Methods for improving the reliability of nuclear power plants

Abstract

To provide a nuclear power plant reliability improvement method for improving the reliability of a nuclear power plant by performing efficient preventive maintenance for flow accelerated corrosion through application of maintenance with importance on reliability taking a corrosion potential as a main monitoring index.SOLUTION: A nuclear power plant reliability improvement method has: a selection step of selecting a piping part with a high risk of flow accelerated corrosion; a monitoring step of monitoring the corrosion rate of the flow accelerated corrosion at the selected piping part; a preventive maintenance execution step of reducing the corrosion rate at the piping part; a correction step of controlling a water quality parameter for cooling water flowing in the piping part; an improvement step of replacing the material of the selected piping part when the corrosion rate does not fall within a target range even if the correction step is performed; and a lifecycle management step of managing the corrosion rate of piping of the nuclear power plant through the selection step, monitoring step, preventive maintenance step, correction step, and improvement step.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for improving the reliability of nuclear power plants.

Background Art

[0002] In thermal power plants and nuclear power plants, carbon steel is widely used as a pipe material. It is known that carbon steel pipes cause flow-accelerated corrosion (FAC) under specific conditions where a fluid with a high flow rate flows. FAC is caused mainly by electrochemical corrosion depending on the flow rate. Since FAC causes a reduction in the wall thickness of the pipe, preventive measures against FAC are applied.

[0003] In the case of nuclear power plants, carbon steel is widely used for the out-of-reactor pipes installed outside the pressure vessel. At the design stage of the plant, a corrosion allowance is provided in advance for carbon steel pipes through which high-flow-rate cooling water flows. The corrosion allowance is provided in anticipation of the wall thickness reduction due to FAC during the operation period of the plant. During the operation period, the wall thickness reduction of the corrosion allowance is periodically inspected.

[0004] In recent years, for carbon steel pipes, not only providing a corrosion allowance in advance but also optimizing preventive maintenance throughout the operation period is desired. From the viewpoints of improving the safety of the plant, improving the economy such as the equipment utilization rate, and coping with high aging, an efficient countermeasure against FAC is required. For carbon steel pipes that are likely to cause FAC, it is desired to optimize the inspection time and replacement time and ensure the equipment reliability with a low-cost and reasonable countermeasure.

[0005] In the case of nuclear power plants, as measures to suppress FAC, in terms of materials, the material is changed from carbon steel to corrosion-resistant materials such as low-alloy steel and stainless steel. Also, in terms of water quality management, oxygen injection into the cooling water is performed. FAC occurs when the dissolved oxygen concentration drops to 15 to 20 ppb or less. When the dissolved oxygen concentration drops, it becomes difficult to form an oxide film on the inner surface of the pipe, the protection of the base material is lost, and FAC is likely to occur.

[0006] In boiling water reactors (BWRs), oxygen injection into the feedwater and condensate systems is widely practiced both domestically and internationally. The steam generated in the reactor contains oxygen produced by the radiolysis of water. However, when the steam condenses in the condenser, much of the oxygen remains in the gas phase. As a result, the dissolved oxygen concentration in the condensate often drops to below 10 ppb. Because oxide films are less likely to form on the piping through which the condensate flows, FAC (Fuel-Aid Coefficient) is more likely to occur when high-temperature, high-velocity cooling water flows through it. FAC in such piping is mitigated by oxygen injection.

[0007] In pressurized water reactors (PWRs), oxygen is injected into the secondary system and the pH is adjusted. The cooling water in the secondary system is adjusted to the alkaline side by adding ammonia, etc. It is known that in an alkaline environment, the protection of the base material by the oxide film is less likely to be lost, and even if the dissolved oxygen concentration drops to about 10 ppb by adding hydrazine or degassing, FAC (Fatal Acquisition Coagulation) is less likely to progress.

[0008] The main indicators used for water quality management related to FAC are dissolved oxygen concentration, pH, and iron concentration of the cooling water. The water quality management targets include feedwater, condensate, reactor water, steam condensate, and suppression pool water. For the piping systems through which these cooling waters flow, the amount of wall thinning due to FAC is managed while taking into consideration factors such as the cooling water flow velocity, cooling water temperature, pipe diameter, and the geometric shape of the piping, including elbows and vents.

[0009] Traditionally, in some boiling water reactors (BWRs), hydrogen injection or precious metal injection has been used as a countermeasure against stress corrosion cracking (SCC) in materials in contact with the cooling water. As an indicator of water quality management related to SCC, electrochemical corrosion potential (ECP) is used as a higher-level indicator than dissolved oxygen concentration in the cooling water. It is known that stainless steel becomes less susceptible to SCC when its ECP falls below approximately -300 to -200 mV (vs. SHE).

[0010] Traditionally, nuclear power plants have implemented various countermeasures against various corrosion phenomena such as SCC and erosion-corrosion, based on evaluations of the corrosion rate.

[0011] Patent Document 1 describes a control method for a nuclear power plant that manages the occurrence of stress corrosion cracking in structural members that come into contact with the cooling water of the nuclear power plant. In this method, the ECP of the structural members in contact with the cooling water and the concentration of impurity ions contained in the cooling water are measured. Based on this information, the time to occur when stress corrosion cracking occurs is determined. When the time to occur is shorter than the set time, at least one of the concentrations of ECP and impurity ions is reduced.

[0012] Patent Document 2 describes a system for evaluating pipe wall thinning in power plants and the like. This system constructs a mathematical formula to express the pipe wall thinning rate using the measured pipe temperature or internal fluid temperature, the measured internal fluid moisture level, and the measured internal fluid velocity as parameters. Based on the maximum pipe wall thinning rate in the piping system, the inspection interval, the remaining pipe life, and the annual plan schedule for wall thickness measurement are determined.

[0013] Patent Document 3 describes a method for calculating and evaluating wall thinning due to erosion and corrosion of equipment and piping systems. In this method, an online wall thinning monitoring system is configured for each system in an actual plant. Based on wall thinning data from the actual plant, additional wall thinning measurement data from the actual plant is added, the function formula is corrected, and the wall thinning calculation formula database is added and modified, and the erosion and corrosion factors are ranked and weighted. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] Patent No. 5483385 [Patent Document 2] Japanese Patent Publication No. 2006-138480 [Patent Document 3] Japanese Patent Application Publication No. 8-178172 [Overview of the project] [Problems that the invention aims to solve]

[0015] In conventional nuclear power plants, measures to prevent piping corrosion were generally carried out on an individual basis. For example, in countermeasures against stress corrosion cracking (SCC), measures such as suppressing the progression of SCC, replacing pipes where SCC was found, and verifying the integrity of replaced pipes were carried out individually for each piping section. When operating the plant while tolerating SCC, the amount of progression was evaluated, and work was carried out to continuously inspect SCC that had already occurred.

[0016] The same applies to flow-accelerated corrosion (FAC), where countermeasures were implemented for each individual piping section. During the plant design phase, a corrosion allowance was set for specific piping sections to account for the amount of wall thinning due to FAC during their service life. This amount of wall thinning was individually and periodically inspected. The standard FAC management method involved predicting and managing future wall thinning based on countermeasures and inspections for each individual piping section.

[0017] However, conventional FAC management methods have the problem of involving an enormous number of piping sections. From the perspective of preventive maintenance of FAC throughout the entire plant, it is necessary to periodically inspect a certain percentage of the piping sections where FAC is likely to occur. Through these periodic inspections, it is necessary to continuously confirm that the amount of wall thinning predicted to occur within a specified number of years is within the design range. When the number and scope of items to be managed are enormous, inspection costs, plant operating rates, and human error become problematic.

[0018] Patent Document 1 describes a technique for estimating the time of stress corrosion cracking (SCC) initiation using the results of measuring the corrosion potential (ECP) of a material. However, the ECP measurement results are used as input or correction for the model equation. The accuracy of the model equation needs to be confirmed by comparing it with the results of actual measurements. Conventional techniques still suffer from the problem of having to manage an enormous number of materials.

[0019] Estimating corrosion rates based on a model equation allows for the development of future inspection plans. However, when the area under management is vast, the time interval between periodic inspections for a particular piping section becomes longer. There is no guarantee that corrosion thinning exceeding the design range will not occur by the next inspection. Estimation based solely on a model equation does not adequately demonstrate the justification for rationalizing the process by extending inspection time intervals.

[0020] In recent years, the adoption of Reliability-Centered Maintenance (RCM) has been progressing globally in nuclear power plants. A representative document on RCM is AP913 from the Institute of Nuclear Power Operations (INPO). In RCM, in order to ensure established reliability (ER), critical structures, systems and components (SSC), optimal maintenance methods, and the timing of maintenance measures are selected, and an optimal maintenance program is developed.

[0021] In the future, in nuclear power plants, from the perspectives of reducing inspection costs, improving the plant operation rate, and reducing human errors, the introduction of RCM will become important. The introduction of RCM requires the ER of SSCs throughout the plant. However, at present, there is no established ER process based on RCM for FAC that affects the reliability of nuclear power plants. The mainstream countermeasure for preventing conventional FAC is Condition Base Maintenance (CBM), and it is only individually applied for each countermeasure content and management target.

[0022] In RCM, in order to show the state of ER, it is necessary to constantly monitor specific performance indicators. However, regarding FAC, due to the absence of an established ER process based on RCM, there is currently no defined performance indicator to be constantly monitored. If there is a highly valid indicator as a performance indicator to be constantly monitored, it will be possible to ensure the validity of the estimation result when estimating the corrosion rate due to FAC. Even if the time interval of regular inspections is extended, ER of SSCs becomes possible, and thus the realization of preventive maintenance for FAC with rationalized inspections is expected.

[0023] Therefore, an object of the present invention is to provide a method for improving the reliability of a nuclear power plant that improves the reliability of a nuclear power plant by performing efficient preventive maintenance against flow-accelerated corrosion (FAC) through the application of reliability-centered maintenance (RCM) with corrosion potential (ECP) as a main monitoring indicator.

