Corrosion environment mitigation method for pressurized water reactor and pressurized water nuclear power plant

Abstract

To provide a corrosion environment mitigation method for a pressurized water reactor capable of maintaining a low corrosion potential at a liquid-contacting portion in contact with a primary coolant of the PWR, and achieving both suppression of SCC in stainless steel and suppression of SCC in nickel-based alloys, and provide a pressurized water nuclear power plant using the same.SOLUTION: A corrosion environment mitigation method for a pressurized water reactor includes the steps of: performing hydrogen injection; performing noble metal injection; and measuring the electrode potential at a liquid-contacting portion. An injection point at which the noble metal injection is performed is located downstream of a regenerative heat exchanger P33 in a chemical and volume control system P300. The dissolved hydrogen concentration in the primary coolant is controlled such that a difference between the electrode potential of the liquid-contacting portion and the equilibrium potential of the oxide formation reaction of nickel becomes greater than a target potential difference. A pressurized water nuclear power plant includes a primary cooling system P100, a secondary cooling system P100, a chemical and volume control system P300, a hydrogen injection device, a noble metal injection device P31, and a potential sensor. The noble metal injection device P31 is connected to the downstream side of the regenerative heat exchanger P33.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a method for mitigating the corrosion environment of a pressurized water reactor and a pressurized water nuclear power plant that suppress stress corrosion cracking of structural members by noble metal injection.

Background Art

[0002] The equipment and piping of nuclear power plants are made of stainless steel, nickel-based alloys, etc. These materials have high susceptibility to stress corrosion cracking (SCC) due to the superposition of material factors, mechanical factors, and environmental factors. Therefore, in nuclear power plants, measures are taken to prevent SCC in order to maintain the integrity of the plant. In recent years, prevention of SCC has also been promoted from the viewpoints of improving the operating rate of nuclear reactors, improving economic efficiency, and coping with high aging.

[0003] As measures against SCC, measures aimed at improving the corrosion resistance of materials, relaxing tensile stress, and relaxing the corrosion environment are being implemented. In boiling water reactors (BWRs) at home and abroad, hydrogen injection is widely carried out as a measure to prevent SCC of structural members in contact with light water, which is the cooling water. Patent Documents 1 and 2 disclose technologies related to hydrogen injection.

[0004] Under a high radiation dose such as inside a pressure vessel, radiolysis of water occurs, and oxygen and hydrogen peroxide are generated in the cooling water. The cooling water circulating through the pressure vessel has a high oxygen concentration and hydrogen peroxide concentration, resulting in a corrosion environment in which SCC is likely to occur. In hydrogen injection, hydrogen gas is injected into such cooling water to cause a recombination reaction between oxygen, hydrogen peroxide, and hydrogen. By consuming oxygen and hydrogen peroxide in the cooling water through the recombination reaction, the corrosion environment to which the material in contact with the cooling water is exposed is mitigated.

[0005] In a BWR, the corrosion potential (Electrochemical Corrosion Potential: ECP) of the structural members in contact with the cooling water is used as an index representing the effect of preventive maintenance against SCC. It is known that when the corrosion potential of stainless steel becomes lower than -300 to -200 mV vs. SHE, the occurrence of SCC is suppressed. Therefore, the plant is operated under a target value of the corrosion potential lower than about -300 mV vs. SHE.

[0006] On the other hand, in a Pressurized Water Reactor (PWR), hydrogen injection into the primary cooling system has been carried out since the early stage of the practical application of PWR. As an index representing the corrosion environment of the contact part, the dissolved hydrogen concentration of light water, which is the primary coolant, is used. In the safety regulations of PWR, the allowable range of the dissolved hydrogen concentration is set at 15 to 50 mL / kg. In the water chemistry management guidelines of PWR, the allowable range of the dissolved hydrogen concentration during power operation is set at 25 to 35 mL / kg. The operation management value of the dissolved oxygen concentration in the primary coolant is set at 5 ppb or less.

[0007] According to previous studies, in a PWR, the condition of the electrode potential of the structural members necessary to suppress SCC of stainless steel is considered to be about -400 mV vs. SHE or lower. In the range of the dissolved hydrogen concentration during the operation of PWR, it is considered that the corrosion potential of the structural members has decreased to near the hydrogen electrode potential. Therefore, based on the combined conditions considering the temperature, pH or dissolved hydrogen concentration of the primary coolant, the corrosion potential to be ensured in a PWR is -800 to -700 mV vs. SHE.

[0008] When using a technique to lower the corrosion potential of the structural members, it is necessary to grasp the corrosion potential of the structural members in contact with the coolant, and it is necessary to measure the electrode potential of the structural members with high precision. There are few cases of monitoring the corrosion potential in a PWR, but there are cases of measuring the electrode potential at the upper part of the reactor core or outside the reactor. Generally, a potential sensor is installed inside the pressure vessel or in the piping connected to the pressure vessel, and based on the measurement results by the potential sensor, the corrosion potential of the structural members is controlled under predetermined conditions.

[0009] In addition, in a BWR, noble metal injection is performed as a measure to prevent SCC of structural members that come into contact with cooling water. In noble metal injection, a solution of a noble metal compound is injected into the cooling water to deposit a noble metal on the surface of the structural members that come into contact with the cooling water. Noble metals such as platinum group metals precipitate as particles on the surface of the structural members and catalyze the recombination reaction. Therefore, when noble metal injection is performed, even when the amount of hydrogen injection is small, it becomes possible to efficiently mitigate the corrosion environment.

[0010] Patent Document 3 describes forming a thin film of at least one metal belonging to the platinum group metals on the surface of the mixed oxide outer skin on the surface of a stainless steel member exposed to the high-temperature and high-pressure water in a nuclear reactor. The technology of Patent Document 3 targets a boiling water reactor.

[0011] Patent Document 4 describes a method for suppressing erosion and cracking of metal parts in a nuclear reactor, which includes a step of forming a catalytic surface on the parts and a step of generating a stoichiometric excess amount of a reducing agent in the water of the nuclear reactor to substantially reduce the concentration of the oxidized form on the surface to zero. The technology of Patent Document 4 aims to suppress erosion and cracking in metal parts in a nuclear reactor, particularly in the high-concentration primary and secondary systems of a PWR.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0013] In recent years, in pressurized water reactors (PWRs), the problem of primary water stress corrosion cracking (PWSCC) of nickel-based alloys used in the primary cooling system has become prominent. Alloy 600, a nickel-based alloy, is used as the structural material and welding material of the primary cooling system. Conventionally, hydrogen injection into the primary coolant has been carried out in PWRs. However, PWSCC of nickel-based alloys has been confirmed since the 1980s and has been recognized as an important problem since around 2000.

[0014] PWSCC of nickel-based alloys has been widely confirmed in pressure vessels, pressurizers, steam generators, primary cooling system pipes, etc. To address such problems, measures have been taken such as converting the materials in contact with the primary coolant to Alloy 690 with a high Cr content, relaxing the stress of the materials in contact with the primary coolant, and reducing the operating temperature at the locations where SCC occurs. However, all of these are limited countermeasures, and a countermeasure with a wider range of effects is desired.

[0015] PWSCC of nickel-based alloys is considered to be related to the equilibrium potential of the nickel oxide formation reaction. There is a concept that when the potential of the material is close to the equilibrium potential of the oxide formation reaction, the oxide film becomes unstable, the occurrence time of SCC becomes shorter, or the propagation rate of SCC becomes faster, resulting in the manifestation of PWSCC. The equilibrium potential of the oxide formation reaction depends on the concentrations of oxidizing species and reducing species. It has been confirmed that when the dissolved hydrogen concentration is sufficiently high or sufficiently low, the occurrence time of SCC becomes longer and the propagation rate of SCC becomes slower.

[0016] Therefore, as a countermeasure against PWSCC of nickel-based alloys, a method of controlling the dissolved hydrogen concentration of the primary coolant is being studied so that the electrode potential of the wetted part in contact with the primary coolant does not approach the equilibrium potential of the nickel oxide formation reaction. Such a method is roughly classified into a method of making the dissolved hydrogen concentration of the primary coolant higher than the reference value corresponding to the equilibrium potential and a method of making the dissolved hydrogen concentration of the primary coolant lower than the reference value corresponding to the equilibrium potential.

[0017] When using a method to lower the dissolved hydrogen concentration of the primary coolant, it is necessary that the corrosion potential of the wetted part in contact with the primary coolant is below the required value necessary to suppress SCC of stainless steel. Even after the hydrogen supplied by hydrogen injection undergoes a recombination reaction with oxygen or hydrogen peroxide generated by radiolysis of water, it is necessary to maintain a highly reducing condition. On the other hand, when using a method to increase the dissolved hydrogen concentration of the primary coolant, it is necessary to consider the problem of hydrogen embrittlement. The hydrogen absorption amount of the fuel cladding tube needs to be below the threshold value necessary to suppress hydrogen embrittlement.

[0018] In the technologies described in Patent Documents 1 to 2, hydrogen injection is performed, but noble metal injection is not. In the technology described in Patent Document 3, noble metal injection for BWR is performed. On the other hand, the technology described in Patent Document 4 targets PWR and is said to suppress erosion and cracking in metal parts in the high-concentration primary and secondary systems of PWR. However, in Patent Document 4, the implementation of noble metal injection during operation is not assumed. It is required to execute measures to suppress SCC during the common period including during the operation of PWR to suppress both SCC of stainless steel and SCC of nickel-based alloys.

[0019] Therefore, an object of the present invention is to provide a method for mitigating the corrosion environment of a pressurized water reactor capable of maintaining a low corrosion potential of the wetted part in contact with the primary coolant of PWR and achieving both suppression of SCC of stainless steel and suppression of SCC of nickel-based alloys, and a pressurized water nuclear power plant using the same.

Means for Solving the Problems

[0020] In order to solve the above problems, a method for mitigating the corrosion environment of a pressurized water reactor according to the present invention includes a step of injecting hydrogen into the primary coolant of the pressurized water reactor, a step of injecting a noble metal into the primary coolant, and a step of measuring the electrode potential of a liquid contact portion that is in contact with the primary coolant at one or more locations including the inside of the pressure vessel. The injection point for injecting the noble metal is downstream of the regenerative heat exchanger of the chemical volume control system. The dissolved hydrogen concentration of the primary coolant is controlled such that the difference from the equilibrium potential of the nickel oxide formation reaction represented by the following formula (1); Ni + H 2 O⇔NiO + H 2 is greater than a preset target potential difference.

