JP2004170278A - Chemical decontamination method and system for radioactive chemical - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate a process for reducing ferric ion to ferrous ion, to provide a decomposition velocity higher than that by oxalic acid, and to provide decontamination performance equivalent to that of oxalic acid. <P>SOLUTION: This method/system is provided with a reduction-dissolving process S2 for bringing a surface of a radioactive chemical having a radioactive oxide skin on its surface into contact with a reducing decontaminant solution dissolved with a mono-carboxylic acid and a dicarboxylic acid, and an oxidation-dissolving process S6 for bringing the surface of the radioactive chemical into contact with an oxidizing decontaminant solution dissolved with an oxidant. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention chemically dissolves metal oxides containing radioactive substances attached to the surface of structural components such as piping, equipment, and structural members installed in radiation handling facilities such as nuclear power plants, The present invention relates to a method and an apparatus for chemical decontamination of activated components to be removed from wastewater.
[0002]
[Prior art]
In a radiation handling facility, an oxide film containing a radionuclide adheres or forms on an inner surface of a structural component that comes into contact with a fluid containing a radioactive substance during operation. As the operation period becomes longer, the radiation dose increases around the pipes and equipment, and the worker's exposure dose increases during periodic inspection work or demolition work during facility decommissioning. In order to reduce the exposure of workers, a chemical decontamination method of chemically dissolving and removing an oxide film has been put to practical use.
[0003]
To date, various chemical decontamination methods have been proposed, including the step of oxidizing and dissolving chromium-based oxides in an oxide film with an oxidizing agent, and the method of reducing and dissolving iron-based oxides, the main component in the oxide film, with a reducing agent. There is known a method in which steps are combined.
[0004]
Patent Document 1 listed below describes a chemical decontamination method using an aqueous solution of permanganic acid as an oxidizing agent and an aqueous solution of dicarboxylic acid (oxalic acid) as a reducing agent. This decontamination method uses low-concentration oxidizing permanganic acid and oxalic acid, which can be decomposed into carbon dioxide and water, to reduce the amount of secondary waste generated compared to previous chemical decontamination methods. Can be reduced. This method has already been used in decontamination work of nuclear power facilities.
[0005]
Patent Document 2 listed below describes a chemical decontamination method using an aqueous solution of ozone as an oxidizing agent and an aqueous solution of oxalic acid as a reducing agent. Ozone is decomposed into oxygen, and oxalic acid is decomposed into carbon dioxide and water, so it is attracting attention as a decontamination technology that can further reduce secondary waste.
[0006]
Patent Literature 3 listed below proposes a method of decontaminating stainless steel metal waste in an organic acid (oxalic acid or formic acid) aqueous solution. In this method, a metal having a potential more negative than the oxidation-reduction potential of stainless steel is brought into contact, and the stainless steel base material is dissolved and decontaminated. Since the treatment with the organic acid aqueous solution alone is used, the decontamination operation is simple, and the metal base material is dissolved, so that it is effective as a method for decontaminating the radioactivity level of metal waste to the level of general industrial waste.
[0007]
Regarding a method for treating a decontamination waste liquid, Patent Document 4 discloses a method for treating an oxalic acid aqueous solution. Since Fe ³⁺ in the oxalic acid aqueous solution forms a complex with oxalic acid and becomes an anion, it is reduced to Fe ²⁺ by irradiating ultraviolet rays (hν) as shown in the following formula (1).
[Fe (C ₂ O ₄ ) ₃ ] ³⁻ + hν → Fe (C ₂ O ₄ ) ₂ + 2CO ₂ (1)
[0008]
Thereby, Fe ²⁺ in the oxalic acid aqueous solution can be separated by the cationic resin. Further, when oxalic acid is decomposed, as shown in the following formulas (2) and (3), the hydroxyl radical (OH.) Generated by the reaction between hydrogen peroxide (H ₂ O ₂ ) and Fe ²⁺ Decomposes oxalic acid into carbon dioxide and water by oxidizing power.
