CA3099352C - Method for decontaminating oxide layer - Google Patents

Abstract

The present disclosure relates to a method for decontaminating an oxide layer, capable of efficiently decontaminating the oxide layer. In detail, the present disclosure relates to a method for decontaminating an oxide layer, including (a) supplying decontamination process water including sulfuric acid (B2SO4), hydrazine (N2B4), and copper sulfate (CuSO4) to a pipeline system, to decontaminate an oxide layer in the pipeline system; (b) mixing Ba(OH)2 with the decontamination process water, formed after step (a) decontaminating the oxide layer, to adjust a pH of the decontamination process water to have a range of 8.2 to 9.4, to perform a precipitation reaction;(c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is performed, to remove an insoluble substance from the decontamination process water; and (d) reusing the decontamination process water, formed after step (c) from which the insoluble substance has been removed, as the decontamination process water of step (a).

Description

METHOD FOR DECONTAMINATING OXIDE LAYER
[0001] Intentionally left blank.
BACKGROUND
1. FIELD

[0002] The present disclosure relates to a method for decontaminating an oxide layer, capable of efficiently decontaminating the oxide layer, and more particularly, to a method for effectively decontaminating a relatively thick oxide layer formed in a carbon steel material system.
2. DESCRIPTION OF RELATED ART

[0003] Decontamination for a system is essential for maintenance of a nuclear power plant in step, and dismantling of an aged nuclear power plant. Decontamination for a primary system of a light-water nuclear power plant maybe carried out in a number of decontamination cycles, and accordingly, a considerable amount of decontamination waste liquid containing radionuclides may be generated. Currently, although chemical Date Recue/Date Received 2022-02-23 decontamination processes using organic acids may be widely used for decontamination of the primary system of a light-water nuclear power plant, there are problems that waste liquid generated from such processes is not easy to treat, and radioactive waste, such as waste ion exchange resins, is generated as waste in the decontamination process.

[0004] In order to solve these problems, the Korea Atomic Energy Research Institute (KAERI) has developed a hydrazine-based reductive metal ion decontamination (HyBRID) process using inorganic acids, as disclosed in Korean Patent No. 1601201. Since decontamination waste liquid generated in this process may be effectively treated by precipitation and decomposition reactions, this process has an advantage of greatly reducing radioactive waste, as compared to the conventional commercial decontamination process.

[0005] Meanwhile, a thickness of an oxide layer formed in a system of a heavy-water reactor made of a carbon steel material, in a heavy-water nuclear power plant, may be tens of times greater than a thickness of an oxide layer formed in a system of a light-water reactor (the light-water reactor: 1 to 3pm, the heavy-water reactor: 75pm or more) . In this case, when the existing HyBRID process is used as is to decontaminate a relatively thick oxide layer, a pH of a decontamination agent may greatly increase, and a function of the decontamination agent may be deteriorated. Therefore, a situation in which a Date Recue/Date Received 2020-11-16 considerable amount of sulfuric acid should be used may be accompanied. In this case, a concentration range of ions in the decontamination process water may increase to reach a limit of an amount of an oxide layer, capable of being decontaminated.
Therefore, it is difficult to effectively perform decontamination.

[0006] For this reason, when a relatively thick oxide layer is decontaminated, since used decontamination process water should be discharged and new decontamination process water should be supplied, a process for decontamination becomes inefficient. Furthermore, since a significant amount of decontamination waste liquid may be generated, waste to be disposed may also significantly increase. In addition, in order to reuse used decontamination process water without discharging the used decontamination process water, it is possible to consider a method of removing ionic components by using an ion exchange resin, but in this case, there is a problem that a considerable amount of waste ion exchange resin may be generated as radioactive waste.

[0007] Therefore, there is demand for a technology capable of effectively decontaminating a relatively large amount of oxide layer while minimizing the number of times of discharging used decontamination process water and supplying new decontamination process water to be developed.

Date Recue/Date Received 2020-11-16 SUMMARY

[0008] An aspect of the present disclosure is to provide a method for decontaminating an oxide layer, capable of effectively decontaminating a relatively large amount of corrosive oxides formed in a system of a heavy-water reactor made of a carbon steel material while significantly reducing the occurrence of radioactive waste.