[0021] Moreover, a pressurized water nuclear power plant according to the present invention is connected to the pressure vessel of a pressurized water reactor, has a steam generator, a pressurizer, and a coolant pump, and includes a primary cooling system that circulates the primary coolant between the pressure vessel and the steam generator via the pressurizer, a secondary cooling system that is connected between the steam generator and the condenser, has a turbine, and circulates the secondary coolant between the steam generator and the condenser via the turbine, a chemical volume control system that is connected to the primary cooling system in a bypass manner and adjusts the boron concentration and volume of the primary coolant, a hydrogen injection device that injects hydrogen into the primary coolant, a noble metal injection device that injects a noble metal into the primary coolant, and a potential sensor that measures the electrode potential of a liquid contact portion that is in contact with the primary coolant. The chemical volume control system has a regenerative heat exchanger that exchanges heat between the primary coolant extracted from the primary cooling system and the primary coolant whose boron concentration and volume have been adjusted. The noble metal injection device is connected downstream of the regenerative heat exchanger of the chemical volume control system.

Advantages of the Invention

[0022] According to the present invention, it is possible to provide a method for mitigating the corrosion environment of a pressurized water reactor that can maintain a low corrosion potential of a liquid contact portion in contact with the primary coolant of a PWR and achieve both suppression of SCC of stainless steel and suppression of SCC of nickel-based alloys, and a pressurized water nuclear power plant using the same.

Brief Description of the Drawings

[0023]

Fig. 1

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Fig. 5B

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Mode for Carrying Out the Invention

[0024] Hereinafter, a method for mitigating the corrosion environment of a pressurized water reactor according to an embodiment of the present invention, and a pressurized water nuclear power plant using the same will be described with reference to the drawings. In the following figures, the same reference numerals are given to common configurations and duplicate descriptions are omitted.

[0025] The corrosion environment mitigation method for a pressurized water reactor according to this embodiment relates to a method for mitigating the corrosion environment that causes stress corrosion cracking (SCC) in a structural member that comes into contact with the primary coolant in a pressurized water reactor (PWR). In this corrosion environment mitigation method, by performing noble metal injection in a PWR, SCC of the stainless steel used in the primary cooling system is suppressed, and SCC of the nickel-based alloy used in the primary cooling system is also suppressed.

[0026] The corrosion environment mitigation method according to this embodiment includes a step of injecting hydrogen into the primary coolant of a pressurized water reactor, a step of injecting noble metals into the primary coolant of a pressurized water reactor, and a step of measuring the electrode potential of a liquid contact part that comes into contact with the primary coolant at one or more locations including the inside of the pressure vessel of the pressurized water reactor. Examples of the liquid contact part include the surface side part of a metal structural member that constitutes reactor equipment or piping, and materials placed in an environment similar to such a part. Examples of the material of the structural member include stainless steel and nickel-based alloys.

[0027] In the corrosion environment mitigation method according to this embodiment, the dissolved hydrogen concentration of the primary coolant of a PWR is controlled by hydrogen injection and noble metal injection. Further, by measuring the electrode potential of the liquid contact part that comes into contact with the primary coolant, the corrosion potential of the liquid contact part, which is the result of controlling the dissolved hydrogen concentration, is grasped. By controlling the dissolved hydrogen concentration of the primary coolant, the corrosion potential of the liquid contact part that comes into contact with the primary coolant is controlled to mitigate the corrosion environment that causes SCC.

[0028] Specifically, in the corrosion environment mitigation method according to this embodiment, the dissolved hydrogen concentration of the primary coolant is controlled such that the difference between the electrode potential of the liquid contact part that comes into contact with the primary coolant and the equilibrium potential of the nickel oxide formation reaction is greater than the target potential difference. That is, the dissolved hydrogen concentration of the primary coolant is controlled so that the electrode potential of the liquid contact part does not approach the equilibrium potential of the nickel oxide formation reaction. Specifically, control is performed to make the dissolved hydrogen concentration of the primary coolant sufficiently higher than the reference value corresponding to the equilibrium potential of the nickel oxide formation reaction, or control is performed to make the dissolved hydrogen concentration of the primary coolant sufficiently lower than the reference value corresponding to the equilibrium potential of the nickel oxide formation reaction.

[0029] Figure 1 shows the relationship between the dissolved hydrogen concentration in the primary coolant of a PWR and the generation time and propagation rate of cracks due to stress corrosion cracking. Figure 1 shows the results of analyzing the behavior of PWSCC in the wetted part in contact with the primary coolant at 330°C. In Figure 1, the horizontal axis represents the dissolved hydrogen concentration [mL / kg] in the primary coolant of the PWR. The vertical axis represents the generation time until cracks occur, and the horizontal axis represents the propagation rate at which cracks propagate. The dashed curve is the analysis result of the crack generation time. The solid curve is the analysis result of the crack propagation rate.

[0030] As shown in Figure 1, the crack generation time in PWSCC takes a minimum value with respect to the dissolved hydrogen concentration in the primary coolant during operation. Also, the crack propagation rate in PWSCC takes a maximum value with respect to the dissolved hydrogen concentration in the primary coolant during operation. When the dissolved hydrogen concentration in the primary coolant is sufficiently high or sufficiently low, the crack generation time becomes long. Also, when the dissolved hydrogen concentration in the primary coolant is sufficiently high or sufficiently low, the crack propagation rate becomes slow.

[0031] Therefore, conventionally, as countermeasures against PWSCC of nickel-based alloys, methods of making the dissolved hydrogen concentration in the primary coolant sufficiently higher than the concentration that causes the minimum value of the crack generation time or the maximum value of the crack propagation rate, or methods of making the dissolved hydrogen concentration in the primary coolant sufficiently lower than the concentration that causes the minimum value of the crack generation time or the maximum value of the crack propagation rate have been studied.

[0032] PWSCC of nickel-based alloys is considered to be related to the equilibrium potential of the nickel oxide formation reaction. When the electrode potential of the structural member in contact with the primary coolant is close to the equilibrium potential of the nickel oxide formation reaction, the oxide film on the surface of the structural member becomes unstable, the crack generation time becomes short, and the crack propagation rate becomes fast, and there is a concept that the occurrence and progress of PWSCC become a problem. The nickel oxide formation reaction is represented by the following formula (1). Ni + H 2 O ⇔ NiO + H 2 ···(1)

[0033] Figure 2 is a diagram showing the relationship between the dissolved hydrogen concentration of the primary coolant and the state of nickel. Figure 2 shows a citation from a report by J. W. Cobble et al. (“High-temperature Thermodynamic Data for Species in Aqueous Solution”, EPRI NP-2400, (1982)). In Figure 2, the horizontal axis represents the dissolved hydrogen concentration [mL / kg] of the primary coolant of a PWR. The vertical axis represents the temperature [°C] of the primary coolant of a PWR. The curves indicate the phase boundary lines corresponding to the equilibrium state between metallic nickel (Ni) and nickel oxide (NiO).

[0034] As shown in Figure 2, during the power operation of a PWR, the dissolved hydrogen concentration is set within the range of 25 to 35 mL / kg based on the water chemistry management guidelines of the PWR. Also, during the power operation of a PWR, the temperature of the primary coolant is approximately 290°C on the inlet side of the pressure vessel and approximately 330°C on the outlet side of the pressure vessel. In such ranges of dissolved hydrogen concentration and temperature, the oxide formation reaction of nickel can reach an equilibrium state. In the equilibrium state, since the crack generation time is minimized or the crack propagation rate is maximized, the occurrence and progression of SCC in nickel-based alloys may become a problem.

[0035] Under the conditions shown in Figure 2, when the dissolved hydrogen concentration of the primary coolant is reduced to about 12 mL / kg or less, regardless of the temperature of the primary coolant, the region where NiO is stable is formed, so SCC of nickel-based alloys can be suppressed. By using a method to lower the dissolved hydrogen concentration of the primary coolant, the reaction equilibrium can be shifted to the side where NiO increases compared to the method of increasing the dissolved hydrogen concentration of the primary coolant. NiO is relatively easy to dissolve, which leads to the suppression of the adhesion of Ni to nuclear fuel materials. Therefore, the method of lowering the dissolved hydrogen concentration of the primary coolant is considered advantageous in that it can reduce the amount of waste generated by activation and the risk of exposure due to spent fuel.

[0036] However, when using a method to lower the dissolved hydrogen concentration of the primary coolant, the corrosion potential of the wetted part in contact with the primary coolant needs to be lower than the required value necessary to suppress SCC of stainless steel. In order to achieve both suppression of SCC of stainless steel and suppression of SCC of nickel-based alloys, it is necessary to maintain a highly reducing condition even after the hydrogen supplied by hydrogen injection undergoes a recombination reaction with oxygen or hydrogen peroxide generated by radiolysis of water. On the other hand, when using a method to increase the dissolved hydrogen concentration of the primary coolant, it is necessary to consider the problem of hydrogen embrittlement. The hydrogen absorption amount of the fuel cladding tube needs to be lower than the threshold value necessary to suppress hydrogen embrittlement.

[0037] In response to such problems, in the corrosion environment mitigation method according to the present embodiment, noble metal injection is performed on the primary coolant of a PWR. Also, control is performed to make the dissolved hydrogen concentration of the primary coolant sufficiently higher than a preset reference value, or to make the dissolved hydrogen concentration of the primary coolant sufficiently lower than a preset reference value, so that the electrode potential of the wetted part in contact with the primary coolant does not approach the equilibrium potential of the nickel oxide formation reaction.

[0038] As the reference value, a predetermined dissolved hydrogen concentration corresponding to the equilibrium potential of the nickel oxide formation reaction is preset. The reference value corresponding to the equilibrium potential of the nickel oxide formation reaction can be set according to the water quality of the primary coolant based on the operating conditions of the plant, etc. As the target potential difference, an arbitrary potential difference that exceeds the fluctuation range and measurement error during the operation of the plant can be set so that the electrode potential of the wetted part is sufficiently far from the equilibrium potential of the nickel oxide formation reaction.

[0039] In the corrosion environment mitigation method according to this embodiment, among the control for increasing the dissolved hydrogen concentration of the primary coolant and the control for decreasing the dissolved hydrogen concentration of the primary coolant, it is preferable to perform at least the control for decreasing the dissolved hydrogen concentration of the primary coolant. When the control for decreasing the dissolved hydrogen concentration is performed, the reaction equilibrium of the nickel oxide formation reaction can be shifted to the NiO side. Therefore, while reducing the amount of waste generated and the risk of exposure, the effect of reducing the amount of hydrogen injection by noble metal injection can also be effectively obtained.

[0040] ≪Overview of BWR≫ Next, the configuration of a BWR to which noble metal injection has been applied in advance and the configuration for performing hydrogen injection and noble metal injection into the cooling water, which is light water, will be described.

[0041] FIG. 3 is a diagram schematically showing the configuration of a BWR. As shown in FIG. 3, the reactor coolant system P500 of the BWR includes a reactor P501, a turbine P503, a condenser P504, etc. The reactor coolant system P500 is provided with a feed water system P510. Further, a recirculation system P520 and a reactor purification system P530 are connected to the reactor coolant system P500.

