FR3108771A1 - Oxide layer decontamination process - Google Patents

Abstract

Disclosed is a method for decontaminating an oxide layer, capable of effectively decontaminating the oxide layer. In detail, the present disclosure relates to a method of decontaminating an oxide layer, comprising (a) supplying decontamination process water containing sulfuric acid (H2SO4), hydrazine (N2H4) and copper sulphate (CuSO4) from a pipe circuit, to decontaminate an oxide layer in the pipe circuit; (b) mixing Ba(OH)2 with the decontamination process water, formed after step (a) of decontaminating the oxide layer, to adjust the pH of the decontamination process water of so that it is between 8.2 and 9.4, to carry out a precipitation reaction; (c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is carried out, to remove an insoluble substance from the decontamination process water; and (d) reusing the decontamination process water, formed after step (c) in which the insoluble substance was removed, as the decontamination process water of step (a). FIG. 1

Description

Oxide layer decontamination process

REFERENCE TO RELATED REQUESTS

This application claims the benefit of priority to Korean patent application No. 10-2020-0036610 filed on March 26, 2020 to the Korean Intellectual Property Office.

BACKGROUND

APPLICATION DOMAIN

The present disclosure relates to a method of decontaminating an oxide layer, capable of effectively decontaminating the oxide layer, and more particularly, to a method of effectively decontaminating a relatively thick oxide layer formed in a steel circuit. carbon.

DESCRIPTION OF THE STATE OF THE ART

Decontamination of a circuit is essential for the maintenance of an operating nuclear power plant and the dismantling of an aging nuclear power plant. The decontamination of a primary circuit of a light water nuclear power plant can be carried out in several decontamination cycles, and therefore a considerable amount of liquid decontamination waste containing radionuclides can be produced. Currently, although chemical decontamination processes using organic acids can be widely used for the decontamination of the primary circuit of a light water nuclear power plant, there are problems associated with the difficulty of treating the liquid wastes produced by these processes and the production of radioactive waste, such as ion exchange resins, as waste during the decontamination process.

To solve these problems, the Korean Research Institute for Atomic Energy (KAERI) has developed a method of decontamination of metal ions by reduction based on hydrazine (hydrazine-based reductive metal ion decontamination, HyBRID) using inorganic acids. , as disclosed in Korean Patent No. 1601201. Since the liquid decontamination waste produced by this process can be effectively treated by means of precipitation and decomposition reactions, this process has the advantage of significantly reducing radioactive waste, compared to to the conventional commercial decontamination process.

Moreover, the thickness of an oxide layer formed in a circuit of a heavy water reactor made of carbon steel, in a heavy water nuclear power plant, can be tens of times greater than the thickness of an oxide layer formed in a circuit of a light water reactor (light water reactor: 1 to 3 μm, heavy water reactor: 75 μm or more). In this case, when the existing HyBRID process is used as it is to decontaminate a relatively thick oxide layer, the pH of a decontaminating agent can greatly increase, and a function of the decontaminating agent can be degraded. This can therefore lead to a situation requiring the use of a significant amount of sulfuric acid. In this case, the ion concentration range in the decontamination process water may increase to reach the limit amount of an oxide layer that can be decontaminated. It is therefore difficult to carry out effective decontamination.

Therefore, when a relatively thick oxide layer is decontaminated, since the used decontamination process water has to be discarded and a new supply of decontamination process water is required, a decontamination process becomes inefficient. In addition, since a large amount of liquid decontamination waste can be produced, the waste to be disposed of can also increase significantly. In addition, in order to reuse the used decontamination process water without discarding the used decontamination process water, it is possible to consider a method of removing ionic components using an ion exchange resin, but in this In this case, the problem is that a considerable amount of ion exchange resin can be produced as radioactive waste.

There is therefore a need to develop a technology capable of effectively decontaminating a relatively large amount of oxide layer while limiting the number of discharges of the decontamination process water used and of new decontamination process water supplies.

ABSTRACT

One aspect of the present disclosure is to provide a method of decontaminating an oxide layer capable of effectively decontaminating a relatively large amount of corrosive oxides formed in a circuit of a heavy water carbon steel reactor while reducing significantly the production of radioactive waste.

