KR20210120210A - Method for decontaminating oxide layer - Google Patents

Abstract

The present invention relates to a method for efficiently decontaminating a corrosive oxide layer. Specifically, the present invention provides a method for decontaminating a corrosive oxide layer. The method comprises the steps of: (a) decontaminating a corrosive oxide film by applying, to a pipe line system, decontamination process water containing a pipeline system, sulfuric acid (H_2SO_4), hydrazine (N_2H_4), and copper sulfate (CuSO_4); (b) mixing Ba(OH)_2 with the decontamination process water by which the corrosive oxide layer is decontaminated, to adjust the pH to 8.2 to 9.4, and performing a precipitation reaction; (c) removing insoluble substances by filtering out the decontamination process water subjected to the precipitation reaction; and (d) reusing the decontamination process water from which the insoluble substances are removed, as the decontamination process water in step (a).

Description

Method for decontaminating oxide layer

The present invention relates to a decontamination method of a corrosive oxide film capable of efficiently decontamination of the corrosive oxide film, and more particularly, to a method for effectively decontaminates a thick corrosive oxide film in a carbon steel system.

In order to maintain and dismantle an aging nuclear power plant in operation, system decontamination is essential. The decontamination of the primary system of a light water reactor nuclear power plant is performed through a number of decontamination process cycles, and accordingly, a significant amount of decontamination waste containing radionuclides is generated. Currently, a chemical decontamination process using organic acids is widely used for the purpose of decontamination of the primary system of light water reactors, but waste liquid treatment is not easy, and radioactive waste such as waste ion exchange resin is generated in a significant amount as decontamination process waste.

In order to solve this problem, the Korea Atomic Energy Research Institute has developed a HyBRID (Hydrazine Based Reductive metal Ion Decontamination) decontamination process technology using an inorganic acid as shown in Patent Registration No. 1601201. The decontamination waste generated in this decontamination process can be effectively treated through precipitation and decomposition reactions, so it has the advantage of greatly reducing radioactive waste compared to the existing commercial decontamination process.

On the other hand, in the case of a heavy water reactor system made of carbon steel, the thickness of the corrosion oxide film is several tens of times higher than that of the light water reactor system (light water reactor: 1-3 μm, heavy water reactor: 75 μm or more). At this time, when a thick corrosive oxide film is decontaminated using the existing HyBRID process technology as it is, the pH is greatly increased and the function of the decontaminant is deteriorated, so that a significant amount of sulfuric acid must be used. However, in this case, the concentration of ions in the decontamination process water increases, and the amount of the corrosion oxide film that can be decontaminated reaches a limit, making it difficult to effectively decontaminate.

For this reason, in the case of decontamination of a thick corrosive oxide film, the decontamination process becomes inefficient because the used decontamination process water must be discharged and new decontamination process water must be used. Furthermore, since a significant amount of decontamination waste liquid is generated, there is a problem in that the waste to be disposed of is also significantly increased. In addition, in order to reuse the used decontamination process water without discharging it, ion components in the decontamination process water can be removed using ion exchange resin, but in this case, a significant amount of waste ion exchange resin is generated as radioactive waste. do.

Therefore, technology development that can effectively decontaminate a large amount of corrosive oxide film while minimizing the number of times of discharging the used decontamination process water and injecting new decontamination process water must be accompanied.

An object of the present invention is to provide a decontamination method of a corrosive oxide film that can efficiently decontaminate thick corrosive oxides in a system made of carbon steel with heavy water while remarkably lowering the generation of radioactive waste.

In one aspect of the present invention, the present invention (a) sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ), and copper sulfate (CuSO ₄ ) The process system (pipe line system) Decontamination of the corrosive oxide film by applying to; _{(b) performing a precipitation reaction by mixing Ba(OH) 2} with the decontamination process water in which the corrosive oxide film has been decontaminated to adjust the pH to 8.2 to 9.4; (c) filtering the decontamination process water subjected to the precipitation reaction to remove insoluble substances; and (d) reusing the decontamination process water from which insoluble substances have been removed as the decontamination process water of step (a).

