JPS61241700A - Decontamination method using fixed-bed electrolytic cell - 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] [Field of Application of the Invention] The present invention relates to a decontamination method for removing oxide films adhering to metal surfaces, and in particular to decontamination methods for removing oxide films containing radionuclides adhering to nuclear power plant equipment and piping. This invention relates to a decontamination method that efficiently dissolves and processes waste products generated during dissolution in the same electrolytic cell.

[Background of the invention]

In the equipment and piping of the primary cooling system of nuclear power plants,
C'o in the cooling water when an oxide film is formed on the metal surface.
It is known that radioactive nuclides such as ``yMn'' are incorporated into this film, increasing the linear layer ratio on the surface of equipment and piping, which tends to increase the radiation dose of workers during periodic inspections and repairs. It is being In order to represent the exposure doses of workers, etc., decontamination technology that dissolves and removes the oxide film containing radioactive nuclei and seeds is essential. Initially, acids and complexing agents were mainly used as decontamination solutions for dissolving and removing oxide films, as described in JP-A-53-731. Since this decontamination liquid is highly acidic, it is effective from the viewpoint of removing the oxide film, but there is a risk that the base material, that is, the metal material, will also be dissolved, and there remains a risk that the base material will be corroded by the residual decontamination liquid. Because nuclear plants require a high level of safety, these decontamination methods were not necessarily suitable for nuclear plants.

Therefore, a method of decontamination by lowering the concentration of decontamination solution was researched and developed in order to alleviate the corrosion of the base material. However, by lowering the concentration of the decontamination solution, the decontamination effect decreased. In response to this, a method has been devised in which a reducing agent is further added to reduce the dissolution rate of the oxide film and thereby enhance the decontamination effect. The reducing agent is consumed in dissolving the oxide film, and in order to enhance the decontamination effect, a method is required to reduce and regenerate the oxidized reducing agent. One such method is a method of electrolytically reducing a decontamination solution, which is described in JP-A-57-85980.

In order to reductively regenerate the liquid, problems such as inefficiency remain with conventional electrolytic cell structures and electrode materials.

Additionally, it is necessary to dispose of waste fluids generated as a result of decontamination. CAN-DE, a typical removal method developed in Canada
The CON method uses a mixture of anionic and cationic resins to remove decontamination fluid and dissolved contaminants. As described above, when treating waste liquid, a separate waste liquid treatment device is required, making it difficult to simplify the decontamination device.

[Purpose of the invention]

The purpose of the present invention is to efficiently dissolve the oxide film containing radionuclides on the surfaces of equipment, piping, etc. by suppressing corrosion of the base material, and to dissipate the decontamination solution containing the eluted radionuclides in the same electrolytic tank. The purpose of the present invention is to provide a decontamination method that has the function of allowing treatment.

[Summary of the invention]

The method of the present invention dissolves and removes an oxide film containing radionuclides such as G o "HM n " attached to the surfaces of nuclear power plant equipment and piping, etc., and also removes radionuclides generated due to the dissolution of the oxide film. The solution containing the above is processed in the same electrolytic cell. Bypass piping will be connected to the object to be decontaminated, and a decontamination agent adjustment and degassing tank, fixed bed electrolyte layer, and circulation device will be installed. A fixed bed electrolytic cell is divided into an anode chamber and a cathode chamber by a diaphragm provided between flat electrodes, and each electrode chamber is filled with conductive solid particles. The membrane that separates the anode chamber and the cathode chamber is a cation exchange membrane that can prevent the diffusion of oxygen generated by electrolysis and selectively transmit hydrogen ions, or a cation exchange membrane that can selectively permeate hydrogen ions.
Use a diaphragm that has this function.

The oxide film that adheres to equipment and piping is magnetite (F
e304), hematite (a-Fez Oa)
is the main component and is often deposited non-uniformly. These oxide films are dissolved and ionized by reduction.

The cathode chamber of the fixed bed electrolytic cell has the function of regenerating the reducing agent contained in the decontamination liquid that has deteriorated due to dissolution of the oxide film and strengthening the reducing power. The filling material in the cathode chamber is preferably solid particles that can efficiently regenerate and strengthen the degraded reducing agent, and graphite particles, metal particles with a large hydrogen overvoltage, or solid particles whose surfaces are coated with these metals are suitable.

On the other hand, in the anode chamber of a fixed bed electrolyzer, the solution containing radionuclides generated by the dissolution of the oxide film is electrically adsorbed and
It has the function of occlusion processing. Activated carbon and graphite particles or inorganic adsorbents are suitable as the solid particles to fill this anode chamber.

Further, the decontamination liquid used in the present invention comprises a complexing agent and a reducing agent. As a complexing agent, ethylenediaminetetraacetic acid, citric acid, etc.
As a reducing agent, a metal ion with a low oxidation-reduction potential such as 6) - ascorbic acid, Cr"+ or V2+ is preferable. The decontamination solution should be heated at 60 to 90°C before being supplied to the object to be decontaminated. It is desirable to keep the dissolved acid concentration sufficiently low.

