JPH0459599B2 - - Google Patents

Description

[Detailed Description of the Invention] (Field of Application of the Invention) The present invention is directed to
This method relates to the method of dissolving and removing deposited iron oxide film, especially the method for dissolving and removing radioactive iron oxide film that has adhered or accumulated on the surfaces of piping, equipment, and fuel assemblies through which cooling water passes through nuclear power plants. Regarding methods suitable for

(Prior art) A radioactive oxide film is formed on the inside of piping, equipment, fuel assemblies, etc. that come into contact with the primary cooling water of a nuclear power plant, and this causes an increase in the surface dose rate of the plant, so regular inspections are required. Removal of these oxide films, that is, decontamination, is necessary from the standpoint of reducing radiation exposure. This removal usually requires that only the oxide film formed on the surface of the metal material be selectively dissolved and separated, while the base material, that is, the metal material itself, can be used thereafter without being dissolved.

Such decontamination methods can be broadly classified into chemical methods, physical methods including mechanical methods, and electrochemical methods. These methods have advantages and disadvantages. The chemical decontamination method uses a decontamination agent that is a blend of an acid, a reducing agent, a complexing agent, and an inhibitor selected in consideration of the characteristics of the oxide film. Although this method is superior in terms of the dissolution rate of the iron oxide film, there remains the risk of also dissolving the metal base material and the inevitable corrosion caused by the residual liquid. Physical decontamination methods include brushing using brush friction, spraying high-pressure water, and mechanically peeling off the film by applying vibrations to the solution or metal base material using ultrasonic waves. The decontamination rate varies greatly depending on the shape of the object, and it may not be applicable if the object is a thin pipe or has a complicated shape. Electrochemical decontamination methods are typified by an anodic polarization method in which a direct current is applied to a metal base material to make it itself an anode, and a cathode polarization method to use it as a cathode. The former corresponds to an electrolytic polishing method that dissolves the base material and the oxide film, and the latter corresponds to the reduction dissolution method that reduces and dissolves only the oxide film. These methods are excellent in principle, but in practice, they are unstable because they are not effective unless the appropriate electrolyte and electrolytic conditions are selected according to the composition and shape of the target object. is left behind.

As described above, the various conventional decontamination methods described above have many problems in terms of stability and reliability when applied to radioactive equipment and the like.

(Objectives of the Invention) Compared to conventional decontamination methods that use a single method, the present invention uses a composite method to selectively dissolve only the iron oxide film without dissolving the base material, and requires complicated piping and decontamination. It is an object of the present invention to provide a method for dissolving an iron oxide film that is excellent in that equipment, etc. can be efficiently decontaminated, there is almost no damage to the base material due to residual liquid, and furthermore, the electrolytic efficiency is high and the electrolytic operation is simple. purpose.

(Summary of the Invention) The present invention aims to remove an iron oxide film that has adhered or accumulated on the surface of piping or equipment that is used in contact with water, while bringing an electrolyte into contact with the piping or equipment that is the object. A method for removing an iron oxide film in which the object is cathodically polarized by performing an electrolytic operation with a counter electrode at a potential lower than the anticorrosion potential of the metal base material to reduce, ionize, and dissolve the film. , an external electrolytic cell and a liquid path reaching the object from the cathode chamber of the external electrolytic cell are provided, and when the cathode of the external electrolytic cell and the object are grounded and at the same potential, the liquid is discharged from the cathode chamber. A method for removing an iron oxide film, characterized in that the electrolytic operation is performed by passing a leakage current to the object through a liquid path, and in combination with the electrolytic operation, ultrasonic vibration is applied to the object. The main points are as follows.

