JPS58210200A - Method for dissolving iron oxide film - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] This relates to a method for dissolving deposited iron oxide films, particularly for dissolving radioactive iron oxide films that adhere and accumulate on the inside of piping and equipment through which cooling water, etc., of nuclear power plants pass. Regarding suitable methods.

A radioactive iron oxide film is formed on the inside of piping, equipment, 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, and it is desirable to remove it. Especially if the dose rate exceeds the permissible value, or even if the nuclear power plant itself is dismantled,
So-called system decontamination is required to remove radioactive iron oxide coatings from plant piping and equipment systems. This type of system decontamination has not been conducted in Japan, and has only been carried out at nuclear power plants in Canada and the United States. The difficulty of this decontamination is that it does not dissolve the carbon steel or stainless steel that is the base material of piping and equipment, and removes iron 2,3 oxide containing radioactive ions on the surface.
Since only the 4,3 iron oxide film must be dissolved, it is necessary to use an appropriate decontamination method and to take into account the effect of residual decontamination agents on the base material.

Decontamination methods include the proven chemical decontamination method and the electrochemical decontamination method for which the applicant previously applied for a patent
No. 162458). The chemical decontamination method uses a decontamination agent that is a blend of acid, reducing agent, complexing agent, and inhibitor selected in consideration of the characteristics of the iron 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 base material and the risk of corrosion due to residual liquid.

On the other hand, electrochemical decontamination methods can be divided into two.

One is a cathode polarization method, and the other is a method in which the concentration of the reducing solution is increased by diaphragm electrolysis, thereby injecting electrons into the iron oxide film. The former method involves polarizing the iron oxide film between a counter electrode and a counter electrode in order to adjust its potential, but this method requires a counter electrode facing the film to be dissolved, which requires large-scale decontamination or complicated piping systems. Decontamination is difficult. The latter electron injection method is, in principle, an excellent method that allows selective dissolution of only the iron oxide film, but the electrolytic bath that strengthens the reducing power and its cathode materials are limited, and the stability of decontamination performance is affected. , reliability remains a problem.

Compared to these existing or under development methods, the present invention has the following advantages:
The point is that only the iron oxide film is selectively dissolved without dissolving the base material.
It is possible to decontaminate complex piping and equipment systems, there is little worry about residual liquid, there is a high degree of freedom in selecting electrolytic cells and their cathode materials, and they are simple, stable, and reliable. The purpose of this invention is to provide an excellent method for dissolving iron oxide films.

The characteristics of the iron oxide film dissolving method of the present invention are as follows: An electrolytic tank is provided outside the piping or equipment in which the iron oxide film is to be melted, and a liquid path is provided to allow lines of electric force to reach the film from the electrolytic tank. The electrolytic cell is connected to the inside of the piping or equipment, and the leakage current from the electrolytic tank is passed through this liquid path to the metal base material of the piping or equipment, including the coating, under a cathode potential lower than the natural potential. The purpose is to generate cathodic polarization, thereby cathodically dissolving the film.

First, the present invention will be generally explained. The component of the iron oxide film that adheres to the base material piping is magnetite (ve5o4).
It is classified as hematite (α-Fe2es). The dissolution mechanism involving electrons between these oxides and the base material carbon steel or stainless steel is as follows.

Fe→Fe +2e″″・River・River・Seal・ (1) Fe304+8H”+2e−→3 Fe
2" +4 H2O・"' (2) Fe203+6
H"+2e-→2 Fe 2++ 3 H2O...(
-f) In other words, in iron, the oxidation reaction that releases electrons proceeds as shown in equation (1).5On the other hand, in magnetite, the reduction reaction that takes in protons (H+) and electrons proceeds as shown in equation (2). As shown in equation (3), the reduction reaction also progresses for hematite, although the number of protons is different from that for magnetite. Taking advantage of the fact that iron oxide undergoes a reduction reaction, a cathode reaction occurs in the base material that has an iron oxide coating.If a current is passed under an appropriate constant cathode potential lower than the natural potential, The iron oxide film dissolves without dissolving the base metal iron ions.

