JPS6224195A - Method of reducing radioactivity of nuclear power plant - 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 nuclear power plants, and in particular, radioactivity of structural materials used in contact with liquid in which radioactive substances are dissolved, such as secondary cooling system piping. The present invention relates to a method for suppressing the adhesion of substances.

[Prior art]

The piping, pumps, valves, etc. used in the primary cooling system of the BWR plant are made of stainless steel, Stellite, etc. (hereinafter referred to as components). When these metals are used for a long time, they are damaged by corrosion, and their constituent metal elements are eluted into reactor cooling water (hereinafter referred to as cooling water) and brought into the reactor. Most of the eluted metal elements become oxides and adhere to the fuel rods, where they are exposed to neutron irradiation. the result,
b0CO% "C0%" Radionuclides such as Cr, , and Mn are generated.These radionuclides are re-eluted and become ions or insoluble solid components (hereinafter referred to as cladding).
floating as. A part of it is removed by a demineralizer for reactor water purification, but the rest adheres to the surfaces of structural members while circulating in the primary cooling water system. Therefore, the dose rate on the surface of the component increases, and radiation exposure of workers during maintenance and inspection becomes a problem.

Therefore, in order to reduce the amount of attached radioactive substances, methods have been proposed for suppressing the elution of the metal elements that are the source of radioactive substances. For example, there are methods to suppress corrosion of structural members by using materials with good corrosion resistance or by injecting oxygen into the water supply system.

However, no matter which method is used, the water supply system, etc.
- It is not possible to sufficiently suppress corrosion of the constituent members of the secondary cooling water system, and - it is not possible to sufficiently reduce the radioactive substances in the secondary cooling water, so the surface dose rate increases due to the adhesion of radioactive substances to the constituent members. do.

Additionally, methods for removing radioactive substances deposited on structural members have been studied and implemented. Removal methods include (1) mechanical cleaning, (2) cleaning by electrolysis, and (3) chemical cleaning. However, methods (1) and (2) make it difficult to remove radioactive substances that are tightly adhered to the surface of the component.
Furthermore, it is not possible to systematically decontaminate a wide area. (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. In this method, even if the dose rate is temporarily low, if the component is exposed again to a liquid that dissolves radioactive substances at a high concentration, it will be rapidly recontaminated.

A method of forming an oxide film on the surface of a component in advance to suppress the adhesion of radioactive substances was disclosed in Japanese Patent Application Laid-Open No. 55-121197.
This method is known from Japanese Patent Application Laid-Open No. 59-37490. 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 differs depending 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 component after being immersed in a liquid in which radioactive substances are dissolved also changes. Since it depends on the properties of the existing film, a sufficient film may not necessarily be formed.

[Purpose of the invention]

An object of the present invention is to provide a method for reducing radioactivity in a nuclear power plant that comes into contact with high-temperature, high-pressure pure water containing radioactive substances.

[Summary of the invention]

The present invention provides a method for reducing radioactivity by forming an oxide film on the surface of a component made of metal that comes into contact with high-temperature, high-pressure reactor water containing radioactive substances before the component is exposed to the reactor water. After performing a first oxidation treatment in which the component is heated in a high-temperature atmosphere, the atmosphere in the first oxidation treatment is adjusted so as to form an oxide film that is denser than the oxide film obtained by the first oxidation treatment. A radiation reduction method characterized by performing a second oxidation treatment by heating in an atmosphere with a higher specific metal ion concentration.

That is, in order to suppress the adhesion of radioactive substances to structural members that come into contact with reactor cooling water containing radioactive substances, the present invention first applies a relatively porous but sufficient film thickness to the members through a primary treatment. a second oxide film is formed, and then a second
The subsequent treatment aims to form a thin but dense film.

In order to form a thick porous film in the first treatment, oxidation treatment is performed using heated water or steam with low oxidizing properties, and in order to form a thin but dense film in the second treatment, the concentration of metal ions is increased. This is achieved by oxidation treatment using heated water or steam. Here, metal ions are ions of metals that can have a positive charge of two or more valences among metals whose atomic number, standard electrode potential, and electronegativity are all lower than Fe, namely Be5Mg, Aj! ,
It means Cas V, Cr-, Mn ions.

The present invention proposes that radioactive substances contained in high-temperature, high-pressure water are taken in when constituent members are corroded by the high-temperature, high-pressure water and an oxide film is formed. It is based on the theory that it can suppress the

In other words, although the film grows thick only with the first treatment in heated water with low oxidizing properties, this film has insufficient corrosion inhibiting effect in the environment of reactor cooling water, so the effect of inhibiting the adhesion of radioactive substances is insufficient. is not enough.