Means for Solving the Problem

[0024] To solve the above problems, a method for improving the reliability of a nuclear power plant according to the present invention includes a selection step of selecting a piping section with a high risk of flow-accelerated corrosion, a monitoring step of monitoring the corrosion rate of flow-accelerated corrosion of the selected piping section, a preventive maintenance execution step of reducing the corrosion rate of the piping section, a corrective step of controlling the water quality parameters of the cooling water flowing through the piping section, an improvement step of replacing the material of the selected piping section when the corrosion rate does not fall within the target range even after performing the corrective step, and a life cycle management step of managing the corrosion rate of the piping of the nuclear power plant by the selection step, the monitoring step, the preventive maintenance execution step, the corrective step, and the improvement step. In the monitoring step, the corrosion potential of the material in contact with the cooling water of the selected piping section is continuously monitored, and the corrosion rate of flow-accelerated corrosion of the piping section estimated based on the corrosion potential is monitored. Alternatively, in the lifecycle management step, a database is constructed for each nuclear power plant and each piping section based on at least one of the following: data representing the corrosion rate accumulated during the repetition of the selection step, the monitoring step, the preventive maintenance execution step, the corrective step, and the improvement step; data representing the wall thickness inspection results measured for the piping section; and data representing the corrosion rate of flow-accelerated corrosion collected at other plants. Based on the database, the time interval for wall thickness inspection for each piping section is extended, and the reliability of the equipment against flow-accelerated corrosion for each piping section is ensured by monitoring the corrosion potential of each piping section during the extended period.

Effect of the Invention

[0025] According to the present invention, it is possible to provide a method for improving the reliability of a nuclear power plant that improves the reliability of the nuclear power plant by performing efficient preventive maintenance against flow-accelerated corrosion (FAC) by applying reliability-centered maintenance (RCM) with the corrosion potential (ECP) as the main monitoring index.

Brief Description of the Drawings

[0026] [Figure 1] It is a block diagram showing a method for improving the reliability of a nuclear power plant according to an embodiment of the present invention. [Figure 2] It is a diagram showing an example of a nuclear power plant to which the method for improving the reliability of a nuclear power plant according to an embodiment of the present invention is applied. [Figure 3] It is a diagram showing an example of a reactor coolant purification system of a nuclear power plant. [Figure 4] It is a diagram showing a specific example of the flow of the method for improving the reliability of a nuclear power plant according to an embodiment of the present invention. [Figure 5] It is a diagram showing an application example of the flow of the method for improving the reliability of a nuclear power plant according to an embodiment of the present invention to a BWR. [Figure 6]This figure shows an example of a nuclear power plant to which the reliability improvement method according to an embodiment of the present invention is applied. [Modes for carrying out the invention]

[0027] The following describes a method for improving the reliability of a nuclear power plant according to one embodiment of the present invention. In the following figures, common components are denoted by the same reference numerals, and redundant explanations are omitted.

[0028] Methods for improving the reliability of nuclear power plants The reliability improvement method for a nuclear power plant according to this embodiment relates to a method for ensuring the reliability of structures, systems, and equipment (SSC) against flow-accelerated corrosion (FAC) by applying reliability-focused maintenance (RCM). In RCM, the monitoring target is the piping section predicted to have a high risk of FAC. The performance indicator monitored continuously can be the corrosion potential (ECP) of the monitored target. While the wall thickness of the piping section can also be used as a performance indicator, it is preferable to use the ECP.

[0029] In this specification, continuous monitoring means continuously collecting information through measurements to the extent that the trends of the object being measured can be grasped over most of the time range of the measurement cycle. Continuous monitoring may include short periods of measurement suspension for purposes such as changing measuring equipment, within a portion of the measurement cycle performed at arbitrary time intervals.

[0030] In the reliability improvement method for nuclear power plants according to this embodiment, the ER process for FAC is realized by measuring performance indicators such as ECPs that are subject to monitoring, estimating the corrosion rate of FACs (FAC rate) based on the measurement results of performance indicators such as ECPs, and implementing measures for FACs based on the estimated FAC rate throughout the plant's lifecycle. By applying RCM, the measures for FACs are optimized, the targets of periodic inspections involving actual measurements are optimized, and the time intervals for periodic inspections are optimized. Through these optimizations, efficient preventive maintenance for FACs with streamlined inspections is realized, thereby improving the reliability of nuclear power plants.

[0031] The piping sections to be monitored can be any appropriate section of the piping in a nuclear power plant. The monitored piping sections may be sections consisting of a single pipe, sections consisting of a part of a single pipe, or sections consisting of multiple interconnected pipes. Measures against FACs can be applied to the sections containing the monitored piping sections.

[0032] Figure 1 is a block diagram showing a method for improving the reliability of a nuclear power plant according to an embodiment of the present invention. As shown in Figure 1, the reliability improvement method for a nuclear power plant 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 1 is based on the diagram of INPO's AP913, with newly devised monitoring indicators and each step for FAC.

[0033] Selection step S1 is a step in which equipment or parts are classified and important elements are selected to be monitored. In selection step S1, piping parts of a nuclear power plant that are at high risk of flow-accelerated corrosion (FAC) are selected.

[0034] Monitoring step S2 is a step in which the performance of the system or elements is monitored. In monitoring step S2, performance indicators such as ECP of the piping section selected for monitoring are monitored, and the corrosion rate of flow-accelerated corrosion (FAC) of the piping section (FAC rate) estimated based on the performance indicators is monitored.

[0035] Preventive maintenance execution step S3 is the step in which preventive maintenance measures are implemented for equipment conditions, etc. In preventive maintenance execution step S3, preventive maintenance measures are implemented to reduce the corrosion rate (FAC rate) of flow-accelerated corrosion (FAC) in the piping section selected as the monitoring target.

[0036] Correction step S4 is a step in which corrective measures such as improved maintenance are implemented. In corrective step S4, corrective measures are implemented to control the water quality parameters of the cooling water flowing through the piping section selected as the monitoring target.

[0037] Improvement step S5 is a step in which corrective measures are implemented to continuously improve equipment reliability. In improvement step S5, corrective measures are implemented to replace the materials of the piping section selected for monitoring.

[0038] Lifecycle management step S6 is a step in which the lifespan extension of systems and elements is managed through monitoring. In lifecycle management step S6, the corrosion rate (FAC rate) of flow-accelerated corrosion (FAC) in the piping of nuclear power plants is managed by repeating the selection step S1, monitoring step S2, preventive maintenance execution step S3, corrective step S4, and improvement step S5.

[0039] In steps S3 to S5, various FAC measures are taken for the piping sections selected for monitoring in step S1, so that the FAC velocity falls within the target range. The execution of steps S3 to S5 reduces the FAC risk of the piping sections selected in step S1. As a result, the ranking of FAC risk among the piping sections in the plant changes. In step S6, in response to this change in ranking, the piping sections to be monitored are re-selected, and the measurement of performance indicators for the monitored sections, estimation of FAC velocity based on the measurement results of the performance indicators, and FAC measures are repeated.

[0040] Furthermore, monitoring step S2 can be performed at any appropriate stage. For example, monitoring step S2 can be performed at will after the execution of selection step S1, preventive maintenance execution step S3, corrective step S4, improvement step S5, etc. Based on the results of monitoring step S2 after the execution of each step, the selection of the next step or the necessity of performing the step can be determined.

[0041] According to the reliability improvement method for nuclear power plants of this embodiment, high-risk piping sections for FAC are monitored, and performance indicators such as ECP are constantly monitored. As a result, the FAC speed of the monitored section, estimated based on performance indicators such as ECP, can be managed to fall within the target range. Therefore, the FAC risk for the entire structure, systems, and equipment (SSC) of the nuclear power plant can be reduced to a level where preventive maintenance can be reasonably implemented (As Low As Reasonably Practicable: ALARP). In configuration management, it will be shown that each SSC is in the expected state as designed and performing as expected, thus demonstrating that the equipment reliability of the plant in terms of FAC is good.

[0042] Therefore, according to the reliability improvement method for nuclear power plants of this embodiment, the reliability of nuclear power plants can be improved by performing efficient preventive maintenance against FAC through the application of RCM. Individual measures to prevent FAC can be linked with management procedures applied to the entire plant through the introduction of RCM. Through such linkage, it becomes possible to build a process that improves the reliability of the entire plant over the long term and continuously throughout the entire lifecycle of the nuclear power plant. Conventionally, it was necessary to confirm the amount of pipe wall thinning due to FAC through periodic inspections involving actual measurements of a vast number of pipe parts. In contrast, with the application of RCM, from a long-term perspective, some of the periodic inspections of wall thinning due to FAC can be replaced by continuous monitoring of monitoring indicators such as ECP. Since it is possible to extend the time interval between inspections, reduce the frequency of inspections, and reduce the number of parts to be inspected, inspections can be streamlined and inspection costs can be reduced while ensuring equipment reliability.

[0043] The reliability improvement method for nuclear power plants according to this embodiment can be applied to any type of nuclear power plant. Examples of nuclear power plant types include boiling water reactors (BWRs), advanced boiling water reactors (ABWRs), pressurized water reactors (PWRs), natural uranium graphite-moderated carbon dioxide-cooled reactors (GCRs), advanced gas-cooled reactors (AGRs), high-temperature gas reactors (HTRs), heavy water reactors (HWRs), molten salt reactors (MSRs), and fast breeder reactors (FBRs).

[0044] <<Examples of Nuclear Power Plant Configurations>> Figure 2 shows an example of a nuclear power plant to which the reliability improvement method according to the present invention is applied. Figure 2 illustrates a boiling water reactor (BWR). As shown in Figure 2, the nuclear power plant P100 is equipped with a reactor P1, a turbine P3, a condenser P4, a reactor coolant purification system, a feedwater system, and the like. The nuclear power plant P100 is designed to allow for a combination of hydrogen injection and precious metal injection during operation.

[0045] Reactor P1 is housed inside containment vessel P11. Reactor P1 is equipped with a pressure vessel P12. Inside pressure vessel P12 is the reactor core P13, and a cylindrical shroud P15 is installed around the core P13. The shroud P15 is supported inside pressure vessel P12 by shroud supports P41 which are installed inside pressure vessel P12.

[0046] Core P13 is loaded with multiple fuel assemblies. Each fuel assembly is formed by arranging multiple fuel rods in a grid pattern inside a channel box. Each fuel rod consists of multiple fuel pellets made of nuclear fuel material, housed inside a cladding tube. Core P13 also contains a neutron instrumentation tube P38 for measuring neutron flux.

