JPS6083000A - Method of decontaminating radioactive contaminated metal - 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 decontaminating radioactively contaminated metal parts, and in particular, to surface contamination generated from nuclear facilities such as nuclear power plants and nuclear waste enrichment plants. The present invention relates to a method suitable for decontaminating real metal waste.

[Background of the invention]

Nuclear power plants generate radioactively contaminated metal waste such as equipment, piping, and tools during periodic inspections and various repair and modification works. Currently, these radioactively contaminated metals are cut to some extent and then filled into drums and stored within nuclear power plants. The number is about 150 to 200 per year, but the cumulative amount is increasing year by year, and when nuclear power plants are decommissioned and dismantled in the future, tens of thousands of drums will be generated from radioactively contaminated metal waste alone. do. Therefore, it is strongly desired to decontaminate and reduce the volume of radioactively contaminated metal parts.

Radioactive metal waste can be broadly classified into tools brought in during work and door materials from equipment inside the power plant.

In the former case, radioactive isotopes adhere to tools from equipment during periodic inspections and modification work, resulting in contamination of their surfaces. On the other hand, the latter type of equipment contamination is caused by the iron-based oxide (crud) deposited in the reactor core being activated by neutron irradiation, and the activated crud is carried to equipment such as the primary cooling system and main steam system. Contamination is caused by deposition on the surface of these devices, or by penetration of radioactive metals into the oxide film layer.Quantitatively, the latter is by far the most common -30 to 30% during periodic inspections conducted every 4U years. Approximately 50 tons will be removed, and up to 20,000 tons will be removed when the furnace is dismantled.

Decontamination methods for these surface-contaminated gold wires include physical methods such as LSL, high-speed jet water cleaning, and ultrasonic cleaning;
7.1: Can be roughly divided into two chemical methods such as de-dying.

Since the number of radioactive metals on the surface of the tool is only 7, it can be easily decontaminated by the physical v<1 method.

On the other hand, the contaminated oxide film is one in which radioactive metals are incorporated into the oxide film layer, and the contaminated oxide film cannot be removed sufficiently by physical methods alone, making it impossible to use chemical methods. Even in the chemical method, if only two washes are used, the mouth of Pe304, which has a strong spinel-type crystal fF4 structure, will be removed. It takes a long time to remove the chemical film and is not practical. However, in the electrolytic decontamination method, the proboscis of the object to be decontaminated is immersed in an electrolytic solution as an anode, and electricity is applied to forcibly dissolve the anode surface. The skin can be completely removed.

Social leather products contaminated with radioactivity due to electrolysis,
< As a method of decontaminating the metal surface of l-1, acid-resistant,
There are two methods: 5H three-electrode electrolysis in a concentrated strong acid aqueous solution such as sulfuric acid (Japanese Patent Application Laid-open No. 140300/1983), and a method of anodic electrolysis in a neutral salt aqueous solution (Japanese Patent Application Laid-Open No. 57-76500). In the method using strong acids (βZ), the removal performance of oxide films or metal surfaces is superior to that of neutral salts, but because the metal containing radioactivity removed by electrolysis becomes ions and dissolves in the strong acid. Processing of waste acid has become complicated, which is the main cause of increased costs and secondary waste. On the other hand, in the method using a neutral salt aqueous solution, the oxide film or metal removed by decomposition at 1;° C. becomes a hydroxide and precipitates, making waste liquid treatment easier. However, even in this method, the environment is similar to that of a nuclear power plant.

Moreover, it has the disadvantage that it is hard to remove the oxide film (Fe304) having a strong strength and a spinel structure which is generated at a temperature of 270''C5 pressure and 0 atmospheres.

In other words, in a decontamination method in which electrolysis is performed using the object as an anode, the oxide film formed on the surface of the object itself becomes soluble i.
．．・It utilizes the phenomenon in which the oxide film peels off as a result of the melting of the underlying metal layer of the oxide film.・For this reason, it can be easily applied even inside a strong oxide film. It has excellent electrolytic ability in solutions containing large amounts of penetrating hydrogen ions and halogen ions, but in most neutral salt solutions such as nitrates and sulfates (1), the ability to remove the λ film is extremely poor.&'↓Figure 1 shows the results of electrolyzing a steel sheet with a 100 μm thick film, but in order to completely remove the film, a matrix of at least twice the weight of the film is required. The material must be electrolyzed and polished with a sodium sulfate solution for over an hour.