H ₂ O ₂ + Fe ²⁺ → Fe ³⁺ + OH ⁻ + OH · (2)
H ₂ C ₂ O ₄ + 2OH · → 2CO ₂ + 2H ₂ O (3)
[0009]
[Patent Document 1]
Japanese Patent Publication No. 3-10919 [Patent Document 2]
Japanese Patent Application Laid-Open No. 2000-81498 [Patent Document 3]
Japanese Patent Application Laid-Open No. Hei 9-113690 [Patent Document 4]
Japanese Unexamined Patent Publication No. 9-510784
[Problems to be solved by the invention]
The techniques described in Patent Documents 1 to 4 described above are decontamination techniques effective for reducing the exposure of workers during periodic inspections and inspections of nuclear power plants and the like. However, when oxalic acid is used as a reducing agent, Fe ³⁺ becomes An ultraviolet device for reducing Fe ²⁺ is required. When the line to be decontaminated becomes large, the amount of decontamination liquid increases, and accordingly, the scale of the ultraviolet device also increases, and the cost of the device increases. Further, when the decomposition time of oxalic acid is long, there is a problem that the whole decontamination period is long.
[0011]
In addition, the technology described in Patent Document 3 uses formic acid as a decontamination agent. Formic acid electrochemically dissolves a metal base material, and is used for decontamination to maintain the soundness of equipment. I can not use it. In addition, there is a problem that sufficient decontamination performance cannot be obtained because the oxide film and iron oxide formed on the device surface cannot be dissolved and removed simply by formic acid treatment alone.
[0012]
Therefore, the present invention does not require a process and an apparatus for reducing trivalent iron ions to divalent iron ions, has a higher decomposition rate than oxalic acid, and has the same decontamination performance as oxalic acid. It is an object of the present invention to provide a chemical decontamination method and apparatus.
[0013]
[Means for Solving the Problems]
The invention according to claim 1 is a chemical decontamination method, in which a surface of an activated component having a radioactive oxide film on its surface is brought into contact with a reducing decontamination solution in which monocarboxylic acid and dicarboxylic acid are dissolved, And an oxidizing and dissolving step of bringing the surface of the activation component into contact with an oxidizing decontamination liquid in which an oxidizing agent is dissolved.
[0014]
The invention according to claim 2 is configured such that the reduction dissolving step and the oxidation dissolving step are performed alternately a plurality of times.
The invention according to claim 3 is configured such that the monocarboxylic acid is formic acid and the dicarboxylic acid is oxalic acid.
[0015]
The invention according to claim 4 is configured such that the oxidizing agent constituting the oxidizing decontamination liquid is any one of ozone, permanganate, and permanganate.
The invention according to claim 5 is configured so that Fe ²⁺ ions and Fe ³⁺ ions eluted in the reducing decontamination liquid are separated and removed from the decontamination liquid by a cationic resin.
[0016]
In the invention according to claim 6, the monocarboxylic acid remaining in the reducing decontamination liquid is decomposed into hydrogen gas and the dicarboxylic acid is decomposed into carbon dioxide gas and water by ozone dissolved in the oxidizing decontamination liquid. Configuration.
The invention according to claim 7 is configured such that hydrogen peroxide or ozone remaining in the reducing decontamination liquid is decomposed into water or oxygen by ultraviolet rays.
[0017]
The invention according to claim 8 is characterized in that the reducing decontamination liquid contains carbonic acid, carbonate or bicarbonate, boric acid or borate, sulfuric acid or sulfate, phosphoric acid or phosphate or phosphoric acid as a corrosion inhibitor for the activated component. It is configured to add any one of hydrogen salts.
[0018]
The invention according to claim 9 is a chemical decontamination apparatus, which includes a decontamination tank that accommodates an activation component having a radioactive oxide film on a surface and contacts the surface with a decontamination liquid, and that is connected to the decontamination tank, A circulation system for circulating the decontamination solution, wherein the circulation system is a drug injection device for injecting a reducing agent and a hydrogen peroxide composed of monocarboxylic acid and dicarboxylic acid into the decontamination solution, An ion exchange device for separating and removing metal ions and an ozone generator for injecting ozone into the decontamination liquid are provided.
