JP3866402B2 - Chemical decontamination method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a chemical decontamination method for chemically decontaminating metal oxides containing radioactive substances attached to piping and equipment installed in a nuclear power generation facility.
[0002]
[Prior art]
In piping and equipment systems installed in nuclear power generation facilities, an oxide film containing a radioactive substance adheres to or is generated on the inner surfaces of the piping and equipment, etc., during operation. For this reason, the radiation dose around pipes and equipment has increased, and problems have arisen due to an increase in radiation exposure dose during periodic inspection work.
[0003]
As part of its removal measures, chemical decontamination is partly effective. Various methods of chemical decontamination are presented. For example, there is a method of oxidizing and dissolving an oxide film with a mixed aqueous solution of nitric acid and potassium permanganate, and then reducing and dissolving the oxide film with a mixed solution of vanadium divalent ions and an organic acid. This method is effective when dissolving an oxide film having a high chromium oxide content.
[0004]
[Problems to be solved by the invention]
However, in the above decontamination method, the decontamination agents nitric acid, manganese, potassium and vanadium become secondary waste after the decontamination, so a large amount of secondary disposal is required when the piping equipment system of nuclear facilities is decontaminated. There was a problem that things were generated.
[0005]
Further, for example, in Japanese Patent Laid-Open No. 6-99193, in a chemical decontamination method for dissolving and removing piping and equipment clad of nuclear power plant, oxidation dissolution treatment is performed with a cleaning solution using persulfuric acid and persulfate, followed by cleaning. A method is described in which a reducing agent such as ascorbic acid, erythorbic acid, or oxalic acid is added to the waste liquid to perform reductive dissolution.
[0006]
This publication uses persulfuric acid and persulfate, but does not disclose a production method for persulfuric acid. Persulfuric acid is not commercially available. A large amount of chemical decontamination agent is required to dissolve and remove the cladding of piping and equipment of nuclear power plants. Therefore, the technique which can mass-produce this persulfuric acid easily with a certain method becomes a subject.
[0007]
In order to solve the above-mentioned problems, the inventors of the present application have found that persulfuric acid can be easily produced in large quantities from peroxodisulfate (hereinafter referred to as persulfate) as a result of intensive studies.
[0008]
In addition, the inventors of the present application conducted a dissolution test of a test piece provided with an oxide film in a water quality environment simulating the water quality conditions of a nuclear power plant using the persulfuric acid aqueous solution obtained in large quantities from the above method as a decontamination agent, In addition to confirming the properties, a manufacturing method using persulfuric acid as a decontamination agent, a decontamination method using the decontamination agent, and a method for treating a decontamination waste liquid were established.
[0009]
The present invention has been made to solve each of the above-mentioned problems, can mass-produce an aqueous solution of persulfuric acid as a decontamination agent, can easily treat the decontamination waste liquid after the completion of decontamination, and is a secondary waste. It aims at providing the chemical decontamination method with little generation | occurrence | production amount. It is another object of the present invention to provide a chemical decontamination method that has a higher decontamination effect than current decontamination methods and can reduce exposure during decontamination work.
[0010]
[Means for Solving the Problems]
The invention corresponding to claim 1 is a chemistry that generates an aqueous solution of peroxodisulfuric acid from an aqueous solution of peroxodisulfate and oxidizes and dissolves an oxide film containing a radionuclide on the surface of the object to be decontaminated by the oxidizing power of the peroxodisulfuric acid. In the decontamination method, the peroxodisulfuric acid is formed by an exchange reaction between a hydrogen ion of a cation exchange resin and a salt of peroxodisulfate.
[0011]
The invention corresponding to claim 2 is characterized in that the peroxodisulfate comprises at least one of sodium peroxodisulfate, potassium peroxodisulfate, or ammonium peroxodisulfate.
[0012]
Corresponding to claim 3 invention produces an aqueous solution of peroxodisulfuric sulfate from an aqueous solution of ammonium peroxodisulfate, oxidizing dissolve oxide film containing radioactive nuclides decontaminated object surface by oxidizing power of the peroxodisulfuric sulfate In the chemical decontamination method, the ammonium ion separated by electrodialysis of the ammonium peroxodisulfate is recovered in an alkaline aqueous solution, and then the ammonium ion is converted to ammonia in the alkaline aqueous solution and recovered in the gas phase. To do.