When decontamination of a system with heavy water made of carbon steel by the method of the present invention, since decontamination process water can be reused without discharging decontamination process water, the generation of decontamination waste solution is greatly reduced, and further, the amount of decontamination agent used is reduced to 50-70%. Since it is reduced, it is possible to reduce the amount of finally generated decontamination waste by at least 30% compared to the existing technology. In addition, the present invention is very simple and can be easily applied to existing processes, and in particular, since it can be easily applied to industries and technologies that are in the process of transferring technology to primary decontamination technology for light water reactors, it is a process technology for decontamination of heavy water systems. It is expected to have great commercial use.

In addition, since the method of the present invention does not use an ion exchange resin at all, unlike overseas decontamination technologies proposed as heavy water carbon steel system decontamination, it is expected that the amount of decontamination waste generated can be reduced by more than 60% compared to foreign technologies. . In addition, the final decontamination waste is not a waste ion exchange resin that is difficult to treat, but a solid BaSO ₄ and metal hydroxide with high stability, so handling and stabilization are very easy. Considering these advantages and the applicability of the simple process, it is judged that it will be easy to secure the demand for overseas heavy water system decontamination technology.

1 is a schematic flowchart of a method for decontamination of a corrosive oxide film according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the embodiment of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

The thick corrosive oxide film in the heavy water carbon steel system is mostly composed of iron oxide. Therefore, it is possible to decontaminate the corrosive oxide film using only a reduction process among chemical decontamination processes.

To this end, the present invention provides a method for efficiently decontamination of the corrosive oxide film.

Specifically, the present invention (a) sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ), and copper sulfate (CuSO ₄ ) by applying the decontamination process water containing the process system (pipe line system) to the corrosion oxide film decontamination; _{(b) performing a precipitation reaction by mixing Ba(OH) 2} with the decontamination process water in which the corrosive oxide film has been decontaminated to adjust the pH to 8.2 to 9.4; (c) filtering the decontamination process water subjected to the precipitation reaction to remove insoluble substances; and (d) reusing the decontamination process water from which insoluble substances have been removed as the decontamination process water of step (a).

The above method is a method that can effectively decontaminate a thick corrosive oxide film in a carbon steel system with heavy water. Decontamination containing sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ), copper sulfate (CuSO ₄ ), etc. After performing the reduction decontamination process in the existing HyBRID decontamination process using process water, a significant amount of sulfuric acid (SO ₄ ) ions and iron (Fe) ions (< 1ppm) that cause deterioration of decontamination performance are removed In addition, by adding a small amount of decontamination agent in the reduction process, reuse is possible at a level similar to that of the initial decontamination process water. For this reason, since the decontamination process water used in the reducing decontamination process can be controlled and used again as the decontamination process water conditions of the initial reducing decontamination process without separate discharge, it is possible to effectively decontaminate the thick corrosive oxide film in the system made of carbon steel.

In the present invention, the decontamination process water may refer to a solution containing a decontamination agent in the process water flowing through the process system.

Accordingly, the process system may be a cooling system of a nuclear power plant, and in this case, the cooling system may be a carbon steel system, and further, the carbon steel system may be a heavy water cooling system. The heavy water cooling system has a corrosive oxide film that is several tens of times thicker than that of the light water primary system, for example, the thickness of the corrosive oxide film formed on the primary system of the light water reactor may be 1-3 μm, and formed in the heavy water cooling system The thickness of the corrosion oxide layer may be 50 to 75 μm.

In the corrosion oxide film formed on the surface of the cooling system of a nuclear power plant, NiFe ₂ O ₄ , Fe ₃ O _{4 ,} or Fe ₂ O _{3 ,} etc. are formed due to high dissolved oxygen in the solution in the cooling system. Therefore, the corrosion oxide film to be decontaminated in the present invention may contain iron oxide, and the iron oxide is decontaminated by dissolving in the decontamination process water of the present invention.