[0047] An annular downcomer P17 is formed between the inner surface of the pressure vessel P12 and the outer surface of the shroud P15. Multiple jet pumps P21 are installed at the bottom of the pressure vessel P12. The outlets of the jet pumps P21 are located below the downcomer P17. The jet pumps P21 discharge the cooling water present in the downcomer P17 of the pressure vessel P12 into the lower plenum, which is the space at the bottom of the pressure vessel P12.

[0048] The water supply system is connected to the pressure vessel P12. The water supply system consists of the water supply piping P10, water supply pump P5, condensate purification device P6, water supply pump P7, low-pressure water heater P8, high-pressure water heater P9, etc. These devices are installed on the water supply piping P10 in this order. The water supply piping P10 connects the condenser P4 to the pressure vessel P12 via a pipeline.

[0049] A hydrogen injection device P16 is connected to the feedwater system via a hydrogen injection pipe P18. The hydrogen injection pipe P18 is connected to the section of the feedwater pipe P10 between the condensate purification device P6 and the feedwater pump P7. The hydrogen injection pipe P18 is equipped with an open / close valve P19. The hydrogen injection device P16 is a device for injecting hydrogen, and it injects hydrogen gas into the cooling water supplied to the reactor P1.

[0050] A precious metal injection device P31 is connected to the feedwater system via a precious metal injection pipe P32. The precious metal injection pipe P32 is connected to the section of the feedwater pipe P10 between the high-pressure feedwater heater P9 and the pressure vessel P12. The precious metal injection pipe P32 is equipped with an open / close valve P33. The precious metal injection device P31 is a device for injecting precious metals, and it injects a solution of precious metal compounds into the cooling water supplied to the reactor P1.

[0051] The reactor coolant purification system is connected to the pressure vessel P12. The purification system consists of the bottom drain piping P34, purification system piping P20, purification system isolation valve P23, regenerative heat exchanger P25, non-regenerative heat exchanger P26, purification system pump P24, reactor water purification device P27, etc. These devices are installed on the purification system piping P20 in this order. The bottom drain piping P34 is connected to the bottom of the pressure vessel P12. The purification system piping P20 connects one end of the bottom drain piping P34 to the section of the feedwater piping P10 between the high-pressure feedwater heater P9 and the pressure vessel P12.

[0052] The purification system is connected to an oxygen injection device P43 via an oxygen injection pipe P44. The oxygen injection pipe P44 is connected to the section of the purification system piping P20 between the pressure vessel P12 and the regenerative heat exchanger P25. The oxygen injection pipe P44 is equipped with an open / close valve P45. The oxygen injection device P43 is a device for injecting oxygen and hydrogen peroxide, and it injects oxygen gas and an aqueous solution of hydrogen peroxide into the cooling water withdrawn from the reactor P1.

[0053] A recirculation system is connected to the pressure vessel P12. The recirculation system consists of recirculation piping P30 and a recirculation pump P37, etc. Recirculation piping P30 connects the pressure vessel P12 from the bottom to the top of the down cam P17 via a pipeline that passes outside the pressure vessel P12. Recirculation piping P30 branches upstream of the recirculation pump P37. The branched piping joins the purification piping P20 via an openable / closable valve P23.

[0054] A corrosion potential sensor P35a is installed in the purification system. The corrosion potential sensor P35a is installed on branch pipe P34a, which branches off from the bottom drain pipe P34. The corrosion potential sensor P35a is attached to flange P36 connected to branch pipe P34a. Branch pipe P34a is connected to the cooling water sampling line, etc.

[0055] A corrosion potential sensor P35b is installed in the recirculation system. The corrosion potential sensor P35b is installed in the recirculation system piping P30. The corrosion potential sensor P35b is attached to flange P36 connected to the recirculation system piping P30.

[0056] The corrosion potential sensors P35a and P35b measure the corrosion potential (ECP) of materials in contact with the cooling water under the conditions of the cooling water drawn from the pressure vessel P12. The corrosion potential sensors P35a and P35b are equipped with a working electrode that measures the potential of the material and a reference electrode that generates a reference potential in the cooling water. The corrosion potential sensors P35a and P35b are installed to verify the effectiveness of hydrogen injection and precious metal injection in suppressing stress corrosion cracking (SCC).

[0057] Furthermore, corrosion potential sensors to confirm the effectiveness of SCC suppression can also be installed inside the pressure vessel P12. In Figure 2, corrosion potential sensor P35c is installed in the lower plenum. Corrosion potential sensor P35d is installed in the core 13. These corrosion potential sensors P35c and P35d measure the corrosion potential (ECP) of materials in contact with the cooling water under the cooling water quality inside the pressure vessel P12.

[0058] In the nuclear power plant P100 shown in Figure 2, cooling water is supplied to the inside of the pressure vessel P12 from the feedwater piping P10 of the feedwater system. The cooling water is ejected from the feedwater sparger above the down cam P17 and supplied to the reactor core P13 by the jet pump P21. In the reactor core P13, the cooling water is heated by the heat generated by the nuclear fission of the nuclear fuel material. The heated gas-liquid two-phase flow of cooling water is separated into steam and water in a gas-liquid separator.

[0059] The water separated in the steam-water separator descends through the downcomer P17 and returns to the cooling water. Meanwhile, the steam separated in the steam-water separator is dehumidified in the steam dryer and then sent to the turbine P3 through the main steam piping P2. The turbine P3 is connected to the generator. Electricity is generated by the rotation of the turbine P3 using steam. The steam that has consumed energy in the turbine P3 is cooled in the condenser P4 and returns to the cooling water.

[0060] Cooling water is supplied from the condenser P4 to the pressure vessel 12 through the feedwater piping P10. After being discharged from the condenser P4, the cooling water is pressurized by the condensate pump P5 and introduced into the condensate purification unit P6. The condensate purification unit P6 removes impurities from the cooling water. The purified cooling water is pressurized by the feedwater pump P7 and enters the low-pressure feedwater heater P8, and then introduced into the high-pressure feedwater heater P9. In the low-pressure feedwater heater P8 and the high-pressure feedwater heater P9, the cooling water is gradually heated to a temperature suitable for the plant's thermal efficiency.

[0061] Turbine P3 and high-pressure feedwater heater P9 are connected to each other by extraction piping P14. Extracted steam, extracted from the steam supplied to turbine P3, bypasses condenser P4 and is introduced to high-pressure feedwater heater P9, and then to low-pressure feedwater heater P8. It then joins feedwater piping P10 upstream of condensate pump P5. In low-pressure feedwater heater P8 and high-pressure feedwater heater P9, the heat from the extracted steam is used to gradually heat the cooling water.

[0062] The cooling water inside pressure vessel P12 contains metal corrosion products that were mixed in during the feedwater supply, as well as metal corrosion products generated by the corrosion of the structural materials of pressure vessel P12. Therefore, a certain percentage of the cooling water inside pressure vessel P12 is removed and purified in the reactor coolant purification system. The cooling water inside pressure vessel P12 is drawn from the lower side of pressure vessel P12 towards the purification system piping P20 by the purification system pump P24.

[0063] Cooling water withdrawn from pressure vessel P12 is introduced into regenerative heat exchanger P25, and then into non-regenerative heat exchanger P26. In regenerative heat exchanger P25 and non-regenerative heat exchanger P26, the cooling water is cooled to approximately 50°C, a temperature suitable for purification. The cooled cooling water is then introduced into reactor water purification unit P27. Reactor water purification unit P27 removes impurities such as metal corrosion products contained in the cooling water. The purified cooling water is then introduced into regenerative heat exchanger P25. In regenerative heat exchanger P25, the cooling water is heated to a temperature suitable for the plant's thermal efficiency.

[0064] When reactor P1 is shut down, all control rods are inserted into core P13. The insertion of all control rods stops the nuclear fission chain reaction of the nuclear fuel material. The heat remaining in the equipment inside core P13 and pressure vessel P12 is removed by the evaporation of the cooling water. However, once the temperature of the cooling water drops to a certain level, the efficiency of heat removal by evaporation decreases. Therefore, when the temperature of the cooling water reaches about 150°C, the residual heat removal system is activated. The residual heat removal system cools the core structure and equipment by performing heat exchange with the pressure suppression pool water and spraying the cooling water.

[0065] At the nuclear power plant P100, hydrogen injection, or a combination of hydrogen injection and precious metal injection, is used as a measure to suppress stress corrosion cracking (SCC) of materials in contact with the cooling water. In hydrogen injection, hydrogen gas is injected into the cooling water supplied to reactor P1 by hydrogen injection device P16. In precious metal injection, a solution of precious metal compounds is injected into the cooling water supplied to reactor P1 by precious metal injection device P31.

[0066] Factors involved in the occurrence of SCC include mechanical factors such as tensile stress applied to the material, environmental factors to which the material is exposed, and material factors such as the chemical composition of the material. In cooling water, oxygen and hydrogen peroxide can be generated by the radiolysis of water. Oxygen and hydrogen peroxide are environmental factors of SCC, and the higher the concentration, the more SCC progresses. Therefore, in nuclear power plant P100, hydrogen injection and precious metal injection are performed during operation to suppress SCC in materials that come into contact with the cooling water.

[0067] Hydrogen injection is a technique that involves injecting hydrogen gas into the cooling water, causing it to react with oxygen and hydrogen peroxide in the cooling water to return them to water. The hydrogen injected into the feedwater system is ejected along with the cooling water from the feedwater sparger above the downcomer P17 and mixes with the reactor water inside the pressure vessel P12. Because the gamma ray dose rate around the downcomer P17 is moderate, the recombination reaction between oxygen and hydrogen peroxide is promoted. As oxygen and hydrogen peroxide are consumed by the recombination reaction, the corrosive environment is mitigated.

[0068] Precious metal injection is a technique that involves injecting a solution of a precious metal compound into the cooling water to deposit the precious metal onto the surface of materials that come into contact with the cooling water. Platinum group compounds, such as sodium hexahydroxoplatinate, are used as the precious metal compound. The solution of the platinum group compound becomes a colloidal solution of platinum group oxides upon gamma irradiation, depositing platinum group elements onto the surface of materials that come into contact with the cooling water. Because the platinum group elements catalyze recombination reactions, the corrosive environment is mitigated with a small amount of hydrogen injection.

