Method for reducing volume of extraction solvent waste

Abstract

To provide a technique of reducing the volume of an extraction solvent waste including a nuclear material, for example.SOLUTION: A diluent in which an extractant is dissolved is added to a sample including a nuclear material, for example, the nuclear material is adsorbed to the extractant so that a complex is formed. After that, an aqueous solution containing the complex is separated by a solvent extraction technique. The aqueous solution containing the separated complex is irradiated with plasma so that the aqueous solution is separated into a nonflammable waste liquid and nonflammable gas, whereby the volume of the aqueous solution is reduced. If the aqueous solution is radioactive, the waste liquid of which volume has been reduced is subjected to vitrification and becomes suitable for geological disposal.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a technology for reducing the volume of a solution containing rare earths, rare metals, etc., extracted by solvent extraction, or nuclear material. [Background technology]

[0002] Solvent extraction is used as one of the methods to recycle rare earths and rare metals used in industrial products, such as lithium used in mobile phone batteries, and is also used to recover nuclear materials from spent nuclear fuel generated at nuclear power plants.

[0003] Taking the latter as an example, spent nuclear fuel generated at nuclear power plants is processed in reprocessing plants to recover reusable nuclear materials such as uranium and plutonium. The aforementioned solvent extraction method is often used to recover nuclear materials. Figure 7 shows an example of the solvent extraction method.

[0004] As shown in Figure 7, the spent nuclear fuel generated at a nuclear power plant contains fission products that are not to be recovered (non-extractable elements) in addition to the nuclear materials to be recovered (extractable elements). The spent nuclear fuel is dissolved in nitric acid or the like to produce an aqueous solution (spent nuclear fuel solution), and the nuclear materials to be recovered are ionized in the aqueous solution.

[0005] This spent nuclear fuel solution is mixed with an organic solvent in which an extractant such as TBP or CMPO is dissolved in a diluent such as dodecane or kerosene to form a mixed solution, and the ionized nuclear material, etc. are adsorbed onto the extractant to form a complex.

[0006] Then, by utilizing the difference in specific gravity between the complexes and fission products, the solution is separated into an aqueous solution containing extractable elements (nuclear materials, etc.) and an aqueous solution containing non-extractable elements (fission products, etc.). The aqueous solution containing non-extractable elements is vitrified and stored in an underground primary storage facility where it is sufficiently cooled. An artificial barrier is then formed using a thick metal container (overpack) and a buffer material made of clay that is difficult for water to pass through. The overpack is then buried in an underground burial facility located in a stratum more than 300 m deep, forming a natural barrier for geological disposal.

[0007] On the other hand, the aqueous solution containing the separated extractable elements (nuclear materials, etc.) contains flammable diluents such as dodecane, making it difficult to mix it into the vitrified waste.

[0008] As a method for disposing of an aqueous solution containing extractable elements, various disposal techniques have been proposed, such as those shown in Patent Documents 1 and 2 below. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2014-230603 A [Patent Document 2] Japanese Patent Application Publication No. 6-160593 Summary of the Invention [Problem to be solved by the invention]

[0010] In the technology described in Patent Document 1, when radioactive alkaline waste liquid is supplied to a storage tank, a first oxidative decomposition process is performed using titanium oxide as a photocatalyst, and then a second oxidative decomposition process is performed using ultraviolet light irradiation. These two decomposition processes decompose trace amounts of DBP (dibutyl phosphate) that have been radioactively decomposed from TBP. The resulting decomposition product is then neutralized with nitric acid to form a neutralized product that can be disposed of as a glass solidification.

[0011] However, the amount of waste liquid that can be oxidatively decomposed using titanium oxide or ultraviolet light irradiation is not large, and it is thought to be difficult to dispose of large amounts of waste liquid.

[0012] The technology described in Reference 2 involves adding calcium hydroxide, powdered glass, alkali and alumina cement as additives to the pyrophosphate-based pyrolysis product obtained by pyrolysis of waste solvents such as TBP, dodecane or kerosene generated in the solvent extraction method, mixing the mixture, and then hydrothermally synthesizing the mixture at high temperature and pressure. This makes it possible to suppress cracking in the hydrothermal body, thereby forming a compression-molded hydrothermal body with a low leaching rate, and enabling solidification processing with high radioactivity containment performance.

[0013] However, when an aqueous solution containing extractable elements is vitrified in addition to the aqueous solution containing non-extractable elements described above, the amount of vitrified waste to be buried underground naturally increases, and this shortens the period until the temporary storage facilities for temporarily storing the vitrified waste until it is cooled and the underground burial facilities for burying the vitrified waste underground after cooling reach their limits.

