JP2015152554A - Radioactive waste liquid treatment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radioactive waste liquid treatment method capable of facilitating the glass solidification treatment of waste liquid produced in a spent nuclear fuel re-treatment process.SOLUTION: After a tributyl phosphate is added to a solution in which spent nuclear fuel is dissolved to separate and extract radioactive substances, a dibutyl phosphate and a monobutyl phosphate produced as radioactive deterioration products of the tributyl phosphate are transferred into an alkaline solution, a substance that produces precipitates by reacting with at least one of the dibutyl phosphate and the monobutyl phosphate is dissolved in the alkaline solution obtained after transfer, and the alkaline solution obtained after precipitation is subjected to a glass solidification treatment.

Description

  The present invention relates to a radioactive liquid waste treatment method for treating liquid waste generated in a nuclear fuel reprocessing process.

  As a nuclear fuel reprocessing process, the PUREX process using tributyl phosphate (TBP) as an extraction solvent is being implemented in various countries. In the Purex process, a nitric acid solution containing uranium or plutonium is mixed with an organic solvent containing tributyl phosphate or dodecane to form a complex of uranium or plutonium with tributyl phosphate. Extract radioactive elements to the organic solvent side. After that, the organic solvent from which radioactive elements have been removed by various chemical treatments is reused, but a part of tributyl phosphate is decomposed by radiation and becomes a degraded component such as dibutyl phosphate (DBP) or monobutyl phosphate (MBP). The organic solvent containing such radiation degradation products is washed with an alkaline solution and regenerated into a reusable organic solvent. It is desirable that the radiation deteriorated material transferred into the alkaline solution in this cleaning is removed from the alkaline solution before the disposal of the alkaline solution. This is because the radiation-degraded product is a phosphoric acid ester compound that is easily foamed, so that the alkaline solution is foamed during disposal in the glass melting furnace, which makes it difficult to control the temperature.

JP 2007-050386 A

In general, as a method for removing phosphate esters such as dibutyl phosphate and monobutyl phosphate from an aqueous solution, decomposition by UV irradiation, electrolysis, decomposition by other accelerated oxidation methods, and the like are known. However, since the alkaline solution contains a radioactive substance, there is a problem that treatment under a high dose is forced and maintenance becomes difficult. Furthermore, since the decomposition treatment facility becomes large, there is a problem that the initial cost of introducing the facility becomes large.
In addition to such decomposition treatment, there is also known a method for precipitating and removing phosphorus compounds by adjusting the pH of the alkaline solution to the acidic side (see Patent Document 1), but the removal efficiency is not necessarily satisfactory. Instead, there is a need for a simpler method with better removal efficiency.

  The present invention has been made to solve the above-described problems, and provides a radioactive liquid waste treatment method capable of facilitating vitrification treatment of liquid waste generated in a spent nuclear fuel reprocessing process.

In order to solve the above problems, the present invention provides the following means.
In the method for treating radioactive liquid waste according to one aspect of the present invention, an extraction step of separating and extracting radioactive substances by adding tributyl phosphoric acid to a solution in which spent nuclear fuel is dissolved, and after the extraction step, A transition step for transferring dibutyl phosphate and monobutyl phosphate generated as radioactive degradation products of tributyl phosphate to an alkaline solution, and a reaction with at least one of the dibutyl phosphate or monobutyl phosphate in the alkaline solution obtained after the transition step And a solidification step of dissolving a substance that forms a precipitate and a vitrification treatment to the alkali solution obtained after the precipitation step.

  In this radioactive liquid waste treatment method, by precipitating at least one of dibutyl phosphoric acid or monobutyl phosphoric acid in the alkali solution, it is possible to suppress the foaming of the alkali solution during the vitrification treatment. Therefore, the vitrification treatment of the alkali solution is performed. It can be done easily.

  In the radioactive liquid waste treatment method, it is preferable to perform a removal step of removing the generated precipitate from the alkaline solution after the precipitation step, and subject the alkaline solution obtained after the removal step to the solidification step. .

  By removing the precipitate from the alkali solution, foaming in the subsequent vitrification treatment can be more reliably suppressed and the volume of the alkali solution can be reduced, so that the vitrification treatment can be performed more efficiently. it can.

  Moreover, in the said radioactive waste liquid processing method, it is preferable to make the said sediment settle in the said removal process.