Generally, the iron oxide film that is targeted for system decontamination in nuclear power plants is extremely thin, on the order of microns, and only requires a small amount of current to dissolve it. In order to pass a current, a counter electrode is provided opposite the film, and polarization is required between the electrode and the counter electrode. Since this is not possible, an external electrolytic cell is connected to the piping with a liquid path that allows electric lines of force to reach the oxide film from the outside, and the leakage current from the electrolytic cell passing through this is transferred to the piping containing the iron oxide film. The medium used in this case is preferably a complexing agent or reducing agent solution that has ionic conductivity and good affinity or reactivity with iron oxide. A high p value is preferable because current flows more easily.

Next, the present invention will be explained by examples. FIG. 1 is a diagram schematically showing an example of the iron oxide film removal flow for carrying out the method of the present invention. A system 2 for recirculating decontamination liquid is connected to piping 1 of a plant whose iron oxide film is to be removed. Between both ends of the recirculation system 2 is an electrolytic cell 3. Storage tank with heating source 4. Ion collector5. A liquid feeding pump P is arranged.

The electrolytic cell 3 is partitioned by an ion exchange membrane 6 so as to perform diaphragm electrolysis, and is separated into an anode chamber 7 and a cathode chamber 8. Although the anode and cathode of the electrodes of the electrolytic cell 3 may be made of the same material, it is desirable that the surface area of the cathode is smaller than that of the anode in order to facilitate leakage current. The anodic chamber solution should be a highly conductive solution that does not change even when subjected to electrolytic oxidation, and the catholyte should be a decontamination solution containing a complexing agent or reducing agent that is highly reactive with the iron oxide film. . A ground 9 is connected to the cathode of the electrolytic power source and the pipe 1 so that they have the same potential. In such a system, when decontaminating the piping 1, close both valves shown in the diagram of the piping 1 near both end connections of the decontamination liquid recirculation system 2, and pump P recirculates the decontamination liquid in the direction of the arrow. When circulated, the decontamination liquid receives an electric charge in the cathode chamber 8, and this electric charge conducts through the liquid and enters the pipe 1, where it passes through the iron oxide film and is received by the earth 9. Piping 1 as the charge moves
The dissolution of the iron oxide film on the inside of the container progresses, and it is eluted into the decontamination solution as iron ions. The iron ions eluted into this solution are removed by an ion collector 5 composed of an adsorbent or the like, and then the decontamination solution is heated to a set temperature in a storage tank 4 and placed in an electrolytic tank 3.
It returns to the cathode chamber 8 and is further recirculated.

Experimental examples showing the effectiveness of the method of the present invention will be explained below.

First, experiments were conducted on the polarization characteristics of iron oxide and base material. Using a pellet (surface area 1 cm) made by firing and pressure-molding magnetite powder as the iron oxide, and a specimen of SUS 304 stainless steel (surface area 1-) as the base material, the husband's current and Polarization characteristics were determined by measuring potential changes.

Electrolytic support solution is 0.002 M/l EDTA・2NH4
The polarization characteristics of magnetite determined in the above experiment are shown in Figure 2.
Furthermore, the polarization characteristics of SUS 304 are shown in FIG.

As shown in Figure 2, magnetite exhibits the characteristic that the direction of current flow changes depending on the way the potential is swept, and this is due to the semiconducting properties of magnetite and changes in the electric double layer at the gas-liquid interface. Therefore, considering a sweep in the positive direction, −o, s
On the other hand, as shown in Figure 3, 5US3Q4 exhibits characteristics different from those of magnetite, which indicates that the electric double layer accompanying film formation is small, and that the electric double layer in the forward sweep is Anode dissolution is -0.4V
This shows that it is exceeded at a potential above. In this way, the potential at which dissolution occurs can be determined from the polarization characteristics of each specimen.

Next (C) The dissolution test conducted in this experiment will be described.

The general configuration of the dissolution test apparatus is shown in Figure 4.

This is diaphragm electrolytic cell 10. Sample dissolution tank 11. Temperature adjustment tank 1
2. Liquid pump 13. This is a series configuration of flow meters 14. The amount of retained liquid is 51. As for the electrical system, the cathode of the DC power supply 15 of the electrolytic cell 10 is grounded 16, and the melting 1aI
The conductive base material of the specimen 17 to be installed in IK is grounded 18. The potential and current of the specimen are measured using a potentiometer 20 connected to the saturated agaric electrode 19 in the dissolution tank 11 and the base material of the specimen, and an ammeter 21 connected between the base material of the specimen 17 and ground. I went. The specimens are magnetite and SUS 30.
4 was used. The magnetite specimen is a fired product connected to a SUS substrate and coated on one side with an adhesive, and has a surface area of 10 crn. The 5US3Q4 specimen is coated on one side with adhesive, similar to the magnetite specimen.
The surface area is 12z. Each of the above devices 10, 14,
12.0, 002'M/l EDTA through 13 -
The 2Nu4 liquid 61 was circulated at a rate of 250 ml/min at a set temperature of about 60°C.