According to another aspect of the present disclosure, a method of decontaminating an oxide layer comprises (a) supplying the decontamination process water containing sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ) and copper sulphate (CuSO ₄ ) from a pipe circuit, to decontaminate an oxide layer in the pipe circuit; (b) mixing Ba (OH) ₂ with the decontamination process water, formed after the oxide layer decontamination step (a), to adjust the pH of the decontamination process water of so that it is between 8.2 and 9.4 in order to carry out a precipitation reaction; (c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is carried out, to remove an insoluble substance from the decontamination process water; and (d) reusing the decontamination process water, formed after step (c) in which the insoluble substance has been removed, as the decontamination process water of step (a).

The above aspects, features and advantages, as well as other aspects, features and advantages of the present disclosure, will be better understood from the following detailed description together with the accompanying drawings, in which:

: is a flowchart illustrating a method of decontaminating an oxide layer according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Preferred embodiments of the present disclosure are described below with reference to the accompanying drawings. However, the embodiments of the present disclosure can be modified to present many other forms and the scope of the present disclosure is not limited to the embodiments described below.

A relatively thick oxide layer in a carbon steel circuit of a heavy water reactor can be composed primarily of iron oxide. For example, the oxide layer can be 99% by weight or more of iron oxide. It is therefore possible to decontaminate the oxide layer using only a reduction process, among the chemical decontamination processes.

The present disclosure provides a method for effectively decontaminating the oxide layer.

In detail, the present disclosure provides a method of decontaminating an oxide layer, comprising (a) supplying decontamination process water containing sulfuric acid (H ₂ SO ₄ ), hydrazine ( N ₂ H ₄ ) and copper sulphate (CuSO ₄ ) from a pipe circuit, to decontaminate an oxide layer in the pipe circuit; (b) mixing Ba (OH) ₂ with the decontamination process water, formed after the oxide layer decontamination step (a), to adjust the pH of the decontamination process water of so that it is between 8.2 and 9.4 in order to carry out a precipitation reaction; (c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is carried out, to remove an insoluble substance from the decontamination process water; and (d) reusing the decontamination process water, formed after step (c) in which the insoluble substance has been removed, as the decontamination process water of step (a). In the present disclosure, "the decontamination process water from step (b)" refers to the decontamination process water formed after step (a) and supplied to step (b) and "the decontamination process water from step (c) ”refers to the decontamination process water formed after step (b) and supplied to step (c).

The above method may be a method capable of effectively decontaminating a relatively thick oxide layer in a carbon steel circuit of a heavy water reactor. In this case, the decontamination process water containing sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ), copper sulfate (CuSO ₄ ), etc. at predetermined concentrations can be used to perform a reduction decontamination step in an existing HyBRID decontamination process, significant amounts of sulfuric acid ions (SO ₄ ) and iron ions (Fe) (<1ppm), which result in degradation of decontamination performance, can be practically eliminated, and furthermore, a decontamination agent in a reduction step can be added in a relatively small amount to reuse this decontamination process water in the same way as the process water decontamination used in the initial step. Therefore, since the decontamination process water used in the reduction decontamination step can be reused under the same conditions as the decontamination process water of the initial reduction decontamination step without separate discharge of decontamination process water, it is possible to effectively decontaminate the relatively thick oxide layer in the carbon steel circuit.

In the present disclosure, the decontamination process water can refer to a solution in which a decontamination agent is included in the process water circulating in a pipeline system.

The pipe circuit can therefore be a cooling circuit of a nuclear power plant. In this case, the cooling circuit can be a carbon steel circuit, and moreover, the carbon steel circuit can be a cooling circuit of a heavy water reactor. An oxide layer with a thickness several tens of times greater than the thickness of a primary circuit of a light water reactor can form in the cooling circuit of the heavy water reactor. For example, the thickness of an oxide layer formed on the primary circuit of the light water reactor may be 1 to 3 μm, and the thickness of an oxide layer formed on the cooling circuit of the reactor at heavy water can be 50 to 75 μm.

NiFe ₂ O ₄ , Fe ₃ O ₄ , Fe ₂ O ₃ or other can form in an oxide layer formed on the surface of a cooling circuit of a nuclear power plant, due to the presence of a relatively high amount of dissolved oxygen in a cooling system solution. The oxide layer as a decontamination target of the present disclosure can therefore contain iron oxide and the iron oxide can be decontaminated by dissolving in the decontamination process water of the present disclosure.

