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JP-7855443-B2 - Methods for improving the reliability of nuclear power plants

JP7855443B2JP 7855443 B2JP7855443 B2JP 7855443B2JP-7855443-B2

Inventors

  • 和田 陽一
  • 伊藤 剛
  • 清水 亮介

Assignees

  • 日立ＧＥベルノバニュークリアエナジー株式会社

Dates

Publication Date
20260508
Application Date
20220719

Claims (9)

  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, in which 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 .
  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, in the selection step, selecting the piping section based on the importance classification of the structures, systems, and equipment of the nuclear power plant using at least one of a risk analysis tool and human risk analysis.
  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 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, 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.

Description

This invention relates to a method for improving the reliability of nuclear power plants. Carbon steel is widely used as a piping material in thermal and nuclear power plants. Carbon steel piping is known to develop flow-accelerated corrosion (FAC) under specific conditions where high-velocity fluids flow. FAC is primarily caused by electrochemical corrosion dependent on flow velocity. Because FAC leads to a reduction in pipe wall thickness, preventative measures against FAC are applied. In nuclear power plants, carbon steel is widely used in the out-of-reactor piping installed outside the pressure vessel. During the plant's design phase, a corrosion allowance is pre-defined for the carbon steel piping through which high-velocity cooling water flows. This corrosion allowance accounts for the amount of wall thinning due to FAC (Fuel Cell Acceleration) during the plant's service life. During service, the amount of wall thinning due to corrosion is periodically inspected. In recent years, for carbon steel piping, it has become desirable not only to incorporate corrosion allowances in advance, but also to optimize preventive maintenance throughout the service life. Efficient countermeasures against FAC (Fiber Accumulation) are required from the perspectives of improving plant safety, enhancing economic efficiency such as equipment utilization rates, and addressing aging infrastructure. For carbon steel piping, which is prone to FAC, it is desirable to optimize inspection and replacement timings and ensure equipment reliability through low-cost and rational measures. In the case of nuclear power plants, measures to suppress FAC include, on the material side, changing from carbon steel to corrosion-resistant materials such as low-alloy steel and stainless steel. Furthermore, on the water quality management side, oxygen is injected into the cooling water. FAC occurs when the dissolved oxygen concentration drops to 15-20 ppb or below. When the dissolved oxygen concentration decreases, it becomes difficult for an oxide film to form on the inner surface of the piping, the protection of the base material is lost, and FAC is more likely to occur. 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. Therefore, the dissolved oxygen concentration in the condensate often drops to below 10 ppb. Because oxide film formation is less likely to occur in the piping through which the condensate flows, FAC (Fuel-Acid Culmination) is more likely to occur when high-temperature, high-velocity cooling water flows through it. FAC in such piping is mitigated by oxygen injection. 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 an alkaline state by adding ammonia, etc. It is known that in an alkaline state, the protective oxide film on the base material is less likely to be lost, and even if the dissolved oxygen concentration drops to around 10 ppb due to the addition of hydrazine or degassing, FAC (Fatal Acquisition Coagulation) is less likely to progress. The main indicators used for water quality management related to FAC (Fuel Cell Aqueduct) are dissolved oxygen concentration, pH, and iron concentration of the cooling water. 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 considering factors such as the cooling water flow velocity, cooling water temperature, pipe diameter, and the geometric shape of the piping, including elbows and vents. Conventionally, in some boiling water reactors (BWRs), hydrogen injection and precious metal injection have been used as countermeasures against stress corrosion cracking (SCC) in materials in contact with the cooling water. As an indicator for 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). Traditionally, nuclear power plants have implemented various countermeasures against corrosion phenomena such as SCC (Steel Cross-Corrosion) and erosion-corrosion, based on evaluations of corrosion rates. 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. This method measures the ECP of the structural m