JPS6184597A - Method of inhibiting adhesion of radioactive substance on nuclear power plant constitution member - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an atomic couplant, and in particular to a structural material used in contact with liquid in which radioactive materials are melted, such as secondary cooling water system piping. This invention relates to an atomic couplant to which a method of suppressing the adhesion of radioactive substances is applied.

[Background of the invention]

Pipes, pumps, valves, etc. used in the primary cooling water system of nuclear power plants are made of stainless steel, Stellite, etc. When these metals are used for a long period of time, they are subject to corrosion damage, and their constituent metal elements are leached into the primary cooling water and brought into the reactor.

Most of the eluted total elements become oxides and adhere to the fuel rods.
Subject to neutron irradiation. As a result, “Co.

Radioactive nuclides such as "Coe siCr" and "Mn" are generated. These radionuclides are re-eluted into the primary cooling water and suspended as ions or insoluble solid components (hereinafter referred to as cladding). Some of the floating water is removed by demineralizers used to purify reactor water, but the rest adheres to the surfaces of structural materials, mainly made of stainless steel, while circulating through the primary cooling water system. For this reason, the dose rate on the surface of fir plantations is high, and radiation exposure for workers during maintenance and inspection has become a problem.

Therefore, in order to reduce the amount of attached radioactive substances, methods are being implemented to suppress the elution of the metal elements that are the source of radioactive substances. That is, methods include using materials with good corrosion resistance or injecting oxygen into the water supply system to suppress corrosion of structural materials. However, no matter which method is used, it is not possible to completely prevent the corrosion of the Saki Afforestation in the secondary cooling water system, including the water supply system, and it is not possible to eliminate radioactive materials in the secondary cooling water, so structural materials The increase in surface dose rate due to the adhesion of radioactive materials to the surface remains a problem.

Additionally, methods for removing radioactive substances adhering to structural materials are being studied and implemented. Removal methods include (1) mechanical cleaning, (2) cleaning by electrolysis, and (3) chemical cleaning. However, methods (1) and (2) have problems such as difficulty in removing radioactive substances that are tightly adhered to the surface of structural materials, and inability to systematically decontaminate a wide area. Method (3) is widely used. (3
) method uses chemicals such as acid solutions to dissolve the oxide film on the steel surface through a chemical reaction and remove the radioactive substances present in the film. The problem with this method is that even if the dose rate is temporarily reduced, if the structural material is exposed again to a liquid that dissolves high concentrations of radioactive substances, it will quickly become recontaminated.

Therefore, the idea of forming an oxidized film on the surface of structural materials in advance to suppress the adhesion of radioactive substances was proposed in JP-A-55-121.
No. 197, Japanese Unexamined Patent Publication No. 59-37498, etc. However, the adhesion behavior of radioactive substances differs significantly depending on the properties of the oxide film that is formed in advance. For example, the behavior of radioactive ions.

It depends on the charge state of the oxide film that has been formed in advance, and the growth rate of the oxide film that is newly formed on the surface of the structural material after being immersed in the liquid in which the radioactive material is dissolved also depends on the properties of the existing film. It's going to change. therefore,
It is necessary to carry out the oxidation treatment by the method most suitable for the liquid to which the structural material is applied.

[Purpose of the invention]

The purpose of the present invention is to eliminate these problems and to eliminate radioactive substances by using a material whose parts that come into contact with the liquid are oxidized as a structural material that is used in contact with liquid in which radioactive substances are dissolved. An object of the present invention is to provide an atomic couplant that suppresses the adhesion of.

[Summary of the invention]

As a result of various analyzes (for example, G
, Romeo, Procaedings of Th.
e 7th International Congress
ss on Metallic Corrosion.

P1456.1978) to b and the other radionuclides have γ-ray decay energy intensities of 1.17 and 1.33.
Because of its high MeV and long half-life of 5.26 years,
Once attached to structural materials, it causes an increase in the surface dose rate over a long period of time. therefore.

In order to reduce the dose rate, the key is how to suppress this @0CO adhesion.