[0014] In addition, since it is difficult to build new disposal sites, the issue is how to utilize the existing storage and burial facilities for as long as possible. Therefore, it is necessary to reduce the volume of radioactive waste generated by nuclear power plants as much as possible in order to reduce the amount of waste that needs to be disposed of.

[0015] In view of the above problems, an object of the present invention is to disclose a technique for reducing the volume of an aqueous solution containing extractable elements separated by a solvent extraction method. [Means for solving the problem]

[0016] The invention described in claim 1 is characterized in that, in an organic solvent to which an extractant and a diluent are added to a sample containing extractable elements such as nuclear material and non-extractable elements such as fission products, the extractable elements are adsorbed onto the extractant to form a complex, the organic solvent is separated into an aqueous solution containing the non-extractable elements and an aqueous solution containing the complex by a solvent extraction method, and the aqueous solution containing the complex is irradiated with plasma to reduce the volume of the aqueous solution containing the complex.

[0017] The invention as recited in claim 2 is characterized in that the plasma as recited in claim 1 is microwave plasma.

[0018] The invention described in claim 3 is characterized in that the extractant described in either claim 1 or claim 2 is a phosphorus-based extractant or an amide-based extractant.

[0019] The invention described in claim 4 is characterized in that the diluent described in either claim 1 or claim 2 is a hydrocarbon such as dodecane or kerosene, an alcohol such as octanol, or an ether such as dihexyl ether. Effect of the Invention

[0020] According to the invention described in claim 1, radioactive waste can be reliably reduced in volume. After the volume reduction, the waste can be made into a state that can be vitrified for geological disposal, and the existing storage and burial facilities can be used for a long period of time.

[0021] According to the invention described in claim 2, the amount of energy required to generate plasma can be reduced compared to the case of using thermal plasma. Also, since the reactivity can be kept low, a strong reactor is not required, and the device can be constructed at low cost.

[0022] According to the invention described in claim 3, by adsorbing the extractable elements to the extractant to form a complex, the specific gravity of the complex is made different from that of the non-extractable elements, and the extractable elements and non-extractable elements can be separated by a solvent extraction method.

[0023] According to the invention described in claim 4, the physical properties such as specific gravity can be improved and the concentration of the extractant can be adjusted to an appropriate concentration. In addition, the desired properties of a diluent can be satisfied, which are (1) low water solubility, (2) low loss due to entrainment of droplets, (3) high flash point, (4) good separation property, (5) high dissolving power, and (6) non-toxicity. [Brief description of the drawings]

[0024] [Figure 1] 1 is a conceptual diagram showing a method for reducing the volume of radioactive waste according to the present invention. [Diagram 2] 1 is a table showing physical properties of waste liquid. [Diagram 3] 1 is a table showing plasma decomposition conditions and products. [Figure 4] 4 is a table showing the material balance before and after decomposition of the waste liquid in the case of No. 1 in FIG. 3. [Diagram 5] 4 is a table showing the material balance before and after decomposition of the waste liquid in the case of No. 2 in FIG. 3. [Figure 6] 4 is a table showing the material balance before and after decomposition of the waste liquid in the case of No. 3 in FIG. 3. [Figure 7] FIG. 1 is a conceptual diagram showing the process of separating extractable elements from spent nuclear fuel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Hereinafter, an embodiment of the present invention will be described with reference to Figures 1 to 7. Figures 1 and 7 are conceptual diagrams of a method for reducing the volume of radioactive waste according to the present invention.

[0026] The radioactive waste of the present invention is intended to be a spent nuclear fuel solution formed by dissolving nuclear material generated from a nuclear power plant in an aqueous solution of nitric acid or the like, as shown in Fig. 7. The solution contains non-extractable elements such as fission products in addition to extractable elements such as nuclear material.

[0027] Examples of nuclear materials include uranium, plutonium, neptunium, americium, and curium. Examples of fission products include ruthenium, palladium, rhodium, technetium, zirconium, cesium, and strontium, but may also include other elements.

[0028] The spent nuclear fuel solution is prepared by mixing an organic solvent in which an extractant such as TBP or CMPO has been dissolved in a diluent such as dodecane or kerosene.

[0029] The diluent may be a hydrocarbon such as dodecane or kerosene, an alcohol such as octanol, or an ether such as dihexyl ether, and may be any diluent capable of improving physical properties such as specific gravity and adjusting the extractant to an appropriate concentration. In addition, the desired properties of the diluent are selected from those that satisfy the following: (1) low water solubility, (2) low loss due to entrainment of droplets, (3) high flash point, (4) good separation property, (5) high dissolving power, and (6) non-toxicity.