On the other hand, secondary treatment with heated water with a high concentration of metal ions forms a very thin and dense film, but it is easily damaged and deteriorates, so this film is also thin in the reactor cooling water environment. Therefore, the film is easily destroyed, and the corrosion inhibiting effect,
As a result, the effect of suppressing the adhesion of radioactive substances cannot be sufficiently exerted.

However, it has been found that when treated in heated water with low oxidizing properties to form a thick film, and then treated in heated water with high metal ion concentration, the subsequent corrosion inhibition effect is very significant.

The reason why the film becomes thicker when the oxidation property is low is that the iron oxide that forms the oxide film is somewhat easily dissolved in the environment of reactor cooling water, so the film tends to become porous, and oxidation progresses through these pores. This is because an oxide film grows.

On the other hand, in heated water with a high ion concentration of a metal whose atomic number, standard electrode potential, and electronegativity are all lower than that of Fe, a thin, dense film is formed due to the formed oxide containing the metal. This is because it is less soluble in high-temperature water than iron oxides, and the oxides with small crystal grains form a thin, dense layer that suppresses subsequent film growth. Although such a dense film has a high corrosion inhibiting effect, it is easily destroyed and cannot be sufficiently effective in an environment of reactor cooling water. Therefore, forming a thick and dense film has a great effect of inhibiting corrosion in the reactor cooling water environment and, in turn, inhibiting the adhesion of radioactive substances.

Therefore, if a porous but thick film is first formed in an environment with low oxidizing properties and then treated in an environment with a high concentration of metal ions, the pores in the oxide film will be filled with dense oxides. , the effect of inhibiting corrosion after coming into contact with the nuclear reactor environment, that is, the effect of inhibiting the adhesion of radioactive substances, is large. FIG. 1 is a model diagram of oxide film growth in a two-stage treatment process.

A first oxide film 15 is formed on the surface of the component 17 made of metal in a first oxidation treatment step, and then a second oxide film 16 is formed on the metal surface in a second oxidation treatment step. Depending on the treatment method, a dense film similar to the second oxide film may be formed on the base of the first oxide film 15.

These oxidation treatments can be performed using Kashima water, steam, highly purified non-oxidizing heated gas, and heated water in which metal salts are dissolved.

For example, boiling water reactor cooling water typically contains 200 ppb.
of dissolved oxygen, and the oxidizing power of your hands depends on this dissolved oxygen concentration. Therefore, in the simplest case, 200℃
The first treatment step uses the above pure water to reduce the dissolved oxygen concentration to 200PP.
It is preferably lower than b, particularly 40 to 100 PPb. When the dissolved oxygen concentration is 40 ppb or less, the formation of an oxide film is suppressed, while when it is 200 ppb or more, the protective property of the film is strong.

Therefore, corrosion is suppressed and a film of a predetermined thickness is not formed.

The second treatment step is to raise the metal ion concentration higher than the metal ion concentration in the reactor water by 5 to 1000 PPb, especially 100 to 400 PPb.
Preferably, it is PPb. The temperature of the solution containing this metal ion is 200 to 300°C, and the dissolved oxygen concentration is 400°C.
PPb or less. The processing time of each processing step is 100-5
00 hrs is preferred.

It should be noted that these treatments do not necessarily need to be performed by distinguishing between the first treatment step and the second treatment step; for example, the treatment may be performed while continuously changing the metal ion concentration from around oppb to around 100OPPb, or The oxidation conditions are not necessarily carried out in two stages, but can be carried out in several stages by changing the oxidation conditions.

The circumstances by which the present invention was discovered will be described in more detail.

Radioactive substances dissolved in reactor water are incorporated into the oxide film that forms on the surface of stainless steel during its formation process. In high-temperature water, the oxide film grows inward (towards the base metal) mainly at the interface between the film and the base metal, and the radioactive substances diffuse inward within the film and then enter the oxide film at the same interface. It is captured. The flux of radioactive substances (JO) can be expressed by equation (1).

Here, d Dioxide film thickness ko: Constant of proportionality D: Diffusion coefficient CI = Concentration of radioactive substances in reactor water C2: Concentration of radioactive substances at the film/metal interface Also, the thickness of the oxide film d) is the constant of proportionality (kl) and the oxide film if(m,), d=, m.........), so Jo can be expressed again by equation (3).

