JP2003090897A - Chemical decontamination method and device for carbon steel member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove iron oxalate left in a carbon steel member to suppress the adhesion accelerating phenomenon of radioactivity in a method for decontaminating a radioactive nuclide by oxalic acid treatment. SOLUTION: This chemical decontamination method for removing the radioactive nuclide from the carbon steel member by use of oxalic acid comprises treating the carbon steel member with formic acid to remove iron oxalate after removing the radioactive nuclide of the member with the oxalic acid. According to this, the iron oxalate can be effectively removed in a short time while ensuring the soundness of the carbon steel member.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a radionuclide from a carbon steel member contaminated with a radionuclide in contact with a solution containing radioactivity in a facility for handling a solution containing radioactivity, such as a nuclear power plant or a reprocessing plant. TECHNICAL FIELD The present invention relates to a chemical decontamination method and apparatus for chemically removing carbon steel members.

[0002]

2. Description of the Related Art In facilities such as nuclear power plants and reprocessing plants that handle solutions containing radioactivity, parts that come into contact with solutions containing radioactivity generally have excellent chemical resistance, corrosion resistance at high temperatures, and mechanical strength. Therefore, expensive materials such as stainless steel and nickel-based alloys are used.

However, in a boiling water nuclear power plant, stress corrosion cracking (hereinafter referred to as SC) occurs in stainless steel in an aqueous solution environment where hot primary cooling water containing radioactivity flows.
There is a special circumstance in which there is a corrosion form called C) and caution is required when using stainless steel.

It goes without saying that SCC is a corrosion mode that eventually leads to brittle fracture and is widely recognized as a fatal defect as a structural material. SCC is an event that occurs when material factors, stress factors, and environmental factors are superimposed, and it is said that SCC will not occur if one of the factors can be removed.

In a boiling water nuclear power plant, in order to prevent SCC, "a stainless steel 316L is used as an alternative material to the conventional material 304 stainless steel for the material factor.
Adopt steel. For stress factors, heat treatment is added to alleviate the residual stress during welding. For environmental factors, the concentration of oxygen and hydrogen peroxide in the environment is reduced by degassing the condenser at startup and injecting hydrogen during operation. , Etc. are planned and applied to the actual plant one after another.
Their effectiveness has been demonstrated.

In the conventional boiling water nuclear power plant, it is said that the generation of SCC can be sufficiently suppressed by applying material countermeasures and residual stress relaxation treatment to the reactor recirculation system piping in which hot primary cooling water containing radioactivity flows. Although stainless steel is still used as the material, it is said that SCC will not occur in the same environment in place of stainless steel from the viewpoint of further safety in the reactor water purification system and residual heat removal system. Carbon steel members are often used as piping materials.

A carbon steel member generally used in a boiling water nuclear power plant is, for example, a carbon steel pipe for high pressure piping as defined by JIS (G3455). This JI
S is specified for a carbon steel pipe having a high working pressure at 350 ° C. or lower. Carbon steel pipes for high-pressure pipes have higher tensile strength, yield point (or yield strength) than stainless steel,
The mechanical properties indicated by the value such as elongation are not particularly inferior, but there is a large difference between the two in corrosion properties in an aqueous solution environment.

Carbon steel generally means a malleable steel containing 0.04 to 1.7% of carbon among iron-carbon alloys containing carbon as a main component. Practical carbon steel is silicon in addition to carbon. It is known to contain manganese, phosphorus, sulfur as well as oxygen, hydrogen, nitrogen and other trace elements, which influence mechanical properties.

Carbon steel pipes for high-pressure pipes used in boiling water nuclear power plants are prepared by adjusting the alloy components in consideration of the operating environment at high temperature and high pressure, and by "annealing" or "normalizing". Sufficient mechanical strength is secured, but since it is a kind of carbon steel in a broad sense, it does not contain chromium, nickel, etc. as an alloying component that suppresses corrosion in an aqueous solution environment. It is easily corroded by just immersing it in still water so that rust, which is an iron oxide, is generated on the surface.

As described above, carbon steel is extremely inexpensive as compared with stainless steel because it does not contain an alloy component that suppresses corrosion, but it is important to consider the environment in which it is used before use. .

Since carbon steel is liable to corrode in the general industry, it is often used with the help of an appropriate anticorrosion method such as surface coating, corrosion inhibitor, and cathodic protection, but it is a rare case but reaction In some cases, the product covers the steel surface to block the corrosive environment, or causes a voluntary anticorrosion effect such as slowing the reaction rate, so that it may be practically used without the aid of other anticorrosion methods.

It has been reported that the protective property of a dense magnetite layer formed on a steel surface in weak alkaline high temperature water such as a boiler evaporation tube can be sufficiently put to practical use. Further, the rust layer formed on the pipe for transferring fresh water at room temperature also has some protection. Further, carbon steel is used for concentrated sulfuric acid tanks and pipes because ferrous sulfate formed in concentrated sulfuric acid at room temperature becomes a protective film because of its poor solubility and causes passivation.

Although these examples are typical examples of successful use of carbon steel without the anticorrosion method, the anticorrosion effect is maintained only under limited environmental conditions, and the conditions are satisfied. It is said that if it is not done, it will soon lose its effect and the corrosion will proceed rapidly.

Further, carbon steel generally has a high solubility of iron ions even in a weakly acidic environment, and does not form a protective layer of corrosion products, so it is not practically used. It is said that even in a neutral environment, the pH locally drops below the rust layer and corrosion is accelerated.

Corrosion in high-temperature water is caused by a reaction between a metallic material used and a corrosive substance constituting a corrosive environment to generate a reaction product, thereby reducing the wall thickness of the metallic material or reducing mechanical properties. Cause deterioration. Therefore, the composition, structure, and properties of the reaction product largely influence the progress of high-temperature corrosion.

The reaction product is generated and grows at the interface between the metal and the corrosive environment. However, since diffusion in the metal is accelerated at a high temperature, the metal as shown in the internal oxidation, carburization, nitriding, and hydrogen corrosion. Even in the interior, changes often occur from the surface.

When the reaction product is formed on the metal-corrosion environment interface, that is, on the metal surface, the state and properties of the reaction product have a great influence on the corrosion rate. When the reaction product is a solid phase under the corrosion conditions, this is called scale, but scale occurs at the metal-corrosion environment interface with the progress of the reaction as in the case of oxidation or sulfurization, and the metal and the corrosion environment grow when they grow. Acts to isolate.

If the scale is made of a dense crystal layer of the reaction product and there are no pores or gaps, the subsequent ion diffusion in the scale will proceed as a reaction process. Diffusion in the solid phase is generally slow because it occurs through lattice defects within the grains or grain boundaries.

Also, since the diffusion path becomes longer as the scale layer grows, the corrosion rate becomes slower with time. That is, it can be said that the solid-phase scale suppresses the corrosion rate to a greater or lesser degree and has a protective property although it may not be sufficient in view of the expected life of the material.

High-temperature high-pressure water flows in the reactor water purification system and the residual heat removal system in which carbon steel members are used in a boiling water nuclear power plant, and the quality of the water contains almost no impurities. It is almost neutral. FIG. 13 schematically shows a piping system diagram around a nuclear reactor of a boiling water nuclear power plant. According to FIG. 13, a flow of primary cooling water around the nuclear reactor and a carbon steel member are used. The parts will be explained.

The reactor water in the reactor pressure vessel 1 is forcibly circulated by a reactor recirculation system consisting of a reactor recirculation pipe 2 connected to the reactor pressure vessel 1 and a recirculation pump 3. Although the boiling water nuclear power plant has a plurality of reactor recirculation systems, FIG. 13 shows an example of two systems.

In the old reactor type, a natural circulation type boiling water nuclear power plant that does not have a recirculation system still exists.
On the other hand, in recent improved boiling water nuclear power plants, a reactor type that does not have a nuclear reactor recirculation system but uses a plurality of internal pumps for forced circulation of the reactor water is also manufactured.

A nuclear reactor water purification system is connected to the suction side of the nuclear reactor recirculation system recirculation pump 3. The reactor water purification system takes in the reactor water from the inside of the reactor pressure vessel 1 through the carbon steel pipe 4 from the suction side and feeds it with a circulation pump 5, a regenerated heat exchanger 6, a non-regenerated heat exchanger 7 Cooling to a temperature (40 ° C. or lower) suitable for the resin in the filter desalting tower 8, impurities in the reactor coolant are removed by the filter desalting tower 8, and the regenerative heat exchanger 6
After recovering heat by the reactor, the reactor pressure vessel 1
It is configured to return to.

Although each component is shown as one unit in FIG. 13, it usually has two or three units. The carbon steel pipe 4 is used to connect the components of the reactor water purification system at the parts shown in the figure, and the temperature conditions are such that the maximum reactor water temperature is 288 ° C and the minimum is about 40 ° C.
There are ℃ and temperature range. The maximum pressure condition is 6.9 MPa, which is the reactor water pressure. The reactor water purification system is usually operated not only during normal operation of the plant but also during shutdown.

On the other hand, the residual heat removal system is provided mainly for removing the decay heat of the core when the reactor is shut down, and the reactor water is branched from the suction pipe of the reactor recirculation pump 3. The carbon steel pipe 10 draws water, the pump 11 sends water, the heat exchanger 12 cools it, and it returns to the reactor pressure vessel 1 through the reactor recirculation discharge pipe. A part of the cooled reactor water may be sprayed from a head spray nozzle provided on the top portion 13 of the reactor pressure vessel 1 to cool the head portion in the reactor pressure vessel 1.

A bypass line 14 is connected to the heat exchanger 12, and the amount of heat exchange can be adjusted by adjusting the bypass flow rate. Since the residual heat removal equipment is in a standby state filled with pool water in the pressure control room as one system (low pressure water injection system) of the emergency core cooling system during plant operation, cleaning and warming up of the system are required prior to operation. is there.

By design, the pressure in the reactor pressure vessel 1 is 9.
When the pressure drops to 5 kg / cm ² , the function as an emergency core cooling facility is no longer required, so the system cleaning can be started, and the pressure in the reactor pressure vessel 1 becomes 7.7 kg / cm ^2.
You can drive from cm ² .

The carbon steel pipe 10 in the residual heat removal system is shown in FIG.
It is used to connect components such as pumps and heat exchangers at the parts shown in. The temperature condition will vary depending on the time when the residual heat removal equipment is turned on at the time of shutdown, but it is about 1
The temperature is generally about 50 ° C., and this value is the upper limit for the temperature at which the carbon steel pipe comes into contact with the liquid.

Carbon steel, which is obviously inferior in corrosion resistance to stainless steel in a high temperature and high pressure water environment such as a reactor water purification system and a residual heat removal system of a boiling water nuclear power plant, can be used as a reaction product produced at high temperature. This is because it is expected to have a spontaneous anticorrosion effect, and the reaction product acts to suppress corrosion to some extent.

However, in designing carbon steel pipes, the thickness of the pipes is determined with a margin for sufficient mechanical strength by adding a thickness called "rotation margin" in consideration of the thinning due to the corrosion reaction.

