JP4879869B2 - Method for selectively separating cesium from high-level radioactive liquid waste - Google Patents

Description

The present invention provides selective use of a dioctyloxo-calix [4] -crown-6 or calix [4] -crown-6 derivative from a solution containing a trivalent actinoid or fission product of each valence state. The present invention relates to a method for separating and recovering cesium.
In particular, the present invention relates to reprocessing of spent nuclear fuel (SF) generated from a nuclear power generation reactor, waste treatment and disposal, and selectively separates only cesium from high-level radioactive liquid waste in the nuclear fuel cycle. It relates to the method of recovery.

The nuclear fuel cycle is classified into reprocessing for the purpose of separating and recovering uranium and plutonium, and high-level radioactive liquid waste processing that occurs with the reprocessing. This high-level radioactive liquid waste contains most of the fission products and minor actinides (MA), is desalted and concentrated, stabilized as a vitrified body, and then disposed of in a deep repository.
In addition, group separation in which various radioactive substances in the high-level radioactive liquid waste are separated and recovered into several groups according to the half-life, chemical properties of elements, and the like has been studied. As the separation target of this group separation, 1) transuranics and technetium (Tc) with long half-life, 2) strontium (Sr) and cesium (Ce), which are heat sources and radiation sources, and 3) valuable as resources And platinum group elements.

  Separation of radioactive cesium reduces the amount of heat generation that affects the cooling period on the ground up to the geological disposal of high-level radioactive liquid waste, and reduces the external exposure due to γ-rays from cesium-137 The use conditions (thermal restriction conditions) of the disposal site for high-level radioactive liquid waste vitrified materials are eased, and it becomes possible to significantly reduce disposal costs, and geological disposal by cesium-135, a long-lived nuclide. It can be expected to reduce the environmental burden, such as the reduction of geological and groundwater contamination.

By separating these elements into groups, the economic and environmental burden of high-level radioactive waste disposal can be reduced and resources can be used effectively. The high-level radioactive liquid waste after reprocessing has a nitric acid concentration of around 3 mol / dm ³ , and a disposal method for the high-level radioactive liquid waste under this nitric acid concentration condition has not yet been established, and heat treatment is performed with borosilicate glass. Then, the disposal method to make a vitrified body is taken. The purpose of vitrifying high-level radioactive liquid waste is to consolidate long-lived radionuclides over a long period of time by solidifying the liquid radioactive liquid waste. Properties required for these solidified bodies are required to be stable in radiation, heat, mechanical properties, and chemical properties. However, cesium contained in the solidified body is one of the nuclides with the highest calorific value among the fission products. During the cooling (and storage) period of 30 to 50 years for the vitrified body, it is removed in the storage facility. In addition to the need for heat, it is a gamma-ray emitting nuclide, which places a heavy burden on the integrity of the solidified body and the maintenance of facilities, including the shielding of radiation.

Examples of methods for separating radioactive cesium, which is a γ-ray emitting nuclide and is a heating element, have been reported so far. First, there is a method of efficiently separating cesium from a nitric acid concentration of 2.5 mol using an inorganic ion exchanger carrying ammonium molybdate phosphate (see, for example, Patent Document 1).
However, this method has a problem in that the inorganic ion exchanger is generated as waste and it is necessary to treat this waste.

Next, a method of separating cesium in a nitric acid-containing aqueous solution using an insoluble ferrocyanide-based adsorbent can be mentioned (for example, see Patent Document 2).
However, in this method, it is necessary to coexist additives such as hydrazine derivatives as an antioxidant during the separation of cesium, and the adsorbent is generated as radioactive waste, and it is necessary to treat the waste. Has a problem.

Moreover, the method of isolate | separating cesium from the industrial effluent containing the other alkali metal using poly (hydroxyarylene) polymer resin is mentioned (for example, refer patent document 3). This method involves the use of solutions containing alkali metal cations, particularly polymer resins containing various poly (hydroxyarylene) ligands, to remove, concentrate and separate cesium cations from nuclear waste effluents. It is.
However, this method is a technique similar to ion exchange, and finally, radioactive solid waste is generated, and when placed in a high radiation field, the adsorption efficiency due to decomposition of the polymer resin and decomposition of the ligand is reduced. Degradation, ligand itself and resin detachment are listed as disadvantages.

