RU2782220C2 - Carbamides for separation of uranium (vi) and plutonium (iv) without plutonium (iv) reduction - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the use of carbamide of the general formula (I), where R¹, R², and R³, the same or different, are a linear or branched alkyl group with 1-12 carbon atoms, R⁴ is a hydrogen atom or a linear or branched alkyl group with 1-12 carbon atoms, and where carbamide contains a total number of carbon atoms from 17 to 25, as an extractant for complete or partial separation of uranium (VI) and plutonium (IV), without plutonium (IV) reduction to plutonium (III), based on an aqueous solution A1 obtained by dissolution of spent nuclear fuel in nitric acid. In this case, the use provides: a) co-extraction of uranium (VI) and plutonium (IV) from the aqueous solution A1, while co-extraction includes at least a contact in an extractor of the aqueous solution A1 with an organic solution S1 containing carbamide in an organic solvent, and separation of aqueous and organic solutions; b) re-extraction of plutonium (IV) and an uranium (VI) fraction from the organic solution obtained at the stage a) during re-extraction containing at least one contact in the extractor of the organic solution S1 with an aqueous solution A2 containing 0.1-0.5 mol/l of nitric acid, and separation of organic and aqueous solutions; c) complete or partial extraction of the uranium (VI) fraction contained in the aqueous solution obtained at the stage b), while extraction contains at least one contact in the extractor of the aqueous solution with an organic solution S2 containing carbamide in an organic solution, and separation of aqueous and organic solutions; as a result, an aqueous solution containing plutonium (IV), but without uranium (VI), or a mixture of plutonium (IV) and uranium (VI), and an organic solution containing uranium (VI) without plutonium (IV) are obtained. The invention also relates to new carbides.

EFFECT: carbamides of the formula (I) in the presence of an aqueous phase with strong acidity provide coefficients of uranium (VI) and plutonium (IV) distribution, allowing for quantitative co-extraction of these two elements from the aqueous phase, and, in the presence of an aqueous phase with moderate acidity, provide coefficients of uranium (VI) and plutonium (IV) separation, at which uranium (VI) is separated from plutonium (IV) without the need for reduction of plutonium (IV) to plutonium (I).

(I)

16 cl, 1 dwg, 3 tbl, 1 ex

Description

Technical field

The invention relates to the field of processing spent nuclear fuel.

More precisely, the invention relates to the use of carbamides (i.e., urea derivatives) as extractants for the complete or partial separation of uranium (VI) from plutonium (IV) based on an aqueous solution obtained by dissolving spent nuclear fuel in nitric acid, by means of liquid-phase extraction without the use of any reduction of plutonium(IV) to plutonium(III). It also belongs to the new carbamides with extremely useful extractive properties.

In particular, the invention finds application in the processing of spent nuclear fuel based on uranium (in particular, uranium oxides - UOX) or uranium and plutonium (in particular, mixed oxides of uranium and plutonium - MOX).

Prior Art

The PUREX process, used worldwide in all spent nuclear fuel treatment plants (Ag in France, Rokkasho in Japan, Sellafield in England, etc.), uses tri-n-butyl phosphate (or TBP) as an extractant for uranium recovery. (VI) and plutonium (IV) by liquid-phase extraction from aqueous solutions obtained by dissolving these types of nuclear fuel in nitric acid.

In this method, TBP is used as a 30% solution (v/v) in an aliphatic solvent (hydrogenated tetrapropylene (HTP), n-dodecane or kerosene). In this field, this organic solution is usually referred to as a solvent.

Regeneration of uranium and plutonium by the PIREX method is carried out in several cycles:

- first cycle: purification of uranium (VI) and plutonium IV) (called "1CUPu") in order to deactivate these elements in relation to americium, curium and fission products with the separation of uranium (VI) and plutonium (IV) into two water streams, starting from this first cycle;

- second cycle: purification of uranium (VI) (referred to as ZCU) in order to complete the decontamination of this element to meet the specifications stipulated in the ASTM standards for uranium as a target product, and

- second cycle, and in some plants this is the third cycle: purification of plutonium (IV) (denoted as "2CPu" and "3CPu" respectively) in order to complete the decontamination of this element to meet the specifications stipulated in the ASTM standards for plutonium as a target product , and its enrichment to the conversion into oxide PuO ₂ .

The PUREX process parameters are satisfactory and reference to the experience gained since the start-up of plants in which this method is applied is positive.

However, the use of tri-n-butyl phosphate (TBP) has limitations that make it impossible for this extractant to achieve the goals of simplicity, compactness, and improved safety for future spent nuclear fuel processing plants.

The main of these limitations is that the separation of uranium and plutonium into two aqueous streams makes it necessary to reduce plutonium (IV) to plutonium (III) (because when using TBP, the separation factor of uranium (VI) and plutonium (IV) is insufficient regardless of the acidity of the aqueous solution used for such separation) and, consequently, the use of reducing and antinitrogen agents in significant quantities, which form, due to decomposition, unstable and reactive products requiring appropriate safety measures.

The supply of extractants for the quantitative co-extraction of uranium and plutonium from an aqueous solution obtained by dissolving spent nuclear fuel in nitric acid, followed by complete or partial separation of these two elements without the need to reduce plutonium (IV) to plutonium (III), was the reason for a certain amount of work .

Thus it has been proposed to use symmetrical N,N-dialkylamides such as N,N-di(2-ethylhexyl)-3,3-dimethylbutanamide (or DEHDMBA) in PCT WO 2017/017207, hereinafter [D1], and also asymmetric N,N-dialkylamides such as N-methyl-N-octyl-2-ethylhexanamide (or MOEHA) in PCT WO 2017/017193, hereinafter referred to as [D2]. In both cases, N,N-dialkylamides showed very encouraging properties.

Meanwhile, three studies have been published in the literature on the extraction of uranium and plutonium from a nitrogen aqueous solution by means of carbamides. We are talking about a study conducted by E.K. Dukes and T.H. Sidall, using N,N,N',N'-tetra-n-butylurea, which was reported in Journal of Inorganic and Nuclear Chemistry 1966, 28(10), pp. 2307-2312, hereinafter referenced [3], and also about the study by G.M. Chumakova et al. also with N,N,N',N'-tetra-n-butylurea, published in "Radiochemistry" 1980, 22(2), pp. 213–217, further reference [4], and about an earlier research conducted by B.G. Vats et al. with N,N-diethyl-N',N'-diisobutylurea (or DEDiBU) and N,N-diethyl-N,N'-di-n-octylurea (or DEDOU) published in Dalton Transactions 2016, 45(25) , pp. 10319 – 10325, further [5].

These studies revealed the potential of tetraalkyl carbamides as extractants for uranium (VI) and plutonium (IV). However, none of them allowed to see the use of compounds of this type for the co-extraction of uranium (VI) and plutonium (IV) from an aqueous solution obtained by dissolving spent nuclear fuel, and for the complete or partial separation of uranium (VI) and plutonium (IV) without reduction of plutonium (IV) to plutonium (III).

However, it turned out that in the framework of their work, the authors of the invention found that some carbamides have extractive properties, namely:

- in the presence of an aqueous phase with a strong acidity, such as aqueous solutions have after dissolving spent nuclear fuel in nitric acid, they provide partition coefficients for uranium (VI) and plutonium (IV) allowing quantitative co-extraction of these two elements from a given aqueous phase, and

- in the presence of an aqueous phase with moderate acidity, they provide U(VI) / Pu(IV) separation factors, at which uranium (VI) is separated from plutonium (IV) without the need to reduce plutonium (IV) to plutonium (III), and this separation can be total or partial as desired.

And it is on these findings that the present invention is based.