[0042] The reactor P501 is installed inside a containment vessel P511. The reactor P501 includes a reactor pressure vessel P512 containing a reactor core P513. The reactor core P513 is surrounded by a cylindrical shroud P515 installed inside the containment vessel P511. A fuel assembly is loaded into the reactor core P513. The fuel assembly is formed by housing a plurality of fuel rods in a lattice-shaped channel box. The fuel rods contain a plurality of fuel pellets made of nuclear fuel material inside a fuel cladding tube.

[0043] An annular downcomer P517 is formed between the inner surface of the pressure vessel P512 and the outer surface of the shroud P515. A plurality of jet pumps P521 are installed in the downcomer P517. The outlet of the jet pump P521 is arranged at the lower part of the downcomer P517. The lower part of the downcomer P517 communicates with a lower plenum P551 at the lower part of the pressure vessel P512. The bottom of the lower plenum P551 is called the furnace bottom. The shroud P515 is supported inside the pressure vessel P512 by a shroud support P523.

[0044] The feed water system P510 is composed of a feed water pipe connecting between the condenser P504 and the pressure vessel P512 and equipment installed in this pipe. In the feed water pipe, a condensate purification device P505, a condensate booster pump P506, a feed water pump P507, a low-pressure feed water heater (not shown), and a high-pressure feed water heater P509 are installed in this order from upstream to downstream.

[0045] The recirculation system P520 is composed of a recirculation system pipe connecting between the lower part of the pressure vessel P512 and the jet pump P521 via the outside of the pressure vessel P512 and equipment installed in this pipe. A recirculation pump P544 is installed in the recirculation system pipe.

[0046] The reactor purification system P530 is composed of a purification system pipe connecting between the recirculation system P520 and the feed water system P510 and equipment installed in this pipe. In the purification system pipe, a purification system pump P524, a regeneration heat exchanger (not shown), a non-regeneration heat exchanger (not shown), and a filtration demineralizer P527 are installed.

[0047] In a BWR, a hydrogen injection device P516 is connected between the condensate purification device P505 and the condensate booster pump P506 of the feed water system P510 via a hydrogen injection pipe P518. The hydrogen injection device P516 is a device for injecting hydrogen, which injects hydrogen gas into the cooling water. Also, downstream of the high-pressure feed water heater P509 of the feed water system P510, a noble metal injection device P31 is connected via a noble metal injection pipe P32. The noble metal injection device P31 is a device for injecting noble metals, which injects a solution of a noble metal compound into the cooling water.

[0048] Inside the pressure vessel P512, the cooling water of the downcomer P517 is sucked into the jet pump P521 and injected into the lower plenum P551. The cooling water is supplied from the lower plenum P551 to the reactor core P513. Also, the cooling water of the downcomer P517 is forcibly circulated to the reactor core P513 through the recirculation system piping by the recirculation pump P544. The cooling water supplied to the reactor core P513 is heated by the heat of the nuclear fission reaction of the nuclear fuel material, and part of it becomes steam.

[0049] The steam generated in the reactor core P513 is sent from the pressure vessel P512 through the main steam pipe P502 to the turbine P503. By rotating the turbine P503 with the steam, the generator connected to the turbine P503 rotates to generate electricity.

[0050] The steam discharged from the turbine P503 condenses into water in the condenser P504. The condensed water is supplied inside the pressure vessel P512 through the feed water system P510. After the impurities are removed by the condensate purification device P505, the condensate is pressurized by the condensate booster pump P506 and further pressurized by the feed water pump P507. Then, after being heated by the low-pressure feed water heater and the high-pressure feed water heater P509, it is fed into the pressure vessel P512. The extraction steam extracted from the main steam pipe P502 and the turbine P503 is sent to the high-pressure feed water heater P509 and the low-pressure feed water heater through the extraction pipe (not shown) and used as a heat source for heating the feed water.

[0051] The cooling water that is cyclically supplied to the pressure vessel P512 may contain metal corrosion products. The metal corrosion products are generated by the corrosion of structural members forming the reactor equipment and piping, or are mixed into the feed water. Therefore, a certain proportion of the cooling water is extracted and purified in the reactor purification system P530. The cooling water taken out to the recirculation system P520 is sent to the reactor purification system P530 by the purification system pump P524. Then, it is cooled to about 50 °C by the regenerative heat exchanger and the non-regenerative heat exchanger. After that, it is passed through the filtration demineralizer P527 to remove the metal corrosion products. The purified cooling water is heated in the regenerative heat exchanger and then returned to the feed water system P510.

[0052] During the operation of a BWR, in order to prevent SCC of the structural members in contact with the cooling water, hydrogen injection, or a combination of hydrogen injection and noble metal injection is carried out. Hydrogen injection is a technique in which hydrogen gas is injected into the cooling water to react hydrogen with oxygen or hydrogen peroxide in the cooling water and return it to water. Noble metal injection is a technique in which a solution of a noble metal compound is injected into the cooling water to deposit a noble metal that acts as a catalyst on the surface of the material in contact with the cooling water. As the noble metal compound, sodium hexahydroxoplatinate or the like is used.

[0053] In hydrogen injection, hydrogen gas is injected into the cooling water supplied to the pressure vessel P512 by the hydrogen injection device P516. The hydrogen injected into the feed water jets out from the feed water sparger installed above the downcomer P517 and mixes with the reactor water. The reactor water may contain oxygen or hydrogen peroxide generated by the radiolysis of water. Under the gamma-ray dose of the downcomer P517, the recombination reaction of oxygen or hydrogen peroxide and hydrogen is promoted. Since the oxidation species that cause corrosion are consumed by the recombination reaction, SCC of the structural members in contact with the cooling water is suppressed.

[0054] In noble metal injection, a solution of a noble metal compound is injected into the cooling water supplied toward the pressure vessel P512 by the noble metal injection device P31. The noble metal compound injected into the water supply adheres noble metal particles to the surface of the structural member in contact with the cooling water. A part of the noble metal reaches the core 513 and adheres to the surface of the nuclear fuel. The noble metal catalyzes the recombination reaction between oxygen or hydrogen peroxide and hydrogen, promoting the consumption of oxidation species that are factors causing corrosion. Since the consumption of oxidation species that are factors causing corrosion is promoted, it becomes possible to suppress the injection amount of hydrogen gas.

[0055] When treating hydrogen peroxide as an equivalent amount of oxygen of 1 / 2, when hydrogen is present in a molar ratio of 2 or more, which is a stoichiometric ratio with respect to oxygen, hydrogen becomes excessive with respect to oxygen, and the recombination reaction proceeds near -500 mV vs. SHE, which is the hydrogen oxidation-reduction potential at neutral pH. When the corrosion potential of the material in contact with the cooling water is mixed and decreases to around -500 mV vs. SHE, the condition of -230 mV vs. SHE or less, which is necessary as a countermeasure against SCC, is achieved. During power operation, the electrode potential of the structural member in contact with the cooling water is measured and the corrosion potential, which is an index of the effect, is monitored so that such conditions are maintained.

[0056] The noble metal injection device P31 may be connected to the reactor purification system P530 instead of the water supply system P510. For example, the noble metal injection device P31 may be connected downstream of the regenerative heat exchanger of the purification system piping on the water supply system P510 side of the reactor purification system P530. However, when the noble metal injection device P31 is connected downstream of the regenerative heat exchanger, the distance between the injection point of the noble metal injection and the pressure vessel P512 becomes long. Since most of the injected noble metal adheres to the intermediate piping, the amount of noble metal adhering to the inside of the pressure vessel P512 decreases.

[0057] In a BWR where hydrogen injection or noble metal injection is carried out, a potential sensor for measuring the corrosion potential is installed to confirm the effects of hydrogen injection or noble metal injection. The potential sensor may be installed in a branch line branched from the bottom drain line. The bottom drain line is a pipe connected between the bottom of the pressure vessel P512 and the purification system P530. One end of the branch line is connected to the bottom drain line, and the other end is connected to a sampling line or the like branched from the recirculation system P520.

[0058] The potential sensor is attached to a T-shaped pipe branched from a pipe or a flange pipe connectable to a pipe. In the case of a BWR, the cooling water at the bottom of the furnace flows through the recirculation system P520 and the reactor purification system P530. Therefore, the corrosion potential in the water quality at the bottom of the furnace is measured by the potential sensor. In the case of an Advanced Boiling Water Reactor (ABWR), the potential sensor can be installed, for example, by connecting a flange pipe to the purification system pipe. Alternatively, it can be installed in a branch line branched from the bottom drain line. Therefore, in the case of an ABWR, the corrosion potential in the water quality above the downcomer P517 is measured by the potential sensor.

[0059] In addition, the potential sensor may be installed in a neutron instrumentation tube. The neutron instrumentation tube is installed in the lower plenum P551 and the reactor core P513. By modifying the existing neutron instrumentation tube, the potential sensor is built into the inside of the neutron instrumentation tube. An opening communicating with the outside is provided in the neutron instrumentation tube. The measurement of the corrosion potential is performed by allowing cooling water to flow into the inside of the neutron instrumentation tube through the opening.

[0060] The structural members such as the reactor equipment and piping of the BWR are mainly made of austenitic stainless steels such as SUS316L, SUS304L, SUS316L, and SUS316NG with nitrogen added to SUS316L, or nickel-based alloys. For example, the shroud P515 is made of SUS304L, SUS316L, etc. The recirculation system piping is made of SUS316NG, etc. Since high strength is required for the shroud support P523, it is made of Alloy 600. Alloy 182 and Alloy 82 are used for the welded parts and build-up parts such as nozzles and the bottom of the reactor.

[0061] <<Outline of PWR>> Next, the configuration of the PWR according to the present embodiment to which noble metal injection is applied and the configuration for performing noble metal injection on the primary coolant, which is light water, will be described.

[0062] FIG. 4 is a diagram schematically showing the configuration of the PWR. As shown in FIG. 4, the PWR includes a primary cooling system P100, a secondary cooling system P200, a chemical volume control system P300, etc. The primary cooling system P100 and the secondary cooling system P200 are thermally coupled via a steam generator P15, but are independent as coolant flow paths. The chemical volume control system P300 is connected to the primary cooling system P100.

[0063] The primary cooling system P100 includes a reactor P1, a pressurizer P14, a steam generator P15, etc. The primary cooling system P100 is connected to a pressure vessel P12. The primary cooling system P100 circulates the primary coolant between the pressure vessel P12 and the steam generator P15 via the pressurizer P14. The primary cooling system P100 communicates with the inside of the heat exchange fine tubes built in the steam generator P15.

[0064] The reactor P1 includes a pressure vessel P12 containing a reactor core P13. A fuel assembly is loaded into the reactor core P13. The fuel assembly is formed by housing a plurality of fuel rods in a lattice-shaped channel box. The fuel rods contain a plurality of fuel pellets made of nuclear fuel material in a fuel cladding tube.