Furthermore, there is a method in which the separation of cesium by the crown calix [4] arene compound is selectively extracted using a solvent extraction method. (For example, see Patent Document 4). This method differs from the method using the ion exchangers of Patent Documents 1 to 3 described above in that a liquid-liquid extraction method is employed.
However, this method requires the use of ortho-nitrophenyl hexyl ether in an organic solvent that dissolves the crown calix [4] allene, and this ortho-nitrophenyl hexyl ether has potential potential for nitration, such as by radiolysis. There are dangers and there are difficulties in industrial use. Crown calix [4] allene is hardly soluble in nonpolar solvents and the like, and solvent selection is a problem.

Further, there is an adsorption separation method in which cesium is adsorbed and separated using a mixed inorganic ion exchanger of mordenite and hydrous titanic acid or an inorganic ion exchanger such as zeolite or antimonic acid (for example, see Non-Patent Document 1). . In this method, cobalt dicarbollide (CCD) is dissolved in nitrobenzene, and cesium in an aqueous solution is selectively separated and recovered by a solvent extraction method.
However, this method also has a problem that the ion exchanger is generated as waste after separating and recovering cesium, and it is necessary to treat the waste.

Furthermore, the solvent extraction method using the organic solvent which melt | dissolved cobalt dicarbollide (CCD) in highly polar nitrobenzene is mentioned (for example, refer nonpatent literature 2). In this method, cobalt dicarbollide (CCD) is dissolved in nitrobenzene, and cesium in an aqueous solution is selectively separated and recovered by a solvent extraction method.
However, in the cobalt dicarbollide system used in this method, a harmful solvent (diluent) such as nitrobenzene is used, and it is difficult to obtain the extraction reagent because the source of the extraction reagent is limited. There is a problem that metal salts (solid waste) remain when incinerated after use.

Accordingly, there is a need for further improvements in methods for selectively separating (and recovering) cesium from high-level radioactive liquid waste.
JP 2000-84418 A JP-A-5-317697 Japanese Patent No. 3853361 Japanese Patent No. 3773252 Marsh (SF March), 2 others, (Distriutions of 14 elements on 63 adsorbers from three simultant solutions (acidic-dissolved sludge, acidified supernate and alkaline supernate) for Hanford HLW Tank 102-SY), "LA-12654", ( United States), Los Alamos National Laboratory, 1994, p. 1-96 SD Reilly, 2 others, Cobalt (III) Dicarbolide A potential 137Cs and 90Sr Waste Extraction Agent., "LA-11695", ( United States), Los Alamos National Laboratory, 1992, p. 1-19

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an improved method for separating cesium from high-level radioactive liquid waste.

A method for separating cesium from a high-level radioactive liquid waste according to one embodiment of the present invention includes dioctyloxo-calix [4] -crown-6 (DOC [4] C6) or calix [4] -crown-6 derivatives. , a solution consisting of an alcohol and n- dodecane monovalent 6 to 9 carbon atoms, and dissolved at a 0.005 mole ^/ dm 3 to 0.03 mol / dm ^3, to prepare a calix solution preparation A high-level radioactive waste solution containing a fission product containing cesium is added to the calix solution prepared in the process and the preparation process, and the cesium is extracted into the calix solution, and extracted by the extraction process And a back-extraction step of back-extracting cesium with a solution in which nitric acid of 0.01 mol / dm ³ or less is dissolved.
In addition, a method for separating cesium from a high-level radioactive liquid waste according to another embodiment of the present invention includes dioctyloxo-calix [4] -crown-6 or a derivative of calix [4] -crown-6 having 6 carbon atoms. to 9 of monohydric alcohols and N, to a solution consisting of N- dialkyl carboxamide, dissolved so as to be 0.005 mol ^/ dm 3 to 0.1 mol / dm ^3, to prepare a calix solution preparation A high-level radioactive waste solution containing a fission product containing cesium is added to the calix solution prepared in the process and the preparation process, and the cesium is extracted into the calix solution, and extracted by the extraction process And a back-extraction step of back-extracting cesium with a solution in which nitric acid of 0.01 mol / dm ³ or less is dissolved. The monovalent alcohol having 6 to 9 carbon atoms is preferably at least one alcohol selected from the group consisting of 1-hexanol, 1-octanol, 1-nonanol and cyclohexanol.