Description of the invention

The subject of the invention is primarily the use of carbamide of general formula (I):

(I),

(I)

where R ¹ , R ² , R ³ , identical or different, mean a linear or branched alkyl group with 1-12 carbon atoms, a cycloalkyl group with 3-12 carbon atoms or a cycloalkyl group with 4-13 carbon atoms;

R ⁴ means a hydrogen atom, a linear or branched alkyl group with 1-12 carbon atoms, a cycloalkyl group with 3-12 carbon atoms or a cycloalkyl group with 4-13 carbon atoms;

as an extractant for the complete or partial separation of uranium (VI) from plutonium (IV) without reduction of plutonium (IV) into plutonium (III) based on an aqueous solution of A1 obtained by dissolving spent nuclear fuel in nitric acid.

Such application includes:

a) co-extraction of uranium (VI) and plutonium (IV) from an aqueous solution, while co-extraction involves at least one contact in the extractor of an aqueous solution A1 with an organic solution S1 containing urea in an organic solvent, and the separation of aqueous and organic solutions;

b) stripping the plutonium (IV) and uranium (VI) fraction from the organic solution obtained in step a), wherein the stripping comprises at least one bringing the organic solution S1 into contact with an aqueous solution A2 in the extractor containing 0.1 –0.5 mol/l of nitric acid, and separation of organic and aqueous solutions;

c) total or partial extraction of the uranium (VI) fraction present in the aqueous solution obtained in step b), wherein the extraction comprises at least one bringing the aqueous solution into contact with an organic solution S2 containing urea in the organic solution, extractor and separation of aqueous and organic solutions;

the result is an aqueous solution containing plutonium (IV) and no uranium (VI) or a mixture of plutonium (IV) and uranium (VI) and an organic solution containing uranium (VI) but no plutonium (IV).

In the above, as well as below, the expressions “when the content is from ... to ...”, “... from ... to ...”, “contained from ... to ...” are equivalent and mean limit values.

The expression "straight or branched alkyl group with 1-12 carbon atoms" means any straight-chain or branched alkyl group containing 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 atoms carbon, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-amyl, sec-amyl, isoamyl, n-hexyl, isohexyl, n-heptyl, isoheptyl, n-octyl, isooctyl, n-nonyl, isononyl, n-decyl, isodecyl, n-undecyl, n-dodecyl, 2-methylbutyl, 2-methylbutyl, 3-methylpentyl, 2-methylhexyl, 3-methylhexyl, 2-methylheptyl, 2-methioctyl, 2-ethylhexyl, 1,5-dimethylhexyl, 2,4,4-trimethylamyl, 2-ethylheptyl, 1,2-dimethylheptyl, 2,6-dimethylheptyl, 3,5,5-trimethylhexyl, 2-methylnonyl, 3,7-dimethyloctyl, 2,4,6-trimethylheptyl, 2-butylhexyl and other groups.

By "cycloalkyl group with 1-12 carbon atoms" is meant any cycloalkyl group with one or more fused or bridged rings, and these rings can be substituted by one or more alkyl groups, if the total number of carbon atoms contained in this cycloalkyl group (together with substituents, if any) is 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12; such a cycloalkyl group is, for example, cyclopropyl, cyclobutyl, cycloamyl, cyclohexyl, cycloheptyl, cyclooctyl, decahydronaphthalenyl, bicyclopropyl, bicyclohexyl, cycloamyl or cyclohexyl groups substituted with one or more methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl groups. , tert-butyl, sec-butyl and other groups.

By "cycloalkyl group with 4-13 carbon atoms" is meant any alkyl group substituted with a cycloalkyl group, provided that the total number of carbon atoms in the alkyl group with its substituent is 4, 5, 6, 7, 8, 9, 10, 11 , 12 or 13; such a cycloalkyl group is, for example, 3-cyclohexylpropyl, 3-cyclohexylbutyl, 3-cyclohexyl-2-methylbutyl, 4-cyclohexyl-1-methylbutyl, 4-cyclohexyl-2-methylbutyl, 4-cyclohexyl-3-methylbutyl, 4-cyclohexyl -1-ethylbutyl, 4-cyclohexyl-2-ethylbutyl, 4-cyclohexyl-1-propylbutyl, 4-cyclohexyl-2-propylbutyl, 4-cyclohexyl-amyl, 4-cyclohexyl-2-methylamyl and other groups.

At the same time, the expressions "aqueous solution" and "aqueous phase" are equivalent and interchangeable, just as the expressions "organic solution" and "organic phase" are equivalent and interchangeable.

While cycloalkyl groups can be considered as alkyl groups in which the single carbon chain or main carbon chain is cyclic, a useful urea of the invention can be characterized as a tetraalkyl urea if R ⁴ is other than a hydrogen atom, or alternatively as tri- alkyl urea if R ⁴ means a hydrogen atom ^. This feature will be used below.

According to the invention, the total number of carbon atoms in urea is preferably 17 to 25.

If the urea is tetraalkyl, it is preferred that R ¹ , R ² , R ³ and R ⁴ represent a linear or branched alkyl group with 1-12 carbon atoms.

However, it is preferable that:

R ¹ and R ² were the same, R ³ and R ⁴ were the same but different from R ¹ and R ² or

R ¹ and R ⁴ were the same, R ² , R ³ were the same but different from R ^1, and R ⁴ or

R ¹ , R ² , R ³ , R ⁴ were all the same.

If R ¹ and R ² are the same, R ² and R ⁴ are the same and different from R ¹ and R ² , then R ¹ and R ² are preferably a linear or branched alkyl group with 1 to 5 carbon atoms, and R ³ and R ⁴ are preferably a linear or branched alkyl group with 6 to 10 carbon atoms, with the total number of carbon atoms contained in the urea being preferably 19, 21 or 23.

Similarly, when R ¹ and R ⁴ are the same, R ² and R ³ are the same but different from R ¹ and R ⁴ , and R ¹ and R ⁴ are preferably a linear or branched alkyl group with 1 to 5 carbon atoms, when where R ² and R ³ are preferably a linear or branched alkyl group with 6 to 10 carbon atoms, the total number of carbon atoms in the urea is preferably 19, 21 or 23.

If R ¹ , R ² , R ³ , R ⁴ are the same, they are preferably a linear or branched alkyl group with 4 to 8 carbon atoms, with particular preference being given to n-butyl, n-amyl, n-hexyl, n-heptyl and n-octyl groups.

If the carbamide is tri-alkyl, then preferably R ¹ , R ² and R ³ are a linear or branched alkyl group with 1-12 carbon atoms, preferably 2-10 carbon atoms, more preferably 4-8 carbon atoms.

However, it is preferable that R ¹ be equal to R ² , moreover, that R ¹ , R ² and R ³ are the same, in this case R ¹ , R ² and R ³ mean preferably a linear or branched alkyl group from 6-8 carbon atoms, with n-hexyl, n-heptyl, n-octyl and 2-ethylhexyl groups being particularly preferred.