[0009] According to another aspect of the present disclosure, a method for decontaminating an oxide layer, includes (a) supplying decontamination process water including sulfuric acid (H2SO4), hydrazine (N2B4), and copper sulfate (CuSO4) to a pipeline system, to decontaminate an oxide layer in the pipeline system; (b) mixing Ba(OH)2 with the decontamination process water, formed after step (a) decontaminating the oxide layer, to adjust a pH of the decontamination process water to have a range of 8.2 to 9.4, to perform a precipitation reaction; (c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is performed, to remove an insoluble substance from the decontamination process water; and (d) reusing the decontamination process water, formed after step (c) from which the insoluble substance has been removed, as the decontamination process water of step (a).
BRIEF DESCRIPTION OF DRAWINGS

Date Recue/Date Received 2020-11-16

[0010] The above and other aspects, features, and advantages of the present disclosure will be more clearly understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:

[0011] FIG. 1 is a schematic flowchart illustrating a method for decontaminating an oxide layer according to an embodiment of the present disclosure.
DE TAILED DESCRIPTION

[0012] Hereinafter, preferred embodiments of the present disclosure will be described with reference to the accompanying drawings. However, embodiments of the present disclosure may be modified in various other forms, and the scope of the present disclosure is not limited to embodiments described below.

[0013] A relatively thick oxide layer in a carbon steel material system of a heavy-water reactor may be mostly composed of iron oxide. For example, 99 wt% or more of the oxide layer may be composed of iron oxide. Therefore, it is possible to decontaminate the oxide layer using only a reduction process, among chemical decontamination processes.

[0014] The present disclosure provides a method for efficiently decontaminating the oxide layer.

[0015] In detail, the present disclosure provides a method for decontaminating an oxide layer, including (a) supplying decontamination process water including sulfuric Date Recue/Date Received 2020-11-16 acid (H2SO4) , hydrazine (N2H4) , and copper sulfate (CuSO4) to a pipeline system, to decontaminate an oxide layer in the pipeline system; (b) mixing Ba (OH)2 with the decontamination process water, formed after step (a) decontaminating the oxide layer, to adjust a pH of the decontamination process water to have a range of 8.2 to 9.4, to perform a precipitation reaction;
(c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is performed, to remove an insoluble substance from the decontamination process water; and (d) reusing the decontamination process water, formed after step (c) from which the insoluble substance has been removed, as the decontamination process water of step (a) .
In the present disclosure "the decontamination process water of step (b) " means the decontamination process water formed after step (a) and supplied to the step (b) , and "the decontamination process water of step (c) " means the decontamination process water formed after step (b) and supplied to the step (c) .

[0016] The above method may be a method capable of efficiently decontaminating a relatively thick oxide layer in a carbon steel material system of a heavy-water reactor. In this case, decontamination process water containing sulfuric acid (H2SO4) hydrazine (N2H4) copper sulfate (CuSO4), and the like in predetermined concentrations may be used to perform a reductive decontamination step in an existing HyBRID decontamination Date Recue/Date Received 2020-11-16 process, significant amounts of sulfuric acid (SO4) ions and iron (Fe) ions (< 1ppm) , which cause deterioration of decontamination performance, may be almost removed, and further, a decontamination agent in a reduction step may be added in a relatively small amount, to reuse this decontamination process water in a manner similar to the decontamination process water used in the initial step. For this reason, since the decontamination process water used in the reductive decontamination step may be used again under conditions of the decontamination process water of the initial reductive decontamination step without separately discharging the decontamination process water, it is possible to effectively decontaminate the relatively thick oxide layer in the system made of a carbon steel material.

[0017] In the present disclosure, the decontamination process water may refer to a solution in which a decontamination agent is included in process water flowing through a pipeline system.

[0018] Therefore, the pipeline system may be a cooling system of a nuclear power plant. In this case, the cooling system may be a system made of a carbon steel material, and further, the system made of a carbon steel material may be a cooling system of a heavy-water reactor. An oxide layer having a thickness of several tens of times more than a thickness of a primary system of a light-water reactor may be formed in the cooling system of the heavy-water reactor. For example, a thickness of an oxide Date Recue/Date Received 2020-11-16 layer formed on the primary system of the light-water reactor may be 1 to 3pm, and a thickness of an oxide layer formed on the cooling system of the heavy-water reactor may be 50 to 75pm.