[0069] Hydrogen injection or precious metal injection lowers the corrosion potential (ECP) of materials in contact with the cooling water. In the case of precious metal injection, if hydrogen is present at a ratio of more than twice the stoichiometric ratio of water to oxygen (H:O=2:1), a recombination reaction proceeds at around -500mV (vs. SHE). The combination of the recombination reaction and material corrosion lowers the potential to around -500mV (vs. SHE). Hydrogen injection or precious metal injection, accompanied by a decrease in ECP, suppresses the occurrence and progression of SCC.

[0070] Without hydrogen injection, the dissolved oxygen concentration in the cooling water is typically around several hundred ppb. However, with hydrogen injection, the dissolved oxygen concentration drops to a few ppb. This decrease in dissolved oxygen concentration reduces the susceptibility of materials in contact with the cooling water to SCC (Steel Chloride Cancellation). However, hydrogen injection or precious metal injection further reduces the dissolved oxygen concentration, making FAC (Fiber Acquisition Coagulation) more likely to occur. FAC significantly thins carbon steel piping. Carbon steel piping is primarily used in the external sections of pressure vessel P12.

[0071] Figure 3 shows an example of a reactor coolant purification system in a nuclear power plant. Figure 3 shows an example of the configuration of the reactor coolant purification system of a BWR shown in Figure 2. As shown in Figure 3, in the purification system of the nuclear power plant P100, the regenerative heat exchanger P25 and the non-regenerative heat exchanger P26 are each composed of multiple stages of heat exchangers.

[0072] The regenerative heat exchanger P25 consists of three stages of heat exchangers P25a, P25b, and P25c. The non-regenerative heat exchanger P26 consists of two stages of heat exchangers P26a and P26b. The heat exchangers are connected to each other via connecting pipes P42a, P42b, P42c, P42d, and P42e. Generally, the regenerative heat exchanger P25 consists of three stages, and the non-regenerative heat exchanger P26 consists of two stages. However, the number of stages of the heat exchangers is not particularly limited.

[0073] Cooling water extracted from reactor P1 is sent to regenerative heat exchanger P25 via purification system piping P20. The cooling water is cooled in heat exchange with the cooling water purified in reactor water purification system P27 in the heat exchangers P25a, P25b, and P25c of each stage. It is then introduced to non-regenerative heat exchanger P26 via connecting pipe P42d. The cooling water is cooled in heat exchange with auxiliary cooling water in the heat exchangers P26a and P26b of each stage. It is then pressurized by purification system pump P24 and introduced to reactor water purification system P27.

[0074] The cooling water is cooled in the regenerative heat exchanger P25 and the non-regenerative heat exchanger P26, and then impurities are removed in the reactor water purification unit P27. The purified cooling water is sent to the regenerative heat exchanger P25. In the heat exchangers P25a, P25b, and P25c of each stage of the regenerative heat exchanger P25, the cooling water is heated by heat exchange with cooling water withdrawn from the reactor P1. It is then sent to the feedwater piping P10 and supplied to the reactor P1 together with condensate.

[0075] In the regenerative heat exchanger P25 and the non-regenerative heat exchanger P26, the cooling water drawn from the reactor P1 is cooled to a temperature suitable for purification. In the regenerative heat exchanger P25, the cooling water purified in the reactor water purification system P27 is heated to a temperature suitable for the plant's thermal efficiency. The non-regenerative heat exchanger P26 is supplied with auxiliary cooling water from the auxiliary cooling system. The auxiliary cooling water circulates through the auxiliary cooling system, passing through each stage of the non-regenerative heat exchanger P26, and is cooled by heat exchange with seawater, etc.

[0076] Flow-accelerated corrosion (FAC) is an electrochemical corrosion process involving mass transfer resulting from the dissolution of oxide films. FAC depends on the flow velocity of the cooling water, as well as the chemical composition of the piping, the quality of the cooling water, the temperature of the cooling water, and the hydrodynamic properties of the cooling water in the piping. The FAC rate increases with increasing cooling water velocity. It also increases with higher cooling water temperatures, generally reaching a maximum value around 130-150°C. Furthermore, it accelerates when the dissolved oxygen concentration of the cooling water falls below approximately 15 ppb.

[0077] During operation of the nuclear power plant P100, the temperature of the cooling water drawn from the pressure vessel P12 is high, around 280°C. The temperature of the cooling water drawn into the purification system decreases as it passes through each stage of the regenerative heat exchanger P25 and the non-regenerative heat exchanger P26. Downstream of the non-regenerative heat exchanger P26, the temperature of the cooling water falls below 80°C. However, it remains relatively high upstream and downstream of the purification system. Furthermore, the flow velocity in the purification system is high, around several m / s, due to the small diameter of the pipes.

[0078] Furthermore, during the operation of the P100 nuclear power plant, the injection of hydrogen or precious metals reduces the oxygen and hydrogen peroxide concentrations in the cooling water in the purification system. In particular, the injection of precious metals makes it easier for excess hydrogen to be generated, causing the amount of hydrogen to exceed the stoichiometric ratio of oxygen to water. The oxygen and hydrogen peroxide in the cooling water may be completely consumed by recombination reactions before reaching the downstream side of the purification system.

[0079] Furthermore, the connecting pipes P42a, P42b, P42c, P42d, and P42e, as well as the sections of the purification system piping P20 that are outside the pressure vessel P12, are generally made of carbon steel. These pipes may have structures that affect the flow velocity. Examples of structures that affect the flow velocity include elbows, vents, T-junctions, orifices, and valves.

[0080] The piping that makes up the purification system, in particular the heating-side connecting pipe P42a that connects the first-stage heat exchanger P25a and the second-stage heat exchanger P25b of the regenerative heat exchanger P25, the heating-side connecting pipe P42b that connects the second-stage heat exchanger P25b and the third-stage heat exchanger P25c, and the cooling-side connecting pipe P42f that connects the second-stage heat exchanger P25b and the first-stage heat exchanger P25a, are made of carbon steel and are exposed to high flow rates and high temperatures, making them piping sections with a high risk of FAC (Fuel Acid Accident) when the oxygen concentration of the cooling water decreases.

[0081] In nuclear power plant P100, piping sections with a high risk of FAC (Fuel-Assisted Accident) can be selected for monitoring, and measures to suppress FAC in these monitored areas, such as oxygen injection or hydrogen peroxide injection, can be performed. Oxygen injection and hydrogen peroxide injection can be performed during plant operation, similar to hydrogen injection and precious metal injection.

[0082] Oxygen injection is a technique that increases the oxygen concentration of cooling water by injecting oxygen gas into the cooling water. Hydrogen peroxide injection is a technique that increases the hydrogen peroxide concentration of cooling water by injecting an aqueous solution of hydrogen peroxide into the cooling water. When the concentration of the oxidizing agent in the cooling water is increased, an oxide film is formed on the surface of the material that comes into contact with the cooling water.

[0083] For example, in the case of carbon steel, hematite (Fe2O3) can be formed. Compared to magnetite (Fe3O4), hematite has lower solubility and forms a dense oxide film. By increasing the concentration of the oxidizing agent in the cooling water, the oxide film is stably maintained even if parameters related to mass transfer, such as flow rate, increase, thus suppressing the FAC rate of the material in contact with the cooling water.

[0084] In Figure 3, a corrosion potential sensor P35f is installed in the heat-dissipating connecting pipe P42a that connects the first stage heat exchanger P25a and the second stage heat exchanger P25b of the regenerative heat exchanger P25. In addition, a corrosion potential sensor P35g is installed in the heat-receiving connecting pipe P42f that connects the second stage heat exchanger P25b and the first stage heat exchanger P25a of the regenerative heat exchanger P25.

[0085] Corrosion potential sensors P35f and P35g are sensors for measuring the corrosion potential (ECP) of materials in contact with cooling water. Corrosion potential sensors P35f and P35g consist of a working electrode for measuring the potential of the material and a reference electrode for generating a reference potential in the cooling water. Corrosion potential sensors P35f and P35g can be installed to monitor the FAC rate of connecting pipes P42a and P42f.

[0086] ≪ER process for FAC≫ Next, the equipment reliability assurance (ER) process for flow-accelerated corrosion (FAC) implemented in the reliability improvement method for nuclear power plants according to this embodiment will be described. In the following description, the application of the ER process for FAC to a BWR will be used as an example. However, similar applications are possible to PWRs and other types of plants by appropriately selecting the monitoring targets.

[0087] The ER process for FAC can be started from any of the steps S1 to S5 shown in Figure 1. Typically, it starts from selection step S1 to identify the monitoring targets to be continuously monitored.

[0088] (Selection step S1) In selection step S1, piping sections at high risk of flow-accelerated corrosion (FAC) in a nuclear power plant are selected. In this step, piping systems at high risk of FAC are selected as monitoring targets for continuous monitoring. Then, from among the piping sections included in those piping systems, piping sections at high risk of FAC are selected.

[0089] The selection of piping systems and components is based on parameters affecting FAC and the importance classification of structures, systems, and equipment (SSC). Based on these, piping systems and components are ranked according to their FAC risk. Then, piping systems with high FAC risk, and the piping components within those systems with the highest FAC risk, are selected as targets for continuous monitoring.

[0090] Parameters that affect FAC include the flow velocity of the cooling water, the chemical composition of the piping, the quality of the cooling water, the temperature of the cooling water, and the hydrodynamic properties of the cooling water in the piping. Regarding the quality of the cooling water, these include oxygen concentration, hydrogen peroxide concentration, electrical conductivity, pH, dissolved hydrogen concentration, and the concentration of impurities such as iron ions.

[0091] The importance classification is a system that categorizes the importance of Safety Safety Components (SSCs) that possess safety functions. In this classification, SSCs in nuclear power plants are divided into Anomaly Prevention Systems (PS) and Anomaly Mitigation Systems (MS). Both PS and MS are further classified into multiple classes (1-3) according to the importance of their safety functions. Based on this importance classification, it is possible to prioritize monitoring targets from the perspective of nuclear power plant safety.

[0092] The selection of piping systems and components can be carried out using risk analysis tools, human risk analysis, or a combination of both. Risk analysis tools include those that quantify risk and perform automated quantitative analysis. Human risk analysis includes analyses based on experts, practitioners, meetings of these groups, empirical analysis, and deductive analysis. By using these methods, piping systems and components can be appropriately ranked according to FAC risk while ensuring objectivity.

[0093] Piping sections selected for monitoring include those with high cooling water flow velocities, those with cooling water temperatures close to 130-150°C, those with low concentrations of oxidizing agents in the cooling water, and those made of carbon steel. The piping system to be selected includes these sections, has a long section outside the pressure vessel P12, and a long section made of carbon steel. For example, a water purification system or a water supply system could be selected.

[0094] The piping sections selected for monitoring are preferably the piping that constitutes the purification system, in particular the heating-side connecting pipe P42a that connects the first-stage heat exchanger P25a and the second-stage heat exchanger P25b of the regenerative heat exchanger P25, the heating-side connecting pipe P42b that connects the second-stage heat exchanger P25b and the third-stage heat exchanger P25c, and the cooling-side connecting pipe P42f that connects the second-stage heat exchanger P25b and the first-stage heat exchanger P25a.

[0095] After the selection step S1 is completed, the system can proceed to any of steps S2 to S5. However, typically, the system proceeds to monitoring step S2 in order to evaluate the FAC speed of the monitored system. Steps S2 to S5 involve monitoring the ECP and taking action against the FAC for the most recent piping system and piping section selected in step S1.

[0096] (Monitoring step S2) In monitoring step S2, performance indicators of the piping section selected for monitoring are monitored, and the flow-accelerated corrosion (FAC) rate (FAC rate) of the piping section, estimated based on the performance indicators, is monitored. Examples of performance indicators include corrosion potential (ECP). Performance indicators such as ECP of the monitored section are continuously monitored throughout the plant's service life. The FAC rate is estimated based on performance indicators such as ECP.

[0097] The ECP of piping sections can be measured using a corrosion potential sensor. As long as the corrosion potential sensor has the required heat and pressure resistance, is equipped with a reference electrode that generates a reference potential, and is operable within the plant's operating temperature range, any suitable electrode-type sensor can be used. The corrosion potential sensor can be installed at appropriate locations depending on the piping section selected for monitoring.

[0098] ECP monitoring may be performed using existing corrosion potential sensors installed near the selected piping section, or using new corrosion potential sensors newly installed near the selected piping section. Corrosion potential sensors can be attached to pipe fitting flanges, T-joints, or guide openings formed in the perimeter wall of pipes. Flanges and T-joints can be connected in the middle of the piping system to which the selected piping section belongs.

[0099] Since FAC rate has an electrochemical corrosion aspect, it can be expressed as a function of the corrosion current at the corrosion potential (ECP) of the material. By determining the correlation between FAC rate and ECP for each piping section, the FAC rate can be estimated by applying the ECP monitoring results to this correlation.

[0100] The correlation between FAC speed and ECP can be determined by measuring the wall thickness and ECP of the selected piping section. While the ECP of the selected piping section is continuously monitored, the wall thickness of the piping section can be measured intermittently through inspections involving actual measurements. Since ECP is an important parameter related to FAC speed, continuously monitoring at least the ECP allows for highly accurate estimation of FAC speed.

[0101] Since FAC velocity involves aspects of mass transfer, it can also be estimated in conjunction with other parameters that affect FAC. These other parameters affecting FAC may be used to calibrate the correlation between FAC velocity and ECP, or as input for hydrodynamic analysis that incorporates mass transfer. The correlation between FAC velocity and ECP can also be determined by multivariate regression using one or more of the parameters that affect FAC. Hydrodynamic analysis can be performed using one or more of the parameters that affect FAC as input.

[0102] For example, in fluid dynamics analysis, a two-dimensional calculation system can be constructed that represents the cross-section of the piping section selected for monitoring. In such a calculation system, a basic equation is calculated using one or more of the parameters affecting FAC as input. Examples of basic equations include model equations representing mass transfer and model equations representing fluid dynamics behavior. In fluid dynamics analysis, the effects of erosion-corrosion, droplet impact erosion, etc., can also be taken into account.

[0103] Solving these fundamental equations simultaneously allows us to estimate mass transfer and FAC rates at the material surface. Since the results of the mass transfer analysis reflect the electrochemical properties of the material surface, they can be validated by comparison with ECP. By continuously monitoring ECP and using other parameters that affect FAC, the validity of the FAC rate estimated based on ECP can be ensured.

[0104] It is preferable to estimate the FAC rate in conjunction with ECP by using at least one of the following parameters: oxygen concentration of the cooling water, hydrogen peroxide concentration of the cooling water, electrical conductivity of the cooling water, iron ion concentration of the cooling water, and pH of the cooling water. The oxygen concentration and hydrogen peroxide concentration of the cooling water affect the stability of the oxide film and the solubility of iron constituting the piping. In addition, the electrical conductivity of the cooling water is related to the concentration of iron ions, etc., and is constantly monitored by the equipment installed. Therefore, by using these parameters in combination, the FAC rate can be estimated with high accuracy within a reasonable inspection cost range.

[0105] One of the features of the reliability improvement method for nuclear power plants according to this embodiment is that performance indicators such as ECP of piping components are used as continuously monitored indicators. This is based on the definition that the state in which FAC does not affect the operation of the BWR is considered the "ideal performance" of the BWR in terms of FAC, and that it is important for achieving ER to clearly demonstrate through continuous monitoring that performance indicators such as ECP are in a state suitable for suppressing FAC.

[0106] The FAC rate is affected by the corrosion resistance of the material. However, the corrosion resistance of the material cannot be constantly monitored during plant operation and must be analyzed during plant construction or shutdown. Therefore, in the reliability improvement method for nuclear power plants according to this embodiment, the chemical composition of the material, such as the amount of Cr, is not used as a constantly monitored indicator. However, the chemical composition of the material, such as the amount of Cr, can be measured intermittently through inspections involving actual measurements.

[0107] In monitoring step S2, it is possible to confirm whether the FAC rate estimated based on performance indicators such as ECP falls within an acceptable target range for preventive maintenance. The FAC rate estimated based on performance indicators can be compared with a preset threshold that defines the upper limit of the target range for the FAC rate. The threshold can be set to any value depending on the size of the corrosion allowance provided in the piping section being monitored, the length of the plant's service life, etc. The size of the corrosion allowance is set based on the location of the piping section, the material of the piping section, the acceptableness of the FAC, etc.

[0108] After the execution of monitoring step S2, the system can proceed to one of steps S3 to S6, depending on the result of comparing the estimated FAC speed based on performance metrics with the threshold.

[0109] For example, if the FAC speed is below the threshold, the FAC speed falls within the target range, and the process can proceed to lifecycle management step S6. On the other hand, if the FAC speed exceeds the threshold, the FAC speed does not fall within the target range, and the process can proceed to corrective step S4.

[0110] Alternatively, if the FAC speed exceeds the threshold and it is predicted that the FAC speed will not fall within the target range even after performing the corrective step S4, the process can proceed to the improvement step S5. Furthermore, if the FAC speed exceeds the threshold, it is predicted that the FAC speed will not fall within the target range even after performing the corrective step S4, and preventive maintenance measures have not been implemented, the process can proceed to the preventive maintenance execution step S3.

[0111] (Preventive maintenance execution step S3) In preventive maintenance execution step S3, preventive maintenance measures are implemented to reduce the corrosion rate of flow-accelerated corrosion (FAC) in the piping sections selected for monitoring. These preventive maintenance measures include oxygen injection, hydrogen peroxide injection, and titanium oxide injection, which are technologies that mitigate the corrosive environment of FAC. In addition, in preventive maintenance execution step S3, standard inspections can be performed on the piping sections selected for monitoring, as well as on other piping sections.

[0112] Oxygen injection, hydrogen peroxide injection, and titanium dioxide injection can be performed for each piping system. In the nuclear power plant P100 shown in Figure 2, the oxygen injection device P43 is connected to the purification system piping P20. Oxygen injection by the oxygen injection device P43 is a preventive maintenance measure for the piping section belonging to the purification system. In practice, the oxygen injection device is used to protect the feedwater piping P10 and is installed in the condensate system, similar to the hydrogen injection device P16. Hydrogen peroxide injection devices and titanium dioxide injection devices can also be connected to the purification system piping P20 and the feedwater piping P10.

[0113] Preventive maintenance measures can include oxygen injection, hydrogen peroxide injection, titanium dioxide injection, or any combination of one or more of these. Since hydrogen peroxide and titanium dioxide solutions are provided as liquids, gas cylinders and gas piping are not required, making handling and piping easier. Also, unlike gases, they can be easily pressurized, allowing them to be injected into each system using small pumps.

[0114] The injection rates of oxygen gas, hydrogen peroxide solution, and titanium dioxide can be set so that the FAC rate of the selected piping section falls within an acceptable target range for preventive maintenance. The behavior of the FAC rate in relation to these injection rates can be confirmed in advance by measuring the wall thickness of the piping section and performing hydrodynamic analysis that takes mass transfer into account.

[0115] Injecting oxygen or hydrogen peroxide can increase the oxygen or hydrogen peroxide concentration of the cooling water in the piping system to which the selected piping section belongs. This makes it easier for an oxide film to form on the surface of the piping section belonging to that system, thus reducing the FAC rate in the selected piping section and nearby piping sections. Injecting titanium dioxide also reduces the solubility of the oxide film, thus reducing the FAC rate. By performing these injections, the integrity of the piping section against FAC can be ensured not only for the selected piping section but also for nearby piping sections.

[0116] In preventive maintenance execution step S3, standard inspections involving actual measurements can be performed on the piping sections selected for monitoring, as well as on other piping sections. Unlike continuous monitoring, standard inspections are performed intermittently as needed. Standard inspections include measuring the wall thickness of the piping sections. Wall thickness can be measured using, for example, an ultrasonic thickness gauge. The composition of the materials is usually managed using the material composition table from the time of plant construction. However, if a problem occurs due to FAC, the chemical composition of the materials and oxide film is also analyzed. The chemical composition can be measured using, for example, X-ray fluorescence (XRF) analysis. Examples of chemical composition include the amount of Cr in the base material and the amount of Fe and O in the oxide film.

