JPH0672954B2 - Dissolution method of oxide - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for dissolving an oxide attached to a metal surface, and particularly to an oxide containing a radionuclide attached to a device / part of a nuclear power plant. The present invention relates to a method for dissolving an oxide, which is capable of effectively dissolving and removing, and suppressing corrosion damage of constituent materials.

[Background of the Invention]

In nuclear power plants, as the operating period elapses, oxides containing radioactive nuclides ( ⁶⁰ Co, ⁵⁴ Mn, etc.) adhere to the equipment and piping in the primary cooling water system, and the activity tends to increase. For this reason, the exposure dose to operators and workers tends to increase during plant operation and periodic inspections, and a decontamination technique for dissolving and removing oxides containing radionuclides is required.

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

In general, as a method of removing oxides attached to a metal surface, a method in which a counter electrode is provided and a current is directly applied to a constituent material to electrolyze, that is, an electrolytic polishing method is known. This method
A high current is applied to dissolve and exfoliate the oxide together with the constituent materials, which is not suitable in terms of suppressing corrosion of the constituent materials. Further, the chemical decontamination method using a chemical agent is one in which an acid and a complexing agent are main components as represented by JP-A-53-731, and these decontamination solutions have a relatively high pH. Since it is low, it is effective for dissolving oxides. However, there is a risk that the constituent materials will also dissolve. Particularly in a nuclear power plant, a high degree of safety is required, and therefore the above-mentioned decontamination method has not always been suitable for reused equipment and parts. On the other hand, a decontamination solution of a neutral solution was used to alleviate the corrosion of the constituent materials, and the decontamination solution was electrolyzed to regenerate the decontamination solution, that is, the reducing agent, and adhered to the surface of the decontamination target. A method of dissolving the oxide was invented. One of the decontamination methods using this method is JP-A-57-85980. In this decontamination method, there are a method of injecting electrons by directly cathodic polarization to the decontamination object and dissolving the oxide, and a method of injecting and dissolving electrons from the decontamination solution side. Since the object of decontamination is lowered to the anticorrosive potential of the constituent material, the overvoltage becomes large, which is not preferable in terms of energy efficiency. Moreover, the judgment at the end of decontamination was unclear. Further, as a similar technique, there is JP-A-59-83800. This technique is one in which the potential of the decontamination target is set to −1.0 V vs. SCE or less to dissolve the oxide, and has an advantage that decontamination can be performed easily. However, as in the above method, an overvoltage becomes large during cathodic polarization, which is not preferable in terms of energy efficiency.

As described above, the conventional decontamination method can dissolve the oxide adhering to the metal surface, but it is not preferable because of the problem of corrosion of the constituent materials during decontamination. The decontamination end point was also unclear.

[Object of the Invention]

An object of the present invention is to dissolve and remove oxides adhering to equipment and parts in a nuclear power plant and the like electrochemically while suppressing the corrosion of constituent materials, and also to clarify the decontamination end point. To obtain the dissolution method of.

[Outline of Invention]

In order to achieve the above object, the present invention is to immerse a decontamination object having an oxide attached thereto in an electrolytic solution, and electrically contact or connect a sacrificial electrode having a base potential lower than that of the object to the object. The target from −0.3V vs. SCE to −0.65V vs. SCE.
The potential of the object is reduced and dissolved, and the potential of the object accompanying the dissolution of the oxide is monitored. When the potential of the object reaches −0.65 V vs. SCE, the dissolution is completed. It is a point.

[Examples of the invention] The present invention effectively dissolves an oxide containing a radionuclide ( ⁶⁰ Co, ⁵⁴ Mn, etc.) attached to the surface of equipment and parts of a nuclear power plant, and at the same time, decontaminates The change in electric potential is detected to determine the end point of decontamination.

Oxides adhering to the surfaces of equipment and parts in the primary cooling water system of a nuclear power plant are Fe ₃ O ₄ (magnetite), α-F
e ₂ O ₃ (hematite) is the main component, and radionuclides that contribute to the dose rate when forming these oxides ( ⁶⁰ Co, ⁵⁴ Mn, etc.)
Contains. In this way, the oxides generated by the oxidation of the surface and the radionuclide contained in the oxides are
It can be dissolved and removed in an ionic state by utilizing the oxidation-reduction reaction. The oxide can be dissolved by receiving an electron (reduced), that is, by lowering the potential. However, in order to suppress the corrosion of the constituent materials when the oxide is dissolved, it is necessary to set the potential in a region where the constituent materials do not corrode.

In the present invention, carbon steel having a base potential lower than that of the decontamination target is brought into electrical contact with the carbon steel to form a battery.
As a result, the potential of the decontamination target is lowered, and the oxide on the surface is reduced and dissolved. The potential of the decontamination target at the time of contact with carbon steel indicates a mixed potential with the potential of carbon steel and depends on the area ratio with the carbon steel.The potential of the decontamination target should be controlled by adjusting the area ratio. You can When the constituent material is stainless steel, it has a uniform metal dissolution region between -0.65V and -0.75V, and at a potential below this, the overvoltage becomes large, which is not preferable in terms of energy efficiency. The present invention is to control the potential in the region where the material is not dissolved and where the oxide is dissolved (-0.3 V to -0.6 V (vs SCE), and the area of carbon steel is 0.2 with respect to the decontamination target. Depending on the decontamination target, a counter electrode is provided for the target to be decontaminated and electrolysis is performed by an external power source to dissolve the oxide.

Further, when the carbon steel is brought into contact with the decontamination target, the potential of the target decreases with the dissolution of the oxide on the surface of the target.
This decontamination operation enters the metal dissolution zone depending on the purpose of decontamination-
In some cases, the decontamination may be continued at 0.65V or even if a potential is applied to this area, the metal surface may be uniformly dissolved, and the activated surface and radionuclide may be completely removed. As described above, the decontamination operation can be applied by the above method when it is applied to reusable equipment / parts and when it is applied to equipment / parts to be discarded.

The decontamination solution consists of a solution containing a complexing agent and an organic acid,
Use a neutral solution of pH 5-7. At the time of application, the decontamination solution should be thoroughly degassed with an inert gas and heated to 60 to 90 ° C before use.

Next, specific examples will be described below.

FIG. 1 and FIG. 2 are diagrams showing a method of dissolving oxides in equipment / parts of a nuclear power plant.

FIG. 1 shows a carbon steel sacrificial electrode 2 connected to a decontamination target 3,
An example in the case of adding a reduction operation is shown. Object 3
Oxide is removed as the electrode 2 is dissolved. Decontamination liquid 4
Ar or N ₂ gas is injected from the injection pipe 5 to be deaerated, and the temperature is raised by the heater 6 for heating.

In the decontamination tank 1, a circulation line 7 for stirring the decontamination liquid,
A circulation pump 8 is provided. Also, drain port 9, vent
It also has 10. At the time of decontamination, since the potential changes as the oxide of the decontamination target progresses, the reference electrode 11 is installed in the decontamination tank and monitored by the potentiometer 12. By monitoring the electric potential, it is determined whether or not the target substance has entered the active dissolution area, and the end of decontamination is determined. This operation differs depending on the purpose of decontamination, and the decontamination is terminated when the potential reaches the active dissolution range for the equipment / parts, etc. which are not to be corroded for the purpose of reuse. For equipment and parts that are allowed to corrode within the permissible limits, even if they reach the active dissolution area, decontamination is continued and the metal surface is uniformly dissolved (several μm) to remove the radionuclide and the activated surface area. remove.

FIG. 2 shows an embodiment in which electrolysis is performed by a power source 13 from the outside to dissolve the oxide of the decontamination target. Also in this case, similarly to FIG. 1, the object which is not desired to be corroded is electrolyzed at a potential nobler than the active dissolution region and at a potential at which the oxide is dissolved. In this case, the potential is set to -0.3 to -0.65 V at the saturated sweet koh electrode (SCE) of the reference electrode in a neutral solution of pH 5 to 7 using the complexing agent and the organic acid as components. As a result, similarly to the method shown in FIG. 1, it is possible to suppress the corrosion of the decontamination object and dissolve only the oxide.