Therefore, the following preferred carbamides can be cited by way of example:

- N,N,N',N'-tetra-n-butylurea (or TBU), corresponding to the General formula (I), where: R ¹ , R ² , R ³ and R ⁴ mean n-butyl group;

- N,N,N',N'-tetra-n-amylurea (or TPU), corresponding to the General formula (I), where: R ¹ , R ² , R ³ and R ⁴ mean n-amyl group;

- N,N,N',N'-tetra-n-hexylurea (or THU), corresponding to the General formula (I), where: R ¹ , R ² , R ³ and R ⁴ mean n-hexyl group;

- N,N,N',N'-tetra-n-octylurea (TOU), corresponding to the General formula (I), where: R ¹ , R ² , R ³ and R ⁴ mean n-octyl group;

- N,N'-di-n-butyl-N,N'-di-n-hexylurea (or DBDHU) corresponding to the general formula (I), where: R ¹ and R ⁴ mean n-butyl group, R ² and R ³ means n-hexyl group;

- N,N'-di-n-heptyl-N,N'-di-n-propylurea (or DHDPU) corresponding to the general formula (I), where R ¹ and R ⁴ mean n-propyl group, R ² R ³ represent an n-heptyl group;

- N,N'-diethyl-N,N'-di-n-octylurea (or sym-DEDOU) corresponding to the general formula (I), where R ¹ and R ⁴ mean an ethyl group, R ² and R ³ mean n- octyl group;

- N,N'-dimethyl-N,N'-di-n-nonylurea (or DMDNU) corresponding to the general formula (I), where: R ¹ and R ⁴ mean a methyl group, R ² and R ³ mean n-nonyl group;

- N,N,N'-tri-n-octylurea (or TrOU), corresponding to the General formula (I) where: R ¹ , R ² and R ³ mean n-octyl group, R ⁴ means a hydrogen atom;

- N,N,N'-tri(2-ethylhexyl)urea (or TrEHU) corresponding to the general formula (I), where: R ¹ , R ² and R ³ mean a 2-ethylhexyl group, R ⁴ means a hydrogen atom, and

- N,N-di(2-ethylhexyl)-N'-n-octylurea (or DEHOU) corresponding to the general formula (I), where: R ¹ and R ² mean 2-ethylhexyl group, R ³ means n-octyl group , R ⁴ means a hydrogen atom.

Of these carbamides, N,N,N',N'-tetra-n-butylurea, N,N,N',N'-tetra-n-hexylurea, N,N,N',N'-tetra-n -pentylurea, N,N'-di-n-butyl-N,N'-di-n-hexylurea, N,N'-di-n-heptyl-N,N'-di-n-propylurea, N,N '-diethyl-N,N'-di-n-octylurea and N,N'-dimethyl-N,N'-di-n-nonylurea due to the fact that they have a particularly high selectivity for uranium (VI) compared to plutonium (IV) with moderate acidity, i.e. in the presence of an aqueous phase containing 0.1 to 0.5 mol/l nitric acid.

According to the invention, the organic solutions S1 and S2 preferably contain 0.5-2 mol/l, preferably 1-1.4 mol/l, urea.

The organic solvent is preferably an acyclic hydrocarbon or a mixture of acyclic hydrocarbons, for example, n-dodecane, hydrogenated tetrapropylene (or TPH), kerosene, Isane ^TM IP-185T or Isane ^TM IP-175T, preferably TPH.

As for the aqueous solution obtained by dissolving spent nuclear fuel, it usually contains nitric acid in an amount of 3 to 6 mol/l.

Preferably, step a) further comprises decontaminating the organic solution obtained by co-extraction of uranium (VI) and plutonium (IV) with respect to americium, curium and fission products, such deactivation comprising at least bringing into contact with organic solution extractor with aqueous solution A3 containing 1–6 mol/l nitric acid, and separation of organic and aqueous solutions. If necessary, this decontamination is supplemented by the deacidification of the organic solution thus deactivated, whereby such deacidification comprises at least bringing into contact in the extractor the organic solution obtained by the deactivation with an aqueous solution of A4 containing 0.1-1 mol/l , preferably 0.5 mol/l nitric acid, and separation of organic and aqueous solutions.

It is also preferred that the contacting in the extractor in which step b) of the organic solution obtained in step a) with the aqueous solution A2 involves countercurrent circulation of the organic and aqueous solutions at a flow rate of O/A (organic solution/aqueous solution) , preferably greater than 1, preferably equal to or greater than 3, more preferably equal to or greater than 5, so that plutonium (IV) enriching stripping, i.e. re-extraction of plutonium (IV) with the formation of an aqueous solution in which the concentration of plutonium (IV) exceeds its concentration in the organic solution from which it was re-extracted.

According to the invention, the uranium (VI) present in the organic solution obtained in step c) can be re-extracted from this solution by at least bringing the organic solution into contact in an extractor with an aqueous solution A5 containing not more than 0.05 mol / l, preferably not more than 0.01 mol/l nitric acid, then separate the organic and aqueous solutions. This back-extraction can be carried out at room temperature or at a temperature of 40 to 50°C, which facilitates stripping of uranium (VI). However, it can be carried out at an O/A flow rate greater than 1, so that the uranium (VI) is re-extracted, accompanied by enrichment.

Of the suitable carbamides according to the invention, some, such as the tetraalkyl carbamides N,N,N',N'-tetra-n-butylurea, N,N,N',N'-tetra-n-hexylurea, N,N, N',N'-tetra-n-octylurea and N,N,N',N'-tetra-n-pentylurea are known in the art.

Others, such as, on the one hand, tetra-alkyl carbamides N,N'-di-n-butyl-N,N'-di-n-hexylurea, N,N'-di-n-heptyl-N,N '-di-n-propylurea, N,N'-diethyl-N,N'-di-n-octylurea, N,N'-dimethyl-N,N'-di-n-nonylurea (or DMDNU), and, on the other hand, the tri-alkyl carbamides N,N,N'-tri-n-octylurea, N,N,N'-tri(2-ethylhexyl)urea and N,N-di(2-ethylhexyl)-N'- n-octylureas were not known to the inventors and have never been described in the prior art.

Therefore, also the subject of the invention is a tetraalkyl urea of the above general formula (I), in which R ¹ and R ⁴ are the same and mean a linear alkyl group with 1 to 4 carbon atoms, R ² and R ³ are the same and mean a linear alkyl group with 6–9 carbon atoms, while the total number of carbon atoms in urea is 21.

This tetraalkyl urea can be DBDHU, sym-DEDOU, or DMDNU urea, with sym-DEDOU and DMDNU being preferred.

As is known per se, this tetra-alkyl urea can be obtained in particular by the reaction of an N,N'-dialkyl urea of the formula RNHC(O)NHR, where: R has the same meaning as R ² , R ³ in urea containing haloalkane HalR', eg iodalkane IR', where R' has the same meaning as R ¹ and R ⁴ in urea in an organic medium, eg tetrahydrofuran and in the presence of a metal hydride such as sodium hydride.

Also the subject of the invention is a trialkyl urea of a separate formula (II):

(II),

(II)

where: R ¹ , R ² and R ³ , the same or different, mean a linear or branched alkyl group with 8-12 carbon atoms or a cycloalkyl group with 3-12 carbon atoms, provided that at least one of the values of R ¹ , R ² or R ³ will be other than a cycloalkyl group.

In addition, the total number of carbon atoms contained in the urea of the individual formula (II) is preferably 17 to 25.

In this urea, R ¹ , R ² and R ³ are preferably each a linear or branched alkyl group, with n-octyl and 2-ethylhexyl groups being particularly preferred.

However, it is preferred that R ¹ is the same as R ² and that R ¹ , R ² and R ³ are the same with each other.

According to the invention, the urea of particular formula (II) is preferably the TrOU, TrEHU or DEHOU ureas, with the TrOU and TrEHU ureas being particularly preferred.

As is known per se, this urea can be obtained by reacting a secondary amine of the formula R ¹ R ² NH, where R ¹ and R ² have the meanings given above, with an isocyanate of the formula R ³ NCO, where R ³ has the meaning given above, in organic medium, such as anhydrous dichloromethane.

In addition to having very useful extractive properties, carbamides also have the advantage that they, like their decomposition products, contain only carbon, hydrogen, oxygen and nitrogen atoms, as a result of which they are completely ashable and do not create secondary hazardous waste.

Other features and advantages of the invention will become apparent from the following description.

In any case, it goes without saying that the following description serves to explain the subject matter of the invention and should not be taken as limiting the subject matter at all.

Brief description of the figure

In FIG. 1 is a schematic diagram of a preferred embodiment of the invention, with boxes 1-4 representing multi-level extractors that are traditionally used in the processing of spent nuclear fuel (mixer-settlers, pulse columns or centrifugal extractors); organic phases are indicated by a solid line, aqueous phases by a dotted line.