To this end, the decontamination process water of the present invention applied to the process system is sulfuric acid (H ₂ SO ₄ ) 25-32mM, hydrazine (N ₂ H ₄ ) 30-70mM, and copper sulfate (CuSO ₄ ) 0.4-0.6mM It may contain a concentration of, preferably 27-30mM sulfuric acid, 45-55mM hydrazine, and may contain copper sulfate at a concentration of 0.45-0.55mM.

At this time, if the concentration of sulfuric acid exceeds 32mM, the pH of the decontamination process water may be lowered, so that corrosion of the base material of the process system may increase, and if it is less than 25mM, the decontamination effect may decrease. In addition, when the concentration of hydrazine exceeds 70 mM or is less than 30 mM, the dissolution performance of the corrosive oxide film may be deteriorated, thereby reducing the decontamination effect. Furthermore, when the concentration of copper sulfate exceeds 0.6 mM, a copper precipitate is formed, an excess of ions is formed, and an additional process may be required to remove the ion component, and when it is less than 0.4 mM, the decontamination effect may be reduced.

Meanwhile, the process of decontamination of the corrosive oxide film by applying the decontamination process water to the process system in step (a) may be performed at a temperature of 93-97° C. for 6 to 10 hours, preferably at a temperature of 95° C. It can be carried out for 6 to 10 hours. However, the time for performing the process may vary depending on the decontamination scale.

_{Furthermore, in the present invention, when Ba(OH) 2} is mixed with the decontamination process water obtained by decontamination of the corrosive oxide film in step (b), the pH of the decontamination process water is 8.2 to 9.4, preferably the pH is 8.35 to 8.65. It can be mixed as much as possible. When the Ba(OH) ₂ is mixed, if the pH of the decontamination process water is less than 8.2, the iron ions in the decontamination process water do not decrease to less than 1 ppm, and further, a large amount of fine iron hydroxide particles in the decontamination process water are generated and filtered. Efficiency is also reduced, so there may be a problem in that the removal of iron is not effective. If the pH exceeds 9.4, a significant amount or most of the reusable decontamination agent in the decontamination process water is removed, resulting in an increase in waste and decontamination agent usage. can occur

To this end, Ba(OH) ₂ in step (b) may be mixed so as to have a mole number of 50 to 70% of the sulfate ions in the decontamination process water, and preferably to a mole number of 55 to 60%. have. When the Ba(OH) ₂ is mixed at less than 50% of the mole number of sulfate ions in the decontamination process water, iron ions in the decontamination process water are not removed to less than 1 ppm, and further fine iron hydroxide particles in the decontamination process water are Because it is generated in large amounts, the filtration efficiency is also reduced, which may cause an ineffective removal of iron components in the decontamination process water. There may be problems such as an increase in the amount of the agent used.

In the decontamination process water in which step (b) has been performed, various insoluble substances are generated due to the precipitation reaction. The insoluble materials may be, for example, BaSO ₄ , Fe(OH) ₂ , Cu(OH) ₂ , and the like. When the insoluble substances remain in the decontamination process water in which step (b) is performed, it may interfere with the reuse of the decontamination process water. Therefore, it is necessary to remove the insoluble substance.

To this end, the present invention performs the step (step (c)) of filtering the decontamination process water subjected to the precipitation reaction. The above step is to remove the insoluble material formed by the precipitation reaction, and may be performed using a filtration device used in the art. For example, the filtration device may be a filtration filter having pores having a diameter of 0.2-0.45 μm. In the decontamination process water that has been filtered as described above, most insoluble substances are removed.