  In the method for separating cesium from the high-level radioactive waste liquid, the high-level radioactive waste liquid may be a high-level radioactive waste liquid obtained by separating uranium and plutonium from spent fuel in a light water reactor. The high-level radioactive liquid waste can also be a high-level radioactive liquid waste after reprocessing of MOX (plutonium-uranium mixed oxide fuel) spent fuel. Further, the high-level radioactive liquid waste can be a high-level radioactive liquid waste obtained by removing at least a part of uranium, plutonium, neptunium, etc. from the spent fast breeder reactor (FBR) spent fuel. Furthermore, the high level radioactive effluent may be a high level radioactive effluent containing americium, curium and fission products. The high-level radioactive liquid waste can also be a high-level radioactive liquid waste containing neptunium, americium, curium and fission products.

According to the method for separating cesium from the high-level radioactive liquid waste of the present invention, dioctyloxo-calix [4] -crown-6 or a derivative of calix [4] -crown-6 is monovalent with 6 to 9 carbon atoms. Since a calix solution dissolved in a mixed solvent of alcohol and n-dodecane or N, N-dialkylcarboxamide is used, cesium can be selectively extracted and further back-extracted using a nitric acid solution having a predetermined concentration. Cesium can be selectively separated without generating solid radioactive waste such as exchangers and inorganic ion exchangers.
Further, dioctyloxo-calix [4] -crown-6 or a derivative of calix [4] -crown-6 used as an extractant or a monovalent alcohol or n-dodecane having 6 to 9 carbon atoms used as a solvent (diluent). Alternatively, N, N-dialkylcarboxamide is composed of carbon, oxygen, hydrogen, and nitrogen, so it can be incinerated, so no solid waste is generated compared to ion exchange and other solvent extraction methods. Has the advantage.

The method for separating cesium from the high-level radioactive liquid waste according to the present invention comprises dioctyl oxo-calix [4] -crown-6 (DOC [4] C6) or calix [4] -crown-6 derivatives. Preparation of preparing a calix solution (organic phase) by dissolving in a solution consisting of ˜9 monohydric alcohol and n-dodecane to 0.005 mol / dm ^{3 to} 0.03 mol / dm ³ Add a high-level radioactive waste solution (spent fuel solution) containing fission products containing cesium to the calix solution (organic phase) prepared in the process and the preparation step, and add it into the calix solution (organic phase). An extraction step for extracting cesium, and an aqueous solution (diluted nitric acid) in which nitric acid of 0.01 mol / dm ³ or less is dissolved in cesium in the calix solution (organic phase) extracted in the extraction step And a back-extraction step of back-extracting to an aqueous phase. The calix solution (organic phase) preparation step is carried out by using a dioctyloxo-calix [4] -crown-6 (DOC [4] C6) or calix [4] -crown-6 derivative having 6 to 9 carbon atoms. In a solution composed of a monohydric alcohol and N, N-dialkylcarboxamide, dissolved so as to be 0.005 mol / dm ^{3 to} 0.1 mol / dm ³ (0.005 mol / dm ³ or more). A calix solution (organic phase) can also be prepared. This will be described in detail below.

  Dioctyloxo-calix [4] -crown-6 (DOC [4] C6) used in the method for separating cesium from the high-level radioactive liquid waste of the present invention has the structure shown in FIG. As a derivative of calix [4] -crown-6, a dioctyloxo-group in dioctyloxo-calix [4] -crown-6 is a linear aliphatic having 6 to 10 carbon atoms (other than 8). And those substituted with a hydrocarbon oxo (group), such as dialkyloxo-calix [4] -crown-6, specifically, dihexyloxo-calix [4] -crown-6, diheptyloxo-calix [ 4] -crown-6, dinonyloxo-calix [4] -crown-6, didecanyloxo-calix [4] -crown-6 and the like. Of these, dioctyl oxo-calix [4] -crown-6 is preferred from the viewpoints of ease of synthesis, large Cs extraction partition ratio, solubility in a diluent, and the like. Dioctyloxo-calix [4] -crown-6 or calix [4] -crown-6 derivatives in the calix solution extract cesium as an extractant.

The dioctyloxo-calix [4] -crown-6 (DOC [4] C6) or calix [4] -crown-6 derivative used in the present invention is a known method such as formalin and a predetermined substituent. Dioctyloxo-calix [4] -crown-6 is prepared and used by heating under reflux in diphenyl ether with phenol having sodium and sodium hydroxide.
In the present invention, the calix solution is a solvent that uses the above-mentioned dioctyloxo-calix [4] -crown-6 or calix [4] -crown-6 derivative used as an extractant as a diluent described later. , For example, a solution obtained by dissolving in a monohydric alcohol having 6 to 9 carbon atoms, n-dodecane, N, N-dialkylcarboxamide.