  By precipitating a precipitate containing a hardly soluble complex, at least one of dibutyl phosphoric acid or monobutyl phosphoric acid can be easily removed.

  Further, in the radioactive liquid waste treatment method, the precipitate is preferably a complex of at least one of the dibutyl phosphoric acid or monobutyl phosphoric acid and iron ions (III).

  Thereby, since a complex with extremely low solubility can be formed, the removal efficiency of the complex can be increased.

  In the radioactive liquid waste treatment method, in the precipitation step, the iron ion (II) is added to the alkaline solution, and then the iron ion (II) is oxidized to produce the iron ion (III). Is preferably formed.

  Thereby, since iron ion (III) can be made into the state melt | dissolved in the solution uniformly, complex formation efficiency can be improved.

  Further, in the radioactive liquid waste treatment method, it is preferable that the precipitate is a complex of at least one of dibutyl phosphoric acid or monobutyl phosphoric acid and zirconium ion (IV).

  Thereby, since a complex with extremely low solubility can be formed, the removal efficiency of the complex can be increased.

  Moreover, in the said radioactive waste liquid processing method, it is preferable to aggregate the said precipitate in the said precipitation process by further adding a flocculant to the said alkaline solution.

  The aggregated precipitate can be more easily removed by a known method such as sedimentation or filter filtration.

  According to the present invention, by precipitating at least one of dibutyl phosphoric acid or monobutyl phosphoric acid in the alkali solution, it is possible to suppress foaming of the alkali solution at the time of vitrification treatment, so that the vitrification treatment of the alkali solution is easy. Can be done.

It is the chart which showed the processing flow of the spent nuclear fuel. In an example of embodiment of the present invention, it is a mimetic diagram showing signs that a complex of Fe ³⁺ and DBP is formed on the surface of a salt of Fe ³⁺ in a solution. In one example embodiment of the present invention, after dissolving the salts of Fe ²⁺ in solution, by the Fe ³⁺ by oxidizing Fe ^2+, complexes of Fe ³⁺ and DBP in solution It is the schematic diagram which showed a mode that it formed. In an example of an embodiment of the present invention, it is a mimetic diagram showing signs that solid precipitate separation of the formed poorly soluble complex deposit is carried out with a horizontal sedimentation tank.

Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings.
First, the situation where dibutyl phosphoric acid (DBP) and monobutyl phosphoric acid (MBP) are generated as radiation degradation products that are the targets of this embodiment will be described.

  As shown in the processing flow of FIG. 1, nuclear fuel (fuel rods) is finely sheared in a reprocessing plant, dissolved in nitric acid, and separated from insoluble matters such as a fuel cladding tube. Uranium and plutonium dissolved in the nitric acid solution are extracted by contact mixing into an organic solvent composed of hydrocarbons such as tributyl phosphate (TBP) and dodecane (extraction step). In this extract, the radiation of uranium and plutonium and nitric acid gradually decompose TBP, generating radiation-degraded products such as DBP and MBP. By contacting and mixing the extract and an alkaline solution that is an aqueous solution containing sodium hydroxide and / or sodium carbonate, TBP remains in the organic solvent, and DBP and MBP move to the alkaline solution (transition step). The TBP remaining in the organic solvent is reused, while the DBP and MBP transferred to the alkaline solution become a high-level waste liquid that is concentrated together with the contaminated radioactive material. The high level waste liquid is vitrified and subjected to an appropriate storage process (solidification step). However, DBP and MBP cause foaming of high-level waste liquid, and there is a problem that it becomes difficult to control the temperature of the melting furnace when vitrifying.

  Therefore, in the present embodiment, for the purpose of precipitating at least one of DBP and MBP in the alkaline solution before vitrification, in the alkaline solution, the precipitate reacts with at least one of DBP or MBP. Is dissolved to form a complex of DBP and metal ion and / or a complex of MBP and metal ion (precipitation step). Since these complexes that are hardly soluble or insoluble in the alkaline solution become precipitates (solids generated in the solution), it is difficult to cause the alkaline solution (waste liquid) to foam in the solidification step. Further, the precipitate can be removed from the alkaline solution by a known solid-liquid separation method (removal step). By removing the precipitate from the alkali solution, the vitrification treatment of the alkali solution can be performed more easily.