[0019] NiFe204, Fe304, Fe2O3, or the like may be formed in an oxide layer formed on a surface of a cooling system of a nuclear power plant, due to relatively high dissolved oxygen in a solution in the cooling system. Therefore, the oxide layer as a decontamination target of the present disclosure may contain iron oxide, and the iron oxide may be decontaminated by being dissolved in the decontamination process water of the present disclosure.

[0020] To this end, initial decontamination process water of the present disclosure supplied to the pipeline system may contain the sulfuric acid (H2SO4) in a concentration range of 25 to 32mM, the hydrazine (N2H4) in a concentration range of 30 to 70mM, and the copper sulfate (CuSO4) in a concentration range of 0.4 to 0.6mM. Preferably, the initial decontamination process water may contain the sulfuric acid in a concentration range of 27 to 30mM, the hydrazine in a concentration range of 45 to 55mM, and the copper sulfate in a concentration range of 0.45 to 0.55mM.

[0021] In this case, when a concentration range of sulfuric acid exceeds 32mM, a pH of the decontamination process water may be lowered to increase corrosion of a base material of the pipeline system, and when a concentration range of sulfuric acid Date Recue/Date Received 2020-11-16 is less than 25mM, a decontamination effect may decrease. In addition, when a concentration range of hydrazine exceeds 70mM
or is less than 30mM, a dissolution performance of the oxide layer may decrease to reduce a decontamination effect.
Furthermore, when a concentration range of copper sulfate exceeds O. 6mM, a precipitate of copper may be formed, and excessive ionic components may be formed. In this case, an additional process may be required to remove ionic components.
When a concentration range of copper sulfate is less than 0.4mM, an decontamination effect may be reduced.

[0022] A process of supplying the decontamination process water to the pipeline system to decontaminate the oxide layer in step (a) may be performed at a temperature range of 93 to 97 C for 6 to 10 hours, and preferably at a temperature of 95 C
for 6 to 10 hours. A time period for performing the above process may be changed, depending on a scale of decontamination.

[0023] Furthermore, in step (b) of the present disclosure, when the Ba (OH) 2 is mixed with decontamination process water of step (b) formed after decontaminating the oxide layer, the mixing step may be configured that a pH of the decontamination process water of step (b) may be 8.2 to 9.4, preferably 8.35 to 8.65.
In the case that the Ba (OH) 2 is mixed, when a pH of the decontamination process water is less than 8.2, iron ions in the decontamination process water may not decrease to less than 1 ppm, and further, a relatively large amount of fine iron Date Recue/Date Received 2020-11-16 hydroxide particles in the decontamination process water may be generated to reduce filtration efficiency. Therefore, there is a problem that iron components may not be effectively removed.
When a pH of the decontamination process water exceeds 9.4, a significant amount or a majority of a decontamination agent, capable of being reused, in the decontamination process water may be removed, to have problems resulting in an increase in amounts of waste and a decontamination agent to be used, and the like.

[0024] To this end, in step (b) , the Ba (OH)2 may be mixed to have a molar number of 50 to 70% of sulfate ions in the decontamination process water of step (b) , and preferably, a mole number of 55 to 60% of sulfate ions in the decontamination process water. When the Ba (OH) 2 is mixed to have a molar number of less than 50% of sulfate ions in the decontamination process water of step (b) , iron ions in the decontamination process water may not decrease to less than 1 ppm, and further, a relatively large amount of fine iron hydroxide particles in the decontamination process water may be generated to reduce filtration efficiency. Therefore, there is a problem that iron components may not be effectively removed. When the molar number of sulfate ions in the decontamination process water exceeds 70%, a significant amount or most of a decontamination agent, capable of being reused, in the decontamination process water may be removed, to have problems resulting in an increase in Date Recue/Date Received 2020-11-16 amounts of waste and a decontamination agent to be used, and the like.

[0025] Various insoluble substances may be generated in the decontamination process water formed after step (b) , due to the precipitation reaction. The insoluble substances may be, for example, BaSO4, Fe (OH) 2, Cu (OH) 2, or the like. When the insoluble substances remain in the decontamination process water formed after step (b) , it may interfere with reuse of the decontamination process water. Therefore, it is necessary to remove the insoluble substance.

