JPS63149598A - Oxidation processing method after chemical decontamination 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 [Industrial Application Field] The present invention relates to an oxidation treatment for suppressing rapid recontamination after chemical decontamination of an operating nuclear power plant.

[Conventional technology]

Piping used in the primary cooling water system of nuclear power plants,
Pumps, valves, etc. are made of stainless steel, Stellite, etc. (hereinafter referred to as
(abbreviated as "constituent materials"). When these metals are used for a long period of time, they are damaged by corrosion, and the constituent metal elements are eluted into the reactor top oil and water (hereinafter referred to as top oil and water) and brought into the reactor. Most of the eluted metal elements become oxidized and deposited on the fuel rods, where they are exposed to neutron irradiation.

As a result, 60Co, 58Co, 51Cr,
54Mn etc. is produced by a radioactive injection nucleator. These radionuclides are re-eluted into the primary cooling water and suspended as ions or insoluble solid components (hereinafter referred to as cladding).

A part of it is removed by a demineralizer for reactor water purification, but the rest adheres to the surfaces of constituent materials while circulating in the primary cooling water system. For this reason, the dose rate on the surface of the component material increases, and radiation exposure of workers during maintenance and inspection has become a problem.

In this case, the adhesion of 60CO, which has a large decay energy and a long half-life, 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, materials with good corrosion resistance may be used for the constituent materials, or oxygen may be injected into the water supply system to suppress corrosion of the constituent materials.

However, no matter which method is used, the water supply system, etc.
- It is not possible to sufficiently suppress the corrosion of the constituent materials of the ceiling water cooling system, and - it is not possible to sufficiently reduce the radioactive substances in the ceiling water cooling system, so the surface dose due to the layering of radioactive substances on the constituent materials. The rate increase port still remains a problem.

Additionally, methods for removing radioactive substances attached to constituent materials have been 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 it is difficult to remove radioactive substances that adhere strongly to the surface of the component, and it is not possible to systematically decontaminate a wide area. At present, method (3) is widely used.

Method (3) involves dissolving and removing the oxide film on the vA surface through a chemical reaction using a chemical such as an organic acid solution, and removing the radioactive substances present in the film. This method may be applied to some pipes and pumps in a plant, but for the purpose of reducing the dose rate level of the entire plant, it is applied to the entire reactor structure, piping around the reactor, and equipment, so-called systematic chemical removal. Dyed is preferable. There are two ways to perform chemical decontamination: one with fuel assemblies loaded in the core, and one without.

[Problem that the invention seeks to solve]

Application of chemical decontamination to the entire nuclear power plant system, that is, systematic chemical decontamination, is effective in reducing overall dose rate levels. However, after chemical decontamination, the oxide film is removed by the decontamination solution and the active surface of the steel is exposed.
There is a problem in that radioactive materials rapidly re-accumulate due to the rapid regeneration of the coating as the plant restarts operations.

An object of the present invention is to provide a rounding method that suppresses rapid recontamination after performing systematic chemical decontamination.

[Means for solving problems]

The above purpose is to decontaminate the nuclear power plant system, remove the decontamination grade containing radioactive materials from the system, and conduct nuclear heating or recirculation pump operation to
This is achieved by flowing oil and water into the system with the temperature raised to 280° C. or higher due to mouth heat, oxidizing the water contact surfaces of the system components, and then shifting to output operation.

[Effect]

Generally, when a steel material covered with an oxide film is immersed in a decontamination solution, the film is dissolved, and in the process, the radioactive substances contained in the film are removed from the steel material.

Therefore, at the end of the decontamination work, the coating is removed from the steel and the active surface of the metal is exposed.

From the viewpoint of corrosion, there is a problem in particular in that metal grain boundaries are exposed. It is known that corrosion starts at defective areas where the atomic arrangement is disordered, such as at grain boundaries. (for example,
Otani, “Plasticity and Corrosion Reaction of Metals JP39, Sangyo Tosho”
Therefore, if the steel material is exposed to high temperature primary cooling water when the plant is restarted under these conditions, significant corrosion will occur.