By the way, in a nuclear power plant,
There is a strong causal relationship between the corrosion of structural members in contact with liquid and the generation of radioactivity as follows. In other words, the generation of radioactivity is considered as follows. First, corrosion products are generated on the surfaces of various wetted materials in water supply systems and condensate systems, and some of them are released into water. The released corrosion products migrate along the water stream and are eventually carried into the reactor.

For example, in the boiling water nuclear power plant, the corrosion products brought into the reactor include soluble nickel ions and chromate ions generated from the feed water heater heat transfer tube made of stainless 304 steel, ,
There are insoluble iron particles from the condensate system components.
When these soluble or insoluble corrosion products are brought into the reactor pressure vessel 1, some of them temporarily adhere to the surface of the fuel cladding tube due to the evaporation-drying phenomenon or the eutectoid reaction.

These reactions are particularly remarkable in a system involving boiling. In fact, in a boiling water nuclear power plant, the boiling phenomenon occurs over almost the entire length except for the lower part of the fuel cladding tube of about 4 m, and therefore corrosion occurs. The product adhesion phenomenon is remarkable. In addition, boiling also occurs in the upper part of the fuel cladding in the pressurized water nuclear power plant, and the adhesion phenomenon occurs similarly, though not as severely as in the boiling water nuclear power plant.

Corrosion products attached to the fuel cladding are intensely activated by neutron irradiation. The corrosion product once attached to the fuel and activated is called a radioactive corrosion product or a fuel clad. Most of the radioactive corrosion products are immobilized on the fuel cladding tube and taken out of the system during the fuel exchange performed when the plant is shut down, but only a small part of them is separated or dissolved during operation or at the time of shutdown. It is released again into the reactor water in the form of particles and ions.

Particulate particles of the released radioactive corrosion product adhere to the site such as the bottom of the reactor where the primary cooling water stays, horizontal pipes, etc. due to the gravity sedimentation phenomenon. On the other hand, the ionic corrosion product is incorporated into the corrosion film when stainless steel or carbon steel in which high-temperature primary cooling water flows is corroded by a mechanism such as eutectoid or isotope exchange represented by the following formula. ⁶⁰ Co ²⁺ + Fe ₂ O ₃ + H ₂ O = ⁶⁰ CoFe ₂ O ₄ + 2H ^{+ 60} Co ²⁺ + CoFe ₂ O ₄ = ⁶⁰ CoFe ₂ O ₄ + Co ²⁺

As a source of radioactivity, there is a case where the core structural material is directly irradiated with neutrons and radiated, and a part thereof is dissolved or desorbed to increase the concentration in the reactor water. This occurs from stellite material is cobalt-based alloy used in the pin or roller member of the control rod ⁵⁸
There are Co and ⁶⁰ Co.

The fuel spring used to fix the fuel cladding tube in the channel is made of nickel-based alloy Inconel material, but the same nuclide is also generated and released into the reactor water to be released. Raising the radioactivity concentration.

The radioactivity contained in the reactor water in the present nuclear power plants is not due to the fissile products produced by the nuclear reaction of the nuclear fuel such as uranium and plutonium, but the radioactive corrosion products as described above. The re-released radioactivity and the radioactivity generated from the activated core structural material. The main nuclides are ⁵⁸ Co, ⁶⁰ Co, ⁵¹ Cr,
⁵⁴ Mn and ⁵⁹ Fe.

In the primary system piping which is in contact with the solution containing radioactivity, it corrodes due to contact with high temperature and high pressure water to form a corrosive film. At this time, ionic radioactivity is incorporated into the film. In the equilibrium state, the amount of radioactivity accumulated in the film is proportional to the amount of the corroded film as shown in the following equation, and the amount of the corroded film of the material is important.

A = k × C × M A: Radioactive adhesion amount (Bq / cm ² ) C: Radioactivity concentration (Bq / ml) M: Corrosion film amount (g / cm ² ) k: Constant (-)

In particular, when stainless steel and carbon steel are compared with each other, it is obvious that carbon steel has a high corrosion rate and a large amount of corrosion film, and therefore the amount of accumulated radioactivity also increases. These have not only been reported in large numbers in laboratory scale corrosion tests using a device (often called a high temperature and high pressure loop) that can simulate the high temperature and high pressure water environment of the primary reactor system, but in fact It has also been confirmed at a nuclear power plant in.

If radioactive material adheres to the surface of the primary piping of the reactor or the surface of the component equipment, the radiation dose rate around it increases, and it is needless to say that the worker is exposed during the inspection of itself, which is a completely different work. However, there are many cases where workers are exposed to radiation only in the vicinity. For this reason, an increase in radiation dose rate causes an increase in work cost and inspection days, resulting in an increase in power generation cost as well as a large social problem in hygiene management if the exposure dose of workers is high.

As described above, in order to avoid the increase in power generation cost and the exposure problem, it is necessary to suppress the increase in the radiation dose rate due to the adhesion of radioactivity to the surface of the primary system piping and the constituent equipment. It is an important theme in power plants and reprocessing plants.

In order to suppress the rise in the dose rate on the surface of the primary system piping and the constituent equipment, it is understood that there are the following measures in consideration of the processes of generation, transfer and accumulation of radioactivity described above.

(1) A corrosion product which becomes a radioactive corrosion product is not generated. (2) Do not bring corrosion products that become radioactive corrosion products into the reactor. (3) The generation of radioactivity from the core structure material is suppressed. (4) Keep the radioactivity concentration in the primary cooling water low. (5) Do not allow radioactivity to adhere to the reactor primary system piping or the surface of component equipment. (6) Remove the radioactivity adhering to the reactor primary system piping and component equipment surfaces.

In the present invention, among these, (6) a technique for removing the radioactivity adhering to the reactor primary system piping and the surface of the component equipment, and (5) no radioactivity adhering to the reactor primary system piping and the surface of the component equipment It is about technology. Specifically, we will provide a chemical decontamination method that makes it difficult for redeposition of radioactivity after decontamination, especially for the parts of reactor primary system piping and components that are made of carbon steel members. It seeks to contribute to the achievement of this theme.

By the way, techniques for removing the radioactivity adhering to the surface of the reactor primary system piping and the constituent equipment have been disclosed up to now in various patent publications. There is known a method in which a step of reducing and dissolving an iron-based oxide as a component with oxalic acid and a step of oxidizing and dissolving chromium oxide with an oxidizing agent are combined.

For example, in JP-A-3-10919, a method using permanganic acid as an oxidizing agent and dicarboxylic acid as a reducing agent is disclosed. In JP-A-2000-81298, ozone water is used as an oxidizing agent and a reducing agent is used. Each method using oxalic acid is disclosed.

Regarding decontamination of a carbon steel member which does not contain chromium and does not require an oxidation step, JP-A-10-123
Japanese Patent No. 293 discloses a decontamination method using oxalic acid and oxalate. In general, oxalic acid has a mild dissolving power for metal base materials, is easy to handle and post-process, and is inexpensive. Therefore, the current chemical decontamination methods often include oxalic acid. Reduction decontamination agents are widely used.

[0050] On the other hand, measures for suppressing radioactivity from adhering to the reactor primary system piping and component equipment surfaces have been studied for a long time, and when classified by technology, oxide film application / prefilming, surface treatment / surface modification. Quality, water quality control, iron injection / iron concentration control, radioactivity concentration reduction technology,
There are plant operations, operations, facilities, etc.

However, as the technology actually adopted in the boiling water nuclear power plant, the electrolytic treatment of the reactor recirculation pipe by the surface treatment / surface modification technology, the zinc ion injection, the iron injection by the water quality control technology, There are only a few cases of iron injection due to intentionally bypassing the condensate filtration filter with iron concentration control.

Further, these measures are designed mainly for stainless steel, and are different from the radioactivity adhesion suppression of carbon steel members intended by the present invention. It should be noted that in recent years, there are some proposals targeting carbon steel, but since there is no report that these measures have been adopted in actual plants, their effectiveness and feasibility are currently questioned. Is.

[0053]

Oxalic acid, which is widely used as a reduction decontamination agent, is used as a decontamination target during the chemical decontamination of carbon steel members in the reduction decontamination process using oxalic acid. The reaction of a large amount of iron ions eluted from a carbon steel member with oxalic acid produces iron oxalate on the surface of the carbon steel member. This iron oxalate has a chemical formula of Fe ₂
It is expressed as C ₂ O ₄ and is usually estimated to be a dihydrate.

Iron oxalate has a property that it is easily dissolved in acid, but is hardly dissolved in water. Further, iron oxalate is unstable at high temperatures, and when heated in an aqueous solution, it undergoes a morphological change and generally changes to hematite (αFe ₂ O ₃ ). At that time, if transition metal ions such as nickel coexist in the environment, these ions are easily incorporated and ferrite formation is caused by the following reaction formula. Ni ²⁺ + Fe ₂ O ₃ + H ₂ O = NiFe ₂ O ₄ + 2H ⁺

Cobalt ion (Co ²⁺ ) and cobalt 60
Ions ( ⁶⁰ Co ²⁺ ) are very similar in chemical properties to nickel ions, and are taken into ferrite by the same reaction formula.

The above phenomenon is simulated in a carbon steel part after chemical decontamination using a reduction decontamination agent containing oxalic acid in an actual machine. First, although the radioactivity is removed from the surface of the carbon steel member by the chemical decontamination treatment using a reducing decontamination reagent containing oxalic acid, iron oxalate is produced and remains as a by-product. When the starting process of the plant progresses and the temperature of the circulating water in contact with the carbon steel member producing iron oxalate rises, the iron oxalate gradually transforms into hematite particles.

When ionic radioactivity such as cobalt 60 ion coexists at that time, the hematite particles themselves become
Part of the particles in the mokushi become ferrite and take in radioactivity. Particles that have taken up radioactivity increase the amount of radioactivity adhering to carbon steel when they remain in the carbon steel part, and become insoluble iron oxide particles when they migrate to other parts and become a low flow rate part or Move to retention area.

Unlike the reagent hematite produced by high-temperature calcination, hematite transformed from iron oxalate does not undergo a thermal history above the reactor water temperature, so it has low crystallinity and a reaction with transition metals and the like. Reactivity is high, such as maintaining sex.

Therefore, both the particles that have become hematite remaining on the surface of the carbon member and the hematite particles that have migrated to the low-velocity portion and the retention portion continuously function as the radioactive attachment site during plant operation, and the saturated attachment amount Radioactivity will continue to be attached until.

The inventors of the present invention conducted the following test to decontaminate the carbon steel member with oxalic acid to produce iron oxalate on the surface of the carbon steel member and iron oxalate. When immersing the carbon steel member in high temperature water, iron oxalate undergoes morphological transition to hematite, and when it further migrates, the radioactivity in high temperature water is surely taken in. It has been confirmed that it easily remains.