[0019]
The invention according to claim 10 is also a chemical decontamination apparatus, which includes a decontamination tank that accommodates an activated component having a radioactive oxide film on the surface and contacts the surface with a decontamination liquid, and that is connected to the decontamination tank. A circulation system for circulating the decontamination solution, wherein the circulation system comprises a decontamination solution containing a reducing agent comprising monocarboxylic acid and dicarboxylic acid and an oxidizing agent comprising hydrogen peroxide and permanganic acid or permanganate. It is configured to include a drug injection device for injection and an ion exchange device for separating and removing metal ions in the decontamination solution.
An eleventh aspect of the present invention is configured such that the circulation system includes a liquid phase decomposition device that decomposes hydrogen peroxide and ozone remaining in the decontamination liquid by ultraviolet rays.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a method and an apparatus for chemically decontaminating an activated component according to an embodiment of the present invention will be described with reference to the drawings.
As shown in FIG. 1, the method for chemical decontamination of an activated component according to the present embodiment includes a step S1 of setting the activated component in a chemical decontamination apparatus, and the step of removing oxides on the surface of the activated component from formic acid and oxalic acid. A step S2 of reducing and dissolving with a decontamination solution comprising a mixed aqueous solution, a separation step S3 of separating and removing metal ions such as iron dissolved in the decontamination solution in the step S2 from the decontamination solution by an ion exchange resin, A reducing agent decomposition step S4 for decomposing formic acid and oxalic acid remaining without being consumed in S2 with ozone and hydrogen peroxide, and a separation step S5 for separating and removing the ozone and hydrogen peroxide remaining in step S4 from the decontamination liquid. A step S6 of oxidizing and dissolving an oxide film on the surface of the activation component with a decontamination solution in which ozone is dissolved, and a separation step S7 of separating and removing the oxidation product in the step S6 from the decontamination solution. Comprising the step S8 for discharging decontaminated effluent. Note that the oxidation dissolution step S6 and the separation step S7 may be performed before the reduction dissolution step S2.
[0021]
FIG. 2 is an example of a chemical decontamination apparatus that decontaminates used equipment generated as a replacement part by a periodic inspection of a nuclear power plant. That is, the chemical decontamination apparatus for activated parts of the present embodiment includes a decontamination tank 1 for storing the decontamination liquid 1a, and a circulation system 2 connected to the decontamination tank 1 and circulating the decontamination liquid 1a. The circulation system 2 is connected to a circulation pump 3, a heater 4, a chemical injection device 5, a liquid phase decomposition device 6, a cationic resin tower 7, a mixed bed resin tower 8, a mixer 9 and an ozone generator 10. Have been. The mixed bed resin tower 8 is filled with a mixture of a cationic resin and an anionic resin. Further, the decontamination tank 1 is connected to a gas phase decomposition tower 11 and an exhaust blower 12.
[0022]
The activated component 13 to be decontaminated is installed in the decontamination tank 1 and immersed in the decontamination liquid 1a. Alternatively, although not shown, a shower of the decontamination liquid 1a is received. The decontamination liquid 1a is pumped by the circulation pump 3, circulated in the circulation system 2, and returned to the decontamination tank 1.
[0023]
When reducing and dissolving the oxide film on the surface of the activation component 13, a reducing aqueous solution containing formic acid and oxalic acid is supplied to the circulation system 2 from the chemical injection device 5. Iron ions dissolved in the reducing decontamination liquid are separated and removed by the cation resin tower 7.
[0024]
After the reduction decontamination, the reducing decontamination liquid is injected with ozone gas from the ozone generator 10 into the circulation system 2 through the mixer 9 or supplied with hydrogen peroxide from the chemical injection device 5 to generate carbon dioxide gas. Decomposes in water. Further, metal ions dissolved in the decontamination liquid 1a are removed in the cationic resin tower 7. When ozone or hydrogen peroxide remains when the decontamination liquid 1a is passed through the cationic resin tower 7, the ozone is decomposed into oxygen by irradiating the liquid phase decomposition device 6 with ultraviolet rays. Hydrogen peroxide decomposes into oxygen and water.
[0025]
When oxidizing and dissolving the oxide film on the surface of the activation component 13, the ozone gas from the ozone generator 10 is injected into the circulation system 2 via the mixer 9, and the ozone water is supplied to the decontamination liquid 1a. The ozone gas discharged from the decontamination tank 1 is sucked into the exhaust blower 12, decomposed in the gas phase decomposition tower 11, and discharged to the existing exhaust system. After the decontamination is completed, the decontamination liquid in the apparatus is passed through the mixed-bed resin tower 8 to be purified.