[0013]
The invention corresponding to claim 4 is characterized in that the ammonium gas recovered in the gas phase is dissolved in an aqueous potassium bromide solution, and then ozone gas is supplied to the aqueous solution to convert it into nitrogen gas.
[0017]
In the present invention, the peroxodisulfuric acid used (hereinafter abbreviated as persulfuric acid) is an emergency redox potential of 2.01 V in the following redox system, as described in the electrochemical handbook, etc. It is an oxidizing agent with strong oxidizing power.
[0018]
_{^{S 2 O 8 2- + 2e -}} = 2SO 4 2- + 2.01V (1)
Therefore, if this reagent can be used for chemical decontamination, it is possible to effectively oxidize and dissolve the oxide film containing radioactive substances in the piping equipment system.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
In the first embodiment of the present invention, a method for producing an aqueous persulfate solution from an aqueous solution of persulfate using an ion exchange resin will be described.
[0020]
FIG. 1 shows an example of an apparatus for a method for producing persulfuric acid. A persulfate aqueous solution 2 is passed through a supply tank 1 through a resin tower 5 containing a cation exchange resin 4 with a pump 3, and the aqueous solution that has passed through the resin tower 5 is recovered in a recovery tank 6. In this operation, the hydrogen ion of the cation exchange resin (R-2H) and the salt ion of persulfate are exchange-reacted to produce an aqueous persulfate solution 7.
[0021]
Sodium peroxodisulfate selected as persulfate (hereinafter abbreviated as sodium persulfate), potassium peroxodisulfate (hereinafter abbreviated as potassium persulfate) and ammonium peroxodisulfate (hereinafter abbreviated as ammonium persulfate) And the exchange reaction between the cation exchange resin and the cation exchange resin.
Na ₂ S ₂ O ₈ + R-2H → H ₂ S ₂ O ₈ + R-2Na (2)
K ₂ S ₂ O ₈ + R-2H → H ₂ S ₂ O ₈ + R-2K (3)
(NH ₄ ) ₂ S ₂ O ₈ + R-2H → H ₂ S ₂ O ₈ + R-2 (NH ₄ ) (4)
[0022]
In order to confirm the above reaction, an aqueous solution 2 having a sodium persulfate concentration adjusted to 20 mmoldm ⁻³ is placed in a supply tank 1 and supplied to a resin tower 5 containing a cation exchange resin 4 by a pump 3 to be contained in a recovery tank 7. The sodium concentration was measured with an inductively coupled radio frequency plasma analyzer. As a result, the sodium concentration in the supply tank 1 was 1300 ppm, whereas the sodium concentration in the recovery tank 6 was 10 ppm or less, and it was confirmed that persulfuric acid was produced.
[0023]
Thus, a commercially available aqueous persulfate solution can be produced by an exchange reaction between a persulfate and a cation exchange resin. In addition, since persulfuric acid can be produced in a general treatment facility, the used ion exchange resin generated in the production process can be easily disposed of.
[0024]
Next, in the second embodiment of the present invention, a method for producing an aqueous solution of persulfate from an aqueous solution of persulfate by electrodialysis will be described. FIG. 2 shows the principle of electrodialysis for producing persulfuric acid. In electrodialysis in which an anode 8 and a cathode 9 and a cation exchange membrane 10 are arranged therebetween, an anode chamber 11 partitioned by the anode 8 and the cation exchange membrane 9 is shown. Then, an aqueous solution 12 of persulfate is supplied, and an alkaline solution 14 is supplied to the cathode chamber 13 partitioned by the cathode 9 and the cation exchange membrane 10 to perform electrodialysis.
[0025]
Here, an ammonium persulfate aqueous solution is selected as the aqueous solution 12 of persulfate, and a sodium hydroxide (NaOH) aqueous solution is selected as the alkaline aqueous solution 7. When electrodialysis is performed in this state, ammonium ions in the aqueous solution of ammonium persulfate are converted into a cation exchange membrane. It moves to the cathode chamber 13 through 10. Ammonium ions that have migrated to the cathode chamber 13 migrate into the gas phase over time as ammonia (NH ₃ ) gas in an aqueous sodium hydroxide solution by an ion equilibrium reaction described below.
NH ₄ ⁺ + OH ⁻ → NH ₃ ↑ + H ₂ O (5)
[0026]
On the other hand, an aqueous solution 15 of persulfuric acid [peroxodisulfuric acid (H ₂ S ₂ O ₈ )] in which persulfate ions and hydrogen ions are combined is generated and recovered in the anode chamber 11, and an aqueous sodium hydroxide (NaOH) solution is recovered from the cathode chamber 13. 16 are collected.
