Dissymmetric N,N-dialkylamides, the synthesis thereof and uses of same

Abstract

Novel dissymmetric N,N-dialkylamides which meet the following formula (I): ##STR00001##
where R represents a linear or branched alkyl group at C.sub.8 to C.sub.15. A method for synthesizing these N,N-dialkylamides, and to the uses of same as extractants, alone or in admixture, in order to extract uranium and/or plutonium from an aqueous acid solution, or to totally or separate uranium from plutonium from an aqueous acid solution and, in particular, an aqueous solution resulting from dissolving spent nuclear fuel in nitric acid. Further, a method for processing an aqueous solution resulting from the dissolution of a spent nuclear fuel in nitric acid, allowing the uranium and plutonium contained in the solution to be extracted, separated and decontaminated in a single cycle, without requiring any plutonium reduction operation, and in which one of the aforementioned N,N-dialkylamides or a mixture of same is used as extractant. Applications for the method include the processing of spent nuclear fuels, in particular comprising uranium (e.g. UOX) or uranium and plutonium (e.g. MOX).

Claims

1. An N,N-dialkylamide of formula (I): ##STR00006## where R is a linear or branched alkyl group having 8 to 15 carbon atoms.

2. The N,N-dialkylamide of claim 1, wherein the alkyl group comprises no more than 12 carbon atoms.

3. The N,N-dialkylamide of claim 2, wherein the alkyl group is an n-octyl, n-decyl, n-dodecyl, 2-ethylhexyl or 2-ethyloctyl group.

4. A method for synthesizing an N,N-dialkylamide of formula (I): ##STR00007## where R is a linear or branched alkyl group having 8 to 15 carbon atoms, comprising a reaction of a halide of formula (II): ##STR00008## where X is a halogen atom, with an amine of formula HN(CH.sub.3)R where R is a linear or branched alkyl group having 8 to 15 carbon atoms.

5. A method for extracting uranium(VI) and/or plutonium(IV) from an acid aqueous solution, comprising at least one contacting of the acid aqueous solution with an organic solution comprising an N,N-dialkylamide or mixture of N,N-dialkylamides of formula (I): ##STR00009## where R is a linear or branched alkyl group having 8 to 15 carbon atoms, as extractant in an organic diluent, followed by a separation of the aqueous solution from the organic solution.

6. The method of claim 5, wherein the organic solution comprises from 1 mol/L to 2 mol/L of the N,N-dialkylamide or mixture of N,N-dialkylamides.

7. A method for separating totally or partially uranium(VI) from plutonium(IV), uranium(VI) and plutonium(IV) being in an acid aqueous solution, comprising the following steps: a) a co-extraction of uranium and plutonium from the aqueous solution, the co-extraction comprising at least one contacting of the aqueous solution with an organic solution comprising an N,N-dialkylamide or mixture of N,N-dialkylamides of formula(I): ##STR00010## where R is a linear or branched alkyl group having 8 to 15 carbon atoms, as extractant in solution in an organic diluent, followed by a separation of the aqueous solution from the organic solution; b) a stripping of plutonium, in oxidation state +IV, and of a fraction of uranium from the organic solution resulting from step a), the stripping comprising at least one contacting of the organic solution with an aqueous solution comprising from 0.1 mol/L to 0.5 mol/L of nitric acid, followed by a separation of the organic solution from the organic solution; and c) an extraction of all or part of the uranium fraction contained in the aqueous solution resulting from step b), the extraction comprising at least one contacting of the aqueous solution with an organic solution identical to the organic solution of step a), followed by a separation of the aqueous solution from the organic solution; whereby there are obtained an aqueous solution comprising either plutonium without uranium, or a mixture of plutonium and uranium, and an organic solution comprising uranium without plutonium.

8. The method of claim 7, wherein the organic solution of step a) comprises from 1 mol/L to 2 mol/L of the N,N-dialkylamide or mixture of N,N-dialkylamides.

9. The method of claims 7, wherein the acid aqueous solution is an aqueous solution resulting from a dissolution of a spent nuclear fuel in nitric acid.

10. A single-cycle method for processing an aqueous solution resulting from a dissolution of a spent nuclear fuel in nitric acid, the aqueous solution comprising uranium, plutonium, americium, curium and fission products including technetium, the cycle comprising the following steps: a) a co-extraction of uranium and plutonium from the aqueous solution, the co-extraction comprising at least one contacting, in an extractor, of the aqueous solution with a first organic solution comprising a N,N-dialkylamide or mixture of N,N-dialkylamides of formula (I): ##STR00011## where R is a linear or branched alkyl group having 8 to 15 carbon atoms, as extractant in solution in an organic diluent, followed by a separation of the aqueous solution from the organic solution; b) a decontamination of the organic solution resulting from step a) with respect to americium, curium and fission products, the decontamination comprising at least one contacting, in an extractor, of the organic solution with an aqueous solution comprising from 0.5 mol/L to 6 mol/L of nitric acid, followed by a separation of the organic solution from the organic solution; c) a partitioning of the uranium and plutonium contained in the organic solution resulting from step b) into an aqueous solution comprising either plutonium without uranium, or a mixture of plutonium and uranium, and an organic solution comprising uranium without plutonium, the partitioning comprising: c.sub.1) a stripping of plutonium, in oxidation state +IV, and of a fraction of uranium from the organic solution resulting from step b), the stripping comprising at least one contacting, in an extractor, of the organic solution with an aqueous solution comprising from 0.1 mol/L to 0.5 mol/L of nitric acid, followed by a separation of the organic solution from the organic solution; c.sub.2) an extraction of all or part of the uranium fraction contained in the aqueous solution resulting from c.sub.1), the extraction comprising at least one contacting, in an extractor, of the aqueous solution with a second organic solution identical to the organic solution of step a), followed by a separation of the aqueous solution from the organic solution; d) a decontamination of the organic solution resulting from c.sub.1) with respect to technetium, the decontamination comprising: d.sub.1) a stripping of technetium, in oxidation state +IV, from the organic solution resulting from c.sub.1), the stripping comprising at least one contacting, in an extractor, of the organic solution with an aqueous solution comprising from 0.1 mol/L to 3 mol/L of nitric acid and at least one reducing agent capable of reducing technetium from oxidation state +VII to oxidation state +IV, followed by a separation of the organic solution from the organic solution; d.sub.2) an extraction of the uranium fraction contained in the aqueous solution resulting from d.sub.1), the extraction comprising at least one contacting, in an extractor, of the aqueous solution with an organic solution identical to the organic solution of a), followed by a separation of the aqueous solution from the organic solution; e) a stripping of uranium from the organic solution resulting from d.sub.1), the stripping comprising at least one contacting, in an extractor, of the organic solution with an aqueous solution comprising no more than 0.05 mol/L of nitric acid, followed by a separation of the organic solution from the organic solution; and f) a regeneration of the organic solution resulting from step e); whereby a first and a second aqueous solution are obtained, decontaminated with respect to americium, curium and fission products including technetium, the first aqueous solution comprising either plutonium without uranium, or a mixture of plutonium and uranium, and the second aqueous solution comprising uranium without plutonium.

