JP2007240370A - Method for separating plutonium into water phase by utilizing difference of nitric acid concentration from uranium and plutonium recovered in organic phase including branched n,n-dialkyl amide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method using N,N-dialkyl amide having a functional group (C=O) having unidentate coordination as an extraction agent, concerning a method for separating and recovering uranium and plutonium from nitric acid aqueous solution including various radioactive nuclides. <P>SOLUTION: In this method for separating and recovering uranium and plutonium from nitric acid aqueous solution, uranium VI and plutonium IV are extracted and separated from the nitric acid aqueous solution into an organic phase by using N,N-dialkyl amide having C=O which is a functional group having unidentate coordination activity as the extraction agent, and then uranium VI and plutonium IV are extracted and separated from the organic phase into a water phase separately by reverse extraction by using the nitric acid aqueous solution having a different concentration as an extraction agent. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for separating and recovering uranium and plutonium from an aqueous nitric acid solution containing various radionuclides. Specifically, the present invention extracts uranium (VI) and plutonium (IV) present in a nitric acid aqueous solution, and N, N-dialkylamide having a monodentate coordination active functional group (C═O) as an extractant. And then extracting and separating plutonium into an aqueous phase by back extraction using an aqueous nitric acid solution as an extractant.

  The present invention is particularly useful for selectively separating uranium and plutonium in spent nuclear fuel dissolved nitric acid solution obtained in the first stage of spent nuclear fuel reprocessing from other fission products.

  As a method for reprocessing spent nuclear fuel, a solvent extraction method using an organic solvent containing tributyl phosphate, which is an organophosphorus monodentate ligand, is widely used industrially. In this method, spent nuclear fuel material is dissolved in an aqueous nitric acid solution, and uranium (U) and plutonium (Pu) are extracted from this solution into an organic phase using an organic solvent containing tributyl phosphate. This is a method of separating plutonium from uranium in an organic phase into an aqueous phase using an aqueous nitric acid solution. In order to separate uranium and plutonium, it is necessary to adjust the valence to trivalent so that plutonium is not extracted into the organic phase, and the valence is reduced from Pu (IV) to Pu (III) using a reducing agent. Then, Pu (III) is separated and recovered from the U (VI) in the organic phase into the aqueous phase using a dilute aqueous nitric acid solution.

  The conventional spent nuclear fuel reprocessing method as described above has the following drawbacks due to tributyl phosphate used as an extractant in the first extraction operation.

  That is, the first drawback is that tributyl phosphate is decomposed by chemical factors or radiation. The decomposition products dibutyl phosphate, monobutyl phosphate, and phosphoric acid are almost all ions coexisting in the reprocessing solution, for example, U (IV), U (VI), Pu (IV), Pu (III ), Zr (IV), etc., form an insoluble salt, which causes a reduction in the recovery rate of uranium and plutonium.

  And the 2nd fault is that the organic solvent which uses tributyl phosphate as an extracting agent cannot be processed easily by incineration. This type of organic solvent does not consist only of volatile components, and since it contains phosphorus in the composition, incineration residues remain, resulting in the generation of solid waste.

Furthermore, in the conventional method, the following drawbacks also exist due to the reducing agent used for adjusting the valence of plutonium from Pu (IV) to Pu (III). That is, commonly used reducing agents are, for example, valence (II) iron, valence (IV) uranas or hydroxylamine (NH ₂ OH), which are oxidized by nitrous acid formed from nitric acid. It is easy and unstable in the presence of nitric acid. In order to avoid this oxidation reaction, it is necessary to add a nitrous acid decomposing agent such as hydrazine (NH ₂ —NH ₂ ) separately to the nitric acid solution.

  However, even if hydrazine is added, there is a further problem if the spent nuclear fuel contains technetium. Tributyl phosphate, an extractant, extracts a large amount of technetium present in the spent nuclear fuel solution, but the presence of this technetium promotes oxidative decomposition of hydrazine by nitrous acid. As a result, since hydrazine that stabilizes the reducing agent of Pu (IV) is consumed, oxidation of the reducing agent proceeds to reduce the reduction efficiency of plutonium, and back extraction of Pu (III) into the aqueous phase is prevented. It will be.

  Due to the use of hydrazine as a nitrous acid decomposing agent, the action of its decomposition product, hydrazoic acid, also becomes a problem. That is, hydrazoic acid that is extremely volatile and extracted into an organic solvent forms unstable or explosive salts. In addition, an ammoniacal compound, which is one of the decomposition products of hydrazine, exhibits similar properties, although not as much as hydrazoic acid. Thus, the increased risk of explosion within the spent nuclear fuel reprocessing facility is clearly undesirable for process management and facility management.