[0027]
For sodium persulfate and potassium persulfate selected as persulfates, sodium ions and potassium ions migrate to the cathode chamber 13 through the cation exchange membrane 10 as in the case of ammonium persulfate. The alkaline (NaOH) aqueous solution 14 is preferably a sodium hydroxide aqueous solution when electrodialyzing a sodium persulfate aqueous solution, and a potassium hydroxide aqueous solution when electrodialyzing a potassium persulfate aqueous solution.
[0028]
Thus, a commercially available persulfate aqueous solution can be produced from a persulfate by electrodialysis. In addition, since persulfuric acid can be produced at a general treatment facility other than a nuclear facility, a used alkaline aqueous solution generated during the production process can be easily disposed of. The used alkaline aqueous solution can be stored and used again for the production of persulfuric acid.
[0029]
Next, in the third embodiment of the present invention, a method of selecting ammonium persulfate as a persulfate and decomposing ammonia gas generated in the process of producing an aqueous solution of persulfate by electrodialysis to nitrogen gas will be described with reference to FIG. To do.
[0030]
This is because ammonium ions cause eutrophication of water, and in recent years, water quality standards for nitrogen content released into the environment tend to be stricter. That is, when ammonia gas separated from ammonium persulfate dissolves in wastewater and is present in a large amount, it causes water eutrophication and is harmful to the environment.
[0031]
Therefore, in the present embodiment, first, ammonium ions are separated from the ammonium persulfate aqueous solution 17 by electrodialysis in the separation step 18 to generate the persulfate aqueous solution 19. Since the separated ammonia ions move into the gas phase, the ammonium ions are decomposed into nitrogen gas 21 in the decomposition step 20.
[0032]
The ammonium gas 20 transferred into the gas phase is decomposed into nitrogen gas 22 in the decomposition step 21 by the reaction shown in the following formulas (6) and (7) by bromine ions (BR ⁻ ) and ozone gas (O ₃ ). And detoxify.
Br ⁻ + H ₂ O + O ₃ → HBrO + OH ⁻ + O ₂ (6)
3HBrO + 2NH ₃ → 3Br ⁻ + 3H ⁺ + N ₂ + 3H ₂ O (7)
[0033]
In order to confirm this decomposition reaction, ammonium gas separated from persulfate was introduced into a potassium bromide aqueous solution having a potassium bromide concentration of 0.02 mol dm ^-3 for a predetermined time to dissolve the ammonium gas in the aqueous solution.
[0034]
Next, ozone gas was supplied for a predetermined time to decompose ammonium ions (ammonia). As a result, the ammonium ion in the potassium bromide aqueous solution before supplying ozone gas, which was about 200 ppm, decreased to 10 ppm or less by supplying ozone gas.
[0035]
As described above, ammonium ions generated in the process of producing persulfuric acid as a decontamination agent from ammonium persulfate can be decomposed into nitrogen gas, and thus do not adversely affect the environment.
[0036]
Next, in the fourth embodiment of the present invention, an implementation flow in the case of decontaminating a piping equipment system of a nuclear power generation facility using persulfuric acid generated from persulfate will be described with reference to FIG.
[0037]
First, as shown in the first embodiment of the present invention, an aqueous persulfuric acid solution as a decontamination agent is produced in the oxidizing agent generation step 23. Next, the oxide film on the surface of the piping system which is the decontamination target device 24 is reduced and dissolved with an oxalic acid aqueous solution in the first reduction and dissolution step 25. Here, iron oxide (Fe ₂ O ₃ ), which is the main component of the oxide film, dissolves in the reaction shown below.
[0038]
Fe ₂ O ₃ + (COOH) ₂ + 4H ⁺
→ 2Fe ²⁺ + 2CO ₂ + 3H ₂ O (8)
The dissolved Fe ²⁺ or radio nuclides such as Co ²⁺ are adsorbed and removed by the cation exchange resin in the first separation step 26. Next, in the first decomposition step 27, excess reducing agent (oxalic acid) is decomposed into carbon dioxide (CO ₂ ) and water by ultraviolet rays and hydrogen peroxide, or ultraviolet rays and ozone gas.