[0065] The primary coolant system P100 is composed of the primary coolant piping P10 that returns from the pressure vessel P12 to the pressure vessel P12 via the pressurizer P14 and the steam generator P15 in this order, and the equipment installed in this piping. In the primary coolant piping P10, the pressurizer P14, the steam generator P15, and the circulation pump P16 are installed in this order from upstream to downstream.

[0066] Inside the pressure vessel P12, the primary coolant flows downward along the outer periphery of the reactor core P13, rises from the bottom of the pressure vessel P12, and is supplied to the reactor core P13. The primary coolant supplied to the reactor core P13 is heated by the heat of the nuclear fission reaction of the nuclear fuel material and flows out from the upper part of the pressure vessel P12 into the primary coolant piping P10.

[0067] The primary coolant flowing out of the primary coolant piping P10 flows into the steam generator P15 after passing through the connection point of the pressurizer P14. The pressurizer P14 generates steam to pressurize the primary coolant so that the primary coolant does not boil even at a high temperature exceeding about 300°C. In the steam generator P15, heat exchange occurs between the secondary coolant supplied to the outside of the thin tube and the primary coolant supplied to the inside of the thin tube, so that the secondary coolant is heated and the primary coolant is cooled. The primary coolant cooled by the heat exchange is returned to the pressure vessel P12 through the primary coolant piping P10.

[0068] The chemical volume control system P300 includes a regenerative heat exchanger P33, a non-regenerative heat exchanger P34, a resin tower P35, a volume control tank P37, a chemical injection system P46, etc. The chemical volume control system P300 is connected to the primary coolant system P100 in a bypass manner to adjust the boron concentration and volume of the primary coolant.

[0069] The chemical volume control system P300 is composed of the chemical volume control piping P30 connected to the primary coolant piping P10 and the equipment installed in this piping. In the chemical volume control piping P30, the regenerative heat exchanger P33, the non-regenerative heat exchanger P34, the resin tower P35, the volume control tank P37, and the chemical injection system P46 are installed in this order from upstream to downstream. A hydrogen injection device (not shown) for injecting hydrogen into the primary coolant is connected to the volume control tank P37.

[0070] Part of the primary coolant flowing through the primary cooling system piping P10 is extracted into the chemical volume control system piping P30. The primary coolant flows into the regenerative heat exchanger P33, the non-regenerative heat exchanger P34, the resin column P35, and the volume control tank P37 in this order. In the regenerative heat exchanger P33, heat exchange is performed between the primary coolant extracted from the primary cooling system P100 flowing into the system and the primary coolant with adjusted boron concentration and volume flowing out of the system, so that the primary coolant on the inflow side is cooled and the primary coolant on the outflow side is heated. In the non-regenerative heat exchanger P34, heat exchange is performed between the primary coolant and the cooling water P102, and the primary coolant on the inflow side is further cooled.

[0071] In the resin column P35, ion exchange treatment is performed on the primary coolant to remove impurity ions derived from metal corrosion products and the like. For the ion exchange treatment, cation exchange resins or mixed bed resins of cation exchange resins and anion exchange resins are used. In the volume control tank P37, the volume of the primary coolant is adjusted. Also, hydrogen injection is performed to adjust the dissolved hydrogen concentration of the primary coolant. The chemical injection system P46 is equipped with a device for injecting boric acid and a device for injecting lithium hydroxide. The primary coolant with adjusted boron concentration and volume is returned to the downstream side of the primary cooling system piping P10 after being heated in the regenerative heat exchanger P33.

[0072] The secondary cooling system P200 includes a water supply system P20, a main steam piping P22, a turbine P23, a condenser P24, etc. The secondary cooling system P200 is connected between the steam generator P15 and the condenser P24. The secondary cooling system P200 circulates the secondary coolant between the steam generator P15 and the condenser P24 via the turbine P23. The secondary cooling system P200 communicates with the outside of the thin tubes for heat exchange built into the steam generator P15.

[0073] The water supply system P20 of the secondary cooling system P200 is composed of a water supply pipe connecting between the condenser P24 and the steam generator P15, and equipment installed on this pipe. In the water supply pipe, a condensate purification device (not shown), a condensate booster pump, a low-pressure feedwater heater, a deaerator, a feedwater pump P27, and a high-pressure feedwater heater P29 are installed in this order from upstream to downstream.

[0074] The secondary coolant is supplied from the condenser P24 to the steam generator P15 by the feedwater pump P27. The secondary coolant is heated by the high-pressure feedwater heater P29 and then supplied to the steam generator P15. The secondary coolant is supplied to the outside of the thin tubes built into the steam generator P15. In the steam generator P15, the secondary coolant is heated by heat exchange and turns into steam.

[0075] The steam of the secondary coolant is sent from the steam generator P15 to the turbine P23 through the main steam pipe P22. By rotating the turbine P23 with the steam, the generator connected to the turbine P23 rotates to generate electricity. As the turbine P23, a high-pressure side turbine and a low-pressure side turbine are provided. A moisture separation heater is installed between the high-pressure side turbine and the low-pressure side turbine. The steam discharged from the high-pressure side turbine is separated from moisture by the moisture separation heater and then introduced into the low-pressure side turbine.

[0076] The steam discharged from the turbine P23 condenses into water in the condenser P24. The secondary coolant, which is the condensed water, is supplied to the steam generator P15 through the water supply system P20. The secondary coolant has impurities removed by a condensate purification device (not shown) and then is pressurized by a condensate booster pump. Then, after being heated by the low-pressure feedwater heater, it is deaerated by the deaerator. After that, after being further pressurized by the feedwater pump P27, it is heated by the high-pressure feedwater heater P29 and then supplied to the steam generator P15. The extraction steam extracted from the main steam pipe P22 and the turbine P23 is sent to the high-pressure feedwater heater P29 and the low-pressure feedwater heater through an extraction pipe (not shown) and used as a heating source for heating the feedwater.

[0077] In a PWR, since the primary cooling system P100 and the secondary cooling system P200 are separated, even if noble metal injection is carried out in the feed water system P20, noble metals cannot be attached to the inside of the pressure vessel P12 or the surface of the components of the primary cooling system P100. When carrying out noble metal injection in a PWR, it is necessary to determine a specific injection point different from that in a BWR.

[0078] ≪Oxidation Species Concentration in the Primary Coolant of PWR≫ Next, the results of the analysis of the oxidation species concentration in the primary coolant of a PWR will be described.

[0079] Figures 5A and 5B are diagrams showing the results of analyzing the relationship between the dissolved hydrogen concentration and the oxidation species concentration of the primary coolant of a PWR. Figures 5A and 5B show the results of calculating the oxidation species concentration of the primary coolant when the dissolved hydrogen concentration of the primary coolant is changed, using a radiation decomposition model. The radiation decomposition model is a model that simulates the behavior of reactions considering energy absorption and generation rates for various chemical species generated by the radiation decomposition of water.

[0080] Figure 5A shows the results for inside the reactor of a PWR. Figure 5B shows the results for outside the reactor of a PWR. The analysis inside the reactor was carried out for the central part of the core. In Figures 5A and 5B, the solid curve indicates the hydrogen peroxide concentration of the primary coolant. The dashed curve indicates the oxygen concentration of the primary coolant.

[0081] As shown in Figure 5A, inside the reactor of a PWR, when the dissolved hydrogen concentration of the primary coolant is 0 mL / kg, the oxygen concentration and the hydrogen peroxide concentration become high concentrations of about 1000 ppb. However, when the dissolved hydrogen concentration increases, the oxygen concentration and the hydrogen peroxide concentration decrease rapidly. The hydrogen peroxide concentration levels off at a decrease to about 1 ppb when the dissolved hydrogen concentration is 5 mL / kg or more. On the other hand, the oxygen concentration decreases to about 0.01 ppb or less when the dissolved hydrogen concentration is about 5 mL / kg, becoming a concentration that can be ignored in the primary coolant.

[0082] On the other hand, as shown in FIG. 5B, outside the reactor of a PWR, when the dissolved hydrogen concentration of the primary coolant is 0 mL / kg, the oxygen concentration and the hydrogen peroxide concentration are as high as about 1000 ppb. However, if the dissolved hydrogen concentration increases even slightly, the oxygen concentration and the hydrogen peroxide concentration will decrease rapidly. Even when the dissolved hydrogen concentration is 5 mL / kg or less, the oxygen concentration and the hydrogen peroxide concentration will decrease to about 0.0001 ppb or less, becoming negligible concentrations in the primary coolant.

[0083] Therefore, in a PWR, even if the current dissolved hydrogen concentration of 25 - 35 mL / kg set in the PWR water chemistry management guidelines is reduced to about 5 mL / kg, it can be said that the possibility of an increase in the oxygen concentration and the hydrogen peroxide concentration becoming a problem is low. As a countermeasure against PWSCC of nickel-based alloys, even if a method of lowering the dissolved hydrogen concentration of the primary coolant is used, the influence on the oxidizing species concentration is small both inside and outside the reactor. Therefore, it can be said that by the method of lowering the dissolved hydrogen concentration of the primary coolant, it is possible to achieve both the suppression of SCC of stainless steel and the suppression of SCC of nickel-based alloys.

[0084] FIG. 6 is a diagram showing the relationship between the dissolved hydrogen concentration of the primary coolant of a PWR, the electrode potential of the wetted part in contact with the primary coolant, and the composition of the oxidation-reduction system of the primary coolant. FIG. 6 shows the results of calculating the oxidizing species concentration of the primary coolant when the dissolved hydrogen concentration of the primary coolant is changed, using a radiation decomposition model. Also shown are the results of calculating the corrosion potential of the wetted part in contact with the primary coolant, using the mathematical formulas described in the literature.

[0085] As the formula for calculating the corrosion potential, the simple formula described in the literature by Takiguchi et al. (“Optimization of Dissolved Hydrogen Concentration for Control of Primary Coolant Radiolysis in Pressurized Water Reactors”, J. Nuclear Science and Technology, 2004, 41(5), p601-609) was used. This simple formula is a formula for calculating the corrosion potential of a material using the concentration ratio of the oxidizing agent and reducing agent in the core as a parameter. The composition of the redox system was determined as the concentration ratio of oxidizing agents such as oxygen, hydrogen peroxide, and radicals, and reducing agents such as hydrogen and hydrated electrons.

[0086] As shown in Fig. 6, in the current range of dissolved hydrogen concentration of 25 to 35 mL / kg set in the PWR water chemistry management guidelines, the corrosion potential of the wetted part in contact with the primary coolant decreased to about -0.2 V vs. SHE. Also, when the dissolved hydrogen concentration was about 5 mL / kg, the corrosion potential of the wetted part in contact with the primary coolant decreased to about 0 V vs. SHE. When the dissolved hydrogen concentration was lower than 5 mL / kg, the corrosion potential of the wetted part in contact with the primary coolant exceeded 0 V vs. SHE.