[0026]
The oxide film formed on the stainless steel surface can be dissolved and removed by formic acid alone in combination with the oxidation treatment, but iron oxide is hardly dissolved by this formic acid alone. In order to dissolve the iron oxide, in this embodiment, about 1/10 of oxalic acid is added to the formic acid concentration. Formic acid can be decomposed in a short time with hydrogen peroxide alone as described below, and if it is a low concentration of oxalic acid, it can be decomposed in a short time with ozone, permanganic acid and potassium permanganate used for the oxidation treatment, Significant reduction in the decontamination work period is possible.
[0027]
Ozone, permanganate, and permanganate (potassium permanganate) can be used as the oxidizing agent to oxidize the surface of the activation component. Improving the dissolution and removal speed of the oxide film by combining with formic acid Can be.
[0028]
Since the equilibrium constant of the complex formation reaction between Fe ²⁺ ion and Fe ³⁺ ion and formic acid is small, both ions can be adsorbed and separated by the cationic resin. Therefore, there is no need for a device for reducing Fe ³⁺ ions to Fe ²⁺ ions as in the case of using oxalic acid.
[0029]
Formic acid can be decomposed in a short time by hydrogen peroxide alone, but oxalic acid is not easily decomposed by hydrogen peroxide alone. Oxalic acid remaining after formic acid decomposition is decomposed by ozone, permanganic acid and potassium permanganate used in the oxidation treatment. Since the oxalic acid concentration is about 1/10 of the formic acid concentration, it can be decomposed in a short time.
[0030]
Hydrogen peroxide or ozone remaining after the decomposition treatment promotes deterioration of the cationic resin. In order to prevent this, hydrogen peroxide is decomposed into water and ozone is decomposed into oxygen by ultraviolet rays.
[0031]
Furthermore, when the formic acid concentration approaches the lower limit of detection, the ozone concentration in the decontamination liquid increases. Since there is a concern about the corrosion of the equipment due to the oxidizing power of ozone, a decomposition aid is added to suppress the base material corrosion of the equipment and maintain the material integrity of the equipment.
[0032]
Next, test data for confirming the oxide film dissolving performance of the chemical decontamination method of the present embodiment using the chemical decontamination apparatus shown in FIG. 2 will be described. The water quality condition of the primary system of the boiling water nuclear power plant was simulated, and an oxide film dissolution test was performed for 3,000 hours on a SUS304 specimen provided with an oxide film.
[0033]
The test results are shown in FIG. The vertical axis in the figure indicates the weight loss of the oxide film, and the horizontal axis indicates the formic acid concentration.印 indicates the result of treatment with an aqueous solution of formic acid followed by treatment with an aqueous solution of formic acid, and △ indicates the result of treatment with an aqueous solution of permanganic acid followed by treatment with an aqueous solution of formic acid. Also, for comparison with the present embodiment, results of treatment with an oxalic acid aqueous solution after treatment with an ozone aqueous solution (marked with ▽) and results of treatment with a formic acid aqueous solution alone (marked with □) are also shown as conventional examples.
[0034]
The ozone treatment conditions are a concentration of 5 ppm and a temperature of 80 ° C. for 2 hours, and the permanganate treatment conditions are a concentration of 300 ppm and a temperature of 95 ° C. for 2 hours. The formic acid treatment conditions were immersion for 1 hour at a concentration of 100 to 50,000 ppm and a temperature of 95 ° C. The oxalic acid treatment conditions were immersion for 1 hour at a concentration of 2000 ppm and a temperature of 95 ° C.
[0035]
As can be seen from the test results, the oxide film hardly dissolves in the formic acid treatment alone (concentration 2000 ppm). On the other hand, when the ozone treatment and the formic acid treatment were used in combination in this embodiment, the oxide film was dissolved together with the formic acid concentration. When compared at a formic acid concentration of 1000 ppm or more, a result about 5 times larger than that of the formic acid alone treatment was obtained. The result is equivalent to the conventional combination of the ozone treatment and the oxalic acid treatment.