First, a carbon steel round bar material having an outer diameter of 6 mm (S20
From C), a round bar-shaped test body having an outer diameter of 4 mm and a length of 70 mm was manufactured. Further, both end surfaces had a depth of about 10 mm and had a female thread structure of M2. The test body was previously provided with an oxide film in a high temperature and high pressure aqueous solution in order to simulate the surface condition before decontamination. The temperature condition was 288 ° C. of the reactor water purification system where the carbon steel member is used at the highest temperature, and the treatment time was 500 hours. By this treatment, the test body having a metallic luster changed to a state covered with a black oxide film.

Next, the test body provided with this oxide film was subjected to decontamination treatment with oxalic acid. The decontamination conditions and decontamination procedure are as follows. First, the reagent oxalic acid dihydrate was dissolved in a beaker to prepare a 2000 ppm oxalic acid solution. Magnetite (Fe ₃ O ₄ ) was added to this to adjust the iron ion concentration to 150 ppm. The test body was put into this solution and kept at 80 ° C. for 5 hours. During heating, the mixture was stirred with a stirrer to make the concentration uniform. The appearance of the test body after decontamination with oxalic acid was yellowish, and it was estimated that iron oxalate was produced.

Next, the test body was immersed in a 2N hydrochloric acid solution as a preliminary means for removing iron oxalate. At this concentration of hydrochloric acid, the carbon steel base material is also easily dissolved, so caution is required, but it was confirmed from the appearance change that iron oxalate was dissolved and removed in just a few seconds.

Three types of tests were carried out: a test body in which iron oxalate remained, a test body in which iron oxalate was removed by preliminary means, and a receiving material (untreated material in which no oxide film was applied). Radioactivity was tested on the body.

As to the method of assembling the test body, as shown in FIG.
mm and a wall thickness of 1 mm, and inserted into an outer cylinder tube 47, and a fixture composed of a set screw 47a and a perforated ring 47b was attached to both ends of the outer cylinder tube 47 to obtain a test body assembly 46a. The test body assembly 46a was connected in series using a stainless steel joint to form a test section.

The apparatus used for the radioactivity deposition test was a high temperature and high pressure loop capable of simulating the high temperature and high pressure water condition of a boiling water nuclear power plant, and the operating conditions of the test section were a temperature of 280 ° C., a pressure of 7.8 MPa and a flow velocity. It was 32 cm / sec. The water quality conditions were a dissolved oxygen concentration of 230 ppb and a dissolved hydrogen concentration of 20 ppb, and a continuous water flow test for 1000 hours was performed.

During operation of the device, ⁶⁰ C as a radioactive tracer
o solution was injected and adjusted to a concentration of 0.02 Bq / ml in loop water. FIG. 15 shows the result of measuring the amount of attached radioactivity of the taken-out test body after the loop operation was completed.

As is clear from FIG. 15, the amount of radioactivity attached to the test body in which the iron oxalate remained was about 5 of the other test bodies.
It was twice as many. The radioactivity of the test body from which iron oxalate was removed by the preliminary means was almost the same as that of the receiving material. Further, in FIG. 15, the test body is subjected to ultrasonic cleaning treatment, and the radioactivity desorbed by this treatment is separated as loose and the remaining radioactivity is classified as tight. It can be seen that the ratio of loose radioactivity is extremely high in the test body in which oxalic acid remains.

FIG. 16 shows the result of observing each step of the surface of the test body on which iron oxalate remained by a scanning electron microscope (SEM).
(A)-(c). In FIG. 16, (a) is an observation result after applying an oxide film, (b) is a chemical decontamination treatment with oxalic acid, and (c) is an observation result after a radioactivity adhesion test. Figure 1
After the oxide film was applied as shown in FIG. 6 (a), the surface of (a) was in a state in which polyhedral crystals covered the surface.

The main components of these crystals are ferrite type oxides such as nickel ferrite and cobalt ferrite. As shown in FIG. 16 (b), rounded particles were generated on the surface of the test body after the oxalic acid treatment, and the particles were aggregated to form large particles having a maximum length of about 5 μm. Was observed. These crystals are iron oxalate particles.

On the other hand, as shown in FIG. 16 (c), fine particles of 1 micron or less were produced after the radioactive adhesion test. Although some of the fine particles were removed by the ultrasonic cleaning treatment, they were collected and analyzed by powder X-ray diffraction. As a result, it was confirmed that hematite (αFe ₂ O ₃ ) was the main crystal component. By the way, according to the radioactivity analysis, a large amount of radioactivity was contained in the fractions dropped by the ultrasonic cleaning treatment.

Since hematite has no attachment sites for divalent ions, it is presumed that the attached radioactivity is attached to the surface layer of hematite by an adsorption reaction or only the surface layer is ferritic. It is an undeniable fact that hematite particles that have undergone morphological transition from iron oxalate contained a large amount of radioactivity regardless of the mechanism of attachment.

When a carbon steel member was subjected to a decontamination treatment using oxalic acid from a series of radioactivity adhesion tests and SEM observation results, radioactivity was removed but iron oxalate was produced as a by-product. Furthermore, it was found that iron oxalate was transformed into hematite by the increase in the temperature of the solution to which it was subsequently contacted, and the radioactivity in the solution was taken in during the transformation.

In the radioactive adhesion test, the surface flow velocity was 32 cm /
sec and the average flow velocity in the actual reactor water purification system piping (2
.About.3 m / sec), the Reynolds number indicating the flow condition is about an order of magnitude slower, but exceeds 5000, which is in the turbulent flow region. For this reason, it is evaluated that there is no extreme difference from the actual equipment when comparing the dropout characteristics due to flow. From these results, it was judged that there is a large tendency for loose radioactivity to remain on the surface of carbon steel members even after decontamination with oxalic acid in actual equipment.

From the above, the iron oxalate produced in the actual machine remains as it is unless it is forcibly removed after the decontamination treatment, and there is a risk that the amount of radioactivity attached to the carbon steel member is significantly accelerated. It was found that Of course, iron oxalate does not remain at the production site, and if it is released into the primary cooling water, it becomes a site for attaching radioactivity, and there is a risk of forming hot spots where a large amount of radioactivity is locally attached.

After the oxalic acid decontamination, in order to dissolve and remove iron oxalate while maintaining the soundness of the material, the oxalic acid decomposition / purification process for decomposing and purifying oxalic acid while controlling the iron concentration is performed. There is a proposal that iron residue can be prevented.

Further, there has been proposed a method of injecting an oxidizing agent such as hydrogen peroxide to decompose iron oxalate, but all of them have a problem that it takes a long time to completely remove iron oxalate. . Further, it has been pointed out that iron oxalate remaining after decontamination of oxalic acid interferes with various treatments for the purpose of suppressing the attachment of radioactivity and cancels the suppressing effect.

The present invention has been made to solve the above problems, and a first object of the present invention is to use a reducing decontamination agent containing oxalic acid from a carbon steel member such as installed equipment and piping. The iron oxalate existing after the chemical decontamination was effectively removed in a very short time while maintaining the soundness of the material, and the carbon steel member was cleaned to suppress the acceleration phenomenon of radioactivity adhesion. Moreover, it is another object of the present invention to provide a method for chemically decontaminating a carbon steel member capable of preventing the occurrence of hot spots.

A second object of the present invention is to positively irradiate a carbon steel member cleaned by removing iron oxalate by adding a coating treatment step for forming a protective coating capable of suppressing the deposition of radioactivity. It is intended to provide a method for chemically decontaminating a carbon steel member capable of suppressing the adhesion.

Further, a third object of the present invention is to carry out a periodic inspection,
It is an object of the present invention to provide a chemical decontamination device for carbon steel members, which can reduce the radiation exposure of workers during construction and can reduce the cost during inspection and construction.

[0081]

The invention according to claim 1 is
In the chemical decontamination method of a carbon steel member to remove the radionuclide from the carbon steel member contaminated with radionuclide using a reducing decontamination reagent containing oxalic acid, after removing the radionuclide from the carbon steel member, It is characterized in that the iron oxalate produced in the carbon steel member is removed by subjecting the carbon steel member to an acid treatment in which an acid solution is brought into contact with the carbon steel member.

In the present invention, the metal surface of the carbon steel member is acid-dissolved, so that the iron oxalate formed on the surface can be removed in a short time. As a result, it is possible to suppress the phenomenon of radioactivity adhesion acceleration after the restart of service of the carbon steel member due to the residual iron oxalate. Iron oxalate is easily dissolved by acid treatment with any acid.

Any acid referred to here is chlorine (Cl),
Non-metals such as sulfur (S), nitrogen (N), phosphorus (P)
Hydrochloric acid (HCl), sulfuric acid formed by the bonding of the acid group containing hydrogen
(H ₂SO_Four), Nitric acid (HNO_Three), Phosphoric acid (H₃PO_Four)
Inorganic acids, which collectively refer to
All organic acids with acidity are covered.

Examples of organic acids include naturally occurring acetic acid,
There are carboxylic acids such as oxalic acid, tartaric acid and benzoic acid, and as the acid having the smallest molecular weight, there is formic acid shown in the invention of claim 2. In addition, including salts of inorganic and organic acids that show acidity in an aqueous solution such as ammonium citrate,
It goes without saying that all of these can be applied in an actual machine as long as the conditions (concentration and temperature) are adjusted.

However, in the case of assuming the construction with an actual machine, the dissolution rate and dissolution efficiency of iron oxalate are important when selecting the acid species to be used and setting the conditions such as the concentration and temperature thereof. It becomes more important to consider the melting of the carbon steel member itself. That is, the most preferable condition is that iron oxalate can be efficiently dissolved while suppressing the dissolution of the base material.

In an actual machine, various steel grades, which are generically called carbon steel in terms of material but differ in their alloy composition, are used, and the melting characteristics of the base material are slightly different. In addition, depending on the conditions, there is also a means of using a dissolution inhibitor called an inhibitor together to suppress the dissolution of the base material.

Furthermore, the construction cost is also an important factor. Equipment for acid treatment and equipment for treatment of acid solution after treatment directly affect the cost. For example, when an inorganic acid is used as the acid solution, the treatment of the acid solution after the dissolution treatment is generally recovered with an ion exchange resin or the like, which is advantageous in terms of facility.

Inorganic acids generally have a higher degree of dissociation than organic acids, and the order of effectiveness as acids is orders of magnitude higher. Therefore, the acid concentration that brings about the same effect of removing iron oxalate is far lower than that of the organic acid in the case of the inorganic acid, and the amount of the ion exchange resin required for recovering the inorganic acid can be small.

However, in this case, it is necessary to sufficiently evaluate the influence of the residual acid solution on the start-up of the plant. On the other hand, when an organic acid is used, even if the treatment of the acid solution is incomplete, the residual acid is decomposed by the start-up of the plant, so that the influence is small and the organic acid is more advantageous.

Further, since the organic acid can be oxidatively decomposed to be mineralized into carbon dioxide gas and water, it is possible to prevent the components from remaining by performing the decomposition operation before starting the plant. Therefore, in actual construction, it is necessary to compare and examine the optimum conditions for the target carbon steel member, and as a result, it is necessary to find a method that does not affect the soundness of the material most and can be implemented at low cost.

