JPS6267500A - Method and device for chemically decontaminating radioactivecontaminant - 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 The present invention relates to a method and a decontamination device for chemically decontaminating radioactive contaminants in nuclear power plants and the like.

The steel primary piping, equipment, and systems at the Maruti-Niwagi-ketsu Nuclear Power Plant are becoming increasingly contaminated with radioactivity due to reasons such as radioactive contaminants being incorporated into corrosion products that adhere to the inner surfaces over time. do. Therefore, radiation exposure increases during maintenance inspections or modification work on piping, equipment, etc. Therefore, decontamination is carried out by injecting a decontamination liquid containing dissolved chemicals into the secondary system piping, some of the equipment, and the system including these, heating it, and passing the liquid through it. This chemical decontamination method is considered to be an effective method due to its ability to remove contaminants stuck to the metal surface of the material and the ease of decontamination operations on the inner surface of the object to be decontaminated, and has many achievements mainly overseas. has been reported.

As chemicals, organic acids, chelating agents, and the like are often used because they have a strong ability to dissolve metal oxides, which are corrosion products, and cause less damage to the metal base material itself due to corrosion. Among these chemicals, organic acids with relatively small molecular weights, such as oxalic acid and formic acid, are relatively valuable in terms of decontamination properties. It is considered an excellent method because it can be transformed into water or water. Among these, oxalic acid in particular is a chemical that has a high reducing power, has a high ability to dissolve metal oxides, and has good decomposition properties in the treatment of decontamination waste liquid.

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However, oxalic acid may form ferrous oxalate, which has low solubility in water when divalent iron is a dissolved metal, resulting in longer decontamination times or in the case of easily soluble carbon steel materials. However, there is a drawback that precipitates of ferrous oxalate adhere to the surface of the object to be decontaminated.

On the other hand, among chemical decontamination methods using oxalic acid, a decontamination method using an aqueous solution containing a mixture of oxalic acid and hydrogen peroxide has been reported. This method uses a decontamination solution that is a pre-mixed mixture of oxalic acid and hydrogen peroxide, so the decontamination solution exhibits oxidizing properties on the object to be decontaminated due to the influence of hydrogen peroxide, which has strong oxidizing power. As mentioned above, it is not suitable for removing corrosion products in the primary system piping, that is, contaminants trapped in oxides such as iron, as it does not have reducing properties.
For example, it is used to dissolve uranium in nuclear fission products through oxidation.

As mentioned above, among the methods of using oxalic acid as a decontamination chemical, in the case of the method of using oxalic acid alone, ferrous oxalate precipitates tend to adhere to the object to be decontaminated, and A decontamination method using an aqueous solution containing a mixture of oxalic acid and hydrogen peroxide has a drawback in that the ability to remove primary corrosion products due to the reducing action of the decontamination agent is poor.

Therefore, the present invention provides a chemical decontamination method that overcomes the drawbacks of the prior art described above, suppresses the adhesion of ferrous oxalate precipitates to objects to be decontaminated, and is also excellent in the ability to remove corrosion products. and equipment.

The present invention solves the above problems by oxidizing ferrous oxalate produced in the decontamination process with hydrogen peroxide and converting it into water-soluble ferric oxalate. It is something.

That is, the method for chemically decontaminating radioactive contaminants according to the present invention is as follows:
Oxalic acid is used in a chemical decontamination method that decontaminates radioactively contaminated steel pipes, equipment, and systems containing them by heating and flowing an aqueous solution containing dissolved chemicals. The present invention is characterized in that an aqueous solution is used as a decontamination liquid, heated and passed through the object to be decontaminated, and then an aqueous hydrogen peroxide solution is mixed into the decontamination liquid, heated, and passed through the decontamination liquid.

When decontaminating radioactive contaminants such as primary system piping using an oxalic acid aqueous solution, if the material of the object to be decontaminated is carbon steel or if the decontamination time is prolonged (then the Corrosion products such as iron oxides that incorporate contaminants are dissolved as iron ions, but among these ions, ferrous ions form oxalic acid and ferrous oxalate, which has low solubility in water. easy.

At that time, if hydrogen peroxide is present in the solution, ferrous ions are oxidized to become ferric ions, which react with oxalic acid to produce ferric oxalate, which has a relatively high solubility, so that the precipitation is No problems occur. However, if hydrogen peroxide is mixed in advance with an aqueous oxalic acid solution and used as a decontamination solution, the decontamination solution becomes oxidizing, which reduces the ability to dissolve corrosion products such as iron oxides that incorporate contaminants. , the decontamination ability will decrease. Therefore, in the present invention, after decontamination with oxalic acid is performed, hydrogen peroxide is added to oxidize the generated ferrous oxalate precipitate and convert it into ferric oxalate, which is dissolved in the liquid.

Further, when a precipitate of ferrous oxalate is formed, suspended matter is generated in the liquid, and the light transmittance of the liquid is reduced. Therefore, a light transmission rapid measurement system consisting of a light source, a light sensor, and a transparent plate was installed in the circulation path for flowing the decontamination liquid, and the change in the light transmittance of the decontamination liquid was detected, and the hydrogen peroxide aqueous solution supply system was installed. It is advantageous to send a signal to the decontamination solution and add an aqueous hydrogen peroxide solution to the decontamination solution.

In the chemical decontamination device according to the present invention, a decontamination liquid supply pipe and a hydrogen peroxide aqueous solution supply pipe are opened, and a light source, a light sensor, and It is characterized in that it incorporates a light transmittance measuring device made of a transparent plate, and is provided with a controller that operates a valve of a hydrogen peroxide aqueous solution supply pipe in response to a signal from the measuring device.

Actual drawing An embodiment of the chemical decontamination apparatus of the present invention is shown in FIG.

In this embodiment, the chemical decontamination device includes an object to be decontaminated 1, a circulation pump 3, a heater 4, a decontamination liquid tank 5, a hydrogen peroxide tank 10, a decontamination waste liquid treatment device 6, and a light transmittance measurement device. 1
2. It is composed of a controller 13 and piping and valves connecting these. The object to be decontaminated 1 is connected to a circulation pump 3 and a heater 4 by piping via seal portions 2 at both ends. Piping that connects the object to be decontaminated 1, the circulation pump 3, the light transmittance measuring device 12, the valve 7, and the heater 4 forms a circulation path.

A supply pipe for the decontamination liquid from the decontamination liquid tank 5 is opened in the piping between the object to be decontaminated 1 and the circulation pump 3 via a valve 9.
Furthermore, a supply pipe for hydrogen peroxide water from a hydrogen peroxide water tank 10 is opened between the opening and the circulation pump 3 via an electromagnetic valve 11. In addition, the light transmittance measuring device 12
From the pipe connecting the valve 7 and the valve 7, a pipe connected to the decontamination waste liquid processing station 6 via the valve 8 is drawn out.

The light transmittance measuring device 12 and the electromagnetic valve 11 are configured such that an electrical signal is sent to the light transmittance measuring device 12 via the controller 13.
It is electrically connected so that it is sent from the electromagnetic valve 11. When ferrous oxalate is generated in the decontamination solution and the light transmittance decreases, the electromagnetic valve 11 is opened and the hydrogen peroxide aqueous solution is set to be supplied into the solution.

Furthermore, a pipe for an air vent valve 18 is drawn out from the pipe between the object to be decontaminated 1 and the heater 4 .

In the embodiment shown in Fig. 1, special joints are used for the seal portions 2 at both ends of the object 1 to be decontaminated, but for example, a nozzle may be used to pour the decontamination liquid into a range of piping partitioned off by a valve. You can also do this.

Next, the operation of the above device will be explained. First, decontamination in decontamination liquid tank 5? Prepare rL14. The prepared decontamination liquid 14 is forced into the decontamination liquid circulation path from the decontamination liquid tank 5 through the valve 9. At this time, the air vent valve 18 is opened, and the air in the circulation path is extracted from the air vent valve 18. When the circulation path is filled with the decontamination liquid, the circulation pump 3 is driven and the decontamination liquid circulates within the path. At that time, the heater 4 is activated to heat the decontamination liquid. The higher the temperature, the better the decontamination ability, but if the temperature exceeds 100°C, the piping will require high pressure specifications, which is undesirable, and generally 85 to 95
A temperature of °C is preferred.

When the oxalic acid aqueous solution, which is a decontamination liquid, flows through the pipe of the object to be decontaminated 1, magnetite and nickel ferrite, which are iron and nickel oxides adhering to the inner surface of the pipe, in other words, corrosion products, are reduced by the oxalic acid, and the liquid It dissolves while reacting with the hydrogen ions inside. As a result, radioactive elements such as Cor=O incorporated into the corrosion products are removed together.

When magnetite and nickel ferrite are dissolved, ferrous ions are present in the decontamination solution. When the material of the object 1 to be decontaminated is carbon steel, the base material is also relatively easy to melt, so the concentration of ferrous ions becomes high. Further, even when decontaminating SUS materials, if the decontamination time is long, for example, several hours, the concentration of ferrous ions in the decontamination solution will increase. Ferrous ions react with oxalate ions in the liquid to produce ferrous oxalate, which has low solubility in water, and a precipitate is formed. According to the observations of the present inventors, temperature and the concentration of both ions are involved in the production of ferrous oxalate, but this phenomenon in the decontamination solution is due to the solubility of ferrous oxalate itself. It is known that ions maintain a supersaturated dissolved state up to a fairly high concentration of each ion, and that when the concentration exceeds a certain level, precipitates rapidly form.

However, if the precipitate is generated during the night, the radioactive elements such as Coco, which were removed from the inner surface of the object 1 by the decontamination action and dissolved in the decontamination solution, will return to the surface of the object 1 to be decontaminated. There is a possibility that it will stick to the inner surface. Therefore, after decontamination with the oxalic acid decontamination liquid is performed for a certain period of time, hydrogen peroxide water 17 is injected from hydrogen peroxide water W110 into the decontamination liquid based on a signal from a light transmittance measuring device 12, which will be described later. The injection operation may be performed by temporarily stopping the circulation pump 3, withdrawing a portion of the decontamination liquid from the valve 8 to the decontamination waste liquid processing device 6, and opening the air purge valve 18.

When hydrogen peroxide is mixed into the decontamination solution, the ferrous ions dissolved in the solution are oxidized and become ferric ions, which become precipitates and become suspended in the solution, causing decontamination. Ferrous oxalate particles adhering to the inner surface of the object are also oxidized to ferric oxalate, which is dissolved in the decontamination solution.

By adding the hydrogen peroxide solution 17 in this way, the ferrous oxalate precipitate can be dissolved, but if the hydrogen peroxide solution is added too early, the reducing power of oxalic acid will not be fully exerted. . If the hydrogen peroxide solution is added too late, too much ferrous oxalate precipitate will form, which will reduce the efficiency of decontamination. Therefore, it is preferable to provide a light transmittance measuring device 12 in the circulation path of the decontamination liquid, and to determine the timing of addition of the hydrogen peroxide solution based on changes in the light transmittance.

That is, when a precipitate of ferrous oxalate is formed in the decontamination solution, the light transmittance of the decontamination solution decreases, so this is detected by the light transmittance measuring device 12 and the amount of the precipitate is estimated. I can do it.

In this way, it is advantageous to supply the hydrogen peroxide solution while monitoring the amount of precipitate produced in the decontamination solution. Through the above operations, the reducing power of the oxalic acid decontamination solution can be sufficiently maintained, and any ferrous oxalate precipitates can be dissolved by adding hydrogen peroxide.

When the above operations are completed, the decontamination liquid is sent as waste liquid 15 to the decontamination waste liquid processing device 6 via the valve 8, treated, and discharged. In the decontamination waste liquid treatment device 6, residual oxalic acid in the waste liquid is decomposed by oxidation treatment such as electrolytic treatment, and dissolved metals are easily treated as electrodeposited or precipitated. The waste liquid contains hydrogen peroxide, which acts as an oxidizing agent in the oxidation treatment mentioned above.
Facilitate processing.

Example 1 A radioactively contaminated pipe made of carbon steel was used as an object to be decontaminated, and decontamination was performed using the apparatus shown in FIG. Fill the circulation path with a 4% by weight aqueous oxalic acid solution, which is a decontamination solution, and heat the decontamination solution to 90°C.
circulated. Hydrogen peroxide solution was added midway according to the signal from the light transmittance measuring device. Decontamination took place for a total of 4 hours.

Experiments were conducted with various amounts of hydrogen peroxide added, and the decontamination effect was expressed as the decontamination factor aDF, which is the surface dose rate of the pipe before decontamination divided by the surface dose rate of the pipe after decontamination. , the results are shown in Figure 2. As can be seen from FIG. 2, as the amount of hydrogen peroxide added increases, the DF increases, and the DF becomes constant at a certain amount of H2O2 added. Therefore, the addition of hydrogen peroxide improved the decontamination effect.

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According to the present invention, it is possible to suppress the adhesion of ferrous oxalate precipitates to the object to be decontaminated, and to perform chemical decontamination that is excellent against secondary corrosion products.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing one embodiment of the decontamination apparatus of the present invention, and FIG. 2 is a graph showing the relationship between the amount of hydrogen peroxide added and the decontamination coefficient.

Claims (3)

[Claims] (1) A chemical decontamination method that decontaminates radioactively contaminated steel pipes, equipment, and systems containing them by heating and flowing an aqueous solution containing dissolved chemicals. is characterized in that an aqueous oxalic acid solution is used as a decontamination liquid, heated and passed through the object to be decontaminated, and then an aqueous hydrogen peroxide solution is mixed into the decontamination liquid, heated, and passed through the decontamination liquid. Chemical decontamination methods for radioactive contaminants. (2) The light transmittance of the decontamination liquid is measured in the circulation path through which the decontamination liquid flows, and a hydrogen peroxide aqueous solution is added to the decontamination liquid in response to the change in the light transmittance. Method described. (3) Light transmittance consisting of a light source, optical sensor, and transparent plate in which the decontamination liquid supply pipe and the hydrogen peroxide aqueous solution supply pipe are opened, and the decontamination liquid circulation path is equipped with a circulation pump and a heating device. 1. A chemical decontamination device for radioactive contaminants, comprising a built-in measuring device and a controller that operates a valve of a hydrogen peroxide aqueous solution supply pipe in response to a signal from the measuring device.