Dissymmetric N,N-dialkylamides used particularly for separating uranium(VI) from plutonium(IV), synthesis thereof and uses of same

Abstract

A dissymmetric RN,N-dialkylamides of formula (I) in which: R.sup.1 represents a linear C.sub.1 to C.sub.4 alkyl, R.sup.2 represents a linear C.sub.1 to C.sub.10 alkyl, and R.sup.3 represents a linear or branched C.sub.6 to C.sub.15 alkyl, where R.sup.3 is different from a n-octyl, n-decyl, n-dodecyl, 2-ethylhexyl and 2-ethyloctyl group when R.sup.1 represents a n-butyl group and R.sup.2 represents an ethyl group. A method for synthesising the N,N-dialkylamides, and uses of same for extracting uranium and/or plutonium from an aqueous acid solution or for fully or partially separating the uranium from the plutonium contained in an aqueous acid solution and a solution resulting from the dissolution of spent nuclear fuel in nitric acid. A method for treating an aqueous solution resulting from the dissolution of spent nuclear fuel in nitric acid, which allows the uranium and the plutonium contained in the solution to be extracted, separated and decontaminated in a single cycle.

Claims

1. A method for separating totally or partially uranium(VI) from plutonium(IV) from an acid aqueous solution A1, comprising: a) a co-extraction of uranium and plutonium from the aqueous solution A1, the co-extraction comprising at least one contacting of the aqueous solution A1 with an organic solution S1 comprising an N,N-dialkylamide or a mixture of N,N-dialkylamides of formula (I): ##STR00004## wherein: R.sup.1 is a linear alkyl group having 1 to 4 carbon atoms; R.sup.2 is a linear alkyl group having 1 to 10 carbon atoms; R.sup.3 is a linear or branched alkyl group having 6 to 15 carbon atoms; provided that R.sup.3 differs from an n-octyl, n-decyl, n-dodecyl, 2-ethylhexyl and 2-ethyloctyl group when R.sup.1 is an n-butyl and R.sup.2 is an ethyl group; in an organic diluent, followed by a separation of the aqueous solution A1 from the organic solution S1; b) a stripping of plutonium, in oxidation state +IV, and of a fraction of uranium from the organic solution S1 resulting from a), the stripping comprising at least one contacting of the organic solution S1 with an aqueous solution A2 comprising from 0.1 mol/L to 0.5 mol/L of nitric acid, followed by a separation of the organic solution S1 from the aqueous solution A2; and c) an extraction of all or part of the uranium fraction contained in the aqueous solution A2 resulting from b), the extraction comprising at least one contacting of the aqueous solution A2 with an organic solution S2 identical to the organic solution S1 of a), followed by a separation of the aqueous solution A2 from the organic solution S2; whereby there are obtained an aqueous solution comprising plutonium without uranium, or a mixture of plutonium and uranium, and an organic solution comprising uranium without plutonium.

2. The method of claim 1, wherein the organic solution S1 of a) comprises from 1 mol/L to 2 mol/L of the N,N-dialkylamide or mixture of N,N-dialkylamides.

3. The method of claim 1, wherein the acid aqueous solution A1 is an aqueous solution resulting from a dissolution of a spent nuclear fuel in nitric acid.

4. The method of claim 1, wherein R.sup.1 is a methyl, ethyl or n-propyl group.

5. The method of claim 1, wherein R.sup.2 is an n-propyl, n-butyl, n-pentyl, n-hexyl or n-heptyl group.

6. The method of claim 1, wherein R.sup.3 is an n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl or 2-ethyloctyl group.

7. The method of claim 1, wherein: R.sup.1 is a methyl group, R.sup.2 is an n-butyl group and R.sup.3 is an n-nonyl group; or R.sup.1 and R.sup.2 are each an n-propyl group and R.sup.3 is an n-octyl group; or R.sup.1 is an n-propyl group, R.sup.2 is an n-pentyl group and R.sup.3 is an n-hexyl group.