On the other hand, the structural materials for the pressure vessels, piping, pumps, valves, etc. around the plant furnace are made of stainless steel and Stellite.

It is made of Inconel and carbon steel, but stainless steel accounts for 97% of the water contact area. Therefore, suppressing the adhesion of radioactive substances to stainless steel is the most effective way to reduce exposure.

Radioactive nuclides dissolved in reactor water are incorporated into the oxide film formed on the surface of stainless steel during its formation process. According to a study by one of the inventors, the adhesion rate of radionuclides shows a correlation with the film growth rate, so it was assumed that suppressing film growth would lead to a reduction in adhesion.

The increase in the coating amount (m) of stainless steel in the reactor water environment is (
As shown in equation 1), it is expressed by the logarithmic side of time (1).

m = a logt + b
(1) Here, a and b are constants, that is, the growth rate decreases as the film grows. Therefore, by forming an appropriate non-radioactive oxide film in advance, it is possible to suppress the formation of a new film after immersion in a solution containing dissolved radioactive substances, and in turn, it is possible to suppress the formation of a new film after immersion in a solution containing dissolved radioactive substances. 6. The present inventors created a metal structural material that was immersed in a liquid containing dissolved radioactive substances by forming an appropriate non-radioactive oxide film on the metal structural material used in contact with the liquid containing dissolved radioactive substances. At the same time, we focused on the fact that later corrosion can be suppressed and the growth of the film can be kept small.At the same time, the growth of the film depends on the amount of Ni in the film. It has been found that when the amount is more than % by weight, it becomes significantly smaller.

The present invention was obtained based on the above findings, and the present invention is based on the fact that the proportion of Ni in all the metals constituting the oxide film on the surface of the part of the metal structural material that comes into contact with the liquid in which radioactive substances are dissolved is An oxide film having a content of 12% by weight or more is formed in advance.

The proportion of Ni in all metals constituting the oxide film (C or less, simply referred to as N1 content) may be 12% by weight or more, but
The upper limit depends on the form of the film formed. The highest content is when the oxide film is composed of Ni alone (content = 100% by weight), and the oxide film takes the form of NiO or Ni304. However, the oxide film usually contains α- or two-Fe. 03 (for example, NiFe, 04), etc. are mixed, and as the amount of this spinel oxide increases, the protective property of the oxide film increases and the effect of suppressing the adhesion of radioactive substances increases.

Therefore, since the Ni content in the case of NiFe2O4 alone is about 34% by weight, it is desirable that the Ni content of the oxide film formed in advance is 12% by weight to 34% by weight.

In order to adjust the Ni content of the oxide film to an appropriate range, the conditions for the pre-oxidation treatment applied to the surface of the metal structural material may be selected. As such pre-oxidation treatment, a method is used in which a metal structural material such as a stainless steel spout is immersed for a predetermined time in water at an appropriately selected temperature, dissolved oxidation concentration, Ni ion concentration, etc.

In the present invention, adhesion of radioactive substances can be suppressed by forming an oxide film with an appropriate Ni content on the surface of the metal structural material in advance by oxidation treatment and disposing this in an atomic couplant.

Note that G o forms a spinel type oxide with iron.

Similar effects are expected for Zn, Cu, Cd, Mg, Ca, and Sr.

In addition, when piping, equipment, etc. used in an atomic couplant is decontaminated by chemical methods, etc., and reused.

Due to the decontamination process, the oxide film on the surface of the structural material has been dissolved and peeled off, so the gold scrap base is exposed, and the amount of radioactive substances deposited upon reuse changes over time in the same way as when unused material. Therefore, by performing the pre-oxidation treatment of the present invention and then reusing it, it is possible to suppress the adhesion of radioactive substances.