[0036]
Also, the combined use of permanganate treatment and formic acid treatment of the present embodiment also has an effect of dissolving the oxide film, and the amount of dissolution is smaller than that of the combination treatment with ozone, but the result is about three times larger than that of the treatment with formic acid alone. Was. In addition, potassium permanganate was selected as a permanganate, treated with potassium permanganate at a concentration of 300 ppm (immersion at a temperature of 95 ° C. for 2 hours), and then treated with formic acid (concentration of 2000 ppm at a temperature of 95 ° C. for 1 hour). The same effect was obtained by performing ()).
[0037]
Next, FIG. 4 shows a test result confirming dissolution of iron oxide (Fe ₃ O ₄ ) in the chemical decontamination method of the present embodiment. At a formic acid concentration of 2000 ppm and a temperature of 95 ° C., the horizontal axis indicates the oxalic acid concentration, and the vertical axis indicates the dissolved Fe concentration. As can be seen from the test results, formic acid alone did not dissolve the iron oxide, but the addition of oxalic acid dissolved the iron oxide and increased the iron concentration almost in proportion to the oxalic acid concentration.
[0038]
As described above, in the chemical decontamination method of the present embodiment, ozone, permanganic acid or permanganate is used as the oxidation treatment, and the mixed decontamination solution of formic acid and oxalic acid is used as the reduction treatment, so that the stainless steel can be used. The oxide film and iron oxide formed on the surface can be efficiently dissolved.
[0039]
Since the radioactive substance is incorporated in the oxide film on the surface of the activated component, the radioactive substance can be decontaminated from the surface of the activated component by dissolving and removing the oxide film, thereby reducing the exposure of workers. The oxide film on the stainless steel surface can be removed by using formic acid alone in combination with the oxidation treatment.However, since formic acid alone hardly dissolves iron oxide, it is decontaminated compared to a mixed decontamination solution of formic acid and oxalic acid. Performance is considered poor.
[0040]
When permanganic acid or permanganate is used for the oxidation treatment in the chemical decontamination apparatus shown in FIG. 2, the ozone generator 10, the mixer 9 and the gas phase decomposition tower 11 are unnecessary, and only the chemical injection apparatus 5 is used. Can perform chemical decontamination of activated parts.
[0041]
Next, a third test result will be described with reference to FIGS. FIG. 5 shows the result of separating Fe ²⁺ ions and Fe ³⁺ ions dissolved in a mixed decontamination solution of formic acid (2000 ppm) and oxalic acid (200 ppm) of the present embodiment using a cationic resin, and FIG. FIG. 7 shows the results of a separation test from a single decontamination solution of oxalic acid (2000 ppm) alone.
[0042]
Explaining the test results from the conventional example, Fe ²⁺ and Fe ³⁺ ions dissolved in the formic acid-only decontamination solution shown in FIG. 6 could be separated by the cationic resin. In the oxalic acid-only decontamination solution shown in FIG. 7, Fe ²⁺ ions could be separated by the cationic resin, but Fe ³⁺ ions could not be separated. This is because Fe ³⁺ forms a complex with oxalic acid and exists as an anion as shown in the above formula (1). In order to separate Fe ³⁺ ions, ultraviolet rays (hν) are irradiated to reduce them to divalent Fe compounds as shown in the above formula (1), or they are separated by an anionic resin in the form of oxalic acid complexes. There is a need.
[0043]
On the other hand, in the mixed decontamination solution of formic acid and oxalic acid of the present embodiment shown in FIG. 5, Fe ²⁺ ions and Fe ³⁺ ions could be separated by the cation resin in the same manner as the formic acid single decontamination solution. This is because, as shown in the following equation (4), since the H ⁺ of Fe ³⁺ ions and formic acid to form the oxalic acid complexes have been replaced, Fe ³⁺ ion is believed that could be separated by cation resins.
^{_{[Fe (C 2 O 4)}} 3] 3- + 6HCOOH + Fe (COOH) 3 + 3H 2 C 2 O 4 ... (4)
[0044]
As described above, when a mixed aqueous solution of formic acid and oxalic acid is used as the reducing agent, an ultraviolet device and a step of reducing Fe ³⁺ ions are not required as compared with the oxalic acid aqueous solution. It becomes possible.
