JP7385505B2 - Chemical decontamination methods - Google Patents

Description

The present disclosure relates to chemical decontamination methods.

At nuclear power plants, decontamination work is performed to remove radionuclides attached to equipment, piping, etc. during maintenance work during operation and dismantling work during decommissioning. In particular, piping members (mainly made of stainless steel and Inconel) that make up nuclear power plant systems are exposed to high temperature and high pressure environments during operation, so an oxide film is formed on the surface and radionuclides are contained in the film. is taken in. A method called chemical decontamination is known as a method for removing this oxide film and decontaminating members.

For example, Patent Document 1 discloses a configuration in which an oxide film (contaminants in piping) is oxidized and dissolved with permanganate ions, and then the oxide film is reduced and dissolved with oxalic acid. Further, Patent Document 1 discloses a configuration in which metal cations eluted in a chemical cleaning solution (processing solution) are removed by adsorption to a cation exchange resin.

Patent No. 3945780

However, in the chemical decontamination method as disclosed in Patent Document 1, it is necessary to discard the ion exchange resin that has adsorbed metal cations from the oxide film. As a result, secondary waste may increase.

The present disclosure has been made to solve the above problems, and aims to provide a chemical decontamination method that can reduce the amount of secondary waste generated during chemical decontamination. .

In order to solve the above problems, a chemical decontamination method according to the present disclosure includes a step of removing an oxide film containing nickel ferrite in a system of a nuclear power plant using treated water, and removing the oxide film. After carrying out the process, the system is chemically decontaminated using the treated water, and the step of removing the oxide film includes adjusting the dissolved hydrogen concentration and temperature of the treated water in the system in advance. The range above the upper limit of the specified range includes a step of modifying nickel ferrite from the oxide film into metal nickel and magnetite , and the range above the upper limit of the specified range includes the step of modifying the nickel ferrite from the oxide film into metal nickel and magnetite. This is a range in which the nickel ferrite is modified to become the metallic nickel and the magnetite .

According to the chemical decontamination method of the present disclosure, it is possible to suppress the amount of secondary waste generated when chemical decontamination is performed.

1 is a diagram illustrating an example of an object to be decontaminated, which is a target of a chemical decontamination method according to an embodiment of the present disclosure. It is a flow diagram of the chemical decontamination method in this embodiment. It is a figure showing the prescribed range of dissolved hydrogen concentration and temperature of treated water set in the above-mentioned chemical decontamination method. It is a flowchart of the process of removing an oxide film in the said chemical decontamination method. It is a flowchart of the process of performing chemical decontamination in the above-mentioned chemical decontamination method.

Hereinafter, embodiments for carrying out the chemical decontamination method according to the present disclosure will be described with reference to the accompanying drawings. However, the present disclosure is not limited to this embodiment.

(Objects to be decontaminated)
Embodiments according to the present disclosure will be described below with reference to FIGS. 1 to 5. First, the object Z to be decontaminated, which is the target of the chemical decontamination method S1 in which surplus water is generated, will be described. The objects Z to be decontaminated to be chemically decontaminated are parts such as piping, containers, various equipment, etc. that constitute the system 100 of the nuclear power plant, and are parts that come into contact with reactor water.

(Configuration of nuclear power plant)
Here, as a nuclear power plant, for example, as shown in FIG. 1, there is a nuclear power plant P including a pressurized water reactor 50. This nuclear power plant P includes a pressurized water reactor 50 in which fuel rods 51 and the like are stored, and a pressurizer 52 that pressurizes the primary cooling water (light water) in order to suppress boiling of the primary cooling water (light water) in the pressurized water reactor 50. , a steam generator 53 that turns secondary cooling water into steam using the heat of the primary cooling water, a primary coolant pump 54 that returns the primary cooling water from the steam generator 53 to the pressurized water reactor 50, and a steam generator 53. a steam turbine 56 that is driven by the steam generated in A water supply pump 59 for returning water to the generator 53 is provided.

The pressurized water nuclear reactor 50 and the steam generator 53 are connected through primary cooling water pipes 55a and 55b. The steam generator 53 and the steam turbine 56 are connected by a steam pipe 55c. The condenser 58 and the steam turbine 56 are connected by a water supply pipe 55d.