Detailed description of particular embodiments

Example 1. Synthesis of tetra-alkyl and tri-alkyl carbamides

1. 1. N,N'-di-n-butyl-N,N'-n-hexylurea or DBDHU

Urea DBDHU is synthesized by:

- reactions of n-hexyl isocyanate (5 ml, 34.3 mmol) with n-hexylamine (0.3 mol/l, 1.2 gram equivalent) in anhydrous dichloromethane to give N,N'-di-hexylurea,

- reactions of N,N'-di-n-hexylurea (7.6 g, 33.4 mol) with iodobutane (5 gram equivalents) in tetrahydrofuran (THF).

In this case, a solution of n-hexyl isocyanate was added dropwise at 0°C to a solution of n-hexylamine in anhydrous dichloromethane. The reaction mixture was shaken for 1 h at 0°C, then brought to room temperature and shaken overnight. The solvent was evaporated under reduced pressure and the product was purified on silica gel with a mixture of dichloromethane and methanol (95/5 v/v) to obtain 7.6 g of N,N'-di-n-hexylurea (yield: >97%).

Then a suspension of NaH at 60% by weight (8 gram equivalents) in THF was added portionwise to a solution of N,N'-di-n-hexylurea in THF at 0°C. The reaction mixture was shaken for 2 hours at 0°C, then iodobutane was added dropwise. The mixture was brought to room temperature, then to reflux. Overnight, the mixture was cooled to 0°C and water was added to it. The aqueous phase was extracted with diethyl ether. The organic phase was then washed with a saturated sodium chloride (NaCl) solution and dried over sodium sulfate (Na ₂ SO ₄ ). The solvent was evaporated under reduced pressure, the resulting solid was purified on silica gel with a mixture of cyclohexane and ethyl acetate (99/1 - 90/10 v/v) to obtain 10.5 g of DBDHU urea (yield: >95%). A description of the properties of this carbamide is given below.

NMR ¹ H (CDCl ₃ , 400 MHz) δ (ppm): 3.12 (m, 8H); 1.50 (m, 8H); 1.28 (m, 16H); 0.92(m, 12H)

¹³ C NMR (CDCl ₃ , 101 MHz) δ (ppm): 165; 48.5; 48.1; 31.6; 30.1; 27.9; 26.7; 22.6; 20.2; 14.0; 13.9

MS (Positive Mode Electrospray Ionized (ESI)): 341 (MH ⁺ ); 682 (2MH ⁺ ); 703 (2M Na ⁺ )

GC-HRMS (EI positive mode): >99% purity; exact calculated weight for C ₂₁ H ₄₄ N ₂ O: 340.3448; received: 340.3412

I.2 - N,N'-di-n-heptyl-N,N'-di-n-propyl urea or DHDPU:

Urea DHDPU was synthesized by:

- reactions of n-heptyl isocyanate (5 g, 35.4 mmol) with n-heptylamine (0.3 mol/l, 1.2 gram equivalent) in anhydrous dichloromethane to give N,N'-di-n-heptylurea, and

- reactions of N,N'-di-n-heptylurea (8.8 g, 34.3 mmol) with iodopropane (5 gram equivalents) in THF;

following the same protocol as described above in paragraph 1.1 for the synthesis of DBDHU.

Thus, 11.2 g of DHDPU urea (yield: >96%) was obtained, the properties of which are described below:

NMR ¹ H (CDCl ₃ . 400 MHz) δ (ppm): 3.07 (m, 8H); 1.50 (m, 8H); 1.25 (m, 16H); 0.86(m, 12H)

¹³ C NMR (CDCl ₃ . 101 MHz) δ (ppm): 163.7; 48.5; 46.7; 30.2; 27.5; 26.4; 25.4; 21.0; 19.6; 12.4; 9.8

MS (ESI in positive mode): 341 (MH ⁺ ); 363 (MNa ⁺ ); 682 (2MH ⁺ ); 703 (2M Na ⁺ )

GC-HRMS (EI in positive mode): purity: 99.1%; exact calculated weight for C ₂₁ H ₄₄ N ₂ O: 340.3448; received: 340.3438

I.3 - N,N'-diethyl-N,N'-di-n-octylurea or sym-DEDOU:

Urea sym-DEDOU was synthesized by:

- reactions of n-octyl isocyanate (5 g, 32.2 mmol) with n-octylamine (0.3 mol/l, 1.2 gram equivalent) in anhydrous dichloromethane to obtain N,N'-di-n-octylurea and

- reactions of N,N'-di-n-octylurea (8.9 g, 31.3 mmol) with iodoethane (5 gram equivalents.) in THF;

following the same protocol as described above in paragraph 1.1 for the synthesis of DBDHU.

10.2 g of sym-DEDOU urea was obtained (yield: >96%), the properties of which are described below.

NMR ¹ H (CDCl ₃ , 400 MHz) δ (ppm): 3.14 (q, J = 7 Hz, 4H _Et ); 3.07(m, 4H _Oct ); 1.48 (m, 4H _Oct ); 1.25(m, 20H _Oct ); 1.08 (t, J = 7 Hz, 6H _Et ); 0.86(m, 6H _Oct )

¹³ C NMR (CDCl ₃ , 101 MHz) δ (ppm): 164.9; 47.8; 42.9; 31.8; 29.4; 29.3; 28.0; 27.1; 22.6; 14.1; 13.2

MS (ESI in positive mode): 341 (MH ⁺ ); 363 (MNa ⁺ ); 703 (2M Na ⁺ )

GC-HRMS (EI in positive mode): purity: 99.5%; exact calculated weight for C ₂₁ H ₄₄ N ₂ O: 340.3448; received: 340.3437

I.4 - N,N'-dimethyl-N,N'-di-n-nonylurea or DMDNU:

Urea DMDNU was synthesized by reacting N,N'-di-n-methylurea (2 g, 22.7 mmol) with iodononane (5.3 ml, 40.2 mmol, 2 gram equivalent) in THF.

Here, a solution of N,N'-dimethylurea in THF was added dropwise at 0° C. to a suspension of NaH at 60% by weight (7.2 g, 45 mmol, 2 gram equivalent) in THF. The reaction mixture was shaken for 1 hour at 0°C, then iodononan was added dropwise. The mixture was brought to room temperature and then to reflux. Overnight, the medium was cooled to 0°C and water was added. The aqueous phase was extracted with diethyl ether. The organic phase was then washed with a saturated NaCl solution and dried over Na ₂ SO _{4 .} The solvent was evaporated at low pressure, the resulting solid was purified on silica gel with a mixture of cyclohexane and ethyl acetate (99/1-90/10 v/v) to give 7.8 g of urea DMDNU (yield: >98%). Description of properties is given below.

NMR ¹ H (CDCl ₃ , 400 MHz) δ (ppm): 3.08 (t, J = 7.5 Hz, 4H); 2.76 (s, 6H); 1.52 (m, 4H); 1.25(m, 24H); 0.87(m, 6H)

¹³ C NMR (CDCl ₃ , 101 MHz) δ (ppm): 165.4; 50.6; 36.5; 31.8; 29.5; 29.4; 29.3; 27.6; 26.9; 22.6; 14.1

MS (ESI in positive mode): 341 (MH ⁺ ); 682 (2MH ⁺ ); 703 (2M Na ⁺ )

GC-HRMS (EI in positive mode): purity: 99.2%; exact calculated weight for C ₂₁ H ₄₄ N ₂ O: 340.3448; received: 340.3454

I.5 - N,N,N'-tri-n-octylurea or TrOU:

Urea TrOU was synthesized by reacting di-n-octylamine (1.5 ml, 4.9 mmol, 1 gram equivalent) with n-octyl isocyanate (1.7 ml, 9.7 mmol, 2 gram equivalent) in anhydrous dichloromethane ( 0.1 mol/l).

