JPH0335385B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel power generation plant that prevents corrosion of metal materials in a system where pure water containing dissolved oxygen comes into contact with the metal materials. In power generation plants where pure water containing dissolved oxygen comes into contact with metallic materials, the metallic materials are corroded, and when the pure water is still, the degree of corrosion becomes more pronounced as the dissolved oxygen concentration increases. about 40
Corrosion occurs in metal materials when dissolved in amounts of 30,000 ppb to 30,000 ppb, and corrosion prevention measures are required. In particular, in BWR (boiling water reactor) power plants, condensate and
Water supply piping is exposed to water with a dissolved oxygen concentration as high as 5 to 8 ppm when exposed to the atmosphere, and the metal materials of the piping, especially carbon steel, are severely corroded. Corrosion products (commonly composed of iron oxides and called crud) generated as a result of corrosion are brought into the reactor at the time of plant startup and adhere to the fuel rods, reducing thermal efficiency. There is a risk of damaging it. Furthermore, after being activated, the crud attached to the fuel rods peels off and re-attaches to the reactor recirculation system piping, etc., increasing the surface dose rate of the piping, etc. There is also the risk of causing an increase in For these reasons, corrosion prevention measures for condensate and water supply system piping during shutdown of BWR power plants have become an important issue. Conventionally, a water draining and drying method called hot drain-off has been used in some plants to prevent corrosion of piping in systems where pure water containing dissolved oxygen and metal materials come into contact, especially when BWR power plants are shut down. . This method involves draining water before the supply water is completely cooled when plant operation is stopped, and drying the piping surface using residual heat. However, it cannot be applied to all plants due to differences in plant structure. There are also problems with the disposal of the large amount of radioactive waste fluid that is generated as a result of draining water. Furthermore, this draining and drying method is complicated to operate, so it is not suitable for short-term shutdowns. Additionally, Japanese Patent Application Laid-Open No. 52-85696 describes a method for preventing corrosion of water supply and condensate system piping by adjusting the degree of vacuum in the condenser and maintaining the dissolved oxygen concentration at 50 to 200 ppb before starting the reactor. is disclosed. but,
Controlling the dissolved oxygen concentration alone cannot sufficiently protect the piping described above from corrosion. As other corrosion prevention methods, in thermal power plants, when the operation is stopped for a short period of time, a full water storage method with the addition of hydrazine is adopted, and when the operation is stopped for a long period of time, a dry storage method is adopted where water is drained and then nitrogen gas is filled. However, in a BWR plant, the added hydrazine must be removed before startup, and the nitrogen gas must also be degassed, so these methods are not recommended.
It cannot be applied to corrosion protection in BWR power plants. An object of the present invention is to provide a power generation plant that prevents corrosion of metal materials in a system where pure water containing dissolved oxygen comes into contact with metal materials of a condenser and water supply system when the turbine of the power generation plant is stopped. A more specific object of the present invention is to provide a power generation plant that prevents corrosion of condensate and water supply system piping when the turbine of a BWR power generation plant is stopped. The present invention relates to a steam turbine rotated by steam, a condenser that returns the steam to pure water, a desalination device that removes salts from the pure water, and a heating source that heats the pure water and turns it into steam again. In a power generation plant having a water supply system that supplies the pure water to a water supply system, a recirculation piping is provided that connects the water supply system and a condenser, and the piping,
Constructing a closed circulation path for the pure water in the condenser, demineralizer, and water supply system, and having a phantom demineralizer that maintains the specific conductivity of the pure water at 0.5 μS/cm or less within the path. Located in a power generation plant featuring: In particular, nuclear reactors and thermal power are used as heating sources. In the system of the present invention, pure water has dissolved oxygen, but the dissolved oxygen concentration in pure water is within a range where metal materials come into contact with pure water containing dissolved oxygen and cause corrosion, causing problems in corrosion protection. Although it varies depending on the type of material, the specific amount is about 40 to about 30,000 ppb. As mentioned above, when a BWR plant is shut down, the condensate and water supply piping is exposed to the atmosphere and comes into contact with water with a dissolved oxygen concentration of 5 to 8 ppm, so it is important to prevent corrosion of the metal materials of these piping. The invention is particularly suitable. In the present invention, the metal material targeted for corrosion protection is water containing dissolved oxygen, especially water with a dissolved oxygen concentration of about
Particular examples where corrosion protection becomes a problem in contact with water of 40 to about 30,000 ppb include carbon steel, low alloy steel, stainless steel, copper and its alloys. The present invention effectively prevents corrosion of carbon steel, which is the material for condensate and water supply piping in BWR power plants. In the present invention, the specific conductivity of pure water is 0.5μS/cm.
One of the requirements is to maintain the specific conductivity below, and the means for adjusting it in this way is, for example, passing pure water through a demineralizer filled with granular positive and negative ion exchange resin to lower the specific conductivity. This can be achieved by In the present invention, the specific conductivity of pure water is 0.5μS/cm.
If it is kept below, the corrosion rate of metal materials will be significantly reduced and effective corrosion protection can be achieved.In particular, if it is kept below 0.1μS/cm, the corrosion protection effect will be even more remarkable. but,
If the specific conductivity of pure water exceeds 0.5 μS/cm, no effective anticorrosion effect can be expected. One of the other requirements of the present invention is to have a pump that flows pure water, and flowing pure water means that the pure water does not remain stationary on the surface of the metal material. The degree of flow of pure water may be such that the purpose of the present invention is achieved together with maintaining the specific conductivity at a specific value, and the flow rate
The normal value is about 0.2 to 1 cm/S or more. To flow pure water, it is sufficient to move the water using, for example, a low-pressure pump. Flowing pure water not only helps keep the specific conductivity below 0.5 μS/cm, but also helps prevent the formation of local batteries caused by local differences in dissolved oxygen concentration in pure water, which can cause corrosion. prevention). In addition, a very thin oxide film is generally formed on metal surfaces that come into contact with pure water by reaction with dissolved oxygen in the pure water, and this oxide film prevents corrosion from progressing due to its lowering effect. However, this oxide film has extremely fine cracks, and there is a risk that corrosion will progress through these cracks, but flowing pure water supplies dissolved oxygen to the metal through these cracks. It is thought that this prevents corrosion from progressing in the cracked area by forming an oxide film on the cracked area. The temperature at which the present invention is carried out is usually 30 to 40°C, and the time can be determined as appropriate depending on the mode of implementation. Example 1 The specific conductivity of water was lowered using a demineralizer, and the anticorrosion effect at different specific conductivities was investigated. The carbon steel shown in Table 1 was immersed in water with a dissolved oxygen concentration of 8 ppm, and the corrosion rate was measured at a flow rate of 0.2 cm/S and a temperature of 30°C.