The invention according to claim 2 proposes a concrete example which can be implemented in an actual machine based on the above-mentioned examination. However, by any means, the iron oxalate produced after chemical decontamination with oxalic acid is forcibly removed, and the radioactivity adheres after the resumption of service of the carbon steel member due to the iron oxalate residue. As described above, the acceleration phenomenon can be suppressed. The invention according to claim 2 is characterized in that a formic acid solution is used as the acid solution for the acid treatment.

Other organic acids having an acid dissociation constant similar to that of formic acid include carboxylic acids such as acetic acid, citric acid, tartaric acid, and succinic acid, and several kinds such as aromatic carboxylic acids. It is thought to have the effect of removing iron oxide,
Among them, formic acid having the lowest molecular weight and simple structure is easily decomposed.

Therefore, by using formic acid, which is easily mineralized, among organic acids that can be decomposed into carbon dioxide gas and water, it is possible to suppress an increase in the amount of secondary waste and to introduce it for the period of decomposition operation and decomposition. It is possible to minimize the energy consumed.

The invention according to claim 3 is characterized in that hydrogen peroxide or ozone is used alone or in combination as an auxiliary agent for the formic acid treatment. FIG. 17 compares and explains the effect of removing iron oxalate when using formic acid and also using these acid treatment aids together. 95 carbon steel test pieces
After immersing in 2000ppm oxalic acid solution at ℃ for 2 hours to produce yellow iron oxalate, hydrogen peroxide alone, formic acid alone, formic acid + hydrogen peroxide combined treatment, formic acid +
The ozone combined treatment was performed for 10 minutes, and the amount of iron oxalate removed was determined.

All temperature conditions are 80 ° C. and formic acid concentration is 0.0
2 mol / L, hydrogen peroxide concentration is 0.03 mol / L,
The ozone injection amount was 2 g / L / h. In the conventional hydrogen peroxide treatment as a method for removing iron oxalate, the reaction on the surface of the test piece was slow, and a large amount of iron oxalate remained after the treatment for 10 minutes.

On the other hand, in the formic acid treatment, most of the iron oxalate on the surface was removed in 10 minutes, and about 5 times as much iron oxalate was removed as in the hydrogen peroxide treatment. Further, with formic acid + hydrogen peroxide and formic acid + ozone treatment, iron oxalate was almost removed in the same manner for 10 minutes, showing a more excellent iron oxalate removing effect than the treatment with formic acid alone. This is the effect of acid dissolution of carbon steel by formic acid (the following formula) M (metal base material) + nH ⁺ → M ^{n +} + (n / 2) H ₂ in addition to Fe ²⁺ ions eluted from carbon steel This is because a hydroxyl radical (.OH) having a strong oxidizing power is generated by the reaction of the following formula with hydrogen peroxide or ozone which is an auxiliary agent, and the dissolution is promoted.

H ₂ O ₂ + Fe ²⁺ → Fe ³⁺ + OH ⁻ + · OH O ₃ + H ₂ O + Fe ²⁺ → Fe ³⁺ + OH ⁻ + · O
H + O ₂ Thus, according to the present invention of claim 3, it is possible to effectively remove more quickly iron oxalate.

The invention according to claim 4 is characterized in that a decomposition step of anodic oxidation of the formic acid solution by electrolysis or oxidative decomposition of hydrogen peroxide or ozone alone or in combination is added to the electrolysis. To do.

When electrolysis is carried out in formic acid, the formic acid is oxidized and decomposed on the surface of the anode into carbon dioxide gas and water. Electrolysis can be decomposed without adding chemicals, and it is possible to reduce the cost of chemicals and the cost of equipment related to chemicals addition, and to suppress an increase in the amount of secondary waste generated. Also, formic acid is decomposed into carbon dioxide gas and water by the oxidizing agent. As shown in Table 1, ozone and hydrogen peroxide have oxidizing power.

[0100]

[Table 1]

Therefore, formic acid is decomposed by the direct oxidation of these oxidizing agents. Furthermore, hydroxyl radicals produced by the reaction of these oxidizers with Fe ²⁺ ions eluted from carbon steel
As shown in Table 1, (.OH) has a very strong oxidizing power and effectively decomposes formic acid. HCOOH + 2 · OH → CO ₂ + 2H ₂ O

Further, since hydrogen peroxide and ozone become oxygen and water when consumed by decomposition of formic acid, they also do not become a waste source, and it is possible to suppress an increase in the amount of radioactive secondary waste generated. It is possible. When the above-mentioned anodic oxidation by electrolysis and oxidative decomposition by hydrogen peroxide or ozone are used together, the decomposition efficiency is further improved. Therefore, according to the invention of claim 4, the formic acid used for the acid treatment does not generate secondary waste,
It can be decomposed efficiently.

According to a fifth aspect of the present invention, an iron component of the formic acid solution is converted into divalent iron ions by cathodic reduction by electrolysis, and the formic acid solution is decomposed by the divalent iron ions and hydrogen peroxide. It is characterized by adding.

When electrolysis is carried out in a solution containing iron ions, the iron ions are divalently reduced on the cathode surface. As described above, the reaction between Fe ²⁺ and hydrogen peroxide produces a hydroxyl radical having strong oxidizing power, and formic acid decomposes. Therefore, according to the invention of claim 5, formic acid can be efficiently decomposed, and an increase in the amount of secondary waste generated can be suppressed.

In the invention according to claim 6, the decomposition step of anodizing the formic acid solution by electrolysis and the decomposition step of reacting divalent iron ions produced by cathodic reduction with hydrogen peroxide are used in combination. Characterize. According to the invention of claim 6, it is possible to more efficiently decompose formic acid and suppress an increase in the amount of secondary waste generated by using the above decomposition step together.

According to a seventh aspect of the present invention, in the fifth aspect, a decomposition step using oxidative decomposition with hydrogen peroxide or ozone is added to the decomposition step by electrolysis of the formic acid solution. According to the invention of claim 7,
By using the above decomposition method together, formic acid can be decomposed more efficiently, and an increase in the amount of secondary waste generated can be suppressed.

The invention according to claim 8 is characterized in that phosphoric acid or a phosphate is added as an auxiliary agent to the decomposition step in which oxidative decomposition by hydrogen peroxide or ozone is also used. Corrosion of the metal base material due to hydroxyl radicals generated by the reaction of Fe ²⁺ ions eluted in the solution by the acid treatment with hydrogen peroxide or ozone can be suppressed. Anions such as phosphate ions are considered to have an effect of removing hydroxyl radicals and have a function of suppressing corrosion of the metal surface.

Further, since phosphoric acid and phosphate react with the metal surface to form a protective film of a sparingly soluble tertiary phosphate (Fe ₃ (PO ₄ ) ₂ etc.), the corrosion inhibiting effect is further enhanced. Therefore, according to the present invention, it is possible to suppress excessive attack of the metal base material by radicals in the formic acid decomposition step using hydrogen peroxide or ozone.

The invention according to claim 9 is the chemical decontamination method for a carbon steel member, wherein a radionuclide is removed from a carbon steel member contaminated with a radionuclide by using a reducing decontamination agent containing oxalic acid. A step of removing the radionuclide from the step of removing the iron oxalate produced in the carbon steel member by performing an acid treatment of contacting the carbon steel member after removal of the radionuclide with an acid solution, and the iron oxalate. It is characterized in that a coating treatment step is formed on the carbon steel member after removal to form a protective coating for suppressing the adhesion of radioactivity.

By forming a protective film on a clean carbon steel member from which iron oxalate has been removed, various harmful effects caused by the residual iron oxalate can be removed, and the original effect of the protective film can be maximized. There is a great merit that can be demonstrated.

According to a tenth aspect of the present invention, in the ninth aspect, the protective film for suppressing the adhesion of radioactivity is obtained by contacting a nickel metal layer formed by electroless plating of nickel or a nickel metal layer with fluorine gas. It is characterized in that it is a passive film of nickel fluoride produced.

In the electroless plating treatment, a plating solution is filled in the inside of the chamber and metal ions are reduced by the action of a reducing agent to form a metal nickel layer at the site. When the surface of the carbon steel provided with the metallic nickel layer is re-immersed, the contact with the primary coolant containing radioactivity is initially cut off and the corrosion does not proceed. However, this state does not last long and the metallic nickel layer dissolves very slowly.

In addition, minute defects and cracks are generated in the metallic nickel layer due to the influence of heat. As a result, the base material gradually comes into contact with liquid at the boundary between the metallic nickel layer and the base material and inside the crack.
At this time, iron atoms in the base material generate oxides, but since excess nickel ions are present in the surroundings, a composite oxide with nickel (nickel ferrite) is formed.

Although iron atoms continue to be supplied from the base material thereafter, both are crystallized as nickel ferrite, and finally the surface and cracks are all covered with nickel ferrite. Nickel ferrite is a thermodynamically stable oxide, and if it reaches this state, corrosion does not proceed even though it is a carbon steel with a large amount of corrosion. There is also an advantage that the nickel ferrite crystal contains almost no radioactivity such as cobalt 60.

This is because the radioactivity is incorporated into nickel ferrite at the lattice position of the divalent ion, and this amount is determined by the concentration ratio of the divalent ion, cobalt 60 ion, and other divalent ions. It depends. That is,
This is because when nickel ferrite is generated in this event, excess nickel ions are present, and the concentration ratio of cobalt 60 ions taken in is extremely small.

Further, according to the method of the passive film of nickel fluoride formed by bringing the nickel metal layer formed by the electroless plating of nickel into contact with the fluorine gas, the nickel metal layer hardly dissolves. Also, since it remains as a protective film even after resuming service, it is possible to prevent corrosion of carbon steel, and as a result, it is possible to suppress adhesion of radioactivity.

In the invention according to claim 11, the protective film for suppressing the radioactivity adhesion is 300 to 600 ° C. with respect to the carbon steel member.
It is an oxide film produced by heat treatment in a gas phase containing oxygen or ozone.

It is generally known that when a steel material is subjected to an oxidation treatment at a service temperature or higher, a strong oxide film is formed and the progress of corrosion after service can be suppressed. This is a technique classified into oxide film application / pre-filming among the techniques for suppressing the radioactivity from adhering to the surface of the reactor primary system piping and component equipment described above.

Although this technology was described as being mainly for stainless steel, the present inventors expected that it would be effective for carbon steel as well, and prepared a test body that had been subjected to an oxidation treatment in air to produce a radiation. A radioactive adhesion test was carried out to compare the radioactive adhesion amounts.

Conditions for the oxidation treatment are 300 ° C., 400 ° C., 5
Heat treatment was performed for 5 hours in air under four conditions of 00 ° C. and 600 ° C., and the loop test was performed. As a result, 400 ℃
However, the amount of radioactivity adhering was smaller than that of the test sample without oxidation treatment under any of the other temperature conditions, and it was confirmed that the air oxidation treatment can suppress the attachment of radioactivity. Was done.

According to the twelfth aspect of the invention, any one of hypophosphite, a borohydride compound and hydrazine is used as a reducing agent for electroless plating, and citric acid or tartaric acid is used as a complexing agent for electroless plating. It is characterized by using.