11. The method of claim 10, wherein the organic solution of step a) comprises from 1 mol/L to 2 mol/L of the N,N-dialkylamide or mixture of N,N-dialkylamides.

12. The method of claim 10, wherein the aqueous solution of step b) comprises from 4 mol/L to 6 mol/L of nitric acid.

13. The method of claim 12, wherein step b) further comprises a de-acidification of the organic solution, the de-acidification comprising at least one contacting of the organic solution with an aqueous solution comprising from 0.1 mol/L to 1 mol/L of nitric acid, followed by a separation of the organic and aqueous solutions.

14. The method of claim 10, wherein the contacting of the organic and aqueous solutions in the extractor of c.sub.1) comprises a circulation of the organic and aqueous solutions with a ratio of the flow of organic solution to the flow of aqueous solution higher than 1.

15. The method of claim 10, wherein the reducing agent is uranous nitrate, hydrazinium nitrate, hydroxylammonium nitrate, acetaldoxime or a mixture thereof.

16. The method of claim 10, wherein the extractor of d.sub.1) is heated to a temperature of 30 C. to 40 C.

17. The method of claim 10, wherein d.sub.2) comprises an acidification of the aqueous solution resulting from d.sub.1), to bring a concentration of nitric acid in the aqueous solution to a value of at least 2.5 mol/L, the acidification comprising an adding of nitric acid to the extractor of d.sub.2).

18. The method of claim 10, wherein the extractor of step e) is heated to a temperature of 40 C. to 50 C.

19. The method of claim 10, wherein the contacting of the organic and aqueous solutions in the extractor of step e) comprises a circulation of the organic and aqueous solutions with a ratio of the flow of organic solution to the flow of aqueous solution higher than 1.

20. The method of claim 10, wherein the organic solution resulting from step f) is divided into a first and a second fraction, the first fraction forming the first organic solution of step a) and the second fraction forming the second organic solution of c.sub.2).

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 illustrates by two straight lines the variation in the logarithm of the distribution coefficients, denoted D.sub.M, of uranium and of plutonium such as obtained with extraction tests conducted with an N,N-dialkylamide of the invention, as a function of the logarithm of the free concentration (in mol/L) of this free N,N-dialkylamide in the organic phase used in these extraction tests.

(2) FIG. 2 gives a flow diagram of the method of the invention for processing an aqueous nitric solution resulting from the dissolution of spent nuclear fuel; in this Figure, rectangles 1 to 7 represent multi-stage extractors such as those conventionally used for the processing of spent nuclear fuel (mixer-settlers, pulsed columns or centrifugal extractors); the organic phases are symbolised by solid lines whilst the aqueous phases are symbolised by dotted lines.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

ISynthesis of N,N-Dialkylamides of the Invention

(3) As previously mentioned, the N,N-dialkylamides of the invention can be obtained with the following reaction scheme A:

(4) ##STR00004##
where X=halogen atom and R=linear or branched C.sub.8 to C.sub.15 alkyl group.

(5) When the amine denoted 4 in the above scheme is not commercially available, it can be obtained with the following reaction scheme B:

(6) ##STR00005##
where R=linear or branched C.sub.8 to C.sub.15 alkyl group.

I.1Synthesis of N-methyl-N-octyl-2-ethylhexanamide or MOEHA

(7) MOEHA, which meets the above formula (I) where R is an n-octyl group, is synthesized from 2-ethylhexanoyl chloride and N-methyl-N-octylamine, in the presence of sodium hydroxide in water (reaction scheme A).

(8) For this purpose, sodium hydroxide (30% NaOH-112 g-0.839 mole-1.19 eq.), water (100 g) and N-methyl-N-octylamine (100 g-0.698 mole-1 eq.) are placed in a fully equipped 500 mL reactor. The system is placed under agitation, the set temperature is 4 C. The 2-ethylhexanoyl chloride (136.5 g-0.839 mole-1.19 eq.) is poured at a bulk temperature of between 14 C. and 17 C. (pour time: 90 minutes). The progress of the reaction is controlled and shows the presence of 0.6% of residual amine. The formation is obtained of 90% of MOEHA and 8.8% of an unknown impurity. The medium is heated to 50 C. for 30 minutes to consume the residual amine. The medium is then cooled to 20 C. and decanted. The organic phase is washed twice with 100 mL water to obtain 208 g of reaction product.

(9) The MOEHA is subsequently obtained with 98.3% purity (measured by gas phase chromatography coupled to a flame ionization detectoror GC-FID) after two distillations under pressure (of 2 and 8 mbar respectively).

(10) .sup.13C NMR (100 MHz, CDCl.sub.3, 25 C.) (ppm): 3.37 (t, J=7.0, 2H, 2H.sub.A); 3.28 (t, J=7.0, 2H, 2H.sub.B); 3.00 (s, 3H, CH.sub.3A); 2.92 (s, 3H, CH.sub.3B); 2.61 -2.42 (m, 2H, H.sub.2Aet H.sub.2B); 1.90-1.77 (m, 2H, CH.sub.2A); 1.71-1.35 (m, 10H, 2CH.sub.2A and 3CH.sub.2B); 1.34- 1.11 (m, 28H, 7CH.sub.2A and 7CH.sub.2B); 0.94-0.77 (m, 18H, 3CH.sub.3A and CH.sub.3B)

(11) .sup.1H NMR (400 MHz, CDCl.sub.3, 25 C.) (ppm): 176.2; 176.0 (CO.sub.A and CO.sub.B); 50.1; 48.1 (C.sub.A and C.sub.B); 43.2; 42.9 (C.sub.2A and C.sub.2B); 35.6; 33.8 (CH.sub.3A and CH.sub.3B); 32.8; 32.7; 31.9; 31.9; 30.1; 30.0; 29.5; 29.4; 29.4; 29.3; 29.2; 27.5; 27.5; 27.0; 26.9; 26.2; 23.1; 23.0; 22.8 (20CH.sub.2); 14.2; 14.2; 14.1; 14.1 (2CH.sub.3A and 2CH.sub.3B); 12.3; 12.2 (CH.sub.3A and CH.sub.3B)