[0026] To this end, the present disclosure may perform a step (c) of filtering the decontamination process water formed after step (b) in which the precipitation reaction is performed. The above step (c) may be performed to remove the insoluble substance formed by the precipitation reaction, and may be performed using a filtration device used in the art. For example, as the filtration device, a filter having pores having a diameter range of 0.2 to 0 . 45pm may be used. Most of the insoluble substances may be removed from the decontamination process water filtered as described above.

[0027] Since most of the components (hydrazine, sulfate ions, copper ions, and the like) present in the initial decontamination process water remain in the decontamination process water from which most of the insoluble substances are removed, the decontamination process water may be used again Date Recue/Date Received 2022-02-23 as the initial decontamination process water of step (a) .
Therefore, hydrazine, sulfuric acid, and copper sulfate may be mixed with the decontamination process water formed after step (c) , to prepare decontamination process water in a manner similar to the decontamination process water used in step (a) .
Therefore, the reusing step may be performed by mixing at least one of sulfuric acid (H2SO4) , hydrazine (N2H4) , or copper sulfate (CuSO4) with the decontamination process water from which the insoluble substances is removed, to regenerate the decontamination process water.

[0028] In this case, by comparing concentrations of hydrazine, sulfuric acid, and copper sulfate contained in the decontamination process water used in step (a) , to concentrations of hydrazine, sulfuric acid, and copper sulfate contained in the decontamination process water formed after step (c) , hydrazine, sulfuric acid, and/or copper sulfate, as a component lacking therein, may supplement and be mixed with the decontamination process water formed after step (c) .

[0029] For example, hydrazine, sulfuric acid, and copper sulfate may be mixed with the decontamination process water regenerated in step (d) , to include the sulfuric acid (H2SO4) in a concentration range of 25 to 32mM, the hydrazine (N2H4) in a concentration range of 30 to 70mM, and the copper sulfate (CuSO4) in a concentration range of 0.4 to 0.6mM.

[0030] The regenerated decontamination process water may be Date Recue/Date Received 2020-11-16 supplied to the pipeline system again, to decontaminate an oxide layer. Therefore, the present disclosure may include a step of using the decontamination process water recycled in step (d) as the initial decontamination process water of step (a). In this case, steps (a) to (d) may be repeated several to tens of times without discharging process water. For efficiency of the method, 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 process water.

[0031] For example, the present disclosure may repeatedly perform a step of regenerating initially used decontamination process water, a step of decontaminating an oxide layer reusing the regenerated decontamination process water, and an step of regenerating the reused decontamination process water, to significantly reduce an amount of a decontamination agent, as compared to the existing decontamination process.

[0032] Hereinafter, the present disclosure will be described in more detail by specific examples. The following examples are only examples to assist in an understanding of the present disclosure, and the scope of the present disclosure is not limited thereto.
Example 1) Preparation of Decontamination Process Water

[0033] Sulfuric acid, hydrazine, and copper sulfate were supplied to and mixed with 1 L of distilled water, to have the Date Recue/Date Received 2022-02-23 sulfuric acid at a concentration of 20mM, the hydrazine at a concentration of 31.2mM, and the copper sulfate at a concentration of 0.5mM, as conditions of decontamination process water after a reduction step, in a HyBRID
decontamination process.

[0034] Thereafter, assuming that an oxide layer in a carbon steel material of a primary system made of iron oxide was decontaminated, iron sulfate was dissolved in the decontamination process water such that a concentration of iron ions was 350 ppm. In this case, 350 ppm of iron ions were adjusted using the iron sulfate, and as a result, it was confirmed that a final concentration of sulfuric acid was 27mM.

[0035] This was used as decontamination process water in which an decontamination step was performed in a cooling system of a heavy-water reactor, to prepare used decontamination process water in which step (a) of the present disclosure was performed.
2) Removal of Sulfate Ion and Iron Ion

[0036] Ba(OH)2 was used to remove sulfate ions and iron ions in the prepared and used decontamination process water.