Most of the components (hydrazine, sulfate ion, copper ion, etc.) present in the initial decontamination process water remain in the decontamination process water from which insoluble substances are mostly removed, so it can be used again as the decontamination process water. Therefore, by mixing hydrazine, sulfuric acid and copper sulfate with the decontamination process water in which step (c) has been performed, it is possible to prepare decontamination process water at a level similar to that of the decontamination process water used in step (a). Therefore, the reusing step may be performed by regenerating the decontamination process water by mixing _{sulfuric acid (H 2} SO ₄ ), hydrazine (N ₂ H ₄ ), and copper sulfate (CuSO ₄ ) with the decontamination process water from which insoluble substances have been removed. have.

At this time, by comparing the concentrations of hydrazine, sulfuric acid and copper sulfate contained in the decontamination process water used in step (a) with the concentrations of hydrazine, sulfuric acid and copper sulfate contained in the decontamination process water used in step (c), The components of hydrazine, sulfuric acid and copper sulfate are mixed with the decontamination process water in which step (c) has been performed.

For example, in the decontamination process water regenerated in step (d), sulfuric acid (H ₂ SO ₄ ) is 25-32 mM, hydrazine (N ₂ H ₄ ) is 30-70 mM, and copper sulfate (CuSO ₄ ) is 0.4-0.6 Sulfuric acid, hydrazine and copper sulfate may be mixed to include at a concentration of mM.

The decontamination process water regenerated as described above may be applied to the process system again and used to decontaminate the corrosive oxide film. Accordingly, the present invention may include the step of using the decontamination process water regenerated in step (d) as the decontamination process water of step (a). In this case, steps (a) to (d) may be repeated several to several tens of times without discharging the process water. However, for the efficiency of the process, the repetition may be performed, for example, about 2 to 20 times. Therefore, steps (a) to (d) may be repeated 2 to 20 times without discharging the process water.

That is, the present invention regenerates the initially used decontamination process water, decontaminates the corrosion oxide film again using the regenerated decontamination process water, and repeats the process of regenerating the used decontamination process water. Compared to that, the amount of decontamination agent used can be significantly reduced.

Hereinafter, the present invention will be described in more detail through specific examples. The following examples are merely examples to help the understanding of the present invention, and the scope of the present invention is not limited thereto.

Example

1) Preparation of decontamination process water

As the conditions of the decontamination process water for the reduction process during the HyBRID decontamination process, the concentration of sulfuric acid 20mM (since iron sulfate is used to dissolve iron ions, sulfuric acid is added at a lower content than 25-32mM), hydrazine 31.2mM, and copper sulfate 0.5mM Sulfuric acid, hydrazine and copper sulfate were added to 1 L of distilled water and mixed.

Thereafter, assuming that the corrosion oxide film in the carbon steel of the primary system made of iron oxide was decontaminated, iron sulfate was dissolved in the decontamination process water so that the concentration of iron ions was 350 ppm.

Using this as the decontamination process water that was decontaminated in the cooling system with heavy water, the decontamination process water used in step (a) of the present invention was prepared.

2) Removal of sulfate ions and iron ions

_{Ba(OH) 2} was used to remove sulfate ions and iron ions in the prepared, used decontamination process water.

Based on the number of moles of sulfate ions, the amount of Ba(OH) ₂ used was changed, and the results of measuring the pH and concentration of each component of the used decontamination process water are shown in Table 1.

division Molar ratio (Ba(OH) ₂ /SO ₄ ^2- ) pH Concentration (ppm) hydrazine sulfate ion iron ions copper ion Comparative Example 1 0 2.61 1,010 2,600 300 31 Comparative Example 2 0.45 8.17 1,000 1,460 1.843 0.282 Example 1 0.5 8.27 1,020 1,310 0.714 0.322 Example 2 0.55 8.36 1,000 1,180 0.536 0.216 Example 3 0.6 8.63 1,030 1,020 0.332 0.150 Example 4 0.7 9.31 1,010 790 0.150 0.046 Comparative Example 3 0.75 9.62 1,010 640 0.121 0.038

As a result, as shown in Table 1, it was confirmed that iron ions could be removed to less than 1 ppm even by using only 50% of the total amount of _{Ba(OH) 2 required to precipitate all sulfate ions.} At this time, it was possible to remove the sulfate ion at half the concentration as expected in theory, and it was confirmed that most of the copper ion was removed in the form of a precipitate under all conditions.

