JP2001081542A - Method of manufacturing stainless steel member for nuclear power plant and nuclear power plant - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the adhesion of a radioactive material to a stainless steel member by depositing a Cr concentrated layer on the surface of a stainless steel member in contact with nuclear reactor cooling water containing a radio active material by primary treatment and thereafter depositing an Fe-Ni concen trated layer on the outside of the Cr concentrated layer by secondary treatment. SOLUTION: By electrolytic polishing and chemical polishing in which Fe in a base material is selectively eluted to relatively increase the concentration of Cr in the surface, a Cr concentrated layer 12 is formed on the surface of a constituting member 11 for a nuclear power plant Also, by heat tretment at >=700 deg.C, the diffusing rate of Cr is made higher than of Fe and Ni, and Cr is concentrated on the surface to deposit the Cr concentrated layer 12. Chromate treatment, electrolytic treatment, chemical vapor deposition, sputtering or the other similar methods are suitable for depositing the Cr concentrated layer 12. Next, by executing heating treatment at <=500 deg.C, two layer oxide in which the external layer is composed of an Fe-Ni concentrated layer 13, and the internal layer is composed of the Cr concentrated layer 12 is deposited, so that the corrosion of the constituting member 11 can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention provides a stainless steel member used in contact with a liquid in which a radioactive substance is dissolved, such as a primary cooling water pipe for a nuclear power plant, for example, by performing a radioactive substance adhesion suppression treatment. Nuclear power plant.

[0002]

2. Description of the Related Art Pipes, pumps, valves and the like used in a primary cooling water system of a nuclear power plant are made of stainless steel, stellite and the like (hereinafter abbreviated as constituent members). When these metals are used for a long period of time, they are damaged by corrosion, and the constituent metal elements are eluted into reactor cooling water (hereinafter abbreviated as cooling water) and brought into the reactor. Most of the eluted metal elements become oxides and adhere to the fuel rods, and undergo neutron irradiation. As a result, 60-Co, 58-Co, 51-Cr, 54-
Radionuclides such as Mn are produced. These radionuclides are re-eluted into primary cooling water to form ions or insoluble solid components.
(Hereinafter, referred to as cladding). Some of them are removed by a desalter for purifying reactor water, but the rest accumulates on the surface of components such as carbon steel and stainless steel while circulating through the primary cooling system. For this reason, the dose rate on the surface of the component member becomes high, and there is a problem of radiation exposure of workers when performing maintenance and inspection.

In general, as a method of preventing an increase in the surface dose rate of a structural material, a method of removing a radioactive substance attached to the structural material has been studied and implemented. There are currently three removal methods.

(1) Mechanical cleaning method (2) Chemical cleaning method (3) Electrolytic cleaning method The method of (1) is mainly applied to parts, for example, cleaning the surface with high-pressure jet water. . However, in this method, it is difficult to remove a radioactive substance having strong adhesion, and a wide area cannot be systematically decontaminated. In fact, even if the dose rate is temporarily reduced by this method, the dose rate tends to increase again after long-term use.

The method (2) uses an agent such as an acid solution to dissolve the oxide film on the surface of the component by a chemical reaction and remove radioactive substances present in the film. The problem with this method is the corrosion damage of the structural material by the chemical. That is, when dissolving the film, the structural material may be damaged by corrosion, and a trace amount of the chemical remaining after decontamination may cause stress corrosion cracking of the structural material.

The method (3) has the same problem as the method (1).

[0007] Further, in order to reduce the amount of radioactive material attached, a method of suppressing elution of a metal element which is a source thereof has been implemented. For example, there is a method of using a material having good corrosion resistance or a method of injecting oxygen into a water supply system to suppress corrosion of the constituent members.

However, any of the methods cannot sufficiently reduce the corrosion of the components of the primary cooling water system including the water supply system, and cannot sufficiently reduce the radioactive substances in the primary cooling water. Therefore, an increase in the surface dose rate due to the accumulation of radioactive material in the constituent members still remains as a problem. In particular, it is urgently necessary to suppress the adhesion of the radioactive material to stainless steel members.

[0009] Techniques for reducing the adhesion of radioactive substances include, for example, JP-A-60-262955 and JP-A-62-2419.
No. 5 has proposed a method for reducing the radioactivity of a nuclear power plant. Japanese Unexamined Patent Publication No. Sho 60-262955 discloses a technique for passivating the surface of a stainless steel pipe in a gaseous oxygen source at 150 ° C. to 450 ° C. in order to prevent accumulation of radioactive material on the surface of the pipe. is there.

Japanese Patent Application Laid-Open No. Sho 62-24195 discloses a method of forming a thick porous oxide film and then forming a thin dense oxide film.