In this dissolution test, the current in the external electrolytic cell 10 was adjusted to keep the current to the test specimen constant.

The amount of dissolved iron ions at this time was determined by analyzing the liquid sampled with the valves 22 before and after the sample dissolving tank 11. Figure 5 shows the test results for magnetite.

As can be seen from this figure, changing the cathode current IC changes the cathode potential E. is hardly affected, but the iron ion dissolution rate Vm changes correspondingly. Therefore, it can be seen that iron ions dissolve depending on the strength of the cathodic current flowing through the magnetite. On the other hand, stainless steel (SUS 30
The test results for 4) are shown in Figure 6.

As this figure shows, in the case of this sample, almost no dissolution of iron ions is observed even when the same cathode current IC as in the case of the magnetite sample is applied. =0.25mA.

Only a slight dissolution was observed at Ec'' of 0.4V.

Therefore, τ5tlS 304 has a cathode potential of 10, 4
It can be seen that if it is made smaller than V, it will dissolve easily.

From the dissolution and decomposition characteristics of iron ions shown in Figures 5 and 6 and Figures 2 and 3 above, in order to dissolve the iron oxide film, the cathode potential EC should be lower than -0.8V. The cathode potential of SUS 304 generated by this current is smaller than that of SUS 30.
It can be seen that No. 4 does not dissolve. Therefore, it can be seen that if a cathode current is applied to the SUS 304 stainless steel base material to which the magnetite film is attached under the cathode potential of the above value, only the film can be selectively dissolved without causing dissolution of the base material.

As is clear from the above description, according to the present invention, iron oxide films adhering to the inner surfaces of complex piping and equipment can be removed without dissolving the base metal, and in particular, by providing a counter electrode in opposition to the iron oxide film. An excellent effect can be obtained by dissolving it without the need for it.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a system for removing iron oxide film on the inner surface of piping by implementing the method of the present invention, and Figures 2 and 3 are experimental graphs showing the polarization characteristics of magnetite specimens and stainless steel specimens, respectively. , FIG. 4 is a schematic diagram of the dissolution test apparatus used to confirm the effectiveness of the present invention, and FIGS. 5 and 6 are experimental graphs showing the dissolution characteristics of magnetite specimens and stainless steel specimens, respectively. . 1... Piping 2... Decontamination liquid recirculation system 3... Electrolytic tank 4... Storage tank 5... Ion collector 6... Diaphragm 7... Anode chamber
8...Cathode chamber 9...Earth 10...
・Electrolytic tank 11... Sample dissolution tank 12... Temperature adjustment jet 13... Pump 14... Flow meter 15
... Power supply 16 ... Earth 17 ... Test piece 18 ... Earth 19 ... Sweet tooth electrode 20 ... Potentiometer 21 ... Ammeter
22... Valve r Te angle -1-+ Agent Honta Ko outside] ;;・ I 11 Figure 2, Figure 4, Kotobuki Figure 5 t (a) 1, f shi 6 1'? 4 He-A + 3 ÷, c-One-r)-1t(
fO

Claims (1)

[Claims] 1. A method for dissolving an iron oxide film adhering to the inner surface of piping or equipment through which water is passed, in which an electrolytic bath is provided outside the piping or equipment, and the electrolysis is carried out up to the film. The electrolytic cell is connected to the inside of the above-mentioned piping or equipment through a liquid path through which electric lines of force reach from the tank, and the leakage current from the electrolytic cell is transferred through the liquid path to the metal base of the piping or equipment containing the coating. 1. A method for dissolving an iron oxide film, which comprises passing the material under a cannidic potential lower than its natural potential to cause cathodic polarization, thereby cathodically dissolving the film. 2. The iron oxide film according to claim 1, characterized in that the electrolytic cell is partitioned with a diaphragm, its cathode and the metal base material are grounded, and the catholyte of the electrolytic cell is allowed to flow into the liquid path. Dissolution method. 3. The iron oxide film according to claim 1.2, wherein the catholyte contains a complexing agent or reducing agent that has ionic conductivity and has good affinity with iron oxide. Dissolution method.