[Table] Figure 1 shows the relationship between the corrosion rate of carbon steel and the specific conductivity of water. The corrosion rate is significantly reduced in pure water with a specific conductivity of less than 0.5μS/cm. specific conductivity
Corrosion rate with pure water of 0.1μS/cm is approximately 1mg/d
m ² /month, and in this case, the carbon steel had a metallic luster in appearance, no pitting corrosion was observed, and the anticorrosion effect was remarkable. Example 2 The relationship between water flow rate and corrosion rate was investigated at various specific conductivities. That is, the temperature is 30°C, the dissolved oxygen concentration is 8 ppm, and the specific conductivity is 2.12 μS/cm and 1.07 μS/cm, respectively.
Carbon steel (first
Table) was immersed to determine the corrosion rate of carbon steel at various flow rates. The results are shown in Figure 2. When the specific conductivity of water is about 1 μS/cm or more, the corrosion rate increases as the water flow rate increases, but when the specific conductivity of water is about 0.5 Below μs/cm, the corrosion rate tends to decrease as the water flow rate increases. Example 3 Carbon steel (Table 1) was immersed in water with a dissolved oxygen concentration of 5 ppm, the specific conductivity of the water was 0.1 μS/cm, the flow rate was 1 cm/S,
Changes in corrosion weight loss over time were measured at a temperature of 35°C. The results are shown in Figure 3 (showing the relationship between water flow and corrosion weight loss), and in flowing water, the corrosion weight loss was kept small and almost constant regardless of the passage of time, as shown by curve A. However, when the flow was once stopped and the state was brought to a standstill, the corrosion weight loss reached approximately 50 mg/dm ² after approximately 70 hours as shown by curve B, and corrosion progressed. However, when water was allowed to flow again, the corrosion loss remained constant and corrosion did not progress any further. For reference, the same experiment as above was conducted in still water, and as shown by curve C, the corrosion weight loss increased with time. From the above, it can be seen that flowing pure water is essential for corrosion prevention according to the present invention from the perspective of corrosion weight loss, but this is also clear from the perspective of corrosion rate, which is approximately 1 mg in flowing water. /dm ² /month,
The concentration in still water was approximately 300 mg/dm ² /month. Example 4 Carbon steel at different dissolved oxygen concentrations (Table 1)
The anticorrosive effect of Carbon steel was immersed in high-purity water with a specific conductivity of 0.1 μS/cm, and the corrosion rate was measured at a flow rate of 1 cm/S and a temperature of 35°C. The results are shown in Figure 4 (showing the relationship between corrosion rate and dissolved oxygen concentration), and the corrosion rate suddenly decreases when the dissolved oxygen concentration exceeds 40 ppb, as shown by solid line A, which corresponds to the open atmosphere condition. At dissolved oxygen concentrations of 8 to 8 ppm, the corrosion rate was approximately 1 mg/dm ² /month. According to this, it can be seen that the anticorrosion effect is exhibited at dissolved oxygen concentrations of 40 ppb or higher. In addition, in still water, the corrosion rate increased as the dissolved oxygen concentration increased, as shown by dotted line B, and reached about 300 mg/dm ² /month at 5 to 8 ppm. Comparing this result with that in flowing water, it can be seen that flowing pure water is essential for corrosion prevention. Example 5 The corrosion or anticorrosion effect of carbon steel (Table 1) at different specific conductivities was investigated. Keep the specific conductivity of pure water at 0.2 to 0.5μS/cm,
Dissolved oxygen concentration 40ppb, flow rate 1cm/S, temperature 35℃,
The carbon steel surface after immersion was observed using a scanning electron microscope under the condition of immersion time of 3960 hours. As a result, it was confirmed that the metal surface was covered with a thickness on the order of μm by crystals mainly composed of magnetite with a grain size of about 1 μm. The carbon steel surface after immersion was similarly observed under the same conditions as above, with the specific conductivity of pure water being kept at 0.1 μs/cm or less. As a result, it was confirmed that the crystal grain size was approximately 0.2 μm, and the metal surface was covered with an extremely dense thin film layer on the order of Å. Based on the above results, the specific conductivity of pure water is 0.2