(12) MS (EI), m/z (I %): 269 (5%) [M]+, 240 (22%) [M-C.sub.2H.sub.5]+, 226 (22%) [M-C.sub.3H.sub.7]+, 212 (40%) [M-C.sub.4H.sub.9]+, 198 (25%) [M-C.sub.5H.sub.11]+, 170 (100%) [M -C.sub.7H.sub.15]+, 142 (5%) [C.sub.9H.sub.20N]+, 127 (5%) [C.sub.8H.sub.15O]+

(13) HRMS (EI): m/z calculated for [MH]+(C.sub.17H.sub.35NO) 269.2714; found 269.2672

I.2Synthesis of N-decyl-N-methyl-2-ethylhexanamide or MDEHA

(14) MDEHA, which meets above formula (I) where R is an n-decyl group, is synthesized following reaction scheme A from 2-ethylhexanoyl chloride and N-decyl-N-methylamine, in the presence of triethylamine (Et.sub.3N) in anhydrous dichloromethane (DCM) (reaction scheme A).

(15) For this purpose, the dichloromethane (100 mL), Et.sub.3N (21.2 g-0.21 mole-1.48 eq.) and N-decyl-N-methylamine (24 g-0.14 mole-1 eq.) are placed in a fully equipped 500 mL reactor. The system is placed under agitation and cooled to 0 C. The 2-ethylhexanoyl chloride (25 g-0.15 mole-1.1 eq.) is then poured at a bulk temperature of between 5 C. and 16 C. (pour time: 45 minutes). Under agitation, the bulk temperature gradually rises to ambient temperature. After 90 minutes, the progress of the reaction is controlled and shows that there no longer remains any initial amine but that there remains 5% of 2-ethylhexanoyl chloride. There is formation of 88.9% of MDEHA and 4.6% of an unknown impurity. The medium is then successively washed with twice 100 mL of a 10% sodium hydroxide solution, then twice with 100 mL of a 1 N hydrochloric acid solution and with 100 mL of a 5% sodium carbonate solution. The organic phase is concentrated under reduced pressure to obtain 43.6 g of an oil. This oil contains 89% of MDEHA and 9.46% of the unknown impurity.

(16) The MDEHA is subsequently obtained with 99.4% purity (measured by GC-FID) after two distillations under pressure (1.5 mbar).

(17) MS (EI), m/z (I %): 297 (3%) [M]+, 268 (13%) [M-C.sub.2H.sub.5]+, 254 (15%) [M-C.sub.3H.sub.7]+, 240(12%) [M-C.sub.4H.sub.9]+, 226 (12%) [M-C.sub.5H.sub.11]+, 198 (100%) [M-C.sub.7H.sub.15]+

(18) HRMS (EI): m/z calculated for [M]+(C.sub.19H.sub.39NO) 297.3026; found 297.3000.

I.3Synthesis of N-dodecyl-N-methyl-2-ethylhexanamide or MDdEHA

(19) MDdEHA, which meets above formula (I) where R is an n-dodecyl group, is synthesized from 2-ethylhexanoyl chloride and N-dodecyl-N-methylamine, in the presence of Et.sub.3N in anhydrous DCM (reaction scheme A).

(20) For this purpose, the DCM (150 mL), Et.sub.3N (22.7 g-0.223 mole-1.49 eq.) and N-dodecyl-N-methylamine (30 g-0.15 mole-1 eq.) are placed in a fully equipped 500 mL reactor. The system is placed under agitation and cooled to about 0 C. The 2-ethylhexanoyl chloride (26 g-0.16 mole-1.06 eq.) is then poured at a bulk temperature of between 0 C. and 2 C. (pour time: 40 minutes). Under agitation, the bulk temperature gradually rises to ambient temperature. After 4 hours, the progress of the reaction is controlled and shows that there no longer remains any starting amine. There is formation of 97% of MDdEHA. The medium is successively washed twice with 100 mL of a 10% sodium hydroxide solution and once with 100 mL of a 5% sodium carbonate solution. The organic phase is then concentrated under reduced pressure to obtain 55.5 g of an oil.

(21) The MDdEHA is subsequently obtained with 99.5% purity (measured by GC-FID) after a single distillation under much reduced pressure (0.7 mbar).

(22) MS (EI) m/z (I %): 325 (3%) [M]+, 296 (11%) [M-C.sub.2H.sub.5]+, 282 (12%) [M-C.sub.3H.sub.7]+, 268 (15%) [M-C.sub.4H.sub.9]+, 254 (10%) [M-C.sub.5H.sub.11]+, 226 (100%) [M-C.sub.7H.sub.15]+

(23) HRMS (EI): m/z calculated for [M]+(C.sub.21H.sub.43NO) 325.3339; found 325.3325.

I.4Synthesis of N-2-ethylhexyl-N-methyl-2-ethylhexanamide or M(2-EH)EHA

(24) M(2-EH)EHA, which meets above formula (I) where R is a 2-ethylhexyl group, is synthesized from 2-ethylhexanoyl chloride and N-methyl-2-ethylhexanamine, in the presence of Et.sub.3N in anhydrous DCM (reaction scheme A), the N-methyl-2-ethylhexanamine having been previously synthesized from 2-ethylhexylamine (reaction scheme B).

Synthesis of N-methyl-2-ethylhexanamine

(25) To a solution of 2-ethylhexylamine (15.0 mL-90.5 mmol-1 eq.) in anhydrous tetrahydrofuran (THF) (70 mL) is added dropwise at 0 C., using an addition funnel, a solution of di-cert-butyl dicarbonate (Boc.sub.2O-23.7 g-108.0 mmol-1.2 eq.) in anhydrous THF (30 mL). The mixture is left under agitation for 20 hours at ambient temperature and then concentrated under a controlled vacuum (0.150 mbar/20 C.). The crude oil obtained is purified by silica gel chromatography (elution: 100% DCM) and, after controlled vacuum concentration (0.035 mbar/40 C.), gives compound 3 of reaction scheme B where R is a 2-ethylhexyl group (21 g) in the form of a colourless oil.

(26) To a solution of compound 3 (5.0 g-21.8 mmol-1 eq.) in anhydrous THF (100 mL) cooled to 0 C. is added dropwise over 10 minutes a solution of aluminium hydride and lithium (LiAlH.sub.4) at 2.4 mol/L in THF (13.6 mL-32.7 mmol-1.5 eq.). The mixture is left to return to ambient temperature and then heated to 50 C. for 16 hours. After return to ambient temperature, the reaction mixture is carefully hydrolysed with successive dropwise additions of ethylacetate (1 mL), water (1.2 mL), 12 N sodium hydroxide (2.5 mL) then water (2.4 mL). After 20 minutes under strong agitation, the mixture is filtered on a Bchner. The filtrate is concentrated under a controlled vacuum (0.035 mbar, 15 C.) to give N-methyl-2-ethylhexanamine (3.0 g) in the form of a colourless oil. This oil is used at the following step without additional purification.

