JP2016114414A - Separation removal method for toxic elements contained in radioactive waste liquid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separation removal method for toxic elements contained in a radioactive waste liquid, capable of simply selectively recovering each of the toxic elements from the radioactive waste liquid that contains the toxic elements each having a super long half-life.SOLUTION: The separation removal method for the toxic elements contained in the radioactive waste liquid comprises: a wet type electrolysis process 10, in which an electrode is immersed in the radioactive waste liquid that contains the toxic elements having super long half-life, and an electrolytic reaction is performed with a voltage to be applied being adjusted on the basis of a standard electrode potential corresponding to the toxic elements such that each of the toxic elements is selectively reduced and deposited on the electrode; and an adsorption process 11 after the deposition of the toxic elements, where cesium contained in the radioactive waste liquid is adsorbed in an adsorbent.SELECTED DRAWING: Figure 1

Description

  Embodiments of the present invention relate to a technique for separating and removing harmful elements from high-level radioactive liquid waste generated when reprocessing spent fuel generated at a nuclear power plant.

  When spent fuel generated at nuclear power plants is reprocessed, high-level radioactive liquid waste containing fission products (FP) and minor actinides (MA) is generated. At present, high-level radioactive liquid waste is to be disposed of as a solidified glass.

  High-level radioactive liquid waste contains radionuclides with ultra-long half-lives, and it is necessary to evaluate and manage the exposure risk over the long term due to the effects of these nuclides. For this reason, the cost of securing disposal sites and managing them is a problem.

  Pd (palladium) -107 (half-life: 6.5 million years), Cs (cesium) -135 (half-life: 2.3 million years) as radionuclides with a very long half-life and high content in high-level radioactive liquid waste ), Zr (zirconium) -93 (half-life: 15.3 million years), Se (selenium) -79 (half-life: 1.13 million years).

  If these radionuclides can be separated and recovered from high-level radioactive waste, and processed and disposed of, the amount of waste and disposal situations can be reduced, safety can be improved, and useful elements can be recycled.

Conventionally, research has been carried out to separate elements in high-level radioactive liquid waste into four groups: a transuranium element group, a strontium / cesium group, a technetium / platinum group element group, and other element groups.
So far, techniques for efficiently separating radionuclides from high-level radioactive liquid waste such as solvent extraction, ion chromatography, and molten salt electrolysis have been developed (for example, Patent Documents 1 to 3). The separated radioactive nuclides such as long-lived radionuclides can be extinguished by a transmutation facility such as an accelerator-driven furnace (ADS).

JP 2011-169888 A Japanese Patent No. 4114076 Japanese Patent No. 4504247

In order to improve the safety of radioactive waste liquid by separating and removing harmful elements from high-level radioactive liquid waste, it is necessary to transmutate the harmful elements and finally eliminate them. Each must be separated with high purity.
In particular, development of a simple process technique capable of selectively separating and removing Pd, Se, Cs, and Zr, which are harmful elements having a very long half-life in a high-level radioactive liquid waste and having a large content, is required. .

  The present invention has been made in consideration of such circumstances, and separation and removal of harmful elements contained in radioactive liquid waste that can easily and selectively recover each of harmful elements from radioactive liquid waste containing harmful elements having an ultra-long half-life. It aims to provide a method.

  In the method for separating and removing harmful elements contained in a radioactive liquid waste according to an embodiment of the present invention, the electrode is immersed in a radioactive liquid waste containing a harmful element having an ultra-long half-life, and is based on a standard electrode potential corresponding to the harmful element. And adjusting the applied voltage to perform an electrolytic reaction, selectively reducing each of the harmful elements to deposit on the electrode, and depositing the harmful elements on the radioactive waste liquid. An adsorption step of adsorbing cesium on the adsorbent.

  The embodiment of the present invention provides a method for separating and removing harmful elements contained in a radioactive liquid waste that can easily and selectively recover each of the harmful elements from the radioactive liquid waste containing a harmful element having an ultra-long half-life.

The flowchart which shows the isolation | separation removal method of the harmful element contained in the radioactive waste liquid which concerns on 1st Embodiment. The flowchart which shows the modification of the separation-and-removal method of the harmful element contained in the radioactive waste liquid which concerns on 1st Embodiment. The flowchart which shows the isolation | separation removal method of the harmful element contained in the radioactive waste liquid which concerns on 2nd Embodiment. The flowchart which shows the isolation | separation removal method of the harmful element contained in the radioactive waste liquid which concerns on 3rd Embodiment. The flowchart which shows the modification of the separation-and-removal method of the harmful element contained in the radioactive waste liquid which concerns on 3rd Embodiment.