8. A single-cycle method for processing an aqueous solution A1 resulting from a dissolution of a spent nuclear fuel in nitric acid, the aqueous solution A1 comprising uranium, plutonium, americium, curium and fission products including technetium, the cycle comprising: a) a co-extraction of uranium and plutonium from the aqueous solution A1, the co-extraction comprising at least one contacting, in an extractor, of the aqueous solution A1 with an organic solution S1 comprising an N,N-dialkylamide or a mixture of N,N-dialkylamides of formula (I): ##STR00005## wherein: R.sup.1 is a linear alkyl group having 1 to 4 carbon atoms; R.sup.2 is a linear alkyl group having 1 to 10 carbon atoms; R.sup.3 is a linear or branched alkyl group having 6 to 15 carbon atoms; provided that R.sup.3 differs from an n-octyl, n-decyl, n-dodecyl, 2-ethylhexyl and 2-ethyloctyl group when R.sup.1 is an n-butyl and R.sup.2 is an ethyl group; in an organic diluent, followed by a separation of the aqueous solution A1 from the organic solution S1; b) a decontamination of the organic solution S1 resulting from a) with respect to americium, curium and fission products, the decontamination comprising at least one contacting, in an extractor, of the organic solution S1 with an aqueous solution A2 comprising from 1 mol/L to 6 mol/L of nitric acid, followed by a separation of the organic solution S1 from the aqueous solution A2; c) a partitioning of the uranium and plutonium contained in the organic solution S1 resulting from 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 S1 resulting from b), the stripping comprising at least one contacting, in an extractor, of the organic solution S1 with an aqueous solution A3 comprising from 0.1 mol/L to 0.5 mol/L of nitric acid, followed by a separation of the organic solution S1 from the aqueous solution A3; c.sub.2) an extraction of all or part of the uranium fraction contained in the aqueous solution A3 resulting from c.sub.1), the extraction comprising at least one contacting, in an extractor, of the aqueous solution A3 with an organic solution S2 identical to the organic solution S1 of a), followed by a separation of the aqueous solution A3 from the organic solution S2; d) a decontamination of the organic solution S2 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 S2 resulting from c.sub.1), the stripping comprising at least one contacting, in an extractor, of the organic solution S2 with an aqueous solution A4 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 S2 from the aqueous solution A4; d.sub.2) an extraction of the uranium fraction contained in the aqueous solution A4 resulting from d.sub.1), the extraction comprising at least one contacting, in an extractor, of the aqueous solution A4 with an organic solution S3 identical to the organic solution S1 of a), followed by a separation of the aqueous solution A4 from the organic solution S3; e) a stripping of the uranium from the organic solution S3 resulting from d.sub.1), the stripping comprising at least one contacting, in an extractor, of the organic solution S3 with an aqueous solution A5 comprising no more than 0.5 mol/L of nitric acid, followed by a separation of the organic solution S3 from the aqueous solution A5; and f) a regeneration of the organic solution S3 resulting from 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 plutonium without uranium, or a mixture of plutonium and uranium, and the second aqueous solution comprising uranium without plutonium.

9. The method of claim 8, wherein the organic solution S1 of a) comprises from 1 mol/L to 2 mol/L of the N,N-dialkylamide or mixture of N,N-dialkylamides.

10. The method of claim 8, wherein the aqueous solution A2 of b) comprises from 4 mol/L to 6 mol/L of nitric acid.

11. The method of claim 8, wherein b) further comprises a de-acidification of the organic solution S1, the de-acidification comprising at least one contacting of the organic solution S1 with an aqueous solution A6 comprising from 0.1 mol/L to 1 mol/L of nitric acid, followed by a separation of the organic solution S1 from the aqueous solution A6.

12. The method of claim 8, wherein the contacting of the organic solution S1 and the aqueous solution A3 in the extractor of c.sub.1) comprises a circulation of the organic solution S1 and the aqueous solution A3 in the extractor with a ratio of the organic solution S1 flowrate to the aqueous solution A3 flowrate higher than 1.

13. The method of claim 8, wherein d.sub.2) comprises an acidification of the aqueous solution A4 resulting from d.sub.1), to bring the concentration of nitric acid in the aqueous solution A4 to a value of at least 2.5 mol/L, the acidification comprising an addition of nitric acid to the extractor of d.sub.2).