[0030] Examples of extractants include phosphorus extractants such as TBP, CMPO, D2EHPA, and DBBP, and amide extractants such as TODGA, TEHDGA, and Malonamide. These extractants adsorb extractable elements and form complexes, which can create a difference in specific gravity between the extractable and non-extractable elements.

[0031] The mixture is separated into an aqueous solution containing extractable elements and an aqueous solution containing non-extractable elements by a solvent extraction method. The aqueous solution containing non-extractable elements is vitrified and then disposed of in a geological formation.

[0032] On the other hand, the volume of the aqueous solution containing the extractable elements is reduced by irradiating it with plasma from the outside. In this embodiment, the case where TBP (tributyl phosphate) is used as the extractant and dodecane is used as the diluent is described.

[0033] As shown in Figure 1(a), when an aqueous solution containing extractable elements is irradiated with plasma from the outside, the TBP in the aqueous solution produces phosphoric acid as shown in Figure 1(b). The phosphoric acid contains nuclear materials and other substances, and this becomes the waste after volume reduction.

[0034] The plasma is assumed to be microwave plasma, which requires less energy to generate than thermal plasma. In addition, since the reactivity is low, a strong reactor is not required, and the device can be manufactured at low cost.

[0035] In addition to phosphoric acid, plasma irradiation also produces water, carbon dioxide, and other substances, which are released into the atmosphere as gases.

[0036] Next, the volume reduction effect of liquid radioactive waste (hereinafter referred to as waste liquid) diluted with 30% TBP and 70% dodecane by weight is described when it is decomposed using microwave plasma. Note that dodecane is used to improve physical properties such as specific gravity and to adjust the concentration of the extractant to an appropriate concentration, and the above weight ratio is an example, and the weight ratio should be adjusted as appropriate.

[0037] Calculating the volume ratio from the weight ratio of TBP and dodecane and the density of both, we get 25% TBP and 75% dodecane. The calculation results are shown in Figure 2. Using the values ​​in Figure 2, we calculate the density of the waste liquid with a weight ratio of 30% TBP and 70% dodecane to be 0.81 kg / L.

[0038] Here, when the phosphorus contained in TBP is oxidized, it takes on various forms, such as orthophosphoric acid (H3PO4), diphosphoric acid (H4P2O7), metaphosphoric acid (HPO3), diphosphorus pentoxide (P2O5), and polyphosphoric acid (HO(HPO3)nH).

[0039] When decomposed using plasma, there is a tendency for it to be decomposed into molecules with smaller molecular weights, and orthophosphoric acid (H3PO4) and diphosphate (H4P2O7) are produced depending on whether or not there is water in the experimental system. Figure 3 shows the decomposition conditions and products.

[0040] When waste liquid is decomposed under sufficient oxygen with an oxygen ratio of 1 or more, TBP and dodecane decompose as shown in Chemical Formula 1 below. The products are carbon dioxide, water, and orthophosphoric acid (H3PO4). Under these conditions, water is generated after the reaction, so even if diphosphoric acid (H4P2O7), metaphosphoric acid (HPO3), or diphosphorus pentoxide (P2O5) are produced, they will react with water to become orthophosphoric acid (H3PO4).

[0041] [ka]

[0042] For example, when 100 [L] of waste liquid is decomposed by plasma under the reaction shown in [Chemical Formula 1], the mass balance is shown in Figure 4. Note that the volume of the gas is at standard conditions (0°C, 1 atm), and the density of orthophosphoric acid (H3PO4) is 1.892 [kg / L].

[0043] Of the products of decomposition, carbon dioxide is released into the atmosphere. Water is released into the atmosphere as water vapor. As a result, the remaining orthophosphoric acid (H3PO4) becomes reduced-volume radioactive waste that contains extractable elements (nuclear materials, etc.).

[0044] As shown in Figure 4, the volume reduction rate is 89% on a weight basis and 95.3% on a volume basis.

[0045] When water is present in the experimental system, if the wastewater is decomposed under oxygen-deficient conditions (oxygen ratio less than 1), TBP and dodecane will decompose as shown in Chemical Formula 2 below. The products are carbon monoxide, hydrogen, and orthophosphoric acid (H3PO4).

[0046] [ka]

[0047] Of the products of decomposition, carbon monoxide and hydrogen are released into the atmosphere, while the remaining orthophosphoric acid (H3PO4) becomes reduced-volume radioactive waste that contains extractable elements (nuclear materials, etc.).

[0048] The mass balance when 100 [L] of waste liquid is decomposed under these conditions is shown in Figure 5. As in Figure 4, the volume of gas is at standard conditions (0°C, 1 atm), and the density of orthophosphoric acid (H3PO4) is 1.892 [kg / L].