Therefore, in order to eliminate these drawbacks, various studies have been conducted over the past several years regarding the possibility of using extractants other than tributyl phosphate. As a typical example, use of an amide-based compound has been proposed. According to the research results of Oak-Ridge Laboratories, Savannah River Laboratories and Comitato Nazionale per l 'Energia Nucleare Laboratories on N, N-dialkylamides having a functional group with monodentate coordination activity (C = O), Species extractants have been shown to have sufficient affinity for the hexavalent and tetravalent ions of actinoids, while having low affinity for major fission products. It is also known that it has sufficient resistance to radiolysis and chemical degradation and has low solubility in aqueous solutions. Furthermore, this kind of substance is easy to synthesize and produce (see, for example, Patent Document 1, Non-Patent Documents 1 and 2).
Japanese Patent Publication No. 6-104573 Effect of Structure of N, N-Distributed Amides on, TH Siddall, III, “Effects of Structure of N, N-Distributed Amides on Their Extraction of Actinide and Zirconium Nitrates and of Nitric Acid ”,“ Journal of Physical Chemistry ”, American Chemical Society, (USA), December 1960, vol. 64, p.1863-1866 GM Gasparini, 1 other, “Application of N, N-Dialkyl Aliphatic Amides in the Separation of Some Actinides”, “Separation. "Separation Science and Technology", Marcel Dekker Inc., (United States), 1980, 15 (4), p.825-844

  In the method for separating and recovering uranium and plutonium from an aqueous nitric acid solution containing various radionuclides, the present invention has a functional group having a monodentate coordination activity (C═O) instead of tributyl phosphate used in the prior art. It is an object of the present invention to provide a method of using N, N-dialkylamide as an extractant.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive research, and as a result, N, N-dialkylamide having a specific structure is used as an extractant to separate and recover uranium (VI) and plutonium (IV). Further, it was discovered that uranium (VI) and plutonium (IV) can be separated and recovered by back-extraction by using aqueous nitric acid solutions having different concentrations as extraction agents, and the present invention has been completed. .

That is, the present invention is a method for separating and recovering uranium and plutonium from an aqueous nitric acid solution containing various radionuclides,
Extracting and separating uranium (VI) and plutonium (IV) into the organic phase from the aqueous nitric acid solution using N, N-dialkylamide having C═O, which is a functional group having a monodentate coordination activity, as an extractant, Back-extracting the uranium (VI) and plutonium (IV) separately from the organic phase into the aqueous phase by back extraction by using different aqueous nitric acid solutions as extractants;
The N, N-dialkylamide has the following formula:
R ₁ C (O) NR ₂ R ₃
(In the formula, R ₁ is a C _{4 to} C ₈ branched alkyl group, which is a branched alkyl group in which a methyl group or an ethyl group is bonded to an alpha carbon or a beta carbon, and R ₂ and R ₃ are each represented by Independently, it is a C _{4 to} C ₈ linear alkyl group, or R ₁ is a C _{4 to} C ₈ linear alkyl group, and R ₂ and R ₃ are each independently C ₄ A branched N, N-dialkylamide having a structure of ˜C ₈ branched alkyl group in which a methyl group or an ethyl group is bonded to the alpha carbon or the beta carbon). .

  In the process of the invention, the N, N-dialkylamide is preferably of the formula:

N, N-di- (2-ethyl) hexylbutanamide having the formula:

N, N-di- (2-ethyl) hexylhexanamide having the formula:

Selected from the group consisting of N, N-di-hexyl- (2-ethyl) hexanamide having

In the method of the present invention, the extraction separation of uranium (VI) and plutonium (IV) preferably uses an aqueous nitric acid solution having a concentration that gives the distribution ratio of uranium and plutonium as D _U > 1 and D _Pu <1. Then, plutonium (IV) is back-extracted and separated, and then uranium (VI) is back-extracted and separated using a nitric acid aqueous solution having a concentration that gives a distribution ratio of uranium as D _U <1.

  Furthermore, the extraction separation of uranium (VI) and plutonium (IV) is more preferably performed by back-extraction separation of plutonium (IV) from the organic phase to the aqueous phase using a relatively high concentration aqueous nitric acid solution, This is done by back-extracting uranium (VI) from the organic phase to the aqueous phase using a relatively low concentration aqueous nitric acid solution.