14. The method of claim 8, wherein the contacting of the organic solution S3 and the aqueous solution A5 in the extractor of e) comprises a circulation of the organic solution S3 and the aqueous solution A5 in the extractor with a ratio of the organic solution S3 flowrate to the aqueous solution A5 flowrate higher than 1.

15. The method of claim 8, wherein the aqueous solution A1 is an aqueous solution resulting from a dissolution of a spent nuclear fuel in nitric acid.

16. The method of claim 8, wherein R.sup.1 is a methyl, ethyl or n-propyl group.

17. The method of claim 8, wherein R.sup.2 is an n-propyl, n-butyl, n-pentyl, n-hexyl or n-heptyl group.

18. The method of claim 8, wherein R.sup.3 is an n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl or 2-ethyloctyl group.

19. The method of claim 8, wherein: R.sup.1 is a methyl group, R.sup.2 is an n-butyl group and R.sup.3 is an n-nonyl group; or R.sup.1 and R.sup.2 are each an n-propyl group and R.sup.3 is an n-octyl group; or R.sup.1 is an n-propyl group, R.sup.2 is an n-pentyl group and R.sup.3 is an n-hexyl group.

Description

BRIEF DESCRIPTION OF THE FIGURE

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

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

I—Synthesis of N,N-Dialkylamides of the Invention

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

(3) ##STR00003##
with R.sup.1═C.sub.1-C.sub.4 linear alkyl group; R.sup.2═C.sub.1-C.sub.10 linear alkyl group; and R.sup.3═C.sub.6-C.sub.15 linear or branched alkyl group.

(4) For this reaction, in a round bottom flask fitted with a septum and magnetic stir bar and under a nitrogen atmosphere, the DCC (1.2 eq.) and HOBt (1.2 eq.) are dissolved in 2-methyltetrahydrofurane (MeTHF) at 0.1 mol/L. Then, the carboxylic acid of formula (II) (1 eq.) is added and the reaction medium is left under agitation for 10 minutes at ambient temperature. The amine of formula (III) (1 eq.) is next added dropwise and the reaction medium left under agitation overnight at ambient temperature.

(5) Thereafter, the reaction medium is filtered on Célite™. The filtrate is washed three times with an aqueous sodium carbonate-saturated solution (Na.sub.2CO.sub.3) and once with an aqueous sodium chloride-saturated solution (NaCl). The organic phase is collected, dried over anhydrous magnesium sulfate (MgSO.sub.4), filtered and concentrated in a Rotavapor.

(6) The reaction product is purified by silica column chromatography (elution with heptane/AcOEt: 100:0 at 75:25, v/v) and the N,N-dialkylamide of formula (I) is obtained in the form of two rotamers (colourless or pale yellow liquid, yield: from 60% to quantitative).

I.1—Synthesis of N-methyl-N-nonyl-2-methylhaxanamide or MNMHA

(7) MNMHA, which meets above formula (I) wherein R.sup.1 is a methyl group, R.sup.2 is an n-butyl group whilst R.sup.3 is an n-nonyl group, was synthesized as described above from 2-methylhexanoic acid and N-methyl-N-nonylamine. The characterizations thereof were the following:

(8) TLC (silica gel): R.sub.f=0.33 (heptane/AcOEt 8:2, v/v)

(9) .sup.1H NMR (400 MHz, CDCl.sub.3) δ (ppm): 3.33 (m, 1H, NCH.sub.2); 3.20 (m, 1H, NCH.sub.2); 2.92 and 2.83 (2s, 3H, NCH.sub.3, 2 rotamers); 2.56 (m, 1H, CH); 1.63-1.26 (m, 4H, 2 CH.sub.2); 1.25-1.11 (m, 16H, 8 CH.sub.2); 1.00 (m, 3H, CH.sub.3); 0.79 (m, 6H, 2 CH.sub.3)

(10) .sup.13C NMR (100 MHz, CDCl.sub.3) δ (ppm): 176.7; 176.3; 49.8; 47.8; 35.7; 35.4; 35.2; 34.2; 33.9; 33.6; 31.8; 31.8; 29.8; 29.7; 29.5; 29.5; 29.4; 29.3; 29.2; 29.2; 28.9; 27.2; 26.8; 26.7; 22.8; 22.7; 22.6; 22.6; 18.0; 17.4; 14.0; 14.0; 13.9; 13.9

(11) IR: ν=2956, 2924, 2855, 1641 (C═O), 1465 cm.sup.−1

(12) MS (ESI positive mode): m/z 270 [M+H].sup.+, 333 [M+ACN+Na].sup.+, 562 [2M+Na].sup.+

(13) HRMS (EI positive mode): calculated for C.sub.17H.sub.35NO: 269.2719; found: 269.2723.