On the other hand, the rate at which radioactive substances are incorporated into the film is expressed by equation (4).

Here, kz: proportionality constant Here, the accumulation rate (J) of radioactive substances is J = Jo = J
Since it is l, C2 can be eliminated from equations (3) and (4). .

By the way, when the rate of accumulation of radioactive substances is determined by the diffusion process, it is expressed by the following equation (6).

Equation (6) shows that the accumulation rate (J) of radioactive substances is the diffusion coefficient (D
) and the radioactive material (C8) in the reactor water, and is inversely proportional to the amount of coating, that is, the thickness of the coating. Therefore, it is sufficient to form a dense and thick oxide film. The other is to reduce the radioactive concentration in the reactor water, that is, cobalt and nickel that are activated in the reactor core, and ions and metal oxides such as iron that promote the activation of these metals, are released into the reactor water due to corrosion of structural members. It's best to prevent it from happening. Corrosion of this component can also be suppressed by forming a dense and thick oxide film.

From the above, it can be seen that in order to suppress the accumulation of radioactive materials, it is best to form a dense and thick oxide film on the components that come into contact with reactor water before nuclear heating begins in a nuclear power plant.

By the way, in the case of stainless steel as a structural member, research by the present inventors shows that the deposition rate of radioactive substances is correlated with the film growth rate, so suppressing film growth will lead to a reduction in adhesion. He was presumed to be deaf.

That is, the reason why the deposition rate of the radioactive substance shows a correlation with the growth rate of the film is that the radioactive substance is taken in at the growth point of the film. Therefore, if the growth of the film is suppressed, the frequency at which radioactive substances are taken in decreases, that is, the uptake is suppressed. The increase in the coating amount (m, ) of stainless steel in a cooling water environment is expressed by the logarithmic law of time (1) as shown in equation (7).

m = a log(bt+1) ---(7)
Here, a and b are constants, that is, the growth of the film is fast in the initial stage, and as the film grows, the growth rate decreases. Afterwards, a suitable non-radioactive oxide film is formed in advance (this is particularly effective in suppressing the uptake of radioactive substances and elution of metal ions. In other words, the oxide film is immersed in a solution in which radioactive substances are dissolved). It is possible to suppress the subsequent formation of a new film, and in turn, it is possible to suppress the adhesion of radioactive substances that often occur during film formation.

The present inventors have discovered that adhesion of radioactive materials can be suppressed by forming an appropriate non-radioactive oxide film on metal components used in contact with nuclear reactor cooling water containing dissolved radioactive materials. Focusing on the fact that the deposition rate of substances depends on the thickness and density of the oxide film formed in advance, we investigated the film formation conditions and found that after oxidation treatment under mildly acidic conditions, the concentration of specific metal ions was 5. PPb
They found that further oxidation treatment under the above conditions significantly reduced the size.

In the case of implanting metal ions, the following method of implanting an aqueous solution of a metal salt is preferable.

13 e: Be(NO+)z, Be504M g
: Mg(C1hCOO)z, Mg5O*, Mg(
CJsOv)z.

Mg(OH)z, Mg(N(h)z, Mg1(po
4A1: 8l (OH):l, A 1 (SO4
)3. Eight i! (N(h):l.

8 IPO4 Ca: Ca(CH3COO)z, Ca5Oa,
Ca(CJsO+)z.

Ca(OH)z, CaHPO4, CaCo1V
: VOCZO4, VO50aCr : Cr(NOa
)z, Crz(So<)+Mn: Mn(HCO
O)z, Mn(NO3)zlMn(lIzPO4)z
+Mn5041 Mn(CH3COO)21 MnCO
There are several types of nuclear power plants, and the present invention can be applied to any of them. In boiling water nuclear power plants, the reactor pressure vessel, recirculation system piping, primary coolant purification system piping, etc. are in contact with reactor water containing radioactive materials, and in pressurized wooden nuclear plants, the reactor pressure vessel, the inside of the reactor Structural materials and steam generators are in contact with similar reactor water. Therefore, by applying the structural material subjected to the oxidation treatment of the present invention to all or part of structural materials made of stainless steel, Inconel, carbon steel, and Stellite that come into contact with liquids containing radioactive materials, the adhesion of radioactive materials can be suppressed. This can further reduce the exposure of workers to radiation.