Synthesis of M(2-EH)EHA

(27) To a solution of N-methyl-2-ethylhexanamine (3.0 g-20.9 mmol-1 eq.) in anhydrous DCM (40 mL), cooled to 0 C., are added dropwise Et.sub.3N (4.4 mL; 31.4 mmol; 1.5 eq.), then 2-ethylhexanoyl chloride (3.6 mL-20.9 mmol-1 eq.). The mixture is left under agitation for 20 hours at ambient temperature after which water (40 mL) is added. The phases are decanted and the aqueous phase is extracted with DCM (240 mL). The organic phases are combined, dried over sodium sulfate, filtered and concentrated under reduced pressure. The crude oil obtained is purified by silica gel chromatography (elution: 100% DCM) to give a 98.5% pure fraction (HPLC at 210 nm) of M(2-EH)EHA (2.6 g) in the form of a pale yellow oil. With the recovered impure fractions, the yield of the synthesis from 2-ethylhexanamine is estimated to be 71%.

(28) .sup.13C NMR, DEPT (CDCl.sub.3) (ppm): 176.6 (CO), 53.8 (CH.sub.2), 51.5 (CH.sub.2), 43.1 (CH.sub.3), 42.7 (CH.sub.3), 38.9 (CH), 37.3 (CH), 36.1 (CH), 34.6 (CH), 32.5 (CH.sub.2), 30.5(CH.sub.2), 29.9(CH.sub.2), 28.7 (CH.sub.2), 25.9(CH.sub.2), 23.6(CH.sub.2), 23.1(CH.sub.2), 22.9(CH.sub.2), 14.0(CH.sub.3), 12.1(CH.sub.3), 10.8(CH.sub.3), 10.5(CH.sub.3).

(29) .sup.1H NMR (CDCl.sub.3) (ppm): 3.16 (d, 2H, 1CH.sub.2, rotamer 1), 3.30 (d, 2H, 1CH.sub.2, rotamer 2), 2.90 (s, 3H, 1CH.sub.3, rotamer 1), 2.98 (s, 3H, 1CH.sub.3, rotamer 2), 2.54 (m, 1H, 1CH), 1.61 (m, 3H, 1CH.sub.2+1CH), 1.42 (m, 2H,CH.sub.2), 1.25 (m, 12H, 6CH.sub.2), 0.84 (m, 12H, 4CH.sub.3).

(30) MS (ESI.sup.+), m/z: 270.3 [MH].sup.+, 292.3 [MNa].sup.+

I.5Synthesis of N-2-ethyloctyl-N-methyl-2-ethylhexanamide or M(2-EO)EHA

(31) M(2-EO)EHA, which meets above formula (I) where R is a 2-ethyloctyl group, is synthesized from 2-ethylhexanoyl chloride and N-methyl-N-2-ethyloctanamine, in the presence of Et.sub.3N in anhydrous DCM (reaction scheme A), the N-methyl-N-2-ethyloctanamine having been previously synthesized from 2-ethyloctylamine (reaction scheme B) which itself is obtained by coupling 1-bromohexane and butyronitrile, followed by reduction of the resulting 2-ethyl-octanenitrile.

Synthesis of N-methyl-N-2-ethyloctanamine

(32) To a solution of diisopropylamine (11.3 mL-80.0 mmol-1 eq.) in anhydrous THF (42 mL) cooled to 78 C., n-butyllithium is added (n-BuLi-2.5 M in hexanes; 32.0 mL-80.0 mmol-1 eq.). Agitation is maintained for 10 minutes at 78 C. after which the butyronitrile (7.0 mL-80.0 mmol-1 eq.) is added dropwise. Agitation is maintained for 10 minutes at 78 C. and 1-bromohexane (11.3 mL-80.0 mmol-1 eq.) is then added dropwise. The mixture is left under agitation for 20 hours with gradual return to ambient temperature. A saturated ammonium chloride solution is added (40 mL) followed by diethylether (Et.sub.2O-50 mL). The phases are decanted and the aqueous phase is extracted with Et.sub.2O (250 mL). The combined organic phases are dried over sodium sulfate, filtered and evaporated under controlled pressure (15 C./0.050 mbar). The crude oil obtained is purified by silica gel chromatography (elution: cyclohexane/DCM) to give 2-ethyloctane-nitrile (60 weight % solution of 24 g with DCM/cyclohexane, i.e. estimated 9.8 g) in the form of a pale yellow solution.

(33) To a solution of 2-ethyloctanenitrile (7.18 g-45.6 mmol-1 eq.) in anhydrous THF (100 mL), cooled to 0 C., is added dropwise over 10 minutes a solution of LiAlH.sub.4 (2.4 M in THF; 38.1 mL-91.3 mmol-2 eq.). The mixture is returned to ambient temperature then heated to 50 C. for 16 hours. After return to 0 C., the reaction mixture is carefully hydrolysed with successive dropwise additions of ethylacetate (3mL), water (3.3 mL), 12 N sodium hydroxide (7 mL) then water (6.7 mL). After 20 minutes under strong agitation, the mixture is filtered on a Bchner. The filtrate is concentrated under a controlled vacuum (0.150 mbar, 35 C.) to give 2-ethyloctylamine (7.0 g in 55 weight % solution in THF) in the form of a colourless oil. This oil is used at the following step without additional purification.

(34) The synthesis of N-methyl-N-2-ethyloctanamine from the 2-ethyloctyl-amine thus obtained is performed by following a similar operating protocol to the one previously described for the synthesis of N-methyl-N-ethylhexanamine.

Synthesis of M(2-EO)EHA

(35) To a solution of N-methyl-N-2-ethyloctanamine (5.6 g-33.0 mmol-1 eq.) in anhydrous DCM (60 mL), cooled to 0 C., are added dropwise Et.sub.3N (6.8 mL-49.5 mmol-1.5 eq.) and then 2-ethylhexanoyl chloride (5.6 mL-33.0 mmol-1 eq.). The mixture is left under agitation for 20 hours at ambient temperature after which water (60 mL) is added. The phases are decanted and the aqueous phase is extracted with DCM (260 mL). The combined organic phases are dried over sodium sulfate, filtered and concentrated under reduced pressure. The crude oil obtained is purified by silica gel chromatography (elution: 100% DCM) to give a 98.3% pure fraction (HPLC at 210 nm) of M(2-EO)EHA (2.6 g) in the form of a pale yellow oil. With the recovered impure fractions, the yield of the step is estimated to be 64%.