[0087] In Fig. 6, since the calculation results using the radiolysis model were input, there is a possibility that a higher corrosion potential was calculated than in the actual machine. In the calculation using the radiolysis model, the calculation results may be affected because it may be affected by the error of the model, parameters based on data obtained in a research reactor are used, and measurement errors occur in the acquisition of data in the research reactor.

[0088] However, the results shown in Fig. 6 suggest that when using the method of lowering the dissolved hydrogen concentration in the primary coolant as a countermeasure against PWSCC of nickel-based alloys, if the dissolved hydrogen concentration becomes extremely low, the corrosion potential of the wetted part in contact with the primary coolant can increase. Therefore, as a countermeasure against PWSCC of nickel-based alloys, the method of increasing the dissolved hydrogen concentration in the primary coolant may be more advantageous than the method of lowering the dissolved hydrogen concentration in the primary coolant.

[0089] On the other hand, when the dissolved hydrogen concentration was reduced to about 5 mL / kg, the molar ratio of hydrogen to oxygen in the primary coolant exceeded 1000, and hydrogen became a large excess relative to oxygen. When noble metal injection was carried out in the range where the dissolved hydrogen concentration was 1 mL / kg or more, a calculation result was obtained in which the electrode potential of the wetted part in contact with the primary coolant was -0.7 V vs. SHE or less. Also, when noble metal injection was carried out in the current range of dissolved hydrogen concentration of 25 to 35 mL / kg set in the water chemistry management guidelines for PWRs, a calculation result was obtained in which the electrode potential of the wetted part in contact with the primary coolant was -0.75 V vs. SHE or less.

[0090] The pH of the primary coolant of a PWR at high temperature is about 6.8 to 7.3. Therefore, it can be said that when noble metal injection is carried out in a PWR, even if the dissolved hydrogen concentration is reduced to a certain extent, the electrode potential of the wetted part in contact with the primary coolant can be reduced to about -0.8 to -0.75 V vs. SHE. According to previous research (Takao Tsuruta, Proceedings of the Spring Academic Conference, Corrosion and Prevention Association, 1983, p78), in a PWR, the condition of the electrode potential of the structural members necessary to suppress SCC of stainless steel is considered to be about -400 mV vs. SHE or less within the pH range during PWR operation.

[0091] Therefore, it can be said that when noble metal injection is performed in a PWR, the corrosion potential of the wetted portion in contact with the primary coolant can be reduced to -0.4 V vs. SHE or lower, which is necessary to prevent SCC of stainless steel. Even when a method of lowering the dissolved hydrogen concentration in the primary coolant is used as a countermeasure against PWSCC of nickel-based alloys, it can be said that SCC of stainless steel can also be suppressed by performing noble metal injection. Further, even when a method of increasing the dissolved hydrogen concentration in the primary coolant is used, it can be said that the electrode potential of the wetted portion can be maintained below the hydrogen oxidation-reduction potential by performing noble metal injection.

[0092] Therefore, according to the corrosion environment mitigation method for a pressurized water reactor and the pressurized water nuclear power plant using the same according to the present embodiment, by performing noble metal injection in a PWR, it becomes possible to control the dissolved hydrogen concentration in the primary coolant so that the electrode potential of the wetted portion does not approach the equilibrium potential of the nickel oxide formation reaction. By controlling the dissolved hydrogen concentration in the primary coolant to be sufficiently higher than the reference value of the dissolved hydrogen concentration corresponding to the equilibrium potential of the nickel oxide formation reaction, or by controlling the dissolved hydrogen concentration in the primary coolant to be sufficiently lower than the reference value of the dissolved hydrogen concentration corresponding to the equilibrium potential of the nickel oxide formation reaction, it is possible to maintain the corrosion potential of the wetted portion in contact with the primary coolant of the PWR at a low level and achieve both suppression of SCC of stainless steel and suppression of SCC of nickel-based alloys. By appropriately controlling the dissolved hydrogen concentration, SCC of the structural members in contact with the primary coolant can be suppressed, and the reliability of the PWR equipment can be improved.

[0093] ≪Injection Point of Noble Metal Injection in PWR≫ Next, the injection point of noble metal injection into the primary coolant of a PWR will be described.

[0094] FIG. 7 is a diagram showing an example of the injection point of noble metal injection in the primary cooling system of a PWR. FIG. 7 schematically shows the configuration of the primary cooling system P100 of a PWR that operates during output operation in order to show the location where noble metal injection is performed on the primary coolant of the PWR. A chemical volume control system P300 is connected to the primary cooling system P100. As shown in Fig. 7, the injection point of noble metal injection into the primary coolant of the PWR can be downstream of the regenerative heat exchanger P33 of the chemical volume control system P300.

[0095] The primary cooling system piping P10 is formed by the hot leg P17, the crossover leg P18, and the cold leg P19. The hot leg P17 is the section from the pressure vessel P12 to the steam generator P15. The crossover leg P18 is the section from the steam generator P15 to the circulation pump P16. The cold leg P19 is the section from the circulation pump P16 to the pressure vessel P12.

[0096] The primary coolant supplied to the reactor core P13 is heated by the heat of the nuclear fission reaction of the nuclear fuel material and then flows out from the upper part of the pressure vessel P12 to the hot leg P17. The primary coolant flowing out to the hot leg P17 flows into the steam generator P15 after passing through the connection point of the pressurizer P14. The primary coolant cooled in the steam generator P15 flows through the crossover leg P18, is pressurized by the circulation pump P16, then flows through the cold leg P19, and returns to the pressure vessel P12.

[0097] In this embodiment, the injection point of noble metal injection in the PWR is the chemical volume control system P300 connected to the primary cooling system P100. This is because when performing noble metal injection in the PWR, it is necessary to use the system that is operating during the output operation. Also, the injection point of noble metal injection is preferably a location close to the pressure vessel P12. Since the primary cooling system P100 of the PWR does not have a feed water system like that of the BWR, it can be said that the chemical volume control system P300 connected to the cold leg P19 close to the pressure vessel P12 is appropriate.

[0098] A part of the primary coolant cooled by heat exchange in the steam generator P15 is extracted from the crossover leg P18 into the chemical volume control system P300. The primary coolant flowing into the chemical volume control system P300 is cooled by the regenerative heat exchanger P33 and then further cooled by heat exchange with the cooling water P102 in the non-regenerative heat exchanger P34. The cooling water P102 is supplied from, for example, the auxiliary cooling system.

[0099] The cooled primary coolant is passed through resin columns P35 to remove impurity ions. As the resin columns P35, a plurality of treatment columns P35a and P35b are provided in parallel. The primary coolant from which impurity ions have been removed is passed through filter P36 to remove suspended substances and the like, and then the volume and dissolved hydrogen concentration are adjusted in volume control tank P37. Hydrogen injection is carried out in volume control tank P37.

[0100] An aqueous solution in which boric acid is dissolved or an aqueous solution in which lithium hydroxide is dissolved is injected into the primary coolant whose volume and dissolved hydrogen concentration have been adjusted. The pure water for preparing the chemical solution is supplied from pure water tank P38 by supply pump P39 toward the downstream of volume control tank P37. Boric acid is injected into the pure water from chemical tank P40 by supply pump P41. Also, lithium hydroxide is injected into the pure water from chemical injection device P42. Also, in a plant where zinc injection is carried out, zinc is injected from chemical injection device P43.

[0101] The primary coolant whose boron concentration, volume, etc. have been adjusted in chemical volume control system P300 is sent to regenerative heat exchanger P33 by injection pump P44. The primary coolant is heated in regenerative heat exchanger P33 and then returned to cold leg P19. Also, a part of the primary coolant whose boron concentration, volume, etc. have been adjusted in chemical volume control system P300 is supplied to circulation pump P16 through seal water injection pipe P45. In circulation pump P16, the primary coolant is used as the seal water of the pump.

[0102] In FIG. 7, noble metal injection device P31 is connected downstream of injection pump P44, downstream of the branch point to seal water injection pipe P45, and downstream of regenerative heat exchanger P33 in chemical volume control system piping P30. The noble metal compound injected by noble metal injection device P31 enters cold leg P19, reaches pressure vessel P12, and deposits noble metal on the surfaces of structural members in the reactor.

[0103] The downstream of the regenerative heat exchanger P33 is a section close to the pressure vessel P12. Therefore, if the injection point of the noble metal injection is downstream of the regenerative heat exchanger P33, most of the noble metal injected into the primary coolant can reach the inside of the pressure vessel P12, especially the surface of the structural members constituting the reactor core P13, while reducing the loss due to adhesion to the intermediate piping. Therefore, the recombination reaction can be promoted in the reactor where oxygen and hydrogen peroxide are easily generated.

[0104] If the noble metal injection device P31 is connected upstream of the injection pump P44, most of the noble metal injected into the primary coolant will adhere to the thin pipes built into the regenerative heat exchanger P33, making it difficult to reach the inside of the pressure vessel P12. Also, if the noble metal injection device P31 is connected upstream of the branch point to the seal water injection pipe P45, the noble metal will be supplied to the circulation pump P16, which may cause abnormal vibration, corrosion, etc. due to the adhesion of the noble metal. On the other hand, if it is downstream of the injection pump P44 and the branch point to the seal water injection pipe P45, while ensuring the soundness of the circulation pump P16, most of the noble metal can be used to promote the recombination reaction in the reactor.

[0105] When implementing noble metal injection using the chemical volume control system P300, the electrode potential of the wetted part in contact with the primary coolant is measured. The electrode potential of the wetted part in contact with the primary coolant can be measured online by installing a potential sensor P50 inside the pressure vessel P12 or in the primary cooling system P100. The potential sensor P50 includes a working electrode that measures the potential of the material in contact with the primary coolant and a reference electrode that generates a reference potential in the primary coolant. By measuring the electrode potential of the wetted part, the effect of noble metal injection is confirmed.

[0106] When implementing noble metal injection, the dissolved hydrogen concentration of the primary coolant is controlled so that the difference between the electrode potential of the wetted part in contact with the primary coolant and the equilibrium potential of the nickel oxide formation reaction is greater than a preset target potential difference. The dissolved hydrogen concentration of the primary coolant can be controlled by adjusting the injection amount of hydrogen gas in hydrogen injection, the injection amount of the solution of the noble metal compound in noble metal injection, etc.