By using these reducing agent and complexing agent, the electroless plating treatment of the present invention makes it possible to prevent hydroxide precipitation in an alkaline solution, control the plating rate, and prevent decomposition of the plating solution. The metal layer produced by this plating treatment contains 90 to 92% nickel and 8 to 1 phosphorus.
It is an amorphous alloy containing 0%, and even if the film thickness is increased, the growth of crystals in the film does not proceed and there is an effect that a uniform surface can be obtained.

The invention according to claim 13 is characterized in that the electroless plating is regulated to a thickness of 20 μm or less and the passive film is regulated to 3 μm or less. In electroless plating, if the thickness is remarkably increased, not only the amount of dissolved nickel increases but also cracks are likely to occur due to thermal stress. Therefore, considering application in a nuclear power plant, 20 μm or less is a reasonable thickness. . On the other hand, it is sufficient that the thickness of the passive state film of nickel fluoride is 3 microns or less.

The invention according to claim 14 is characterized in that the oxygen concentration of the heat treatment in the gas phase containing oxygen is 1% by volume or more. Here, the reason why the concentration of oxygen in the gas phase is 1% by volume or more is based on the following experimental results. That is, the inventors of the present invention simultaneously manufacture a test body with a different oxygen concentration when manufacturing the test body with a different treatment temperature of the oxide film, and use it for the radioactivity adhesion test. The surveyed oxygen concentration was three conditions of 1, 5 and 21% by volume. However, the amount of radioactivity attached to the manufactured test bodies was the same at any oxygen concentration. Therefore, the present inventors have presumed that the same effect can be obtained when the oxygen concentration is 1% by volume or more, and have reached the present invention proposal.

According to the fifteenth aspect of the present invention, a buffer tank and a decontaminating solution prepared by injecting and mixing water flowing out from the buffer tank and a decontaminating agent are brought into contact with the carbon steel member to be decontaminated, Decontamination solution circulation line for flowing the decontamination solution into the buffer tank, and oxalic acid solution injection connected to the decontamination solution circulation line between the water outflow side of the buffer tank and the carbon steel member to be decontaminated. Device, hydrogen peroxide injection device and formic acid injection device, an oxygen gas supply source connected between the formic acid injection device and the inflow side of the decontamination target carbon steel member, and the decontamination target carbon steel member is heated And a heating device.

It was a technically difficult task to heat-treat large-diameter pipes and large-scale equipment already installed in a nuclear power plant or the like to give an oxide film on the liquid contact surface. This can be achieved by covering the carbon steel member to be decontaminated with a band-shaped or ribbon-shaped band type heater, or by disposing a high-frequency induction heating device or the like. In particular, when using a high frequency induction heating device, not only can it handle high processing temperatures, but it can also be easily heated to an arbitrary depth by changing the frequency of the high frequency. The member can be heated to form an oxide film.

[0127]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of a chemical decontamination method for carbon steel members according to the present invention will be described with reference to FIG. Figure 1
Shows an example of a basic system configuration of a chemical decontamination apparatus in which the present embodiment is applied to a decontamination target made of carbon steel corresponding to the piping etc. of the reactor coolant purification system shown in FIG. .

In FIG. 1, reference numeral 15 is a carbon steel member to be decontaminated, which is shown as an example of decontaminating the inner surface of the pipe.
One end of a circulation line 16 is connected to both ends of the decontamination target carbon steel member 15, and the other end of the circulation line 16 is connected to a buffer 17 which also serves as a tank. A circulation pump 18 having a heater 19 for heating the decontaminating liquid flowing out from the buffer tank 17 on the circulation line 16 on the side where the decontaminating liquid flows out.
Are branched and connected, and the decontamination liquid heated by the heater 19 flows into the buffer tank 17.

Further, in the circulation line 16 on the side where the decontamination solution flows out from the buffer tank 17 and flows into the decontamination target carbon steel member 15, the oxalic acid solution adjusting tank 2 is arranged along the downstream side.
Injection pump 21 connected to 0, hydrogen peroxide injection device 3
3, the injection pump 32 connected to the formic acid adjustment tank 31, the decontamination solution injection pump 22, the bypass line 43 for bypass connection to the electrolytic device 24 having the anode 34 and the cathode 35 are sequentially connected.

On the other hand, the circulation line 1 in which the decontamination solution flows out from the carbon steel member 15 to be decontaminated and flows into the buffer tank 17.
6 is a water quality measuring device 30 for monitoring the decontamination water quality along the downstream side, a liquid feed pump 23 for collecting the decontamination liquid, a pump 45 for purifying the decontamination liquid, a cation exchange resin tower 25 and a mixing pump. A bypass line 44 including the floor resin tower 26 is sequentially connected. A moisture separator 27, a filter 28, and an exhaust blower 29 are sequentially connected to the top of the buffer tank 17 to perform exhaust gas treatment.

Next, the decontamination operation by the above decontamination apparatus will be described below. A heater 1 attached to a buffer tank 17 by circulating water inside the chemical decontamination apparatus system shown in FIG.
After the temperature is raised at 9, the oxalic acid adjusting tank 20 injects the oxalic acid whose concentration is adjusted to a predetermined concentration when injected into the decontamination target carbon steel member 15 into the circulation line 16 through the injection pump 21. To do. An oxalic acid decontamination solution is circulated through the decontamination target carbon steel member 15 to dissolve the oxide film containing the radionuclide attached to the inner surface of the decontamination target carbon steel member 15.

The water quality measuring device 30 constantly monitors and measures the water quality of the decontamination liquid flowing out from the outlet of the carbon steel member 15 to be decontaminated. The measurement items are the temperature, pH, conductivity, and redox potential of the decontamination solution, and the decontamination solution temperature, oxalic acid concentration, and redox potential are monitored by these measured values. The decontamination solution should be sampled periodically to analyze the radionuclide concentration and dissolved metal concentration.

When the radionuclide concentration in the decontamination solution is high and when it is necessary to remove Fe ²⁺ ions for controlling the oxidation-reduction potential and the iron ion valence, water is passed through the cation exchange resin 25 to make it radioactive. Nuclides and eluted metals are collected and removed.
The valence of the iron ions in the decontamination solution is adjusted by applying a DC voltage to the anode 34 and the cathode 35 in the electrolysis device 24 to reduce Fe ³⁺ to Fe ²⁺ at the cathode 35, and the anode 34. The oxidation reaction of Fe ²⁺ to Fe ³⁺ is carried out.

These reactions occur at the same time in the electrolyzer 24. When the area ratio of the anode 34 to the cathode 35 is larger than 1, Fe ²⁺ generation becomes dominant, and when the polarity of electrolysis is reversed, F ^{2+ is} generated.
The production of e ³⁺ becomes dominant, and the reaction can be controlled in each direction of oxidation and reduction as needed. The oxalic acid process is continued until the elution of radionuclide tends to be saturated, and then the oxalic acid decomposition process is started.

When a voltage suitable for oxalic acid decomposition is applied to the anode 34 and the cathode 35 of the electrolysis device 24 for electrolysis, oxalic acid is oxidatively decomposed on the surface of the anode 34. At this time, if an oxidizing agent such as hydrogen peroxide or ozone is used together, the oxalic acid decomposition is further promoted.

Since the method of electrolysis and the combined use of these oxidizing agents can also be used for the decomposition of formic acid solution after the iron oxalate removal step, the electrolysis apparatus and the ozone generator or the hydrogen peroxide injection apparatus are connected to oxalic acid. If it is used for both the purpose of decomposition and the decomposition of formic acid solution, the decontamination device can be used more reasonably.

Gases generated from the system during oxalic acid decontamination and oxalic acid decomposition (CO ₂ generated by consumption and decomposition of oxalic acid, H ₂ and O ₂ generated during electrolysis) are exhausted from the buffer tank 17. Moisture separator 2 sucked by blower 29
7, processed by the filter 28 and discharged to the exhaust system.
After the oxalic acid concentration has dropped to a predetermined concentration, the mixed bed resin tower 2
Water is passed to 6 to clean the decontamination solution.

A series of steps from the oxalic acid decontamination step to the oxalic acid decomposition step and the purification step are repeated a plurality of cycles according to the degree of decrease in the dose of the carbon steel member 15 to be decontaminated.
The acid treatment for removing iron oxalate is performed before the purification step of the final decontamination cycle. When the oxalic acid concentration is sufficiently reduced by the oxalic acid decomposition step, the formic acid solution adjusted to a predetermined concentration by the formic acid adjusting tank 31 is injected by the injection pump 32.

Formic acid adjusting tank 31 and injection pump 32
May be provided separately from the oxalic acid injection system, but if the formic acid adjustment tank 31 and the injection pump 32 are washed with water at the time of oxalic acid injection to prevent the mixture of residual oxalic acid when using formic acid, oxalic acid injection It is possible to rationalize the decontamination equipment without problems as it is shared with the system.

The formic acid solution is supplied to the decontamination target carbon steel part 15 and circulated to remove iron oxalate remaining on the inner surface of the decontamination carbon steel member 15. As shown in FIG. 17, when the acid treatment with formic acid is used together with hydrogen peroxide or ozone as an auxiliary agent, the effect of removing iron oxalate is further improved.

The effect of the concentration of the formic acid solution on the amount of iron oxalate removed will be described with reference to FIG. FIG. 2 shows the dependence of the amount of iron oxalate removed on the concentration of formic acid. The test piece of the carbon steel member was immersed in a 2000 ppm oxalic acid solution at 95 ° C for 2 hours to sufficiently generate yellow iron oxalate, and then at 80 ° C for 1 hour.
Immerse in a formic acid solution of 0 to 2000 ppm for 10 minutes,
The amount of iron oxalate removed was determined.

When the concentration of formic acid was lower than 1000 ppm, no significant change was observed in the condition of the test piece during immersion. At a formic acid concentration of 1000 ppm or more, gas was generated from the surface of the test piece, and the carbon steel member base material of the test piece was dissolved. As is clear from FIG. 2, the amount of iron oxalate removed greatly increased from the formic acid concentration of around 1000 ppm.

Therefore, the concentration of formic acid during the acid treatment is 1000 pp.
It is possible to remove iron oxalate in a shorter time by carrying out under the condition of m or more. When it is determined that the removal of iron oxalate has reached a steady state by the water quality measuring device 30 and the sampling analysis of the formic acid solution, the formic acid decomposition step is performed. Formic acid and an equivalent amount of hydrogen peroxide are injected into the circulation line 16 from the hydrogen peroxide injection device 33.

Formic acid is oxidatively decomposed by the direct oxidation with hydrogen peroxide and the hydroxyl radical generated by the reaction between Fe ²⁺ eluted from carbon steel and hydrogen peroxide. At this time, if electrolysis is performed by the electrolysis device 24 for decomposing oxalic acid, formic acid is decomposed by anodic oxidation, so that the decomposition treatment can be performed more efficiently.

CO ₂ which is a decomposition product of formic acid, and exhaust gases of H ₂ and O ₂ generated when electrolysis is also used are sucked from the buffer tank 17 into the exhaust blower 29, as in the case of oxalic acid decomposition. It passes through the moisture separator 27 and the filter 28 and is discharged to the existing exhaust system. After the decomposition of formic acid, the removed iron oxalate and the eluted metal components are collected and removed by the mixed bed resin 26, and purified to a water condition that allows drainage.
By the above chemical decontamination operation, the decontamination can be performed on the carbon steel of the decontamination target carbon steel member 15 without the residual iron oxalate.