(36) .sup.13C NMR, DEPT (CDCl.sub.3) (ppm): 176.2 (CO), 53.8 (CH.sub.2), 51.5 (CH.sub.2), 43.2 (CH.sub.3), 42.7 (CH.sub.3), 39.0 (CH), 37.3 (CH), 36.1 (CH), 34.7 (CH), 32.5 (CH.sub.2), 31.8 (CH.sub.2), 30.8 (CH.sub.2), 29.8 (CH.sub.2), 29.7 (CH.sub.2), 26.5 (CH.sub.2), 26.0 (CH.sub.2), 23.7 (CH.sub.2), 22.9 (CH.sub.2), 22.6 (CH.sub.2), 14.0(CH.sub.3), 12.1(CH.sub.3), 10.8(CH.sub.3), 10.5(CH.sub.3)

(37) .sup.1H NMR (CDCl.sub.3) (ppm): 3.34 (m, 2H, 1CH.sub.2, rotamer 1), 3.22 (m, 2H, 1CH.sub.2, rotamer 2), 3.03 (s, 3H, 1CH.sub.3, rotamer 1), 2.95 (s, 3H, 1CH.sub.3, rotamer 2), 2.59 (m, 1H, 1CH), 1.65 (m, 3H, 1CH.sub.2+1CH), 1.49 (m, 2H, 1CH.sub.2), 1.29 (m, 16H, 8CH.sub.2), 0.90 (m, 12H, 4CH.sub.3)

(38) MS (ESI.sup.+), m/z: 298.4 [MH].sup.+, 320.4 [MNa].sup.+

IIExtracting Properties of the N,N-Dialkylamides if the Invention

II.1Acquisition of the Distribution Coefficients of Uranium and Plutonium, and of the Separation Factors FSU/Pu, from a Synthetic Aqueous Solution of Uranium and Plutonium, for MOEHA, MDEHA and MDdEHA

(39) First, extraction tests were carried out using: as organic phases: solutions comprising 1.4 mol/L MOEHA, MDEHA or MDdEHA in TPH; and as aqueous phases: aliquots of an aqueous solution comprising 90 g/L of uranium(VI), about 70 mg/L of plutonium(IV) and 4.15 mol/L of HNO.sub.3.

(40) Stripping tests were then conducted using: as organic phases: the organic phases obtained after the foregoing extraction tests; and as aqueous phases: aliquots of an aqueous solution comprising 0.1 mol/L of HNO.sub.3.

(41) Each of these tests was conducted by placing in contact an organic phase with an aliquot of aqueous solution, in a tube under agitation, for 15 minutes at 25 C. The O/A volume ratio used was 1 for the extraction tests and 1 for the stripping tests. These phases were separated from one another after centrifugation.

(42) The concentrations of uranium and plutonium were measured in the separated organic and aqueous phases by X-ray fluorescence for uranium and by -spectrometry for plutonium.

(43) Table 1 below, for each tested N,N-dialkylamide, gives the concentrations of uranium denoted [U].sub.org, such as obtained in the organic phases after the extraction tests, the distribution coefficients of uranium denoted D.sub.U, and of plutonium denoted D.sub.Pu, such as obtained after the extraction and stripping tests, the concentrations of nitric acid denoted [HNO.sub.3].sub.aq., such as obtained in the aqueous phases after the extraction and stripping tests, and the U/Pu separation factors denoted FS.sub.U/Pu, such as obtained after the stripping tests.

(44) This Table also gives the experimental results obtained under the same operating conditions but using solutions comprising N,N-dialkylamides of the prior art as organic phases, namely: a solution comprising 0.9 mol/L of N,N-di(2-ethylhexyl)-isobutanamide (or DEHiBA) and 0.5 mol/L of N,N-di(2-ethylhexyl)-n-butanamide (or DEHBA) in TPH, these two N,N-dialkylamides being proposed in reference [3] under the names DOiBA and DOBA; and a solution comprising 1.4 mol/L of N,N-di(2-ethylhexyl)-3,3-dimethylbutanamide (or DEHDMBA) in TPH, this N,N-dialkylamide being proposed in reference [1] under the name DOTA.

(45) TABLE-US-00001 TABLE I Organic phase MOEHA MDEHA MdDEHA DEHiBA + DEHBA DEHDMBA 1.4M/TPH 1.4M/TPH 1.4M/TPH 0.9M + 0.5M/TPH 1.4M/TPH Extraction [U].sub.org. 68 69 69 77 65 (g/L) D.sub.U 4.1 2.4 3.7 4.0 4.4 D.sub.Pu 2.2 1.8 1.7 0.74 1.0 [HNO.sub.3].sub.aq. 4.16 4.17 4.17 4.14 4.14 (mol/L) Stripping D.sub.U 0.81 0.83 0.82 0.63 0.61 D.sub.Pu 0.046 0.056 0.058 0.057 0.033 FS.sub.U/Pu 17.4 14.9 14.2 11.0 18.6 [HNO.sub.3].sub.aq. 0.50 0.52 0.57 0.58 0.54 (mol/L)

(46) This Table shows that, under strong acidity, the N,N-dialkylamides of the invention extract uranium(VI) (D.sub.U9VI)2.4) as well as the prior art N,N-dialkylamides, but they extract more strongly plutonium(IV) (D.sub.Pu(IV)1.7) than the latter.

(47) It also shows that plutonium(IV) can subsequently be easily stripped from the organic phase using an aqueous nitric solution of low acidity ([HNO.sub.3]=0.5 M) whereas the uranium preferably remains held in this organic phase (FS.sub.U/Pu>14).

II-2Study on the Stoichiometry of Complexes Formed by MOEHA with Uranium and Plutonium:

(48) Extraction tests were conducted using: as organic phases: solutions respectively comprising 0.1 mol/L, 0.5 mol/L, 0.75 mol/L, 1.0 mol/L, 1.25 mol/L, 1.5 mol/L and 2 mol/L of MOEHA in TPH; and as aqueous phases: aliquots of an aqueous solution comprising 2 g/L of uranium(VI), 1 mol/L of HNO.sub.3 and 2 mol/L of LiNO.sub.3, and aliquots of an aqueous solution comprising 1.710.sup.4 mol/L of plutonium(IV), 1 mol/L of HNO.sub.3 and 2 mol/L of LiNO.sub.3.

(49) For these tests, each organic phase was placed in contact with an aliquot of aqueous solution, in a tube under agitation, for 15 minutes at 25 C., with an O/A volume ratio of 1. These phases were then separated from one another after centrifugation.

