C 25°C 30°C 40°C slope 4.2

slope 4.1 slope 3.8 slope 3.3 slope 2.1

-2 - 1.6 - 1.2 - 0.8 - 0.4 0

log DNd/(1+β[NO3- ])

10°C 20°C 25°C 30°C slope 4.2 40°C

-2 - 1.6 - 1.2 - 0.8 - 0.4 0

log DNd/(1+β[NO3- ])

10°C 20°C 25°C 30°C 40°C slope 4.2

-2 - 1.6 - 1.2 - 0.8 - 0.4 0

log DNd/(1+β[NO3- ])

10°C 20°C 25°C 30°C 40°C

Experimental conditions : DMDOHEMA in dodecane; [NaNO3] = 2.2 M ; [Nd(III)] = 0.102 M

Figure 7: Variation of log DNd / (1 +β[NO₃^-]) function of log [free diamide] at different temperatures.

° The extraction of Ln(III) and Am(III) nitrates by bis-malonamides prepared at UAM were studied by CIEMAT scientists and some experiments were also done at CEA-DEN. As mentioned above, chlorinated instead of aliphatic diluents were used for the experiments in order to obtain sufficient solubility of the extractant(s) in the diluent. Figure 8 presents some results related to Am(III) extraction from nitric acid aqueous solutions by several bis- malonamides. Similar data were observed for Ln(III) nitrate extractions.

Extraction of Am(III)

10^-3 10^-2 10^-1 1 10

2.0 3.0 4.0 5.0 6.0 Concentration [HNO₃]_aq, M

Distribution coefficient, D

UAM-046 UAM-048 UAM-049 UAM-050 UAM-051 UAM-052

Extraction of Am(III)

10^-3 10^-2 10^-1 1 10

2.0 3.0 4.0 5.0 6.0 Concentration [HNO₃]_aq, M

Distribution coefficient, D

UAM-046 UAM-048 UAM-049 UAM-050 UAM-051 UAM-052

Experimental conditions: shaking time 10 minutes, ligand concentration 0.1M in tetrachloroethane.

Figure 8: Extraction of Am(III) by some bis-malonamides.

nuclear science and technology