[0117] Standard inspection results can be accumulated as data for each inspection during the process of repeated, intermittent inspections. The data accumulated during the repetition process can be compiled into a database as time-series data for each piping section. The database of wall thickness inspection results can be used to verify the FAC rate estimated based on performance indicators. The database of material chemical composition inspection results can be used to calibrate the correlation between FAC rate and ECP, and as input for hydrodynamic analysis.

[0118] According to preventive maintenance execution step S3, preventive measures can be taken to suppress the occurrence and progression of FAC in the piping sections selected for monitoring. After the execution of preventive maintenance execution step S3, the system can proceed to monitoring step S2. In monitoring step S2, the FAC rate after the execution of preventive maintenance execution step S3 can be confirmed.

[0119] (Correction step S4) In corrective step S4, corrective measures are implemented to control the water quality parameters of the cooling water flowing through the piping section selected for monitoring. In this step, water quality parameters related to the cooling water quality, which are among the parameters that affect FAC, are controlled to reduce the FAC rate of the piping section selected for monitoring. It is preferable that corrective step S4 be performed after the execution of preventive maintenance execution step S3, via monitoring step S2.

[0120] Water quality parameters include the oxygen concentration of the cooling water, the hydrogen peroxide concentration of the cooling water, the electrical conductivity of the cooling water, the pH of the cooling water, the dissolved hydrogen concentration of the cooling water, and the concentration of impurities such as iron ions in the cooling water. Of these water quality parameters, it is preferable to control at least one of the oxygen concentration, hydrogen peroxide concentration, and electrical conductivity of the cooling water, due to their significant impact on FAC and their high controllability.

[0121] The oxygen and hydrogen peroxide concentrations in the cooling water can be increased by increasing the amount of oxygen gas injected into the cooling water, increasing the amount of aqueous hydrogen peroxide injected, or decreasing the amount of hydrogen injected within a range that does not affect SCC. Conversely, the oxygen and hydrogen peroxide concentrations in the cooling water can be decreased by decreasing the amount of oxygen gas injected into the cooling water, decreasing the amount of aqueous hydrogen peroxide injected, or increasing the amount of hydrogen injected within a range that does not affect radioactive behavior.

[0122] Furthermore, the oxygen and hydrogen peroxide concentrations in the cooling water can be controlled through reactor operation management to the extent that it does not affect operation. For example, they can be increased by reducing the reactor output, increasing the amount of cooling water in the reactor, or decreasing the temperature of the cooling water in the reactor. Conversely, they can be decreased by increasing the reactor output, decreasing the amount of cooling water in the reactor, or increasing the temperature of the cooling water in the reactor.

[0123] The electrical conductivity of the cooling water can be controlled by adjusting the concentration of impurities and the amount of titanium dioxide injected. Examples of impurities include metal ions such as iron ions, chloride ions, and sulfate ions. The concentration of impurities can be analyzed offline by sampling through a sampling line. The concentration of impurities can also be controlled by adjusting the purification capacity of condensate purification systems, reactor water purification systems, etc.

[0124] In corrective step S4, the cooling water quality parameters are controlled so that the FAC rate estimated based on the ECP falls within an acceptable target range for preventive maintenance. In corrective step S4, the FAC rate corrected by controlling the water quality parameters can be compared with a preset threshold. As with monitoring step S2, any numerical value can be set as the threshold.

[0125] According to the corrective step S4, highly responsive measures can be taken to immediately suppress the occurrence and progression of FAC in the piping section selected for monitoring. After executing the corrective step S4, the process can proceed to monitoring step S2 in order to evaluate the FAC rate of the monitored section. Subsequently, depending on the result of comparing the FAC rate corrected by controlling the water quality parameters with the threshold, the process can proceed to one of steps S5 to S6.

[0126] For example, if the FAC rate corrected by controlling water quality parameters exceeds a threshold, the process can return to correction step S4. In the subsequent correction step S4, the control of the water quality parameters can be modified so that the FAC rate falls within an acceptable target range for preventive maintenance.

[0127] If, as a result of the change in control settings, the FAC rate corrected by the control of water quality parameters is below the threshold, the FAC rate falls within the target range, and the process can proceed to life cycle management step S6. On the other hand, if the FAC rate corrected by the control of water quality parameters exceeds the threshold, the FAC rate does not fall within the target range, and the process can proceed to improvement step S5.

[0128] Controlling water quality parameters increases the oxygen and hydrogen peroxide concentrations in the cooling water, weakening the effectiveness of hydrogen injection and precious metal injection in suppressing stress corrosion cracking (SCC). Therefore, there are upper limits to increasing the injection rate of oxygen gas or aqueous hydrogen peroxide solution for corrosion prevention maintenance. Furthermore, there are range limitations to control through reactor operation management and control of the electrical conductivity of the cooling water. For this reason, if the FAC rate does not fall within the target range even after changing the control settings of the water quality parameters, the process proceeds to improvement step S5, where materials in the piping section are replaced.

[0129] In corrective step S4, the cooling water quality parameters are controlled so that the FAC rate estimated based on the performance indicator falls within an acceptable target range for preventive maintenance. However, the control of the water quality parameters can also be performed using ECP as the indicator instead of FAC rate. When ECP is used as the indicator, the cooling water quality parameters can be controlled so that ECP falls within a predetermined target range.

[0130] (Improvement Step S5) In improvement step S5, corrective measures are implemented to replace the material of the piping section selected for monitoring. Replacement of piping material can be performed if the flow-accelerated corrosion (FAC) corrosion rate does not fall within the target range after corrective step S4, or if continuous measures are required after corrective step S4. Replacement of piping material can also be suggested by the computer managing the ER process for FAC, based on the FAC rate monitoring results.

[0131] The materials used in piping sections can be replaced with appropriate materials depending on the required purpose and material cost, as long as integrity against FAC (Factory Air Conditioning) is ensured. However, the longer the piping section to be replaced, the higher the material cost. Also, materials with higher corrosion resistance are generally more expensive. Furthermore, weldability and the necessity of welding inspections differ for each material, which affects maintenance costs. Therefore, it is preferable to select an appropriate material according to the location and length of the piping section.

[0132] The material of the piping section can be changed to the same type of material as the current piping section, as long as substantial thinning due to corrosion has not occurred. Alternatively, it can be changed to a material with higher corrosion resistance than the current piping section. From the viewpoint of ensuring long-term integrity, it is preferable to change the material of the piping section to a material with higher corrosion resistance. The material of the piping section may be changed to a different metal species, or to a different chemical composition within the same metal species.

[0133] The materials used in piping can be replaced with carbon steel with an impurity level of Cr of 0.3 mass% or less, low-alloy steel with an impurity level of Cr of 0.3 mass% to several mass%, or stainless steel. Low-alloy steel is alloy steel in which the total amount of alloying elements is 5 mass% or less. Examples of low-alloy steel include alloy steel containing approximately 0.3 to several mass% of Cr, 0.3 mass% or more of Ni, and 0.08 mass% or more of Mo. Examples of stainless steel include low-carbon stainless steel containing 0.03 mass% or less of C.

[0134] According to improvement step S5, continuous measures can be taken to suppress the occurrence and progression of FAC in the long term for the piping sections selected for monitoring. After the execution of improvement step S5, it is possible to proceed to any of steps S2-S3 or S6. In monitoring step S2, the FAC rate after the execution of improvement step S5 can be confirmed. In preventive maintenance execution step S3, preventive maintenance measures can be implemented on the materials after the execution of improvement step S5.

[0135] (Lifecycle Management Step S6) In lifecycle management step S6, the corrosion rate of flow-accelerated corrosion (FAC) in nuclear power plant piping is managed by repeating the selection step S1, monitoring step S2, preventive maintenance execution step S3, corrective step S4, and improvement step S5. Lifecycle management step S6 consists of repeating at least some of steps S1 to S5.

[0136] In lifecycle management step S6, the piping sections to be monitored are updated according to the risk of FAC with each iteration of steps S1 to S5. Then, for the piping sections selected for monitoring, performance indicators such as ECP are measured, the FAC rate is estimated based on the performance indicators, and measures against FAC are taken based on the estimated FAC rate. By selecting measures against FAC and determining whether or not to implement measures against FAC, ER (Effective Recovery) for FAC in the nuclear power plant piping is achieved.

[0137] In the lifecycle management step S6, the inspection conditions for the standard inspections performed in the preventive maintenance execution step S3 can be modified based on the FAC rate estimated based on performance indicators. The modification of inspection conditions applies to the monitored items selected in the selection step S1, as well as conventional scheduled inspection items other than those being monitored. Examples of inspection conditions include the frequency of inspections, the number of piping sections to be inspected, and the scope of piping sections to be inspected.

[0138] During the repetition of steps S1 to S5, if the FAC speed is below a threshold in monitoring step S2, the time interval for inspection can be extended, the number of piping sections to be inspected can be reduced, or the scope of the piping sections to be inspected can be narrowed. By making such changes to the inspection conditions, inspections involving actual measurements can be streamlined throughout the entire plant.

[0139] Measurement results of performance indicators such as ECP, estimated FAC velocity results based on performance indicators, and inspection results from standard inspections can be accumulated as data for each piping section during the repetition of steps S1 to S5. The data accumulated during the repetition process can be compiled into a database as time-series data for each piping section. Data representing the measurement results of performance indicators, data representing the estimated FAC velocity results, and data representing the wall thickness inspection results measured for each piping section during standard inspections can be accumulated for each nuclear power plant.

[0140] In lifecycle management step S6, a maintenance management database can be constructed for each nuclear power plant and each piping section based on data representing the estimated FAC rate accumulated during the iteration of steps S1 to S5, data representing the wall thickness inspection results measured for piping sections accumulated during the iteration of steps S1 to S5, and at least one of the data representing the FAC rate collected at other plants.

[0141] The maintenance management database can be used to validate FAC rates estimated based on performance indicators. The validation involves comparing the estimated wall thinning based on FAC rate estimation results collected at the nuclear power plant with the measured wall thinning based on wall thickness inspection results collected at the same plant. Alternatively, it can compare the wall thinning based on FAC rates collected at the same nuclear power plant with the wall thinning based on FAC rates collected at other nuclear power plants.