Meanwhile, a solution containing di-n-octylamine and n-octyl isocyanate in anhydrous dichloromethane was shaken for 4.5 hours at ambient temperature. The reaction mixture was then washed twice with 1M hydrochloric acid and twice with saturated sodium bicarbonate (NaHCO ₃ ). The organic phase was dried over magnesium sulfate (MgSO ₄ ), concentrated under reduced pressure, the residue was purified by flash chromatography on silica gel with dichloromethane/ethyl acetate (95/5; v/v) to give 1.9 g of TrOU as a colorless oil (yield: 100 %). The properties are described below.

NMR ¹ H (CDCl ₃ , 400 MHz) δ (ppm): 4.25 (s, 1H); 3.20 (q, J = 5.4 Hz, 2H); 3.14 (q, J = 7.7 Hz, 4H); 1.50 (m, 6H); 1.46 (m, 30H); 0.87 (t, J = 7.2 Hz, 9H)

¹³ C NMR (CDCl ₃ , 101 MHz) δ (ppm): 157.8; 47.5; 41.0; 32.0; 30.6; 29.6; 29.5; 29.4; 28.8; 27.2; 22.8; 14.2

IR (KBr, ν _max /cm ^-1 ): 3345; 2955; 2922; 2853; 1622; 1533; 1465; 1376; 1274; 767; 722

MS (ESI in positive mode): 397 (MH ⁺ ), 816 (2MNa ⁺ )

HRMS (ESI in positive mode): calculated for C ₂₅ H ₅₃ N ₂ O: 397.4158; received: 397.4156.

Elemental analysis (%) (C ₂₅ H ₅₂ N ₂ O + 0.18AcOEt): calculated: C 74.88; H 13.06; N 6.79; obtained: C, 75.17; H 12.74; N 6.88

I.6 - N,N,N'-tri(2-ethylhexyl)urea or TrEHU:

TrEHU was synthesized from di(2-.ethylhexyl)amine (1.49 ml, 4.92 mmol, 1 gram equivalent) and 2-ethylhexyl isocyanate (1.76 ml, 9.84 mmol, 2 gram equivalent) in anhydrous dichloromethane (0.1 mol/l).

Meanwhile, a solution containing di(2-ethylhexyl)amine and 2-ethylhexyl isocyanate in anhydrous dichloromethane was shaken for 5 hours at room temperature. The reaction mixture was then washed twice with 1M hydrochloric acid and twice with saturated NaHCO ₃ solution. The organic phase was dried over MgSO ₄ , concentrated under reduced pressure and the residue was purified by flash chromatography on silica gel with a dichloromethane/ethyl acetate gradient (99/1 - 90/10 v/v) to give 1.94 g TrEHU as a thick colorless oil (yield: 99%). The properties are described below.

NMR ¹ H (CDCl ₃ , 400 MHz) δ (ppm): 4.26 (s, 1H); 3.17 (t, J = 5.7 Hz, 2H); 3.09 (m, 4H); 1.62 (m, 2H); 1.46(m, 25H); 0.88(m, 18H)

NMK ¹³ C (CDCl ₃ , 101 MHz) δ (ppm): 158.2; 51.6; 43.6; 39.8; 38.4; 31.2; 30.7; 28.9; 24.4; 24.0; 23.1; 14.1; 11.0; 10.8.

IR (KBr, ν _max /cm ^-1 ): 3348; 2957; 2925; 2859; 1617; 1534; 1459; 1378; 1240; 765; 727

MS (ESI in positive mode): 397 (MH ⁺ ); 816 (2MNa ⁺ )

HRMS (ESI in positive mode): calculated for C ₂₅ H ₅₃ N ₂ O: 397.4158; received: 397.4156

Elemental analysis (%) (C ₂₅ H ₅₂ N ₂ O): calculated C 75.69; H 13.21; N 7.06; obtained: C, 75.49; H 13.45; N 6.91

I.7 - N,N-di (2-ethylhexyl) -N'-n-octylurea or DEHOU:

Urea DEHOU was synthesized from di(2-ethylhexyl)amine (1.49 ml, 4.92 mmol, 1 gram equivalent) and n-octyl isocyanate (1.75 ml, 9.84 mmol, 2 gram equivalent) in anhydrous dichloromethane (0.1 mol/l) following the same protocol as the protocol described above in paragraph 1.6 for the synthesis of carbamin TrEHU. 1.84 g of DEHOU urea was obtained as a colorless oil (94% yield). The properties are described below.

NMR ¹ H (CDCl ₃ , 400 MHz) δ (ppm): 4.25 (t, J = 4.9 Hz, 1H); 3.20 (q, J = 6.1 Hz, 2H); 3.07 (m, 4H); 1.61 (hept, J = 5.6 Hz, 2H); 1.46 (quint, J = 6.4 Hz, 2H); 1.25(m, 26H); 0.86(m, 15H)

¹³ C NMR (CDCl ₃ , 101 MHz) δ (ppm): 158.3; 51.5; 41.0; 38.5; 31.9; 30.8; 30.5; 29.5; 29.4; 29.0; 27.2; 24.1; 23.2; 22.8; 14.2; 10.9

IR (KBr, ν _max /cm ^-1 ): 3344; 2957; 2924; 2857; 1618; 1533; 1459; 1408; 1378; 1241; 765; 725

MS (ESI in positive mode): 397 (MH ⁺ ); 816 (2MNa ⁺ )

HRMS (ESI in positive mode): calculated for C ₂₅ H ₅₃ N ₂ O: 397.4158; received: 397.4154

Elemental analysis (%) (C ₂₅ H ₅₂ N ₂ O): calculated: C 75.69; H 13.21; N 7.06; obtained: C, 75.87; H 13.49; N 6.89

I.8 - N-cyclohexyl-N',N'-di-n-octylurea or CyDOU:

Urea CyDOU corresponding to the above formula (I), in which R ¹ = R ² = n-octyl, R ³ = cyclohexyl and R ⁴ = hydrogen, was synthesized from di-n-octylamine (1.50 ml, 4.87 mmol, 1 gram equivalent) and cyclohexyl isocyanate (1.27 ml, 9.74 mmol, 2 gram equivalent) in anhydrous dichloromethane (0.1 mol/L) using the same protocol as described in paragraph 1 above ,6 TrEHU urea synthesis protocol except that the dichloromethane/ethyl acetate gradient used for flash silica gel chromatography with some gradient was 96/4 - 90/10. This yielded 1.48 g of CyDOU urea as a colorless oil (yield: 83%).

The properties are described below.

NMR ¹ H (CDCl ₃ , 400 MHz) δ (ppm): 4.11 (s, 1H); 3.64 (m, 1H); 3.13 (t, J = 7.6 Hz, 4H); 1.94 (m, 2H); 1.59 (m, 8H); 1.28 (m, 24H); 0.87(t, 6H)

¹³ C NMR (CDCl ₃ , 101 MHz) δ (ppm): 157.1; 49.3; 47.5; 34.3; 32.0; 29.6; 29.4; 28.8; 27.2; 25.9; 25.2; 22.8; 14.2

IR (KBr, ν _max /cm ^-1 ): 3330; 2922; 2853; 1617; 1528; 1451; 1407; 1314; 1251; 1214; 890; 767; 722

MS (ESI in positive mode): 367 (MH ⁺ ); 756 (2MNa ⁺ )

HRMS (ESI in positive mode): calculated for C ₂₃ H ₄₇ N ₂ O: 367.3688; received: 367.3686

Elemental analysis (%) (C ₂₃ H ₄₆ N ₂ O): calculated: C 75.35; H 12.65; N 7.64; obtained: C, 75.33; H 12.80; N 7.47

II. Extractive properties of carbamides

II.1. Tetraalkyl carbamides

Uranium (IV) and plutonium (VI) extraction tests

Extraction tests were carried out using:

- as organic phases: solutions containing either ≈ 0.5 mol/l or ≈ 1.2 mol/l of one of the carbamides: TBU, THU, TOU, TPU, DBDPU, DHDPU, sym-DEDOU and DMDNU in hydrogenated tetrapropylene (TPH) and

- as aqueous phases: aqueous solutions containing 10 g/l uranium (VI), ≈ 200 kBq/ml plutonium (IV) and nitric acid at a concentration of either 4 mol/l (to simulate the acidity inherent in an aqueous solution obtained by dissolving spent nuclear fuel in nitric acid), or 0.5 mol/l (to simulate the acidity inherent in an aqueous plutonium re-extraction solution according to the invention).