To this end, the initial decontamination process water of this disclosure supplied to the pipeline system may contain sulfuric acid (H ₂ SO ₄ ) in a concentration range of 25 to 32 mM, hydrazine (N ₂ H ₄ ) in a concentration range of 30 to 70 mM and copper sulfate (CuSO ₄ ) in a concentration range of 0.4 to 0.6 mM. Preferably, the initial decontamination process water may contain sulfuric acid in a concentration range of 27 to 30 mM, hydrazine in a concentration range of 45 to 55 mM, and copper sulfate in a range of. concentration of 0.45 to 0.55 mM.

In this case, when the concentration range of sulfuric acid is greater than 32 mM, the pH of the decontamination process water may decrease, and thus increase the corrosion of a basic material of the pipeline system, and When the concentration range of sulfuric acid is less than 25mM, the decontamination effect may decrease. In addition, when the concentration range of hydrazine is greater than 70 mM or less than 30 mM, the dissolution performance of the oxide layer may decrease, and thus reduce the decontamination effect. Further, when the concentration range of copper sulfate is greater than 0.6 mM, a precipitate of copper may form and excess ionic components may be formed. In this case, an additional process may be necessary to remove the ionic components. When the copper sulfate concentration range is less than 0.4mM, the decontamination effect can be reduced.

A method of supplying the pipe circuit decontamination process water to decontaminate the oxide layer in step (a) can be carried out at a temperature between 93 and 97 ° C for 6 to 10 hours, and preferably at a temperature of 95 ° C for 6 to 10 hours. The execution time of the above method may vary depending on the extent of the decontamination.

Further, in step (b) of the present disclosure, when the Ba (OH) ₂ is mixed with the decontamination process water of step (b) formed after the decontamination of the oxide layer, the mixing step can be configured so that the pH of the decontamination process water from step (b) can be between 8.2 and 9.4, preferably between 8.35 and 8.65 . In the case where Ba (OH) ₂ is mixed, when the pH of the decontamination process water is less than 8.2, the iron ions of the decontamination process water cannot go below 1 ppm, and on the other hand, a relatively large amount of fine particles of iron hydroxide in the decontamination process water can be produced, and thus reduce the efficiency of the filtration. There is therefore a problem which can prevent efficient removal of the ferrous components. When the pH of the decontamination process water is greater than 9.4, a large amount or most of a decontamination agent, which can be reused, in the decontamination process water can be removed, and generate problems resulting in an increase in the amount of waste and decontamination agent to be used, etc.

To this end, in step (b), the Ba (OH) ₂ can be mixed to reach a molar number of 50-70% sulfate ions in the decontamination process water from step (b) , and preferably a molar number of 55 to 60% of sulfate ions in the decontamination process water. When Ba (OH) ₂ is mixed to achieve a molar number of less than 50% sulfate ions in the decontamination process water from step (b), the iron ions in the decontamination process water do not can not pass below 1 ppm, and on the other hand, a relatively large amount of fine particles of iron hydroxide in the decontamination process water can be produced, and thus reduce the filtration efficiency. There is therefore a problem which can prevent efficient removal of the ferrous components. When the molar number of sulfate ions in the decontamination process water is greater than 70%, a large amount or most of a decontamination agent, which can be reused, in the decontamination process water can be removed. , and generate problems resulting in an increase in the amounts of waste and decontamination agent to be used, etc.

Various insoluble substances can be produced in the decontamination process water formed after step (b), due to the precipitation reaction. Insoluble substances can be, for example, BaSO ₄ , Fe (OH) ₂ , Cu (OH) ₂ or the like. When the insoluble substances remain in the decontamination process water formed after step (b), it can interfere with the reuse of the decontamination process water. It is therefore necessary to remove the insoluble substance.

To this end, the present disclosure can perform a step (c) of filtering the decontamination process water formed after step (b) in which the precipitation reaction is carried out. The above step (c) can be carried out to remove the insoluble substance formed by the precipitation reaction, and can be carried out using a filter device used in the art. For example, it is possible to use as a filtration device a filter whose pores have a diameter of between 0.2 and 0.45 μm. Most insoluble substances can be removed from the filtered decontamination process water as described above.