[0142] When comparing nuclear power plants, it is preferable to compare data from piping sections where the parameters affecting FAC are similar across the nuclear power plants when determining the amount of wall thinning based on FAC rate. It is preferable that the amount of wall thinning based on FAC rate be verified for each nuclear power plant and each piping section by comparing it with measured wall thinning amounts, etc.

[0143] If the comparison results show that the amounts of wall thinning match within a predetermined margin, it can be determined that the amount of wall thinning in the piping section selected for monitoring at the nuclear power plant is likely accurate. In such cases, the fact that the amounts of wall thinning match can be recorded for each repetition of steps S1 to S5. If the matching of wall thinning amounts is confirmed over a predetermined cycle for each repetition of steps S1 to S5, the inspection conditions for the standard inspection performed in preventive maintenance execution step S3 can be changed.

[0144] Modifying inspection conditions based on such a maintenance management database can streamline inspections involving actual measurements. Compared to comparing the FAC speed estimated based on performance indicators in monitoring step S2 with a threshold, the determination is made over a predetermined cycle. Therefore, the validity of streamlining the inspection can be more strongly guaranteed.

[0145] According to Life Cycle Management Step S6, long-term maintenance management against FAC (Factory Action Control) can be performed for the entire plant's piping. Life Cycle Management Step S6, based on continuous monitoring of performance indicators for the monitored piping components, allows for extended inspection intervals, reduced inspection frequency, and a reduction in the number of components inspected for the monitored piping components and other piping components belonging to the same piping system. Life Cycle Management Step S6 can be continued throughout the service life of the nuclear power plant.

[0146] The ER process for FAC implemented in the reliability improvement method for nuclear power plants according to this embodiment, unlike conventional technologies, ensures that the desired performance of the plant is not compromised by continuously monitoring performance indicators such as ECP. Since the monitoring targets are selected according to the risk of FAC, ER can be implemented not only for the piping sections selected for monitoring but also for piping systems other than those monitored. It is possible to reduce the amount of inspection without causing a decrease in safety due to uncertainty. Therefore, such an ER process can simultaneously achieve improved equipment reliability and rationalization of equipment maintenance. It becomes possible to review the items to be inspected and to ensure the safety of workers in the radiation-exposed environment.

[0147] Conventional water quality management for FAC (Fuel Cell Corrosion) primarily uses dissolved oxygen concentration, pH, and iron concentration as indicators. However, these indicators have the problem that even if measurements are taken, it is not possible to determine whether the FAC is occurring at the measurement point or upstream of the measurement point. In contrast, using ECP (Energy Corrosion Potential) as an indicator allows for precise determination of the FAC location because the local corrosive environment at the sensor installation is measured. The ECP of the piping material represents the corrosive environment of the cooling water flowing through the piping. Therefore, corrosion potential sensors installed to counter SCC (Steel Chain Corrosion) can also be used to counter FAC. ECP fluctuates under the influence of pH. Furthermore, it is affected by mass transfer determined by flow conditions, and therefore fluctuates under the influence of cooling water velocity, pipe diameter, pipeline geometry, and cooling water temperature. For this reason, ECP is a useful indicator for precisely determining the location of FAC.

[0148] ≪Specific examples of ER processes for FAC≫ Next, a specific example of the ER process for FAC implemented in the reliability improvement method for nuclear power plants according to this embodiment will be described. Figure 4 is a diagram showing a specific example of the flow of the reliability improvement method for nuclear power plants according to the embodiment of the present invention.

[0149] As shown in Figure 4I, in selection step S1, piping sections with a high FAC risk are selected. Piping section selection may be done on a plant-wide basis or on a piping system-wide basis. For example, if there are piping sections A to H, these piping sections are ranked according to their FAC risk. Piping section A, which has the highest FAC risk, is selected as the monitoring target for continuous ECP monitoring.

[0150] As shown in Figure 4, II, in monitoring step S2, the ECP of the piping section selected for monitoring is monitored, and the FAC velocity of the piping section estimated based on the ECP is monitored. The monitored FAC velocity needs to be controlled so that it falls within a predetermined performance target range during the plant's operating cycle from startup to shutdown. The performance target range can be set in advance for each plant operating cycle. The consistency of the monitored FAC velocity with wall thickness inspection data, which is measured intermittently through inspections involving actual measurements, is confirmed.

[0151] As shown in Figure 4, III, depending on the FAC rate of the piping section estimated based on ECP, one of the following steps is performed: preventive maintenance execution step S3, corrective step S4, or improvement step S5. In preventive maintenance execution step S3, for example, oxygen injection is performed. In corrective step S4, water quality parameters are controlled. In improvement step S5, materials in the piping section are replaced.

[0152] When oxygen is injected in preventive maintenance execution step S3, the FAC rate decreases in the piping section belonging to the piping system to which the oxygen injection was performed. However, the degree of decrease in FAC rate varies depending on the piping material, temperature, cooling water flow conditions, and the geometric shape of the piping. For example, in piping section A selected as the monitoring target, water quality parameters are controlled in corrective step S4 so that the FAC rate falls within the target range.

[0153] In preventive maintenance execution step S3 and corrective step S4, the FAC rate may not decrease to the target range. For example, piping sections B to E were ranked as high risk in selection step S1, but the FAC rate did not decrease to the target range. For such piping sections, the materials of the piping section are replaced in improvement step S5 in order to continuously improve equipment reliability. Whether or not the FAC rate decreases to the target range may be determined based on the FAC rate estimated based on ECP, or based on past inspection results or operating experience at other plants.

[0154] In lifecycle management step S6, the piping sections are re-selected in accordance with changes in the risk ranking of FAC, and the measurement of ECPs under monitoring, estimation of FAC rates based on ECP measurement results, and measures against FAC are repeated. Through this ER process, the risk of FAC becomes ALARP. As the FAC risk decreases for the piping sections selected for monitoring, the reduction in FAC risk can also be demonstrated for other piping sections belonging to the same piping system as those sections. Therefore, the long-term reliability of the nuclear power plant can be improved.

[0155] ≪Example of applying the ER process to BWRs for FAC≫ Next, an example of applying the ER process to a BWR to the FAC implemented in the reliability improvement method for nuclear power plants according to this embodiment will be described. Figure 5 is a diagram showing an example of applying the flow of the reliability improvement method for nuclear power plants according to the present invention to a BWR.

[0156] As shown in Figure 5I, the FAC rate differs for each piping section in a BWR. In selection step S1, piping sections with a high risk of FAC are selected from candidates such as the reactor coolant purification system and the feedwater system. These piping systems have long sections outside the pressure vessel and make extensive use of carbon steel pipes, making them high-risk piping systems for FAC. Candidate piping sections include connecting pipes to the regenerative heat exchanger, purification system piping, and feedwater piping.

[0157] Each stage of the heat exchanger is equipped with connecting pipes on the tube side and connecting pipes on the shell side. For example, in the case of a three-stage heat exchanger, there are a total of four potential locations, including both the tube and shell sides. For example, the connecting pipe with the highest risk of FAC is selected as the target for continuous monitoring of ECP. Piping sections where the risk of FAC is predicted to be low can be excluded from the selection candidates in advance. For example, piping sections where the cooling water is at a low temperature are predicted to have a low risk of FAC.

[0158] As shown in Figure 5, section II, when preventive maintenance execution step S3 and corrective step S4 are performed, the FAC rate decreases for each piping section. For example, the connecting pipe selected in selection step S1 is controlled so that the FAC corrosion rate falls within the target range. If the FAC rate does not fall within the target range due to oxygen injection, a reduction in the amount of hydrogen injected or an increase in the amount of precious metal injected can be combined within a range that does not affect SCC. However, as in piping section B, there may be piping sections where the FAC rate does not decrease easily depending on the piping material, temperature, cooling water flow conditions, and the geometric shape of the piping.

[0159] As shown in III of Figure 5, when improvement step S5 is performed, the FAC rate decreases significantly in the piping section where the material has been replaced. For example, piping sections where the FAC rate does not decrease easily, such as piping section B, are selected as monitoring targets in subsequent cycles, and the material is replaced in improvement step S5. In addition, in piping upstream or downstream of the purification system, such as the bottom drain piping P34, and in piping downstream of the water supply system, oxygen injection is difficult because the cooling water is at a high temperature. In such piping sections, countermeasures are taken by replacing the material in improvement step S5.

[0160] As shown in IV of Figure 5, when lifecycle management step S6 is performed, the FAC risk becomes ALARP for all piping components. For example, the connecting pipe was the piping component with the highest FAC risk, but the FAC risk is reduced by various measures. By continuously demonstrating the reduction in the FAC risk of the connecting pipe through ECP's continuous monitoring, it becomes possible to demonstrate that the FAC risk has also decreased for piping components other than the connecting pipe.

[0161] Therefore, by executing lifecycle management step S6, assuming continuous monitoring of the ECP of the monitored piping section, it is possible to extend the time interval between inspections, reduce the frequency of inspections, or reduce the number of inspection points for the monitored piping section and other piping sections belonging to the same piping system. This allows for the rationalization of equipment maintenance, such as inspections, while ensuring the safety of workers in the radiation-exposed environment and ensuring equipment reliability.

[0162] ≪Extreme Variation 1≫ In monitoring step S2, in addition to performance indicators such as ECP of the piping section, auxiliary performance indicators can also be continuously monitored. Examples of auxiliary performance indicators include at least one of the following: oxygen concentration of the cooling water, hydrogen peroxide concentration of the cooling water, electrical conductivity of the cooling water, iron ion concentration of the cooling water, and pH of the cooling water. Another example is the amount of substance deposited on the surface of the piping section material by precious metal injection or titanium oxide injection.

[0163] The correlation between these auxiliary performance indicators and FAC speed is periodically determined. Based on this correlation, the FAC speed of the piping section can be estimated. Continuous monitoring of these auxiliary performance indicators ensures equipment reliability even in piping sections where corrosion potential sensors cannot be installed, or during periods when ECP cannot be measured due to corrosion potential sensor failure, etc.

[0164] Furthermore, the correlation between auxiliary performance indicators and FAC rate can be affected by reactor output and may change depending on the burnup of nuclear fuel, the position of control rods, the amount of cooling water in the core, etc. Therefore, it is difficult to continue using the correlation between auxiliary performance indicators and FAC rate over the long term. For this reason, it is preferable to periodically determine and update such correlations with each fuel change in the reactor or at predetermined operating intervals. For example, it is preferable to update them at least every five years or so, when all the fuel in the reactor is replaced.