Each of these experiments was carried out with contact in the tube while shaking the organic phase with the aqueous phase for 30 minutes at 25°C. The volume ratio O/A was 1.

After centrifugation and phase separation, the concentration of uranium in aqueous phases was measured with an atomic emission spectrometer with an induced plasma jet (or ICP-AES), and the concentration of uranium in organic phases was determined by re-extraction of this element in a solution of nitric acid at a concentration of 0.01 mol/l and by measuring with ICP-AES in the aqueous phase formed during this re-extraction. The plutonium concentrations were measured in aqueous and organic phases by means of α spectrometry.

Table 1 below shows, for each concentration at which urea was tested, the uranium partition coefficients, denoted as D_U,_,and plutonium, designated as D_Ptobtained for aqueous phases at 4 mol/l HNO_3, and 0.5 mol/l HNO_3,as well as the separation factors, obtained for aqueous phases at 0.5 mol/l HNO₃.

For comparison, the table also includes the results of extraction tests under the same test conditions, but using solutions containing N,N-dialkylamides known from the prior art as organic phases, namely:

- a solution containing 0.5 mol/l of urea DEHDMBA (N,N-di(2-ethylhexyl)-3,3-dimethibutyramide) in hydrogenated tetrapropylene (TRH), proposed in the source [1],

- a solution containing either 0.5 mol/l or 1.2 mol/l MOEHA (N-methyl-N-octyl-2-ethylhexanamide in TPH, proposed in the source [2].

Table 1

Tested compounds [C]
(mol/l) [ _HNO3 ]
(mol/l) D _U D _Pu FS _U/Pu

TBU

TBU 0.48 four 2.8 0.15 0.5 0.11 0.004 26 1.2 four 15.9 3.8 0.5 0.96 0.027 36

THU 0.51 four 4.0 0.33 0.5 0.15 0.009 16 1.3 four 17.5 2.6 0.5 1.0 0.045 23

TOU 0.52 four 5.1 1.3 0.5 0.26 0.048 5.4 1.2 four 16.5 1.9 0.5 1.1 0.33 3.4

TPU 1.2 four 18.4 2.15 0.5 0.80 0.011 73

DBDHU 1.2 four 18.7 2.22 0.5 0.77 0.010 74

DHDPU 1.2 four 18.8 2.37 0.5 0.74 0.008 94

sym-DEDOU 1.2 four 18.5 1.32 0.5 0.68 0.004 170

DMDNU 1.3 four 39.1 33.5 0.5 2.74 0.092 thirty

DEHDMBA 0.50 four 2.5 0.31 0.5 0.036 0.003 13

MOEHA 0.50 four 1.9 0.22 0.5 0.050 0.004 13 1.2 four 7.5 4.3 0.5 0.31 0.016 twenty

[C] = concentration of the tested compound in the organic phase;

[HNO ₃ ] = concentration of nitric acid in the aqueous phase.

From this table, it can be seen that whether strong acidity ([HNO ₃ ] = 4 mol/l) or moderate acidity ([HNO ₃ ] = 0.5 mol/l), the uranium(IV) partition coefficients obtained for the eight tetraalkyl carbamides all exceed the coefficients obtained for N,N-dialkylamides DEHDMBA and MOEHA at a similar concentration of the organic phase, which confirms the pronounced ability of tetraalkyl carbamides to extract uranium (VI) from a highly deoxidized aqueous phase.

In particular, whether at strong acidity ([NHO ₃ ] = 4 mol/l) or at moderate acidity (HNO ₃ ] = 0.5 mol/l), the distribution coefficients of uranium (VI) obtained for carbamides TPU, DBDHU, DHDPU, sym-DEDOU, DMDNU at a concentration of about 1.2 mol/l in the organic phase are more than twice the coefficients obtained at a similar concentration of MOEHA in the organic phase.

Also, this table indicates that at strong acidity ([HNO ₃ ] = 4 mol/l) and at a concentration of 0.5 mol/l of the organic phase, carbamides TBU and TOU also create plutonium (IV) partition coefficients that are greater than the coefficients obtained at a similar concentration of the organic phase in DEHDMBA and MOEHA.

In contrast, at a concentration of 1.2 mol/l of the organic phase, the carbamides TBU, THU, TPU, DBDHU, DHDPU and sym-DEDOU form plutonium (IV) partition coefficients that are less than the coefficient obtained at a similar concentration of the organic phase in MOHE, however, these coefficients the distributions remain, however, very satisfactory, since the urea coefficients TBU, THU, TPU, DBDPU, DHDPU are greater than 2, the urea coefficient TOU is about 2 (D _Pu+ = 1.9), the urea coefficient sym-DEDOU is greater than 1.3.

The table also shows that at moderate acidity ([HNO ₃ ]) = 0.5 mol/l), the U(VI) / Pu (IV) separation factors obtained for TBU, THU, TPU, DBDHU, DHDPU, sym- DEDOU and DMDNU exceed the coefficients for N,N-dialkylamides DEHDMBA and MOEHA at a similar concentration of the organic phase.

In particular, when applied at 1.2 mol/l in the organic phase, TBU achieves a U(VI)/Pu(IV) separation factor of 36, almost twice that of the MOEHA at a similar concentration of the organic phase; TPU and DBDHU carbamides make it possible to obtain U(VI)/Pu(IV) separation factors of 73 and 74, respectively, i.e. almost 4 times greater than the coefficient for MOEHA; Urea DHDPU allows you to obtain a U(VI) / Pu (IV) separation factor of 94, i.e. almost 5 times greater than the coefficient for MOEHA; as for the U(VI)/Pu(IV) separation factor obtained for sym-DEDOU, it is 8.5 times greater than that for MOHEA (170 versus 20).

Consequently? this table confirms that tetra-alkyl carbamides can be successfully used as extractants in a process for treating an aqueous solution obtained by dissolving spent nuclear fuel in nitric acid, including the co-extraction of uranium (VI) and plutonium (IV) from such a solution, which is strongly acidic , and provide partial or complete separation of plutonium (IV) and uranium (VI) present in the organic phase obtained by such stripping of plutonium (IV) from this organic phase with an aqueous solution of moderate acidity.

Of the tetraalkyl carbamides, urea DMDNU seems to be particularly useful for the development of such a method, since, on the one hand, at strong acidity, it has a much greater ability to extract uranium (D _U and D _Pu > 30) than the ability of urea MOEHA (D _U = 7 .5 and D _Pu = 4.3) and, on the other hand, it provides moderate acidity at a uranium (VI) / plutonium (IV) separation factor (FS _{U / Pu} = 30¸ which, in turn, is much higher, than the coefficient obtained using MOEHA.

Even if, at strong acidity, it has a weaker ability to extract plutonium than MOEHA, it seems that syn-DEBOU urea is an equally good candidate, since it makes it possible to obtain, at moderate acidity, a particularly high separation factor for uranium ((VI) and plutonium (IV) (FS _U/Pu = 170).

Uranium (VI) filling capability tests

Tests were carried out to determine the ability to fill with uranium (VI) by 4-fold bringing into contact in pipes with shaking of organic phases with a content of 1.2 mol/l of one of the carbamides TPU, DBDHU, DHDPU, sym-DEDOU and DMDNU in TPH with aliquots parts of the aqueous phase containing 200 g/l of uranium (VI) and 3.4 mol/l of nitric acid.