In the system 100 of the nuclear power plant P configured in this way, the decontamination target Z is the primary cooling system member that is in contact with the reactor water, that is, the primary cooling water. The objects to be decontaminated Z include the pressurized water reactor 50, the pressurizer 52, the steam generator 53, the primary coolant pump 54, the primary cooling water pipes 55a and 55b connecting these, the primary cooling water pipes 55a and 55b, etc. There are various valves etc. installed in the These objects to be decontaminated Z are made of stainless steel containing iron as a main component and chromium and nickel, Inconel which is a nickel-based alloy, or the like.

The metal elements constituting the decontamination target Z are slightly eluted into the reactor water, and a portion of them adheres to the surfaces of the fuel rods 51 in the pressurized water reactor 50. When the metal elements attached to the surface of the fuel rod 51 are irradiated with neutron beams from the fuel, they undergo a nuclear reaction and become radioactive nuclides such as chromium, iron, nickel, and cobalt. Most of these radionuclides remain attached to the surface of the fuel rod 51 in the form of oxides, but some of them are eluted into the reactor water or released as insoluble solids. The radionuclides eluted or released into the reactor water adhere to the reactor water contact surface of the object to be decontaminated Z. As a result, an oxide film containing radionuclides is formed on this reactor water contact surface. Therefore, workers working near the object Z to be decontaminated are exposed to radiation from radionuclides in the oxide film formed on the parts that are the object Z to be decontaminated. Here, the oxide film is formed containing nickel ferrite (NiFe ₂ O ₄ ).

(Chemical decontamination method)
The chemical decontamination method S1 is related to system decontamination performed when the above-described nuclear power plant P is decommissioned. In the chemical decontamination method S1, treated water, which is a chemical solution, is circulated inside the pipes and the like that constitute the system 100, which is the object Z to be decontaminated. The treated water is circulated within system 100 by primary coolant pump 54 . As a result, the oxide film containing radionuclides adhering to the inner surface of the object Z to be decontaminated is removed and discarded. As shown in FIG. 2, the chemical decontamination method S1 includes a step S2 of removing an oxide film and a step S3 of performing chemical decontamination.

In step S2 of removing the oxide film and step S3 of chemical decontamination, the dissolved hydrogen concentration and temperature of the treated water are adjusted, respectively. For this reason, the system 100 is provided with a dissolved hydrogen concentration sensor 91 that detects the dissolved hydrogen concentration of the treated water, and a temperature sensor 92 that detects the temperature of the treated water. Further, the dissolved hydrogen concentration of the treated water is adjusted by the hydrogen concentration adjustment section 93. In the embodiment of the present disclosure, the hydrogen concentration adjustment unit 93 supplies hydrogen into the system by supplying the hydrogen concentration to the gas phase portion of the volume control tank of the chemical volume control system. Further, the concentration is adjusted by adjusting the supply amount and the pressurizing pressure. The temperature of the treated water is adjusted by the temperature adjustment section 94. In the embodiment of the present disclosure, the primary coolant pump 54 is used as the temperature adjustment section 94. By adjusting the output of the primary coolant pump 54, the temperature of the treated water is adjusted by the frictional energy generated within the primary coolant pump 54.

(Process of removing oxide film)
In step S2 of removing the oxide film, a part of the oxide film is removed before step S3 of chemical decontamination. Here, a part of the oxide film is a soft cladding, which is an oxide film that easily peels off among the oxide films attached to the object Z to be decontaminated such as piping. In step S2 of removing the oxide film, the oxide film containing nickel ferrite in the system 100 is removed using treated water. As shown in FIG. 4, the step S2 of removing the oxide film includes a step S21 of modifying nickel ferrite (NiFe ₂ O ₄ ) from the oxide film, and a step S22 of removing metallic nickel contained in the oxide film. I'm here.

In the step S21 of modifying the nickel ferrite from the oxide film, as shown in FIG. 3, the dissolved hydrogen concentration and temperature of the treated water in the system 100 are adjusted to a range A1 that is equal to or higher than the upper limit Au of a predetermined range A. . The specified range A is determined based on the relationship between the dissolved hydrogen concentration and temperature of the treated water. Specifically, the prescribed range A is a range in which nickel ferrite contained in the oxide film can exist in a stable form. The specified range A is also called the DH band. The specified range A is preset in order to appropriately manage the dissolved hydrogen concentration of the treated water in step S3 of chemical decontamination.