[0011]

Among the above prior arts, the one described in Japanese Patent Application Laid-Open No. Sho 60-262955 has a drawback that a dense oxide film is formed, but it is very thin and thus easily damaged or deteriorated. Therefore, the effect of inhibiting corrosion and the effect of inhibiting the adhesion of radioactive substances cannot be sufficiently exhibited. Further, the coating described in Japanese Patent Laid-Open No. 24195/1987 has a thick coating, but if the dense layer with a thin surface is damaged, the inner layer is porous, conversely increasing the effective water contact surface area. There is a risk that adhesion will increase.

An object of the present invention is to provide a method of manufacturing a nuclear power plant capable of suppressing the adhesion of a radioactive substance to a stainless steel member used in contact with a liquid in which the radioactive substance is dissolved.

[0013]

As a radionuclide contributing to an increase in dose, 60-Co having a long half-life of 5.26 years and a high decay energy of γ-rays is typical. This radionuclide has γ-ray decay energy intensities of 1.17 and 1.33 MeV
And a long half-life of 5.26 years, it will cause a long-term increase in surface dose rate once it adheres to structural materials. Therefore, in order to reduce the dose rate, it is important how to suppress the adhesion of 60-Co.

The radionuclides dissolved in the reactor water are incorporated into the oxide film formed on the surface by the corrosion of stainless steel during the formation process. Therefore, in order to reduce the surface dose rate, it can be said that it is effective to suppress corrosion of stainless steel in reactor water in which radionuclides are dissolved.

In general, the normal corrosion rate is initially large, but decreases with time. This is because an oxide film is formed on the material surface over time, and this acts as an anticorrosion film. That is, when an anticorrosion film is formed on the surface, diffusion of water, oxygen ions, iron ions, and the like, which affect corrosion, is suppressed by the anticorrosion film. Therefore, if the anticorrosion film is formed on the surface of the component in advance by the pre-oxidation treatment, it is possible to suppress corrosion and to reduce the incorporation and incorporation of radionuclides during the initial film growth period when the corrosion rate is high.

What is important here is the nature of the oxide film formed on the surface of the component by the oxidation treatment. The inventors have found that the outer layer of the stainless steel oxide film has a two-layer structure of an oxide mainly composed of Fe and Ni and having a lower Cr concentration than the base material and an inner layer having a higher Cr concentration than the base material. It has been found that the corrosion resistance is highest.

The present invention comprises the following two steps.

First, the first step is a process for forming a Cr-enriched layer on the surface. As a method, first, electrolytic polishing and chemical polishing can be mentioned. In electrolytic polishing and chemical polishing, Fe in the base material is generally eluted selectively, so that the Cr concentration on the surface becomes relatively high, so that a Cr-enriched layer can be easily obtained on the surface.

It has also been found that Cr is concentrated on the surface of the material by heat treatment at 700 ° C. or higher. At lower temperatures, the diffusion rates of Fe and Ni are higher, so that Fe and Ni are concentrated on the surface, but at higher temperatures, the diffusion rates of Fe, Ni, and Cr are reversed,
Cr is concentrated on the surface, and a Cr-rich layer is formed on the surface.

A third method is to provide a surface Cr layer by chromate treatment, electrolytic plating, chemical vapor deposition, sputtering, and other similar methods.

The second step following the Cr-rich processing is the stabilization processing of the Cr-enriched layer. Generally, Cr has a slower diffusion rate than other constituent elements of stainless steel such as Fe and Ni. That is,
By forming the Cr-enriched layer, migration of chemical species involved in the corrosion reaction can be suppressed, and the corrosion resistance is improved. However, there is a problem that Cr is easily eluted as ions in a BWR reactor water environment. On the other hand, oxides of Fe and Ni are stable in the BWR reactor water environment, and by providing Fe and Ni layers on the outer surface of the Cr-rich layer,
It has been found that elution of the rich layer can be prevented.

Formation of Fe, Ni rich layer on outermost surface is 500 ° C.
This is achieved by performing heat treatment at the following temperature for a certain period of time. As already written, Cr at low temperatures below 500 ° C
Compared with Fe and Ni, the use of the fact that Fe and Ni are more easily diffused allows the outer layer as shown in FIG.
It becomes possible to form a two-layer oxide mainly composed of i and an inner layer composed mainly of Cr.

By the above-described treatment, corrosion of stainless steel can be suppressed, and adhesion of radioactive substances accompanying the corrosion can be greatly reduced.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Example 1) Corrosion when immersed in high-temperature water containing 10 ppb of Co ions at 288 ° C. and 200 ppb of dissolved oxygen for 500 hours after various treatments using SUS316L stainless steel Table 1 shows the measurement results of the speed and the amount of Co attached.