There are many types of atomic couplants, and the present invention can be applied to any of them. For example, in a water-type nuclear reactor reactor, the pressure vessel, recirculation system piping, primary coolant purification system piping, etc. are in contact with reactor water containing radioactive materials, and in a pressurized wood-type nuclear reactor, the pressure vessel, Structural materials and steam generators are in contact with similar reactor water. Therefore, the pre-oxidation treatment of the present invention is applied to all or part of the stainless steel mouth, structural materials made of one or more metals selected from Inconel, carbon chains, and stellite, which come into contact with the liquid containing these radioactive substances. Adhesion of radioactive materials can be suppressed by applying structural materials that
As a result, it is possible to provide a plant in which workers are exposed to less radiation.6 The present invention is designed to reduce the amount of metal components contained in the atomic couplant grooves being released into the primary cooling water as the structural materials corrode when the atomic couplet groove construction materials come into contact with the primary cooling water. It can also be applied when suppressing For example, in metals that contain cobalt as an impurity or constituent element, such as stainless steel, Stellite, and Inconel, which make up piping, pumps, valves, etc., cobalt is eluted into the primary cooling water due to corrosion and is carried into the reactor. It will be done. Most of the eluted cobalt turns into oxides and adheres to the fuel rods, where they are exposed to neutron irradiation. As a result, GoCo, which is a radionuclide, is generated, but by applying the present invention, the release of cobalt can be suppressed.

In addition, in boiling water nuclear couplants, radionuclides such as Go, Co, Cr, and s4Mn combine with iron-based oxides in the reactor water and exist as insoluble components in the primary cooling water. Similar to dissolved components, these undissolved components adhere to the surface of structural materials mainly made of stainless steel chains while circulating through the primary cooling water system.This adhesion process is similar to that of dissolved components. , depends on the growth of an oxide film accompanying corrosion of structural materials. Therefore, the present invention is also effective in suppressing the adhesion of insoluble components.

[Embodiments of the invention]

Example 1 Stainless steel was exposed to flowing reactor water at 130-280°C for 60°C.
After being immersed for 00 hours, the amount of oxide film formed on the surface, the percentage (%) of the amount of Ni element relative to the amount of all metal elements in the film, and the amount of deposited Co were measured. Reactor water! CO
The concentration was 200 μC 1-m-J, and more than 90% existed as ions. Dissolved oxygen concentration is 150-170 ppb,
pH was 6.9-7.2.

Figure 1 shows the results. At 130-250°C, both the amount of oxide film and the amount of Co deposited increase with temperature. However, at temperatures above 250'C, where the Ni content in the oxide film is 12% by weight or more, both amounts decrease. On the contrary, it decreased. This is because the reaction rate follows the Arrhenius law, so the growth rate of the oxide film and the deposition rate of CO both increase as the temperature rises, but above 250°C, the Ni content increases. This is due to the formation of highly protective Ni ferrite (N i F ex○4), which reduces the corrosion rate.

Example 2 Figure 2 shows pure water at 288°C (dissolved oxygen concentration, 200pp).
b) shows the relationship between the amount of oxide film formed on the surface of stainless steel immersed for a certain period of time and the Ni content in the film, in the case of containing 12% by weight or more of Ni.

It was found that the amount of film growth decreased. "C.

Since the amount of Co deposited is proportional to the amount of Co film growth, under conditions where Co film growth is suppressed, Co deposition is also suppressed.

Example 3-2 Figure 3 shows the amount of oxide film formed on the surface of stainless steel when stainless steel was immersed in water containing Ni ions for 6000 hours at 130-280°C.6 The dissolved oxygen concentration was 170 pp.
It was b. The film can be formed to a significant thickness at temperatures above 150°C, and is desirably oxidized at a temperature range of 150-300°C to form a corrosion-resistant film.

Example 4 304 stainless steel was immersed for 90 hours in an aqueous solution containing 10 ppb of Ni ions at 288° C., and the composition of the metal elements forming the oxide film was investigated.

PI was set in the range of 4.4 to 9.4, and the dissolved oxygen concentration was set to 180 to 220 pPb.

The results are shown in Figure 4. In the pH range of 5-7.5,
The Ni content increased, and a film with a Ni content of 30% by weight could be formed, especially at pH 6.

Note that even in an aqueous solution containing ions of other divalent metal elements such as CO ions, it was possible to form a film having a high content of other divalent metal elements such as CO.

It is preferable that the divalent metal element ions be injected as sulfates, nitrates, or chlorides, or inserted into water as either a total conjugate element or an alloy, and then eluted.