I.2—Synthesis of N-methyl-N-octyl-2-propylpentanamide or MOPPA

(14) MOPPA, which meets formula (I) above where R.sup.1 and R.sup.2 are both an n-propyl group whilst R.sup.3 is an n-octyl group, was synthesized from 2-propylpentanoic acid and N-methyl-N-octylamine. The characterizations thereof were the following:

(15) TLC (silica gel): R.sub.f=0.37 (heptane/AcOEt 8:2, v/v)

(16) .sup.1H NMR (400 MHz, CDCl.sub.3) δ (ppm): 3.25 (t, J=7.5 Hz, 1H, NCH.sub.2); 3.17 (t, J=7.6 Hz, 1H, NCH.sub.2); 2.89 and 2.80 (2s, 3H, NCH.sub.3, 2 rotamers); 2.50 (m, 1H, CH); 1.55-1.34 (m, 4H, 2 CH.sub.2); 1.31-1.09 (m, 16H, 8 CH.sub.2); 0.77-0.72 (m, 9H, 3 CH.sub.3)

(17) .sup.13C NMR (100 MHz, CDCl.sub.3) δ (ppm): 176.1; 175.8; 49.8; 47.8; 40.9; 40.7; 35.3; 35.3; 35.3; 33.5; 31.7; 31.6; 29.2; 29.2; 29.2; 29.1; 28.9; 27.2; 26.8; 26.7; 22.5; 22.5; 20.8; 20.7; 14.2; 14.1; 14.1; 13.9; 13.9.

(18) IR: ν=2955, 2925, 2856, 1639 (C═O), 1464 cm.sup.−1

(19) MS (ESI positive mode): m/z 270 [M+H].sup.+, 333 [M+ACN+Na].sup.+, 562 [2M+Na].sup.+

(20) HRMS (EI positive mode): calculated for C.sub.17H.sub.35NO: 269.2719; found: 269.2727.

I.3—Synthesis of N-methyl-N-hexyl-2-propylheptanamide or MHPHepA

(21) MHPHepA, which meets above formula (I) where R.sup.1 is an n-propyl group, R.sup.2 is an n-pentyl group whilst R.sup.3 is an n-hexyl group, was synthesized from 2-propylheptanoic acid and N-methyl-N-hexylamine. The characterizations thereof were the following:

(22) TLC (silica gel): R.sub.f=0.31 (heptane/AcOEt 8:2, v/v)

(23) .sup.1H NMR (400 MHz, CDCl.sub.3) δ (ppm): 3.30 (dt, J=1.8 Hz, J=7.1 Hz, 1H, NCH.sub.2); 3.21 (dt, J=2.2 Hz, J=6.9 Hz, 1H, NCH.sub.2); 2.94 and 2.85 (2s, 3H, NCH.sub.3, 2 rotamers); 2.53 (m, 1H, CH); 1.59-1.39 (m, 4H, 2 CH.sub.2); 1.36-1.14 (m, 16H, 8 CH.sub.2); 0.83-0.76 (m, 9H, 3 CH.sub.3).

(24) .sup.13C NMR (100 MHz, CDCl.sub.3) δ (ppm): 176.2; 176.0; 49.9; 47.9; 41.2; 41.0; 35.5; 35.4; 35.4; 33.7; 33.2; 33.1; 32.1; 32.0; 31.6; 31.5; 29.0; 27.5; 27.3; 27.2; 26.5; 26.5; 22.6; 22.5; 20.9; 20.8; 14.3; 14.2; 14.0; 14.0; 13.9.

