JPH07318691A - Method for separating radioactive isotope from high level wastes - Google Patents

Abstract

PURPOSE:To obtain a method for separating a trivalent ion of a radioactive isotope, e.g. a transuranic element, contained in an acid solution of high level wastes with high partition ratio. CONSTITUTION:A mixture of a bidentate ligand organic phosphonate, hydrophobic strong acid anions, and an organic solvent is brought into contact, as an extraction agent or an adsorbent, with an acid solution of high level wastes in order to separate trivalent ions of radioactive isotope contained therein. Preferably, the biclentate ligand organic phosphonate includes dibutyl diethylcarbamoyl methylphosphonate and the hydrophobic strong acid anion includes dicarbollide, molybdenum phosphate, and tungsten silicate.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for separating radioisotopes from high-level waste generated from various parts of a nuclear fuel cycle, in particular radioactive aqueous solutions. Such radioactive aqueous solutions include various waste liquids generated from nuclear fuel reprocessing processes and plutonium handling facilities, and similar waste liquids generated by research activities at research institutions.

[0002]

2. Description of the Related Art An effluent containing a transuranic element generated from a nuclear fuel cycle is solidified in a stable state after separating and recovering the transuranic element as much as possible, and is separately stored as alpha waste. Such waste will be deeply disposed in the future. In order to reduce the amount of alpha waste that is subject to such high-cost disposal as much as possible, if the content of transuranium elements in the waste is less than the specified amount, classify it as non-alpha waste and dispose it in a shallow layer. Is being considered. Therefore, it is necessary to develop a method for separating transuranium elements selectively and with high recovery rate, and it is currently being sought globally. In particular, for the high-level liquid waste generated from the reprocessing step, the method of separating and recovering the transuranium elements contained therein has become a central issue in the group separation process.

A well-known method for separating transuranic elements is a method using di-2-ethylhexyl phosphoric acid (DEHPA) or a strongly acidic cation exchanger. They are suitable for separating transuranium elements from alkaline, neutral or weakly acidic aqueous solutions, but it is difficult to separate Am ³⁺ and Cm ³⁺ from aqueous solutions with an acid concentration of 0.1 mol / l or more. Is. The waste liquid after reprocessing the spent nuclear fuel by the Purex method or the like to recover U, Pu, etc. is often a nitric acid solution having a high concentration. Therefore, it is practical to apply the above method to such waste liquid. is not.

On the other hand, in recent years, C = having coordination ability
A bidentate organic compound having two functional groups such as O and P = O has been reported as an extractant capable of separating transuranic elements even from an aqueous solution containing a high concentration of acid. For example,
It has been reported that the trivalent cation of the lanthanide element, Am ³⁺ and Cm ³⁺ are extracted from a high level radioactive liquid waste having an acid concentration of about 1 to 5 mol / l. According to these reports, such a bidentate organic compound is dibutyldiethylcarbamoylmethylphosphonate (DBDECMP).
And dihexyl diethylcarbamoylmethylphosphonate (DHDECMP) and octyl (phenyl) -N, N-diisobutylcarbamoylmethylphosphine oxide (C
MPO) (EP Horwitz and WW Schulz, in Ne
w Separation Chemistry Techniques for Radioact
ive Waste and Other Special Applications (L.
Cecille et al, Eds) Elsevier, London, 1991, p.
21) or N, N-tetraalkylalkylpropanediamide (C. Musikas, in Solvent Extraction
(T. Sekine and S. Kusakabe, Eds) Elsevier, L
ondon, 1992, p297). A common property of these compounds is the binding of trivalent cations without changing the valency of the cations, resulting in a positively charged coordination compound. In an actual system, it is extracted in a form associated with ions of an acid such as nitric acid (negatively charged).

However, even if such a bidentate organic compound is used as an extractant or a chromatographic adsorbent for a waste liquid, the distribution ratio (or distribution coefficient) for trivalent metal ions is not sufficiently large, and the tridentate metal compound is not sufficiently large. Valent metals cannot be removed effectively. In particular, the high decontamination factor required for converting alpha waste to non-alpha waste cannot be obtained. In addition, the amount of bidentate organic compounds dissolved and lost in the water phase in the waste liquid is large and must be constantly replenished, and the compound contained in the water phase must be removed. There is a problem.