The present invention will be comprehensively explained. Base material piping/
The main components of deposits that adhere to equipment are magnetite (Fe ₃ O ₄ ) and hematite (α-Fe ₂ O ₃ ), and there are also other iron compounds such as FeO and FeOOH. contains radioactive ions such as ⁶⁰ C, ⁵⁴ Mn, etc. These components can be dissolved by oxidation reaction or reduction reaction, while the metal base material can be dissolved by oxidation reaction. These reactions are illustrated by the following reaction equations, taking as an example the case of magnetite made of carbon steel as a base material.

Fe→Fe ²⁺ +2e ^- (1) Fe ₃ O ₄ +8H ⁺ +2e ^- →3Fe ²⁺ +4H ₂ O (2) In iron, the oxidation reaction that releases electrons proceeds as shown in equation (1). On the other hand, in magnetite, a reduction reaction that takes in protons (H ⁺ ) and electrons proceeds as shown in equation (2). The rate of dissolution is determined by the potential of the electrode, which depends on the density of electrons. The critical potential at which these dissolutions occur is the potential obtained when immersed in a solution (called the natural potential or anti-corrosion potential). VS.SCE), and about -0.15V (VS.SCE) for magnetite. Therefore, the potential that dissolves magnetite without dissolving carbon steel needs to be lower than -0.7V. A cathode polarization method is effective for applying such potential settings from an external power source. This involves lowering the potential of the material to be dissolved by passing a direct current between it and a counter electrode in a solvent. In order to selectively dissolve only the iron oxide in the metal base material, it is necessary to lower the potential to a level lower than the corrosion protection potential of the base metal.

However, in reality, the components that adhere to the surfaces of piping, equipment, etc. are not just a single component, but multiple components, and moreover, the manner in which they adhere is often non-uniform. In such cases, even if cathode polarization is performed, the material will not dissolve uniformly, and depending on the distribution of the components and the flow of current, sometimes the inside of the coating layer may be partially dissolved or the surface may be unevenly dissolved. It happens. In order to prevent such a situation, in the present invention, the components dissolved by reduction are cathodically polarized, and ultrasonic waves are applied to the areas where the adhesion force of the deposits has weakened to vibrate the base material and the deposits. Due to the difference in number, the deposits are quickly peeled off. Note that the solution used in these operations is preferably a complexing agent solution that suppresses the reaction by forming a complex with iron ions during cathodic polarization dissolution, but when applying ultrasonic vibration, it is preferable to use a complexing agent solution that suppresses the reaction depending on the presence or absence of dissolved gas. Alternatively, pure water or electrolyte solution may be used.

(Embodiments of the Invention) FIG. 1 is a schematic diagram of the flow when implementing the method of the present invention for removing an iron oxide film attached to the inner surface of a pipe.

In this embodiment, the cathode polarization of the pipe is performed by capturing the leakage current from the counter electrode, and ultrasonic vibration is applied. As shown in FIG. 1, a system 2 for recirculating decontamination liquid is connected to piping 1 of a plant whose iron oxide film is to be removed. An electrolytic cell 3, a storage tank 4 equipped with a heating source, a collector 5 for ions and iron oxide particles, and a liquid pump P are provided between both ends of the recirculation system 2. A ground 6 is connected to the piping system 1 so that the cathode of the electrolytic power source of the electrolytic cell 3 and the piping 1 are at the same potential. On the other hand, an ultrasonic vibrator 8 connected to the oscillator 7 for imparting ultrasonic vibration is connected to the pipe 1.
attached to one end of the The electrolytic cell 3 is partitioned by an ion exchange membrane 9 so as to perform diaphragm electrolysis, and is separated into an anode chamber 10 and a cathode chamber 11. The electrodes of the electrolytic cell 3 may be made of the same material as the anode and the cathode, but in order to facilitate leakage current to the pipe 1, it is desired that the surface area of the cathode is smaller than that of the anode. The anodic chamber solution only needs to be a highly conductive solution that does not change even when subjected to electrolytic oxidation, and the catholyte should be a solution containing an oxidizing agent that is highly reactive with the iron oxide film as a decontaminating solution to be passed through the pipe 1. .