The upper limit Au is a line determined by the dissolved hydrogen concentration and temperature. In step S21 of modifying nickel ferrite from the oxide film, when the dissolved hydrogen concentration and temperature of the treated water are adjusted to the upper limit Au of the specified range A or higher, the nickel ferrite contained in the oxide film is modified and becomes metallic nickel and magnetite. Become. Specifically, a reaction as shown in the following formula (1) occurs.
_{NiFe2O4 →} Ni+ _{Fe3O4 ...} (1)

When nickel ferrite is modified, the volume of the oxide film changes, stress is generated in the oxide film, and cracking or peeling may occur in some cases. As a result, much of the oxide film is peeled off from the object Z to be decontaminated. Fragments of the oxide film (separated materials) peeled off from the object Z to be decontaminated are collected by a filter 95 disposed in the system 100 and removed from the system 100.

In addition, in step S21 of modifying the nickel ferrite from the oxide film, in order to keep the dissolved hydrogen concentration of the treated water within the range A1 which is equal to or higher than the upper limit Au of the specified range A, the dissolved hydrogen concentration of the treated water is, for example, 100°C or more and 280°C or less, It is preferable that the amount is 5 cc/kg-H ₂ O or more and 100 cc/kg-H ₂ O or less. Among these, the preferable range of dissolved hydrogen concentration in the treated water is 120° C. or higher and 280° C. or lower, and 10 cc/kg-H ₂ O or more and 100 cc/kg-H ₂ O or lower. Further, a particularly preferable range of the dissolved hydrogen concentration of the treated water is 176° C. or more and 280° C. or less, and 30 cc/kg-H ₂ O or more and 100 cc/kg-H ₂ O or less.

Step S22 of removing metallic nickel contained in the oxide film is performed after step S21 of modifying nickel ferrite from the oxide film. In step S22 of removing metallic nickel contained in the oxide film, as shown in FIG. 3, the dissolved hydrogen concentration and temperature of the treated water in the system 100 are adjusted to a range A2 that is equal to or lower than the lower limit Ad of the specified range A. The lower limit Ad is a line defined by the dissolved hydrogen concentration and temperature. In other words, the region sandwiched between the upper limit Au and the lower limit Ad is the specified range A.

In step S22 of removing metallic nickel contained in the oxide film, a full water oxidation operation is performed in order to adjust the dissolved hydrogen concentration and temperature of the treated water in the system 100 to below the lower limit Ad of the specified range A. In the full water oxidation operation, the treated water is brought into a weakly reducing atmosphere, so that the treated water is below the lower limit Ad. Specifically, under conditions of full water oxidation operation, for example, hydrogen peroxide (H ₂ O ₂ ) is added to the treated water. Under this condition, treated water is circulated within the system 100. As a result, when the dissolved hydrogen concentration and temperature of the treated water in the system 100 are adjusted to below the lower limit Ad, the metallic nickel produced by denaturation in the previous step and the metallic nickel originally contained in the oxide film are dissolved and removed. Ru. Metallic nickel is dissolved by the treated water being circulated during full water oxidation operation. That is, nickel contained in the oxide film is removed. Specifically, a reaction as shown in the following formula (2) occurs.
Ni→NiO
→Ni ²⁺ …(2)

As a result, the nickel is removed, making the oxide film brittle and easily peeling off from the object Z to be decontaminated. Fragments of the oxide film (removed materials) peeled off from the object Z to be decontaminated are collected by the filter 95 disposed in the system 100 or the complementary system and removed from the system 100.

In addition, in step S22 of removing metallic nickel contained in the oxide film, in order to keep the dissolved hydrogen concentration of the treated water within range A2, which is below the lower limit Ad of specified range A, the dissolved hydrogen concentration of the treated water is, for example, 10 cc/kg-H ₂ O or below. It is preferable that Among these, the preferable range of dissolved hydrogen concentration in treated water is 3 cc/kg-H ₂ O or less. Further, a particularly preferable range of the dissolved hydrogen concentration of the treated water is 0.5 cc/kg-H ₂ O or more and 1.0 cc/kg-H ₂ O or less.