  Furthermore, in the method of the present invention, the N, N-dialkylamide is preferably diluted with normal decane.

  According to the method of the present invention, uranium and plutonium in the spent nuclear fuel dissolved nitric acid solution obtained in the first stage of spent nuclear fuel reprocessing can be selectively separated from other fission products.

  Hereinafter, preferred embodiments of the method for separating and recovering uranium and plutonium of the present invention will be described.

  The method of the present invention is a method for separating and recovering uranium and plutonium from aqueous nitric acid solutions containing various radionuclides, and extracting N, N-dialkylamides having C═O, which is a functional group having monodentate coordination activity. Extracting and separating uranium and plutonium from the aqueous nitric acid solution into the organic phase as an agent, and then extracting and separating plutonium from the organic phase into the aqueous phase by back extraction using the aqueous nitric acid solution as an extracting agent.

  The method of the present invention can be applied to an aqueous nitric acid solution having a nitric acid concentration of about 3.0M to about 5.0M. The dissolution adjustment liquid for spent nuclear fuel is usually adjusted to a nitric acid concentration of about 3.0M. The uranium concentration is about 0.5M to about 1.0M at maximum, and plutonium is about 1/100 for spent nuclear fuel in light water reactors and about 1/10 for fast reactors with respect to uranium, and the concentration is about 100 mM at maximum. . The method of the present invention can be suitably used for such a concentration range.

  In the present invention, as an extractant for separating uranium and plutonium from other radionuclides, N, N having a functional group having a monodentate coordination activity (C═O) is used instead of tributyl phosphate conventionally used. -Use dialkylamides. This eliminates all the problems of the prior art associated with the use of tributyl phosphate.

The N, N-dialkylamide used in the present invention has the following formula:
R ₁ C (O) NR ₂ R ₃
(In the formula, R ₁ is a C _{4 to} C ₈ branched alkyl group, which is a branched alkyl group in which a methyl group or an ethyl group is bonded to an alpha carbon or a beta carbon, and R ₂ and R ₃ are each represented by Independently, it is a C _{4 to} C ₈ linear alkyl group, or R ₁ is a C _{4 to} C ₈ linear alkyl group, and R ₂ and R ₃ are each independently C ₄ A branched N, N-dialkylamide having a structure of a branched alkyl group of ˜C _{8, which is} a branched alkyl group in which a methyl group or an ethyl group is bonded to the alpha carbon or the beta carbon. dialkyl monoamide (BAMA).

  The N, N-dialkylamide used in the present invention preferably has the following formula:

N, N-di- (2-ethyl) hexylbutanamide (D2EHBA) having the following formula:

N, N-di- (2-ethyl) hexylhexanamide (D2EHHA) having the following formula:

Selected from the group consisting of N, N-di-hexyl- (2-ethyl) hexanamide (DH2EHA).

In addition, in the present invention, in addition to the specific N, N-dialkylamide described above, either the nitrogen alkyl group or the carbonylalkyl group is a branched group, and the distribution ratio of uranium and plutonium at a specific nitric acid concentration ( Use various N, N-dialkylamides provided that D = N, N-dialkylamide / substance quantity in nitric acid aqueous solution) as D _U > 1 and D _Pu <1 respectively. Can do.

  In the present invention, the use of the N, N-dialkylamide extractant provides the following advantages over the disadvantages caused by the conventionally used tributyl phosphate.

  That is, firstly, secondary amines and carboxylic acids, which are decomposition products of N, N-dialkylamide, are basically water-soluble, and various metal ions are organically decomposed like decomposition products of tributyl phosphate. It does not extract into the phase and does not produce insoluble salts.

  Second, N, N-dialkylamide contains only carbon, hydrogen, oxygen and nitrogen in its composition and can be gasified by incineration, resulting in the amount of solid waste during waste solvent treatment. Can be greatly reduced.

  The N, N-dialkylamide extractant may be diluted with a suitable solvent for the purpose of adjusting physical properties such as viscosity and specific gravity and improving the phase separation from the aqueous phase. The dilution concentration can be determined as appropriate. The dilution solvent is preferably normal dodecane, which is a linear aliphatic hydrocarbon, but is not limited thereto.

  According to the present invention, uranium (VI) and plutonium (IV) can be efficiently extracted and separated into an organic phase from an aqueous nitric acid solution containing various radionuclides by using an N, N-dialkylamide extractant. it can. Uranium (VI) and plutonium (IV) extracted in the organic phase can be separated and recovered separately in the aqueous phase by back extraction using aqueous nitric acid solutions having different concentrations.