  Therefore, the present embodiment includes an extraction step in which tributyl phosphoric acid is added to a solution in which spent nuclear fuel is dissolved to separate and extract radioactive substances, and a radioactive degradation product of the tributyl phosphate after the extraction step. The dibutyl phosphoric acid and the monobutyl phosphoric acid produced as a transition step are transferred to an alkaline solution, and the alkaline solution obtained after the transition step reacts with at least one of the dibutyl phosphoric acid or the monobutyl phosphoric acid to generate a precipitate. A radioactive waste liquid treatment method comprising: a precipitation step for dissolving a substance; and a solidification step for subjecting the alkaline solution obtained after the precipitation step to vitrification treatment.

  Furthermore, this embodiment performs the removal process which removes the said produced | generated precipitate from the said alkaline solution after the said precipitation process, The said alkaline solution from which the said precipitate obtained after the said removal process was removed is used. It is preferable to use for the said solidification process.

The type of the metal ion that dissolves in the alkaline solution to form the complex is not particularly limited as long as it binds to at least one of DBP and MBP to form a hardly soluble or insoluble complex. For example, trivalent iron ions (III), trivalent zirconium (III), tetravalent zirconium ions (IV), ruthenium ions, niobium ions, and the like can be mentioned as suitable ions.
The said metal ion may be used individually by 1 type, and 2 or more types may be used together.

  Here, “slightly soluble” means a solubility of less than 100 mg / L, and “insoluble” means a solubility that is lower than that of hardly soluble. Accordingly, the meaning of insolubility includes the meaning of insolubility.

  The method for removing the precipitate is not particularly limited. For example, the alkaline solution in which the precipitate is formed may be allowed to stand to precipitate the precipitate, or the precipitate may be filtered with a filter.

  The pH of the alkaline solution for dissolving the metal ions is not particularly limited as long as the pH is capable of dissolving the metal ions and forming the hardly soluble complex. Prior to dissolution of the metal ions, the pH may be adjusted in advance by adding an acid or an alkali. Examples of pH suitable for dissolving the metal ion to form the hardly soluble complex include pH 3 and higher.

  The concentration of metal ions dissolved in the alkaline solution is not particularly limited. For example, the concentration of the metal ions dissolved in the same amount or more on a mass basis or a molar basis relative to the concentrations of DBP and MBP contained in the alkaline solution. The method of doing is mentioned.

  For example, in order to remove DBP and MBP in an alkaline solution before vitrification, a complex of DBP and iron ion (III) is obtained by dissolving trivalent iron ion (III) in the alkaline solution. And a complex of MBP and iron ion (III) can be formed. Since these complexes are hardly soluble or insoluble in the alkaline solution, they can be removed as precipitates.

Complex formation between DBP and iron ion (III) can be expressed by the following reaction formula.
3DBP + Fe ³⁺ → Fe (DBP) ₃
The complex formation between MBP and iron ion (III) can be expressed by the following reaction formula.
3MBP + 2Fe ³⁺ → Fe ₂ (MBP) ₃

  Since the solubility of the complex with respect to the alkaline solution is about several hundred μg to several tens mg / L and is extremely low, the complex can be removed from the solution as a precipitate. For example, it is possible to employ a method in which the solution in which the precipitate is generated is allowed to stand, the complex precipitate is settled to the bottom of the water tank, and the supernatant is transferred to another water tank. Moreover, the method of filtering the deposit of the said complex with a filter can also be illustrated.

  The concentration of iron ion (III) dissolved in the alkaline solution is not particularly limited. For example, the concentration of DBP and MBP contained in the alkaline solution is the same or more on the basis of mass or number of moles. A method of dissolving at a concentration is mentioned. Specifically, for example, the concentration range can be 14 to 30 mM.

The method for dissolving iron ions (III) in the alkaline solution is not particularly limited, and for example, trivalent such as iron chloride (III) (FeCl ₃ ), iron sulfate (III) (Fe ₂ (SO ₄ ) ₃ ), etc. The method of dissolving the compound containing the iron ion in the said alkaline solution is mentioned.

  Instead of iron ion (III), trivalent zirconium (III) or tetravalent zirconium ion (IV) may be used. By dissolving at least one of these zirconium ions in the alkaline solution, a poorly soluble complex of DBP and MBP and the zirconium ion is formed, and as in the case of iron ion (III), DBP as a precipitate is formed. And MBP can be removed.