Example 5 When piping, equipment, etc. used in an atomic couplant are decontaminated by chemical methods and reused, the oxide film on the surface of the structural material is dissolved and removed by the decontamination process, so gold IFI
The C base material is exposed, and the amount of radioactive substances deposited upon reuse shows the same change over time as unused material. Therefore, by performing the oxidation treatment of the present invention in advance and then reusing it, it is possible to suppress the adhesion of radioactive substances.

Example 6 There are many types of atomic couplants, and the present invention can be applied to any of them. For example, in a boiling water nuclear couplant, the pressure vessel, recirculation system piping, primary coolant purification system piping, etc. are in contact with metal-containing reactor water, and in a pressurized wooden nuclear couplant, the pressure vessel, the inside of the reactor, etc. Materials, steam generators, etc. are in contact with similar reactor water.

Therefore, the oxidation treatment of the present invention is applied to all or part of the structural material made of one or more metals selected from stainless steel, Inconel, carbon chains, and stellite that comes into contact with the liquid containing these radioactive substances. By applying structural materials, it is possible to suppress the adhesion of radioactive substances, and as a result, it is possible to provide a plant in which workers are exposed to less radiation. In addition, if the present invention is applied to stainless steel piping for a water supply system heat exchanger, it is expected that the elution of Go, Ni, etc. will also be suppressed since it has been recognized that corrosion of stainless steel tT3 can be suppressed.

Example 7 The following method can be considered to apply the present invention to the atomic couplant shown in Example 6.

(1) After the plant is installed, the surfaces of metal materials that will be used in contact with liquids in which radioactive substances are dissolved are subjected to oxidation treatment by the method based on the present invention, and then installed in the plant.

(2) Regarding the existing piping, equipment, etc. of the plant, the following methods can be considered.

(a) Figure 5 is a system diagram of a boiling water type atomic couplant. The chemical substances shown in Example 4 are injected into the primary cooling water using equipment provided at the outlet 6 of the condensate purification system or at the lower water supply level. The injection amount is controlled by sampling the reactor water or feed water and measuring the concentration of divalent metal ions.

(b) Implantation of divalent metal element ions is also possible by installing a metal that releases them. For example, a metal plate is installed inside the condensate hot well 4. This releases each metal element ion into the primary cooling water, increasing the concentration in the reactor water system,
The same effect as (a) can be obtained.

Next, it is also conceivable to place an alloy filter containing a divalent metal element in the condensate purification system 5 or the reactor water purification system 8. This makes it possible to obtain both the effect (a) and the cladding removal effect.

〔Effect of the invention〕

As is clear from the above description, the present invention can suppress the adhesion of radioactive substances to structural materials made of metal by simple means, and has a wide range of applications, especially stainless steel used in atomic couplants. Applied to structural materials,
1! Suppress the increase in quantity rate.

It is suitable for reducing the radiation exposure of workers, has high practical value, and is extremely meaningful industrially.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the temperature dependence of the corrosion behavior of stainless steel according to an example of the present invention, Figure 2 is a diagram showing the relationship between the amount of oxide film and Ni content, and Figure 3 is a diagram showing the relationship between the amount of oxide film and Ni content. FIG. 4 is a diagram showing the relationship between Ni content and PH, and FIG. 5 is a system diagram of the BWR plant. 1...Ni content, 2...G0Coo adhesion amount, 3.
... Oxide film amount, 4... Condensate hot well, 5...
Condensate purification system, 6... Condensate purification outlet, 7... Water supply outlet, 8... Reactor water purification system.

Claims (1)

[Claims] 1. In order to suppress the adhesion of radioactive substances to nuclear power plant components made of iron-based materials that come into contact with reactor cooling water, the water-contact surfaces of these components are treated with heated water, and Fe is added to the water-contact surfaces. and forming a spinel-type oxide containing 12% to 34% by weight of divalent metal elements other than Fe (the percentage of the amount of each divalent metal element to the total amount of metal elements in the oxide film). A method for suppressing the adhesion of radioactive substances to components of a nuclear power plant.