(First embodiment)
Hereinafter, this embodiment is described based on an accompanying drawing.
As shown in FIG. 1, the method for separating and removing harmful elements contained in the radioactive liquid waste according to the first embodiment corresponds to the harmful elements by immersing the electrode in the radioactive liquid waste containing the harmful elements having a very long half-life. Electrolytic reaction was carried out by adjusting the voltage applied based on the standard electrode potential, wet electrolysis process 10 in which each of the harmful elements (Pd, Se) was selectively reduced and deposited on the electrode, and the harmful elements were deposited. And an adsorption step 11 for adsorbing Cs contained in the radioactive liquid waste on the adsorbent.

  Further, after adsorbing Cs on the adsorbent, the electrode is immersed in the radioactive liquid waste to perform an electrolytic reaction, and nitrate ions contained in the radioactive liquid waste are reduced to ammonia ions to change the liquidity to alkaline. A precipitation step 12 for precipitating the contained Zr as a hydroxide is included.

  In FIG. 1, high-level radioactive liquid waste generated when reprocessing spent nuclear fuel is targeted for processing. The high-level radioactive liquid waste is usually an acidic (nitric acid) solution containing nitric acid.

  High-level radioactive liquid waste includes long-lived radionuclides such as Ln (lanthanoid) and An (actinoid), short-lived radionuclides such as Cs and Sr, white metal elements (Ru, Rh, Pd, etc.), rare earths, Zr, Mo In this embodiment, Pd, Se, Cs, and Zr having an ultra-long half-life are targeted for separation and removal.

  The present embodiment is not limited to the separation treatment of high-level radioactive liquid waste, and sodium sulfate waste liquid, sodium borate waste liquid, or sodium chloride waste liquid containing harmful elements having an ultra-long half-life may be treated. good.

  In the wet electrolysis process 10, two electrodes to be an anode and a cathode are immersed in a high-level radioactive liquid waste, and a voltage is applied to the electrodes to perform an electrolytic reaction. Then, the voltage to be applied is adjusted based on the standard electrode potential corresponding to each of Pd and Se, and Pd and Se are selectively reduced and deposited on the cathode. Pd and Se can be collected separately by exchanging the cathode and performing an electrolytic reaction.

  In the wet electrolysis process 10, among the four elements to be separated and recovered, Pd and Se, which are elements with a preferential (low) ionization tendency, are deposited on the cathode and separated and removed. Other elements contained in the waste liquid, such as platinum group elements such as Ru and Rh, may be deposited and recovered by adjusting the applied voltage based on the standard electrode potential corresponding to each element.

  In the adsorption step 11, Cs contained in the high-level radioactive liquid waste after depositing and collecting Pd and Se is adsorbed by the adsorbent, and Cs is separated and recovered from the waste liquid. Examples of the adsorbent include zeolite, ferrocyanide, and silicotitanate. In particular, mordenite, which is a kind of zeolite, is suitable for adsorption of Cs.

  The adsorption method only needs to be able to adsorb the Cs contained in the high-level radioactive waste liquid to the adsorbent, such as a method of passing the waste liquid through a column filled with the adsorbent or a method of introducing the waste liquid into a container holding the adsorbent. There is.

In the precipitation step 12, two electrodes serving as an anode and a cathode are immersed in a high-level radioactive waste liquid after adsorbing Cs on an adsorbent, and a voltage is applied to the electrodes to perform an electrolytic reaction. The applied voltage is adjusted so that the reaction of the following formula (1) occurs in the waste liquid, and nitrate ions contained in the waste liquid are reduced to ammonia ions. Due to the generation of ammonia ions, the liquidity of the high-level radioactive liquid waste changes to alkaline.
NO ₃ ⁻ + 10H ⁺ + 8e ⁻ → NH ₄ ⁺ + 3H ₂ O (1)

Zr contained in the high-level radioactive liquid waste has the property that the form of existence changes depending on the pH in the solution. When the liquidity of the liquid waste changes to alkaline, hydroxide (Zr (OH) ₄ ) As precipitate. Zr (OH) ₄ is separated and recovered by solid-liquid separation of the waste liquid after precipitation.

  In the solidification step 13, Zr is precipitated as a hydroxide and separated and recovered, and then the remaining waste liquid is subjected to a solidification treatment such as glass solidification. The solidified waste can be disposed of as low-level radioactive waste because harmful elements are separated and removed.

  Thus, the wet electrolysis process 10 for depositing and collecting Pd and Se by electrolytic reaction, the adsorption process 11 for separating and collecting Cs by adsorbing the adsorbent, and the precipitation process 12 for collecting and collecting Zr as a hydroxide are combined. By sequentially removing harmful elements, four elements having an ultra-long half-life can be selectively separated and removed by a simple process.