[0107] The target potential difference can be set to any potential difference as long as the electrode potential of the liquid contact part does not approach the equilibrium potential of the nickel oxide formation reaction. For example, for the minimum value of the crack generation time and the maximum value of the crack propagation rate shown in FIG. 1, the target range of the dissolved hydrogen concentration is set so as to be sufficiently far from such minimum and maximum values. A target potential difference with an arbitrary width can be set so as to correspond to such a target range of the dissolved hydrogen concentration. The upper limit value and the lower limit value of the electrode potential set to generate the target potential difference can be compared with the measurement result of the electrode potential.

[0108] The measurement result of the electrode potential of the liquid contact part in contact with the primary coolant is compared with the upper limit value of the electrode potential based on the equilibrium potential of the nickel oxide formation reaction. When the measurement result is equal to or higher than the upper limit value, since it does not approach the equilibrium potential of the nickel oxide formation reaction, the dissolved hydrogen concentration of the primary coolant can be maintained in the current state. On the other hand, when the measurement result is less than the upper limit value, since it is close to the equilibrium potential of the nickel oxide formation reaction, control is performed to increase the dissolved hydrogen concentration of the primary coolant. Alternatively, control is performed to decrease the dissolved hydrogen concentration until the measurement result becomes equal to or lower than the lower limit value of the electrode potential.

[0109] Also, the measurement result of the electrode potential of the liquid contact part in contact with the primary coolant is compared with the lower limit value of the electrode potential based on the equilibrium potential of the nickel oxide formation reaction. When the measurement result is equal to or lower than the lower limit value, since it does not approach the equilibrium potential of the nickel oxide formation reaction, the dissolved hydrogen concentration of the primary coolant can be maintained in the current state. On the other hand, when the measurement result exceeds the lower limit value, since it is close to the equilibrium potential of the nickel oxide formation reaction, control is performed to decrease the dissolved hydrogen concentration of the primary coolant. Alternatively, control is performed to increase the dissolved hydrogen concentration until the measurement result becomes equal to or higher than the upper limit value of the electrode potential.

[0110] The locations for measuring the electrode potential of the liquid-contact part that contacts the primary coolant are preferably two or more locations including the inside of the pressure vessel P12 and the outside of the pressure vessel P12. It is preferable to constantly monitor that the corrosion potential of the liquid-contact part that contacts the primary coolant is within the range of the target potential at which SCC of stainless steel and SCC of nickel-based alloys are suppressed both inside and outside the reactor. When measurements are performed both inside and outside the reactor, the effects of hydrogen injection and noble metal injection can be compared between the inside of the reactor where oxygen and hydrogen peroxide are generated and the outside of the reactor where the concentration of oxidation species is close to steady state.

[0111] In FIG. 7, a potential sensor P50a is installed at the upper part of the reactor core P13 which is a representative point inside the reactor. Also, a potential sensor P50b is installed in a drain pipe branched from the hot leg P17 which is a representative point outside the reactor. As other installation locations, the potential sensor P50 can be installed in a branch line branched from the primary cooling system pipe P10, housed in an instrumentation pipe connected to the pressure vessel P12, or built into instrumentation equipment immersed in the primary coolant.

[0112] Noble metal injection using the chemical volume control system P300 may be carried out during the power operation of the PWR, during the cooling operation of the PWR, or during the shutdown of the PWR. However, from the viewpoint of continuously suppressing SCC of stainless steel and SCC of nickel-based alloys, it is preferably carried out at least during the power operation of the PWR.

[0113] In addition, for a PWR that performs noble metal injection, in addition to a potential sensor, it is preferable to install a measuring instrument that measures one or more of the dissolved hydrogen concentration of the primary coolant, the conductivity of the primary coolant, the pH of the primary coolant, and the ion concentration of the primary coolant. These measuring instruments are preferably installed in a branch line branched from the primary cooling system pipe P10 or the chemical volume control system pipe P30. These measuring instruments can be used for measuring the water quality parameters of the primary coolant. The water quality parameters of the primary coolant can be used for estimating the risk of SCC.

[0114] Figure 8 is a diagram showing an example of the injection point of noble metal injection in the primary cooling system of a PWR. In Figure 8, to show the location where noble metal injection is performed on the primary coolant of the PWR, the configuration of the primary cooling system P100 of the PWR operating during output operation is schematically shown. A chemical volume control system P300 is connected to the primary cooling system P100. As shown in Figure 8, the injection point of noble metal injection into the primary coolant of the PWR can also be a combination of downstream of the regenerative heat exchanger P33 and upstream of the regenerative heat exchanger P33 of the chemical volume control system P300.

[0115] In Figure 8, the noble metal injection device P31 is connected to downstream of the injection pump P44 and downstream of the branch point to the seal water injection pipe P45, and downstream of the regenerative heat exchanger P33 and upstream of the regenerative heat exchanger P33 in the chemical volume control system pipe P30. A downstream noble metal injection device P31a is connected downstream of the regenerative heat exchanger P33. An upstream noble metal injection device P31b is connected upstream of the regenerative heat exchanger P33.

[0116] The noble metal compound injected by the downstream noble metal injection device P31a enters the cold leg P19, reaches the pressure vessel P12, and deposits noble metal on the surface of structural members in the reactor. When the injection point of noble metal injection is downstream of the regenerative heat exchanger P33, most of the noble metal injected into the primary coolant can reach the inside of the pressure vessel P12, particularly the surface of the structural members constituting the reactor core P13, while reducing the loss due to adhesion to the intermediate pipes. Therefore, in the reactor where oxygen and hydrogen peroxide are easily generated, the recombination reaction can be promoted.

[0117] On the one hand, the noble metal compound injected by the upstream noble metal injector P31b adheres the noble metal to the surface of the thin tube built in the regenerative heat exchanger P33. If the injection point of the noble metal injection is upstream of the regenerative heat exchanger P33, even if the chemicals and pure water injected by the chemical volume control system P300 contain oxygen, it is possible to suppress the oxygen from being brought into the primary cooling system piping P10 and the pressure vessel P12. Since the oxygen mixed in the primary coolant is consumed before reaching the downstream injection point, the controllability of the dissolved hydrogen concentration by the downstream injection point can be improved.

[0118] Note that the downstream noble metal injector P31a and the upstream noble metal injector P31b may perform noble metal injection at the same time as each other, or may perform noble metal injection at different times from each other. Also, the downstream noble metal injector P31a and the upstream noble metal injector P31b may be controlled to the same injection amount as each other, or may be controlled to different injection amounts from each other. However, from the viewpoint of maintaining the primary coolant during output operation under sufficient reducing conditions, it is preferable to perform noble metal injection at the same time as each other.

[0119] Fig. 9 is a diagram showing an example of the injection point of noble metal injection in the primary cooling system of a PWR. Fig. 9 schematically shows the configuration of the primary cooling system P100 of the PWR that operates during the temperature reduction operation in order to show the location where noble metal injection is performed on the primary coolant of the PWR. A residual heat removal system P400 is connected to the primary cooling system P100. As shown in Fig. 9, the injection point of noble metal injection into the primary coolant of the PWR can also be downstream of the cooler P49 of the residual heat removal system P400.

[0120] The residual heat removal system P400 includes a residual heat removal system pump P48, a cooler P49, etc. The residual heat removal system P400 is connected to the primary cooling system P100 in a bypass manner to remove the residual heat of the primary coolant after the reactor is shut down. When the operation of the reactor P1 stops, the control rods are inserted into the reactor core P13, and the nuclear fission chain reaction of the nuclear fuel material stops. The heat remaining in the equipment inside the reactor core P13 and the pressure vessel P12 is removed by the evaporation of the primary coolant. However, when the temperature of the primary coolant drops to a certain extent, the heat removal efficiency by evaporation decreases. Therefore, a temperature reduction operation to operate the residual heat removal system P400 is performed.

[0121] The residual heat removal system P400 is composed of a residual heat removal system pipe P47 connected in parallel to the primary cooling system pipe P10 and equipment installed on this pipe. In the residual heat removal system pipe P47, a residual heat removal system pump P48 and a cooler P49 are installed in this order from upstream to downstream.

[0122] During the temperature reduction operation of the PWR, the primary coolant circulating in the primary cooling system pipe P10 is sent to the cooler P49 through the residual heat removal system pipe P47 by the operation of the residual heat removal system pump P48. In the cooler P49, heat exchange between the primary coolant and the cooling water P102 is performed, and the primary coolant is cooled. The cooling water P102 is supplied from, for example, the auxiliary machine cooling system. The primary coolant from which the residual heat has been removed is returned to the cold leg P19.

[0123] In FIG. 9, the noble metal injection device P31 is connected downstream of the cooler P49 in the residual heat removal system pipe P47. The noble metal injection device P31 can be connected to, for example, a vent line downstream of the cooler P49. The noble metal compound injected by the noble metal injection device P31 enters the cold leg P19, reaches the pressure vessel P12, and adheres the noble metal to the surface of the structural members in the reactor.

[0124] If the injection point of noble metal injection is the residual heat removal system P400, there is a limitation that noble metal injection can only be carried out during the operation of the residual heat removal system P400. However, the downstream of the cooler P49 is a section close to the pressure vessel P12. Therefore, most of the noble metals injected into the primary coolant can reach the inside of the pressure vessel P12, especially the surface of the structural members constituting the reactor core P13, while reducing the loss due to adhesion to the intermediate pipes. Therefore, in the reactor where oxygen and hydrogen peroxide are easily generated, the recombination reaction can be promoted.

[0125] ≪Process for Ensuring Equipment Reliability in PWR≫ Next, a process for ensuring equipment reliability to suppress SCC in PWR will be described.

[0126] Conventionally, in nuclear power plants, the introduction of Reliability Centered Maintenance (RCM) has been promoted worldwide. A typical document regarding RCM is AP913 of the Institute of Nuclear Power Operations (INPO) in the United States. In RCM, for the sake of Established Reliability (ER), important Structures, Systems and Components (SSC), the optimal maintenance method, and the timing of implementing maintenance measures are selected to formulate an optimal maintenance program.

[0127] In the pressurized water nuclear power plant according to this embodiment, it is preferable to execute a process for ensuring equipment reliability on the premise of noble metal injection. In this process for ensuring equipment reliability, by constantly monitoring the corrosion potential (ECP) of the wetted part in contact with the primary coolant, measures for the monitored part are executed over the life cycle of the plant to suppress SCC of stainless steel and SCC of nickel-based alloys.

[0128] In the process of ensuring equipment reliability, the state where SCC of stainless steel and SCC of nickel-based alloys do not affect the operation of the PWR is considered as the "should-be performance" of the PWR, and it is constantly monitored that the ECP is within the target range and the PWR is in a suitable state for operation. By introducing such an RCM, inspection costs can be reduced, the plant operation rate can be improved, human errors can be reduced, etc., and the reliability of nuclear power plants can be improved.