Further, the temperature of the treated water to be within the range A2 below the lower limit Ad is preferably 100° C. or below, for example. Among these, the preferred temperature range of the treated water is 40°C or more and 100°C or less.

In addition, in the step S2 of removing the oxide film, the step S21 of modifying nickel ferrite from the oxide film as described above and the step S22 of removing metallic nickel contained in the oxide film may be repeated multiple times. good.

(Process of chemical decontamination)
As shown in FIG. 5, step S3 of chemical decontamination is performed after step S2 of removing the oxide film. The step S3 of chemical decontamination includes an oxidation step S31, a reduction step S32, a decontamination step S33, a decomposition step S34, and a purification step S35. Step S3 of chemical decontamination is performed while the dissolved hydrogen concentration and temperature of the treated water in the system 100 are maintained within a specified range A.

In the oxidation step S31, treated water to which an oxidizing agent has been added is supplied and circulated inside the object Z to be decontaminated. Specifically, permanganic acid is added as an oxidizing agent. In the oxidation step S31, by circulating treated water containing permanganate inside the decontamination target Z, the chromium in the oxide film attached to the decontamination target Z containing stainless steel and nickel-based alloys is converted to Cr. It oxidizes ^as ₆₊ . As a result, in the oxidation step S31, primary treated water, which is treated water containing chromium, which is the radionuclide, is generated.

The reduction step S32 is performed after the oxidation step S31. In the reduction step S32, a trace amount of oxalic acid is added to the primary treated water as a reducing agent. In the reduction step S32, by adding permanganic acid, permanganate ions contained in the primary treated water and precipitated manganese dioxide are decomposed by oxalic acid. That is, in the reduction step S32, oxalic acid is added in an amount that can decompose permanganate ions and precipitated manganese dioxide contained in the primary treated water. Further, by adding a small amount of oxalic acid, nickel is eluted from the object Z to be decontaminated into the primary treatment water.

The decontamination step S33 is performed after the reduction step S32. In the decontamination step S33, a larger amount of oxalic acid is added to the primary treated water as an organic acid than in the reduction step S32. In addition, in the decontamination step S33 of this embodiment, eluted metals are removed by passing secondary treated water, which is primary treated water to which a large amount of oxalic acid has been added, through an adsorbent such as an ion exchange resin. Ru. Specifically, the secondary treated water is circulated and supplied into the object Z to be decontaminated many times. By supplying the secondary treated water into the interior of the object Z to be decontaminated, iron, nickel, and cobalt are eluted from the object Z to be decontaminated into the treated water. Further, when eluted from the object Z to be decontaminated into the secondary treatment water, chromium, iron, nickel, and cobalt are removed using an adsorbent.

The decomposition step S34 is performed after the decontamination step S33. In the decomposition step S34, oxalic acid in the secondary treatment water is decomposed. In the decomposition step S34, for example, oxalic acid in the secondary treated water is decomposed into water and carbon dioxide by irradiating the secondary treated water with ultraviolet rays. Note that in the decomposition step S34, oxalic acid may be decomposed by other methods. As a result, tertiary treated water is produced in which oxalic acid has been decomposed and removed from the secondary treated water.

The purification step S35 is performed after the decomposition step S34. In the purification step S35 of this embodiment, chromium, iron, nickel, and cobalt that could not be removed in the decontamination step S33 are removed. Specifically, in the purification step S35, for example, an adsorbent such as an ion exchange resin is used to remove radionuclides such as chromium, iron, nickel, and cobalt contained in the treated water, and residual substances contaminated with these radionuclides. Decontamination is performed by removing components (ions such as oxalic acid). Thereafter, the dose of the object Z to be decontaminated is measured, and if the dose has decreased sufficiently, the chemical decontamination method S1 is ended. Furthermore, if the dose of the object to be decontaminated Z is still high, the oxidation step S31 to the purification step S35 are repeated as necessary. The decontaminated treated water can be used again in the next cycle.