Complex formation between DBP and zirconium ion (IV) can be expressed by the following reaction formula. The nitrate ions in the reaction formula are derived from the nitric acid used in the process of dissolving the fuel rods in the previous step of the extraction step. However, in the complex formation, the nitrate ion is not an essential component.
^{_{(NO 3 -) 2 + 2DBP}} + Zr 4+ → Zr (DBP) 2 (NO 3 -) 2

Method of dissolving the zirconium ions in the alkaline solution is not particularly limited, for example, zirconium oxide (IV) (ZrO _2), zirconium tungstate _{(IV) (Zr (WO 4} ) 2), zirconium chloride (III) ( Examples thereof include a method in which a compound containing zirconium ions such as ZrCl ₃ ) and zirconium (IV) chloride (ZrCl ₄ ) is dissolved in the alkaline solution.

  The concentration of zirconium ions (III) and zirconium ions (IV) dissolved in the alkaline solution is not particularly limited. For example, the concentration of DBP and MBP contained in the alkaline solution is on a mass basis or a molar basis. The method of making it melt | dissolve by the density | concentration of the same quantity or more is mentioned. Specifically, for example, the concentration range can be 14 to 30 mM.

The complex formation between DBP and ruthenium ions or niobium ions can be represented by the following reaction formula. The nitrate ions in the reaction formula are derived from the nitric acid used in the process of dissolving the fuel rods in the previous step of the extraction step. However, in the complex formation, the nitrate ion is not an essential component. M and n each independently represent 0 or a natural number. “(K) +” represents the valence of ruthenium ion or niobium ion.
^{_{(NO 3 -) n + (}} DBP) m + Ru (k) + → Ru (DBP) m (NO 3 -) n kn-m +
^{_{(NO 3 -) n + (}} DBP) m + Nb (k) + → Nb (DBP) m (NO 3 -) n kn-m +

  A method for dissolving ruthenium ions or niobium ions in the alkaline solution is not particularly limited. For example, a method for dissolving a compound containing ruthenium ions or niobium ions, such as ruthenium oxide, niobium oxide, niobium chloride, or the like, in the alkaline solution. Is mentioned.

  The concentration of ruthenium ions or niobium ions dissolved in the alkaline solution is not particularly limited. For example, the concentration of DBP and MBP contained in the alkaline solution is the same or higher on the basis of mass or mole number. A method of dissolving at a concentration is mentioned. Specifically, for example, the concentration range can be 14 to 30 mM.

  When the metal ions are dissolved in the alkali solution, the formation of the complex proceeds on the surface of the lump composed of the compound containing the metal ions, and a shell composed of a sparingly soluble complex is formed on the surface of the lump. There is. In this case, since the compound inside the shell cannot form a complex, a part of the added compound is wasted without contributing to the complex formation of DBP and MBP.

For example, as shown in FIG. 2, when a mass of Fe ³⁺ salt is added to the alkaline solution, an insoluble Fe (DBP) ₃ shell is formed on the surface of the mass, and Fe inside the shell is formed. ^{Since 3+} cannot form a complex, the utilization efficiency of Fe ³⁺ added as a whole decreases.

  Therefore, when adding the metal ion to the alkaline solution, first, a compound containing a metal ion that is difficult to form a complex with DBP and MBP is dissolved in the alkaline solution, and then in the alkaline solution, The use efficiency can be prevented from being lowered by oxidizing or reducing the dissolved metal ions to generate metal ions capable of forming a complex with DBP or MBP in the alkaline solution.

For example, when a salt of Fe ²⁺ is added instead of Fe ³⁺ in the alkaline solution, Fe ²⁺ does not form a complex with DBP and MBP, so the salt of Fe ²⁺ It disintegrates in the alkaline solution and becomes individual Fe ²⁺ ions, which are uniformly dissolved in the alkaline solution. By oxidizing Fe ²⁺ dissolved in this manner in the alkaline solution to produce Fe ³⁺ , an insoluble or poorly soluble complex of Fe ³⁺ with DBP and MBP can be formed ( (See FIG. 3). As explained here, according to the method of converting the added iron ion (II) into the iron ion (III) in the solution, Fe ³⁺ is not wasted and the utilization efficiency of the added iron ion is increased. Can do.