In such a system, when decontaminating piping 1, close both valves shown in piping 1 near both end connections of decontamination liquid recirculation system 2, and pump P recirculates the decontamination liquid in the direction of the arrow. When the decontamination liquid is circulated and electrolyzed in the electrolytic cell 3, it receives an electric charge in the cathode chamber 9, and this electric charge conducts through the liquid and enters the pipe 1, where it passes through the iron oxide film and is received at the ground 6. It will be done. In other words, a leakage current flows, and along with the movement of charge at this time, the pipe 1
The iron oxide film on the inside of the iron oxide film continues to dissolve and is eluted into the decontamination solution as iron ions. In order to eliminate the non-uniformity of the film thickness due to the progress of elution, next, the vibrator 8 is activated from the ultrasonic oscillator 7 to apply ultrasonic vibration to the solution.
Cleans the inner surface of piping by preventing dissolution and peeling of uneven coatings.

Although FIG. 2 does not correspond to an embodiment of the configuration that is the gist of the present invention, parts or equipment are taken out of the plant system and iron oxides attached to them are removed by cathodic polarization and ultrasonic vibration. This is an example of a test device for removal using a combination of the above-mentioned methods, in which a target component or device 13 is placed in a container 12, and cathode polarization and ultrasonic vibration are applied in the same container 12. It is something to do. An ultrasonic oscillator 14 is attached to the bottom of the insoluble container 12. A DC power supply 15 is provided outside the container. When removing deposits, an electrolytic solution as a decontamination solution is put into the container 12, and an external power source 1 is connected to the object 13.
The grounded cathode of 4 is connected and the anode is connected to the container 12. Object 13 for removing deposits
The target object 13 is cathodically polarized by performing an electrolytic operation with the container 12, which is a counter electrode, at a potential lower than the anticorrosion potential of the metal base material, thereby dissolving the deposits, and then applying ultrasonic vibration to dissolve the deposits. and promotes peeling. The electrolytic solution used at this time is preferably a solution containing a complexing agent. A separate treatment method based on ion exchange resin is used to treat the waste liquid.

Although the experimental apparatus shown in FIG. 2 described above and the embodiment apparatus shown in FIG. 1 for implementing the method of the present invention are different in structure, they are the same in terms of the iron oxide film removal method. That is, the decontamination effect at the working electrode (here, the cathode), in other words, the dissolution rate, is determined by the potential and current at the working electrode (here, the leakage current). By the way, the potential is determined by connecting to the grounded negative side of the DC power source, and the current is determined by determining the distance between the working electrode (cathode) and counter electrode (anode) and the area of the working electrode. Become.

Therefore, by adjusting the distance between the working electrode and the counter electrode or the area of the working electrode in the experimental apparatus shown in FIG. 2, it is possible to make the experimental apparatus equivalent to the example apparatus shown in FIG. can.

Next, an experimental example showing the effectiveness of the method of the present invention will be explained. In the experiment, a 10 x 10 mm square test piece was used, which was made by cutting piping to which radioactive iron oxide had adhered. The experiment was conducted separately for dissolution using cathodic polarization and peeling using ultrasonic vibration. In the dissolution experiment by cathodic polarization, 0.002M/
The lEDTA2NH ₄ solution was heated to a temperature of 80℃, degassed with Ar gas, and dissolved by setting the cathode potential of the test piece to ^-1.0V based on a saturated agaric electrode. Nuclide analysis was performed using a semiconductor detector. On the other hand, in an experiment to peel off and remove ⁶⁰ Co using ultrasonic vibration, a beaker containing pure water was placed in an ultrasonic cleaner, a test piece was placed in it, and the beaker was subjected to ultrasonic vibration. The vibration frequency of the washer was 38KHz.