On the other hand, in boiling water nuclear power plants, the concentration of radioactive materials in the primary cooling water that comes into contact with the components of the water supply system and condensate system is relatively low, so there is little radioactive material adhesion (and the increase in dose is not much of a problem. However, Metal ions and metal oxides released due to corrosion of the components of these systems are carried into the reactor pressure vessel with the feed water, causing an increase in the concentration of radioactive materials in the reactor cooling water. Another important issue is the suppression of corrosion of the structural members of the nuclear power plant.The method of the present invention basically involves the suppression of corrosion of the structural members. By applying a primary treatment with a weak oxidizing property and a secondary treatment with a strong oxidizing property, an oxide film with high protection against corrosion is formed on the surface of the component parts, thereby preventing metals from entering the secondary cooling water. It is possible to reduce the release of ions and metal oxides, and in turn, reduce the amount of radioactive substances deposited in the recirculation system and reactor water purification system.

[Example 1] JIS 5tlS having the chemical composition (wt%) shown in Table 1
304 stainless steel with pure water (liquid) at 250℃ and A
7! (NO3): After various oxidation treatments with an aqueous solution containing I, 288 containing toppb cobalt ions
It was immersed in heated water at ℃ for 500 hours, and the corrosion rate and amount of cobalt deposited were measured. The results are shown in Table 2.

It can be seen that the two-stage treatment according to the present invention has a low corrosion rate (and suppresses the amount of cobalt deposited).

Table 1 Table 2 Table 2 During treatment [10: 200PPb (DO: Dissolved oxygen concentration [Example 2] The heat source is the heat generated by the operation of the pumps in each system of a boiling water nuclear power plant, and nuclear heating is used. First, the two-stage oxidation treatment of the present invention is carried out on the plant by adjusting the amount of dissolved oxygen in the pure water of the reactor secondary cooling water at the time of start-up operation or at the end of decontamination.

Figure 2 is a system diagram of a boiling water nuclear power plant.

The oxidation treatment of the present invention is applied to the cooling reactor water in the reactor 1 - recirculation system 2 -
This can be carried out while circulating the reactor water through the thread path of the reactor water purification system 5. First, the system is filled with pure water, the main steam isolation valve 25 is closed, and the recirculation pump is operated. As a result, the temperature of the water in the system increases at a rate of 3° C./h. As the first treatment step, the dissolved oxygen concentration is set at 40-100 ppb, and the temperature is set at 260-100 ppb.
Circulate heated water at 280°C for 100-500 hours.゛In order to reduce the dissolved oxygen concentration to 40 to 100 ppb, the main steam isolation valve 10 is opened when the temperature of the water in the system reaches 100°C or higher, and the steam is blown into the main steam system 11. This can be achieved by flowing nitrogen gas into the gas phase at the top of the furnace 1. Moreover, temperature adjustment can be achieved by adjusting the amount of heated water flowing into the reactor purification system 4. Predetermined time first
After the next treatment, Be, Mg, AI, Ca, V
A second treatment is performed by increasing the concentration of metal ions selected from %Cr- and M11%. Metal ion concentration is 10-1
000 ppb and the temperature is the same as the first treatment (260-28
Set at 0°C and circulate heated water for 100-500 hours. To increase the metal ion concentration from 10 to 10,009 pb, an aqueous solution of metal salts was injected through the sampling line 12'' of the re-vli ring system. By performing such a two-step oxidation treatment, the adhesion of radioactive substances was reduced. It can be a nuclear power plant.

[Example 3] In the same system as in Example 2, the recirculation pump 3 was operated, and in the first treatment, hydrazine was injected from the sampling line 12'' of the recirculation system to a concentration of 100 to 1.
000 ppb and heated water at 200 to 280°C.
00-500 hours, then in the second treatment Af
f'' as 8β(N(h)z aqueous solution 10-1000p
By implementing a two-step oxidation treatment in which PB is injected and heated water at 200 to 280°C is circulated for 100 to 500 hours, the nuclear power plant can suppress the adhesion of radioactive materials to structural materials. I can do it.

[Example 4] After installing the feed water heater 9 during construction of a boiling water nuclear power plant, 28 heaters 9 were installed before the water supply before connecting it to the plant.
The two-stage treatment of the present invention is carried out by attaching a temporary circulation line to the tank and circulating heated water with adjusted dissolved oxygen using an in-house boiler or a temporary boiler as a heat source. This is to suppress the release of metal ions and metal oxides from the feedwater heater 9, but as can be seen from the system diagram in FIG. Since it contacts only the inner surface of the heater tube 26, the oxidation treatment only needs to be formed on the inner surface of the heater tube 26.

