JPS61237091A - Corrosion preventive method and device for turbine in nuclear power generating 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 a method and apparatus for preventing corrosion of a turbine in a nuclear power plant, and more particularly to a method and apparatus for preventing stress corrosion cracking of a low pressure turbine rotor disk.

[Background of the invention]

Especially in low pressure turbines of modern nuclear power plants,
2Ni used in the turbine rotor disk part
Stress corrosion cracking (hereinafter abbreviated as SCC) has been found to occur in CrMoV steel, and measures to prevent SCC have become an urgent issue.

Conventionally, oxidizing ions such as thribdate ions, chromate ions, and tungstate ions are known as SCC inhibitors for low alloy steels such as 2 NICrMoV steels or stainless steels. Injecting such oxidizers into the steam is undesirably dangerous, as these oxidizers cause a concentration of the agent on the surfaces of the turbine blades. By the way, it is known that oxygen scavengers such as hydrazine and morpholine exist stably in steam without concentrating.

Therefore, a method for suppressing general corrosion of thermal power plant components using these oxygen scavengers has been proposed and has already been put into practical use. That is, there is a so-called gelatile treatment method in which hydrazine is injected as a corrosion prevention method for iron and iron-based alloys in the secondary system, water supply system, and main steam system.

However, in the case of nuclear power plants, the above-mentioned gelatile treatment method has the problem of radiolysis of the drug itself, so even though it is suitable for thermal power plants, it has been difficult to apply to nuclear power plants.

[Purpose of the invention]

The purpose of the present invention is to prevent SCC occurring in the turbine rotor disk of a power generation plant by reducing the oxygen gas in the main steam to water and keeping the oxygen concentration as low as possible. The purpose of the present invention is to provide a method and apparatus for strengthening the functional film.

[Summary of the invention]

The present invention provides a method for achieving the above object in a nuclear power plant in which main steam to be supplied to a turbine contains oxygen gas and hydrogen gas as radiolysis products. ! ! Similarly, the present invention also provides means for controlling the oxygen concentration in the main steam piping between the reactor and the turbine to 5 ppm or less. It is characterized by the fact that it has been provided.

More specifically, the present invention removes harmful oxygen by reducing oxygen gas and hydrogen gas contained in the main steam of a nuclear power plant to water using a water-repellent catalyst. This feature is effective in preventing SCC of turbine disks in nuclear power plants, particularly in preventing SCC of low-pressure turbine disks. Among these, special K
Although the present invention may be applied to selected parts where SCC is likely to occur, it is generally preferable to apply the present invention to the entire turbine foundation.

[Embodiments of the invention]

Prior to a detailed explanation of the present invention, the present inventors' research results regarding SCC of a low pressure turbine disk in a nuclear power plant will be explained below.

According to the above study, it was found that SCC of the turbine disk is caused by oxygen in the main steam.

It is believed that the reason why an oxidizing agent such as oxygen causes SCC is as follows.

In other words, the turbine disk rotates at high speed and is subjected to large stress, and the occurrence of SCC here depends on the rate of formation of a new surface due to severe deformation and the rate of formation of a surface film, that is, the rate of repassivation of the new surface. . The rate of formation of the new surface is controlled by dynamic strain, and the rate of repassivation of the new surface is controlled by the strength of the corrosive action of the environment. It is believed that oxygen causes SCC because an increase in oxygen concentration increases corrosiveness and slows down the repassivation rate.

From the above considerations, it is thought that the reason why reducing oxygen concentration is effective in preventing SCC is that the repair ability of the passive film increases in a low-oxygen environment. It goes without saying that the formation of such a passive film is effective not only against SCC, but also against pitting corrosion, crevice corrosion, and general corrosion.

The present inventors have found that the most effective method for removing oxygen from main steam is to reduce the oxygen in the steam to water using a water-repellent catalyst. In order to prevent turbine corrosion, a method for reducing oxygen using a catalyst was developed. This catalyst-based oxygen reduction method is
This method is extremely effective as a method for preventing SCC in nuclear power plants since it does not have the problem of radiolysis of drugs as described above.