In the method for separating cesium from the high-level radioactive liquid waste, a calix solution (organic) of the dioctyloxo-calix [4] -crown-6 (DOC [4] C6) or calix [4] -crown-6 derivative is used. The concentration in the phase) is appropriately determined depending on the concentration of cesium in the high-level radioactive liquid waste, the concentration of nitric acid, the type of solvent and the blending ratio, etc. If it is 005 mol / dm ³ or more, cesium can be separated and extracted effectively. Preferably, 0.005 mol ^/ dm 3 to 0.1 mol / dm ^3, more preferably 0.005 mol ^/ dm 3 to 0.03 mol / dm ^3.

As a solvent for dissolving dioctyloxo-calix [4] -crown-6 (DOC [4] C6), a solvent in which a monovalent alcohol having 6 to 9 carbon atoms and n-dodecane are mixed can be used. In this case, the mixing ratio (volume ratio) of the monohydric alcohol having 6 to 9 carbon atoms and n-dodecane is preferably 20% by volume to 40% by volume: 80% by volume to 60% by volume, and 25% by volume to 35%. Volume%: 75 volume% to 65 volume% is more preferable, and 30 volume%: 70 volume% is still more preferable. That is, 30% by volume of monovalent alcohol having 6 to 9 carbon atoms and 70% by volume of n-dodecane are used as a solvent, and dioctyl is 0.005 mol / dm ^{3 to} 0.03 mol / dm ^3. More preferred is a calix solution (organic phase) to which oxo-calix [4] -crown-6 (DOC [4] C6) is added. When the blending ratio of the monohydric alcohol having 6 to 9 carbon atoms is less than 20% by volume, the solubility of dioctyloxo-calix [4] -crown-6 is not preferable. Moreover, when the blending ratio (volume ratio) of the monohydric alcohol having 6 to 9 carbon atoms exceeds 40% by volume, the Cs extraction / distribution ratio decreases, which is not preferable.
Examples of the monovalent alcohol having 6 to 9 carbon atoms include 1-nonyl alcohol, 1-octanol, 1-hexanol, cyclohexanol, 2-octanol, and 2-hexanol. Of these, 1-nonyl alcohol, 1-octanol, 1-hexanol, cyclohexanol and the like are preferable because of the solubility of dioctyloxo-calix [4] -crown-6, and 1-octanol is more preferable.

Moreover, as a solvent in which dioctyloxo-calix [4] -crown-6 (DOC [4] C6) is dissolved, a monovalent alcohol having 6 to 9 carbon atoms and N, N-dialkylcarboxyamide were mixed. A solvent can be used. In this case, the blending ratio (volume ratio) of the monohydric alcohol having 6 to 9 carbon atoms and the N, N-dialkylcarboxamide is preferably 85 vol% to 95 vol%: 15 vol% to 5 vol%. 90% by volume: 10% by volume is more preferable. That is, 90% by volume of monohydric alcohol having 6 to 9 carbon atoms and 10% by volume of N, N-dialkylcarboxamide are used as a solvent, and 0.005 mol / dm ^{3 to} 0.1 mol / dm ³ A calix solution (organic phase) to which dioctyloxo-calix [4] -crown-6 (DOC [4] C6) is added is even more preferable. In this solvent system, when the blending ratio of the monohydric alcohol having 6 to 9 carbon atoms is less than 85% by volume, the Cs extraction / distribution ratio is lowered, which is not preferable. Moreover, when the compounding ratio of the monohydric alcohol having 6 to 9 carbon atoms exceeds 95% by volume, the solubility is lowered, which is not preferable.
Examples of N, N-dialkylcarboxamide include N, N-dipentyloctaneamide, N, N-dihexyloctaneamide, N, N-diheptyloctaneamide, N, N-dioctylhexaneamide, and the like. Of these, N, N-dihexyloctaneamide is preferred because it is reported in papers as candidate compounds in the separation of uranium, plutonium and the like, and has abundant basic data. The monovalent alcohol having 6 to 9 carbon atoms is the same as described above. These solvents may contain inevitable impurities to the extent that they do not affect the effects of the present invention, that is, the selective separation of cesium.