By filling conductive solid particles between the flat plate electrodes as described above, the electrode area of each electrode chamber can be increased and the electrolysis efficiency can be improved. As a result, it is possible to regenerate and strengthen the reducing power of the reducing agent and to treat a solution containing radionuclides generated due to the dissolution of the oxide film in the same electrolytic cell. Furthermore, since it also processes solutions, there is no need for incidental equipment for solution processing, making it possible to downsize the decontamination equipment.

[Embodiments of the invention]

The present invention will be described below with reference to Examples.

First, FIG. 1 shows a decontamination flow when the method of the present invention is applied to an actual plant.

The decontamination tank is roughly divided into a deaeration tank 2 and a fixed bed electrolytic tank 3 for the equipment and piping 1 of the object to be decontaminated, and the decontamination liquid is circulated to remove the oxide film in the dissolution tank. The degassing tank 2 is equipped with a decontamination liquid adjustment tank 4 and a degassing N2 or argon cylinder 5.

It is also provided with a heater 6 for heating. The fixed bed electrolytic cell 3, which is the key point in this decontamination flow, is partitioned by a diaphragm into an anode chamber and a cathode chamber, each of which is filled with solid particles, and connected to a DC power source 7 for electrolysis. The decontamination solution consists of a complexing agent and a reducing agent (including metal ions in a low oxidation state such as Cr" and V"), and is adjusted in the decontamination solution adjustment tank 4 and transferred to the degassing tank 2 where it is heated and degassed. is carried out and supplied to the fixed bed electrolytic cell 3 by the circulation pump 8. The decontamination liquid supplied to the electrolytic cell 3 is electrolytically reduced to strengthen its reducing power and to regenerate the degraded reducing agent. This decontamination agent is supplied to the object to be decontaminated. The resulting peeled cladding is removed by a filter 9, and the decontamination solution containing the radionuclide complexed with the complexing agent as the oxide film is dissolved is sent to the electrolytic cell 3 and recovered. As described above, the present invention is characterized by regenerating and strengthening the reducing agent in the decontamination solution and treating the decontamination solution containing eluted radioactive materials in the same electrolytic cell, and when applied to an actual plant. Decontamination methods are effective.

Furthermore, a dissolved oxygen meter 10 and a pH 11 meter are installed in the decontamination flow to monitor the decontamination solution.

FIG. 2 shows the structure of a fixed bed electrolytic cell 8 for the purpose of regenerating the reducing power of the decontamination solution (including strengthening the reducing power) and removing eluted substances. The electrolytic cell 3 has a diaphragm 1 between flat electrodes 12.
3, which is partitioned into a cathode chamber 14 and a melt chamber 15.

The cathode chamber 14 contains graphite particles and Pb with a large hydrogen overvoltage.
, Zn, or other metal particles, or particles coated with these metals. On the other hand, the anode chamber 15 is filled with graphite particles or activated carbon particles. As described above, the purpose of the cathode chamber and anode chamber in the same electrolytic cell is to process the decontamination solution, including regenerating the reducing power of the decontamination solution and removing eluate. The cathode chamber is filled with particles and has a large effective electrode, making it suitable for efficient electrolytic reduction of degraded decontamination fluid. The anode chamber is also filled with particles, making it easy to collect and process the decontamination solution containing radionuclides generated as the oxide film dissolves.

Further, the anode chamber is used for final waste liquid treatment.Examples of the present invention and their results will now be described. First, FIG. 3 shows a test device used to examine the effectiveness of the present invention. As described above, the fixed bed electrolytic cell 16 used in the test was partitioned into an anode chamber and a cathode chamber by a diaphragm, and a PT plated plate was used as a flat electrode. The anode chamber was filled with activated carbon particles, and the cathode chamber was filled with graphite particles. A DC power source was connected to the electrolytic cell, and electrolysis was carried out at a current of 0.4 A. The electrolyzed decontamination agent was supplied to a glass test tank 19 by a circulation pump 18.

The test chamber 19 includes a ribbon heater 20 for heating, a diffuser tube 21 for deaeration, and an electrometer 23 for measuring the potential of the test piece 22.
has been established. The test piece 22 used in the test was one removed from the piping of a nuclear power plant. The test piece is 151m
A lead wire was connected to a wire of approximately 151 m.

In addition, the decontamination solution contains 0.006Mzl-ED as a complexing agent.
TA・2NH4, 0.002M/Q as reducing agent
-L.ascorbic acid and metal reducing ion (c, 2
+) was thoroughly degassed and heated to 80°C before use. In the test, a decontamination solution electrolytically reduced in the cathode chamber of the electrolytic tank is supplied to the test tank, and the radioactivity of the test piece immersed in the test tank is reduced by G.
e (Li) It was measured with a semiconductor detector and evaluated by the Go'' removal rate.

The test results are shown in FIG. Figure 5 shows Co@a on the vertical axis.
The removal rate (%) is shown, and the horizontal axis shows the decontamination time (h). For comparison, a case where the sample is simply immersed in a decontamination solution is also shown. Co' if simply immersed. The removal rate is 10
%, which is low.

On the other hand, it can be seen that a large removal rate can be obtained when the decontamination solution is electrolyzed. Among these, it can be seen that when a fixed bed electrolytic cell is used, it is more effective than a diaphragm electrolytic cell using a flat plate electrode.