(50) The concentrations of uranium were measured in the aqueous phases by inductively coupled plasma atomic emission spectrometry (or ICP-AES) whilst the concentrations of uranium in the organic phases were determined by stripping these elements in water and measuring their concentration by ICP-AES in the aqueous phases resulting from this stripping. The plutonium concentrations were measured in the aqueous and organic phases by -spectrometry.

(51) The results are illustrated in FIG. 1 which, in the form of two straight lines, illustrates the variation in the logarithm of the distribution coefficients, denoted D.sub.M, of uranium and of plutonium, as a function of the logarithm of the free concentration (in mol/L) of MOEHA in organic phase (total concentration of MOEHA corrected for the fraction of nitric acid extracted in organic phase).

(52) This Figure shows that the slope of the straight line corresponding to the extraction of uranium(VI) is close to 2, confirming the formation of a UO.sub.2(NO.sub.3).sub.2(MOEHA).sub.2 complex which conforms to complexes conventionally observed with N,N-dialkylamides.

(53) On the other hand, according to these results, the complex formed by MOEHA with plutonium(IV) appears to involve three molecules of MOEHA per one Pu.sup.4+ cation, thereby giving a Pu:MOEHA stoichiometry of 1:3 (Pu(NO.sub.3).sub.4(MOEHA).sub.3], already observed with other dissymmetric N,N-dialkylamides (reference [5]). The extraction equilibrium of plutonium(IV) by MOEHA can therefore be written as follows:
Pu.sup.4++4NO.sub.3.sub.+3MOEHA.Math.Pu(NO.sub.3).sub.4MOEHA.sub.3

II.3Acquisition of the Distribution Coefficients of Uranium, Plutonium and Fission Products, from an Aqueous Solution Resulting from the Dissolution of Nuclear Fuel Pellets in HNO3, for MOEHA

(54) Extraction tests were performed using: as organic phase: a solution comprising 1.4 mol/L of MOEHA in TPH; and as aqueous phase: an aqueous solution previously obtained by dissolving pellets derived from different irradiated fuels of UOX-BWR type (Boiling Water Reactor) and UOX-PRW type (Pressurised Water Reactor) in 5 M nitric acid.

(55) This aqueous solution comprises 4.3 mol/L of HNO.sub.3 and its element composition is given in Table II below.

(56) TABLE-US-00002 TABLE II Concentration Concentration Activity Element (g/L) Element (g/L) Element (Bq/L) U(VI) 244 Si 0.175 .sup.106Ru 1.1 10.sup.11 Pu(IV) 2.53 Ba 0.570 .sup.134Cs 2.4 10.sup.11 Tc 0.275 Al 0.145 .sup.137Cs 1.1 10.sup.12 Np 0.214 Ca 0.130 .sup.144Ce 9.7 10.sup.10 Zr 1.08 K 0.070 .sup.154Eu 3.6 10.sup.10 Ru 0.510 Mg 0.090 .sup.155Eu 2.2 10.sup.10 Mo 0.106 Na 0.135 .sup.241Am 6.6 10.sup.10 Pd 0.345 Sr 0.210 Fe 0.285

(57) The organic phase, previously equilibrated at 6 mol/L of HNO.sub.3, was placed in contact with the aqueous phase, in a tube under agitation, for 15 minutes at 25 C., with an O/A volume ratio of 2.5.

(58) These phases were then separated from one another after centrifugation.

(59) The concentrations of uranium and plutonium, and the activities of the - isotopes were measured in each of the organic and aqueous phases thus separated, via X-ray fluorescence for uranium and plutonium, and -spectrometry for the - isotopes.

(60) The concentrations of Tc, Np, Zr, Mo and Fe were only able to be measured in the aqueous phase by ICP-AES, and the concentrations of these elements in the organic phase were estimated by the difference between the initial concentrations of said elements in the aqueous phase and those measured at equilibrium after extraction.

(61) The results obtained in terms of aqueous phase acidity denoted [H.sup.+].sub.aq., of uranium and plutonium concentrations in the aqueous and organic phases respectively denoted [U].sub.aq., [U].sub.org., [Pu].sub.aq. and [Pu].sub.org., and of distribution coefficients denoted D, are given in Table III below.

(62) This Table also gives the experimental results obtained under the same operating conditions but using as organic phase a solution comprising 30% (v/v) TBP in TPH.

(63) TABLE-US-00003 TABLE III Organic phase MOEHA 1.4M/TPH TBP 30% (v/v)/TPH [H.sup.+].sub.aq. (mol/L) 5.75 5.9 [U].sub.aq. (g/L) 25 18 [U].sub.org. (g/L) 89 91 [Pu].sub.aq. (mg/L) 340 250 [Pu].sub.org. (mg/L) 91 960 D.sub.U 3.5 4.9 D.sub.Pu 2.7 3.8 D.sub.Tc 1.1 0.69 D.sub.Np 2.2 2.2 D.sub.Zr 0.03 0.10 D.sub.Mo <0.1 D.sub.Fe <0.01 D.sub.(106Ru) 8.5 10.sup.4 2.8 10.sup.4 D.sub.(134Cs) 5.8 10.sup.5 1.5 10.sup.5 D.sub.(137Cs) 7.5 10.sup.5 1.8 10.sup.5 D.sub.(154Eu) 8.6 10.sup.5 3.8 10.sup.4 D.sub.(241Am) 2.3 10.sup.4 1.5 10.sup.4

(64) This Table shows that the use of MOEHA as extractant leads to high distribution coefficients (>>1) for uranium(VI) and plutonium(IV) at an acidity of 5.75 mol/L of HNO.sub.3, despite the strong uranium saturation of the organic phase (89 g of uranium/L).

(65) It also shows that the use of MOEHA as extractant also leads to high separation factors FS.sub.U/PF and FS.sub.Pu/PF, in particular with respect to ruthenium 106, since these are always higher than 3 000. The separation factors FS.sub.U/Am and FS.sub.Pu/Am are also very high.

(66) These results that are very close to those observed under identical conditions but using TBP as extractant, confirm that the N,N-dialkylamides of the invention allow the extraction of uranium and plutonium quantitatively and selectively with respect to americium, curium and the main fission products likely to be contained in an aqueous solution resulting from the dissolution of a spent nuclear fuel in nitric acid, whilst subsequently allowing the partitioning of uranium and plutonium into two aqueous streams, the first comprising uranium without plutonium, and the second containing plutonium with or without uranium, without having to reduce the plutonium which is not the case with TBP.