Although the above-mentioned bidentate organic compound acts in association with a hydrophilic anion, on the other hand, it has been shown from the conventional studies that a strongly hydrophobic anion increases the extraction degree of trivalent metal. Is expected. For example, Eu ³⁺ is extracted by the dicarbolide anion (Czechoslovak patent N
o.216101, inventors J Rais and H. Petrzilova, now extinct). The advantage of this method is that the amount of anion dissolved and lost in the aqueous phase in the waste liquid is small.

However, the feature of this method is that the extraction distribution ratio of trivalent metal is amplified in the region where the concentration of nitric acid is low. For example, the waste liquid is denitrified in advance to a range of 0.3 to 0.5M HNO _3. There must be. Therefore, it is difficult to extract a trivalent metal from a highly acidic waste liquid by this method.

[0008]

OBJECT OF THE INVENTION It is an object of the present invention to provide a method for separating trivalent ions of radioactive isotopes such as transuranium contained in an acidic high level waste solution with a high partition ratio.

[0009]

According to the present invention, a mixture of a bidentate organic phosphonate, a hydrophobic strong acid anion and an organic solvent is contacted with an acidic high level waste solution as an extractant or adsorbent, A method for separating trivalent ions of a radioisotope contained in a high level waste solution is provided.

That is, as a result of earnest studies by the inventor, the above-mentioned method shows that the bidentate organic phosphonate and the hydrophobic strong acid anion exert a synergistic effect, and the distribution ratio to the trivalent metal ion is remarkably increased. I found it.

The bidentate organic phosphonate in the present invention is represented by the following general formula.

[0012] A preferred bidentate organic phosphonate is dibutyldiethylcarbamoylmethylphosphonate, which is represented by the following general formula.

[0013]

The preferred strong hydrophobic acid anion according to the present invention is dicarbolide (H ⁺ [(C ₂ B ₉ H ₈ Cl ₃ ) 2C).
o] ^- ), phosphomolybdic acid (H ₃ PO ₄ · 12MoO ₃ · 12H
₂ O), and silicotungstic acid (H ₄ [SiO ₄ W ₁₂ O
₃₆ ]).

When a mixture of a bidentate organic phosphonate, a hydrophobic strong acid anion and an organic solvent is used as an extractant, for example, this extractant solution and an acidic high level waste solution are mixed and stirred. Next, if both solutions are separated into two phases by a centrifugal separation method, the trivalent metal ion in the waste solution is transferred to the extractant solution and is extracted and separated.

When a mixture of a bidentate organic phosphonate, a hydrophobic strong acid anion and an organic solvent is used as an adsorbent, for example, when an organic solution of the mixture is impregnated with a styrenedivinylbenzene polymer (resin), The resin becomes a strong adsorbent for trivalent metal ions. Alternatively, since the bidentate organic phosphonate, phosphomolybdic acid and silicotungstic acid form a precipitate, this precipitate itself can be used as an adsorbent.

The radioactive isotope ions important as extraction targets in the high-level waste solution are mainly actinides such as Am ³⁺ and Cm ³⁺ , especially trivalent ions of transuranium elements. However, these transuranium elements are highly radioactive and are dangerous to handle. It is known that trivalent ions of lanthanide such as Eu ³⁺ show the same extraction behavior or adsorption behavior as the trivalent ion of actinide. Therefore, if an experiment for confirming the distribution ratio of the trivalent ion of lanthanide is performed, The results can be applied to the trivalent ion of actinide.

[0018]

EXAMPLES Example 1 3.2 mol / l (hereinafter, referred to as “the following, containing a very small amount of Am ³⁺ and Eu ^3+”)
An experiment was conducted to extract Am ³⁺ and Eu ³⁺ from a nitric acid solution (denoted as M).

Nitrobenzene 60 vol% + carbon tetrachloride 4
Dicarbollide (0.045M) and dibutyldiethylcarbamoylmethylphosphonate (hereinafter referred to as DB in 0vol% solution)
Notated as DECMP. 0.065M) was added and mixed to serve as an extraction solvent. 5 ml of this extraction solvent and 5 ml of the above nitric acid solution were collected in a glass container with a stopper and kept at room temperature (23 ± 1
After stirring for about 10 minutes at (° C.), the mixture was separated into two phases by a centrifuge. 1 ml was taken from each phase, placed in a polyethylene rod bottle, and the γ ray of each sample was measured using a γ ray measuring device. The concentration of Am ³⁺ is ²⁴¹ Am of 60 keV γ-ray,
Further, the concentration of Eu ³⁺ was obtained by 105 keV · γ ray of ¹⁵⁵ Eu. As a result, the distribution ratio at equilibrium is D _Am3 = 3.
9, D _Eu3 = 33.