(effect)
In the chemical decontamination method S1 having the above configuration, in the step S21 of modifying the nickel ferrite from the oxide film, the dissolved hydrogen concentration and temperature of the treated water in the system 100 are set to be equal to or higher than the upper limit Au of the specified range A. As a result, the nickel ferrite contained in the oxide film is modified, metallic nickel and magnetite are generated, and the oxide film is cracked and peeled off. As a result, much of the oxide film is peeled off from the object Z to be decontaminated. By physically removing the stripped oxide film using the filter 95, it is possible to suppress the amount of radioactive waste that needs to be removed using an organic adsorbent such as an ion exchange resin in the chemical decontamination step S3. . As a result, it becomes possible to suppress the amount of secondary waste generated during chemical decontamination.

Further, in step S22 of removing metallic nickel contained in the oxide film (including that generated in step S21 of modifying nickel ferrite from the oxide film), the dissolved hydrogen concentration and temperature of the treated water in the system 100 are within the specified range A. It is adjusted to below the lower limit Ad. As a result, when the treated water is circulated under full water oxidation operating conditions, the metal nickel contained in the oxide film is dissolved and the nickel contained in the oxide film is removed. As a result, the oxide film becomes brittle and easily peels off from the object Z to be decontaminated. Therefore, the removal of the oxide film from the object Z to be decontaminated is promoted, and it becomes possible to further suppress the amount of secondary waste generated when chemical decontamination is performed.

Furthermore, by repeating step S21 of modifying nickel ferrite from the oxide film and step S22 of removing metallic nickel contained in the oxide film multiple times, the removal of the oxide film from the object Z to be decontaminated is further promoted. Ru. As a result, it becomes possible to effectively suppress the amount of secondary waste generated during chemical decontamination.

Furthermore, the dissolved hydrogen concentration was adjusted by adjusting the pressure of the treated water and the amount of hydrogen supplied in the volume control tank. Thereby, the dissolved hydrogen concentration can be easily adjusted.

(Other embodiments)
Although the embodiment of the present disclosure has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes design changes within the scope of the gist of the present disclosure. .

For example, in the above embodiment, the step S2 of removing the oxide film includes the step S22 of removing nickel contained in the oxide film, but the method is not limited to this. For example, in step S2 of removing the oxide film, step S22 of removing nickel contained in the oxide film may not be performed, and only step S21 of modifying nickel ferrite from the oxide film may be performed.

Further, in the above embodiment, the pressurizer 52 is used as the hydrogen concentration adjustment section 93, but the method is not limited to this. Other means may be adopted as appropriate as long as the dissolved hydrogen concentration of the treated water can be adjusted.

Further, in the above embodiment, the primary coolant pump 54 is used as the temperature adjustment section 94, but the method is not limited to this method. As long as the temperature of the treated water can be adjusted, other means such as a heater may be used as appropriate.

Further, in the step S22 of removing metallic nickel, it is sufficient that metallic nickel contained in the oxide film can be removed. Therefore, the metallic nickel to be removed may be only the metallic nickel produced by modification in the step S21 of modifying the nickel ferrite from the oxide film, or may be the metallic nickel originally contained in the oxide film before the step S2 of removing the oxide film. It may be only the metal nickel that was previously used, or it may be both of them as in this embodiment.

<Additional notes>
The chemical decontamination method S1 described in the embodiment can be understood, for example, as follows.

(1) The chemical decontamination method S1 according to the first aspect includes a step S2 of removing an oxide film containing nickel ferrite in the system 100 with treated water from a system 100 of the nuclear power plant P; After carrying out the step S2 of removing, the system 100 is chemically decontaminated with the treated water, and the step S2 of removing the oxide film includes the step S3 of chemically decontaminating the system 100 with the treated water. The method includes a step S21 of modifying the nickel ferrite from the oxide film by adjusting the dissolved hydrogen concentration and temperature to be equal to or higher than the upper limit Au of a predetermined range A.

As a result, metallic nickel and magnetite are generated from the nickel ferrite contained in the oxide film, and cracks and peeling occur in the oxide film. As a result, much of the oxide film is peeled off from the system 100. By physically removing the peeled off oxide film, it is possible to suppress the amount of radioactive waste that needs to be removed using an organic adsorbent such as an ion exchange resin in step S3 of chemical decontamination. As a result, it becomes possible to suppress the amount of secondary waste generated during chemical decontamination.