By the way, radioactive piers dissolved in reactor water are incorporated into the oxide film formed on the surface of stainless steel during its formation process. According to the research of the present inventors, in high-temperature water, the oxide film mainly grows inward (toward the base metal side) at the interface between the film and the base metal, and radionuclides diffuse and move inward within the film. It is then incorporated into the oxide film at the same interface.

Therefore, it can be seen that in order to suppress the accumulation of radionuclides, it is sufficient to suppress the diffusion of radionuclides within the oxide film. Furthermore, since the adhesion rate of radionuclides shows a correlation with the film growth rate, it is presumed that suppressing film growth will lead to reduced adhesion.

That is, the reason why the deposition rate of the radionuclide shows a correlation with the growth rate of the film is that the radionuclide is taken in at the growth point of the film. Therefore, inhibiting the growth of the film will reduce the frequency of deflection and incorporation of radionuclides. In other words, uptake is suppressed.

The increase in the film amount (e) of steel materials in the primary cooling water is (1
), a logarithmic relationship holds true with time (1).

m = k, tag (kg t + 1) −
4) Here, k, , is a constant.

That is, as the film grows, its growth rate decreases. Therefore, if a suitable non-radioactive oxide film is formed in advance, it is possible to suppress the formation of a new film after immersion in a liquid containing dissolved radioactive substances, and this will also prevent the adhesion of radioactive substances during film formation. can be suppressed.

However, the corrosion that occurs at the Fushi reactor water interface varies depending on the properties of the oxide film that is previously formed on the metal surface.

Furthermore, since the surface of metals and metals is active after being exposed to the decontamination solution, the re-formed oxide film will have a large effect on the initial oxidation conditions. Therefore, it is necessary to perform the oxidation treatment on the constituent materials after systematic chemical decontamination using a certain appropriate method.

The means described in the above section of the present invention provides a method suitable for oxidation treatment of constituent materials after such systematic chemical decontamination.

〔Example〕

As described above, the present invention removes the decontamination liquid from the system after systematic chemical decontamination of the nuclear reactor, and before power operation, heats the reactor to 280% by heating by nuclear heating or by operating a recirculation pump.
Primary cooling water that has been heated to a temperature of 280°C or higher is passed through the system, but in this case it is desirable that the following conditions be met: 0.
o It has dissolved oxygen of I)I)b and is weakly alkaline (
The pH is 7.8 to 8.6), or the dissolved oxygen concentration is neutral and the dissolved oxygen concentration is 300 to 300 ppb.
00 hours is desirable. The principle behind forming a protective film with high corrosion resistance under such conditions is as follows.

When the pH increases with high-temperature water containing dissolved oxygen, Cr in the stainless steel base material is eluted as chromate ions, while Fe and Ni are stabilized as oxides and their solubility decreases. As a result, the entire metal surface is uniformly covered with a dense oxide film mainly composed of Fe and Ni, starting from the active metal grain boundaries. Boiling water reactor reactor water environment is approximately 200 ppb
of! ! Generally, Cr is easily eluted in an oxidizing atmosphere that dissolves Cr.

Therefore, if a film containing a large amount of Cr is formed in advance, Cr in the film will be redissolved as chromate ions, resulting in increased film defects and loss of protection. However, in the above-mentioned processing environment, a film mainly composed of Fe and Ni with little Cr is formed, so this film is
It exists stably at the R'3 boundary for a long period of time and exhibits a protective effect. In addition, when pi-i exceeds 8.6, Fe, N
Both i are undesirable because they tend to be eluted as ferric acid and nickel acid, leading to damage to the base material.

A similar phenomenon occurs when the dissolved oxygen level is increased in neutral high-temperature water.

In either case, it is desirable that the liquid circulate and flow. This is because, if water is stagnant, oxygen will not diffuse sufficiently to the metal surface, and there is a risk that the film will be formed non-uniformly. Also, the processing time is 150
~300 hours is desirable. This is because no increase in the effect is observed at 300 hours or more, and the effect is small at 150 hours or less.