In addition, for decontamination objects intended for disposal,
Since it is necessary to reduce the surface dose, the electric potential of the object is set in the active dissolution region and the operation of dissolving the oxide and the metal surface is performed.

Further, when carrying out the method of FIG. 1, it is necessary to set the area of the carbon steel electrode. Depending on the electrode area to be connected,
Since the potential of the decontamination target is different, the carbon steel area is set to 0.2 to 1.5 times that of the decontamination target in order to dissolve the oxide while avoiding the active dissolution region.

As described above, when the present invention is used for decontaminating equipment / parts of a nuclear power plant, the decontamination effect can be effectively obtained, and the decontamination apparatus itself can be simplified.

FIG. 3 shows a test apparatus for examining the potential dependence of the constituent materials of the decontamination object and the dissolution of the oxide. This test apparatus is roughly divided into a dissolution tank 14, a constant-potential power supply 15, and a stirrer with a heater for heating, and the decontamination liquid 17 is degassed with Ar gas injected from a degassing diffuser 18. Between the test piece 19 of the constituent material and the oxide and the counter electrode 20, cathodic polarization was performed at a constant potential with the saturated sweet-coil electrode 21 of the reference electrode as a reference. The decontamination solution used consists of a complexing agent and an organic acid,
The solution was heated to 90 ° C. with a 5% strength solution and the procedure was performed. The amount of oxide dissolved was determined by measuring the iron ion in the solution with an atomic absorption spectrophotometer. Further, regarding the dissolution of the constituent materials, the amount of dissolution was obtained by weight reduction before and after the immersion. The result is shown in FIG. The vertical axis represents the amount of iron dissolved (mg / cm ² · h) and the horizontal axis represents the potential (Vvs · SC).
E) is shown. Dissolution of constituent materials is -0.65 to -0.75V
It can be seen that it has an active dissolution region. In contrast,
Oxide dissolution tends to increase as the potential decreases. From this, by suppressing the dissolution of the constituent materials,
To accelerate the dissolution of oxides, under these conditions, -0.3V
It can be seen that the potential should be set between ~ -0.65V.
In order to dissolve the oxide, there are a method of setting an electric potential from the outside in this way and a method of electrically contacting a carbon steel or the like having a base electric potential and lowering it to an appropriate electric potential. Then, in order to reduce the potential to an appropriate level, it is necessary to consider the area ratio of the carbon steel to be brought into contact. FIG. 4 shows a test apparatus that examined changes in potential depending on the area ratio. In this test apparatus, a carbon steel 22 and an oxide 23 are connected to each other in a melting tank 14 via a non-resistance ammeter 24, and a change in potential is measured by a potentiometer 25. Figure 6 shows the results, where the vertical axis is the potential (Vvs · SCE) and the horizontal axis is the oxide (Fe).
₃ O ₄ ) and the area ratio of carbon steel. The test conditions are the same as in the above-mentioned example. As can be seen from FIG. 5 and FIG. 6, in order to suppress the constituent materials and maintain the region in which the oxide is dissolved in the potential decrease due to the contact with carbon steel, the oxide, that is, the surface area of the decontamination target , It is necessary to increase the surface area of carbon steel by 0.2 to 1.5 times.

In order to further demonstrate the effectiveness of the present invention, a specimen removed from the piping of a nuclear power plant was used as a test piece.
Using the test apparatus shown in the figure, an experiment was conducted under the same decontamination conditions as in the basic test. The area of carbon steel connected to the decontamination target was the same area as the target. In addition, the radioactivity of the test piece was measured with a Ge (Li) semiconductor detector and evaluated by the removal rate of ⁶⁰ Co. The results are shown in FIG. In the figure, the vertical axis is ⁶⁰
Co removal rate (%), horizontal axis shows decontamination time. ⁶⁰ Co became saturated in about 7 hours, indicating that the oxide on the surface was almost dissolved and removed. In this way, by electrically connecting the carbon steel and holding it at an appropriate potential,
High decontamination effect can be obtained.