[0129] Figure 10 is a block diagram showing the process of ensuring equipment reliability in a PWR. As shown in Figure 10, the process of ensuring equipment reliability in the PWR according to this embodiment includes a selection step S1, a monitoring step S2, a preventive maintenance execution step S3, a corrective step S4, an improvement step S5, and a life cycle management step S6. Figure 10 is constructed based on the diagram of INPO's AP913 as a process corresponding to a PWR that performs noble metal injection.

[0130] The selection step S1 is a step of classifying equipment or parts and selecting important elements as monitoring targets. The monitoring step S2 is a step of monitoring the performance of systems or elements. The preventive maintenance execution step S3 is a step of executing preventive maintenance measures for equipment conditions, etc. The corrective step S4 is a step of executing corrective measures such as improved maintenance. The improvement step S5 is a step of executing improvement measures for continuously improving equipment reliability. The life cycle management step S6 is a step of managing life extension, etc. by monitoring for systems or elements.

[0131] In the preventive maintenance execution step S3, the corrective step S4, and the improvement step S5, various measures for reducing the risk of SCC are taken for the parts selected as monitoring targets in the selection step S1. By executing the preventive maintenance execution step S3, the corrective step S4, and the improvement step S5, the ranking of the parts with a high risk of SCC among the parts in contact with the primary coolant changes. In the life cycle management step S6, in response to such a change in ranking, while reselecting the parts to be monitored, measures for reducing the risk of SCC are repeated.

[0132] The process for ensuring equipment reliability against SCC can start from any of the steps S1 to S5 shown in Fig. 1. Usually, it starts from the selection step S1 to identify the monitoring targets to be continuously monitored. Hereinafter, the details of each step corresponding to the PWR where noble metal injection is carried out will be described.

[0133] (Selection step S1) In the selection step S1, the parts with a high risk of stress corrosion cracking (SCC) are selected from the parts in contact with the primary coolant. For example, among the parts made of stainless steel or nickel-based alloy in contact with the primary coolant, considering the water quality of the primary coolant in contact with the part and the stress generated during operation, etc., the parts with a high risk of SCC are selected. Then, the part with the highest risk of SCC is selected as the monitoring target for continuous monitoring.

[0134] The selection of the parts can be carried out considering the criteria related to the safety system of the reactor and the criteria related to the pressure boundary. The criteria related to the safety system are the criteria defined or recommended from an engineering perspective, etc., for the design of the cooling system and its related systems provided to mitigate the impact of accidents. The criteria related to the pressure boundary are the criteria defined or recommended from an engineering perspective, etc., for the equipment, piping, etc. included in a predetermined range such as the pressure boundary. Based on the importance of the PWR structures, systems, and components (SSC), the parts in contact with the primary coolant can be ranked according to the risk of SCC.

[0135] The selection of the parts can be carried out using a risk analysis tool, a human risk analysis, or both. Examples of the risk analysis tool include tools that numerically quantify risks and automatically perform quantitative analysis. Examples of the human risk analysis include analysis based on meetings, empirical analysis, deductive analysis, etc. by experts, practitioners, or groups of them. Using these, ranking can be carried out while ensuring objectivity.

[0136] As parts with a high risk of SCC, those made of stainless steel or nickel-based alloys are assumed to be parts where the water quality of the primary coolant in contact with the part and the stress generated during operation are likely to cause SCC. In particular, parts that are difficult to access and difficult to repair or replace, and equipment and structures that constitute the pressure boundary, etc. are candidates. Specific examples of such parts include measurement nozzles at the bottom of the furnace, control rod drive mechanism nozzles at the upper part of the furnace, thermocouple nozzles, etc.

[0137] After the execution of the selection process S1, it is possible to shift to the monitoring process S2 or the improvement process S5. However, usually, in order to evaluate the risk of SCC to be monitored, the process shifts to the monitoring process S2. In the monitoring process S2 and the improvement process S5, for the latest parts selected in the selection process S1, monitoring of ECP, measures to mitigate the corrosion environment, etc. are carried out.

[0138] (Monitoring Process S2) In the monitoring process S2, as a performance index of the part selected as the monitoring target, the corrosion potential (ECP) of the part is monitored. In RCM, in order to show the state of ER, it is necessary to constantly monitor specific performance indexes. In the monitoring process S2, by measuring the electrode potential of the part selected as the monitoring target, the variation with respect to the reference value of ECP, which is an index of the effect, is monitored. By monitoring the relationship between ECP and the reference value, the risk of SCC of the part selected as the monitoring target is judged. The ECP of the monitoring target is constantly monitored during the operation period of the plant.

[0139] The ECP of the part selected as the monitoring target is measured by a potential sensor. As the potential sensor, an existing potential sensor installed in the same system as the part selected as the monitoring target, a new potential sensor newly installed near the part selected as the monitoring target, etc. can be used. The potential sensor can be attached to a flange in the form of a pipe joint, a T-shaped pipe, a guide port formed on the peripheral wall of the pipe, etc.

[0140] The risk of SCC in the selected part to be monitored can be evaluated by referring to the material of the selected part to be monitored, the stress generated in the selected part to be monitored, and the water quality parameters of the primary coolant in contact with the selected part to be monitored, in addition to confirming the variation of ECP. Also, it can be evaluated by referring to the measured results of the occurrence time and progress rate of SCC measured under the same conditions as the selected part to be monitored. Based on the evaluation of the SCC risk thus obtained, the performance of the structures, systems, and components (SSC) of the PWR is monitored.

[0141] In the monitoring step S2, it is preferable to measure one or more of the dissolved hydrogen concentration of the primary coolant, the conductivity of the primary coolant, the pH of the primary coolant, the ion concentration of impurity ions such as iron ions in the primary coolant, and the amount of noble metal adhesion to the wetted part. SCC has an electrochemical corrosion aspect and is affected by the conductivity and pH of the primary coolant. Also, it involves the elution of impurity ions such as iron ions. Therefore, by measuring these, the state of SCC in the selected part to be monitored can be estimated.

[0142] After the execution of the monitoring step S2, it is possible to shift to the correction step S4, the improvement step S5, or the life cycle management step S6. For example, when the risk of SCC is high, it is possible to shift to the correction step 4. Or, when the risk of SCC is high and it is predicted that the risk will not be sufficiently reduced even if the correction step S4 is performed, it is possible to shift to the improvement step S5.

[0143] (Preventive maintenance execution step S3) In the preventive maintenance execution step S3, preventive maintenance measures against stress corrosion cracking (SCC) in the selected part to be monitored are executed. As preventive maintenance measures, at least hydrogen injection and noble metal injection are performed on the primary coolant in contact with the selected part to be monitored. Also, in the preventive maintenance execution step S3, standard inspections can be performed on the selected part to be monitored.

[0144] The injection amount of hydrogen gas and the injection amount of the noble metal compound solution are controlled such that the electrode potential of the selected site to be monitored is within a range below the target potential required to suppress SCC of stainless steel and the difference from the equilibrium potential of the nickel oxide formation reaction is greater than the target potential difference. For example, the dissolved hydrogen concentration of the primary coolant is controlled by hydrogen injection and noble metal injection such that the corrosion potential is -400 mV vs. SHE or less and the difference from the equilibrium potential of the nickel oxide formation reaction is sufficiently large.

[0145] The standard inspection is an inspection for examining the occurrence status of SCC and is carried out with actual measurement. Unlike continuous monitoring, the standard inspection is carried out intermittently when necessary. As the standard inspection, visual inspection, penetrant inspection, ultrasonic flaw detection inspection, etc. can be performed. By the standard inspection, the presence or absence of SCC occurrence, the SCC occurrence time, the SCC propagation rate, etc. are evaluated. By performing the standard inspection, it becomes possible to calibrate the correlation between the ECP continuously monitored and the occurrence status of SCC.

[0146] After the execution of the preventive maintenance execution step S3, it is possible to shift to the monitoring step S2. In the monitoring step S2, it is possible to confirm the corrosion potential, the SCC occurrence time, and the propagation rate after the execution of the preventive maintenance execution step S3.

[0147] (Rectification step S4) In the rectification step S4, rectification measures for controlling the water quality parameters of the primary coolant that comes into contact with the selected site to be monitored are executed. By controlling the water quality parameters, the SCC occurrence time and the propagation rate are reduced to suppress SCC of stainless steel and SCC of nickel-based alloys. As the rectification measures, it is possible to change the conditions of hydrogen injection and change the conditions of noble metal injection. As the water quality parameters, at least the dissolved hydrogen concentration of the primary coolant is controlled.

[0148] In the rectification step S4, corrective measures are executed such that the electrode potential of the site selected as the monitoring target is within a range equal to or lower than the target potential necessary to suppress SCC of stainless steel, and the difference from the equilibrium potential of the nickel oxide formation reaction is larger than the target potential difference. For example, the dissolved hydrogen concentration of the primary coolant is controlled by hydrogen injection and noble metal injection such that the corrosion potential becomes -400 mV vs. SHE or lower and the difference from the equilibrium potential of the nickel oxide formation reaction is within a sufficiently large range.

[0149] As water quality parameters, it is preferable to control one or more of the dissolved hydrogen concentration of the primary coolant, the conductivity of the primary coolant, the pH of the primary coolant, the ionic concentration of impurity ions such as iron ions in the primary coolant, and the amount of noble metal adhered to the wetted part. By controlling these water quality parameters, the corrosion potential, which is an index of the effect, can be controlled more accurately according to the water quality of the primary coolant. When the amount of noble metal adhered is decreasing, the corrosion potential can be adjusted within the target range by re-injecting or increasing the amount of the noble metal compound.

[0150] After the execution of the rectification step S4, it is possible to shift to the monitoring step S2, the improvement step S5, or the life cycle management step S6. For example, it is possible to shift to the monitoring step S2 to confirm the risk of SCC. Alternatively, when the risk of SCC is high and the risk does not sufficiently decrease even after performing the rectification step S4, it is possible to shift to the improvement step S5.

[0151] (Improvement step S5) In the improvement step S5, improvement measures such as replacing the material of the site selected as the monitoring target and measures to relieve the stress of the site selected as the monitoring target are executed. The improvement measures can be carried out when the difference between the electrode potential of the site selected as the monitoring target and the equilibrium potential of the nickel oxide formation reaction does not become larger than a preset target potential difference even after performing the rectification step S4.

[0152] In the improvement measures for material replacement, the material of the selected part to be monitored may be replaced with the same type of material as the current material or a different material from the current material. For example, it can be changed to a material with higher corrosion resistance than the current material. The material of the selected part to be monitored may be changed to a different metal type from the current material or to a different chemical composition among the same metal type. For example, it is possible to change from nickel-based alloy 600 to nickel-based alloy 690, or from alloy 182 to alloy 82, etc.

[0153] In the measures for reducing stress, for the material in contact with the primary coolant, processes such as applying compressive stress or relieving residual stress can be performed. Examples of the process for applying compressive stress include shot peening, water jet peening, ultrasonic shot peening, etc. Examples of the process for relieving residual stress include laser peening, heat treatment of the material in contact with the primary coolant, etc.