On the other hand, since it was confirmed that the concentration of hydrazine hardly changes, hydrazine can be easily prepared as decontamination process water for reduction process decontamination using purified decontamination process water by adding only the amount consumed during the process.

3) Regeneration of decontamination process water

In order to regenerate the decontamination process water purified as described above as the decontamination process water for the reduction process decontamination, hydrazine, sulfate ions and copper ions were added to the purified decontamination process water. At this time, the amount of hydrazine consumed during the decontamination process was added only in an insufficient amount compared to the initial concentration of 1,600 ppm of the decontamination process water.

Each component added is summarized in Table 2, and when each component is added, the pH of the regenerated decontamination process water is set to 2.75.

division molar ratio
(Ba(OH) ₂ /SO ₄ ^2- ) Injection concentration (ppm) hydrazine sulfate ion copper ion Example 1 0.5 600 1,290 31 Example 2 0.55 600 1,420 31 Example 3 0.6 600 1,580 31 Example 4 0.7 600 1,810 31

As a result, as shown in Table 2, it is confirmed that the amount of each component injected is almost similar to the amount removed through precipitation/filtration during the decontamination process. Through this, when the decontamination process water used after reduction process decontamination is purified and then recycled to produce decontamination process water that can be used as decontamination process water, it can be recycled with decontamination process water having a composition almost similar to the initial decontamination process water. It can be seen that

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and variations are possible within the scope without departing from the technical spirit of the present invention described in the claims. It will be apparent to those of ordinary skill in the art.

Claims (10)

(a) sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ), and decontamination of the corrosive oxide film by applying decontamination process water containing copper sulfate (CuSO _{4 ) to the process system (pipe line system);
(b) performing a precipitation reaction by mixing Ba(OH) 2} with the decontamination process water in which the corrosive oxide film has been decontaminated to adjust the pH to 8.2 to 9.4;
(c) filtering the decontamination process water subjected to the precipitation reaction to remove insoluble substances; and
(d) Reusing the decontamination process water from which insoluble substances have been removed as the decontamination process water of step (a).
According to claim 1,
The process system is a system made of carbon steel, a decontamination method of a corrosion oxide film.
According to claim 1,
The process system is a heavy water cooling system, the decontamination method of the corrosive oxide film.
According to claim 1,
The corrosive oxide film includes iron oxide, a decontamination method of the corrosive oxide film.
According to claim 1,
The corrosive oxide film has a thickness of 50-75 μm, a decontamination method of the corrosive oxide film.
According to claim 1,
The decontamination process water of step (a) contains sulfuric acid (H ₂ SO ₄ ) at a concentration of 25-32 mM, hydrazine (N ₂ H ₄ ) at a concentration of 30-70 mM, and copper sulfate (CuSO ₄ ) at a concentration of 0.4-0.6 mM. , Decontamination method of corrosive oxide film.
According to claim 1,
The step (a) is performed at a temperature of 93-97° C. for 6 to 10 hours, a decontamination method of a corrosive oxide film.
According to claim 1,
Ba(OH) ₂ in step (b) is mixed so as to have a molar number of 50 to 70% of sulfate ions in the decontamination process water.
According to claim 1,
The step (d) is performed by regenerating the decontamination process water by mixing _{sulfuric acid (H 2} SO ₄ ), hydrazine (N ₂ H ₄ ), and copper sulfate (CuSO ₄ ) with the decontamination process water from which insoluble substances have been removed. Corrosion Oxide film decontamination method.
According to claim 1,
A method of decontamination of a corrosive oxide film by repeating steps (a) to (d) 2 to 20 times without discharging process water.

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