II.4Acquisition of the Distribution Coefficients of Uranium and Plutonium, and of the Separation Factors FSU/Pu, from a Synthetic Aqueous Solution of Uranium and Plutonium, for M(2-EH)EHA and M(2-EO)EHA

(67) Extraction tests were conducted using: as organic phases: solutions comprising 0.5 mol/L of M(2-EH)EHA or M(2-EO)EHA in TPH; and as aqueous phases: aliquots of aqueous solutions of uranium(VI) (11.5 g/L) doped with plutonium(IV) (0.4 MBq/mL) and comprising either 4 mol/L of HNO.sub.3 or 0.5 mol/L of HNO.sub.3 (to simulate the aqueous phase of low acidity that is typically used to strip plutonium at a U/Pu partitioning step into two aqueous streams).

(68) Each of these tests was performed by placing in contact an organic phase, in a tube under agitation, with an aliquot of aqueous solution for 15 minutes at 25 C. The O/A volume ratio used was 1. These phases were then separated from one another after centrifugation.

(69) The concentration of uranium and the activity of plutonium (.sup.239+240Pu) were measured in the organic and aqueous phases thus separated, using ICP-AES and -spectrometry respectively.

(70) Table IV below, for each tested N,N-dialkylamide, gives the distribution coefficients of uranium denoted D.sub.U, and of plutonium denoted D.sub.Pu, such as obtained, and the U/Pu separation factors, denoted FS.sub.U/Pu, such as obtained with an acidity of 0.5 mol of HNO.sub.3/L.

(71) This Table also gives the experimental results obtained under the same operating conditions but using, as organic phases, aliquots of a solution comprising 0.5 mol/L of MOEHA in TPH.

(72) TABLE-US-00004 TABLE IV Organic phase M(2-EH)EHA M(2-EO)EHA MOEHA 0.5M/TPH 0.5M/TPH 0.5M/TPH [HNO.sub.3] 4M D.sub.U 0.95 0.98 1.64 D.sub.Pu 0.056 0.049 0.185 [HNO.sub.3] 0.5M D.sub.U 0.02 0.02 0.04 D.sub.Pu 0.0015 0.0012 0.0033 FS.sub.U/Pu 13.3 16.7 12.1

(73) This Table shows that the presence of a branching in the alkyl group represented by R in above formula (I) leads to compounds having an excellent U(VI)/Pu(IV) selectivity at low acidity (FS.sub.U/Pu>13), allowing a selective stripping of plutonium from a weakly acidic aqueous phase such as conventionally used at the U/Pu partitioning step, without having to reduce the plutonium.

(74) The distribution coefficients D.sub.U and D.sub.Pu are slightly lower than those obtained with MOEHA, notably due to the steric hindrance brought by the branching. These distribution coefficients can nevertheless be strongly increased by increasing the content of extractant. When considering the formation of a Pu(NO.sub.3).sub.4L.sub.3 complex, such as evidenced in foregoing Example II.2 with MOEHA, an increase of the content of M(2-EH)EHA from 0.5 mol/L to 1.5 mol/L would allow an increase in the distribution coefficient of plutonium by a factor of 27, i.e. from 0.056 to 1.5, with 4 moles/L of HNO.sub.3, a value that is sufficient in a process to co-extract plutonium under strong acidity.

IIIFlow Diagram of the Method of the Invention for Processing an Aqueous Nitric Solution Resulting from Dissolving a Spent Nuclear Fuel

(75) Reference is made to FIG. 2 illustrating a flow diagram of the method of the invention to reprocess an aqueous nitric solution resulting from dissolution of spent nuclear fuel.

(76) As shown in this Figure, the method comprises 8 steps.

(77) The first of these steps, denoted U/Pu co-extraction in FIG. 2, is intended to obtain the joint extraction of uranium and plutonium, the first in oxidation state +VI and the second in oxidation state +IV, from the aqueous nitric solution resulting from the dissolution of a spent nuclear fuel.

(78) Such a solution typically comprises from 3 to 6 mol/L of HNO.sub.3, uranium, plutonium, minor actinides (americium, curium and neptunium), fission products (La, Ce, Pr, Nd, Sm, Eu, Gd, Mo, Zr, Ru, Tc, Rh, Pd, Y, Cs, Ba, . . . ) and a few corrosion products such as iron.

(79) The U/Pu co-extraction step is performed by circulating the dissolution solution in extractor 1, in counter-current to an organic phase (denoted OP in FIG. 2) which comprises from 1 mol/L to 2 mol/L and better still from 1.3 mol/L to 1.5 mol/L of an N,N-dialkylamide of the invention, or of a mixture of N,N-dialkylamides of the invention, in solution in an organic diluent.

(80) This organic diluent is an aliphatic, linear or branched hydrocarbon, such as n-dodecane, TPH, the isoparaffinic diluent marketed by TOTAL under the trade name lsane IP185T, preference being given to TPH.

(81) The second step of the method, denoted FP washing in FIG. 2, is intended to strip the organic phase resulting from U/Pu co-extraction of the fraction of fission products that was extracted from the dissolution solution jointly with the uranium and plutonium.

(82) For this purpose, the FP washing step comprises one or more washing operations of the organic phase resulting from U/Pu co-extraction, each washing operation being performed by circulating this organic phase in extractor 2, in counter-current flow to an aqueous nitric solution having a concentration which may range from 0.5 mol/L to 6 mol/L of HNO.sub.3, but which is preferably from 4 mol/L to 6 mol/L of HNO.sub.3 and better still from 4 to 5 mol/L of HNO.sub.3, so as to facilitate the stripping of ruthenium and technetium.

(83) If the FP washing step is conducted with one or more aqueous solutions of strong acidity, i.e. typically of 3 mol/L of HNO.sub.3 or higher, then this step additionally comprises a de-acidification of the organic phase that is performed by circulating this organic phase in counter-current flow to a weakly acidic aqueous nitric solution, i.e. comprising from 0.1 mol/L to 1 mol/L of HNO.sub.3 such as, for example, an aqueous solution comprising 0.5 mol/L of HNO.sub.3, to prevent too much acid being carried towards the extractor dedicated to the third step denoted Pu stripping in FIG. 2, that would perturb the performance of this third step.

(84) The Pu stripping step, which represents the first step of the U/Pu partitioning, is intended to strip the plutonium in oxidation state +IV and therefore without reducing this plutonium, from the organic phase resulting from FP washing.

(85) This step is performed by circulating this organic phase in extractor 3, in counter-current flow to an aqueous solution comprising from 0.1 mol/L to 0.5 mol/L of HNO.sub.3 and by preferably using an O/A flow ratio higher than 1, preferably of 3 or higher and better still of 5 or higher, so that the plutonium(IV) is stripped in concentrating manner.