Next, a second embodiment according to the present invention will be described with reference to FIG. The decontamination apparatus shown in FIG. 3 is an example of a decontamination apparatus configuration targeting equipment, pipes, and the like made of carbon steel, as in the first embodiment. 3, those parts that are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description of the overlapping parts will be omitted.

The present embodiment has substantially the same basic decontamination equipment structure as that of the first embodiment, except that the ozone generator 36 and the mixer 37 as the ozone injection system are used as the decontamination liquid. This is because it was attached to the circulation line 16 on the inflow side and the ozone decomposing device 38 was attached to the exhaust gas treatment system. Ozone can be used for oxalic acid decomposition, formic acid treatment aid, and formic acid decomposition.

In each step, the ozone gas generated from the ozone generator 36 is injected into the system solution flowing through the inflow side circulation line 16 via the mixer 37 and dissolved. In the oxalic acid decomposition step, the time required for oxalic acid decomposition is significantly increased by using anodic oxidative decomposition in the electrolysis device 24 and oxidative decomposition by ozone and hydroxyl radicals generated from ozone and eluted Fe ^2+. Can be shortened.

In the formic acid treatment step, the hydroxyl radicals generated as the acid treatment aid in the same manner as above promote the dissolution of the carbon steel base material, and the time required for iron oxalate removal can be shortened. In the formic acid decomposition step, formic acid can be decomposed by oxidative decomposition by ozone and the hydroxyl radicals,
At this time, if electrolysis is further performed by the electrolysis device 24 for decomposing oxalic acid, formic acid is decomposed by anodic oxidation, so that the decomposition treatment can be performed more efficiently.

In each of these steps, hydrogen peroxide may be used alone or in combination with ozone. In the decomposition step using hydrogen peroxide or ozone, the addition of phosphoric acid or phosphate as an auxiliary agent has the effect of suppressing the corrosion of the metal surface. When ozone gas is injected into the decontamination system, excess ozone that has not been consumed is mixed into the exhaust gas.

Since the regulation of the exhaust ozone concentration in the working environment in Japan is 0.1 ppm, it is necessary to reduce the ozone in the exhaust gas to 0.1 ppm or less by the ozone decomposing device 38.
As the ozone decomposing device 38, activated carbon or a metal catalyst is effective. Activated carbon is suitable for low-concentration ozone gas of several hundred ppm or less. Ozone reacts with activated carbon as shown below,
Decomposes into oxygen and carbon dioxide. 2O ₃ + C → CO ₂ + 2O ₂

In a decomposition test of low-concentration ozone gas using a honeycomb-type activated carbon filter, 20 ppm of ozone gas was used for 5 m.
By contacting the mt activated carbon filter for about 0.1 seconds, 9
A decomposition efficiency of 9% or more was obtained. It is possible to satisfy the discharged ozone concentration standard by adjusting the amount of activated carbon and the contact time according to the concentration of ozone gas to be treated and the period of use.

When ozone is used for oxalic acid decomposition, formic acid decomposition, etc., most of the ozone is consumed in the decomposition reaction because of the high concentration of the reacting organic substances. Therefore, the ozone concentration in the exhaust gas is low, and a decomposition device using activated carbon can be applied. In addition, since activated carbon is inexpensive, it is generally used as an ozone decomposer.

However, the ozone concentration in the exhaust gas is 1000
When a high concentration of ppm or more or a large amount of ozone treatment is required, the activated carbon is consumed by the decomposition of ozone, so that the required amount of activated carbon increases and the scale of the decomposition device becomes large. For the treatment of such high-concentration ozone, decomposition with a metal oxide catalyst such as manganese or iron is effective. Ozone is decomposed into oxygen by the reaction with the metal catalyst (M). O ₃ + M → O ₂ + MO MO + O ₃ → 2O ₂ + M

In an ozone decomposition test using a catalyst filter made of a metal oxide such as manganese oxide, extremely high decomposition efficiency was obtained, and an inlet ozone gas concentration of 38000 ppm was adjusted to 60%.
Even if it is ventilated for more than 00 hours, the concentration of discharged ozone gas is 0.0
It was 1 ppm or less. The catalyst is not consumed as long as it does not contain a component that becomes a catalyst poison except for mechanical strength deterioration such as pulverization due to reaction heat.

The catalyst poison for the metal catalyst is a metal compound,
They are halogen compounds, nitrogen oxides, sulfur oxides, etc., and none of them is contained in a large amount in the exhaust gas at the time of chemical decontamination. Therefore, using a catalyst, a compact decomposition device
It is possible to treat a high concentration and a large amount of ozone gas.

Further, a chemical treatment in which exhaust gas is passed through a chemical liquid which reacts with ozone can also be used. In particular, if an organic chemical capable of decomposing oxalic acid or the like is used, it is possible to carry out the exhaust ozone treatment without generating secondary waste.

A third embodiment according to the present invention will be described with reference to FIG. The present embodiment is a configuration example of a decontamination apparatus in which a system in which carbon steel and stainless steel are mixed as a decontamination target is a decontamination target and carbon steel and stainless steel can be separated.
When stainless steel is to be decontaminated, an oxidation step is required in addition to the oxalic acid step in order to enhance the decontamination effect. The apparatus shown in FIG. 4 has a configuration in which ozone decontamination equipment for using ozone as an oxidant is added to the decontamination apparatus equipment shown in FIG. 3 described in the second embodiment. The decontamination target carbon steel member 15 and the decontamination target stainless steel member 44 are connected by an isolation valve 39. In addition, in FIG.
The same parts as those in FIG.

When carrying out the ozone oxidation step, the isolation valve 3 is used.
9 is closed, and the decontamination target stainless steel member 40 and the decontamination target carbon steel member 15 are separated. With the circulation line 16 and the stainless steel decontamination line 41, the decontamination liquid is circulated only in the decontamination target stainless steel member 40. The water quality measuring device 30 constantly measures the dissolved ozone concentration in addition to the decontamination liquid temperature, pH, redox potential, conductivity, and the ozone generator 36 from the ozone generator 36 so as to maintain the dissolved ozone concentration range suitable for decontamination. Adjust the ozone gas injection rate.

Carbonate, carbonate, hydrogen carbonate, boric acid,
Addition of borate, sulfuric acid, sulfate, phosphoric acid, phosphate and hydrogen phosphate to the ozone decontamination solution has the effect of suppressing corrosion of stainless steel. Anions generated by dissolving these in the decontamination solution are considered to have the effect of removing highly reactive chemical species generated by the decomposition of ozone in water, and to suppress the corrosion of the metal surface. .

Further, in the case of phosphoric acid or a phosphate, a protective film of a sparingly soluble tertiary phosphate (Fe ₃ (PO ₄ ) ₂ or the like) is formed by reacting with the metal surface, so that the corrosion inhibiting effect is further enhanced. high. In the ozone process, the decontamination solution is periodically sampled, the radionuclide concentration and the eluted metal concentration are analyzed, and the ozone process is continued until the elution of the radionuclide or chromium shows a saturation tendency.

After the ozone step, the remaining oxalic acid is decomposed in a short time by injecting the oxalic acid solution from the oxalic acid adjusting tank 17, and the oxalic acid step is continuously performed. In the oxalic acid step, the decontamination target carbon steel member 15 and the decontamination target stainless steel member 40 are decontaminated by separating or simultaneously passing water as necessary so that both members can obtain a sufficient decontamination effect.

When the formic acid treatment is carried out after the oxalic acid decomposition step of the final cycle, the isolation valve 39 is closed in advance and the stainless steel member 40 to be decontaminated is preliminarily closed so that iron oxalate separated by the formic acid treatment does not deposit on the stainless steel surface. Does not pass water.
Hydrogen peroxide can also be used as an auxiliary agent in the formic acid treatment step. After removing the iron oxalate, the formic acid waste liquid is decomposed by hydrogen peroxide or electrolysis and purified by the mixed bed resin 26.

On the downstream side of the purification process, the isolation valve 39 is opened and the stainless steel member 40 to be decontaminated and the carbon steel member 15 to be decontaminated.
While simultaneously passing water through and, the mixed-bed resin 26 purifies the water quality that can be discharged. By the above operation, it is possible to decontaminate an object to be decontaminated in which carbon steel and stainless steel are mixed, without iron oxalate remaining.

Next, a fourth embodiment of the present invention will be described with reference to FIG. In the present embodiment, as in the third embodiment, a system in which the decontamination target carbon steel member 15 and the decontamination target stainless steel member 40 are mixed is targeted for decontamination, and the carbon steel and the stainless steel can be separated from each other. 2 is an example of the configuration of the decontamination device.
In the decontamination apparatus configuration basically similar to that of the third embodiment, ozone gas generated from the ozone generator 36 can be injected into the decontamination target stainless steel member 40 and the decontamination target carbon steel member 15, respectively. The mixers 37 and 42 are installed.

Ozone can be used for oxidative decontamination agents, oxalic acid decomposition, formic acid treatment aids, and formic acid decomposition. By effectively utilizing ozone in multiple steps, it is possible to perform all operations of decontamination and iron oxalate removal without using hydrogen peroxide.

When carbon steel and stainless steel are mixed and cannot be separated, the oxidative decontamination liquid is also passed through the carbon steel system. When the oxidant solution is passed through the carbon steel system, the oxidant is consumed by the reaction with the carbon steel surface. When the oxidizing agent is consumed in the carbon steel system, there is a problem that the decontaminating effect is lowered because the oxidizing agent is not sufficiently supplied to the stainless steel part.

FIG. 6 (a) shows the permanganate ion concentration when an oxidizing agent containing permanganate ion used for conventional chemical decontamination is passed through a carbon steel system, the actual concentration in the solution and the stainless steel concentration. It is a figure shown as a ratio to the decontamination target concentration required for steel decontamination. Fig. 6 (b) shows the dissolved ozone concentration when ozone is injected into the stainless steel and carbon steel systems under the same solution conditions and ozone injection conditions, and the actual concentration in the solution and the depletion required for decontamination of the stainless steel. It is a figure shown as a ratio to a dyeing target density.

In both FIGS. 6 (a) and 6 (b), the ratio of the metal surface area of the liquid contact portion to the amount of the decontamination liquid is 10 cm ² / cm ³ . Figure 6
As is clear from (a), permanganate ions can be obtained only at about half of the target concentration even if the decontamination target concentration equivalent amount is added. After that, an oxidant was added, but even if the concentration temporarily increased, it decreased within 30 minutes, and the decontamination target concentration could not be obtained even if about 2.5 times the amount equivalent to the target concentration was added. I can't. In order to reach the decontamination target concentration, it is necessary to add a larger amount of chemicals, and the amount of secondary waste increases accordingly.