The effect of suppressing SCC due to oxygen reduction was observed in the temperature range of 60°C or higher. This is considered to be because the formation of the oxide film is controlled by temperature. Further, the oxygen concentration at which the effect of suppressing SCC appears is in the range of 0 to 5 ppm.

Based on the above knowledge, the present invention is designed to control the oxygen concentration in the main steam to 5 ppm or less.

Oxygen gas and hydrogen gas in the main steam supplied to the turbine in nuclear power generation plans are radiolysis products of reactor water. Therefore, in the main steam, oxygen gas is present in an amount of 0.5 mole per mole of hydrogen gas.

In order to reduce the oxygen in the main steam, it is sufficient to recombine the oxygen and hydrogen, and it is most effective to use a water-repellent catalyst for this purpose. However, when the catalyst surface is covered with water molecules, the catalyst activity decreases and the above-described recombination reaction stops. Therefore, water-repellent catalysts are designed to repel water on the catalyst surface. The temperature range in which the water-repellent catalyst exhibits the effect of reducing oxygen concentration is 50°C or higher.

It has long been known that Pt and Pd catalysts are good for combining hydrogen and oxygen. Pt and Pd catalysts are usually supported on alumina or activated carbon in an amount of 0.1 to 0.5 wt%. When these catalysts are used at room temperature, they promote the bonding of hydrogen and oxygen to produce water, but their activity does not last long and eventually they lose their activity altogether. The reason for this is that the surface of the catalyst becomes wet due to the generated water. Therefore, a catalyst whose surface has been treated to make it water repellent, or a tb water-repellent catalyst, is effective in sustaining the reaction.

There are two methods for obtaining a water-repellent catalyst: one is to treat a hydrophilic catalyst to make it water-repellent, and another method is to support the active ingredient on a water-repellent carrier.

An example of a method for preparing a water-repellent catalyst is shown in FIG.

As shown in the figure, a Pt-Ti0□ catalyst is prepared by impregnating a spherical titania carrier with a Teflon dispersion and, after firing, impregnating it with a methanol solution of chloroplatinic acid.

Impregnation with Teflon dispersion, which is a water repellent treatment, is performed using Pt-Tie.
2. It may be applied after making the catalyst. Regardless of the order of water-repellent treatment, the activities of the two are often almost equal. When preparing a water-repellent catalyst using a water-repellent carrier such as porous Teflon, it may be directly impregnated with an ethanol or acetone solution of chloroplatinic acid.

Materials for water repellent treatment include fluorinated graphite, Teflon, polyethylene, etc. Ligropyrene, polystyrene, etc. can be used. Further, as the water-repellent carrier material, a porous material similar to the above-mentioned material can be used.

Note that the passivation treatment by reducing oxygen concentration as in the present invention has the effect of suppressing SCC not only in low alloy steels such as 2NICrMoV steel but also in stainless steel, carbon steel, and 2K-based alloys. This makes it possible to apply it not only to nuclear power plants but also to thermal power plants, chemical plants, etc. as a preventive measure against SCC.

The present inventors investigated the relationship between oxygen concentration and temperature and SCC susceptibility using a constant load method in order to investigate the oxygen concentration and temperature conditions under which the SCC suppressing effect appears.

Figure 1 shows pure water vapor at 60-200℃ (oxygen concentration 0-50℃).
This figure shows the results of investigating the scc behavior of 2 NICrMoV steel in 2 NICrMoV steel (ppm). In the figure, the vertical axis is the oxygen concentration p
The logarithmic scale of pm is plotted, and the temperature scale is plotted on the horizontal axis. The numbers within 0 are the test time, the X mark indicates that corrosion cracking was observed in the rupture test, and the ○ mark indicates that corrosion cracking was observed in the rupture test. It shows what could not be done. As shown in Figure 1, temperature 7
SCC susceptibility is high when the temperature is above 0℃ and the oxygen concentration is 7 ppm or above, and when the temperature is below 60℃ or the oxygen concentration is 5
No SCC susceptibility was observed in the ppm or lower range. That is, the oxygen concentration at which the SCC suppressing effect of the present invention appears is in the range of 5 ppm or less.