[0165] The oxygen concentration and hydrogen peroxide concentration of the cooling water can be increased until the FAC rate decreases, as long as the effect of suppressing SCC by hydrogen injection or precious metal injection is obtained. On the other hand, precious metal injection catalyzes a recombination reaction to maintain ECP near the oxidation-reduction potential of hydrogen. Therefore, when using the amount of deposit due to precious metal injection as a supplementary performance indicator, it is preferable to confirm that hydrogen is present in a stoichiometric ratio of water or higher to oxygen. Within such a range, the amount of deposit due to precious metal injection becomes a highly reliable performance indicator.

[0166] Titanium dioxide injection is a technique that involves injecting a titanium dioxide solution into the cooling water to deposit titanium dioxide on the inner surface of the piping. Titanium dioxide is known to exhibit photocatalytic activity under Cherenkov light, reducing the corrosion potential of materials. Because titanium dioxide is incorporated into the oxide film to form ilmenite (FeTiO3), which has low solubility, it can suppress FAC (Fatal Acid Accumulation).

[0167] The electrical conductivity and iron ion concentration of the cooling water can be reduced to a degree that lowers the FAC rate, within the limits of the cooling water quality and purification capacity. During monitoring of ECP and FAC rate, it may be found that an increase in FAC rate is due to a change in the water quality itself, rather than measures such as oxygen injection or ECP. In such cases, the purification capacity of the cooling water can be increased during corrective step S4 or other steps.

[0168] <<Variation 2>> In monitoring step S2, instead of the ECP of the piping section, the wall thickness of the piping section can be used as the performance indicator to be continuously monitored. The wall thickness of the piping section can be measured directly during plant operation using an ultrasonic thickness gauge or the like that can withstand high-temperature environments. The wall thickness of the piping section can be measured at one or more measurement points, including the piping section selected as the monitoring target. However, unlike ECP, the wall thickness of the piping section is prone to measurement noise. Therefore, from the viewpoint of taking rapid action against FAC and improving the controllability of the action, it is preferable to continuously monitor the ECP.

[0169] <<Variation 3>> Figure 6 shows an example of a nuclear power plant to which the reliability improvement method according to an embodiment of the present invention is applied. Figure 6 illustrates an improved boiling water reactor (ABWR). The above-described reliability improvement method for nuclear power plants can also be applied to other types of nuclear power plants such as ABWRs. As shown in Figure 6, the nuclear power plant P200 is an ABWR and is equipped with a reactor P1, turbine P3, condenser P4, reactor coolant purification system, feedwater system, etc.

[0170] Unlike nuclear plant P100, nuclear plant P200 does not have a recirculation system. Also, nuclear plant P200 is equipped with an internal pump P40 instead of a jet pump P21. Downstream of the bottom drain pipe P34, it branches into a purification system pipe P20 and a residual heat removal system pipe P20a. Downstream of the residual heat removal system pipe P20a is connected to a down camshaft P17 inside the pressure vessel P12.

[0171] Furthermore, the turbine P3, the high-pressure feedwater heater P9, and the low-pressure feedwater heater P8 are connected to each other by extraction piping P14. Extracted steam extracted from the steam is introduced into the high-pressure feedwater heater P9 and the low-pressure feedwater heater P8, bypassing the condenser P4. The drain from the high-pressure feedwater heater P9 is returned to the feedwater. The drain from the low-pressure feedwater heater P8 is returned to the condensate.

[0172] In the nuclear power plant P200, corrosion potential sensor P35a is installed on branch pipe P34a, which branches off from bottom drain pipe P34. Corrosion potential sensor P35e is attached to flange P36, which is connected to residual heat removal pipe P20a. Corrosion potential sensors P35a and P35e are installed to verify the effectiveness of hydrogen injection and precious metal injection in suppressing stress corrosion cracking (SCC).

[0173] In the nuclear power plant P200, corrosion potential sensors for measuring the corrosion potential (ECP) of materials in contact with cooling water can be installed in the connecting pipes P42a, P42f, etc., of the regenerative heat exchanger P25, similar to the nuclear power plant P100. In addition, injection devices for oxygen injection, hydrogen peroxide injection, and titanium oxide injection can be connected to the purification system piping P20 and the feedwater piping P10.

[0174] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. For example, the present invention is not necessarily limited to having all the configurations of the embodiments described above. Some configurations of one embodiment may be replaced with other configurations, some configurations of one embodiment may be added to other forms, or some configurations of one embodiment may be omitted. [Explanation of symbols]

[0175] S1 Selection Step S2 monitoring steps S3 Preventive Maintenance Implementation Steps S4 Correction Step S5 Improvement Steps S6 Lifecycle Management Steps P100 Nuclear power plant P1 reactor P2 Main steam piping P3 Turbine P4 Condenser P5 Condensate Pump P6 Condensate Purification System P7 Water supply pump P8 Low-pressure feedwater heater P9 High-pressure feedwater heater P10 Water supply piping P11 Containment Vessel P12 Pressure Vessel P13 Core P14 Extraction piping P15 Shroud P16 Hydrogen Injection Device P17 Down Cam P18 Hydrogen injection piping P19 Shut-off valve P20 Septic System Piping P21 Jet Pump P23 Purification system isolation valve P24 Purification System Pump P25 Regenerative heat exchanger P26 Non-regenerative heat exchanger P27 Furnace water purification system P30 Recirculation system piping P31 Precious metal injection equipment P32 Precious metal injection piping P33 Shut-off valve P34 Bottom drain piping P35 Corrosion Potential Sensor P36 Flange P38 Neutron Instrument Tube P40 Internal Pump P41 Shroud Support P42 Connecting pipe P43 Oxygen Injection Device P44 Oxygen injection piping P45 Shut-off valve

Claims

1. A selection step to identify piping sections at high risk of flow-accelerated corrosion, A monitoring step to monitor the corrosion rate of flow-accelerated corrosion in the selected piping section, A preventive maintenance step to reduce the corrosion rate of the piping portion, A corrective step to control the water quality parameters of the cooling water flowing through the aforementioned piping section, If the corrosion rate does not fall within the target range even after performing the above corrective step, an improvement step is made to replace the material of the selected piping section. A life cycle management step for managing the corrosion rate of piping in a nuclear power plant, comprising the selection step, the monitoring step, the preventive maintenance execution step, the corrective step, and the improvement step, It has, A method for improving the reliability of a nuclear power plant, comprising the monitoring step, which involves continuously monitoring the corrosion potential of the material in contact with the cooling water of the selected piping section, and monitoring the corrosion rate of flow-accelerated corrosion of the piping section estimated based on the corrosion potential.

2. A method for improving the reliability of a nuclear power plant according to claim 1, A method for improving the reliability of a nuclear power plant, comprising selecting the piping section in the selection step using at least one of a risk analysis tool and human risk analysis based on the importance classification of the structures, systems, and equipment of the nuclear power plant.

3. A method for improving the reliability of a nuclear power plant according to claim 1, A method for improving the reliability of a nuclear power plant, wherein in the monitoring step, the corrosion rate is estimated based on the corrosion potential and at least one of the chemical composition of the piping portion, the quality of the cooling water, the temperature of the cooling water, and the hydrodynamic characteristics of the cooling water in the piping portion.

4. A method for improving the reliability of a nuclear power plant according to claim 1, A method for improving the reliability of a nuclear power plant, wherein in the monitoring step, at least one of the oxygen concentration of the cooling water, the hydrogen peroxide concentration of the cooling water, and the electrical conductivity of the cooling water is measured, and the measurement results are used to calibrate the corrosion rate.

5. A method for improving the reliability of a nuclear power plant according to claim 1, A method for improving the reliability of a nuclear power plant, comprising the steps of oxygen injection, hydrogen peroxide injection, and titanium dioxide injection in the aforementioned preventive maintenance implementation step.

6. A method for improving the reliability of a nuclear power plant according to claim 1, A method for improving the reliability of a nuclear power plant, wherein in the corrective step, at least one of the following is performed: increasing the amount of oxygen injected into the cooling water, increasing the amount of hydrogen peroxide injected into the cooling water, and decreasing the amount of hydrogen injected into the cooling water.

7. A method for improving the reliability of a nuclear power plant according to claim 1, A method for improving the reliability of a nuclear power plant, comprising constructing a database for each nuclear power plant and for each piping part based on at least one of the following: data representing the corrosion rate accumulated during the repetition of the selection step, the monitoring step, the preventive maintenance execution step, the corrective step, and the improvement step in the life cycle management step; data representing the wall thickness inspection results measured for the piping part; and data representing the corrosion rate of flow-accelerated corrosion collected at other plants.

8. A method for improving the reliability of a nuclear power plant according to claim 7, A method for improving the reliability of a nuclear power plant, which involves extending the time interval for wall thickness inspections of each piping section based on the aforementioned database, and ensuring equipment reliability against flow-accelerated corrosion for each piping section by monitoring the corrosion potential of each piping section during the extended period.

9. A selection step for selecting piping sections at high risk of flow-accelerated corrosion, A monitoring step to monitor the corrosion rate of flow-accelerated corrosion in the selected piping section, A preventive maintenance step to reduce the corrosion rate of the piping portion, A corrective step to control the water quality parameters of the cooling water flowing through the aforementioned piping section, If the corrosion rate does not fall within the target range even after performing the above corrective step, an improvement step is made to replace the material of the selected piping section. A life cycle management step for managing the corrosion rate of piping in a nuclear power plant, comprising the selection step, the monitoring step, the preventive maintenance execution step, the corrective step, and the improvement step, It has, In the lifecycle management step, a database is constructed for each nuclear power plant and each piping section based on at least one of the following: data representing the corrosion rate accumulated during the repetition of the selection step, the monitoring step, the preventive maintenance execution step, the corrective step, and the improvement step; data representing the wall thickness inspection results measured for the piping section; and data representing the corrosion rate of flow-accelerated corrosion collected at other plants. A method for improving the reliability of a nuclear power plant, which involves extending the time interval for wall thickness inspections of each piping section based on the aforementioned database, and ensuring equipment reliability against flow-accelerated corrosion for each piping section by monitoring the corrosion potential of each piping section during the extended period.

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