As a comparison, when an experiment was conducted under the same conditions using an extraction solvent of nitrobenzene + carbon tetrachloride + DBDECMP system, it was D _Eu3 = 0.15. Further, when an experiment was conducted under the same conditions using an extraction solvent of nitrobenzene + carbon tetrachloride + dicarbollide system, D _Eu3 = 0.
It was 0006.

Similarly, as a comparison, when a similar experiment was conducted for Cs ⁺ ions under the same conditions, D _Cs = 0.05 was obtained when an extraction solvent of nitrobenzene + carbon tetrachloride + dicarbollide + DBDECMP was used, and nitrobenzene + When a carbon tetrachloride + dicarbollide type extraction solvent was used, D _Cs = 6.5, and the latter, which is a conventional method, had a higher extraction effect.

Example 2 An experiment was conducted to extract Eu ³⁺ from a 3.2M nitric acid solution containing a trace amount of Eu ³⁺ .

Dicarbolide (0.045M) and DBDECMP (0.065M) were added to the 1,2-dichloroethane solution and mixed to obtain an extraction solvent. Eu ³⁺ was extracted from this nitric acid solution using this extraction solvent by the same method and conditions as in Example 1. As a result, the distribution ratio at equilibrium was D _Eu3 = 25.

Further, Eu ³⁺ was extracted from the nitric acid solution in the same manner as above using an extraction solvent prepared by adding dicarbolide (0.045M) and DBDECMP (0.065M) to the chloroform solution and mixing them. As a result, the distribution ratio at equilibrium was D _Eu3 = 9.3.

Example 3 An experiment was conducted to extract Eu ³⁺ from a 3.2M nitric acid solution containing a trace amount of Eu ³⁺ .

To the o-nitrotoluene solution, silicotungstic acid (40 g / l) and DBDECMP (0.1 M) were added,
Mixed and used as an extraction solvent. Eu ³⁺ was extracted from this nitric acid solution using this extraction solvent by the same method and conditions as in Example 1. As a result, the distribution ratio at equilibrium was D _Eu3 = 90.

Next, in order to back-extracted Eu ³⁺ from the organic phase was extracted Eu ^3+, it was a 9M nitric acid solution of the organic phase and equal volume to the stirred equilibrium. As a result, the distribution ratio at equilibrium is D
_Eu3 = 2.8, which is not suitable for back extraction. Therefore, the same back extraction operation was performed using 0.18M DT instead of 9M nitric acid.
When PA Na salt-0.85 M lactic acid aqueous solution and 1 M sodium carbonate aqueous solution were used, the former D _Eu3 = 0.07,
In the latter case, D _Eu3 = 0.005, indicating that back extraction is possible.

Example 4 Equimolar phosphomolybdic acid and DBDECMP were added to a 1M nitric acid solution and mixed by stirring to form a precipitate,
The precipitate was washed and dried. As an adsorbent,
The adsorption partition coefficient of Eu ³⁺ for this adsorbent was measured by the batch method. The results in the 0.1M, 4M and 8M nitric acid systems were 1960, 270 and 220, respectively.

[0029]

As described above, according to the method of the present invention, the bidentate organic phosphonate having C = O and P = O functional groups in the molecule and the hydrophobic strong acid anion exert a synergistic effect. Thus, the distribution ratio for trivalent metal ions in the acidic high level waste solution is significantly increased. Therefore, by using a mixture of a bidentate organic phosphonate, a hydrophobic strong acid anion and an organic solvent, a solvent extraction method, an extraction chromatographic method, an adsorption method, or the like is used to selectively select a transuranium element from a waste liquid with a high partition ratio. Can be separated with. This greatly contributes to cost reduction of waste treatment / disposal and reuse of recovered elements (disappearance treatment by group separation method).

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Claims (4)

[Claims] 1. A solution of a bidentate organic phosphonate, a hydrophobic strong acid anion, and an organic solvent is contacted with an acidic high-level waste solution as an extractant or an adsorbent to obtain a high-level waste solution in the high-level waste solution. Method for separating trivalent ions of radioisotopes contained in. 2. The method of claim 1, wherein the bidentate organic phosphonate is dibutyldiethylcarbamoylmethylphosphonate. 3. The method according to claim 1, wherein the hydrophobic strong acid anion is any one of dicarbolide, phosphomolybdic acid, and silicotungstic acid. 4. The method according to claim 1, wherein the radioisotope is a lanthanide and / or an actinide.