Moreover, the potential of the decontamination target changes momentarily as the surface oxide dissolves, and approaches the potential of the sacrificial electrode (carbon steel).
Enter the active dissolution zone (metal uniform dissolution zone) shown in the figure. On the other hand, the potential of the object is monitored during dissolution, and the potential point that enters the active dissolution region is terminated when it is desired to suppress dissolution of the material. If corrosion is acceptable, melt the surface of the material,
You can continue to be more effective.

In addition to the potential control method described above, a method of directly subjecting the decontamination object to cathodic polarization and setting it to an appropriate potential to dissolve and remove the oxide can be applied.

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

According to this example, it is possible to efficiently dissolve oxides attached to the surface of a metal, particularly oxides attached to equipment / parts of a nuclear power plant, by setting a potential according to the purpose of decontamination. For example, for the purpose of reuse after decontamination and for the object of decontamination that is not desired to be corroded, the potential of the object is cathodic polarized at a potential that does not reach the active dissolution region from the immersion potential. Also,
For an object that is allowed to corrode, set the potential of the object within the allowable limit to the active dissolution potential by an external power source, or in the state of entering the active dissolution area with the dissolution of oxide. It can be divided into the case where decontamination is continued. The present invention employs a method of externally setting the cathode polarization operation with a constant potential power source and a method of controlling the potential of an object to which a sacrificial electrode having a base potential is electrically connected. In this way, the potential is controlled by the above method according to the purpose of decontamination, so that the oxide can be effectively dissolved and removed, and the soundness of the material can be maintained. INDUSTRIAL APPLICABILITY The present invention can be applied not only to a nuclear power plant but also to other plants, can be used for pretreatment of steel sheets, and can be used in various fields.

Therefore, by using the present invention, decontamination of equipment and parts of a nuclear power plant can be easily performed, which facilitates disassembling and inspecting nuclear equipment, and also leads to reduction of radiation exposure of workers. The reliability of the plant can be improved.

〔The invention's effect〕

According to the method of the present invention, it is possible to electrochemically dissolve and remove oxides attached to devices and parts while suppressing corrosion of constituent materials. Further, according to the present invention, there is an effect that the decontamination end point becomes clear.

[Brief description of drawings]

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. A diagram showing the relationship between the amount of dissolved iron and the potential, Fig. 6 is a diagram showing the change in the potential depending on the area ratio of oxide and carbon steel, and Fig. 7 shows the test results confirming the effectiveness of the present invention. FIG. 4 is a diagram showing the relationship between decontamination time and ⁶⁰ Co removal rate. 1 ... decontamination tank, 2 ... carbon steel, 3 ... decontamination target, 5 ...
… Gas injection tube, 6 …… heating heater, 8 …… circulation pump, 11 …… reference electrode, 12 …… potentiometer, 13 …… external power supply.

Front page continuation (72) Inventor Masato Kobayashi 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd. Hitachi Factory (72) Inventor Toshio Sawa 4026 Kujicho, Hitachi City, Ibaraki Japan Hitachi, Ltd. (56) References JP 58-210200 (JP, A)

Claims (3)

[Claims] 1. A decontamination object to which an oxide adheres is immersed in an electrolytic solution, and a sacrificial electrode having a base electric potential lower than that of the object is electrically contacted or connected to the object to remove the object. 0.3V
vs. SCE to −0.65V vs. SCE potential is maintained, the oxide is reduced and dissolved, and the potential of the object is monitored with dissolution of the oxide, and the potential of the object is −0.65V. A method for dissolving an oxide, characterized in that the point at which vs.SCE is reached is the end point of dissolution. 2. A method for dissolving an oxide according to claim 1, wherein the sacrificial electrode is carbon steel. 3. The surface area of the sacrificial electrode according to claim 1 or 2, wherein the surface area of the object is 0.
A method for dissolving an oxide, which is characterized by being 2 to 1.5 times.