(86) The stripping of plutonium(IV), performed at the Pu stripping step, is accompanied by a stripping of a fraction of uranium(VI) that is also contained in the organic phase resulting from FP washing.

(87) The fourth step of the method, which is denoted 1.sup.st U washing in FIG. 2 and which corresponds to the second U/Pu partitioning step, is therefore intended to extract from the aqueous phase resulting from Pu stripping: either the entirety of the uranium contained in this aqueous phase if it is desired that the U/Pu partitioning should lead to an aqueous solution comprising plutonium without uranium, and to an organic solution comprising uranium without plutonium; or the quantity of uranium which, after 1.sup.st U washing, allows an aqueous solution to be obtained comprising uranium and plutonium in a previously chosen ratio, if it is desired that the U/Pu partitioning should lead to an aqueous solution comprising a mixture of plutonium and uranium in this ratio, and to an organic solution comprising uranium without plutonium.

(88) In both cases, 1.sup.st U washing is performed by circulating the aqueous phase resulting from Pu stripping in extractor 4, in counter-current flow to an organic phase having an identical composition to that of the organic phase used at U/Pu co-extraction. The quantity of extracted uranium is adjusted by acting firstly on the O/A flow ratio and secondly on the acidity of the aqueous phase, the extraction of uranium being all the greater the higher the organic phase/aqueous phase flow ratio and the stronger the acidity of the aqueous phase. An addition of HNO.sub.3 of greater or lesser concentration to the aqueous phase circulating in extractor 4 can therefore be provided, as a function of the acidity that it is desired to impart to this aqueous phase.

(89) The fifth step denoted -Tc barrier in FIG. 2 is intended to strip, from the organic phase resulting from Pu stripping, the fraction of technetium that was extracted at U/Pu co-extraction but that was not stripped at FP washing, for the purpose of decontaminating this organic phase with respect to technetium.

(90) It also allows the stripping, from the organic phase resulting from Pu stripping, of the fraction of neptunium that was extracted at U/Pu co-extraction and followed technetium up to -Tc barrier, as well as traces of plutonium that this organic phase may still contain.

(91) This step is performed by circulating the organic phase resulting from Pu stripping in extractor 5, in counter-current flow to an aqueous nitric solution of low acidity, i.e. comprising from 0.1 mol/L to 3 mol/L of HNO.sub.3 and better still 1 mol/L of HNO.sub.3, and comprising one or more reducing agents allowing the reduction of technetiumwhich is contained in the organic phase in oxidation state +VIIto technetium(IV) that is non-extractable by the N,N-dialkylamides, of neptunium(VI) to neptunium(IV) or neptunium (V) that are non-extractable by N,N-dialkylamides under low acidity, and plutonium(IV) to plutonium(III) that is less extractable by N,N-dialkylamides under low acidity than plutonium(IV), without having to reduce uranium(VI).

(92) As reducing agents, the use can therefore be made of uranous nitrate (or U(IV)), hydrazinium nitrate (or NH), hydroxylammonium nitrate (or NHA), acetaldoxime, or a mixture thereof such as a mixture U(IV)/NH, U(IV)/NHA or U(IV)/acetaldoxime, preference being given to a mixture U(IV)/NH or U(VI)/NHA. Gluconic acid can be added to the aqueous solution to reduce phenomena of technetium re-oxidization in the aqueous phase and thereby limit consumption of reducing agent(s).

(93) This step can be conducted at ambient temperature (i.e. 20-25 C.) but preferably it is conducted at a temperature ranging from 30 C. to 40 C. and better still at 32 C. to promote the stripping kinetics of technetium whilst limiting technetium re-oxidization phenomena in the aqueous phase, and hence limit the risk of the technetium, once stripped, of being back-extracted into the organic phase.

(94) The sixth step, denoted 2.sup.nd U washing in FIG. 2 is intended to extract, from the aqueous phase resulting from -Tc barrier, the uranium that was stripped together with the technetium at the preceding step, to prevent the -Tc barrier step from resulting in a too high loss of uranium in the aqueous phase.

(95) It is performed by circulating the aqueous phase resulting from -Tc barrier in extractor 6, in counter-current flow to an organic phase having an identical composition to that of the organic phases used for U/Pu co-extraction and 1.sup.st U washing, after an acidification of this aqueous phase through the addition of concentrated nitric acid, e.g. 10 M, to promote the extraction of uranium.

(96) The seventh step, denoted U stripping in FIG. 2, is intended to strip uranium(VI) from the organic phase resulting from -Tc barrier.

(97) It is performed by circulating the organic phase resulting from -Tc barrier in extractor 7, in counter-current flow to an aqueous nitric solution of low acidity, i.e. comprising no more than 0.05 mol/L of HNO.sub.3 such as, for example, an aqueous solution comprising 0.01 mol/L of HNO.sub.3. This step can be performed at ambient temperature (i.e. 20-25 C.) but it is preferably conducted under heat (i.e. typically at a temperature of 40-50 C.) using an O/A flow ratio higher than 1 so that the uranium(VI) is stripped in concentrating manner.

(98) After these 7 steps, we obtain: two raffinates, which correspond to the aqueous phases respectively leaving extractors 1 and 6, the first comprising fission products as well as americium and curium (Primary raffinate in FIG. 2), and the second comprising technetium, neptunium and possibly traces of plutonium (Secondary raffinate in FIG. 2); the aqueous phase leaving extractor 4, which comprises either decontaminated plutonium or a mixture of decontaminated plutonium and decontaminated uranium, called Pu stream or Pu+U stream accordingly; the aqueous phase leaving extractor 7, which comprises decontaminated uranium, called U stream; and the organic phase leaving extractor 7, which no longer comprises any plutonium or uranium but may contain a certain number of impurities and degradation products (formed by hydrolysis and radiolysis) of the extractant which may have accumulated over the preceding steps.

(99) Therefore, the eighth step, denoted OP washing in FIG. 2, is intended to regenerate this organic phase by subjecting it to one or more washes with a basic aqueous solution, for example a first wash with an aqueous solution of 0.3 mol/L of sodium carbonate, followed by a second wash with an aqueous solution of 0.1 mol/L of sodium hydroxide, then one or more washes with an aqueous nitric acid solution allowing a re-acidification thereof, for example an aqueous solution comprising 2 mol/L of HNO.sub.3, each wash being performed by circulating said organic phase in an extractor in counter-current flow to the aqueous washing solution.

(100) As can be seen in FIG. 2, the organic phase thus regenerated can be recycled back to extractors 1 and 4 to be re-introduced in the processing cycle.

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