It can be seen that an excellent corrosion inhibiting effect, that is, an anticorrosive effect can be obtained when the corrosion rate is maintained at 0.5 to 0.5 μs/cm, particularly 0.1 μs/cm or less. In addition, when looking at the corrosion protection effect in terms of corrosion loss, it is 517mg/dm2 when the specific conductivity is 0.2 to 0.5μs/cm, and 33.2mg/ ^dm2 when the specific conductivity is 0.1μs/cm.
dm ² , and it can be seen from this that a more significant corrosion prevention effect is exhibited by keeping the specific conductivity low. Example 6 Figure 5 is a schematic system diagram of a BWR power plant, where 1 is a nuclear reactor, 2 is a turbine, 3 is a condenser, and 4 is a system diagram of a BWR power plant.
5 is a condensate demineralizer, 5 is a condensate low pressure pump, and 6 is a feed water recirculation line. The corrosion prevention method of the present invention was carried out from the time the turbine operation of the BWR power plant shown in FIG. 5 was stopped to the time it was started. The condensate and water supply system piping of the BWR power plant is mainly made of carbon steel, and the water in contact with it showed a dissolved oxygen concentration of 5 to 8 ppm when exposed to the atmosphere. Water is passed through the condensate demineralizer 4 using a circulation system with a water supply recirculation line 6, and the specific conductivity of the water is measured using a specific conductivity measuring device, and the specific conductivity of the water is measured at 0.5 μs/
Circulation was carried out at a flow rate of 1 cm/s to maintain the flow rate at 1 cm/s or less. After 750 hours, no corrosion was observed in the condensate and water supply piping. In this embodiment, the main components of the condenser 3 are a cooling pipe made of steel alloy and a condenser container made of carbon steel, and their contact areas are approximately 40,000 m 2 and 8,000 m ² , respectively.
^m2 . In addition, the water flows from the condensate low pressure pump 5 through the condensate demineralizer 4 and through the feed water recirculation line 6 to the condenser 3.
The system leading to the system includes a feed water heater and piping.The heating tube of the feed water heater is made of stainless steel, the body of the feed water heater is made of low alloy steel, and the piping is made of carbon steel.The water contact area of each of these is Approx. 15000m ² , Approx. 100m ² and 1500
^m2 . When this embodiment is implemented, the condensate demineralizer 4
When we measured the concentrations of iron, copper, chromium, and nickel in the water at the inlet and outlet, the concentrations of these metals were always below 1 ppb at both measurement points. As is clear from the above description, the present invention provides corrosion protection for metal materials that come into contact with pure water by simple means, and is particularly applicable to corrosion protection of condensate and water supply system piping when the turbine of a BWR power plant is stopped. It is suitable for this purpose, has high practical value, and is of great industrial significance.

[Brief explanation of drawings]

Figure 1 is a diagram of the relationship between specific conductivity and corrosion rate, Figure 2
Figure is a diagram of the relationship between water flow rate and corrosion rate, Figure 3 is a diagram of the relationship between time and corrosion loss, Figure 4 is a diagram of the relationship between dissolved oxygen concentration and corrosion rate, and Figure 5 is a schematic diagram of the BWR power plant system. It is a diagram. 1... Nuclear reactor, 2... Turbine, 3... Condenser, 4... Condensate demineralizer, 5... Condensate low pressure pump,
6... Water supply recirculation line.

Claims (1)

[Scope of Claims] 1. In a power generation plant having a water supply system that supplies pure water to a steam turbine, a condenser, a demineralizer, and a heating source that forms steam, a regeneration system that connects the water supply system and the condenser is provided. Circulation piping is provided, forming a closed path in which the pure water sequentially circulates from the water supply system through the piping, the condenser, the demineralizer, and the water supply system, and a pump that circulates the pure water within the path. and the demineralizer that maintains the specific conductivity of the pure water at 0.5 μS/cm or less. 2. The power generation plant according to claim 1, wherein the heating source is a source furnace.