[0045]
Next, a fourth test result will be described with reference to FIG. FIG. 8 shows a decomposition test result of the mixed aqueous solution of formic acid and oxalic acid (△ mark, Δ mark) of the present embodiment, the conventional aqueous solution of oxalic acid alone (□ mark), and the conventional aqueous solution of formic acid alone (印 mark). The test conditions were as follows: the concentration of both formic acid alone and oxalic acid alone was 2,000 ppm, the mixed aqueous solution was 2,000 ppm formic acid and 100 ppm oxalic acid, the temperature was 90 ° C., and 20 ppm of Fe ions were dissolved in each aqueous solution.
[0046]
In the decomposition method, the mixed aqueous solution first decomposes formic acid with hydrogen peroxide (addition amount: 1.5 equivalents) (marked with △), and then ozone (O ₃ generation amount / liquid amount: 75 g / h / m ³⁾ ) To decompose oxalic acid (▽). The aqueous solution of oxalic acid alone is a combination of ultraviolet light (output / liquid volume: 3 kw / m ³ ) and hydrogen peroxide (addition amount: 1.5 equivalents), and the aqueous solution of formic acid alone is hydrogen peroxide (addition amount: 1. (5 equivalents) alone.
[0047]
Explaining the test results from a conventional example, the aqueous solution of oxalic acid alone was decomposed to an organic carbon concentration of 10 ppm or less in 10 hours by using hydrogen peroxide and ultraviolet light in combination. In addition, formic acid was decomposed with only hydrogen peroxide to an organic carbon concentration of 10 ppm or less in 2 hours.
[0048]
On the other hand, in the mixed aqueous solution of the present embodiment, formic acid is decomposed by hydrogen peroxide alone, but oxalic acid is not decomposed by hydrogen peroxide alone. Then, after formic acid decomposition, oxalic acid was continuously decomposed by ozone used for the oxidation treatment, and the organic carbon concentration was decomposed to 10 ppm or less in less than 4 hours in total. Oxalic acid can also be decomposed by other oxidizing aqueous solutions such as permanganate and potassium permanganate.
[0049]
The reason for not decomposing formic acid with an oxidizing aqueous solution is that ozone alone can be decomposed, but it takes as much time as oxalic acid is decomposed by hydrogen peroxide and ultraviolet rays, so the decomposition time is not shortened. is there. In the case of permanganate and potassium permanganate, the decomposition reaction is very slow, and the above decomposition time is required.
[0050]
As described above, although the mixed aqueous solution of formic acid and oxalic acid has a slower decomposition time than formic acid alone, it can be decomposed in about half the time of oxalic acid that has been used as a decontamination agent. Further, as shown in the above equations (2) and (3), the decomposition of oxalic acid requires an ultraviolet device to generate Fe ²⁺ ions. ^{Since the} step of reducing ³⁺ ions is not required, the overall cost of decontamination work can be reduced.
[0051]
The concentration range of formic acid and oxalic acid used in the reducing decontamination agent is preferably 1000 ppm to 5000 ppm for formic acid and 50 ppm to 300 ppm for oxalic acid in consideration of decontamination performance and reduction time.
[0052]
Next, as a fifth test result, a process of decomposing hydrogen peroxide and ozone remaining after the decomposition process of the mixed decontamination solution of formic acid and oxalic acid will be described. Iron ions and radioactive substances eluted in the decontamination solution are separated by the ion exchange resin. However, if hydrogen peroxide and ozone remain in the decontamination solution, the oxidative deterioration of the ion exchange resin is promoted. there is a possibility. In order to prevent this, the decontamination liquid is irradiated with ultraviolet rays (hν), and hydrogen peroxide and ozone are decomposed into water and oxygen by the reactions shown in the following equations (5) and (6).