Next, a method for adjusting the dissolved oxygen a and pH of -Amanbe oil water will be described. Dissolved oxygen 87 to 300 to s o
o In order to adjust to ppb, pure water (l@oxygen concentration 5 to 8 p
It is effective to inject a certain amount of pm) into the reactor via the control rod drive water system. Oxygen can also be provided directly from a cylinder containing oxygen at high pressure. There are several ways to adjust the pH to alkaline. One is to directly inject an alkaline solution from the outside via the water supply system, control rod drive water system, or reactor water purification system, and the other is to load the cation exchange resin with alkali elements.
This is a method that uses the distribution equilibrium with the hydrogen ion concentration in the overhead water to elute.

By performing the above oxidation treatment after chemical decontamination and then proceeding to output operation, it is possible to suppress corrosion of the sybrant components, suppress redeposition of radioactive substances, and provide a plant with low radiation loss.

Each embodiment of the present invention will be described below.

Example 1 A method of applying the method of the present invention to an actual machine will be described based on FIG. After chemical decontamination of the FA child reactor 1, reactor water recirculation system 2, and reactor water purification system 3, the decontamination liquid is removed and the system is filled with pure water again. Thereafter, the recirculation pump 5t is operated without fuel 4 loaded, or the fuel 4 is loaded and nuclear heating operation is performed without output. This method keeps the water temperature inside the reactor above 280°C. in this case,
The steam bypasses the turbine 6, and the amount of circulating water is kept to just a few points at the rated output. In this state, oxygen is supplied via the water supply system 8, the reactor water purification system 3, or the control rod drive water system 9 to bring the dissolved oxygen in the reactor 1 to 300 to 5 degrees.
Q Adjust to Oppb or inject alkali to maintain pH between 768 and 8.6. Driving in this state 1
After running for 50 to 300 hours, shift to output operation.

Example 2 SUS 304 and 316L samples were immersed in a BWR reactor water environment and then decontaminated with a chemical decontamination liquid mainly containing organic acids. Thereafter, the decontamination solution was removed, the surface of the sample was sufficiently washed and washed, and then oxidation treatment was performed under the conditions shown in 1.fi.

Thereafter, the material was immersed in the reactor water environment again and the amount of 60% CO deposited was compared with that of conventional untreated material. The results are shown in Table 1. As is clear from this, compared to the untreated material, all of the treated materials according to the present invention were able to suppress the adhesion amount of 60CO to 20 to 40 inches.

Table 1 Example 3 After preparing samples similar to those in Example 2 for SUS304 and 316L, the temperature was 280-290°C and the dissolved oxygen concentration was 150.
150 at different pH using ~300 ppb hot water.
Oxidation treatment was performed for ~300 hours, and 6
The effect of suppressing 0CO adhesion was investigated. The results are shown in Figure 2. Note that the pH was in the range of 4 to 10, and the acidic side was adjusted with sulfuric acid, and the alkaline side was adjusted with sodium hydroxide or lithium hydroxide. As is clear from FIG. 2, the amount of 60Co deposited was most suppressed by the oxidation treatment adjusted in the range of pi-17.8 to 8.6.

Example 4 Samples similar to those in Example 2 were prepared for SUS a 04 and 316L, and the dissolved oxygen concentration was adjusted in the range of 0 to 3000 ppb using neutral high temperature water of 280 to 290°C. Oxidation treatment was performed for ~300 hours. After that, B
Immersed in WR reactor water at 60C.

The amount of adhesion was investigated. The results are shown in Figure 3. As is clear from the figure, the amount of 60Co attached is from 300 to 300 l# degree of dissolved oxygen.
The greatest suppression was achieved by oxidation treatment adjusted in the range of 500 pPb.