As described above, it has been demonstrated that the fixed bed electrolytic cell used in the present invention is suitable for electrolytic reduction of a decontamination agent and is effective in decontamination.

Next, we will present the results of a study on the effectiveness of another function of the fixed bed electrolyzer. Figure 4 shows the test equipment used in the study. The test equipment was an open-type diaphragm electrolytic cell 24 filled with activated carbon particles 25 in both electrode chambers, and the test was carried out in a batch manner.

As a treatment liquid (waste liquid), a complex of Fe ions [Fe-ED
TA)- was adjusted to 110 PP and placed in the electrolytic cell bipolar chamber, and the solution in the anode chamber was sampled and analyzed at each hour.
The effectiveness of waste liquid treatment was confirmed. Further, the electrolysis current was 0.4A constant current electrolysis as in the previous example.

The results are shown in FIG. In FIG. 6, the vertical axis shows the concentration of Fe-EDTA as waste liquid, and the horizontal axis shows the electrolysis time. As is clear from the figure, Fe-ED in the waste liquid increases as the electrolysis time progresses.
The TA concentration decreased, indicating that it was adsorbed on the particle surface. This demonstrated that it is possible to treat waste liquid in the anode chamber of an electrolytic cell filled with solid particles.

Based on the above test results, by using fixed bed electrolysis during decontamination, the reducing power of the reducing agent in the decontamination solution is regenerated and strengthened in the cathode room, and in the anode room, the waste liquid containing eluted radionuclides is collected using the same electrolysis method. It has been demonstrated that this can be done in a tank.

〔Effect of the invention〕

According to the present invention, an oxide film on a metal surface can be efficiently removed by electrolytically reducing a decontamination agent using a fixed bed electrolytic cell and supplying the decontamination agent to an object to be decontaminated. Furthermore, since the oxide film is reduced and dissolved away from the decontamination liquid side, it can be applied to complex equipment, piping, etc. Furthermore, waste liquid containing radioactive nuclides generated due to dissolution of the oxide film can be collected and processed within the same electrolytic cell. This eliminates the need for incidental equipment such as waste liquid treatment, which is effective in simplifying decontamination equipment.

[Brief explanation of drawings]

Figure 1 is a diagram showing a decontamination device when the present invention is applied to an actual plant, Figure 2 is a structural diagram of a fixed bed electrolyte layer used in the present invention, and Figures 3 and 4 are diagrams showing the effectiveness of the present invention. Fig. 5 is a schematic diagram of a test device for demonstrating the effectiveness of the present invention, and Fig. 5 is a test result showing the effectiveness of the present invention, and a diagram showing the relationship between the removal rate of Co" by electrolytic reduction of the decontamination solution and the decontamination time. FIG. 6, like FIG. 5, shows the test results showing the effectiveness of the present invention and is a diagram showing the study results of waste liquid treatment. 1. Object to be decontaminated, 2. Degassing tank , 3... Fixed bed electrolytic cell, 4... Decontamination adjustment tank, 8... Circulation pump, 9... Filter, 10... Dissolved oxygen meter, 1
DESCRIPTION OF SYMBOLS 1... PH meter, 12... Flat plate electrode, 13... Diaphragm, 14... Cathode chamber, 15... Anode chamber.

Claims (1)

[Claims] 1. In a method for removing oxide films adhering to metal surfaces of nuclear power plant equipment, piping, etc., decontamination is performed using a fixed bed electrolytic cell in which conductive solid particles are filled between flat electrodes. The degraded reducing agent in the liquid is regenerated by electrolytic reduction,
A fixed-bed electrolytic cell is used, which is supplied to the object to be decontaminated, ionizes and removes the oxide film, and electrically adsorbs or occludes a decontamination solution containing radioactive ions in the same fixed-bed electrolytic cell. Decontamination method. 2. The fixed bed electrolytic cell used in the decontamination method is divided into a cathode chamber and an anode chamber by a diaphragm, and the cathode chamber is filled with graphite particles and metal particles with a large hydrogen overvoltage, or particles coated with these metals, and the anode chamber is filled with graphite particles and metal particles with a large hydrogen overvoltage, or particles coated with these metals. A decontamination method using a fixed bed electrolytic cell according to claim 1, characterized in that the cell is filled with activated carbon particles, graphite particles, or an inorganic adsorbent such as ferrite or titanium oxide. 3. The fixed bed electrolytic cell regenerates the reducing power of the reducing agent in the deteriorated decontamination solution in the cathode chamber, and processes the decontamination solution containing radioactive ions in the anode chamber.
A decontamination method using the fixed bed electrolytic cell described in Sections 1 and 2. 4. The decontamination solution used in the decontamination method is a neutral solution containing a complexing agent and a reducing agent with a pH of 5 to 7, and further reduces the dissolved oxygen concentration in the decontamination solution to about several tens of ppb. A decontamination method using a fixed bed electrolytic cell according to claims 1 and 2, characterized in that the electrolytic cell is heated to 60 to 90°C and supplied to the object to be decontaminated.