[0037] Based on the number of moles of sulfate ions, an amount of Ba(OH)2 was changed, and Table 1 illustrated the results in which a pH of the used decontamination process water and a concentration of each of the components were measured.
[Table 1]
Molar Ratio pH Concentration (ppm) Date Recue/Date Received 2020-11-16 (Ba(OH)2/E042) Hydrazine Sulfate Ion Iron Ion Copper Ion CE1 0 2.61 1,010 2,600 350 31 CE2 0.45 8.17 1,000 1,460 1.843 0.282 TEl 0.5 8.27 1,020 1,310 0.714 0.322 1E2 0.55 8.36 1,000 1,180 0.536 0.216 1E3 0.6 8.63 1,030 1,020 0.332 0.150 1E4 0.7 9.31 1,010 790 0.150 0.046 CE3 0.75 9.62 1,010 640 0.121 0.038 *IE: Inventive Example, CE: Comparative Example

[0038] As a result, as illustrated in Table 1, it was confirmed that iron ions were removed to less than 1 ppm, even when only 50% of a total amount of Ba(OH)2 required to precipitate all sulfate ions was used. In this case, it was confirmed that sulfate ions were removed in a concentration range half of that as theoretically expected, and most of copper ions were removed in the form of precipitates under all conditions.

[0039] Since it was confirmed that a concentration range of hydrazine was almost unchanged, hydrazine may be easily prepared as decontamination process water for reduction process decontamination by using purified decontamination process water, when supplementing only an amount thereof consumed during the process thereinto.

[0040] 3) Regeneration of Decontamination Process Water

[0041] In order to regenerate the purified decontamination process water as the decontamination process water for reduction process decontamination, hydrazine, sulfate ions, and copper ions were added to the purified decontamination Date Recue/Date Received 2020-11-16 process water. In this case, only an insufficient amount of hydrazine, consumed during the decontamination process, as compared to 1,600 ppm, the concentration of the initial decontamination process water, was added.

[0042] Each of the added components was summarized in Table 2. In this case, when each of the components was added, a pH
of the regenerated decontamination process water was set to be 2.75.
[Table 2]
Molar Ratio Concentration to be supplied (ppm) (Ba(OH)2/S0421 Hydrazine Sulfate Ion Copper Ion TEl 0.5 600 1,290 31 1E2 0.55 600 1,420 31 1E3 0.6 600 1,580 31 1E4 0.7 600 1,810 31 *IE: Inventive Example

[0043] As a result, as illustrated in Table 2, amounts of the components to be supplied were almost the same as amounts removed by precipitation/filtration during the decontamination process, respectively. By this, it can be seen that, when the decontamination process water used after performing the reduction process decontamination was purified and then regenerated to produce decontamination process water, it was possible to regenerate the decontamination process water having a composition, almost similar to the initial decontamination process water.

[0044] When a system of a heavy-water reactor made of a carbon Date Recue/Date Received 2020-11-16 steel material is decontaminated by a method for decontaminating an oxide layer according to an aspect of the present disclosure, since decontamination process water may be reused without discharging the decontamination process water, the occurrence of decontamination waste liquid may be greatly reduced. In addition, since an amount of a decontamination agent to be used may be reduced by 50 to 70%, an amount of decontamination waste liquid finally generated may be reduced by at least 30%, as compared to the existing process. In addition, the method according to an aspect of the present disclosure may be relatively simple and may be thus easily applied to the existing process. In particular, since the method according to an aspect of the present disclosure may be easily applied to the decontamination technology for the primary system of the light-water reactor, it is judged that commercial use as a process for decontaminating the system of the heavy-water reactor may be relatively high.

[0045] In addition, since the method according to an aspect of the present disclosure does not use an ion exchange resin at all, in a different manner to the conventional decontamination technology proposed for decontaminating a system of a heavy-water reactor made of a carbon steel material, it is expected that an amount of decontamination waste generated may be reduced by more than 60%, as compared to the conventional decontamination technology using the ion exchange resin. In Date Recue/Date Received 2020-11-16 addition, since decontamination waste finally generated is a solid BaSO4 and metal hydroxide, having high stability, not a waste ion exchange resin that may be difficult to treat, handling and stabilization may be very easy. Considering these advantages and applicability of a relatively simple process, it is judged that a demand for decontamination technology of decontaminating a system of a heavy-water reactor made of a carbon steel material may be satisfied.