The extraction behavior of uranium (VI) and plutonium (IV) with aqueous nitric acid depends on the concentration of uranium and / or plutonium to be recovered, the type, concentration, operating temperature, etc. of the N, N-dialkylamide extractant used. Change. Therefore, the conditions for back extraction with aqueous nitric acid solution take into account the dependence of the distribution ratio of uranium and plutonium (D = the amount of substances in the organic phase containing N, N-dialkylamide / the amount of substances in the aqueous nitric acid solution) on the concentration of nitric acid. Specifically, plutonium (IV) is back-extracted and separated using an aqueous nitric acid solution having a concentration that gives a distribution ratio of uranium and plutonium as D _U > 1 and D _Pu <1. Next, uranium (VI) is back-extracted and separated using an aqueous nitric acid solution having a concentration that gives the uranium distribution ratio as D _U <1. The concentration of the nitric acid aqueous solution is a relatively high concentration for the back extraction of plutonium (IV), and a relatively low concentration for the back extraction of uranium (VI). Typically, about 1.0 M aqueous nitric acid is used for back extraction separation of plutonium (IV), and about 0.5 M aqueous nitric acid is used for back extraction separation.

  In the present invention, as described above, the following advantages are provided by separating uranium and plutonium extracted and separated into an organic phase using a difference in nitric acid concentration.

  That is, the plutonium reduction step (Pu (IV) to Pu (III)) required in the conventional method is not required, the process can be simplified, and addition of a reducing agent, a nitrous acid decomposing agent, or the like is not required. Secondary reaction can be suppressed.

  Since the N, N-dialkylamide extractant used in the present invention shows an extraction amount comparable to tributyl phosphate for high concentrations of uranium, the method of the present invention is directly applied to the current light water reactor fuel reprocessing. Is possible. Moreover, in the prior art which handled extraction of high concentration uranium with N, N-dialkylamide, a polar aromatic solvent (toluene, 1,3,5-trimethylbenzene) was used as a diluent solvent for the extractant. Many examples have been used (for example, see Non-Patent Document 2). This is for the purpose of suppressing the formation of the third phase, which is the second organic phase concentrated when the amount of the metal complex extracted into the organic phase exceeds the solubility in the organic solvent. is there. However, there is a possibility that an explosive compound may be produced as a by-product due to the reaction between the aromatic solvent as a diluting solvent and the nitric acid as an aqueous phase. In the method of the present invention, such a potential danger can be eliminated by using nonpolar normal decane, which is also used in the reprocessing step, as a diluent solvent.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited by these Examples.

Example 1
In this example, as the extraction solvent, the following formula:

A normal dodecane solution containing N, N-di- (2-ethyl) hexylbutanamide having the following formula was used.

Uranium and plutonium ions were extracted from a nitric acid solution containing a trace amount of uranium and plutonium by using a normal dodecane solution containing 1.0M D2EHBA as an extraction solvent. Nitrate concentration dependence and separation characteristics of the extraction distribution ratio uranium at equilibrium (VI) and plutonium (IV) _{(D U} and _{D Pu)} shown in FIG. In FIG. 1, when the nitric acid concentration was 5.0 M, the extraction distribution ratios were D _U = 17.3 and D _Pu = 2.93, respectively. When the nitric acid concentration was 1.0 M, D _U = 1.56 and D _Pu = 0.41, and it was possible to efficiently recover plutonium by back extraction. Furthermore, when the nitric acid concentration was 0.5 M, D _U = 0.40 and D _Pu ≦ 0.01, and uranium could be efficiently recovered by back extraction.
(Example 2)
In this example, as the extraction solvent, the following formula:

A normal dodecane solution containing N, N-di- (2-ethyl) hexylhexanamide having the following formula was used.

Uranium and plutonium ions were extracted from a nitric acid solution containing a very small amount of uranium and plutonium by using a normal dodecane solution containing 1.0M D2EHHA as an extraction solvent. Nitrate concentration dependence and separation characteristics of the extraction distribution ratio uranium at equilibrium (VI) and plutonium (IV) _{(D U} and _{D Pu)} shown in FIG. In FIG. 2, when the nitric acid concentration is 5.0 M, the extraction distribution ratios were D _U = 16.1 and D _Pu = 8.25, respectively. When the nitric acid concentration was 1.0 M, D _U = 1, 41 and D _Pu = 0.12. Plutonium could be efficiently recovered by back extraction. Furthermore, when the nitric acid concentration was 0.5M, D _U = 0.43 and D _Pu = 0.03, and uranium could be efficiently recovered by back extraction.
(Example 3)
In this example, as the extraction solvent, the following formula:

A normal dodecane solution containing N, N-di-hexyl- (2-ethyl) hexanamide having the following formula was used.