Each contact was carried out at 25°C for 30 minutes at an O/A ratio of 2.

After centrifugation and phase separation, the concentration of uranium in the organic phases was measured after the re-extraction of this element from a solution of nitric acid at a concentration of 0.01 mol/l; the ICP-AES instrument measured the concentration in the aqueous phase after stripping.

Table II below shows for each tested urea the concentrations of uranium (VI) in g/l, which were obtained in the organic phases after each of the 4 contacts.

For comparison, this table also contains the results of tests for the ability to fill with uranium (VI), carried out under the same experimental conditions, but using solutions containing 1.2 mol/l MOEHA in TPH as organic phases.

Table II

Tested compounds Uranium (VI) concentration in organic phases, g/l Contact 1 Contact 2 Contact 3 Pin 4 TPU 88 124 134 143 DBDHU 91 127 131 131 DHDPU 91 129 136 136 sym-DEDOU 91 125 138 130 DMDNU 95 133 138 137 MOEHA 87 124* --- ---

- After the dissolution of the 3rd phase.

After the second contact, the organic phase containing MOEHA formed a third phase. The addition of 300 ml of the organic phase (preliminarily balanced by contact with nitric acid) made it possible to eliminate this third phase: the saturation of the organic phase with uranium (VI) was therefore 124 g/l, i.e. approximately 87% of the theoretical filling capacity of the organic phase (taking into account the uranium/extractant stoichiometry = ½).

On the other hand, under the same conditions, the carbamides did not separate and it was possible to fill the organic phase containing urea with uranium (VI) in an amount from 130 to 143 g/l, which is more than 88% of the theoretical ability to fill the organic phase (taking into account the stoichiometry uranium/extractant = ½).

These results show that carbamides have a strong uranium (VI) loading capacity, which is compatible with the development of a nuclear fuel treatment process, and that they even eliminate the organic phase segregation problems seen with MOENA.

II.2 Tri-alkyl carbamides

Extraction tests were carried out similar to those described above in section II.1, but solutions containing 0.4–0.5 mol/l of one of the carbamides TrOU, TrEHU and DEHOU in hydrogenated tetrapropylene (TPH) were used as organic phases.

The results of these tests are presented below in table III.

Also included in this table for comparison are the results previously entered in Table I for MOEHA at 0.5 mol/L in TPH.

Table III

Tested compounds [C]
(mol/l) [ _HNO3 ]
(mol/l) D _U D _Pu FS _U/Pu

TrOU

Trou 0.52 four 22 60 0.5 2.7 0.037 72

TrEHU 0.43 four 3.5 0.61 0.5 0.52 0.004 121

DEHOU 0.44 four 5.4 1.7 0.5 0.63 0.026 24

MOEHA 0.5 four 1.9 0.22 0.5 0.050 0.004 13

[C] is the concentration of the tested compound in the organic phase;

[HNO ₃ ] - concentration of nitric acid in the aqueous phase.

It can be seen from this table that at strong acidity ([HNO ₃ ] = 4 mol/l) the three tri-alkyl carbamides tested extracted more uranium (VI) and plutonium ((IV) than the N,N-dialkylamide MOEHA because they produced partition coefficients for uranium(VI) and plutonium(IV) which were all greater than those obtained with this N,N-dialkylamide.

It also shows that at moderate acidity ([HNO ₃ ] = 0.5 mol/l), the three tested trialkyl carbamides make it possible to retain uranium (VI) in the organic phase at the same time very effectively (D _U > 0.5) and selectively with respect to to plutonium (IV), since the distribution coefficient of the resulting plutonium (IV) with these carbamides is less than 0.04.

Urea TrEHU is particularly selective as it provides a U(VI)/Pu(IV) separation factor of 121, i.e. 10 times greater than the coefficient of N,N-dialkylamide MOEHA at a comparable concentration.

In addition, this table confirms that tri-alkyl carbamides can be effectively used as extractants in a method for treating an aqueous solution obtained by dissolving spent nuclear fuel in nitric acid, including co-extraction of uranium (VI) and plutonium (IV) from this aqueous solution, being strongly acidic, and the partial or complete separation of plutonium (IV) and uranium (VI) present in this organic solution obtained by co-extraction by stripping plutonium (IV) from this organic phase with an aqueous solution of moderate acidity.

III. Schematic diagram of the preferred application of the invention

Let us turn to Fig. 1, which shows a schematic diagram of a preferred application of the invention.

As can be seen from this figure, the application involves four steps.

In the first step, labeled "Uranium-Plutonium Co-Extraction" in FIG. 1 aims to co-extract uranium (VI) and plutonium (IV) from an aqueous nitrogen solution obtained by dissolving spent nuclear fuel, designated "A1" in FIG. one.

Such a solution contains, as a rule, 3 - 6 mol / l HNO ₃ , uranium, plutonium, minor actinides (in particular, americium and curium), fission products (La, Ce, Pr, Nd, Sm, Eu, Gd, Mo, Zr, Ru, Tc, Rh, Pd, Y, Cs, Ba…), as well as some corrosion products such as iron.

The "U + Pu co-extraction" step is carried out by circulating in the extractor 1 an aqueous solution A1 in countercurrent organic phase, designated S1 in FIG. 1, which contains 0.5 - 2 mol/l, preferably 1.0 - 1.4 mol/l, carbamide of general formula (I) in the form of a solution with an organic solvent.

This organic solvent is preferably an acyclic hydrocarbon or a mixture of acyclic hydrocarbons, for example n-dodecane, hydrogenated tetrapropylene (TPH), kerosene, Isane ^™ IP-175T, preferably hydrogenated tetrapropylene.

The second stage, indicated in Fig. 1 as “PF (fission products) washing”, aims to re-extract from the organic phase formed after the “U + Pu co-extraction”, the fission product fraction extracted from the A1 aqueous solution together with uranium (VI) and plutonium (IV).

In this case, the "PF Wash" stage comprises one or more washing operations of the organic phase obtained after the "U + Pu Co-extraction", each washing operation being carried out with this organic phase circulating in the extractor 2 in countercurrent to the aqueous solution indicated by A3 in FIG. 1 containing nitric acid, the concentration of which can be up to 1-6 mol/l, but preferably is from 4 to 6 mol/l.

If the "PF Wash" step is carried out using a highly acidic aqueous solution, i.e. typically equal to or greater than 3 mol/l HNO ₃ , this step additionally comprises deoxidation of the organic phase, which is carried out by circulating this organic phase in countercurrent to the aqueous solution designated A4 in FIG. 1 and being slightly acidic, i.e. containing from 0.1 to 1 mol/l HNO ₃ , as an aqueous solution containing 0.5 mol/l HNO ₃ to prevent too much acid from entering the extractor intended for the third stage, labeled "Pu stripping" in FIG. . 1 and to avoid performance degradation of this third step.

At the "Pu Re-extraction" stage, plutonium is re-extracted to the +IV oxidation state of the organic phase formed at the "PF Wash" stage.

It is carried out by circulating this organic phase in the extractor 3 in countercurrent to the aqueous solution indicated as "A2" in FIG. 1 containing 0.1–0.5 mol/l HNO ₃ , preferably at an O/A flow rate greater than 1, preferably equal to or greater than 3, more preferably equal to or greater than 5, so that the extraction of plutonium (IV) is accompanied by its enrichment .

The plutonium(IV) stripping in the "Pu Stripping" step is followed by the stripping of the uranium (VI) fraction, which is also present in the organic phase formed during the "PF Wash".