A method has been proposed in which the steel plate is subjected to alternating electrolysis in a neutral salt aqueous solution to completely remove the oxide film (Japanese Patent Laid-Open No. 53-1206).
37), such alternating electrolysis method has not been applied to the decontamination of radioactively contaminated metals for a long time. In addition, Japanese Patent Application Publication No. 53 120637
The invention relates to the removal of oxidized scale generated during processing steps such as rolling and annealing of copper materials.
Fe203 r middle layer: Fe3O4+ inner layer: li”
After applying mechanical scale breaking to the oxide film accumulated on e O), the object is subjected to alternating current electrolysis. However, radioactive metal waste generated from nuclear power plants and other facilities often consists of thick pipes (more than 10 holes) and valves.

It is difficult to apply mechanical scale breaking such as rolling.

One of the unique problems with this method, which targets radioactive metal waste, is the re-deposition of radioactive metals that have been removed from the target object. Particularly in the case of alternating electrolysis, redeposition of radioactive contaminated metals occurs due to plating during cathode electrolysis, so it is necessary to prevent this.

[Purpose of the invention]

An object of the present invention is to provide a method for decontaminating radioactive contaminated metals that can efficiently remove a strong spinel-type oxide film (Fe304) containing radioactivity.

[Summary of the invention]

In the present invention, in a neutral salt aqueous solution, the entire oxide film containing radioactivity on the surface of a radioactively contaminated metal is alternately applied to the cathode, single pole, and electrolysis for ion diffusion under polarization potential regulation as shown in Figure 2. It is designed to be electrolytically removed by repeated alternating electrolysis.

As mentioned above, when the object to be decontaminated (contaminated gold) was electrolyzed as a positive electrode, the base material in the lower layer of the oxide film was dissolved, but on the other hand, when the object was electrolyzed as a cathode, The reduction reaction of the following formula occurs.

Fe3O4+61(20+e−-)3Fe”+100I
(−+l−I2・・・・・・・・・(1) 2H20+26− −+20I−I−+l−I2 −・
Old... (2) The oxide film dissolves in this reaction of equation (1), but the dissolution rate is extremely slow, and the main reaction is (2)
) is the decomposition of water. However, F before the cathode N
The oxide film of e2O3 and Fe3O4 is shown in equation (3)5.
Then, by cathodic electrolysis, it is reduced and transformed into a soft Hiko oxide film mainly composed of FeO.

Fe3O4+e-→ Softening (FeO main body)... Group...
3) Alternating electrolysis utilizes the characteristics of the oxidized film as described above, and involves repeating the process of softening the strong oxide film by cathodic reduction, making it more permeable to ions, and then performing anodic electrolysis. The oxide film is removed efficiently. The softened oxide film increases the permeability of ions.

Next, anodic electrolysis causes the metal base material to dissolve and the oxide film to peel off. By repeating this process, the oxide film of the radioactively contaminated metal can be removed much faster than in the past.

Both the reduction of the oxide film by cathodic electrolysis and the dissolution reaction of the metal base material by anodic electrolysis depend on the temperature of the electrolytic solution, and the reactions are accelerated at high temperatures. Based on this, it is expected that the removal rate of the oxide film of radioactively contaminated metals can be improved by raising the temperature of the electrolyte, but a problem unique to alternating electrolysis is that once the temperature reaches a high temperature, the removed metal becomes radioactive. We discovered the phenomenon of redeposition to the surface of metal waste.

Figure 3 shows the amount of re-deposition when steel materials with oxide films were subjected to alternating electrolysis. The amount of redeposition increases as the electrolyte temperature increases and decreases as the current density increases. According to FIG. 3, conditions for preventing redeposition can be determined from either the electrolyte temperature or current density.

Figure '54 shows sulfur 15i2 thorium (1'Ja25O4
) Shows the polarization potential of steel in a 20wt% electrolyte. The polarization potential reflects the situation at the interface between the π liquid and the electrode (iron (the same material)), and when a current is applied as an anode, as shown in Figure 4, iron dissolves and a passive film of iron forms.

It is divided into oxygen generation areas due to water decomposition. Although the polarization potential changes depending on the temperature, the conditions for preventing re-adhesion shown in FIG. 3 all fall within the region of polarization potential of 1.5 square centimeters or more, which means that water decomposes. From this, police box ML JQ
In order to prevent the redeposition of radioactive metals at 6:00, the general condition is to maintain the polarization potential at 1.5 V or higher during anodic electrolysis and to allow water to decompose. This is the alternating electrolysis mechanism mentioned above.

The oxide film softened by cathode electrolysis is used as an anode iE.
This means that in the process of stripping by solution, the dissolution of iron should be suppressed to a low level and the generation of oxygen gas by one decomposition of water should be the main stream. In order to peel off the oxide film softened by cathodic electrolysis by homopolar electrolysis, it is necessary to dissolve the metal base material at the interface of the oxide film, but this should be minimized in terms of increasing waste costs and preventing redeposition. be. In the iron dissolution region in Heru 14, iron is dissolved at a current efficiency of 100 cm, so dissolved iron ions remain at the electrode interface.
During the next cathode electrolysis, it re-deposit onto the electrode due to the plating phenomenon. On the other hand, in the water decomposition region, the dissolution efficiency of iron is several orders of magnitude, and the efficiency of water decomposition is over 90 orders of magnitude, so the iron ion concentration is slightly less than 100%, and the oxygen gas from water decomposition diffuses. Therefore, there is almost no re-deposition during the next cathode electrolysis.