[0039]
Next, in the oxidation dissolution step 28, the oxide film on the surface of the piping system is oxidized and dissolved with the persulfuric acid aqueous solution produced in the oxidant generation step 23. Here, the hardly soluble chromium oxide (Cr ₂ O ₃ ) in the oxide film is dissolved by the reaction shown below.
Cr ₂ O ₃ + 3S ₂ O ₈ ^2- + 4H ₂ O
→ Cr ₂ O ₇ ^2- + 6SO ₄ ^2- + 8H ⁺ (9)
[0040]
Dichromate ions (Cr ₂ O ₇ ²⁻ ) dissolved here, sulfate ions (SO ₄ ²⁻ ) generated by decomposition of persulfuric acid, and Fe ²⁺ and Fe ³⁺ dissolved from the oxide film are In the separation step 29 of 2, the cation exchange resin and the anion exchange resin are adsorbed and removed.
[0041]
Next, the remaining oxide film is dissolved by the reaction shown in the equation (8) in the second reduction and dissolution step 30, the eluted metal is separated in the third separation step 31, and the excess reducing agent is separated in the second decomposition step 32. Disassemble with.
[0042]
These steps are repeated by measuring the radioactivity concentration in the decontamination solution, the radiation dose in the atmosphere, etc., and returning to the first reduction and dissolution step 25, which is the first decontamination step, as necessary. The decontamination waste liquid after decontamination is already provided as waste water 34 of the decontamination waste liquid, which is almost close to ion exchange water, because the eluted metal and the like are separated by the cation exchange resin and the anion exchange resin in the fourth separation step 33. Can be discharged into a liquid waste treatment system.
[0043]
Next, in order to confirm the effect of the fourth embodiment, a SUS304 test piece provided with an oxide film by simulating the water quality condition of a boiling water reactor with a temperature of 280 ° C. and a dissolved oxygen concentration of 200 ppb. The results of the dissolution test will be described.
[0044]
The test conditions were oxidation dissolution with an aqueous solution of persulfuric acid at a concentration of 20 mmoldm ^{−3 at} a temperature of 95 ° C. and reduction dissolution with an aqueous solution of oxalic acid at an concentration of an oxalic acid of 22 mmoldm ⁻³ and a temperature of 95 ° C. For comparison, oxidation dissolution with an aqueous sodium persulfate solution was performed at a sodium sulfate concentration of 20 mmoldm ⁻³ , and reduction dissolution with an oxalic acid aqueous solution at an oxalic acid concentration of 22 mmoldm ⁻³ and a temperature of 95 ° C.
[0045]
The test results are shown in FIG. In FIG. 5, the vertical axis indicates the amount of oxide film dissolved, and the horizontal axis indicates the type of oxidizing agent. The oxide film dissolved about 20 mgdm ^-3 when oxidized with an aqueous persulfate solution, but dissolved about 1/2 when oxidized with an aqueous sodium persulfate solution. This result shows that the persulfuric acid aqueous solution generated in the oxidizing agent generating step can be applied as an oxidizing agent for chemical decontamination.
[0046]
Next, a decomposition test of oxalic acid as a reducing agent was performed. Two kinds of decomposition methods were tested: an ultraviolet / hydrogen peroxide method and an ultraviolet / ozone gas method. In the ultraviolet / hydrogen peroxide method, an equivalent amount of hydrogen peroxide was placed in a 22 mmoldm ⁻³ aqueous solution of oxalic acid, and this aqueous solution was irradiated with ultraviolet light with an output of 100 W using a high-pressure mercury lamp. As a result, the organic carbon concentration in the aqueous solution decreased to 10 ppm or less in 3 hours.
[0047]
The ultraviolet / ozone gas method was irradiated with ultraviolet rays of output 100w using a high pressure mercury lamp in aqueous solution of oxalic acid 22Mmoldm ^-3, was bubbled ozone gas simultaneously 0.5gdm ^-3. As a result, the organic carbon concentration in the aqueous solution decreased to 10 ppm or less in 4 hours.
[0048]
In addition to ultraviolet rays, the reducing agent can be decomposed by a combination of radiation such as β rays or γ rays and oxygen, hydrogen peroxide, or ozone gas as an oxidizing substance.
[0049]
As described above, the chemical decontamination method according to the present embodiment generates non-commercial persulfuric acid from persulfate, and this makes it possible to effectively treat the oxide film on the surface of piping equipment such as a reactor primary system. The dissolved metal (including radionuclides) in the decontamination waste liquid can be separated by an ion exchange resin.