  FIG. 2 is a flowchart showing a modification of the harmful element separation and removal method according to the first embodiment. The same steps as those in the harmful element separation / removal flow shown in FIG.

  In this modification, ferrocyanide is used as an adsorbent for adsorbing Cs in the adsorption step 11.

The incineration step 14 incinerates the ferrocyanide after adsorbing Cs to decompose the ferrocyanide. Then, the residue after incineration dryness, recovering Cs as an oxide _(Cs 2 O).

In place of the incineration step 14, the ferrocyanide after adsorbing Cs is introduced into an electrolytic solution of an acid solution or an alkaline solution to perform electrolysis, and the electrolytic solution in which Cs is dissolved is heated and evaporated. , Cs may be recovered as an oxide (Cs ₂ O).

(Second Embodiment)
FIG. 3 is a flowchart showing a method for separating and removing harmful elements according to the second embodiment. In addition, the same code | symbol is attached | subjected to the process same as the separation removal flow of the harmful element which concerns on 1st Embodiment shown in FIG. 1, and description is abbreviate | omitted.

  The second embodiment is different from the method for separating and removing harmful elements according to the first embodiment in that the zeolite after adsorbing Cs and the eluent are mixed, and Cs is dissolved in the eluent to obtain Cs. This is a point having a Cs elution step 15 for separating zeolite. In the second embodiment, zeolite is used as the adsorbent for adsorbing Cs in the adsorption step 11.

  In the Cs elution step 15, the eluent is passed through the zeolite, or the eluent is injected into a container holding the zeolite to mix the zeolite and the eluent. Then, Cs adsorbed on the zeolite is desorbed from the zeolite and dissolved in the eluent to separate Cs from the zeolite.

  As an eluent, either an acid solution or an alkaline solution is used. Examples of the acid solution include a nitric acid solution, a sulfuric acid solution, and a hydrochloric acid solution, and a mixed solution obtained by combining these acid solutions may be used. Examples of the alkaline solution include aqueous ammonia, sodium hydroxide solution, potassium hydroxide solution, and calcium hydroxide solution, and a mixed solution obtained by combining these alkaline solutions may be used.

  The zeolite from which Cs has been separated by the Cs elution step 15 is reused as an adsorbent in the adsorption step 11. By separating Cs from the zeolite using the eluent and reusing the zeolite, the amount of secondary waste generated in the separation and removal process can be reduced.

  Further, when ammonia water is used as the eluent, if ammonia adheres to the zeolite from which Cs has been separated by the Cs elution step 15, the adsorption performance of the reused zeolite may be lowered.

The NH ₃ removal step 16 heats the zeolite from which Cs has been separated by the Cs elution step 15 to evaporate and remove the ammonia attached to the zeolite. Thus, even when ammonia water is used as the eluent, it can be reused as an adsorbent without deteriorating the adsorption performance of the zeolite. In addition, it is necessary to perform the heating of the zeolite at a temperature at which the attached ammonia can be removed by evaporation, and 350 ° C. or higher is preferable.

The ammonia removed by evaporation in the NH ₃ removal step 16 is returned to the Cs elution step 15 and reused as the eluent. By reusing ammonia removed from the zeolite, the amount of secondary waste liquid generated can be reduced.

(Third embodiment)
FIG. 4 is a flowchart showing a method for separating and removing harmful elements according to the third embodiment. In addition, the same code | symbol is attached | subjected to the step same as the separation removal flow of the harmful element in 2nd Embodiment shown in FIG. 3, and description is abbreviate | omitted.

  The third embodiment is different from the method for separating and removing harmful elements according to the second embodiment in that the Cs elution step 15 heats the eluent in which Cs is dissolved to evaporate ammonia and recovers Cs as a hydroxide. It is the point which has the heating process 17 to do. In the adsorption step 11, zeolite is used as an adsorbent for adsorbing Cs, and ammonia is used as an eluent in the Cs elution step 15.

  In the heating step 17, the eluent in which Cs is dissolved in the Cs elution step 15 is heated to evaporate ammonia. Then, the residue after evaporation is dried to recover Cs as a hydroxide (CsOH).

  The ammonia evaporated and removed by the heating step 17 is condensed and returned to the Cs elution step 15 and reused as an eluent. Thus, by reusing ammonia in the eluent in which Cs is dissolved in the Cs elution step 15, the amount of secondary waste liquid generated can be reduced.

  FIG. 5 is a flowchart showing a modification of the harmful element separation and removal method according to the third embodiment. The same steps as those in the hazardous element separation / removal flow shown in FIG.

