JPS6225300A - Method of dissolving oxide - 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 method for dissolving oxides attached to metal surfaces, and in particular to dissolving oxides containing radionuclides attached to atomic couplant equipment and parts. The present invention relates to a method for dissolving oxides that can remove target VC and suppress corrosion damage to constituent materials.

[Background of the invention]

For nuclear couplants, as the operating period progresses.

- Radionuclides ("Co,"
There is a tendency for oxides containing ``Mn, etc.'' to adhere and increase radioactivity.For this reason, the exposure dose of operators and workers during plant operation and periodic inspections tends to increase, and radionuclides are There is a need for decontamination technology that dissolves and removes the contained oxides.

When decontaminating equipment and parts to be reused, it is necessary to suppress corrosion of constituent materials during decontamination and efficiently dissolve and remove oxides.

Generally, as a method for removing oxides adhering to a metal surface, a method is known in which a counter electrode is provided and a current is applied directly to the constituent material to cause electrolysis, that is, an electrolytic polishing method. This method is
A high current is applied to dissolve and peel off the oxide along with the constituent materials, so it is not suitable in terms of suppressing corrosion of the constituent materials.

In addition, the chemical decontamination method using chemicals was published in Japanese Unexamined Patent Publication No. 53-7
As typified by No. 31, these decontamination solutions mainly contain acids and complexing agents.

Since the pH is relatively low, it is effective in dissolving oxides. However, there is a risk that the constituent materials may also dissolve. Particularly in nuclear couplants, a high level of safety is required, so the decontamination methods described above are not necessarily suitable for reusable equipment and parts. Knowing this, use a neutral solution decontamination solution,
In addition to alleviating corrosion of constituent materials, a method was invented in which the decontamination solution is electrolyzed to regenerate the decontamination solution, that is, the reducing agent, and to dissolve nona-oxides that adhere to the surface of the object to be decontaminated. One of the decontamination methods using this method is JP-A-57-85980. This decontamination method injects more electrons by directly cathodically polarizing the object to be decontaminated.

There are methods of dissolving oxides and methods of dissolving them by injecting electrons from the decontamination liquid side, but this operation lowers the object to be decontaminated to the anti-corrosion potential of the constituent materials, resulting in a large overvoltage. Unfavorable in terms of energy efficiency. Furthermore, the determination at the end of decontamination was also unclear. Further, as a similar technique, Japanese Patent Application Laid-Open No. 59-83800 can be mentioned. This technique dissolves oxides by lowering the potential of the object to be decontaminated to -1.0 V vs -8 CE, and has the advantage of being easy to decontaminate. However, it is similar to the method described above. When the cathode is polarized, the overvoltage increases, which is not desirable in terms of energy efficiency.

As described above, conventional decontamination methods are capable of dissolving oxides adhering to metal surfaces, but are not preferred because of the problem of corrosion of constituent materials during decontamination. Also, the end point of decontamination was not clear.

[Purpose of the invention]

The object of the present invention is to provide an atomic couplant, etc.

To obtain a method for dissolving oxides that electrochemically dissolves and removes oxides attached to equipment, parts, etc. while suppressing corrosion of I-4 constituent materials, and also allows for a clear decontamination end point. .

[Summary of the invention]

The features of the present invention are as follows:1. An object to be decontaminated containing an oxide is immersed in an electrolytic solution, and a sacrificial electrode having a lower potential than the object is electrically contacted or connected to the object, or a constant voltage power source is applied from an external source to the object. By connecting between an object and a counter electrode, the potential of the object is cathodically polarized between the natural potential and a potential that does not reach the active decomposition range, and oxides attached to the surface of the object are reduced and dissolved. It's about letting people know.

[Embodiments of the invention]

The present invention effectively dissolves oxides containing radioactive nuclides (60Cr+r''Mn, etc.) attached to the surfaces of atomic couplant equipment and parts, and detects potential changes of objects to be decontaminated. It is used to determine the end point.

The oxides adhering to the surfaces of equipment and parts in the primary cooling water system of the atomic couplant are Fe3O4 (magnetite),
α-Fe203 (hematite) is the main component, and radionuclides ("
In addition, the oxide produced by oxidation of the surface of the sea urchin and the radioactive nuclide contained within the oxide can be dissolved into ionic form by using an oxidation-reduction reaction. Oxides can be dissolved by receiving electrons (being converted), that is, by lowering the potential.However, when dissolving oxides, in order to suppress corrosion of the constituent materials, It is necessary to set the potential in a range where the constituent materials do not corrode.