[0046] While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.
***

[0047] In some aspects, embodiments of the present invention as described herein include the following items:
Item 1. A method for decontaminating an oxide layer, comprising:
(a) supplying decontamination process water including sulfuric acid (H2SO4), hydrazine (N2H4), and copper sulfate (CuSO4) to a pipeline system, to decontaminate the oxide layer in the pipeline system;
(b) mixing Ba (OH) 2 with the decontamination process water, formed after step (a), to adjust a pH of the decontamination process water to have a range of 8.2 to 9.4, to perform a Date Recue/Date Received 2022-02-23 precipitation reaction;
(c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is performed, to remove an insoluble substance from the decontamination process water; and (d) reusing the decontamination process water, formed after step (c) from which the insoluble substance has been removed, as the decontamination process water of step (a), wherein the Ba(OH)2 of step (b) is mixed to have a molar number of 50 to 70% of sulfate ions in the decontamination process water formed after step (a).
Item 2. The method of item 1, wherein the pipeline system is a carbon steel material system.
Item 3. The method of item 1 or 2, wherein the pipeline system is a cooling system of a heavy-water reactor.
Item 4. The method of any one of items 1 to 3, wherein the oxide layer comprises iron oxide.
Item 5. The method of any one of items 1 to 4, wherein the oxide layer has 50 to 75 pm thickness.
Item 6. The method of any one of items 1 to 5, wherein the decontamination process water of step (a) contains the sulfuric acid (H2SO4) in a concentration range of 25 to 32 mM, the hydrazine (N2H4) in a concentration range of 30 to 70 mM, and the copper sulfate (CuSO4) in a concentration range of 0.4 to 0.6 mM.

Date Recue/Date Received 2022-02-23 Item 7. The method of any one of items 1 to 6, wherein step (a) is performed at a temperature range of 93 to 97 C for 6 to 10 hours.
Item 8. The method of any one of items 1 to 7, wherein step (d) is performed by mixing at least one of sulfuric acid (H2SO4), hydrazine (N2H4), and copper sulfate (CuSO4) with the decontamination process water formed after step (b) from which the insoluble substance has been removed, to regenerate the decontamination process water.
Item 9. The method of any one of items 1 to 8, wherein the steps (a) to (d) are repeated 2 to 20 times without discharging the decontamination process water.

Date Recue/Date Received 2022-02-23

Claims (9)

1. A method for decontaminating an oxide layer, comprising:
(a) supplying decontamination process water including sulfuric acid (H2SO4), hydrazine (N2H4), and copper sulfate (CuSO4) to a pipeline system, to decontaminate the oxide layer in the pipeline system;
(b) mixing Ba (OH) 2 with the decontamination process water, formed after step (a), to adjust a pH of the decontamination process water to have a range of 8.2 to 9.4, to perform a precipitation reaction;
(c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is performed, to remove an insoluble substance from the decontamination process water; and (d) reusing the decontamination process water, formed after step (c) from which the insoluble substance has been removed, as the decontamination process water of step (a), wherein the Ba(OH)2 of step (b) is mixed to have a molar number of 50 to 70% of sulfate ions in the decontamination process water formed after step (a).

2. The method of claim 1, wherein the pipeline system is a carbon steel material system.

3. The method of claim 1 or 2, wherein the pipeline system is a cooling system of a heavy-water reactor.

4. The method of any one of claims 1 to 3, wherein the oxide layer comprises iron oxide.

5. The method of any one of claims 1 to 4, wherein the oxide layer has 50 to 75 µm thickness.

6. The method of any one of claims 1 to 5, wherein the decontamination process water of step (a) contains the sulfuric acid (H2SO4) in a concentration range of 25 to 32 mM, the hydrazine (N2H4) in a concentration range of 30 to 70 mM, and the copper sulfate (CuSO4) in a concentration range of 0.4 to 0.6 mM.

7. The method of any one of claims 1 to 6, wherein step (a) is performed at a temperature range of 93 to 97°C for 6 to hours.

8. The method of any one of claims 1 to 7, wherein step (d) is performed by mixing at least one of sulfuric acid (H2SO4), hydrazine (N2H4), and copper sulfate (CuSO4) with the decontamination process water formed after step (b) fromwhich the insoluble substance has been removed, to regenerate the decontamination process water.

9. The method of any one of claims 1 to 8, wherein the steps (a) to (d) are repeated 2 to 20 times without discharging the decontamination process water.