The present inventors have studied the oxygen concentration reducing effect at each temperature in order to investigate the optimal temperature conditions for performing oxygen concentration reduction treatment using the water-repellent catalyst described above. The steam used in the experiment had the same composition as the main steam of the BWR power plant.

The table below shows the effect of reducing oxygen concentration by the water-repellent catalyst at a treatment temperature of 25 to 250°C.

As is clear from this table, the effect of reducing oxygen concentration was observed at a treatment temperature of 50° C. or higher.

Table: BWf method for reducing oxygen concentration using the water-repellent catalyst of the present invention?
Fig. 2 shows an example in which this method is applied to prevent SCC in low-pressure turbines of low-pressure turbines. In the diagram, 1 is a double reactor, 2
3 is a high pressure turbine, 3 is a low pressure turbine, 4 is a turbine rotor disk, 5 is a moisture separator, 6 is a main steam pipe, 7 is a generator, 8 is a high pressure pump, 9 and 10 are each provided according to the present invention. This is a recombiner and temperature sensor using a water-repellent catalyst. The recombiner 9 is filled with a water-repellent catalyst as shown in FIG. It is supplied to the turbine 3. A recombiner 9 and a moisture separator 5 are disposed in the middle of the main steam pipe 6 to reduce the oxygen concentration and remove moisture from the main steam. In addition, the temperature sensor 10 installed in the main steam piping 6 to the low pressure turbine 3 detects the temperature of the incoming steam, and the recombiner 9 is linked to this to reduce the oxygen concentration in the main steam by 8 CC. It is designed to control the oxygen concentration that appears.

〔Effect of the invention〕

The experiment whose results are shown in Figure 1 was carried out under the same heat treatment conditions as for the collector, but the stress conditions were evaluated under severe acceleration conditions that were different from the stress level applied to the real-root turbine rotor disk. be.

As shown in Figure 1, the oxygen concentration in the main steam is 16
Since reducing the ppm to 1 ppm increases the SCC rupture life by about 40 times or more, it is easily understood that the plant life will be extended by at least 40 times.

[Brief explanation of drawings]

Fig. 1 is a graph showing the relationship between dissolved oxygen concentration and temperature to obtain the SCC suppressing effect of the present invention, Fig. 2 is a schematic diagram of a BWFtf runt to which the present invention is applied, and Fig. 3 is a graph showing the water-repellent catalyst P.
FIG. 4, which is an explanatory diagram of a method for adjusting t-Ti0□-Teflon, is a schematic diagram of a recombiner used in the present invention. 1... Nuclear reactor (gay 2), 2... High pressure turbine, 3
...Low pressure turbine, 4...Turbine rotor disk, 5...Moisture separator, 6...Main steam piping, 7...Generator, 8...High pressure?
9...Recombiner, lO...Oxygen sensor. - Figure 1 Temperature (tl'J Figure 2

Claims (1)

[Claims] 1. In a nuclear power plant in which the main steam to be supplied to the turbine contains oxygen gas and hydrogen gas as radiolysis products, the oxygen concentration in the main steam is set to 5 pp.
A method for preventing corrosion of a turbine in a nuclear power plant, characterized by controlling corrosion to less than m. 2. In order to control the oxygen concentration in the main steam to 5 ppm or less, the main steam containing oxygen gas is supplied to the turbine in the nuclear power plant through a recombiner using a water-repellent catalyst. The turbine corrosion prevention method according to claim 1, characterized in that: 3. In a nuclear power plant including a nuclear reactor, main steam piping for supplying main steam produced in the nuclear reactor, and a turbine operated by receiving the supply of the main steam, the nuclear power plant includes the nuclear reactor and the turbine. 1. A corrosion prevention device for a turbine in a nuclear power plant, characterized in that means for controlling the oxygen concentration in the main steam to 5 ppm or less is provided in the main steam piping between the main steam and the main steam. 4. A turbine in a nuclear power plant according to claim 1, wherein the means for controlling the oxygen concentration in the main steam to 5 ppm or less is a recombiner using a water-repellent catalyst. Corrosion prevention device. 5. The corrosion prevention device for a turbine in a nuclear power plant according to claim 3 or 4, wherein the turbine is a low-pressure turbine.