Example 5 Samples similar to those in Example 2 were prepared using SUS304 and 316L, and dissolved oxygen was dissolved at 280 to 290°C.
2 to 100~300 pptl:, P) 1 to 7.8~
Dissolved oxygen concentration in high temperature water adjusted to &6 and neutral to 300
Using high temperature water adjusted to ~s o o ppb, 25 ~
After oxidation treatment for different times in the range of 1000 hours,
It was immersed under BWR reactor water conditions and the amount of 60CO deposited was investigated.

The results are shown in Figure 4. As is clear from the figure, 60C
In any treatment, the amount of O adhesion was most suppressed at 150 hours or more, and at 300 hours or more, the suppressive effect did not increase even if the treatment was carried out for a long time.

Example 6 Samples similar to those in Example 2 were prepared using SUS aO4 and 316L, and then subjected to oxidation treatment in the range of 150 to 295°C, and the amount of 60CO deposited under the conditions of a BWR reactor water tank was measured. Oxidation treatment: dissolved oxygen $1100-300ppb,
p) i High temperature water adjusted to 7.8-8.6 and neutral dissolved oxygen #300 to so o PI) fA to b
Conditioned high-temperature water was used, and the treatment time was 150 to 300 hours. The results are shown in Figure 5. As is clear from the figure,
In both treatments, the adhesion of 60CO was most suppressed at temperatures above 280°C.

〔Effect of the invention〕

As is clear from the above explanation, according to the present invention, rapid recontamination after systematic chemical decontamination of a nuclear power plant can be suppressed by simple means, and it is suitable for reducing radiation exposure of workers. This makes it possible to perform oxidation treatment that is easy and has high practical value.

[Brief explanation of the drawing]

Figure 1 is a system diagram of a BWR nuclear power plant to which the present invention is applied, and Figure 2 is a diagram of the relationship between 60CO adhesion amount and treatment pf.
FIG. 3 is a diagram showing the relationship between the amount of 60 CO deposited and the treatment dissolved oxygen concentration, FIG. 4 is a diagram of the relationship between the amount of 60 CO deposited and treatment time, and FIG. 5 is a diagram showing the relationship between the amount of 60 CO deposited and the treatment temperature. 1... Nuclear reactor 2... Reactor water purification system 3.
... Reactor water purification system 4 ... Fuel 5 ... Recirculation pump 6 ... Turbine 7 ... Condensate storage tank
8...Water supply system 9...Control rod drive water system 10...
・Containment vessel 11...Steam system. ``° -ko''--Kotani Kotabe -1 Fig. 1 4゛-kuru 8- Waterless system Fig. 2 treatment pH Fig. 3 m treatment 87@#m 21111 degrees (ρρb) Fig. 4 treatment opening Opening (former figure 5 processing 3 two gardens ('C)

Claims (1)

[Scope of Claims] 1. After chemically decontaminating the reactor structure in contact with the primary cooling water of the nuclear power plant and the system consisting of piping and equipment around the reactor, the decontamination liquid is removed and the system is removed before the power operation of the nuclear power plant. The chemistry of a nuclear power plant is characterized in that the primary cooling water is heated to 280°C or higher by heating by nuclear heating or the operation of a recirculation pump, and then flows through the system to oxidize the water contact surface of the system. Oxidation treatment method after decontamination. 2. Claim 1, wherein the primary cooling water heated to 280°C or higher and flowing into the system during the oxidation treatment has a pH of 7.8 to 8.6 and a dissolved oxygen concentration of 200±100 ppb. Oxidation treatment method after chemical decontamination of the described nuclear power plant. 3 The primary cooling water that is heated to 280°C or higher and is flowed into the system during the oxidation treatment is neutral and has a dissolved oxygen concentration of 30°C.
The oxidation treatment method after chemical decontamination of a nuclear power plant according to claim 1, wherein the oxidation treatment method is 0 to 500 ppb. 4. The oxidation treatment method after chemical decontamination of a nuclear power plant according to claim 1, 2, or 3, wherein the primary cooling water heated to 280° C. or higher is passed through the system for 150 to 300 hours. .