Uranium and plutonium ions were extracted from a nitric acid solution containing a very small amount of uranium and plutonium by using a normal dodecane solution containing 1.0 M DH2EHA as an extraction solvent. Nitrate concentration dependence and separation characteristics of the extraction distribution ratio uranium at equilibrium (VI) and plutonium (IV) _{(D U} and _{D Pu)} shown in FIG. In FIG. 3, when the nitric acid concentration was 5.0M, the extraction distribution ratios were D _U = 3.50 and D _Pu = 1.01, respectively. When the nitric acid concentration was 3.0M, D _U = 1.80 and D _Pu = 0.27, and plutonium could be efficiently recovered by back extraction. Furthermore, when the nitric acid concentration was 0.5 M, D _U = 0.11 and D _Pu ≦ 0.01, and uranium could be efficiently recovered by back extraction.

  The method of the present invention can be applied in current nuclear fuel cycles, particularly in reprocessing plants.

Figure 1 is normal dodecane solution at equilibrium of uranium when used as an extracting solvent (VI) and extraction distribution ratio plutonium (IV) containing D2EHBA: shows a D _U and nitrate concentration dependence and separation characteristics of the D _Pu FIG. 2, n-dodecane solution at equilibrium of uranium when it is used as extracting solvent (VI) and extraction distribution ratio plutonium (IV) containing D2EHHA: shows a D _U and nitrate concentration dependence and separation characteristics of the D _Pu FIG. 3, n-dodecane solution at equilibrium of uranium when it is used as extracting solvent (VI) and extraction distribution ratio plutonium (IV) containing DH2EHA: shows a D _U and nitrate concentration dependence and separation characteristics of the D _Pu FIG.

Claims (5)

A method for separating and recovering uranium and plutonium from an aqueous nitric acid solution containing various radionuclides,
Extracting and separating uranium (VI) and plutonium (IV) into the organic phase from the aqueous nitric acid solution using N, N-dialkylamide having C═O, which is a functional group having a monodentate coordination activity, as an extractant, Back-extracting the uranium (VI) and plutonium (IV) separately from the organic phase into the aqueous phase by back extraction by using different aqueous nitric acid solutions as extractants;
The N, N-dialkylamide has the following formula:
R ₁ C (O) NR ₂ R ₃
(In the formula, R ₁ is a C _{4 to} C ₈ branched alkyl group, which is a branched alkyl group in which a methyl group or an ethyl group is bonded to an alpha carbon or a beta carbon, and R ₂ and R ₃ are each represented by Independently, it is a C _{4 to} C ₈ linear alkyl group, or R ₁ is a C _{4 to} C ₈ linear alkyl group, and R ₂ and R ₃ are each independently C ₄ A branched N, N-dialkylamide having a structure of C ₈ -branched alkyl group, which is a branched alkyl group having a methyl group or an ethyl group bonded to an alpha carbon or a beta carbon thereof. N, N-dialkylamide has the following formula:

N, N-di- (2-ethyl) hexylbutanamide having the formula:

N, N-di- (2-ethyl) hexylhexanamide having the formula:

The process of claim 1, wherein the process is selected from the group consisting of N, N-di-hexyl- (2-ethyl) hexanamide having: Plutonium (IV) is back-extracted using an aqueous nitric acid solution having a concentration ratio of uranium and plutonium as D _U > 1 and D _Pu <1, and then a distribution ratio of uranium is given as D _U <1. The method according to claim 1 or 2, wherein uranium (VI) is back-extracted and separated using a concentrated aqueous nitric acid solution.   Using a relatively high concentration aqueous nitric acid solution, plutonium (IV) is back-extracted from the organic phase to the aqueous phase, and then using a relatively low concentration aqueous nitric acid solution, uranium (VI) is organically separated. The method according to any one of claims 1 to 3, wherein the back-extraction separation is performed from the phase to the aqueous phase. The method according to any one of claims 1 to 4, wherein the N, N-dialkylamide is diluted with normal dodecane.

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