In this way, in order to prevent redeposition during alternating electrolysis,
During anodic electrolysis, it is necessary to reduce the efficiency of dissolving the gold-11 base material to 10 times or less, and decompose the remaining water into water, but the efficiency of dissolving the metal base material and decomposing water depends on the temperature of the electrolyte and the electrolyte temperature. Varies depending on type. Regarding the temperature of the electrolyte, the efficiency of dissolving the metal matrix increases with increasing temperature, but it can be lowered by adding substances that promote the decomposition of water, such as neutral salts such as sodium nitrate and sodium phosphate. can. For example, in a sodium sulfate electrolyte, as shown in FIG. 5, it can be seen that the area where the efficiency of dissolving the metal base material is 10 cm or less and there is no redeposition is expanded by adding 5 cm of sodium nitrate. Also, when the amount of sodium nitrate is 30% or more, the efficiency of dissolving the metal base material increases.
In this case, the range of 3 to 20 inches is desirable because the removal efficiency of the oxide film is greatly reduced.

In this way, by setting an electrolytic region based on a single polarization type position or adding a water decomposition accelerator, it is possible to prevent the re-deposition of dissolved metals, but only a small amount of radioactive metals such as 60CO and 59FC re-deposit. It is desirable to take even more thorough measures to prevent re-deposition, as this will also reduce the decontamination effect. Therefore, it is effective to diffuse trace amounts of metal ions dissolved by anodic electrolysis into the solution before the subsequent cathodic electrolysis begins. As shown in FIG. 2, an auxiliary electrolysis time for polarization potential control was provided between anodic electrolysis and cathodic electrolysis. Redeposition of dissolved metal ions is due to metal electrodeposition (plating) as shown in the following formula.

M”+28 −+ M ・・・・・・=・(4) The potential at which this reaction occurs (standard electrode potential) differs depending on the type of metal, but P to Fe ” ” + C”0” r is - It is about 0,44, -0,28, -0,25V,
At a potential lower than that, the reaction of formula (4) proceeds. Therefore,
After the anodic electrolysis, electrolysis is carried out while keeping the polarization potential at or above the standard electrode potential of the engraving F≦ions, and by stirring the solution, the metal ions dissolved by the anodic electrolysis can be diffused without being redeposited. Furthermore, if the polarization potential is too high, metal dissolution will occur, so it is effective to keep it below -0.1V. Further, the electrolysis time for diffusing the dissolved metal ions with the above-mentioned polarization potential regulated is desirably about 20 to 30 seconds so that the ions can be sufficiently diffused.

Figure 6 shows that carbon steel with a hλ coating of about 10 μm thick was heated in an aqueous solution containing 20 Wt of sodium sulfate and 2 wt% of sodium nitrate at 50'C for a minute (a). Figure 6 shows the results obtained when alternating electrolysis was performed with a It is confirmed that the oxide film is completely removed in about 15 minutes of electrolysis.In this method, the radioactivity intensity is reduced to the bank ground level by removing 15 mg/α2 of the contaminated metal and oxide film in 15 minutes. It is possible to decontaminate up to

Although the radioactive contaminants removed by decontamination remain in the electrolyte, all of them precipitate as hydroxides or oxides in the neutral salt aqueous solution, and the aqueous solution is not contaminated by radioactivity at all.

In the alternating electrolysis of water droplets, the dissolution of the metal base material and the peeling off of the oxide film proceed simultaneously, but the peeled off oxide film does not dissolve and precipitates as it is, and the metal ions eluted by the dissolution of the metal base material are expressed by the following formula. (1), (2) as shown in
It reacts with the hydroxide ion produced by the formula, and all of it becomes hydroxide and precipitates.

Fe3””+30H-→Fe(OH)3 ---(4)
As seen in the reaction equations (1) to (4), only water is consumed in the electrolytic reaction, not neutral salts, and the electrolyte can be continuously supplied by simply replenishing water. and can be used.