In the present invention, carbon steel having a base potential is brought into electrical contact with the object to be decontaminated to form a battery between the object and the carbon steel. This reduces the potential of the object to be decontaminated and reduces and dissolves the oxides on the surface. The potential of the object to be decontaminated when in contact with carbon steel shows a hybrid potential with the potential of carbon steel, and depends on the area ratio to the carbon steel.The potential of the object to be decontaminated when in contact with carbon steel is controlled by adjusting the area ratio. I can do it. When the constituent material is stainless steel, it has a uniform metal melting region between -0.65V and -0.75V, which causes overvoltage to increase at a potential lower than this, which is unfavorable in terms of energy efficiency. The present invention deals with areas where materials do not dissolve and where oxides dissolve (-0,3V to -0,65V (VS
5CE), and the potential is controlled by increasing the area of carbon steel by 0.2 to 1.5 times the area of the object to be decontaminated. Also.

Depending on the object to be decontaminated, a counter electrode may be provided to the object to be decontaminated, and oxidation may be performed using an external source to dissolve the oxide.

Furthermore, when carbon steel comes into contact with objects to be decontaminated.

As the oxide on the surface of the object is dissolved, the potential of the object decreases. Depending on the purpose of decontamination, this decontamination operation may end at -0.65V, which enters the metal uniform dissolution region, or may continue decontamination even if the potential enters this region, uniformly dissolving the metal surface and preventing activation. In some cases, contaminated surfaces and radioactive A species may be completely removed. Based on the above, the above method can be applied to decontamination operations when applied to equipment and parts that are to be reused and when applied to equipment and parts that are to be disposed of.

The decontamination solution consists of a solution containing decontamination and organic acids.
A neutral solution with a pH of 5-7 is used. When applying, the decontamination solution should be sufficiently degassed with an inert gas and at a temperature of 60 to 90%
Heat for tK and use [e].

Next, specific examples will be listed and explained below.

FIGS. 1 and 2 are diagrams showing a method for dissolving oxides of equipment, parts, etc. of atomic couplants.

Figure 1 shows a carbon steel sacrificial electrode 2 connected to a decontamination object 3.
An example is shown in which an f-element operation is added. Object 3
Half of the oxide is removed by dissolving the electrode 2. The decontamination liquid 4 is degassed by injecting Af or N2 gas through an injection pipe 5, and its temperature is raised by a heating heater 6.

In decontamination tank 1, there is circulation to stir the decontamination solution. It is equipped with a line 7 and a circulation pump 8. Also, Drainro 9.
It is also equipped with a vent 10. During decontamination, a reference voltage is installed in the decontamination tank because the level changes as the oxides of the object to be decontaminated progress.

Monitored with potentiometer 12. By monitoring the potential, it is determined whether the target object has entered the active dissolution zone, and the end of decontamination is determined. This operation varies depending on the purpose of decontamination.
Decontamination ends when the active solubility range is reached. Also,
For equipment and parts that are allowed to corrode within acceptable limits, even if they reach the active dissolution zone, decontamination should continue to uniformly dissolve the metal surface (several μm) and remove radionuclides and activated surface layers. .

In FIG. 2, electrolysis is performed using an external power source 13.

This is an example in which oxides of an object to be decontaminated are dissolved. In this case as well, as in Figure 1, for objects that you do not want to corrode, use a potential lower than the active dissolution range.

and electrolyzes at a potential that dissolves the oxide. In this case, the potential is determined by the reference electrode saturated sweet electrode (SCE) in a neutral solution containing a complexing agent and an organic acid and having a pH of 5 to 7.
to -0,3 to -〇, 65 V. This allows the fourth
Similar to the method shown in the figure, corrosion of the object to be decontaminated can be suppressed and only oxides can be dissolved.

Also, for the proboscis, which is subject to decontamination for the purpose of disposal.

Since it is necessary to reduce the per-force per-surface dose, the potential of the object is set in the active dissolution region, and an operation is performed to dissolve the oxide and metal surfaces.

Furthermore, when implementing the method shown in FIG. 1, it is necessary to set the area of the carbon steel electrobalance. Depending on the electrode area to be connected,
This is because the potential of the object to be decontaminated is different.

In order to avoid the active dissolution zone and dissolve oxides, the carbon steel area should be set between 0.2 and 1.5 times the area of the object to be decontaminated.

As described above, when the present invention is used to decontaminate equipment, parts, etc. of an atomic couplant, an effective decontamination effect can be obtained, and the decontamination equipment itself can be simplified.

FIG. 3 shows a test device in which the potential dependence of the constituent materials of the object to be decontaminated and the dissolution of oxides was investigated. This test equipment is roughly divided into 14. Constant potential source 15. The decontamination liquid 17 is degassed with Ar gas injected from the deaeration diffuser 18. Between the constituent materials and the test piece 19 of the oxide and the counter electrode 20, cand fractionation was performed at a constant potential, with the saturation ratio of the reference electrode being 21%. The frozen decontamination solution was heated at 900 °C with a 5% concentration solution containing no complexing agent and organic acid. The dissolved amount of the modified product was determined by measuring iron ions in the melt using an atomic absorption spectrophotometer. Also.

Regarding the dissolution of the constituent materials, the amount dissolved was determined by weight loss before and after immersion. The results are shown in FIG.

The vertical axis shows the iron dissolution amount (mg/-・h), and the horizontal axis shows the potential (Vv
s, 5CE). The dissolution of the constituent materials is -0,
It can be seen that the active solubility range is between 65 and 0.75V. On the other hand, the dissolution of oxides tends to increase as the potential decreases. From this, six conditions are required to suppress the dissolution of the constituent materials and promote the dissolution of the oxide.

It can be seen that the potential should be set between -0.3V and -0.65V. To dissolve oxides, there are two methods: one is to set the potential externally, and the other is to lower the potential to an appropriate level by electrically contacting carbon steel, etc., which has a base potential. In order to lower the potential to an appropriate level using this method, the area ratio of the carbon steel to be brought into contact must be considered. The fourth test device was used to examine changes in potential depending on these area ratios.
Illustrated.

This test device f has charcoal in the melting tank 14. #422 and oxide 23 are connected via a non-resistance trN, total 24, and changes in potential are measured with a potentiometer 25.

Figure 6 shows this result, with the vertical axis plotting the potential (V
vs-S CE )=The area ratio of oxide (Fe304) and carbon steel is shown in the lateral movement. What are the test conditions?

This is the same as in the previous embodiment. As can be seen from Figures 5 and 5g6, in order to suppress the constituent materials and maintain a region where oxides can be dissolved when the potential decreases due to contact with carbon steel, oxides, that is, objects to be decontaminated, must be It is necessary to increase the surface area of carbon steel by 0.2 to 1.5 times the surface area.

In order to further demonstrate the effectiveness of the present invention, a specimen removed from the piping of an atomic couplant was used as a test piece.
An experiment was conducted using the test equipment shown in 15A under the same decontamination conditions as in the basic test 1 above. The area of the carbon steel connected to the object to be decontaminated was set to be the same area as the object. In addition, the radioactivity of the test piece was measured using a Qe(Li) semiconductor detector and evaluated based on the 60Co removal rate. The results are shown in FIG. In the figure, the vertical axis shows the removal rate (%) of 60co, and the horizontal axis shows the decontamination time.

60 CO and reached a saturated state at 1 degree for about 7 hours.

This shows that most of the oxides on the surface have been dissolved and removed. In this way, by electrically connecting carbon steel and maintaining it at an appropriate potential, a high decontamination effect can be obtained.

In addition, the potential of the object to be decontaminated changes moment by moment with the dissolution of oxides on the first surface, approaches the potential of the sacrificial electrode (carbon steel), and the fifth
It enters the active dissolution region (Kan-Kyun - dissolution region) shown in the figure. On the other hand, during dissolution, the potential of the object is monitored, and if the awakening point reaches the active dissolution region and the dissolution is suppressed, the dissolution is terminated. In addition, if the corrosive force y is permissible, the surface of the material may be melted17.
, can be continued for further effect.

In addition to the potential control method described above, a method of directly cathodically polarizing the object to be decontaminated, setting it to an appropriate potential, and dissolving and removing oxides can also be applied.