Thus, in the fourth step, labeled "Wash U" in FIG. 1, it is intended to be extracted from the aqueous phase formed during the “Pu Stripping”:

- or the entire amount of uranium (VI) present in this aqueous phase in the case when it is necessary to obtain at the end of this stage an aqueous solution containing plutonium (IV), but without uranium (VI),

- or the amount of uranium (VI), which ensures the receipt at the end of this stage of an aqueous solution containing uranium (VI) and plutonium (IV) in a given ratio.

In both cases, the "U Wash" is carried out by circulating in the extractor the aqueous phase obtained from the "Pu Stripping" in countercurrent organic phase, designated "S2" in FIG. 1, the qualitative and quantitative composition of which is preferably the same as the same composition of the organic phase S1. The amount of extracted uranium (VI) is controlled by influencing, on the one hand, the O/A ratio and, on the other hand, the acidity of the aqueous phase, while uranium (VI) is actually extracted the better, the greater the ratio between the flow rates of the organic and aqueous phases and acidity of the aqueous phase. The addition of more or less concentrated nitric acid to the aqueous phase circulating in the extractor 4 can be provided depending on the acidity to be imparted to this aqueous phase.

At the end of these four stages you get:

- raffinate corresponding to the aqueous phase 1 leaving the extractor and containing fission products, as well as americium and curium,

- the aqueous phase leaving the extractor 4 and containing either deactivated plutonium (IV) or a mixture of deactivated plutonium (IV) and uranium (VI),

- organic phase leaving the extractor 3 and containing uranium (VI) without plutonium (IV).

This organic phase may be sent directly or after other treatments to an extractor not shown in FIG. 1, in which uranium (VI) is re-extracted from this organic phase, for example, by means of an aqueous solution with a content of not more than 0.05 mol/l nitric acid as an aqueous solution with a content of 0.01 mol/l HNO ₃ at room temperature ( i.e. at 20-25°C) or hot (i.e. typically at 40-50°C) and preferably using an O/A flow ratio greater than 1 so that the uranium(VI) is extracted with providing enrichment.

Mentioned sources

1. International application PCT WO 2017/017207.

2. International application PCT WO 2017/017193.

3.E.K. Dukes and T.H. Sidall, Journal of Inorganic and Nuclear Chemistry 1966, 28(10), pp. 2307–2312.

4.G.M. Chumakova et al. Radiochimiya 1980, 22(2), pp. 213–217.

5.B.G. Vats et al., Dalton Transactions 2016, 45(25), pp. 10319 – 10325.

Claims (35)

1. Application of urea of general formula (I):

(I) where R ¹ , R ² and R ³ , the same or different, mean a linear or branched alkyl group with 1-12 carbon atoms, R ⁴ means a hydrogen atom or a linear or branched alkyl group with 1-12 carbon atoms, and carbamide contains a total number of carbon atoms from 17 to 25, as an extractant for the complete or partial separation of uranium (VI) and plutonium (IV), without reduction of plutonium (IV) to plutonium (III), based on an aqueous solution of A1 obtained by dissolving spent nuclear fuel in nitric acid, while the application provides for: a) co-extraction of uranium (VI) and plutonium (IV) from an aqueous solution A1, wherein the co-extraction includes at least the contact in the extractor of an aqueous solution A1 with an organic solution S1 containing urea in an organic solvent, and the separation of aqueous and organic solutions; b) re-extraction of plutonium (IV) and uranium (VI) fraction from the organic solution obtained in step a) during the re-extraction, which included at least one bringing into contact in the extractor of the organic solution S1 with an aqueous solution A2 containing 0.1– 0.5 mol/l nitric acid, and separation of organic and aqueous solutions; c) total or partial extraction of the fraction of uranium (VI) contained in the aqueous solution obtained in step b), wherein the extraction comprises at least one bringing the aqueous solution into contact in the extractor with the organic solution S2 containing urea in the organic solution , and separation of aqueous and organic solutions; resulting in an aqueous solution containing plutonium (IV) but without uranium (VI) or a mixture of plutonium (IV) and uranium (VI), and an organic solution containing uranium (VI) without plutonium (IV). 2. The use according to claim 1, in which R ⁴ means a linear or branched alkyl group with 1-12 carbon atoms and in which: - R ¹ and R ² are the same, R ³ and R ⁴ are the same, but different from R ¹ and R ² , - R ¹ and R ⁴ are the same, R ² and R ³ are the same, but different from R ¹ and R ⁴ , - R ¹ , R ² , R ³ and R ⁴ are the same. 3. Use according to claim 2, in which R ¹ and R ² are the same and mean a linear or branched alkyl group with 1-5 carbon atoms, R ³ and R ⁴ are the same and mean a linear or branched alkyl group with 6-10 atoms carbon, urea contains a total of 19, 21, or 23 carbon atoms. 4. The use according to claim 2, in which R ¹ and R ⁴ are the same and mean a linear or branched alkyl group with 1-5 carbon atoms, R ² and R ³ are the same and mean a linear or branched alkyl group with 6-10 atoms carbon, urea contains a total of 19, 21, or 23 carbon atoms. 5. The use according to claim 2, in which R ¹ , R ² , R ³ and R ⁴ are the same and mean a straight or branched alkyl group with 4-8 carbon atoms. 6. Use according to claim 1, wherein R ⁴ is a hydrogen atom. 7. Use according to claim 6, wherein R ¹ , R ² and R ³ are the same. 8. The use according to claim 7, in which R ¹ , R ² and R ³ mean a linear or branched alkyl group with 6-8 carbon atoms. 9. Use according to claim 1, wherein the carbamide is N,N,N'-tri-n-octylurea, N,N,N'-tri(2-ethylhexyl)urea, N,N-di(2-ethylhexyl) -N'-n-octylurea, N,N,N',N'-tetra-n-butylurea, N,N,N',N'-tetra-n-amylurea, N,N,N',N'- tetra-n-hexylurea, N,N,N',N'-tetra-n-octylurea, N,N'-di-n-butyl-N,N'-di-n-hexylurea, N,N'-di -n-heptyl-N,N'-di-n-propylurea, N,N'-diethyl-N,N'-di-n-octylurea or N,N'-dimethyl-N,N'-di-n- nonylurea. 10. Application according to any one of paragraphs. 1–9, in which organic solutions S1 and S2 contain carbamide in an amount of 0.5 to 2 mol/l. 11. Application according to any one of paragraphs. 1-9, in which step a) additionally contains the deactivation of the organic solution obtained by co-extraction of uranium (VI) and plutonium (IV), with respect to americium, curium and fission products, while the deactivation comprises at least bringing into contact in an organic solution extractor with an A3 aqueous solution containing 1–6 mol/L nitric acid and separation of the organic and aqueous solutions. 12. Application according to any one of paragraphs. 1-9, in which bringing into contact in the extractor, in which step b) is carried out, the organic solution obtained in step a) with an aqueous solution A2 involves countercurrent circulation of organic and aqueous solutions with an organic solution/aqueous solution volume flow ratio of more than one. 13. Application according to any one of paragraphs. 1-9, which additionally provides for the stripping of uranium (VI) from the organic solution obtained in step c), while the stripping comprises at least bringing the organic solution into contact with an aqueous solution of A5 containing not more than 0.05 mol / l nitric acid, and separation of organic and aqueous solutions. 14. Carbamide of general formula (I):

(I) where R ¹ and R ⁴ are the same and mean a linear alkyl group with 1-4 carbon atoms, R ² and R ³ are the same and represent a linear alkyl group with 6-9 carbon atoms and urea contains a total of 21 carbon atoms. 15. The carbamide according to claim 14, which is N,N'-diethyl-N,N'-di-n-octylurea or N,N'-dimethyl-N,N'-di-n-nonylurea. 16. Urea of a separate formula (II):

(II) which is N,N,N'-tri-n-octylurea, N,N,N'-tri(2-ethylhexyl)urea or N,N-di(2-ethylhexyl)-N'-n-octylurea.

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