[0025]

【table 1】

As can be seen from the table, the effect of suppressing corrosion of stainless steel and adhesion of Co is scarcely obtained only by the Cr-rich treatment of the first step. On the other hand, by performing the Cr stabilization treatment in the second step, the corrosion or Co deposition rate in the simulated reactor water environment was dramatically reduced.

(Example 2) A gas introduction pipe, a heater and a gas generator are provided at one end of a stainless steel pipe, and a high-temperature gas is introduced into the pipe. At this time, it is preferable to attach a heat insulating material to the outer surface of the pipe. As the gas, a gas containing oxygen such as air is used. Further, the gas may include water vapor.

First, in the first step, a high-temperature gas is introduced into the pipe so that the inner surface temperature of the pipe becomes 700 ° C. or higher. In this state, the temperature is maintained for a certain time. The holding time may be sufficient.

In the second step, the heater is adjusted to lower the temperature to 500 ° C. or less. After the temperature reaches the target temperature of 500 ° C. or less, the temperature is maintained for a certain time. In this case, it is necessary to maintain the temperature for at least one hour. When the temperature of the second step is low, it is desirable that the holding time be long.

As described above, the first step is performed at 900 ° C. for 1 hour.
Corrosion in the simulated reactor water environment and the deposition rate of Co were reduced to about 1/4 by treating the two processes at 400 ° C for 10 hours.

(Embodiment 3) This embodiment is an example in which an electrolytic polishing process is performed as a first step. First, stainless steel piping is electropolished by a usual industrial method. Subsequently, the pipe after the electropolishing is sufficiently washed with deionized water, dried after removing chemicals, and placed in a heater. Thereafter, the temperature of the heater is raised to a target temperature of 500 ° C. or lower, and is maintained for at least one hour. During the heat treatment, it is preferable to continuously inject dry air to keep the oxidizing atmosphere in the heater constant. After the oxidation treatment was completed, the temperature in the heater was lowered to the atmospheric temperature, the pipe was taken out, washed with deionized water, and dried.

[0032]

As described above, according to the present invention, radioactive substances can be prevented from adhering to stainless steel members by simple means, and an increase in dose rate of stainless steel members in a nuclear power plant can be suppressed. It is possible to reduce the exposure of the elderly.

[Brief description of the drawings]

FIG. 1 is a cross-sectional configuration diagram showing a process of forming an oxide film by a two-step process of the present invention.

[Explanation of symbols]

11: constituent members, 12: first treatment layer (Cr-enriched layer), 13: second treatment layer (Fe, Ni-enriched layer).

Claims (8)

[Claims] 1. A surface treatment method for forming an oxide film on a surface of a stainless steel member for a nuclear power plant in contact with reactor cooling water containing a radioactive material to reduce the radioactivity of the nuclear power plant, Forming a Fe and Ni enriched layer outside the Cr enriched layer in a second treatment after forming the surface of the stainless steel pipe for a nuclear power plant. 2. In the oxide film forming treatment, the treatment conditions are changed stepwise or continuously in the first treatment for forming the Cr-enriched layer and in the second treatment for forming the Fe- and Ni-enriched layers. 2. The surface treatment method for stainless steel piping for a nuclear power plant according to claim 1, wherein the method is performed. 3. The first treatment is to form a Cr-enriched layer by electropolishing or chemically polishing at least a part of a surface of a stainless steel member in contact with a reactor coolant containing a radioactive substance. Item 4. The surface treatment method for stainless steel piping for a nuclear power plant according to item 1. 4. The first treatment is to form a Cr-enriched layer by bringing at least a part of a surface of a stainless steel member in contact with a reactor coolant containing a radioactive substance into contact with a gas containing oxygen at 700 ° C. or higher. 2. The surface treatment method for a stainless steel member of a nuclear power plant according to claim 1, wherein: 5. The primary treatment is any one of a chromate treatment, an electrochemical deposition, a chemical vapor deposition, a sputtering, and other common methods on at least a part of a surface of a stainless steel member that comes into contact with a reactor cooling water containing a radioactive substance. 2. The surface treatment method for a stainless steel member of a nuclear power plant according to claim 1, wherein a Cr layer is formed by the method. 6. The secondary treatment includes contacting at least a part of the surface subjected to the primary treatment with a gas containing oxygen at 500 ° C. or less.
4. The method for producing a stainless steel member for a nuclear power plant according to claim 3, wherein a Fe and Ni concentrated layer is formed outside the concentrated layer. 7. The surface treatment method for a stainless steel member for a nuclear power plant according to claim 4, wherein the gas containing oxygen is air. 8. The surface treatment method for a stainless steel member for a nuclear power plant according to claim 4, 6 or 7, wherein the gas or air containing oxygen contains water vapor.