[0049] The volume reduction rate is the same as in Figure 4, with a reduction of 89% on a weight basis and 95.3% on a volume basis.

[0050] In the above-mentioned embodiment, orthophosphoric acid (H3PO4) was used as the waste after volume reduction, but orthophosphoric acid (H3PO4) may damage equipment or corrode storage containers. It may also dissolve in the generated water. Therefore, waste liquid containing orthophosphoric acid (H3PO4) may be treated using a coagulation sedimentation method. The reaction formula when the coagulation sedimentation method is performed using slaked lime (calcium hydroxide: Ca(OH)2) is shown below in [Chemical formula 3].

[0051] [ka]

[0052] The calcium phosphate (Ca5(PO4)3(OH)) produced forms sludge. However, sludge contains a lot of water and requires processing such as drying. If it were possible to separate the calcium phosphate from the water, 4.8 L (15.2 kg) of calcium phosphate would be produced from 4.7 L (8.9 kg) of orthophosphoric acid (H3PO4) produced, as shown in Figures 4 and 5. Therefore, the volume reduction effect when stabilizing with calcium phosphate is a reduction of 81.1% on a weight basis and 95.2% on a volume basis.

[0053] When there is no water in the experimental system, and the waste liquid is decomposed under oxygen-deficient conditions with an oxygen ratio of less than 1, TBP and dodecane are decomposed as shown in Chemical Formula 4 below. The products are carbon monoxide, hydrogen, and diphosphate (H4P2O7).

[0054] [ka]

[0055] Of the products of decomposition, carbon monoxide and hydrogen are released into the atmosphere, while the remaining diphosphate (H4P2O7) becomes reduced radioactive waste that contains extractable elements (nuclear materials, etc.).

[0056] The mass balance when 100 [L] of waste liquid is decomposed under these conditions is shown in Figure 6. As in Figures 4 and 5, the volume of gas is at standard conditions (0°C, 1 atm), and the density of orthophosphoric acid (H3PO4) is 1.892 [kg / L].

[0057] The volume reduction rate is a 90.0% reduction on a weight basis and a 95.5% reduction on a volume basis.

[0058] As explained above, when radioactive waste (liquid waste) containing a diluent such as dodecane is decomposed using plasma, carbon can be decomposed into carbon dioxide or carbon monoxide, and hydrogen can be decomposed into water or gaseous hydrogen. Therefore, a significant reduction in volume can be achieved.

[0059] The reduced volume of radioactive waste (liquid waste) can be mixed into vitrified waste and disposed of geologically because the flammable liquid is decomposed by plasma irradiation. Since the waste is sufficiently reduced in volume for geological disposal, it becomes possible to use underground storage and burial facilities for a long period of time.

[0060] In the above examples, TBP was used as the extractant and dodecane as the diluent. However, even if other extractants and diluents as mentioned above are used, the flammable diluent can be decomposed by plasma and the waste liquid after volume reduction can be similarly solidified by glass.

[0061] The contents of the present invention have been explained above using the example of reducing the volume of a solution containing nuclear material extracted by solvent extraction. However, as mentioned above, the present invention can also be applied to reducing the volume of a solution containing rare earths, rare metals, etc. extracted by solvent extraction.

[0062] In other words, there is an advantage to reducing the volume of solutions containing rare earths and rare metals extracted by solvent extraction, making them easier to handle. Therefore, this advantage can be enjoyed by irradiating the waste liquid with plasma to decompose the solution and reduce the volume by releasing the gas that is generated. Note that solutions containing rare earths and rare metals are not vitrified or disposed of in a geological layer even if they are reduced in volume. [Industrial Applicability]

[0063] It can be used for decomposing liquids, and is particularly suitable for reducing the volume of aqueous solutions that contain harmful substances.

Claims

1. A method for reducing the volume of extractant-based waste, comprising the steps of: adding an extractant and a diluent to a sample containing extractable elements such as nuclear material and non-extractable elements such as fission products, and forming a complex by adsorbing the extractable elements to the extractant in an organic solvent; separating the organic solvent into an aqueous solution containing the non-extractable elements and an aqueous solution containing the complex by a solvent extraction method; and irradiating the aqueous solution containing the complex with plasma to reduce the volume of the aqueous solution containing the complex.

2. 2. The method for reducing the volume of extractive solvent waste according to claim 1, wherein the plasma is microwave plasma.

3. 3. The method for reducing the volume of extractant waste according to claim 1, wherein the extractant is a phosphorus-based extractant or an amide-based extractant.

4. 3. A method for reducing the volume of extractive solvent waste as described in claim 1 or claim 2, characterized in that the diluent is a hydrocarbon such as dodecane or kerosene, an alcohol such as octanol, or an ether such as dihexyl ether.

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