(2) The chemical decontamination method S1 according to the second aspect is the chemical decontamination method S1 of (1), in which the step S2 of removing the oxide film is performed after the step S21 of modifying the nickel ferrite is performed. , includes a step S22 of removing metallic nickel contained in the oxide film by controlling the dissolved hydrogen concentration and temperature of the treated water in the system 100 to be equal to or lower than the lower limit Ad of the specified range A.

As a result, nickel contained in the oxide film is separated as metallic nickel. When the treated water is circulated, the metal nickel is dissolved and the nickel contained in the oxide film is removed. This makes the oxide film brittle and easily peels off from the system 100. Therefore, the removal of the oxide film from the system 100 is promoted, and it becomes possible to further suppress the amount of secondary waste generated when chemical decontamination is performed.

(3) The chemical decontamination method S1 according to the third aspect is the chemical decontamination method S1 of (2), in which the step S2 of removing the oxide film includes the step S21 of modifying the nickel ferrite, and the step S21 of modifying the nickel ferrite. Step S22 of removing metallic nickel contained in the oxide film is repeated multiple times.

This promotes the peeling of the oxide film and makes it possible to suppress the amount of secondary waste generated during chemical decontamination.

(4) The chemical decontamination method S1 according to the fourth aspect is the chemical decontamination method S1 according to any one of (1) to (3), wherein in the step S3 of performing the chemical decontamination, the dissolved hydrogen Chemical decontamination within the system 100 is performed while the concentration and temperature are maintained within the specified range A.

Thereby, in step S3 of chemical decontamination, chemical decontamination can be performed while suppressing peeling of the oxide film.

50... Pressurized water reactor 51... Fuel rods 52... Pressurizer 53... Steam generator 54... Primary coolant pump 55a... Primary cooling water pipe 55b... Primary cooling water pipe 55c... Steam pipe 55d... Water supply pipe 56... Steam turbine 57 ... Generator 58 ... Condenser 59 ... Water supply pump 91 ... Dissolved hydrogen concentration sensor 92 ... Temperature sensor 93 ... Hydrogen concentration adjustment section 94 ... Temperature adjustment section 95 ... Filter 100 ... System A ... Specified range A1 ... Range A2 ... Range Ad ...Lower limit Au...Upper limit P...Nuclear power plant (nuclear power plant)
S1...Chemical decontamination method S2...Step of removing the oxide film S21...Step of modifying nickel ferrite from the oxide film S22...Step of removing metallic nickel contained in the oxide film S3...Step of chemical decontamination S31...Oxidation step S32...Reduction process S33...Decontamination process S34...Decomposition process S35...Purification process Z...Decontamination target object

Claims (4)

A step of removing an oxide film containing nickel ferrite in the system of a nuclear power plant using treated water;
a step of chemically decontaminating the system with the treated water after performing the step of removing the oxide film,
The step of removing the oxide film includes:
A step of denaturing nickel ferrite from the oxide film into metallic nickel and magnetite by setting the dissolved hydrogen concentration and temperature of the treated water in the system to be within a predetermined upper limit or higher ,
The range above the upper limit of the specified range is a chemical decontamination method in which the nickel ferrite contained in the oxide film is modified to become the metal nickel and the magnetite. In the step of removing the oxide film, after carrying out the step of modifying the nickel ferrite into the metal nickel and the magnetite , the dissolved hydrogen concentration and the temperature are within the lower limit of the specified range. Including the process of removing metallic nickel contained in the oxide film,
The chemical decontamination method according to claim 1, wherein the range below the lower limit of the specified range is a range in which the metal nickel is dissolved and removed . The chemical according to claim 2, wherein the step of removing the oxide film repeats a plurality of steps of modifying the nickel ferrite into the metal nickel and the magnetite , and removing the metal nickel contained in the oxide film. Decontamination methods. The specified range is a range in which the nickel ferrite contained in the oxide film can exist in a stable form,
The chemical decontamination according to any one of claims 1 to 3, wherein in the step of performing chemical decontamination, the chemical decontamination within the system is performed while the dissolved hydrogen concentration and the temperature are maintained within the specified range. Decontamination methods.

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