Decomposition of hydrogen peroxide: H ₂ O ₂ + hv → O ₂ + 2H ⁺ + 2e ⁻ (5)
Decomposition of ozone: O ₃ + hν → O + O ₂ (6)
[0053]
In order to confirm the above reaction, a decomposition test using ultraviolet light of hydrogen peroxide and ozone remaining in the decontamination liquid (formic acid concentration: 10 ppm or less) was performed. FIG. 9 shows the results of the hydrogen peroxide decomposition test, and FIG. 10 shows the results of the ozone decomposition test. At an output of 3 kW / m ³ of ultraviolet light, hydrogen peroxide having an initial concentration of 20 ppm was decomposed to 1 ppm or less in 1.5 hours, and ozone having an initial concentration of 5.5 ppm was decomposed to 0.1 ppm or less in 12 minutes.
[0054]
As described above, hydrogen peroxide or ozone remaining during or after the decomposition of formic acid in the decontamination liquid can be decomposed by ultraviolet rays, so that the eluted metal ions can be separated without reducing the exchange capacity of the ion exchange resin. . Accordingly, the amount of used ion exchange resin generated as secondary waste can be reduced.
[0055]
In FIG. 2, a liquid phase decomposition device 6 for irradiating ultraviolet rays is used only for decomposing hydrogen peroxide or ozone remaining in the decontamination liquid to secure the integrity of the ion exchange resin. If there is no residual hydrogen peroxide and ozone, or if there is no residual metal ion separation treatment with an ion exchange resin even if it remains, the liquid phase decomposition device 6 is unnecessary.
[0056]
In addition, as a corrosion inhibitor for suppressing corrosion of stainless steel coming into contact with ozone water as an oxidizing agent, there are carbonic acid, carbonate, bicarbonate, boric acid, borate, sulfuric acid, sulfate, phosphoric acid, phosphate, phosphate, and the like. Although it is said that the effect of adding a hydrogen salt is effective, also in the present embodiment, since ozone gas is supplied at the time of oxalic acid decomposition, the above-mentioned corrosion inhibitor is used for the stainless steel base material at the time of oxalic acid decomposition treatment. It was confirmed that it was effective in controlling corrosion.
[0057]
According to the chemical decontamination method and apparatus for an activated component of the present embodiment, the following effects can be obtained. That is, a method of decontaminating a structural component of a radiation handling facility as a decontamination target and chemically dissolving an oxide film containing a radioactive substance generated or adhered to the surface of the activated component as an individual decontamination target. In the decontamination, the decontamination is performed by alternately contacting a reducing decontamination solution in which formic acid, a monocarboxylic acid and oxalic acid, a dicarboxylic acid, are mixed and dissolved, and an oxidizing decontamination solution in which an oxidizing agent is dissolved. Substances can be efficiently removed.
[0058]
In addition, since the Fe ³⁺ ions eluted in the reducing mixed decontamination solution can be separated by the cation resin, a reduction device and a reduction step for reducing Fe ³⁺ ions to Fe ²⁺ ions are not required. Overall costs can be reduced.
[0059]
Furthermore, formic acid in a reducing mixed decontamination solution can be decomposed only with hydrogen peroxide, and oxalic acid at a low concentration can be decomposed in an oxidizing aqueous solution in a short time, so that a divalent Fe compound as a decomposition catalyst is generated. Since the reduction device and the reduction step are not required, the overall cost of the decontamination device can be reduced.
[0060]
【The invention's effect】
According to the present invention, there is no need for a step and an apparatus for reducing trivalent iron ions to divalent iron ions, the decomposition rate is higher than that of oxalic acid, and activation having the same decontamination performance as oxalic acid. A method and apparatus for chemical decontamination of parts can be provided.
[Brief description of the drawings]
FIG. 1 is a flowchart showing a chemical decontamination method for an activated component according to an embodiment of the present invention.
FIG. 2 is a system diagram showing a chemical decontamination apparatus for an activated component according to an embodiment of the present invention.
FIG. 3 is a dissolution curve diagram of an oxide film for explaining the effects of the method and apparatus for chemically decontaminating activated parts according to the embodiment of the present invention.
FIG. 4 is a dissolution curve diagram of an oxide film for explaining the effect of the method and apparatus for chemically decontaminating activated parts according to the embodiment of the present invention.
FIG. 5 is a bar graph showing results of a separation test of iron ions in a reducing mixed aqueous solution using a cationic resin, and illustrating effects of the embodiment of the present invention.