By setting the potential as described above, it becomes possible to suppress corrosion of the constituent materials and efficiently selectively dissolve oxides.

According to this embodiment, it is possible to efficiently dissolve oxides adhering to metal surfaces, particularly oxides adhering to atomic couplant equipment and parts, by setting the potential according to the purpose of decontamination. For example, for a decontaminated object that is intended for reuse after decontamination and is not to be corroded, the potential of the object is cathodically polarized from the immersion potential to a potential that does not reach the active dissolution range. In addition, for objects that are allowed to corrode, the potential of the object should be set to the active dissolution potential by an external power supply within the allowable limit, or the potential of the object should be set to the active dissolution potential due to the dissolution of the oxide. This can be divided into cases where decontamination continues and cases where decontamination continues. In the present invention, the cathode polarization operation is performed by externally setting a constant potential N source, or by electrically dissociating a sacrificial electrode with a low potential to control the potential of the object. In this way, since the potential is controlled by the above method according to the purpose of decontamination, oxides can be effectively dissolved and removed, and the integrity of the material can be maintained.

The present invention can be applied not only to atomic couplants but also to other plants, and can also be used for pretreatment of steel plates, etc., and can be used in many fields.

Therefore, by using the present invention, it is possible to easily decontaminate the equipment and parts of the atomic couplant, which facilitates the disassembly and inspection of nuclear equipment, and also reduces radiation exposure for workers. The reliability of the system can be improved.

〔Effect of the invention〕

According to the method of the present invention, oxides adhering to devices, parts, etc. can be electrochemically dissolved and removed while suppressing corrosion of constituent materials. Another advantage of the present invention is that the end point of decontamination becomes clear.

[Brief explanation of the drawing]

FIGS. 1 and 2 are system diagrams of an apparatus for carrying out the method of the present invention, FIGS. 3 and 4 are schematic diagrams showing a test apparatus for demonstrating the effectiveness of the present invention, and FIG. Figure 6 is a diagram showing the relationship between the amount of dissolved iron and potential; Figure 6 is a diagram showing changes in potential depending on the area ratio of oxide and carbon steel; Figure 7 is a diagram showing the test results confirming the effectiveness of the present invention. 2 is a diagram showing the relationship between decontamination time and 60Co removal rate. DESCRIPTION OF SYMBOLS 1... Decontamination tank, 2... Carbon steel, 3... Decontamination target object, 5... Gas injection pipe, 6... Heating heater, 8...
...Circulation pump, 11...Reference [i, 12...Potentiometer, 13...Outside''t0・
Do-ichi\1, () Agent Patent attorney Katsuo Ogawa -No 1 Hard $5 Hard e IE (V ys, 51:E) Kaya 6 Hard Fe3θ4-Return drop σ Kaya 7 Prison θ 5 /ρ
Shoeigincho (FL)

Claims (1)

[Claims] 1. In a method for dissolving oxides attached to metal surfaces,
An object to be decontaminated containing an oxide is immersed in an electrolytic solution, and a sacrificial electrode having a lower potential than the object is electrically contacted or connected to the object, or a constant voltage power source is applied from an external source to the object. By connecting between an object and a counter electrode, the potential of the object is cathodically polarized between the natural potential and a potential that does not reach the active dissolution region, and the oxide attached to the surface of the object is reduced and dissolved. A method for dissolving oxides characterized by the following. 2. A method for dissolving oxides according to claim 1, characterized in that a change in potential of the object to be decontaminated as the oxides are dissolved is monitored to determine the end point of dissolution. 3. In claim 1, the electrolytic solution mainly contains a complexing agent and an organic acid, has a pH of 4 to 7, and has a voltage of -0.3 V to -0. 65V vs.
, a method for dissolving oxides, characterized in that the potential is set between SCE and SCE. 4. The method for dissolving oxides according to claim 1, wherein the sacrificial electrode is made of carbon steel. 5. In claim 1, cathodic polarization by electrical contact or connection of carbon steel increases the surface area of the carbon steel by 0.2 to 1.5 times the surface area of the object to be decontaminated. Characteristic method of dissolving oxides. 6. The method for dissolving oxides as set forth in claim 1, characterized in that the dissolution is completed when the immersion potential of the object to be decontaminated reaches -0.65V vs. SCE.