On the other hand, in the case of ozone shown in FIG. 6B, the dissolved ozone concentration in the carbon steel system is lower than that in the stainless steel system, but is about 20% lower. Since ozone continuously injects ozone gas into the decontamination liquid, it is considered that the ozone gas is constantly supplied additionally even if it is consumed, and the decrease in concentration is small. If the decrease is about 20%, it is possible to obtain the target ozone concentration by increasing the ozone gas injection amount, and the secondary waste does not increase even if the ozone injection amount is increased.

In the oxalic acid step, the oxalic acid decomposition step and the iron oxalate removal step, if the carbon content is properly flushed or purified so that insoluble components generated from the carbon steel portion do not deposit on the stainless steel portion, It is possible to carry out these steps under conditions that ensure the soundness of both steel and stainless steel at the same time.

Therefore, even if carbon steel and stainless steel are mixed and cannot be isolated, decontamination can be performed without using iron as an oxidative decontamination agent. It can be carried out.

Next, an example of main steps of decontamination when a stainless steel / carbon steel mixed system is to be decontaminated will be described with reference to FIGS. 7 (a) and 7 (b). As shown in FIG. 7 (a), the main decontamination process consists of an oxalic acid treatment, an oxalic acid decomposition, and a purification process in the first cycle, and an ozone treatment, an oxalic acid treatment, and an oxalic acid treatment in the second and third cycles. It consists of a combination of acid decomposition and purification processes. Hereinafter, the process times are shown for comparison. The iron oxalate removal step of the present invention is carried out in the latter stage of the oxalic acid decomposition step of the final cycle.

Iron (II) oxalate is hardly soluble in water, and the solubility of iron (II) oxalate in hot water is 0.026 g / 10.
It is 0 g. When the method of positively removing iron oxalate is not used, oxalic acid and iron are removed so that the concentration of iron oxalate in the liquid is less than this solubility, and the iron oxalate precipitated on the carbon steel surface is removed. Dissolve.

However, if the dissolved components are not removed from the liquid, further dissolution will not proceed. Therefore, in such a method, it takes a long time to dissolve and remove iron oxalate, and at the same time, it takes a long time to purify to the wastewater standard. From the results of decontamination up to now, if iron oxalate remains on the metal surface after decontamination, the oxalic acid decomposition and purification time will be longer than 10 hours compared to the case where iron oxalate does not remain. It is estimated that it will take.

As described above, since the removal of iron oxalate by using formic acid or a combination of formic acid and an auxiliary agent is effective in a short time, it is considered that the formic acid treatment process in an actual machine is completed within 1 hour. Further, FIG. 8 shows the decomposition effect of the formic acid solution by hydrogen peroxide and Fe ²⁺ . When 1.5 times equivalent of hydrogen peroxide was added to a 50,000 ppm formic acid solution in the presence of Fe ^2+, it was within 30 minutes at a temperature of 80 ° C and 90 at 50 ° C.
Decomposed within minutes. Since the decontamination solution temperature before the final purification is expected to be around 60 ° C, the decomposition efficiency is equivalent to the result under the condition of 50 ° C.

Further, the formic acid treatment for removing iron oxalate is effective from the concentration of about 50 ppm which is 1/50 of the concentration in the main decomposition test, as described above. Formic acid concentration 1
If it is used under the condition of about 000ppm, the decomposition time is 50
It is considered that it can be decomposed sufficiently within 1 hour since it was significantly shortened from 90 minutes under the condition of ° C.

As shown in FIG. 7 (b), in comparison with the conventional oxalic acid decomposition / purification process, the iron oxalate removal process is added in the present invention, so the number of processes itself increases. However, the total formic acid treatment and formic acid decomposition treatment steps are 1-2.
It can be carried out in a short time of about 10 hours, and the oxalic acid decomposition / purification process when the iron oxalate removal method is not implemented as described above is 10 hours or more longer than when iron oxalate is removed. Therefore, even if the number of steps is increased, the construction period can be shortened by the present invention as compared with the conventional method.

A fifth embodiment of the present invention will be described with reference to FIG. FIG. 9 is a basic system of a chemical decontamination apparatus in which the chemical decontamination method according to the present embodiment is applied to a decontamination target made of carbon steel corresponding to the piping of the reactor coolant purification system in FIG. An example of the configuration is shown. In FIG. 9, the same parts as those in FIG. 1 are designated by the same reference numerals, and overlapping parts are omitted. In this embodiment, in the first embodiment shown in FIG. 1, valves V1 to V3 are attached to the circulation line 16 and branched from the decontamination solution outlet side of the decontamination target carbon steel member 15 to valves V4 and V4.
5, a plating solution circulation line 48 is provided between the valve V4 and the valve V3, and a pump 50, a plating solution regeneration tank 53, a plating solution tank 51 having a heater 52, and a pump 49 are sequentially connected to the plating solution circulation line 48. There is something I did.

In this embodiment, first, the carbon steel member 1 to be decontaminated by the series of chemical decontamination operations shown in the first embodiment.
Decontamination is performed on No. 5 without leaving iron oxalate. Then, in the final step, a coating treatment step is performed to form a protective coating on the inner surface of the carbon steel member by plating to prevent radioactivity from adhering.

The specific film treatment process is as follows. After completion of a series of chemical decontamination treatments, the valves V1 and V2 are closed to isolate the decontamination target carbon steel member 15 from the decontamination system. Next, the valve V5 is opened to discharge the isolated purified water of the decontamination system out of the system. After that, the valves V3 and V4 are opened and the plating solution circulation line 48 is connected. Pump 4
9 and 50 are activated to allow the plating solution stored in the plating solution tank 51 to flow into the decontamination target carbon steel member 15.

A heater 52 is provided in the plating solution tank 51.
Is provided and the plating liquid is maintained at a predetermined temperature. The plating solution that has passed through the decontamination target carbon steel member 15 is sent to the plating solution reclaiming tank 53, and after adjusting the concentration of the plating solution and removing impurities, returns to the plating solution tank 51. When the decontamination target carbon steel member 15 is brought into contact with a plating liquid having a predetermined concentration and a predetermined temperature, a plating layer is formed.

After the time required for laminating the desired plated layer has elapsed, the valves V3 and V4 are closed and the carbon steel member 15 to be decontaminated 15
Is isolated from the plating solution circulation line 48. Then, the valve V5 is opened to discharge the plating liquid accumulated in the carbon steel member 15 to be decontaminated. Further, the valve V5 is closed, the valves V1 and V2 are opened, the decontamination line is connected to the circulation line 16, and the plating solution is washed using the pump and the purification equipment of the circulation line 16.

Next, a sixth embodiment of the present invention will be described with reference to FIG. In the present embodiment, the fluorine gas cylinder 54 is installed in the component equipment in the fifth embodiment, and other parts are the same as in FIG. In this embodiment, fluorine gas is used for discharging the plating solution after the electroless plating is completed. That is, the valves V3 and V4 are closed and the carbon steel member 15 to be decontaminated is isolated from the plating solution circulation line after a lapse of time when a desired plating layer is laminated.

Thereafter, the valve V5 is opened to discharge the plating solution accumulated in the decontamination target carbon steel member 15. At this time, the fluorine gas is sent from the fluorine gas cylinder 54 to the system, and the plating solution is discharged by the gas pressure. At the same time, the plating layer formed on the surface of the carbon steel member 15 to be decontaminated is simultaneously formed into a passive film.

Next, a seventh embodiment of the present invention will be described with reference to FIG. FIG. 11 is a basic system of a chemical decontamination apparatus in which the chemical decontamination method according to the present invention is applied to a decontamination object made of carbon steel corresponding to the piping of the reactor coolant purification system shown in FIG. An example of the configuration is shown.

In this embodiment, as shown in FIG. 11, the decontamination liquid inflow side circulation line 16 of the carbon steel member 15 to be decontaminated is branched to the downstream side of the valve V2 and the mixed gas cylinder 58 is connected via the valve V3. This is because the heater 55 was installed on the surface of the carbon steel member 15 to be decontaminated while being installed.
A thermometer 56 and a temperature controller 57 are attached to the heater 55. In addition, the mixed gas cylinder 58 is filled with a mixed gas of nitrogen gas containing 1 volume% or more of oxygen.

In the present embodiment, first, the carbon steel member 15 to be decontaminated is decontaminated without residual iron oxalate by the series of chemical decontamination operations in the first embodiment. Then, in the final step, a coating treatment step is performed to form a protective coating on the inner surface of the decontamination target carbon steel member 15 for suppressing the adhesion of radioactivity.

The specific film treatment process is as follows. After completion of a series of chemical decontamination treatments, the valves V1 and V2 are closed to isolate the decontamination target carbon steel member 15 from the decontamination system. Next, the valve V5 connected in series to the valve V1 is opened to discharge the isolated purified water of the decontamination system out of the system. Then, the valve V3 is opened, and the mixed gas containing 1 volume% or more of oxygen is flown into the decontamination target carbon steel member 15 from the mixed gas cylinder 58. When the gas phase portion of the system is sufficiently replaced with the injected gas, the valve V5 is closed.

A heater 5 is attached to the carbon steel member 15 to be decontaminated.
5 is wound, and the film treatment step is performed by adjusting the current value by the temperature controller 57 while measuring the surface temperature of the carbon steel member 15 to be decontaminated by the thermometer 56. Here, the set temperature is in the temperature range of 300 to 600 ° C., and the heating time of 5 hours at the longest is sufficient. This heat treatment forms a protective film on the surface of the pipe, which is effective in suppressing the attachment of radioactivity.

When the carbon steel part to be decontaminated occupies a large area, the oxygen concentration in the mixed gas initially filled may be less than 1% by volume due to the consumption due to the formation of the oxide film. It is necessary to set a higher value, or measure the concentration appropriately, and add oxygen components depending on the situation. However, such work can be avoided in most cases by prefilling with air gas (oxygen 21% by volume, remaining nitrogen gas).

Next, an eighth embodiment according to the present invention will be described with reference to FIG. FIG. 12 is a basic system of a chemical decontamination apparatus in which the chemical decontamination method according to the present embodiment is applied to a decontamination object made of carbon steel corresponding to the piping of the reactor coolant purification system in FIG. An example of the configuration is shown.

In this embodiment, a movable high frequency induction furnace 59 is provided in place of the heater 55 of the carbon steel member 15 to be decontaminated in the seventh embodiment shown in FIG. It is connected to the control device 60.

In this embodiment, first, the carbon steel member 1 to be decontaminated by the series of chemical decontamination operations shown in the first embodiment.
Decontamination of No. 5 is performed without residual iron oxalate. Then, in the final step, a coating treatment step is performed to form a protective coating on the inner surface of the decontamination target carbon steel member 15 for suppressing the adhesion of radioactivity.

The specific film treatment process is as follows. After completion of a series of chemical decontamination treatments, the valves V1 and V2 are closed to isolate the decontamination target carbon steel member 15 from the decontamination system. Next, the valve V5 is opened to discharge the isolated purified water of the decontamination system out of the system.