(25) IR: ν=2956, 2926, 2857, 1638 (C═O), 1459 cm.sup.−1

(26) MS (ESI positive mode): m/z 270 [M+H].sup.+, 333 [M+ACN+Na].sup.+

(27) HRMS (EI positive mode): calculated for C.sub.17H.sub.35NO: 269.2719; found: 269.2732.

II—Extracting Properties of the N,N-Dialkylamides of the Invention

II.1—Acquisition of Uranium Distribution Coefficients

(28) Extraction tests were conducted using: as organic phases: solutions comprising 0.4 mol/L of MNMHA or MOPPA or MHPHepA dans le TPH; and as aqueous phases: aqueous solutions comprising 12 g/L of uranium(VI) and either 4 mol/L of HNO.sub.3 or 0.5 mol/L of HNO.sub.3 (to simulate the aqueous phase of weak acidity which is typically used to strip plutonium at the U/Pu partitioning step into two aqueous streams).

(29) Each of these tests was performed, in a tube and under agitation, by contacting one of the organic phases with one of the aqueous phases for 30 minutes at 25° C. The O/A volume ratio used was 1. These phases were separated from each other after centrifugation.

(30) The concentrations of uranium were measured in the separated organic and aqueous phases by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES).

II.2—Acquisition of Plutonium Distribution Coefficients

(31) Extraction tests similar to those described under item 11.1 above were conducted but using, as aqueous phases, aqueous solutions which comprised 12 g/L of uranium(VI), 4 mol/L of HNO.sub.3 or 0.5 mol/L of HNO.sub.3, and plutonium(IV) (≈0.4 MBq/mL).

(32) After separating the phases, the concentrations of uranium were measured in the organic and aqueous phases by ICP-AES, whilst the activities of .sup.239+240plutonium were measured in the organic and aqueous phases by α-spectrometry.

II.3—Results

(33) Table 1 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 with the aqueous phases at 4 mol/L of HNO.sub.3 and at 0.5 mol/L d′HNO.sub.3, and the U/Pu separation factors denoted FS.sub.U/Pu such as obtained with the aqueous phases at 0.5 mol/L of HNO.sub.3.

(34) This Table also gives the experimental results obtained under the same operating conditions but using, as organic phases, solutions comprising N,N-dialkylamides of the prior art, namely: one solution comprising 0.32 mol/L of N,N-di(2-ethylhexyl)-isobutanamide (or DEHiBA) and 0.18 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 one solution comprising 0.5 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.

(35) TABLE-US-00001 TABLE I Organic phase MNMHA MOPPA MHPHepA DEHiBA 0.32M + DEHDMBA 0.4M 0.4M 0.4M DEHBA 0.18M 0.5M HNO.sub.3 4M D.sub.U 1.22 1.50 1.64 1.43 2.46 D.sub.Pu 0.148 0.129 0.117 0.20 0.28 HNO.sub.3 0.5M D.sub.U 0.026 0.030 0.035 0.030 0.025 D.sub.Pu 0.0012 0.0023 0.0013 0.004 0.0015 FS.sub.U/Pu 22 13 27 7 16

(36) This Table shows that, when used at a concentration de 0.4 mol/L in organic phase, the N,N-dialkylamides of the invention extract uranium(VI) and plutonium(IV) from an aqueous nitric phase sufficiently well (D.sub.U>1; D.sub.Pu>0.1) to allow a quantitative co-extraction of uranium(VI) and plutonium(IV) in a method for processing an aqueous solution resulting from the dissolution of a spent nuclear fuel in nitric acid, in which they will be used at a concentration ranging from 1 mol/L to 2 mol/L.

(37) An excellent U(VI)/Pu(IV) selectivity is reached with a nitric acid concentration of 0.5 mol/L (FS.sub.U/Pu>12) with very low distribution coefficients of plutonium(IV) (D.sub.Pu<0.0030). In particular, MHPHepA allows an FS.sub.U/Pu separation factor of 27 to be obtained.

(38) Therefore, in a method for processing an aqueous solution resulting from the dissolution of a spent nuclear fuel in nitric acid in which the N,N-dialkylamides of the invention will be used at a concentration ranging from 1 mol/L to 2 mol/L, it will be possible to selectively strip plutonium(IV) from the organic phase resulting from the U(VI)/Pu(IV) co-extraction using an aqueous phase comprising 0.5 mol/L of nitric acid.

(39) As also shown in Table 1, the N,N-alkylamides of the invention, with a nitric acid concentration of 0.5 mol/L, exhibit a U(VI)/Pu(IV) selectivity that is higher than that obtained with the DEHiBA/DEHBA mixture of reference [3], whilst having the advantage of being able to be used alone rather than in a mixture.

(40) The performance of the N,N-alkylamides of the invention is close to that of DEHDMBA in reference [1]. However, they have the advantage of being less viscous than the latter (this being one of the limiting parameters for the development of a method with DEHDMBA for processing an aqueous solution resulting from the dissolution of a spent nuclear fuel in nitric acid) since the viscosity at 25° C. of an organic phase comprising 1.2 mol/L of MNMHA or MOPPA in TPH is 2.25 mPa/s and 2.16 mPa/s respectively, whilst it is 3.6 mPa/s for an organic phase comprising 1.27 mol/L of DEHDMBA in TPH.

III—Flowchart of the Method of the Invention for Processing an Aqueous Solution of a Dissolved Spent Nuclear Fuel

(41) Reference is made to FIG. 1 giving a flowchart of the method of the invention for processing an aqueous solution of a spent nuclear fuel dissolved in nitric acid.

(42) As shown in this FIGURE, the method comprises 8 steps.

(43) The first of these steps, denoted «U/Pu co-extraction» in FIG. 1, is intended to extract uranium and plutonium jointly, the first in oxidation state +VI and the second in oxidation state +IV, from the aqueous nitric solution of dissolved spent nuclear fuel.

(44) Said 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 some corrosion products such as iron.

(45) The «U/Pu co-extraction» step is performed by circulating the dissolution solution in extractor 1, in counter-current flow to an organic phase (denoted «OP» in FIG. 1) which comprises from 1 mol/L to 2 mol/L and better still from 1.1 mol/L to 1.4 mol/L, e.g. 1.2 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.

(46) This organic diluent is an aliphatic, linear or branched, hydrocarbon such as n-dodecane, TPH or the isoparaffinic diluent marketed by TOTAL under the trade name Isane IP 185T, preference being given to TPH.

(47) The second step of the method, denoted «FP washing» in FIG. 1, is intended to strip, from the organic phase resulting from «U/Pu co-extraction», the fraction of fission products that was extracted from the dissolution solution jointly with uranium and plutonium.

(48) This «FP washing» step comprises one or more washing operations of the organic phase resulting from «U/Pu co-extraction», each washing operation being conducted by circulating this organic phase in extractor 2, in counter-current flow to an aqueous nitric solution having a concentration possibly ranging from 1 mol/L to 6 mol/L of HNO.sub.3, but 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, to facilitate the stripping of ruthenium and technetium.

(49) If the «FP washing» step is conducted with one or more strongly acidic aqueous solutions, i.e. typically of 3 mol/L d′HNO.sub.3 or higher, then this step also comprises a de-acidification of the organic phase, which is performed by circulating this organic phase in counter-current flow to an aqueous nitric solution of low acidity, i.e. comprising from 0.1 mol/L to 1 mol/L of HNO.sub.3 such as an aqueous solution comprising 0.5 mol/L of HNO.sub.3, to prevent too much acid being carried into the extractor dedicated to the third step, denoted «Pu stripping» in FIG. 1, which would perturb the performance of this third step.

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

(51) 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 preferably using an O/A flowrate ratio higher than 1, preferably of 3 or higher, and better still of 5, to obtain a concentrating stripping of plutonium(IV).

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

(53) Therefore, the fourth step of the method, denoted «1.sup.st U washing» in FIG. 1 and which represents the second step of the U/Pu partitioning, is 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 amount of uranium with which it is possible, at the end of «1.sup.st U washing», to obtain an aqueous solution comprising uranium and plutonium in a previously chosen ratio, if it is desired that 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.

(54) In both cases, the «1.sup.st U washing» is performed by circulating, in extractor 4, the aqueous phase resulting from «Pu stripping» in counter-current flow to an organic phase having the same composition as the organic phase used for «U/Pu co-extraction». The amount of uranium extracted is adjusted by acting both on the ratio of O/A flowrates and on the acidity of the aqueous phase, the extraction of uranium being bettered the higher the organic phase/aqueous phase flowrate ratio and the higher 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 it is desired to impart to this aqueous phase.

(55) The fifth step, denoted «α-Tc barrier» in FIG. 1, is intended to strip, from the organic phase resulting from «Pu stripping», the technetium fraction that was extracted at «U/Pu co-extraction» and not stripped at «FP washing», for the purpose of decontaminating this organic phase with respect to technetium.

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

(57) It is performed by circulating, in extractor 5, the organic phase resulting from «Pu stripping» 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 to reduce technetium—which is contained in the organic phase in oxidation state +VII—to technetium(IV) non-extractable by the N,N-dialkylamides, neptunium(VI) to neptunium(IV) or neptunium(V) which are non-extractable by the N,N-dialkylamides under weak acidity, and plutonium(IV) to plutonium(III) which is less extractable by the N,N-dialkylamides under weak acidity than plutonium(IV), whilst without reducing uranium(VI).

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

(59) This step can be conducted at ambient temperature (i.e. 20-25° C.) but it is preferably 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 phenomena of technetium re-oxidation in aqueous phase and hence limit the risk that the technetium, once stripped, of being re-extracted in the organic phase.

(60) The sixth step, denoted «2.sup.nd U washing» in FIG. 1, is intended to extract, from the aqueous phase resulting from «α-Tc barrier», the uranium that was back-extracted together with technetium at the preceding step, so that the «α-Tc barrier» step does not lead to a large loss of uranium in aqueous phase.

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

(62) The seventh step, denoted «U stripping» in FIG. 1, is intended to strip uranium(VI) from the organic phase resulting from «α-Tc barrier».

(63) It is performed by circulating, in extractor 7, the organic phase resulting from «α-Tc barrier» in counter-current flow to an aqueous nitric solution of very low acidity, i.e. comprising no more than 0.5 mol/L and better still no more than 0.05 mol/L of HNO.sub.3, e.g. an aqueous solution comprising 0.01 mol/L of HNO.sub.3. This step can be conducted at ambient temperature (i.e. at 20-25° C.) but is preferably conducted under heat (i.e. typically at a temperature of 40-50° C.) using an O/A flowrate ratio higher than 1 to obtain a concentrating stripping of uranium(VI).

(64) After these 7 steps, the following are obtained: two raffinates corresponding to the aqueous phases respectively leaving extractors 1 and 6, the first comprising fission products together with americium and curium («Primary raffinate» in FIG. 1), and the second comprising technetium, neptunium and optionally traces of plutonium («Secondary raffinate» in FIG. 1); the aqueous phase leaving extractor 4, which comprises either decontaminated plutonium or a mixture of decontaminated plutonium and uranium, and 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 either plutonium or uranium but may contain a certain number of impurities and degradation products of the extractant (formed by hydrolysis and radiolysis) which may have accumulated over the preceding steps.

(65) Therefore, the eighth step, denoted «OP washing» in FIG. 1, is intended to regenerate this organic phase by subjecting it to one or more washes with an aqueous basic solution, e.g. a first wash with an aqueous 0.3 mol/L solution of sodium carbonate, followed by a second wash with an aqueous 0.1 mol/L solution of sodium hydroxide, then one or more washes with an aqueous nitric acid solution for re-acidification, e.g. 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 wash solution.

(66) As can be seen in FIG. 1, the organic phase thus regenerated can be returned to extractors 1 and 4 to be fed back into the processing cycle.

CITED REFERENCES

(67) [1] FR-A-2 591 213 [2] FR-A-2 642 561 [3] FR-A-2 642 562 [4] Ruikar et al., Journal of Radioanalytical and Nuclear Chemistry 1993, 176(2), 103-111 [5] Prabhu et al., Radiochimica Acta 1993, 60, 109-114 [6] Cui et al., Radiochimica Acta 2005, 93, 287-290 [7] Sun et al., Journal of Radioanalytical and Nuclear Chemistry 2005, 264(3), 711-713