[0050]
Therefore, according to this embodiment, compared with the case where the conventional neutralization treatment is performed, the waste liquid treatment is easy, and the radiation exposure of the worker accompanying the work can be reduced. Moreover, since oxalic acid used for the reducing agent can be decomposed into carbon dioxide gas and water, it can be discharged to an existing liquid waste treatment system.
[0051]
【The invention's effect】
The present invention has the following effects.
(1) By generating persulfuric acid, which is a strong oxidant, from an aqueous solution of persulfate and using it as a decontamination agent, it is possible to effectively dissolve the oxide film on the surface of piping equipment such as the reactor primary system. it can.
[0052]
(2) Since persulfuric acid can be produced in large quantities outside of nuclear power plants, secondary waste (ion exchange resin, alkaline aqueous solution, etc.) generated as a result of its production can be easily disposed of. it can.
[0053]
(3) The elution metal of decontamination waste liquid and sulfuric acid produced by reduction of persulfuric acid can be separated by ion-exchange resin, so it is easier to process decontamination waste liquid than neutralization, etc. Exposure can be reduced.
(4) Since oxalic acid, which is a reducing agent, can be decomposed into carbon dioxide and water, it can be discharged into an existing liquid waste treatment system and the amount of secondary waste generated can be reduced.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an apparatus for producing a persulfuric acid aqueous solution in an embodiment of a chemical decontamination method according to the present invention.
FIG. 2 is a process chart schematically showing the principle of generating a persulfuric acid aqueous solution in the embodiment of the chemical decontamination method according to the present invention.
FIG. 3 is a flowchart showing a decomposition process of a by-product (ammonia) after producing a persulfuric acid aqueous solution in the embodiment of the chemical decontamination method according to the present invention.
FIG. 4 is a flowchart showing a case of decontamination of a piping equipment system of a nuclear power generation facility by the chemical decontamination method according to the present invention.
FIG. 5 is a bar diagram showing the result of an oxide film dissolution test for explaining the effect of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Supply tank, 3 ... Supply pump, 4 ... Cation exchange resin, 5 ... Resin tower, 6 ... Recovery tank, 8 ... Anode, 9 ... Cathode, 10 ... Cation exchange membrane, 11 ... Anode chamber, 13 ... Cathode chamber, 14 ... NaOH, 15 ... H 2 S 2 O 8, 16 ... NaOH, 17 ... aqueous solution of ammonium persulfate, 18 ... ammonium ion separation step, 19 ... persulfate aqueous solution, 20 ... ammonia gas, 21 ... decomposition step, 22 ... Nitrogen gas, 23 ... oxidant generation step, 24 ... decontamination target device, 25 ... first reduction and dissolution step, 26 ... first separation step, 27 ... first decomposition step, 28 ... oxidation dissolution step, 29 ... Second separation step II, 30 ... second reduction and dissolution step, 31 ... third separation step, 32 ... second decomposition step, 33 ... fourth separation step, 34 ... drainage of decontamination waste liquid.

Claims (4)

  In the chemical decontamination method, an aqueous solution of peroxodisulfuric acid is produced from an aqueous solution of peroxodisulfate, and an oxide film containing a radionuclide on the surface of the decontamination object is oxidized and dissolved by the oxidizing power of the peroxodisulfuric acid. Is a chemical decontamination method comprising a product produced by an exchange reaction between a hydrogen ion of a cation exchange resin and a salt of peroxodisulfate. 2. The chemical decontamination method according to claim 1, wherein the peroxodisulfate comprises at least one of sodium peroxodisulfate, potassium peroxodisulfate, or ammonium peroxodisulfate. To form an aqueous solution of ammonium peroxodisulfate aqueous solution from peroxodisulfate sulfuric acid, in a chemical decontamination method of oxidizing dissolving oxide film containing radioactive nuclides decontaminated object surface by oxidizing power of the peroxodisulfuric sulfate, the peroxodisulfuric A chemical decontamination method comprising recovering ammonium ions separated by electrodialysis of ammonium sulfate into an alkaline aqueous solution, and then converting the ammonium ions into ammonia with the alkaline aqueous solution and recovering it in the gas phase. 4. The chemical decontamination method according to claim 3, wherein the ammonium gas recovered in the gas phase is dissolved in an aqueous potassium bromide solution, and then ozone gas is supplied to the aqueous solution to convert it into nitrogen gas.

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