  In the purification step 18, after the precipitation step 12 in which Zr is precipitated and separated as a hydroxide, the radioactive liquid waste is heated to purify the evaporated gas and recover ammonia. The recovered ammonia is condensed and returned to the Cs elution step 15 to be reused as an eluent.

  According to the method for separating and removing harmful elements contained in the radioactive liquid waste of each embodiment described above, a wet electrolysis process for precipitating and collecting Pd and Se by an electrolytic reaction, an adsorption process for adsorbing and separating Cs on an adsorbent, In addition, by combining the precipitation step of precipitating and collecting Zr as a hydroxide, it is possible to selectively separate and collect four elements having an ultra-long half-life by a simple process.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 10 Wet electrolysis process 11 Adsorption process 12 Precipitation process 13 Solidification process 14 Combustion process 15 Cs elution process 16 Moisture removal process 17 Heating process 18 Purification process

Claims (15)

Immerse the electrode in radioactive waste liquid containing harmful elements with an ultra-long half-life, adjust the voltage applied based on the standard electrode potential corresponding to the harmful elements, perform an electrolytic reaction, and select each of the harmful elements A wet electrolysis process in which the electrode is reduced and deposited on the electrode;
An adsorption step of adsorbing cesium contained in the radioactive liquid waste on an adsorbent after the harmful elements are deposited. A method for separating and removing the harmful elements contained in the radioactive liquid waste.   The radioactive liquid waste according to claim 1, wherein the radioactive liquid waste is a high-level radioactive liquid waste generated from spent nuclear fuel, a sodium sulfate waste liquid, a sodium borate waste liquid, or a sodium chloride waste liquid containing the harmful elements. To remove and remove harmful elements in water.   The method for separating and removing harmful elements contained in a radioactive liquid waste according to claim 1 or 2, wherein the harmful elements are palladium, selenium, cesium, and zirconium.   The method for separating and removing harmful elements contained in a radioactive liquid waste according to any one of claims 1 to 3, wherein the adsorbent is zeolite, ferrocyanide, or silicotitanate. The radioactive liquid waste is a nitric acid acid high level radioactive liquid waste,
After adsorbing the cesium to the adsorbent, an electrode is immersed in the radioactive liquid waste to perform an electrolytic reaction, nitrate ions contained in the radioactive liquid waste are reduced to ammonia ions to change the liquidity to alkaline, The method for separating and removing harmful elements contained in the radioactive liquid waste according to any one of claims 1 to 4, further comprising a precipitation step of precipitating zirconium contained in the radioactive liquid waste as a hydroxide. For the adsorbent, using ferrocyanide,
The radioactive material according to any one of claims 1 to 5, further comprising an incineration step of incinerating the ferrocyanide after adsorbing cesium to separate and recover cesium as an oxide. A method for separating and removing harmful elements contained in waste liquid. For the adsorbent, using zeolite,
2. The method according to claim 1, further comprising a Cs elution step of mixing the zeolite after adsorbing cesium and an eluent to dissolve cesium in the eluent and separating the cesium and the zeolite. A method for separating and removing harmful elements contained in the radioactive liquid waste according to claim 5.   The method for separating and removing harmful elements contained in radioactive liquid waste according to claim 7, wherein the eluent is an acid solution or an alkaline solution.   The method for separating and removing harmful elements contained in a radioactive liquid waste according to claim 8, wherein at least one of a nitric acid solution, a sulfuric acid solution, and a hydrochloric acid solution is used as the acid solution.   9. The method for separating and removing harmful elements contained in radioactive liquid waste according to claim 8, wherein the alkaline solution uses at least one of ammonia water, sodium hydroxide solution, potassium hydroxide solution, and calcium hydroxide solution. . As the eluent, the ammonia water is used,
The radioactive waste liquid according to claim 7, further comprising an NH ₃ removal step of heating the zeolite separated in the Cs elution step to evaporate and remove ammonia adhering to the zeolite. Method for separating and removing contained harmful elements. The method for separating and removing harmful elements contained in radioactive liquid waste according to claim 11, wherein the NH ₃ removing step heats the zeolite separated in the Cs elution step at 350 ° C or higher.   The method for separating and removing harmful elements contained in a radioactive liquid waste according to claim 11 or 12, wherein ammonia removed by evaporation is reused as the eluent in the Cs elution step.   14. The method according to claim 11, further comprising a heating step of heating the eluent in which cesium is dissolved in the Cs elution step to evaporate the ammonia and recovering cesium as a hydroxide. A method for separating and removing harmful elements contained in the radioactive liquid waste according to any one of the items.   The method for separating and removing harmful elements contained in radioactive liquid waste according to claim 14, wherein ammonia evaporated by heating in the heating step is collected and reused as the eluent in the Cs elution step.

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