During the oxidation treatment, a temporary circulation line 24 is attached to the entrance and exit of the heater tube 26, as shown in FIG. The circulation line 24 is provided with a circulation pump 18, a heating steam blowing line 20 from the boiler, a steam blowing line 21, and an oxygen injection line 22. 7. First, the target system is filled with pure water and heated while being circulated. As the first treatment, dissolved oxygen is 40 to 100 pb, temperature is 200
-250°C heated water is circulated for 100-500 hours. In order to adjust the dissolved oxygen concentration, the valve of the blow line 21 is opened when the temperature reaches 100° C. or higher, and a part of the steam is blown out. Next, as a second treatment, Aβ1 concentration 1
Heated water of 0 to 1000 ppb and a temperature of 200 to 300°C is circulated for 100 to 500 hours. To prepare the A7!3+ concentration at this time, a small amount of Al (NO
3) Inject the z aqueous solution. Note that such two-stage oxidation treatment only needs to treat the inner surface of the heater tube 26, but if heated water is circulated only inside the heater tube 26, the pressure difference between the inside and outside of the tube will become extremely large, causing the tube to deteriorate. Since problems such as deformation may occur, it is necessary to bypass some of the circulating water to the outside of the tube to reduce the pressure difference.

The above two-stage oxidation treatment reduces the release of metal ions or metal oxides due to corrosion of the feedwater heater heater tube, which reduces the radioactivity concentration in the reactor water, so there is less radioactive material attached. It can be a nuclear power plant.

〔Effect of the invention〕

According to the present invention, it is possible to suppress the adhesion of radioactive substances to structural materials made of metals, and in particular, it has a very remarkable effect of suppressing increases in the dose rate of structural materials such as stainless steel used in nuclear power plants. can get.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional configuration diagram showing the process of forming an oxide film by the two-stage treatment of the present invention, FIG. 2 is a system diagram of a boiling water nuclear power plant that is an embodiment of the present invention, and FIG. It is a system diagram of the temporary circulation line of a feed water heater. ■... Nuclear reactor, 2... Recirculation system, 3... Recirculation pump, 4... Reactor purification system, 5... Reactor water purifier, 6
... Turbine, 7... Condenser, 8... Condensate purification device, 9... Feed water heater, 10... Water supply system, 11...
・Steam type, 12.12″...Oxygen-injected pulp, 13
...Vacuum pump, 14...Exhaust tower, 15...1st
Oxide film, 16... Second oxide film, 17... Component, 18... Pump, 19... Tank, 20...
Steam blow line, 21... Steam blow line, 22...
...Oxygen injection line, 23...Bypass line, 24
...Circulation line, 25...Main steam isolation valve, 26...
・Heater tube.

Claims (7)

[Claims] (1) In a method for reducing the radioactivity of a nuclear power plant by forming an oxide film on the surface of a metal component of a nuclear power plant that comes into contact with reactor cooling water containing radioactive substances, the process for forming the oxide film includes: A method for reducing radioactivity in a nuclear power plant, which comprises forming a thick porous oxide film in a first treatment, and then forming a thin, dense oxide film in a second treatment. (2) In the process of forming an oxide film, the process of forming a thick porous oxide film in the first process and forming a thin and dense oxide film in the second process is carried out from the first process to the second process. 2. A method for reducing radioactivity in a nuclear power plant according to claim 1, characterized in that the method is carried out by changing treatment conditions either individually or continuously. (3) Be the second treatment to form a thin and dense oxide film.
, Mg, Al, Ca, V, Cr, and Mn. . (4) The solution is Be, Mg, Al, Ca, V, Cr, Mn
one or more selected from the group of metal ions consisting of
_3, HPO_4, H_2PO_4, NO_2, NO_
3. The method for reducing radioactivity in a nuclear power plant according to claim 3, characterized in that the method comprises one or more selected from the anion atomic groups consisting of 3, OH, HCOO, SO_4, and CH_3COO. (5) The method for reducing radioactivity in a nuclear power plant according to claim 3, wherein the temperature of the solution is 200 to 300°C. (6) The method for reducing radioactivity in a nuclear power plant according to claim 3, wherein the metal ion concentration of the solution is 5 to 1000 PPb. (7) The method for reducing radioactivity in a nuclear power plant according to claim 3, wherein the dissolved oxygen concentration of the solution is 400 PPb or less.