Since most of the components (hydrazine, sulfate ions, copper ions, etc.) present in the initial decontamination process water remain in the decontamination process water from which most of the insoluble substances are removed, the process water decontamination can be reused as the initial decontamination process water from step (a). Hydrazine, sulfuric acid and copper sulfate can therefore be mixed with the decontamination process water formed after step (c) to prepare the decontamination process water in the same way as water. decontamination process used in step (a). It is therefore possible to carry out the reuse step by mixing at least sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ) or copper sulphate (CuSO ₄ ) with the decontamination process water from which insoluble substances are removed in order to regenerate the decontamination process water.

In this case, by comparing the concentrations of hydrazine, sulfuric acid and copper sulfate contained in the decontamination process water used in step (a) to the 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 missing component, can be added and mixed with the decontamination process water formed after step (c).

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

Regenerated decontamination process water can feed back into the pipeline system to decontaminate an oxide layer. The present disclosure may therefore 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) can be repeated up to tens of times without rejecting the process water. To ensure the efficiency of the process, the repetition can be performed, for example, about 2 to 20 times. Steps (a) to (d) can therefore be repeated 2 to 20 times without rejecting the process water.

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

The present disclosure will be described in more detail hereinafter by way of specific examples. The following examples are only intended to facilitate understanding of the present disclosure and the scope of the present disclosure is not limited thereto.
Example
1) Preparation of decontamination process water

Sulfuric acid, hydrazine and copper sulfate were added to and mixed with 1 L of distilled water, so that sulfuric acid at a concentration of 20 mM was available, hydrazine at a concentration of 31.2 mM and copper sulfate at a concentration of 0.5 mM as the conditions of the decontamination process water after a reduction step, in a HyBRID decontamination process.

Subsequently, considering that an oxide layer of a carbon steel primary circuit composed of iron oxide was decontaminated, iron sulphate was dissolved in the decontamination process water so as to reach an iron ion concentration of 350 ppm. In this case, 350 ppm iron ions were adjusted using iron sulfate, and therefore, it was confirmed that the final concentration of sulfuric acid was 27 mM.

This mixture was used as decontamination process water subjected to a decontamination step in a cooling circuit of a heavy water reactor to prepare the decontamination process water subjected to step (a) of the present disclosure. .
2) Elimination of sulfate ions and iron ions

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

Depending on the number of moles of sulfate ions, the amount of Ba (OH)₂has been modified, and Table 1 shows the results of the measurements of the pH of the decontamination process water used and the concentration of each of the components.
[Table 1] Molar ratio (Ba (OH) ₂ / SO ₄ ^2- ) pH Concentration (ppm) Hydrazine Sulphate ions Iron ions Copper ions CE1 0 2.61 1,010 2,600 350 31 CE2 0.45 8.17 1000 1,460 1,843 0.282 IE1 0.5 8.27 1,020 1310 0.714 0.322 IE2 0.55 8.36 1000 1180 0.536 0.216 IE3 0.6 8.63 1,030 1,020 0.332 0.150 IE4 0.7 9.31 1,010 790 0.150 0.046 CE3 0.75 9.62 1,010 640 0.121 0.038 * IE: Example of the invention, CE: Comparative example

As a result, as shown in Table 1, it was confirmed that the iron ions fell below 1 ppm even when only 50% of the total amount of Ba (OH) ₂ needed to precipitate all the sulfate ions was used. In this case, it was confirmed that the sulfate ions were removed in a concentration range corresponding to half of that theoretically expected, and that most of the copper ions were removed as precipitates under all conditions.

Since it was confirmed that the hydrazine concentration range was virtually unchanged, hydrazine can be easily prepared as a decontamination process water for reduction decontamination with purified decontamination process water, by adding only the amount consumed during the process.

3) Regeneration of decontamination process water

In order to regenerate the purified decontamination process water as the decontamination process water for the reduction decontamination, hydrazine, sulfate ions and copper ions were added to the purified decontamination process water. In this case, only an insufficient amount of hydrazine, consumed during the decontamination process, relative to 1600 ppm, the concentration of the initial decontamination process water, was added.