Thereafter, the valve V3 is opened, and the mixed gas containing oxygen of 1 volume% or more is flown into the carbon steel member 15 to be decontaminated from the mixed gas cylinder 58. When the gas phase portion of the system is sufficiently replaced with the injected gas, the valve V5 is closed. A high-frequency induction furnace 59 is installed in the carbon steel member 15 to be decontaminated, and the coating process is performed by the controller 60 while measuring the surface temperature of the pipe with the thermometer 56.

Here, the set temperature is 300 to 600.
It is sufficient to set the temperature range to ° C and the heating time to 5 hours at the longest. This heat treatment forms a protective film on the surface of the pipe, which is effective in suppressing the attachment of radioactivity. The high frequency induction furnace 59 used in the present embodiment is a handy type that can be easily moved, and can be subjected to a film treatment so as to surround the carbon steel member 15 to be decontaminated.

[0198]

According to the present invention, in a chemical decontamination method for removing a radionuclide from a carbon steel member contaminated with a radionuclide by using a reducing decontamination agent containing oxalic acid, the shu which is present after the chemical decontamination is performed. Iron oxide is effectively removed in an extremely short time while maintaining the soundness of the material, and the inner surface of the carbon steel member to be decontaminated is cleaned to suppress the acceleration phenomenon of radioactivity adhesion. Can prevent the occurrence of hot spots.

Further, according to the present invention, in the final step after chemical decontamination, a protective film capable of suppressing radioactivity adhesion is formed on the carbon steel member cleaned by the removal of iron oxalate, so that its effectiveness is maximized. You can make the most of it.

Further, according to the chemical decontamination apparatus of the present invention, the radiation exposure of the worker at the time of periodic inspection and construction can be reduced, and the inspection and construction period and cost can be reduced. Further, since the iron oxalate can be removed in a shorter time than the conventional oxalic acid decomposition / purification process for preventing the residual iron oxalate, the decontamination process itself can be shortened.

[Brief description of drawings]

FIG. 1 is a first method of chemically decontaminating a carbon steel member according to the present invention.
3 is a system diagram for explaining the embodiment of FIG.

FIG. 2 is a characteristic diagram showing formic acid concentration dependence of the amount of iron oxalate removed in the first embodiment.

FIG. 3 is a second method of chemically decontaminating a carbon steel member according to the present invention.
3 is a system diagram for explaining the embodiment of FIG.

FIG. 4 is a third method of chemically decontaminating a carbon steel member according to the present invention.
3 is a system diagram for explaining the embodiment of FIG.

FIG. 5 is a fourth method of chemically decontaminating a carbon steel member according to the present invention.
3 is a system diagram for explaining the embodiment of FIG.

FIG. 6A is a characteristic diagram showing the relationship between the permanganate ion concentration and time of the oxidant concentration measurement test result in the fourth embodiment, and FIG. 6B is a characteristic diagram for the same dissolved ion concentration.

7A is a main decontamination process diagram in the fourth embodiment, and FIG. 7B is a process diagram showing a comparison between the conventional method and the iron oxalate removal process in FIG.

FIG. 8 is a characteristic diagram showing the results of a formic acid decomposition test in the fourth embodiment.

FIG. 9 is a fifth method of chemically decontaminating a carbon steel member according to the present invention.
3 is a system diagram for explaining the embodiment of FIG.

FIG. 10 is a system diagram for explaining a sixth embodiment of the method for chemically decontaminating a carbon steel member according to the present invention.

FIG. 11 is a system diagram for explaining a seventh embodiment of the method for chemically decontaminating a carbon steel member according to the present invention.

FIG. 12 is a system diagram for explaining an eighth embodiment of the chemical decontamination method for a carbon steel member according to the present invention.

FIG. 13 is a piping system diagram schematically showing the periphery of a nuclear reactor of a boiling water nuclear power plant.

FIG. 14 is an assembly diagram showing an example of assembling a test body in the chemical decontamination method for a carbon steel member.

FIG. 15 is a bar chart showing the results of analysis of the amount of attached radioactivity of the test body in FIG.

16 (a) is a micrograph showing an SEM observation result of the test body in FIG. 14 after applying an oxide film, FIG. 16 (b) is a micrograph after oxalic acid treatment, and FIG. The microscope photograph figure after a test.

FIG. 17 is a bar diagram showing the results of the iron oxalate removal test of the conventional example and the example of the present invention in comparison.

[Explanation of symbols]

1 ... Reactor pressure vessel, 2 ... Reactor recirculation piping, 3 ... Recirculation pump, 4 ... Carbon steel piping, 5 ... Circulation pump, 6 ... Regenerative heat exchanger, 7 ... Non-regenerative heat exchanger, 8 ... Filtration Desalination tower, 9 ...
Water supply system piping, 10 ... Carbon steel piping, 11 ... Pump, 12 ...
Heat exchanger, 13 ... Head spray nozzle, 14 ... Bypass line, 15 ... Decontamination target carbon steel member, 16 ... Circulation line, 17 ... Buffer tank, 18 ... Circulation pump, 19 ...
Heater, 20 ... Oxalic acid solution adjusting tank, 21 ... Injection pump, 22 ... Decontamination solution inflow pump, 23 ... Solution delivery pump, 2
4 ... Electrolyzer, 25 ... Cation exchange resin tower, 26 ... Mixed bed resin tower, 27 ... Moisture separator, 28 ... Filter, 29 ... Exhaust blower, 30 ... Water quality measuring instrument, 31 ... Formic acid adjusting tank,
32 ... Injection pump, 33 ... Hydrogen peroxide injection device, 34 ...
Anode, 35 ... Cathode, 36 ... Ozone generator, 37 ... Mixer, 38 ... Ozone decomposing device, 39 ... Isolation valve, 40 ... Stainless steel member for decontamination, 41 ... Stainless steel decontamination line, 42 ... Mixer, 43, 44 ... Bypass line, 4
5 ... Pump, 46 ... Test body, 46a ... Test body assembly, 47
... Outer cylinder, 47a ... Set screw, 47b ... Perforated ring, 4
8 ... Plating liquid circulation line, 49, 50 ... Pump, 51 ...
Plating liquid tank, 52 ... Heater, 53 ... Plating liquid recycling tank, 54 ... Fluorine gas cylinder, 55 ... Heating heater, 5
6 ... Thermometer, 57 ... Temperature controller, 58 ... Mixed gas cylinder, 59 ... High frequency induction furnace, 60 ... Control device, V1-V5
…valve.

Front page continuation (51) Int.Cl. ⁷ identification code FI theme code (reference) C02F 1/461 C02F 1/72 Z 1/72 1/78 1/78 1/46 101B 101C (72) Inventor Ichiro Inami F-term (reference) 4D050 AA13 AB14 AB16 BB02 BB09 BC07 BC10 BD02 BD03 BD06 4D061 DA08 DB18 DC09 DC10 DC22 EA03 EA04 EB04 ED02 ED03 FA08 Incorporated company Toshiba Yokohama Office 8 term, Shinsugita-cho, Isogo-ku, Yokohama, Kanagawa Prefecture

Claims (15)

[Claims] 1. A chemical decontamination method for a carbon steel member, wherein the radionuclide is removed from the carbon steel member contaminated with the radionuclide by using a reducing decontamination agent containing oxalic acid, the method comprising the step of: The method for chemical decontamination of a carbon steel member, characterized in that the iron oxalate formed on the carbon steel member is removed by performing an acid treatment in which the carbon steel member is brought into contact with an acid solution. 2. The method for chemical decontamination of a carbon steel member according to claim 1, wherein a formic acid solution is used as the acid solution for the acid treatment. 3. A hydrogen peroxide as an auxiliary agent for the acid treatment,
Alternatively, the chemical decontamination method for a carbon steel member according to claim 2, wherein ozone is used alone or in combination. 4. The decomposition step of anodizing the formic acid solution by electrolysis, or adding a decomposition step of oxidative decomposition using hydrogen peroxide or ozone alone or in combination with the electrolysis. Chemical decontamination method for carbon steel members. 5. An iron component of the formic acid solution is converted to divalent iron ions by cathodic reduction by electrolysis, and a decomposing step of decomposing the formic acid solution with the divalent iron ions and hydrogen peroxide is added. The method for chemically decontaminating a carbon steel member according to claim 2. 6. The decomposition step of anodizing the formic acid solution by electrolysis and the decomposition step of reacting divalent iron ions produced by cathodic reduction with hydrogen peroxide are used in combination. A method for chemically decontaminating a carbon steel member as described. 7. The chemical decontamination method for a carbon steel member according to claim 5, wherein a decomposition step using oxidative decomposition with hydrogen peroxide or ozone is added to the decomposition step by electrolysis of the formic acid solution. . 8. The chemical decontamination method for a carbon steel member according to claim 7, wherein phosphoric acid or a phosphate is added as an auxiliary agent to the decomposition step using oxidative decomposition with hydrogen peroxide or ozone in combination. . 9. A chemical decontamination method for a carbon steel member, wherein the radionuclide is removed from the carbon steel member contaminated with the radionuclide by using a reducing decontamination agent containing oxalic acid, wherein the radionuclide is removed from the carbon steel member. A step of removing, a step of removing the iron oxalate generated in the carbon steel member by subjecting the carbon steel member after removal of the radionuclide to an acid solution, and a carbon steel after removal of the iron oxalate A method for chemical decontamination of a carbon steel member, which comprises providing a film treatment step for forming a protective film for suppressing adhesion of radioactivity on the member. 10. The protective film for suppressing the adhesion of radioactivity is a nickel metal layer formed by nickel electroless plating, or a nickel fluoride passivation film formed by bringing the nickel metal layer into contact with fluorine gas. The method for chemical decontamination of a carbon steel member according to claim 9, wherein 11. The protective film for suppressing the adhesion of radioactivity is an oxide film formed by heat-treating the carbon steel member in a gas phase containing oxygen or ozone at 300 to 600 ° C. The chemical decontamination method for a carbon steel member according to claim 9. 12. The method according to claim 10, wherein any one of hypophosphite, a borohydride compound, and hydrazine is used as a reducing agent for the electroless plating treatment, and citric acid or tartaric acid is used as a complexing agent. A method for chemically decontaminating a carbon steel member as described. 13. The method for chemical decontamination of a carbon steel member according to claim 10, wherein the electroless plating has a thickness of 20 μm or less and the passive film has a thickness of 3 μm or less. 14. The chemical decontamination method for a carbon steel member according to claim 11, wherein the oxygen concentration of the heat treatment in the gas phase containing oxygen is maintained at 1% by volume or more. 15. A buffer tank, and a decontaminating solution prepared by injecting and mixing water flowing out from the buffer tank with a decontaminating agent is brought into contact with the carbon steel member to be decontaminated, and the decontaminating solution after the contact is said Oxalic acid solution injection device connected to the decontamination solution circulation line for flowing into the buffer tank and the decontamination solution circulation line between the water outflow side of the buffer tank and the carbon steel member to be decontaminated, hydrogen peroxide injection And a formic acid injection device, an oxygen gas supply source connected between the formic acid injection device and the inflow side of the decontamination target carbon steel member, and a heating device for heating the decontamination target carbon steel member. A chemical decontamination device for carbon steel members, which is characterized in that