FIG. 6 is a bar graph showing the results of a separation test of iron ions in a formic acid aqueous solution using a cationic resin, and illustrating the effects of the embodiment of the present invention.
FIG. 7 is a bar graph showing the results of a separation test of iron ions in an oxalic acid aqueous solution using a cationic resin, and illustrating the effect of the embodiment of the present invention.
FIG. 8 is a curve diagram showing the results of a decomposition test of a reducing mixed aqueous solution and explaining the effect of the embodiment of the present invention.
FIG. 9 is a curve diagram showing the results of a decomposition test of residual hydrogen peroxide and explaining the effect of the embodiment of the present invention.
FIG. 10 is a curve diagram showing the results of a decomposition test of residual ozone and explaining the effect of the embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Decontamination tank, 1a ... Decontamination liquid, 2 ... Circulation system, 3 ... Circulation pump, 4 ... Heater, 5 ... Chemical injection device, 6 ... Liquid phase decomposition device, 7 ... Cation resin tower, 8 ... Mixed bed resin Tower, 9 mixer, 10 ozone generator, 11 gas phase decomposition tower, 12 exhaust blower, 13 activation parts.

Claims (11)

A reducing and dissolving step in which the surface of the activated component having a radioactive oxide film on its surface is brought into contact with a reducing decontamination solution in which monocarboxylic acid and dicarboxylic acid are dissolved, and an oxidation in which an oxidizing agent is dissolved in the surface of the activated component And a step of oxidizing and dissolving the activated component in contact with an aqueous decontamination solution. The method according to claim 1, wherein the reduction dissolution step and the oxidation dissolution step are alternately performed a plurality of times. 3. The method according to claim 1, wherein the monocarboxylic acid is formic acid, and the dicarboxylic acid is oxalic acid. 3. The method according to claim 1, wherein the oxidizing agent constituting the oxidizing decontamination liquid is one of ozone, permanganic acid and permanganate. The method for chemically decontaminating activated parts according to claim 1 or 2, wherein Fe ²⁺ ions and Fe ³⁺ ions eluted in the reducing decontamination liquid are separated and removed from the decontamination liquid by a cationic resin. 2. The monocarboxylic acid remaining in the reducing decontamination liquid is decomposed into hydrogen gas and dicarboxylic acid is decomposed into carbon dioxide gas and water by ozone dissolved in the oxidizing decontamination liquid. Or the method for chemical decontamination of activated parts according to 2 above. 7. The method according to claim 6, wherein the hydrogen peroxide or ozone remaining in the reducing decontamination liquid is decomposed into water or oxygen by ultraviolet rays. Any of carbonic acid, carbonate or bicarbonate, boric acid or borate, sulfuric acid or sulfate, phosphoric acid or phosphate or hydrogen phosphate is added to the reducing decontamination solution as a corrosion inhibitor for the activated component. The method for chemical decontamination of activated parts according to claim 6, characterized in that: A decontamination tank containing a radioactive component having a radioactive oxide film on the surface and contacting the surface with a decontamination liquid; and a circulation system connected to the decontamination tank and circulating the decontamination liquid. The system includes a chemical injection device for injecting a reducing agent comprising monocarboxylic acid and dicarboxylic acid and hydrogen peroxide into the decontamination solution, an ion exchange device for separating and removing metal ions in the decontamination solution, An ozone generator for injecting ozone into a liquid. A decontamination tank containing a radioactive component having a radioactive oxide film on the surface and contacting the surface with a decontamination liquid; and a circulation system connected to the decontamination tank and circulating the decontamination liquid. A system is a chemical injection device for injecting a reducing agent comprising monocarboxylic acid and dicarboxylic acid and an oxidizing agent comprising hydrogen peroxide and permanganic acid or permanganate into the decontamination solution, and a metal in the decontamination solution. A chemical decontamination apparatus for an activated component, comprising: an ion exchange apparatus for separating and removing ions. 11. The chemical decontamination of an activated part according to claim 9, wherein the circulation system includes a liquid phase decomposition device that decomposes hydrogen peroxide and ozone remaining in the decontamination liquid by ultraviolet rays. apparatus.

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