[0154] After the execution of the improvement process S5, it is possible to shift to the monitoring process S2, the preventive maintenance execution process S3, or the life cycle management process S6. For example, it is possible to shift to the monitoring process S2 to confirm the risk of SCC. Alternatively, it is possible to shift to the preventive maintenance execution process S3 to implement preventive maintenance measures for the material after the improvement measures.

[0155] (Life Cycle Management Process S6) In the life cycle management process S6, the risk of stress corrosion cracking (SCC) of the contact part in contact with the primary coolant of the PWR is managed over the operation period of the plant by continuously repeating the selection process S1, the monitoring process S2, the preventive maintenance execution process S3, the correction process S4, and the improvement process S5 over a long period. By managing the risk of SCC, the ER of the plant is improved.

[0156] In the life cycle management process S6, for each repetition of processes S1 to S5, the parts to be monitored are updated according to the risk of SCC. Then, for the parts selected as the monitoring targets, the monitoring of the corrosion potential, the preventive maintenance measures by hydrogen injection and noble metal injection, the corrective measures to correct the water quality parameters, the replacement of materials, and the improvement measures to relieve stress are repeated.

[0157] During the repetition of processes S1 to S5, the measurement results of the corrosion potential and the evaluation results of the occurrence time and progress rate of SCC can be accumulated as data for each part to be monitored. The data accumulated during the repetition process can be used to estimate the occurrence time and progress rate of SCC for the parts selected as the monitoring targets. By estimating the occurrence time and progress rate of SCC, the time interval of regular inspections can be extended, and the monitoring of SCC during that period can be substituted by the monitoring of the corrosion potential.

[0158] According to the above equipment reliability assurance process for SCC, by monitoring the ECP at all times for the parts with a high risk of SCC as the monitoring targets, the occurrence time and progress rate of SCC for the monitoring targets estimated based on the performance indicators can be managed to fall within the target range. Therefore, for the entire structures, systems, and components (SSC) of the nuclear power plant, the risk of SCC can be reduced to a level where preventive maintenance can be reasonably implemented (As Low As Reasonably Practicable: ALARP). In terms of configuration management, since it will be shown that each SSC is in the state it should be as designed and is exhibiting the performance it should, it can be clearly demonstrated that the equipment reliability of the plant against SCC is good.

[0159] Also, according to the equipment reliability assurance process for SCC described above, in order to select the monitoring targets according to the risks of SCC, not only the selected parts but also other parts within the system to which the selected parts belong can ensure equipment reliability. According to the application of RCM, from a long-term perspective, part of the regular inspection of the SCC situation can be replaced by continuous monitoring of the monitoring indicators. Since it is possible to extend the inspection time interval, reduce the inspection frequency, and reduce the inspection parts, while ensuring equipment reliability, the inspection can be rationalized, and the inspection volume and inspection cost can be reduced. Therefore, such an equipment reliability assurance process can simultaneously improve equipment reliability and rationalize equipment maintenance, and can simplify inspections and ensure the safety of operators.

[0160] As described above, the embodiments of the present invention have been described. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the present invention is not necessarily limited to those having all the configurations included in the above-described embodiments. Part of the configuration of one embodiment can be replaced with another configuration, part of the configuration of one embodiment can be added to another form, or part of the configuration of one embodiment can be omitted.

Explanation of Reference Numerals

[0161] P1 Reactor P10 Primary Cooling System Pipe P12 Pressure Vessel P13 Core P14 Pressurizer P15 Steam Generator P17 Hot Leg P18 Crossover Leg P19 Cold Leg P20 Feed Water System P22 Main Steam Pipe P23 Turbine P24 Condenser P27 Feed Water Pump P29 High Pressure Feed Water Heater P30 Chemical Volume Control System Pipe P31 Noble Metal Injection Device P32 Precious Metal Injection Pipe P33 Regenerative Heat Exchanger P34 Non-Regenerative Heat Exchanger P35 Resin Tower P36 Filter P37 Volume Control Tank P38 Pure Water Tank P39 Supply Pump P40 Chemical Tank P41 Supply Pump P42 Chemical Injection Device P43 Chemical Injection Device P44 Injection Pump P45 Seal Water Injection Pipe P46 Chemical Injection System P47 Waste Heat Removal System Pipe P48 Waste Heat Removal System Pump P49 Cooler P50 Potential Sensor P100 Primary Cooling System P200 Secondary Cooling System P300 Chemical Volume Control System P400 Waste Heat Removal System

Claims

1. A step of injecting hydrogen into the primary coolant of a pressurized water reactor; A step of injecting a noble metal into the primary coolant; A step of measuring the electrode potential of a liquid contact portion that comes into contact with the primary coolant at one or more locations including the inside of the pressure vessel, and includes: The injection point for injecting the noble metal is downstream of the regenerative heat exchanger of the chemical volume control system; A method for mitigating the corrosion environment of a pressurized water reactor, wherein the dissolved hydrogen concentration of the primary coolant is controlled such that the difference between the electrode potential and the equilibrium potential of the nickel oxide formation reaction is greater than a preset target potential difference.

2. A method for mitigating the corrosion environment of a pressurized water reactor according to Claim 1, wherein: The injection point for injecting the noble metal is downstream of the regenerative heat exchanger of the chemical volume control system and upstream of the regenerative heat exchanger.

3. A step of injecting hydrogen into the primary coolant of a pressurized water reactor; A step of injecting a noble metal into the primary coolant; A step of measuring the electrode potential of a liquid contact portion that comes into contact with the primary coolant at one or more locations including the inside of the pressure vessel, and includes: The injection point for injecting the noble metal is downstream of the cooler of the residual heat removal system; A method for mitigating the corrosion environment of a pressurized water reactor, wherein the dissolved hydrogen concentration of the primary coolant is controlled such that the difference between the electrode potential and the equilibrium potential of the nickel oxide formation reaction is greater than a preset target potential difference.

4. A method for mitigating the corrosion environment of a pressurized water reactor according to any one of Claims 1 to 3, wherein: The locations for measuring the electrode potential are two or more locations including the inside of the pressure vessel and the outside of the pressure vessel.

5. A method for mitigating the corrosion environment of a pressurized water reactor according to any one of Claims 1 to 3, wherein: The method includes a step of measuring one or more of the dissolved hydrogen concentration of the primary coolant, the conductivity of the primary coolant, the pH of the primary coolant, and the ion concentration of the primary coolant.

6. A method for mitigating the corrosion environment of a pressurized water reactor according to any one of Claims 1 to 3, wherein: A selection step of selecting a portion with a high risk of stress corrosion cracking among the liquid contact portions; A monitoring step of monitoring the corrosion potential of the selected portion; A preventive maintenance execution step of injecting a noble metal into the primary coolant that comes into contact with the selected portion to perform preventive maintenance against the stress corrosion cracking. An adjustment step of adjusting the water quality parameters of the primary coolant in a range where the difference between the monitored electrode potential and the equilibrium potential becomes larger than a preset target potential difference; An improvement step of, when the difference does not become larger than the target potential difference even after executing the adjustment step, replacing the material of the selected part or executing a measure to relieve the stress of the selected part; A life cycle management step of managing the risk of stress corrosion cracking of the liquid contact part by repeating the selection step, the monitoring step, the preventive maintenance execution step, the adjustment step, and the improvement step, which is a method for mitigating the corrosion environment of a pressurized water reactor.

7. A method for mitigating the corrosion environment of a pressurized water reactor according to claim 6, wherein the water quality parameters are one or more of the dissolved hydrogen concentration of the primary coolant, the conductivity of the primary coolant, the pH of the primary coolant, the ion concentration of the primary coolant, and the amount of noble metal adhered to the liquid contact part, which is a method for mitigating the corrosion environment of a pressurized water reactor.

8. A pressurized water reactor having a primary cooling system connected to a pressure vessel of a pressurized water reactor, having a steam generator, a pressurizer, and a coolant pump, and circulating a primary coolant between the pressure vessel and the steam generator via the pressurizer; A secondary cooling system connected between the steam generator and the condenser, having a turbine, and circulating a secondary coolant between the steam generator and the condenser via the turbine; A chemical volume control system connected in a bypass manner to the primary cooling system, for adjusting the boron concentration and volume of the primary coolant; A hydrogen injection device for injecting hydrogen into the primary coolant; A noble metal injection device for injecting noble metal into the primary coolant; A potential sensor for measuring the electrode potential of a liquid contact part in contact with the primary coolant, and comprising: The chemical volume control system has a regenerative heat exchanger for heat-exchanging the primary coolant extracted from the primary cooling system and the primary coolant whose boron concentration and volume have been adjusted; The noble metal injection device is connected downstream of the regenerative heat exchanger of the chemical volume control system, which is a pressurized water nuclear power plant.

9. A pressurized water nuclear power plant according to claim 8, wherein the noble metal injection device is connected downstream of the regenerative heat exchanger of the chemical volume control system and upstream of the regenerative heat exchanger, which is a pressurized water nuclear power plant.

10. It is connected to the pressure vessel of a pressurized water reactor, has a steam generator, a pressurizer, and a coolant pump, and has a primary coolant system that circulates the primary coolant between the pressure vessel and the steam generator via the pressurizer, It is connected between the steam generator and the condenser, has a turbine, and has a secondary coolant system that circulates the secondary coolant between the steam generator and the condenser via the turbine, It is connected to the primary coolant system in a bypass manner, and has a chemical volume control system that adjusts the boron concentration and volume of the primary coolant, It is connected to the primary coolant system in a bypass manner, and has a residual heat removal system that removes the residual heat of the primary coolant after the reactor is shut down, A hydrogen injection device that injects hydrogen into the primary coolant, A noble metal injection device that injects noble metals into the primary coolant, It is equipped with a potential sensor that measures the electrode potential of the wetted part in contact with the primary coolant, The residual heat removal system has a cooler that cools the primary coolant extracted from the primary coolant system, The noble metal injection device is connected downstream of the cooler of the residual heat removal system in a pressurized water nuclear power plant.

11. A pressurized water nuclear power plant according to any one of Claims 8 to 10, wherein the dissolved hydrogen concentration of the primary coolant is controlled such that the difference between the electrode potential and the equilibrium potential of the nickel oxide formation reaction is greater than a preset target potential difference.

12. A pressurized water nuclear power plant according to any one of Claims 8 to 10, wherein the potential sensor is installed at two or more locations including inside and outside the pressure vessel.

13. A pressurized water nuclear power plant according to any one of Claims 8 to 10, wherein a measuring instrument is installed to measure one or more of the dissolved hydrogen concentration of the primary coolant, the conductivity of the primary coolant, the pH of the primary coolant, and the ion concentration of the primary coolant.

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