Each of the added components was summarized in Table 2. In this case, when each of the components was added, the pH of the regenerated decontamination process water was set to 2.75.
[Table 2] Molar ratio (Ba (OH) ₂ / SO ₄ ^2- ) Concentration to provide (ppm) Hydrazine Sulphate ions Copper ions IE1 0.5 600 1290 31 IE2 0.55 600 1,420 31 IE3 0.6 600 1,580 31 IE4 0.7 600 1,810 31 * IE: Example of the invention

Therefore, as shown in Table 2, the amounts of components to be supplied were substantially the same as the amounts removed by precipitation / filtration during the decontamination process, respectively. It can thus be seen that when the decontamination process water used after the reduction decontamination was purified and then regenerated to produce decontamination process water, it was possible to regenerate the decontamination process water with a composition substantially identical to that of the initial decontamination process water.

When a circuit of a carbon steel heavy water reactor is decontaminated by an oxide layer decontamination process according to one aspect of the present disclosure, since the decontamination process water can be reused without discharge decontamination process water, the production of liquid decontamination waste can be significantly reduced. In addition, since the amount of decontamination agent to be used can be reduced by 50 to 70%, the amount of liquid decontamination waste finally produced can be reduced by at least 30% compared to the existing process. In addition, the method according to one aspect of the present disclosure can be relatively simple and therefore can be easily applied to the existing method. In particular, since the method according to one aspect of the present disclosure can be easily applied to the technology of decontaminating the primary circuit of the light water reactor, it is believed that commercial use as a method of decontaminating the circuit of the heavy water reactor can be relatively extensive.

In addition, since the method according to one aspect of the present disclosure does not use ion exchange resin at all, unlike the conventional decontamination technology proposed for the decontamination of a circuit of a heavy water steel reactor. to carbon, it should be possible to reduce the amount of decontamination waste produced by 60% compared to conventional decontamination technology using ion exchange resin. In addition, since the decontamination waste finally produced is _{solid BaSO 4} and metal hydroxide, exhibiting high stability, and not an ion exchange resin which can be difficult to process, handling and stabilization can be very difficult. simple. In view of these advantages and the applicability of a relatively simple process, it is believed that a need for decontamination technology to decontaminate a circuit of a heavy water reactor made of carbon steel can be satisfied.

Although exemplary embodiments have been presented and described above, it will be obvious to those skilled in the art that it is possible to make modifications and variations without departing from the mind and within the scope of the present disclosure as defined by the appended claims.

Claims (10)

Process for decontaminating an oxide layer, comprising:
(a) the supply of decontamination process water containing sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ) and copper sulphate (CuSO ₄ ) to a pipeline system , to decontaminate an oxide layer in the pipe circuit;
b) mixing Ba (OH) ₂ with the decontamination process water, formed after the oxide layer decontamination step (a), to adjust the pH of the decontamination process water so that it be between 8.2 and 9.4 in order to carry out a precipitation reaction;
c) filtering the decontamination process water, formed after step (b) in which the precipitation reaction is carried out, to remove an insoluble substance from the decontamination process water; and
d) reusing the decontamination process water, formed after step (c) in which the insoluble substance has been removed, as the decontamination process water of step (a). A method according to claim 1, wherein the pipe circuit is a carbon steel circuit. Process according to Claim 1, in which the pipe circuit is a cooling circuit of a heavy water reactor. A method according to claim 1, wherein the oxide layer comprises iron oxide. A method according to claim 1, wherein the oxide layer has a thickness of 50 to 75 μm. The method of claim 1, wherein the decontamination process water from step (a) contains sulfuric acid (H ₂ SO ₄ ) in a concentration range of 25 to 32 mM, hydrazine ( N ₂ H ₄ ) in a concentration range of 30 to 70 mM and copper sulfate (CuSO ₄ ) in a concentration range of 0.4 to 0.6 mM. A method according to claim 1, wherein step (a) is carried out at a temperature between 93 and 97 ° C for 6 to 10 hours. Process according to Claim 1, in which the Ba (OH) ₂ of step (b) is mixed so as to obtain a molar number of 50 to 70% of sulfate ions in the decontamination process water formed after l 'step (a). A method according to claim 1, wherein step (d) is carried out by mixing at least sulfuric acid (H ₂ SO ₄ ), hydrazine (N ₂ H ₄ ) or copper sulfate (CuSO ₄₎ ) with the decontamination process water formed after step (b) in which the insoluble substance has been removed, in order to regenerate the decontamination process water. The method of claim 1, wherein steps (a) through (d) are repeated 2 to 20 times without discharge of the decontamination process water.