JPH06105317B2 - Decontamination method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial field of application" The present invention is to sufficiently and reliably decontaminate radioactive substances and other contaminants that remain on the inner wall surfaces of containers and the like that contain radioactive substances. The present invention relates to a decontamination method that can be performed.

"Prior art and its problems" The method of removing contaminants from the inner wall surface of a container, etc., is an electrolytic cleaning method using an acid solution or alkaline solution as an electrolyte solution as a decontamination solution, and a cleaning solution as a decontamination solution. There is an ultrasonic cleaning method using an acid solution, an alkaline solution, an organic solvent, or the like.

However, in all of these methods, the contaminants are dissolved in the decontamination solution such as the electrolyte solution or the cleaning solution to be removed, so that the concentration of the dissolved contaminants in the decontamination solution is high. However, there was a problem that the decontamination effect was significantly reduced. Therefore, if the decontamination effect is to be improved by using the conventional method, the decontamination solution must be constantly replaced with a new one, and thus a large amount of the decontamination solution used for decontamination is required. However, this large amount of decontamination solution must be further treated by waste solution by a treatment method such as coagulation-sedimentation method or evaporation method, so that the cleaning cost is high, and replacement of the decontamination solution is time-consuming and troublesome. There was a problem.

"Means for Solving Problems" The inventors of the present invention have conducted extensive studies to solve the above problems, and have obtained the following findings.

That is, it was presumed that the reason why the contaminants adhering to the container or the like as described above could not be sufficiently removed by the conventional decontamination method was the re-adhesion of the contaminants to the container or the like. Therefore, when various methods were compared and examined, a method of electrolytically cleaning a container or the like to which a contaminant adheres and then ultrasonically cleaning in a poor solvent for the contaminant was significantly larger than the conventional method. We came to know that the removal efficiency was improved. It is considered that this is because the contaminants decontaminated by ultrasonic cleaning are less likely to be dissolved in the decontamination solution, so that re-adhesion to the decontaminated object is small and the decontamination effect is large.

The decontamination method of the present invention has been made based on the above findings. After the object to be decontaminated (for example, a container) to which contaminants are adhered is electrolytically cleaned, it is placed in a poor solvent for the contaminants. It is characterized by ultrasonic cleaning in.

[Operation] In the decontamination method according to the present invention, first, in the electrolytic cleaning step, the stainless steel decontamination target is energized to the electrolyte solution as an anode, whereby the oxide system adhered to the surface of the decontamination target Dissolve the contaminant in the electrolyte solution. By dissolving the contaminant in this way, the contaminant can be rapidly separated from the surface of the article to be decontaminated and transferred into the electrolyte solution. That is, to perform efficient decontamination,
It is essential that the pollutants be dissolved in the electrolyte solution.
However, in this state, reattachment of the pollutant according to the concentration of the pollutant in the electrolyte solution is unavoidable after the power supply is cut off. Therefore, next, by performing ultrasonic cleaning in a poor solvent for the contaminant, the particulate contaminant reattached to the surface of the substance to be decontaminated is separated from the substance to be decontaminated by cavitation. Then, the separated contaminants are not dissolved in the poor solvent, and are dispersed in the form of fine particles in a state of being surrounded by the poor solvent and separated from the substance to be decontaminated by the poor solvent.
Therefore, contaminants are unlikely to reattach to the article to be dyed, and extremely high decontamination efficiency can be obtained.

"Example" Hereinafter, the decontamination method of the present invention will be described in more detail with reference to an example of a container.

The decontamination method of the present invention comprises immersing a stainless steel decontamination material having an oxide attached as a contaminant in an electrolyte solution containing at least one of an acid capable of dissolving the oxide and hydrogen peroxide. , An electrolytic cleaning step in which the contaminant is dissolved in the electrolyte solution to perform electrolytic cleaning by energizing the electrolyte solution with the decontamination target as an anode, and the decontamination target after the electrolytic cleaning is against the contaminant. By immersing in a poor solvent and ultrasonically cleaning in this poor solvent, the contaminants remaining on the surface of the object to be decontaminated are separated from the object to be decontaminated, and an ultrasonic cleaning step of dispersing in the poor solvent is performed. It is characterized by having.

The above-mentioned container is an article to be decontaminated, which is usually
It is made of stainless steel which is not easily corroded by chemicals such as corrosive acids used for removing contaminants.

As the acid that can be used in the electrolyte solution, an acid that dissolves a pollutant oxide is selected, and specifically, oxalic acid, sulfuric acid, nitric acid and the like are preferable. Further, the concentration of acid or hydrogen peroxide in the electrolytic aqueous solution is determined in consideration of the type and concentration of oxides that are pollutants, for example, in an oxalic acid solution is usually about 10 ~ 30 g / l, It is not limited to. Then, electrolytic cleaning using such an electrolyte solution is performed using the stainless steel container to be decontaminated as the anode and the electrolytic cell as the cathode. As the electrolysis conditions, the voltage in the electrolytic cell is about 10 to 20 V, the current density is about 30 to 60 mA / cm ² , and the electrolysis time is 2 to 10
It is said to be about a minute. Further, the temperature of the electrolyte solution during electrolytic cleaning is usually about 20 to 50 ° C. If the temperature is lower than 20 ° C, the electrolytic reaction becomes insufficient and the electrolytic reaction becomes insufficient. It becomes inconvenient.