The experimental results are shown in Figs. 3 and 4. FIG. 3 shows the results of ultrasonic vibration treatment after cathodic polarization. As is clear from the figure, ⁶⁰ Co is gradually removed by cathodic polarization, but approximately 50%
When the dissolution was stopped and then ultrasonic vibration was applied for 20 minutes, the removal rate rapidly increased to 95%. On the other hand, FIG. 4 shows the results of cathodic polarization after ultrasonic vibration treatment. From the same figure, ultrasonic vibration only removes about 25% of ⁶⁰ Co.
It was found that even with subsequent cathodic polarization, a removal rate of only about 65% could be obtained. As can be seen from the comparison of the above two experimental results, the deposits are once reductively dissolved by cathodic polarization, but because the composition and adhesion strength of the deposits are unevenly distributed, the deposits are dissolved from positions where the current easily flows. As a result, the surface becomes uneven with roughness and density, and if vibration is applied in this state, dissolution and peeling due to an increase in surface area will proceed simultaneously in a short period of time. As can be seen from the above experimental results, applying ultrasonic vibration after cathode polarization is most effective for removing deposits, but depending on the nature of the deposits, these operations may be performed in the reverse order or at the same time. Even in the case of electrolytic operation alone, considerable effectiveness can be obtained.

(Effects of the Invention) According to the present invention, it is possible to selectively, uniformly and efficiently remove only the iron oxide film on the surface of the base material without dissolving it, and moreover, it is possible to remove the iron oxide film on the surface of the base material selectively, uniformly and efficiently. The method of the present invention can be applied to objects to be decontaminated such as pipes and equipment, and even when the objects are installed in plants, there is no fear of corrosion due to residual liquid, and It has advantages such as simple electrolytic conditions and electrolytic operations.

[Brief explanation of drawings]

Figure 1 is a flow diagram showing an embodiment of the present invention for removing the iron oxide coating on the inner surface of piping installed in a plant, and Figure 2 is a flow diagram showing parts and equipment taken out of the plant system. A schematic diagram of a test device for removing iron oxide film on the surface of the iron oxide film by cathodic polarization and application of ultrasonic vibration, and FIGS. 3 and 4 are experimental results showing the effectiveness of the method of the present invention.
The figure is a graph showing the film removal rate when ultrasonic waves are applied after cathode polarization, and FIG. 4 is a graph showing the film removal rate when ultrasonic waves are applied before cathode polarization. 1... Piping to be decontaminated, 3... Electrolytic cell, 4... Heating storage tank, 5... Deposit collector, 7... Ultrasonic oscillator, 8... Ultrasonic vibrator, 9... Ion exchange membrane,
10... Anode chamber, 11... Cathode chamber, P... Pump.

Claims (1)

[Claims] 1. In order to remove iron oxide film that has adhered or accumulated on the surface of piping or equipment that is used in contact with water, the metal of the piping or equipment is brought into contact with an electrolytic solution. A method for removing an iron oxide film in which the object is cathodically polarized by performing an electrolytic operation with a counter electrode at a potential lower than the anticorrosion potential of the base material to reduce, ionize, and dissolve the skin, the method comprising: A liquid path is provided from the external electrolytic cell and the cathode chamber of the external electrolytic cell to the object, and when the cathode of the external electrolytic cell and the object are grounded and at the same potential, the liquid is discharged from the cathode chamber. A method for removing an iron oxide film, characterized in that the electrolytic operation is performed by passing a leakage current through the object through a channel, and in combination with the electrolytic operation, ultrasonic vibration is applied to the object. 2. The iron oxide film according to claim 1, wherein the electrolytic solution used in the electrolytic operation contains a complexing agent or reducing agent having good affinity with iron oxide and is heated. Removal method. 3. The method for removing an iron oxide film according to claim 1 or 2, wherein applying the ultrasonic vibration to the object is performed at the same time as or after the electrolytic operation. 4. The method for removing an iron oxide film according to claim 1 or 2, wherein applying the ultrasonic vibration to the object is performed before the electrolytic operation.