The method for dissolving iron ions (II) in the alkaline solution is not particularly limited, and examples thereof include iron chloride (II) (FeCl ₂ ), iron oxide (II) (FeO), iron sulfide (II) (FeS), Examples include a method in which a compound containing a divalent iron ion such as iron (II) sulfate (FeSO ₄ ) or potassium hexacyanoferrate (II) (K ₄ [Fe (CN) ₆ ]) is dissolved in the alkaline solution. .

  The concentration of iron ion (II) dissolved in the alkaline solution is not particularly limited. For example, the concentration of DBP and MBP contained in the alkaline solution is the same or more on the basis of mass or number of moles. A method of dissolving at a concentration is mentioned. Specifically, for example, the concentration range can be 14 to 30 mM.

  The method for oxidizing iron ions (II) dissolved in the alkaline solution to iron ions (III) is not particularly limited. For example, a method of dissolving an oxidizing agent such as hydrogen peroxide or potassium permanganate in the solution. Is mentioned. The addition amount of the oxidizing agent can be appropriately set according to the concentration of iron ion (II) in the solution.

Since the precipitate containing the complex formed in the solution has a small particle size, it may not be easily settled or filtered with a filter. In such a case, a method of adding an aggregating agent for aggregating the precipitate to the solution in which the precipitate is formed can be mentioned.
By adding an aggregating agent, precipitates can be aggregated to form a hardly soluble lump having a large particle size. The hardly soluble precipitate having a large particle size can be settled relatively easily or filtered with a filter. The addition amount of the flocculant can be appropriately set according to the concentration of the precipitate or complex in the solution.

  The kind of the flocculant is not particularly limited, and examples thereof include polyaluminum chloride.

  Moreover, a sediment can be efficiently removed using the horizontal sedimentation tank 10 as shown in FIG. When a solution containing the complex precipitate 13 is introduced from the inlet 11 of the horizontal sedimentation tank in a slow flow, the complex precipitate in the solution settles on the bottom 10 a of the tank and is more than the inlet 11. The supernatant can be discharged from the discharge port 12 provided at a high position. Thereby, the sedimented sediment can be separated from the solution and removed.

  Although the embodiments of the present invention have been described in detail with reference to the drawings, the configurations and combinations of the embodiments in the embodiments are examples, and the addition and omission of configurations are within the scope not departing from the gist of the present invention. , Substitutions, and other changes are possible. Further, the present invention is not limited by the embodiments, and is limited only by the scope of the claims.

DESCRIPTION OF SYMBOLS 10 ... Horizontal sedimentation tank, 10a ... Bottom of horizontal sedimentation tank, 11 ... Inlet, 12 ... Outlet, 13 ... Precipitate

Claims (7)

An extraction process in which tributyl phosphate is added to the dissolved liquid in which spent nuclear fuel is dissolved to separate and extract radioactive substances;
After the extraction step, a transition step of transferring dibutyl phosphate and monobutyl phosphate produced as radioactive degradation products of the tributyl phosphate to an alkaline solution,
A precipitation step in which a substance that reacts with at least one of the dibutyl phosphate or monobutyl phosphate to form a precipitate is dissolved in the alkaline solution obtained after the transfer step;
A solidification step of subjecting the alkaline solution obtained after the precipitation step to a vitrification treatment;
A radioactive liquid waste treatment method comprising:   The removal step of removing the generated precipitate from the alkali solution is performed after the precipitation step, and the alkali solution obtained after the removal step is subjected to the solidification step. Of radioactive liquid waste treatment.   The radioactive waste liquid treatment method according to claim 2, wherein the deposit is settled in the removing step.   The radioactive waste liquid treatment method according to any one of claims 1 to 3, wherein the precipitate is a complex of at least one of the dibutyl phosphoric acid or monobutyl phosphoric acid and iron ions (III).   In the precipitation step, after adding iron ions (II) to the alkaline solution, the complex is formed by oxidizing the iron ions (II) to produce iron ions (III). Item 5. A method for treating radioactive liquid waste according to Item 4.   The radioactive waste liquid treatment method according to any one of claims 1 to 3, wherein the precipitate is a complex of at least one of dibutyl phosphate or monobutyl phosphate and zirconium ion (IV).   The radioactive waste liquid treatment method according to any one of claims 1 to 6, wherein in the precipitation step, the precipitate is further aggregated by adding a flocculant to the alkaline solution.

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