[Embodiments of the invention]

Embodiment 1 FIG. 6 is a block diagram of an apparatus suitable for carrying out the method of the present invention. This device has a power supply of 1. Electrolytic bath 2° Reference electrode 4, cleaning soak 8. It consists of a centrifugal dehydrator 11 and a mixing tank 15 as main components. The radioactive waste 5 is immersed in an electrolytic bath 2 filled with a neutral salt aqueous solution 3, and subjected to alternating electrolysis together with a reference electrode 4. The electrolysis time is preferably 10 to 20 minutes. Contaminants removed by alternating electrolysis precipitate as oxides or hydroxides 6, and are sent to a centrifugal dehydrator 11, where they become sludge 13 dehydrated to a water content of about 80 cm and sent to a mixer 15. On the other hand, the dehydrated liquid after dehydration is passed through the filter 12 and reused as an electrolyte. The dehydrated sludge sent to the mixer is stirred and mixed with water glass 14 of 2 to 3 times the weight by a stirrer 16, and then filled into a drum 17. The filling in the drum solidifies in 48 to 72 hours.

The radioactivity of the waste decontaminated by electrolysis is reduced in the back grout area 1 by washing it with normal tap water 9 in the washing basin 8 by spray washing 10.

However, using the apparatus described above, cathodic electrolysis was carried out for 60 seconds using an aqueous solution of 2Qwt sodium sulfate as the electrolyte and carbon as the counter electrode, with a current density of 0.3A/an2,
Electrolysis for ion diffusion 30 seconds, current density 0 to 0.0
5A/cm2, anodic electrolysis for 30 seconds, upstream density 0.3A/α
2. The contaminating oxide film could be completely removed by carrying out the process for about 20 minutes. Furthermore, the precipitate was centrifuged at 400 Orpm? i:? After being dehydrated in a machine to a water content of 80%, it was mixed with twice the weight of water glass and solidified.
The volume was reduced to 715.

Example 2 The polarization potential measuring device 18 shown in FIG. 7 was added to the device of Example 1.
Automatic ft1l with additional control computer 19
Please take care of me. The polarization potential of 50 radioactive surface-contaminated metals is measured using the polarization potential measuring device 18. During anodic electrolysis, electrolysis is performed at a polarization potential of 1.5 times or more, and during electrolysis for ion diffusion, the polarization potential is measured to radioactive surface contamination. Electrolyze at a standard electrode potential of full bending or higher. During the electrolysis, the polarization potential is appropriately measured, and if it deviates from the above conditions, the control computer 19 controls the current during electrolysis so that electrolysis can always be carried out in the optimal polarization potential range.

Such electrolysis with controlled polarization potential can completely prevent decontamination residue from re-deposition.

￥Example 3 Using the device convex of Example 1 or 2, Na28
0420 wt% (7) 3 to 20 wt% to the electrolyte
Using the added electrolyte solution, conduct electrolysis to prevent ion diffusion at a current density of 0.2 to 0.5 A/an with a radioactively contaminated metal as an electrode (current density of 0 to 0.05 A/an).
2. time 30 seconds), cathodic electrolysis (60S) and anodic electrolysis (30 seconds).
Decontamination was carried out by alternating electrolysis during 0S). As a result, there was no re-deposition of any decontamination residue, and the decontamination was completed to the background level in about 20 minutes.

〔Effect of the invention〕

According to the present invention, a radioactively contaminated oxide film layer can be reliably removed by alternating electrolysis.

During the electrolysis of sweet octopus, the uncontaminated metal base material can be dissolved (because it can be done in a small amount with mome, and the volume of secondary particles can be significantly reduced. Also, a neutral salt aqueous solution can be used as the electrolyte). By using ・The electrolytic dedying process and the process of removing hydroxides etc. generated by the process can be performed at the same time, so the electrolytic treatment time can be shortened and
This has the effect of reducing the cost of chemicals used in Ij1 liquid.

[Brief explanation of the drawing]

The first section is a relationship diagram between electrolysis time and polishing tλ, Figure 2 is a relationship diagram between polarization potential and electrolysis time, and Figure 3 is a relationship diagram between resealing; Figure 4 is a diagram of the relationship between current density and polarization potential, Figure 5 is a diagram of the relationship between current density and electrolyte temperature,
FIGS. 6 and 7 are schematic diagrams of the electrolytic dedying system. 1... Power supply, 2... Electrolytic tank, 8... Cleaning tank, 15
・・・Mixing tank・Hou / Prison electrolysis time open (7? Ljn) (Special) 2nd mouth 3rd solid state C 5th evil, electrolytic high temperature (° Riho 6 mouths

Claims (1)

[Claims] 1. In a method for electrolytically dedying a metal whose surface has been contaminated with a radioactive substance - an aqueous solution 'f prepared by adding an oxidizing compound to an aqueous solution of a neutral salt: used as an electrolytic solution. Oxide films contaminated with radioactivity on the surface of metals to be decontaminated can be removed by alternating electrolysis between anodic electrolysis and cathodic electrolysis, which passes a current smaller than both the anodic electrolysis and cathodic electrolysis. A method for decontaminating radioactively contaminated metals, characterized by removing layers and metal surfaces.