Next, a high level radioactive liquid waste containing cesium is added to the calix solution (organic phase) prepared as described above. Specific examples of high-level radioactive liquid waste include high-level radioactive liquid waste obtained by separating uranium and plutonium from spent fuel in light water reactors, and high-level radioactivity after reprocessing of MOX (plutonium-uranium mixed oxide fuel) spent fuel. Waste liquid, high-level radioactive liquid waste containing americium, curium and fission products, and neptunium from which fast breeder reactor (FBR) spent fuel is at least partially, preferably all removed from uranium, plutonium, neptunium, etc. , High-level radioactive liquid waste containing americium, curium and fission products. Specific examples of the fission product include zirconium, cesium, strontium, molybdenum, ruthenium, rhodium, palladium, and neodymium. These high-level radioactive liquid wastes are usually nitric acid waste liquids, and the concentration thereof is about 2 to 4 mol / dm ³ , and further about 3 mol / dm ³ .
Based on the concentration of cesium in the waste liquid, etc., the high-level radioactive waste liquid (nitric acid waste liquid) contains, for example, a high-level radioactive waste liquid (aqueous phase): calix solution (organic phase) 1 : 0.5-2, in particular, 1: 1 (volume ratio) is added, and cesium is extracted into the calix solution (organic phase).

Next, as described above, cesium is back-extracted into the aqueous phase using a solution (diluted nitric acid) in which nitric acid having a predetermined concentration is dissolved in water in a calix solution (organic phase) from which cesium has been selectively extracted. The concentration of nitric acid in the aqueous phase is appropriately determined based on the concentration of cesium in the organic phase, but is preferably 0.1 mol / dm ³ or less, more preferably 0.01 mol / dm ³ or less, and even more. preferably from 0.001 to 0.01 mol / dm ^3. The ratio (volume ratio) between the nitric acid solution (aqueous phase) and the calix solution (organic phase) in the back extraction is, for example, nitric acid solution (aqueous phase): calix solution (organic phase) = 0.5 to 2: 1. In particular 1: 1.
The cesium solution that has been back-extracted, that is, separated, is collected after performing a predetermined treatment if necessary, and further subjected to a predetermined treatment, and then stored intermediately in an intermediate storage, for example, to sufficiently dissipate heat (heat generation). After that, a geological process for embedding in the geological layer is performed. On the other hand, the radioactive waste liquid from which cesium has been separated and removed is subjected to, for example, a predetermined treatment, and after vitrification, a formation treatment is performed in which it is embedded in the formation.

Next, an example of a method for separating spent fuel solution to which the method for separating cesium from the high-level radioactive waste liquid of the present invention is applied will be described with reference to FIGS.
In the separation method of FIG. 2 (separation method-1), first, uranium is separated as a uranium product, for example, uranium dioxide (UO ₂ ) by a process of recovering hexavalent uranium (U (VI)) from the spent fuel solution. Then, after the uranium is separated by the uranium recovery process, the transuranium element (TRU) is separated by the transuranium element (TRU) recovery process.
The method of separating (and recovering) cesium of the present invention is applied to a high-level radioactive liquid waste from which uranium and transuranium elements (TRU) are separated. The cesium (collected liquid) separated and recovered by the cesium (Cs) separation step is subjected to intermediate storage and then subjected to geological disposal that is buried in the geological formation. Cesium (recovered liquid) can be stored more safely by, for example, generating heat by cesium during intermediate storage, and performing geological disposal buried in the formation after the heat generation is almost finished. On the other hand, the fission product (FP) separated in the Cs recovery / separation process does not contain Cs having a large calorific value, so that the geological disposal is more safely carried out in the geological formation. The separated uranium and transuranium element (TRU) (total TRU recovery liquid) are processed by performing a predetermined process as shown in FIG.

  Next, the separation method (separation method-2) of FIG. 3 includes TRU and fission products after separating and recovering uranium (U) and plutoniun (Pu) from the spent fuel solution in a so-called co-decontamination process. A method (Cs recovery step) of separating (and recovering) cesium from the high-level radioactive waste liquid of the present invention is applied to the solution to selectively separate and recover only cesium (Cs). The separated cesium recovery liquid is subjected to geological disposal after sufficiently dissipating heat in the intermediate storage as described above. On the other hand, the high-level radioactive liquid waste after Cs is separated by the Cs recovery process is vitrified and disposed of.

Thus, according to the method for separating cesium from the high-level radioactive waste liquid of the present invention, dioctyloxo-calix [4] -crown-6 or calix [4] -crown-6 derivatives are converted to carbon atoms having 6 to 6 carbon atoms. Since a calix solution (organic phase) dissolved in a mixed solvent of 9 monohydric alcohol and n-dodecane or N, N-dialkylcarboxyamide is used, cesium can be selectively extracted, and nitric acid (water) with a predetermined concentration. Since it can be back-extracted into an aqueous phase using a solution, cesium can be selectively separated (and recovered) without generating solid radioactive waste such as organic ion exchangers and inorganic ion exchangers.
Further, dioctyloxo-calix [4] -crown-6 or a derivative of calix [4] -crown-6 used as an extractant or a monovalent alcohol having 6 to 9 carbon atoms used as a solvent (diluent) or N, N -Dialkylcarboxamide is composed of carbon, oxygen, hydrogen, and nitrogen and can be incinerated, so it has the advantage that no solid waste is generated compared to ion exchange and other solvent extraction methods. .
Furthermore, by using the solvent extraction method and processing the extracted residual solution as it is with a multistage counter-current apparatus, dilution operations for adjusting the solution for ion exchange and separation in the pH range so far are possible. This eliminates the need to improve the economics of equipment and separation operations. Cesium, especially cesium-137, is a heat-generating radioactive material, so it is necessary to remove heat in the storage facility during the cooling (and storage) period of 30 to 50 years of vitrified glass by separating cesium. In addition to reducing radiation properties, it is possible to reduce the soundness of solidified bodies and the burden of facility maintenance, including radiation shielding.

  Hereinafter, although an example and a comparative example explain the present invention, the present invention is not limited to this.

Example 1
Dioctyloxo-calix [4] -crown-6 (dioctyloxo-calix [4] crown-6) (synthetic product) (hereinafter also referred to as “DOC [4] C6”) is 0.005 mol / dm ³ . Thus, a dioctyloxo-calix [4] -crown-6 solution (organic phase) was prepared by adding to a solution of 30% by volume of 1-octanol and 70% by volume of dodecane. Next, high-level radioactive liquid waste (nitric acid waste liquid) at various nitric acid concentrations as shown in FIG. 4 in which 10 ⁻² concentration (mol / dm ³ ) cesium was dissolved was mixed with nitric acid waste liquid by volume ratio. (Aqueous phase): Organic phase = 1: 1 In addition to the above organic phase, the mixture was thoroughly mixed, and the distribution ratio of cesium was examined.
Further, the distribution ratio of cesium was similarly examined using 1-nonyl alcohol, 1-hexanol and cyclohexanol instead of 1-octanol. FIG. 4 shows the results of examining the effect (influence) of these various alcohols on the extraction distribution ratio of cesium, including the nitric acid concentration dependency.

As understood from FIG. 4, it was confirmed that cesium can be sufficiently extracted by adding these alcohols (monohydric alcohols having 6 to 9 carbon atoms). Furthermore, it was confirmed that cesium can be extracted more favorably in the nitric acid concentration of the high-level radioactive waste liquid (nitric acid waste liquid) in the range of 2 to 4 mol / dm ³ , particularly 3 mol / dm ³ . The extraction distribution ratio of the obtained cesium was in the order of 1-nonyl alcohol <1-octanol <1-hexanol <cyclohexanol.

(Example 2)
As plotted in FIG. 5, dioctyloxocalix [4] -crown-6 (DOC [4] C6) was added at 30% by volume of 1- (1) at various predetermined concentrations (mol / dm ³ ). The relationship between cesium extraction and DOC [4] C6 concentration was examined in the same manner as in Example 1 except that it was dissolved in octanol and 70 vol% n-dodecane solution. At the same time, the concentration of cesium and DOC [4] C6 was investigated by changing the nitric acid concentration in the high-level radioactive liquid waste (nitric acid waste liquid). The result is shown in FIG.

As understood from FIG. 5, it was confirmed that the higher the concentration of DOC [4] C6, the higher the extraction / distribution ratio of cesium (Cs). In particular, an extraction distribution ratio of cesium (Cs) of 8: 2 was obtained under the conditions of nitric acid concentration of 3.0 mol / dm ³ and DOC [4] C6 concentration of 0.05 mol / dm ³ , and sufficient cesium was obtained. It was confirmed that it can be recovered. In addition, when the nitric acid concentration in high level radioactive liquid waste was made high, the tendency for the extraction distribution ratio of cesium to become high was confirmed.

(Example 3)
In Example 3, the influence of the initial cesium concentration on the extraction distribution ratio of cesium was examined. Dioctyloxocalix [4] -crown-6 (DOC [4] C6) is dissolved in a solution of 30% by volume of 1-octanol and 70% by volume of n-dodecane so as to be 0.05 mol / dm ^3. The addition (organic phase) was prepared. Next, 3.0 mol / dm ³ nitric acid-containing high-level radioactive waste liquid (nitric acid waste liquid) containing various concentrations of cesium as plotted in FIG. 6, by volume ratio, nitric acid waste liquid (aqueous phase): In addition to the above organic phase so that the organic phase = 1: 1, it was mixed well, and the influence of the initial cesium concentration on the extraction distribution ratio of cesium from the 3.0 mol / dm ³ nitric acid solution was investigated. . The result is shown in FIG.

As is apparent from the results of FIG. 6, if the initial cesium concentration in the high-level radioactive liquid waste (dissolving fuel) is 0.01 mol / dm ³ or less, a sufficient distribution ratio can be obtained and cesium can be recovered. It was confirmed that

Example 4
In Example 4, the separation selectivity of strontium (Sr), nadium (Na) and europium (Eu), which are considered to have an influence upon cesium extraction, was examined. Dioctyloxocalix [4] -crown-6 (DOC [4] C6) is dissolved in a solution of 30% by volume of 1-octanol and 70% by volume of n-dodecane so as to be 0.05 mol / dm ^3. The addition (organic phase) was prepared. Next, various concentrations of strontium (Sr), nadium (Na), and europium (Eu), which are considered to affect cesium extraction, were dissolved as plotted in FIG. Add high-level radioactive waste liquid (nitric acid solution) at a concentration of nitric acid to the above organic phase and mix well so that the volume ratio of nitric acid waste liquid (aqueous phase): organic phase = 1: 1 As in Example 1, the distribution ratios of strontium (Sr), nadium (Na) and europium (Eu) were examined and compared with the distribution ratio of cesium. The result is shown in FIG.

As is clear from FIG. 7, cesium is strontium (Sr), sodium (Na), and europium (Eu) within the nitric acid concentration range (0.1 mol / dm ³ to 10 mol / dm ³ ) in which the experiment was conducted. It was shown that sufficient separation from each other is possible. In particular, the Cs / Na ratio in FIG. 7 is in the range of 10 ³ to 10 ^{4 at} the assumed nitric acid concentration (2.0 mol / dm ^{3 to} 4.0 mol / dm ³ ), and that Na does not interfere. Indicated.

(Example 5)
In Example 5, the back extraction behavior of cesium with a dilute nitric acid solution was examined. As in Example 1, dioctyloxocalix [4] -crown-6 (DOC [4] C6) was added in an amount of 30% by volume of 1-octanol and 70% by volume of 0.05 mol / dm ³ . As shown in FIG. 8, cesium extracted by dissolving in n-dodecane solution was extracted with varying nitric acid concentration, and the back extraction behavior of cesium with dilute nitric acid solution was examined. The result is shown in FIG.

As is clear from FIG. 8, the extraction distribution ratio of cesium (Cs) becomes 1 or less by setting the nitric acid concentration to 1.0 mol / dm ^3. By reducing the nitric acid concentration, for example, 0.1 mol / dm 3 is obtained. by ³ or less, further, by 0.01 mol / dm ³ or less, it was confirmed that it is possible to sufficiently back extracted cesium extracted in the organic phase .

(Example 6)
Dioctyloxocalix [4] -crown-6 (DOC [4] C6) was mixed with 10% by volume of N, N-dihexyloctaneamide (synthetic product) and 90% by volume so as to be 0.01 mol / dm ^3. In the same manner as in Example 1, a cesium extraction experiment was conducted. As a result, it was confirmed that extraction can be performed at an extraction distribution ratio of 7.0 from a nitric acid concentration of 3.0 mol / dm ³ . Further, it was confirmed that cesium can be back-extracted by using a solution having a nitric acid concentration of 0.001 mol / dm ³ .

(Example 7)
As in Example 6 above, dissolved in 10% by volume of N, N-dihexyloctaneamide and 90% by volume of 1-octanol solution so that the concentration of DOC [4] C6 is 0.01 mol / dm ^3. Using the prepared calix solution, extraction experiments of cesium, strontium, europium and sodium were performed using various nitric acid concentrations (1.0 to 4.0 mol / dm ³ ) as shown in Table 1. Table 1 shows the extraction distribution ratio of each element obtained in the nitric acid concentration range of 1.0 to 4.0 mol / dm ³ .

  As is clear from Table 1, the extraction distribution ratio of strontium, europium and sodium at each nitric acid concentration is 0.01 or less, and even if each element coexists in the nitric acid concentration region used, only cesium can be extracted. confirmed.

  The method for separating cesium from the high-level radioactive waste liquid of the present invention removes uranium (U) and all TRUs from a high-level radioactive waste liquid, for example, spent fuel solution by separating and recovering uranium (U) and all TRUs. Cesium can be separated and removed from the solution. Cesium (especially cesium 137) is an exothermic radioactive substance, and the radioactive waste that remains after vitrification is separated and removed by vitrification, and then buried and stored in a more stable state. Can do.

It is a figure which shows the structure of a dioctyl oxo-calix [4] -crown-6 (DOC [4] C6). It is a figure which shows an example of the separation method (process) of the spent fuel solution to which the method of isolate | separating and collect | recovering cesium from the high level radioactive waste liquid of this invention is applied. It is a figure which shows another example of the separation method (process) of the spent fuel solution to which the method of isolate | separating and collect | recovering cesium from the high level radioactive waste liquid of this invention is applied. It is a graph which shows the addition effect (influence) of the various alcohol which acts on extraction of cesium. It is a graph which shows the density | concentration dependence of DOC [4] C6 which acts on extraction of cesium. Is a graph showing the concentration-dependence of CsNO ₃ on the extraction behavior of cesium. It is a graph which shows the influence of strontium, europium, and sodium with respect to extraction of cesium. It is a graph which shows the back extraction behavior of cesium.

Claims (8)

Dioctyloxo-calix [4] -crown-6 or a derivative of calix [4] -crown-6 is added to a solution comprising a monohydric alcohol having 6 to 9 carbon atoms and n-dodecane in an amount of 0.005 mol / dm. A preparation step of preparing a calix solution by dissolving to ^{3 to} 0.03 mol / dm ³ ;
An extraction step of extracting cesium in the calix solution by adding a high-level radioactive waste solution containing a fission product containing cesium to the calix solution prepared by the preparation step;
Cesium extracted from the high-level radioactive waste liquid, comprising: a back-extraction step of back-extracting the cesium extracted in the extraction step with a solution in which 0.01 mol / dm ³ or less of nitric acid is dissolved. How to separate. Dioctyloxo-calix [4] -crown-6 or a derivative of calix [4] -crown-6 was added to a solution composed of a monovalent alcohol having 6 to 9 carbon atoms and N, N-dialkylcarboxamide. A preparation step of preparing a calix solution by dissolving to 005 mol / dm ^{3 to} 0.1 mol / dm ³ ;
An extraction step of adding cesium into the calix solution by adding a high-level radioactive waste solution containing a fission product containing cesium to the calix solution prepared by the preparation step;
Cesium extracted from the high-level radioactive waste liquid, comprising: a back-extraction step of back-extracting the cesium extracted in the extraction step with a solution in which 0.01 mol / dm ³ or less of nitric acid is dissolved. How to separate.   The monovalent alcohol having 6 to 9 carbon atoms is at least one alcohol selected from the group consisting of 1-hexanol, 1-octanol, 1-nonanol and cyclohexanol. A method for separating cesium from the high-level radioactive waste liquid described.   4. The high-level radioactive waste liquid according to claim 1, wherein the high-level radioactive waste liquid is a high-level radioactive waste liquid obtained by separating uranium and plutonium from spent fuel of a light water reactor. How to separate.   4. The method for separating cesium from high-level radioactive waste liquid according to any one of claims 1 to 3, wherein the high-level radioactive liquid waste is a high-level radioactive liquid waste after reprocessing of MOX spent fuel. .   The high-level radioactive waste according to any one of claims 1 to 3, wherein the high-level radioactive waste is a high-level radioactive waste obtained by removing at least part of uranium, plutonium, and neptunium from FBR spent fuel. A method of separating cesium from liquid. 4. The method for separating cesium from a high-level radioactive liquid waste according to any one of claims 1 to 3, wherein the high-level radioactive liquid waste is a high-level radioactive liquid waste containing americium, curium and fission products. .   The cesium is separated from the high-level radioactive liquid waste according to any one of claims 1 to 3, wherein the high-level radioactive liquid waste is a high-level radioactive liquid waste containing neptunium, americium, curium and fission products. how to.

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