Further, the poor solvent used for ultrasonic cleaning is determined by the type of contaminants, and for example, contaminants such as UO ₂ and U ₃ O ₈
In the case of oxides such as, for example, polar solvents such as water and alcohols in which UO ₂ and U ₃ O ₈ are dissolved at a maximum of only about 8.1 × 10 ⁻⁵ wt% or less are used as a poor solvent for the oxides. The condition of ultrasonic cleaning using such a poor solvent is that the frequency is about 20 to 40 kHz and the output is 2
The cleaning time is usually about 2 to 10 minutes. The temperature of the poor solvent during washing depends on the kind of the poor solvent and the like, but is usually about 20 to 50 ° C.

And the waste liquid from electrolytic cleaning or ultrasonic cleaning is
Since the electrolyte solution or the poor solvent used for ultrasonic cleaning is excellent in decontaminating effect on contaminants, the amount becomes small, so that the waste liquid can be easily disposed by a waste liquid treatment method such as evaporation method. Then, the pollutants in the waste liquid remain as a solid content when the waste liquid is processed by the evaporation method, and therefore are incinerated to become ash. The incinerated ash is enclosed in cement, asphalt, etc. and solidified. When an organic solvent is used as a poor solvent, the organic solvent is incinerated or stored in a storage container.

In this method, if the container to be decontaminated has a simple shape, perform electrolytic cleaning or ultrasonic cleaning and then mechanically clean the container with a brush or the like. You can also If rust is generated on the surface of the container and contaminants adhere to the rust, soak it in a rust remover for 20 to 40 minutes and then wash it with a brush. It is also possible to carry out the washing operation for carrying out as a pretreatment. By adding such a cleaning operation, the decontamination effect can be further improved.

Although the container to be decontaminated is, for example, a container such as a pellet tray in the above embodiment, the decontamination product is not limited to this.

Hereinafter, working examples and effects of the decontamination method of the present invention will be clarified.

(Experimental Example) (Experimental Example 1) A stainless steel pellet tray to which uranium oxide is attached as a contaminant is used as a decontamination target, which is immersed in an oxalic acid solution having a concentration of 10 g / l for electrolytic cleaning, and then the contaminant is removed. Water was used as the poor solvent, ultrasonic cleaning was carried out in this water, and further washing and drying were carried out. Then, in this experimental example, the radioactivity intensity was measured before and after decontamination, and the measurement results are shown in Table 1 together with the decontamination coefficient and the removal rate. The size of the pellet tray and the cleaning conditions for electrolytic cleaning and ultrasonic cleaning are as follows.

Pellet tray Size of tray… Vertical 84 to 100 mm × Horizontal 180 to 189 mm Electrolytic cleaning conditions Voltage… 13 to 16 V, current… 6 to 9 A, time… 4 minutes, tray interval… 6 mm Ultrasonic cleaning conditions Frequency… 26 kHz, output… 300W, anode current… 15.5mA, time… 6 minutes Unit of radioactivity intensity ... cpm (cunt per minute) DF ... Radioactivity intensity before decontamination / Radioactivity intensity after decontamination (Experimental example 2) Contaminants are decontaminated from the pellet tray in substantially the same manner as in Experimental example 1. did. However, electrolytic cleaning was performed using an oxalic acid solution having a concentration of 30 g / l. Then, in this experimental example, the radioactivity intensity was measured before and after decontamination, and the measurement results are shown in Table 2 together with the decontamination coefficient and the removal rate. The size of the pellet tray and the cleaning conditions for electrolytic cleaning and ultrasonic cleaning are as follows.

Pellet tray Size of tray: Vertical 94 to 95 mm x Horizontal 172 to 191 mm Electrolytic cleaning conditions Voltage: 9 to 10 V, current: 10 A, time: 5 minutes, tray interval:
6mm ultrasonic cleaning conditions Frequency… 26kHz, output… 300W, anode current… 15.5mA, time… 6 minutes (Experimental Example 3) Contaminants were decontaminated from the pellet tray in substantially the same manner as in Experimental Example 1. However, the pellet tray was electrolytically cleaned in an oxalic acid solution having a concentration of 25 g / l and then further ultrasonically cleaned.
Then, the radioactivity intensity was measured before and after decontamination, and the measurement results are shown in Table 3 together with the decontamination coefficient and the removal rate. The size of the pellet tray and the cleaning conditions for electrolytic cleaning and ultrasonic cleaning are as follows.

Pellet tray Size of tray: Vertical 98 mm x 178 mm Electrolytic cleaning conditions Voltage: 12 V, current: 10 A, time: 5 minutes, tray interval: 6 mm Ultrasonic cleaning conditions Frequency: 26 kHz, output: 300 W, anode current: 15.5 mA, Time ... 5 minutes (Conventional example 1) A pellet tray similar to that used in Experimental Examples 1 to 3 to which a pollutant composed of a radioactive substance was attached was used as a container for decontamination, and this was immersed in an oxalic acid solution having a concentration of 30 g / l. Electrolytically cleaned. Then, the radioactivity intensity before and after decontamination was measured, and the measurement result was measured with the decontamination coefficient and the removal rate.
Shown in the table. The size of the pellet tray and the cleaning conditions for electrolytic cleaning are as follows.

Pellet tray Size of tray: Vertical 97 mm × Horizontal 176 to 183 mm Electrolytic cleaning conditions Voltage: 10 V, current: 8 A, time: 3 to 8 minutes, tray interval:
6-7.5 mm (Conventional example 2) Almost the same pellet tray as used in Experimental Examples 1 to 3 to which a pollutant consisting of a radioactive substance was adhered was used as a container for decontamination, and 200 g of oxalic acid and 30 ml of peroxide were added to 1 container. It was immersed in a mixed solution containing hydrogen and ultrasonically cleaned. Then, the radioactivity intensities before and after decontamination were measured, and the measurement results are shown in Tables 5 and 6 together with the decontamination coefficient and the removal rate. The size of the pellet tray and the cleaning conditions for ultrasonic cleaning are as follows.

Pellet tray Size of tray: Vertical 98 mm x 178 mm Ultrasonic cleaning conditions Frequency: 26 kHz, output: 300 W, anode current: 15.5 mA, time: 30 minutes (Table 5), 45 minutes (Table 6) (Comparative Example 1) A pellet tray similar to that used in Experimental Examples 1 to 3 to which a pollutant composed of a radioactive substance was attached was used as a decontamination target, and this was immersed in an oxalic acid solution having a concentration of 10 g / l. After electrolytic cleaning, 2N nitric acid was used as a good solvent for contaminants, and ultrasonic cleaning was performed in this good solvent. Then, in this comparative example, the radioactivity intensity was measured before and after decontamination, and the measurement results are shown in Table 7 together with the decontamination coefficient and the removal rate.
The size of the pellet tray and the cleaning conditions for electrolytic cleaning and ultrasonic cleaning are as follows.

Pellet tray Size of tray… Vertical 84 to 100 mm × Horizontal 180 to 189 mm Electrolytic cleaning conditions Voltage… 13 to 16 V, current… 6 to 9 A, time… 4 minutes, tray interval… 6 mm Ultrasonic cleaning conditions Frequency… 26 kHz, output… 300W, anode current… 15.5mA, time 6 minutes As is clear from these results, Experimental Examples 1 to 3 above
Shows a higher decontamination coefficient and a higher removal rate than those of Conventional Examples 1 and 2, and thus it is clear that the decontamination ability of removing contaminants from the container is excellent.

"Effects of the Invention" As described above, according to the decontamination method of the present invention, in the electrolytic cleaning step, the stainless steel decontamination target is used as an anode to energize the electrolyte solution to decontaminate the surface of the decontamination target. By dissolving the oxide-based pollutants adhering to the electrolyte solution in the electrolyte solution, the pollutants are promptly separated from the surface of the article to be decontaminated and transferred to the electrolyte solution. Then, by ultrasonically cleaning the substance to be decontaminated in a poor solvent for contaminants, while the degree of reattachment of the contaminant is mild, the particulate contaminant reattached to the surface of the substance to be decontaminated. Cavitation is used to remove from the body to be decontaminated. The released contaminants are not dissolved in the poor solvent, so they are surrounded by the poor solvent and dispersed in fine particles, and are separated from the decontamination target by the poor solvent, preventing reattachment to the decontamination target. It

Thus, in the decontamination method of the present invention, after most of the contaminants on the surface of the article to be decontaminated are dissolved and removed in the electrolytic cleaning step,
In the ultrasonic cleaning step, the contaminants redeposited after the dissolution removal are removed in a non-dissolved state while preventing further redeposition, so that a high decontamination rate can be obtained without impairing the processing efficiency. Since it is not necessary to frequently exchange the electrolyte solution and the ultrasonic cleaning liquid during decontamination, it is possible to suppress the amount of waste liquid to a minimum.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tsuneo Watanabe 622, Funaishikawa, Tokai-mura, Naka-gun, Ibaraki Prefecture Mitsubishi Nuclear Fuel Co., Ltd. Tokai Works (56) Reference JP-A-60-106998 (JP, A) Japanese Patent Publication Sho 59-33877 (JP, B2)

Claims (1)

[Claims] 1. A stainless steel decontamination material to which an oxide adheres as a contaminant is immersed in an electrolyte solution containing at least one of an acid capable of melting the oxide and hydrogen peroxide, An electrolytic cleaning step in which the contaminants are dissolved in the electrolyte solution to perform electrolytic cleaning by energizing the electrolyte solution with the decontaminating body serving as an anode, and By immersing in a solvent and ultrasonically cleaning in this poor solvent, the contaminants remaining on the surface of the object to be decontaminated are separated from the object to be decontaminated, and an ultrasonic cleaning step of dispersing in a poor solvent is provided. A decontamination method characterized by: