Corrosion potential measurement method and corrosion potential measurement device

Abstract

To provide a corrosion potential measurement method that can be constructed at low cost, can accurately evaluate corrosion potential of a part of a reactor to be protected against SCC, and can appropriately determine timing for re-injection of a precious metal following detachment of the precious metal from a surface of the part to be protected against SCC.SOLUTION: A corrosion potential measurement method of the present invention involves: installing a SUS316 L electrode P35a3 in a corrosion potential measurement manifold P50a through which reactor water flows, to measure corrosion potential of 300-series stainless steel; installing an 82 alloy electrode P35a5 downstream of the SUS316 L electrode P35a3, to measure corrosion potential of 82 alloy; and setting a first substance transfer coefficient based on a reactor water flow condition at the 82 alloy electrode P35a5 to correspond to a second substance transfer coefficient based on a reactor water flow condition facing the 82 alloy, which forms a part of the reactor that is to be protected against stress corrosion cracking.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a corrosion potential measuring method and a corrosion potential measuring device. [Background technology]

[0002] Structural components such as equipment and piping in nuclear power plants are made of structural materials such as stainless steel and nickel-based alloys. These structural materials are susceptible to stress corrosion cracking (hereinafter sometimes referred to as SCC) under certain conditions. Therefore, SCC prevention measures are applied to maintain the integrity of nuclear reactors. In recent years, SCC prevention measures have also been applied from the perspective of improving economic efficiency, such as increasing the capacity factor of nuclear reactors, and from the perspective of responding to aging plants.

[0003] As a measure to prevent SCC, hydrogen injection into reactor water is known (see, for example, Patent Document 1 and Patent Document 2). By injecting hydrogen, hydrogen reacts with oxygen and hydrogen peroxide contained in the reactor water to return it to water, thereby lowering the corrosion potential (hereinafter sometimes referred to as ECP) ​​and suppressing the occurrence of SCC. Furthermore, as a measure to prevent SCC, noble metal injection, in which noble metals such as platinum are injected into reactor water, is known (see, for example, Patent Document 3). This noble metal injection promotes the electrochemical reaction of hydrogen due to the catalytic action of the noble metal, further reducing the ECP. These techniques require accurate measurement of the ECP of structural materials in nuclear power plants in order to evaluate the SCC suppression effect.

[0004] Conventionally, a known ECP measurement device is one in which a sample electrode made of the same material (stainless steel or nickel-based alloy) as the SCC protection target and a reference electrode (e.g., a platinum electrode) are placed near the SCC protection target (see, for example, Patent Document 4). This ECP measuring device makes it possible to accurately determine the ECP in the SCC protection target area by measuring the potential difference between the sample electrode and the reference electrode. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 2687780 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-43051 [Patent Document 3] Japanese Patent Application Publication No. 4-223299 [Patent Document 4] Japanese Patent Application Laid-Open No. 2009-30980 Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, nickel-based alloy weld metal 82 (hereinafter sometimes referred to as 82 alloy) is used in the reactor bottom (e.g., shroud support) of nuclear power plants using boiling water reactors (BWRs). It is known that the ECP of 82 alloy is higher than that of 304 stainless steel (SUS304) and 316L stainless steel (SUS316L), which are used as structural materials inside the reactor. Therefore, in conventional ECP measurement devices (see, for example, Patent Document 4), the ECP measurements of these stainless steels cannot be used to measure the ECP of 82 alloy, and therefore a separate electrode must be installed to measure the ECP of 82 alloy. However, adding an additional electrode for measuring the ECP of Alloy 82 to a conventional ECP measurement device (see, for example, Patent Document 4) and applying it to the bottom of the furnace not only increases costs but also poses the problem of limited access to the ECP measurement device at the bottom of the furnace.

[0007] Furthermore, considering flow-accelerated corrosion (hereinafter sometimes referred to as FAC) of Alloy 82, the rate at which precious metals attached to the surface of Alloy 82 at the reactor bottom are lost may be faster than that of precious metals attached to the surfaces of other structural materials in the reactor (e.g., SUS304 and SUS316L). Therefore, with conventional ECP measurement devices (see, for example, Patent Document 4), even if the measured ECP value increases, it is difficult to distinguish whether the increase is due to a change in reactor water quality or a change caused by the desorption of precious metals. In other words, there is a demand for an ECP measurement device that can be constructed at low cost, that can accurately evaluate the corrosion potential of SCC-protected areas inside the reactor of a nuclear power plant that is subject to constant monitoring, and that can appropriately determine the timing of re-injection of precious metals following detachment of precious metals from the surface of SCC-protected areas.

[0008] The present invention aims to provide a corrosion potential measuring method and a corrosion potential measuring device that can be constructed at low cost, can accurately evaluate the corrosion potential of SCC protection target parts in a reactor, and can appropriately determine the timing for re-injection of precious metal following detachment of precious metal from the surface of the SCC protection target part. [Means for solving the problem]

[0009] The corrosion potential measurement method of the present invention includes a step of adding hydrogen and a noble metal to reactor water, and is a corrosion potential measurement method for a nuclear power plant having a first structural material made of 300 series stainless steel and a second structural material that undergoes flow-accelerated corrosion, the method comprising the steps of: installing a first corrosion potential measurement unit that measures the corrosion potential of the first structural material in a piping through which reactor water flows; and installing a second corrosion potential measurement unit that measures the corrosion potential of the second structural material downstream of the first corrosion potential measurement unit or in parallel with the first corrosion potential measurement unit; A first mass transfer coefficient based on the flow conditions of reactor water in the second corrosion potential measuring section corresponds to a second mass transfer coefficient based on the flow conditions of reactor water facing the second structural material that forms the area to be protected against stress corrosion cracking within the reactor.

[0010] The corrosion potential measuring device of the present invention is a corrosion potential measuring device for use in a nuclear power plant, which comprises a first structural material made of 300 series stainless steel and a second structural material that undergoes flow-accelerated corrosion, and in which a precious metal is attached so that it comes into contact with reactor water containing hydrogen. The corrosion potential measuring device comprises: a first corrosion potential measuring unit installed in a pipe through which reactor water flows and measuring the corrosion potential of the first structural material; and a second corrosion potential measuring unit installed downstream of the first corrosion potential measuring unit or in parallel with the first corrosion potential measuring unit and measuring the corrosion potential of the second structural material, wherein a first mass transfer coefficient based on the flow conditions of the reactor water in the second corrosion potential measuring unit is set to correspond to a second mass transfer coefficient based on the flow conditions of the reactor water facing the second structural material that forms a part in the reactor that is to be protected against stress corrosion cracking. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a corrosion potential measuring method and a corrosion potential measuring device that can be constructed at low cost, that can accurately evaluate the corrosion potential of SCC protection target parts in a reactor, and that can appropriately determine the timing for re-injection of precious metal following detachment of precious metal from the surface of the SCC protection target part. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a diagram illustrating the configuration of a nuclear power plant having a boiling water reactor (BWR) to which an electrochemical corrosion potential measuring device according to a first embodiment of the present invention is applied. [Figure 2] 1 is a diagram illustrating the configuration of an electrochemical corrosion potential measuring device according to a first embodiment of the present invention. [Figure 3] 3 is a diagram illustrating the configuration of a mass transfer coefficient adjusting section in the second corrosion potential measuring section of the corrosion potential measuring device of FIG. 2.

[0023] FIG. [Figure 4] FIG. 4 is a configuration explanatory diagram of a first modified example of the mass transfer coefficient adjusting unit shown in FIG. [Figure 5] FIG. 4 is a configuration explanatory diagram of a second modified example of the mass transfer coefficient adjusting unit shown in FIG. [Figure 6] FIG. 4 is a diagram illustrating the configuration of an electrochemical corrosion potential measuring device according to a second embodiment of the present invention. [Figure 7] FIG. 10 is a diagram illustrating the configuration of an electrochemical corrosion potential measuring device according to a third embodiment of the present invention. [Figure 8A] 1 is a graph showing the relationship between platinum deposition amount and corrosion potential. [Figure 8B] 1 is a graph showing the relationship between changes in reactor water quality and corrosion potential. [Figure 9] 1 is a graph showing the relationship between the operation time of a nuclear power plant and the change in corrosion potential when platinum desorption occurs. [Figure 10] FIG. 10 is a diagram illustrating the configuration of a nuclear power plant having a boiling water reactor (BWR) to which an electrochemical corrosion potential measuring device according to a fourth embodiment of the present invention is applied. [Figure 11] FIG. 11 is a configuration explanatory diagram of a purification system of the nuclear power plant of FIG. [Figure 12] FIG. 10 is a diagram illustrating the configuration of an electrochemical corrosion potential measuring device according to a fourth embodiment of the present invention. [Figure 13] 1 is a diagram illustrating the configuration of a nuclear power plant having an advanced boiling water reactor (ABWR) to which the corrosion potential measuring device of the present invention is applied. [Figure 14] FIG. 10 is a configuration explanatory diagram showing a modified example of the electrochemical corrosion potential sensor. DETAILED DESCRIPTION OF THE INVENTION

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments (embodiments) for carrying out an electrochemical corrosion potential measuring device and an electrochemical corrosion potential measuring method according to the present invention will be described in detail with reference to the accompanying drawings as appropriate. [First embodiment] Before describing the corrosion potential measuring device and the corrosion potential measuring method of this embodiment, a nuclear power plant to which this corrosion potential measuring device is applied will be described. Here, a nuclear power plant using a boiling water reactor (BWR) will be described as an example, but the corrosion potential measuring device of this embodiment can also be applied to nuclear power plants using, for example, an advanced boiling water reactor (ABWR) or a pressurized water reactor (PWR).

[0014] (Nuclear Plant) FIG. 1 is a diagram showing the overall system configuration of a nuclear power plant P100 according to this embodiment. As shown in FIG. 1, the nuclear power plant P100 includes a reactor P1, which is a BWR, a turbine P3, a condenser P4, a feedwater system, a reactor cleanup system, and the like, which will be described later. The reactor P1 is installed in a reactor containment vessel P11. The reactor P1 has a pressure vessel P12 that houses a reactor core P13. A cylindrical shroud P15 installed in the pressure vessel P12 surrounds the reactor core P13.

[0015] The reactor core P13 is loaded with multiple fuel assemblies (not shown). Each fuel assembly includes multiple fuel rods filled with multiple fuel pellets made of nuclear fuel material. 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 and communicate with the lower space (lower plenum (not shown)) of the pressure vessel P12. The bottom of the lower plenum is called the reactor bottom. The structure that supports the shroud P15 is the shroud support P41. Alloy 82 is used for the welds of this shroud support P41. In addition, in the corrosion potential measuring device A1 (see Figure 2) of this embodiment described later, the ECP value at the welded part of the shroud support P41 made of alloy 82 is obtained in a simulated manner as the ECP value of the part to be protected from stress corrosion cracking inside the reactor.

[0016] The feedwater system is configured by arranging a condensate pump P5, a condensate purifier P6, a feedwater pump P7, a low-pressure feedwater heater P8, and a high-pressure feedwater heater P9 in this order on a feedwater pipe P10 connecting the condenser P4 and the pressure vessel P12. A hydrogen injection device P16 is connected to the feedwater pipe P10 between the condensate purifier P6 and the feedwater pump P7 by a hydrogen injection pipe P18. An on-off valve P19 is provided on the hydrogen injection pipe P18. A precious metal injection device P31 is connected to the feedwater pipe P10 between the high-pressure feedwater heater P9 and the pressure vessel P12 by a precious metal injection pipe P32. An on-off valve P33 is provided on the precious metal injection pipe P32.

[0017] The reactor cleanup system is configured by arranging, in this order, a cleanup isolation valve P23, a cleanup pump P24, a regenerative heat exchanger P25, a non-regenerative heat exchanger P26, and a reactor water purification device P27 in cleanup system piping P20, which connects the pressure vessel P12 and the feedwater piping P10. The cleanup system piping P20 is connected to the feedwater piping P10 between the high-pressure feedwater heater P9 and the pressure vessel P12.

[0018] Reactor water present in the downcomer P17 inside the pressure vessel P12 is sucked into the jet pump P21 and led to the lower plenum below the reactor core P13. The reactor water is supplied from the lower plenum to the reactor core P13 and heated by the heat generated by the nuclear fission of the nuclear fuel material contained in the fuel rods of the fuel assemblies. Part of the heated reactor water becomes steam. This steam is led from the pressure vessel P12 through the main steam pipe P2 to the turbine P3, causing it to rotate. A generator (not shown) connected to the turbine P3 is rotated, generating electricity. The steam discharged from the turbine P3 is condensed into water in the condenser P4.

[0019] This water is supplied as feedwater through feedwater piping P10 into the pressure vessel P12. The feedwater flowing through the feedwater piping P10 is pressurized by a condensate pump P5, impurities are removed by a condensate purification device P6, the pressure is further increased by a feedwater pump P7, and the feedwater is heated by a low-pressure feedwater heater P8 and a high-pressure feedwater heater P9. An extraction pipe P14 is connected between the turbine P3 and the low-pressure feedwater heater P8 and the high-pressure feedwater heater P9. Extracted steam extracted from the main steam pipe P2 and the turbine P3 by the extraction pipe P14 is supplied to the low-pressure feedwater heater P8 and the high-pressure feedwater heater P9, respectively, via the extraction pipe P14, and serves as a heating source for the feedwater.

[0020] The reactor water in the pressure vessel P12 contains metal corrosion products contained in the feedwater and products resulting from corrosion of the structural materials inside the pressure vessel P12. Therefore, a certain percentage of the reactor water is purified by the reactor water purification system. The reactor water in the pressure vessel P12 is supplied to the regenerative heat exchanger P25 and the non-regenerative heat exchanger P26 through the purification system piping P20 branched from the recirculation system piping P30 by the operation of the purification system pump P24, and is cooled to approximately 50°C by these heat exchangers. The cooled reactor water passes through the reactor water purification device P27, where the metal corrosion products contained in the reactor water are removed. After being heated in the regenerative heat exchanger P25, the reactor water is combined with the feedwater flowing in the feedwater piping P10 and supplied to the pressure vessel P12.

[0021] When the operation of the reactor P1 is shut down, all control rods (not shown) are inserted into the core P13. By inserting all control rods, the nuclear fission reaction of the nuclear fuel material is stopped, and the operation of the reactor P1 is shut down. The heat remaining in the equipment inside the core P13 and the pressure vessel P12 is removed by the evaporation of reactor water. When the temperature drops to a certain level, the efficiency of heat removal by evaporation of reactor water decreases. Therefore, when the reactor water temperature drops to about 150°C, the equipment inside the core P13 and the pressure vessel P12 is cooled using a residual heat removal system (not shown).

[0022] In such a nuclear power plant P100, hydrogen injection and precious metal injection are combined and performed on the BWR (nuclear reactor P1) as preventive maintenance against the risk of SCC (stress corrosion cracking). Hydrogen injection is performed by injecting hydrogen from a hydrogen injection device P16 into a feedwater pipe P10 through a hydrogen injection pipe P18 with an on-off valve P19 open. The hydrogen injected into the feedwater is sprayed into the reactor from a feedwater sparger (not shown) above the downcomer P17 and mixed with the reactor water. As the added hydrogen passes through the downcomer P17, it recombines with oxygen and hydrogen peroxide in the reactor water due to gamma rays in the downcomer P17 area to produce water. In other words, the added hydrogen consumes dissolved oxygen and hydrogen peroxide, which are environmental factors for SCC, thereby mitigating the corrosive environment. As a result, the ECP (corrosion potential) of structural materials in contact with the reactor water is reduced, thereby suppressing the initiation and progression of SCC.

[0023] Furthermore, precious metal injection is a technology that promotes the effect of hydrogen injection through catalytic action. Precious metal injection is performed by injecting an aqueous solution containing, for example, a platinum (Pt) compound from a precious metal injection device P31 into the pressure vessel P12. Specifically, this aqueous solution is injected into the water supply pipe P10 from the precious metal injection pipe P32 with the on-off valve P33 open. The platinum injected into the feedwater merges with the reactor water and is contained in the reactor water in the form of tiny platinum particles. When the reactor water containing platinum particles comes into contact with structural materials, some of the platinum particles adhere to the surface of the structural materials. The remaining platinum particles adhere to the surface of the fuel.

[0024] On the surface of platinum particles, hydrogen reacts electrochemically with oxygen and hydrogen peroxide in the reactor water to form water. When hydrogen is present in a stoichiometric ratio (H2O = 2:1) greater than that of oxygen (hydrogen peroxide is treated as half the amount of equivalent oxygen), hydrogen becomes excessive relative to oxygen. This causes the reaction between hydrogen and oxygen to proceed at a potential near -500 mV vs. SHE, which is the hydrogen oxidation-reduction potential. This causes the ECP (corrosion potential) of the structural material to mix and drop to near -500 mV vs. SHE. In other words, SCC can be suppressed by controlling the hydrogen injection and precious metal injection so that the ECP falls within the target potential range for SCC preventive maintenance.

[0025] Furthermore, in the nuclear power plant P100, an electrochemical corrosion potential sensor P35a and an electrochemical corrosion potential sensor P35b are installed in the cleanup system piping P20 and the recirculation system piping P30, through which reactor water flows below the downcomer P17. The nuclear power plant P100 measures the ECP (corrosion potential) of the reactor water in the cleanup system piping P20 and the recirculation system piping P30 using the electrochemical corrosion potential sensor P35a and the electrochemical corrosion potential sensor P35b. That is, the nuclear power plant P100 measures the ECP (corrosion potential) as a constant monitoring indicator of whether hydrogen injection and precious metal injection are achieving an SCC suppression effect to improve plant reliability.

[0026] Specifically, in the nuclear power plant P100, a bottom drain line P34 is provided between the bottom of the pressure vessel P12 and the purification system piping P20. The bottom drain line P34 draws reactor water from the bottom of the pressure vessel P12 and passes it through the purification system piping P20. A corrosion potential measurement manifold P50a equipped with an electrochemical corrosion potential sensor P35a is installed in a branch line P34a of the bottom drain line P34. The electrochemical corrosion potential sensor P35a measures the ECP of the reactor water at the bottom of the pressure vessel P12. The branch line P34a is connected to a sampling line (not shown) for reactor water, etc.

[0027] The corrosion potential sensor P35b is mounted on a flange P36 provided on the recirculation system piping P30 or on a corrosion potential measurement manifold (not shown) provided in a sampling line connected to the recirculation system piping P30. The corrosion potential sensor P35b measures the ECP of the reactor water flowing through the recirculation system piping P30.

[0028] Furthermore, in the nuclear power plant P100, the electrochemical corrosion potential sensor P35c and the electrochemical corrosion potential sensor P35d can also be installed in the lower plenum (not shown) of the pressure vessel P12 and the reactor core P13. The electrochemical corrosion potential sensor P35c and the electrochemical corrosion potential sensor P35d are preferably installed in the neutron instrumentation tube P38. The electrochemical corrosion potential sensor P35c is installed at the position of the lower plenum inside the neutron instrumentation tube P38. The electrochemical corrosion potential sensor P35d is installed at the position of the reactor core P13 inside the neutron instrumentation tube P38. Then, ECP measurements are performed by drilling holes in the neutron instrumentation tube P38 so that reactor water in the target region flows into these electrochemical corrosion potential sensors P35c and P35d.

[0029] The structural members of the reactor P1 are made of austenitic stainless steel, such as SUS 316 L. In addition to the aforementioned SUS 316 L, the structural members of the reactor P1 can also be made of austenitic stainless steel such as SUS 304, SUS 304 L, which is SUS 304 with a reduced carbon content, or SUS 316 NG (NG: for nuclear use), which is SUS 316 L with added nitrogen to increase its strength. Furthermore, various electrodes can be constructed using the materials of these structural members. The shroud P15 can be made of SUS304L, SUS316L, or the like. The purification system piping P20 and the recirculation system piping P30 can be made of SUS304, SUS316NG, carbon steel, or the like. Additionally, nickel-based alloys are used in reactor P1. Alloy 600 is used in the hearth because high strength is required. Nickel-based alloy 182 weld metal (Alloy 182) and Alloy 82 are used as weld metals for the welds and overlays in the hearth.

[0030] (Corrosion potential measuring device) Next, the corrosion potential measuring device of this embodiment will be described. FIG. 2 is a diagram illustrating the configuration of the corrosion potential measuring device A1. As shown in FIG. 2, the corrosion potential measurement device A1 is disposed in a corrosion potential measurement manifold P50a provided in a branch line P34a (see FIG. 1) that constitutes the bottom drain line P34 (see FIG. 1). The corrosion potential measuring manifold P50a is made of 300 series stainless steel such as SUS304L, SUS316L, etc. In this embodiment, the corrosion potential measuring manifold P50a is assumed to be made of SUS316L, but is not limited to this.

[0031] 2, the corrosion potential measuring device A1 includes an corrosion potential sensor P35a, an electrometer P51, and a scanner P52. The corrosion potential measuring device A1 also includes a mass transfer coefficient adjusting unit P53 (see FIG. 3) described later. The corrosion potential sensor P35a consists of multiple sensors and multiple electrodes, and is arranged in a row from the upstream side of the corrosion potential measurement manifold P50a in the following order: an iron-type ECP sensor P35a1, an iron-type ECP sensor P35a2, a SUS316L electrode P35a3, a Pt-type ECP sensor P35a4, and an 82 alloy electrode P35a5. The outputs of these sensors and electrodes are connected to the high-side input of the electrometer P51 via the scanner P52. The low-side of the electrometer P51 and the corrosion potential measuring manifold P50a are grounded.

[0032] The iron-type ECP sensors P35a1 and P35a2 directly measure the ECP of the corrosion potential measuring manifold P50a made of SUS316L. The reason for providing the two iron-type ECP sensors P35a1 and P35a2 is to provide redundancy in case one of the iron-type ECP sensors P35a1 and P35a2 unexpectedly fails. The SUS316L electrode P35a3 is made of the same material as the corrosion potential measurement manifold P50a. The SUS316L electrode P35a3 is used to measure the ECP of SUS316L. In principle, the SUS316L electrode P35a3 exhibits the same potential as the corrosion potential measurement manifold P50a. The Pt-type ECP sensor P35a4 is used to measure the ECP of the corrosion potential measuring manifold P50a, including the calibration of the iron-type ECP sensors P35a1 and P35a2 as a hydrogen electrode potential.

[0033] These iron-type ECP sensors P35a1 and P35a2, SUS316L electrode P35a3, and Pt-type ECP sensor P35a4 measure the potential difference between them and the corrosion potential measurement manifold P50a. Because each sensor has a known reference potential, the ECP of each electrode is measured by subtracting the effect of grounding from the measured potential difference of each electrode. The 82 alloy electrode P35a5 is formed of the same material as the 82 alloy in the welded portion of the shroud support P41 (see FIG. 1). The 82 alloy electrode P35a5 is used to measure the ECP of the 82 alloy in the corrosion potential measuring device A1.

[0034] The SUS316L, a 300-series stainless steel that forms the corrosion potential measurement manifold P50a, corresponds to the "first structural material." The iron-type ECP sensors P35a1 and P35a2, the SUS316L electrode P35a3, and the Pt-type ECP sensor P35a4 constitute the "first corrosion potential measurement unit." The 82 alloy corresponds to the "second structural material." The 82 alloy electrode P35a5 constitutes the "second corrosion potential measurement unit." The 82 alloy electrode P35a5 constituting the "second corrosion potential measurement section" is installed downstream of the iron-type ECP sensors P35a1, P35a2, the SUS316L electrode P35a3, and the Pt-type ECP sensor P35a4 constituting the "first corrosion potential measurement section."

[0035] In this embodiment, the second structural material is described using alloy 82 as an example, but is not limited to this. As the second structural material, a nickel-based alloy having a nickel / iron composition ratio of 10 or more and an iron composition of 7.5% or less, or a nickel-based alloy diluted by welding to have a nickel / iron composition ratio of 10 or more and an iron composition of 7% or less, is particularly preferred.

[0036] Secondary structural materials with such compositions are used for welding structural components inside the reactor. However, when exposed to oxidizing reactor water, the corrosion protection of the oxide film formed on their surface is reduced. Furthermore, FAC may occur when exposed to the flow of reactor water. When the ECP is reduced by the injection of precious metals and the atmosphere becomes reducing, the surface oxide film is expected to provide protection. Therefore, structural components made of such secondary structural materials are important targets for SCC protection inside the reactor. Examples of such second structural materials include the above-mentioned 82 alloy and 182 alloy.

[0037] Next, the mass transfer coefficient adjusting section P53 (see FIG. 3) will be described. 3 is a diagram illustrating the configuration of the mass transfer coefficient adjustment unit in the 82 alloy electrode P35a5 (second corrosion potential measurement unit). Note that in FIG. 3, the iron-type ECP sensor P35a2 (see FIG. 2), the SUS316L electrode P35a3 (see FIG. 2), and the Pt-type ECP sensor P35a4 (see FIG. 2) are omitted from the illustration. The mass transfer coefficient adjustment unit P53 simulates the flow conditions of the reactor water to which the 82 alloy electrode P35a5, which is the "second corrosion potential measurement unit" in the corrosion potential measurement device A1, is exposed to the flow conditions of the reactor water to which the weld (82 alloy) of the shroud support P41 (see Figure 1) inside the reactor is exposed.

[0038] The corrosion potential during injection of precious metals is determined by the concentrations of oxygen, hydrogen peroxide, and hydrogen in the reactor water, the amount of precious metals such as platinum attached, and the mass transfer coefficient (second mass transfer coefficient) in the area to be protected from corrosion cracking. The mass transfer coefficient is determined by the linear flow velocity at each position, which is determined from the core flow rate, and the hydraulic equivalent diameter. The mass transfer coefficient is determined in advance during the design of the nuclear power plant P100. The mass transfer coefficient can be set based on the reactor water flow velocity, representative diameter, reactor water temperature, etc., using commonly known approximation formulas or computer-based flow analysis methods.

[0039] The mass transfer coefficient (second mass transfer coefficient) in the area to be protected from corrosion cracking in the furnace bottom is approximately 1 × 10 -3 Specifically, by setting the linear flow velocity of the reactor water in the corrosion potential measurement manifold P50a at 2 to 3 m / s and the piping diameter at 2 to 3 cm, it is possible to simulate the mass transfer coefficient at the welded portion (Alloy 82) of the shroud support P41 (see Figure 1).

[0040] As shown in FIG. 3, the mass transfer coefficient adjusting portion P53 is formed by a reactor water flow path narrowing portion P53a (reducer) in which the reactor water flow path gradually narrows from the upstream side to the downstream side. According to this mass transfer coefficient adjusting unit P53, even when the amount of water flow cannot be set large from the viewpoint of radiation exposure management, the linear flow velocity can be increased so as to satisfy a predetermined mass transfer coefficient.

[0041] Fig. 4 is a configuration explanatory diagram of a first modified example of the mass transfer coefficient adjustment unit P53 shown in Fig. 3. Fig. 5 is a configuration explanatory diagram of a second modified example of the mass transfer coefficient adjustment unit P53 shown in Fig. 3. 4, the mass transfer coefficient adjuster P53 according to the first modification has a flow path adjuster P54 installed in the reactor water flow path of the corrosion potential measurement manifold P50a at a position facing the 82 alloy electrode P35a5. The reactor water flow path area in the corrosion potential measurement manifold P50a other than the position facing the 82 alloy electrode P35a5 is maintained at a normal flow path area. According to the first modification, the reactor water flow path of the corrosion potential measuring manifold P50a is partially narrowed at the position where the 82 alloy electrode P35a5 faces, and the linear flow velocity can be increased.

[0042] As shown in FIG. 5, the mass transfer coefficient adjustment unit P53 according to the second modified example is configured such that the inner diameter of the piping in the corrosion potential measurement manifold P50a is maintained at the normal size, and the electrode portion of the 82 alloy electrode P35a5 is made thin and needle-like and protrudes into the piping of the corrosion potential measurement manifold P50a. According to the second modification, the needle-shaped tip of the 82 alloy electrode P35a5 protruding into the piping is exposed to the flow of reactor water, thereby increasing the mass transfer coefficient. In this case, by using a cylindrical 82 alloy electrode with an outer diameter of about 1 to 2 mm, the mass transfer coefficient can be made equivalent to that at the reactor bottom even with a linear flow velocity of several tens of cm / s.

[0043] (Corrosion potential measurement method) Next, a method for measuring the corrosion potential will be described. As described above, the corrosion potential measuring method of this embodiment includes the step of adding hydrogen and a noble metal to reactor water. Taking platinum (Pt) as an example of a precious metal, platinum adheres to the surface of the corrosion potential measurement manifold P50a (see FIG. 2), the surface of the SUS316L electrode P35a3 (see FIG. 2), the surface of the 82 alloy electrode P35a5 (see FIG. 2), and the like during the precious metal injection process carried out before the nuclear power plant P100 (see FIG. 1) begins operation and the subsequent precious metal injection process carried out for 10 to 14 days during operation. As a result, platinum adheres to the surface of the corrosion potential measurement manifold P50a and the like to the same extent as the amount of platinum that adheres to the parts to be protected inside the reactor. When hydrogen is injected into the reactor water and the molar concentration of hydrogen exceeds 2, the ratio of the molar concentration of oxygen to the molar concentration of water, the platinum deposits exhibit a hydrogen electrode potential due to the electrochemical catalytic action of platinum. Under BWR operating conditions, the platinum deposits exhibit a hydrogen electrode potential of approximately -500 mV vs. SHE.

[0044] In addition, in this corrosion potential measurement method, as described above, iron-type ECP sensors P35a1, P35a2, a SUS316L electrode P35a3, and a Pt-type ECP sensor P35a4, which are the first corrosion potential measurement unit, are installed in a corrosion potential measurement manifold P50a made of 300 series stainless steel (first structural material) through which reactor water flows, to measure the corrosion potential of the corrosion potential measurement manifold P50a.

[0045] In this corrosion potential measurement method, an 82 alloy electrode P35a5 (second corrosion potential measurement unit) made of the same material as the 82 alloy (second structural material) constituting the welded portion of the shroud support P41 (see FIG. 1) is installed downstream of the first corrosion potential measurement unit. This second corrosion potential measurement unit measures the corrosion potential of the 82 alloy (second structural material) exposed to reactor water flowing through the corrosion potential measurement manifold P50a. In addition, this corrosion potential measurement method sets the mass transfer coefficient (first mass transfer coefficient) based on the reactor water flow conditions in the second corrosion potential measurement section (82 alloy electrode P35a5) so that it corresponds to the mass transfer coefficient (second mass transfer coefficient) based on the reactor water flow conditions facing the 82 alloy (second structural material) that forms the weld of the shroud support P41 (see Figure 1), which is the part to be protected from stress corrosion cracking at the bottom of the reactor.

[0046] That is, in this corrosion potential measurement method, the mass transfer coefficient adjustment unit P53 (see Figures 3 to 5) sets the flow conditions of the reactor water in the second corrosion potential measurement unit (82 alloy electrode P35a5) so as to simulate the flow conditions of the reactor water in the part to be protected from stress corrosion cracking at the bottom of the reactor (the welded part of the shroud support P41 (see Figure 1) made of 82 alloy). In this corrosion potential measurement method, the corrosion potential measured by the first corrosion potential measurement unit and the second corrosion potential measurement unit is monitored to monitor the state of precious metal adhesion to the surfaces of the first structural material and the second structural material when the precious metal is injected, the value of the corrosion potential during operation of the nuclear plant, and the increase in corrosion potential due to the decrease in precious metal due to desorption.

[0047] The bottom drain line P34 (see Figure 1) drawn from the bottom of the BWR reactor carries reactor water containing information on the water quality near the reactor bottom. The water quality information includes, for example, the concentrations of oxygen, hydrogen peroxide, and hydrogen, pH, electrical conductivity, and temperature. As mentioned above, by appropriately designing the flow rate and piping diameter of the corrosion potential measurement manifold P50a, it is possible to simulate the reactor water flow conditions in the reactor bottom, particularly the Alloy 82 welds in the shroud support P41 (see Figure 1). "Appropriately simulating" means designing the ECP and FAC so that the mass transfer coefficients are equivalent, since ECP and FAC are affected by mass transfer in terms of flow conditions.

[0048] This allows the effects of ECP and platinum desorption to be measured. If the corrosion potential sensor P35a (see Figure 2) is designed in this way and exposed to reactor water for a long period of time, platinum may be desorbed by FAC relatively faster in the case of Alloy 82 than in the case of SUS316L. In contrast, in this embodiment, the SUS316L electrode P35a3 (see Figure 2) is installed upstream of the 82 alloy electrode P35a5 (see Figure 2), so the ECP of the protected area in the reactor (the welded portion of the shroud support P41 (see Figure 1)) can be properly monitored without being affected by platinum desorbed from the 82 alloy that forms the 82 alloy electrode P35a5.

[0049] Next, the corrosion potential measuring device A1 and the corrosion potential measuring method according to this embodiment will be described in more detail. The corrosion potential measuring device A1 of this embodiment is a corrosion potential measuring device A1 for a nuclear plant P100 comprising a first structural material made of 300-series stainless steel and a second structural material made of 82 alloy, which undergoes flow-accelerated corrosion, and to which a precious metal is added so as to come into contact with reactor water containing hydrogen. The corrosion potential measuring device A1 comprises a first corrosion potential measuring unit (iron-type ECP sensors P35a1, P35a2, SUS316L electrode P35a3, and Pt-type ECP sensor P35a4) installed in a piping (corrosion potential measuring manifold P50a) through which reactor water flows and measures the corrosion potential of the first structural material, and a second corrosion potential measuring unit (82 alloy electrode P35a5) installed downstream of the first corrosion potential measuring unit and measures the corrosion potential of the second structural material, and the mass transfer coefficient based on the flow conditions of the reactor water in the second corrosion potential measuring unit is set to correspond to the mass transfer coefficient based on the flow conditions of the reactor water facing the second structural material, which forms the part in the reactor that is to be protected against stress corrosion cracking.

[0050] Specifically, the second corrosion potential measuring unit of the corrosion potential measuring device A1 has a mass transfer coefficient adjustment unit P53 that adjusts the mass transfer coefficient in the second corrosion potential measuring unit so as to simulate the mass transfer coefficient in the part to be protected from stress corrosion cracking inside the reactor (the welded part of the shroud support P41 (see Figure 1)).

[0051] More specifically, the mass transfer coefficient adjusting section P53 is a reactor water flow path narrowing section P53a that makes the flow velocity of the reactor water in the second corrosion potential measuring section faster than the flow velocity of the reactor water in the first corrosion potential measuring section.

[0052] According to such a corrosion potential measuring device A1, the mass transfer coefficient based on the flow conditions of the reactor water in the second corrosion potential measuring section is set to correspond to the mass transfer coefficient based on the flow conditions of the reactor water facing the second structural material that forms the SCC protection target section in the reactor bottom, so that the corrosion potential in the SCC protection target section in the reactor can be accurately evaluated. Furthermore, this corrosion potential measuring device A1 can measure the corrosion potential in an environment that simulates the reactor water flow conditions of the SCC protection target area at the reactor bottom, and evaluate the corrosion potential of the SCC protection target area at the reactor bottom.Therefore, unlike conventional ECP measuring devices (see, for example, Patent Document 4), which must be installed directly at the SCC protection target area at the reactor bottom, it is possible to construct a corrosion potential measuring mechanism at low cost.

[0053] Furthermore, with this corrosion potential measuring device A1, the corrosion potential is measured using the SUS316L electrode P35a3 and the 82 alloy electrode P35a5, both of which have platinum attached thereto. However, since platinum desorbs from the 82 alloy electrode P35a5 before it does from the SUS316L electrode P35a3, even if the ECP measurement value increases, it is possible to distinguish whether the increase is due to a change in the reactor water quality or a change caused by platinum desorption.

[0054] Furthermore, when making such a determination, the corrosion potential measuring device A1 is not affected by platinum desorbed from the 82 alloy forming the 82 alloy electrode P35a5 (see FIG. 2) because the SUS316L electrode P35a3 (see FIG. 2) is installed upstream of the 82 alloy electrode P35a5 (see FIG. 2). In other words, the corrosion potential measuring device A1 can more accurately determine such a determination. Furthermore, because the corrosion potential measuring device A1 can determine a decrease in corrosion potential due to platinum desorption, it can appropriately determine the timing for re-application of precious metal injection.

[0055] Furthermore, the corrosion potential measurement method of this embodiment includes a step of adding hydrogen and a precious metal to reactor water, and is a corrosion potential measurement method for a nuclear power plant P100 having a first structural material made of 300-series stainless steel and a second structural material made of 82 alloy, which undergoes flow-accelerated corrosion. In this method, a first corrosion potential measurement unit (iron-type ECP sensors P35a1, P35a2, SUS316L electrode P35a3, and Pt-type ECP sensor P35a4) for measuring the corrosion potential of the first structural material is installed in a piping (corrosion potential measurement manifold P50a) through which the reactor water flows, and a second corrosion potential measurement unit (82 alloy electrode P35a5) for measuring the corrosion potential of the second structural material is installed downstream of the first corrosion potential measurement unit, and a mass transfer coefficient based on the flow conditions of the reactor water in the second corrosion potential measurement unit is set to correspond to the mass transfer coefficient based on the flow conditions of the reactor water facing the second structural material, which forms a part in the reactor that is to be protected against stress corrosion cracking. Such a corrosion potential measuring method has the same effects as those of the corrosion potential measuring device A1 described above.

[0056] In addition, in this corrosion potential measurement method, the second structural material is preferably a nickel-based alloy having a portion where the nickel / iron composition ratio is 10 or more and the iron composition is 7.5% or less, and a portion where the nickel / iron composition ratio is 10 or more and the iron composition is 7% or less due to dilution by welding.

[0057] According to this corrosion potential measurement method, when the second structural material having such a composition is used for welding structural members in a reactor, and the ECP is lowered by the injection of a noble metal, creating a reducing atmosphere, the surface oxide film is expected to be protective. That is, in this corrosion potential measurement method, the structural members formed from such second structural materials are important parts to be protected from SCC in the reactor.

[0058] [Second embodiment] Next, a corrosion potential measuring device according to a second embodiment of the present invention will be described. 6 is a diagram illustrating the configuration of an electrochemical corrosion potential measuring device A2 according to a second embodiment of the present invention. Note that in this electrochemical corrosion potential measuring device A2, the SUS316L electrode P35a3 (see FIG. 2) of the electrochemical corrosion potential measuring device A1 (see FIG. 2) is omitted. In the second embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0059] As shown in Figure 6, the corrosion potential measuring device A2 differs from the corrosion potential measuring device A1 of the first embodiment (see Figure 2) in that the 82 alloy electrode P35a5 (second corrosion potential measuring unit) is installed in parallel with the first corrosion potential measuring unit consisting of iron-type ECP sensors P35a1, P35a2, and Pt-type ECP sensor P35a4. Specifically, the 82 alloy electrode P35a5 (second corrosion potential measuring portion) is disposed in a bypass portion of the corrosion potential measuring manifold P50a where the flow of reactor water branches off.

[0060] With this corrosion potential measuring device A2, when platinum adhering to the 82 alloy electrode P35a5 is desorbed by the FAC of the 82 alloy, the first corrosion potential measuring unit (the iron-type ECP sensors P35a1 and P35a2 and the Pt-type ECP sensor P35a4) is not affected by the desorbed platinum. In other words, the corrosion potential measuring device A2 can more accurately distinguish between these two. Furthermore, because the corrosion potential measuring device A1 can determine the decrease in corrosion potential due to platinum desorption, it can appropriately determine the timing for re-application of precious metal injection.

[0061] [Third embodiment] Next, a corrosion potential measuring device according to a third embodiment of the present invention will be described. 7 is a diagram illustrating the configuration of an electrochemical corrosion potential measuring device A3 according to a second embodiment of the present invention. Note that in this electrochemical corrosion potential measuring device A3, the iron-type ECP sensor P35a2 (see FIG. 2), the SUS316L electrode P35a3 (see FIG. 2), and the Pt-type ECP sensor P35a4 (see FIG. 2) of the electrochemical corrosion potential measuring device A1 (see FIG. 2) are omitted. In the third embodiment, the same components as those in the first and second embodiments are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0062] As shown in FIG. 7, the corrosion potential measuring device A3 has an 82 alloy electrode P35a6 that faces the inside of the corrosion potential measuring manifold P50a between the iron-type ECP sensor P35a1 and the 82 alloy electrode P35a5. The 82 alloy electrode P35a6 is made of the same 82 alloy material as the 82 alloy electrode P35a5.

[0063] The tip of the 82 alloy electrode P35a6 is set at a position farther from the corrosion potential measuring manifold P50a than the tip of the 82 alloy electrode P35a5. As a result, the 82 alloy electrode P35a6 is exposed to the slow-flowing reactor water that flows as a vortex from the corrosion potential measurement manifold P50a into the T-shaped pipe P56 in which the 82 alloy electrode P35a6 is placed. On the other hand, the 82 alloy electrode P35a5, which is arranged downstream of the 82 alloy electrode P35a6, is exposed to the fast flowing reactor water flowing through the corrosion potential measurement manifold P50a. As a result, the 82 alloy electrode P35a6 is less susceptible to FAC than the 82 alloy electrode P35a5, and platinum detachment due to FAC occurs later in the 82 alloy electrode P35a6 than in the 82 alloy electrode P35a5. That is, in the corrosion potential measuring device A3, the reactor water flow conditions in the second corrosion potential measuring section equipped with the 82 alloy electrodes P35a5 and P35a6 are set to at least two different flow velocities.

[0064] In addition, in such a corrosion potential measuring device A3, the corrosion potential measured by the first corrosion potential measuring unit composed of the iron-type ECP sensor P35a1 and the second corrosion potential measuring unit composed of the 82 alloy electrodes P35a5 and P35a6 is monitored to monitor the state of precious metal adhesion to the surfaces of the first structural material (300 series stainless steel: SUS316L) and the second structural material (82 alloy) during precious metal injection, the corrosion potential value during operation of the nuclear plant P100 (see Figure 1), and the increase in the corrosion potential associated with the decrease in precious metal due to desorption, and the flow conditions of the reactor water in the second corrosion potential measuring unit are set to realize at least two mutually different reactor water flow velocities, thereby implementing a corrosion potential measuring method.

[0065] According to the corrosion potential measuring device A3 and corrosion potential measuring method of the third embodiment, when the corrosion potential measured by the 82 alloy electrode P35a5 exposed to reactor water under high-velocity flow conditions increases, it is possible to distinguish whether the increase in corrosion potential is due to platinum desorption or a change in water chemistry by comparing the corrosion potential measured by the 82 alloy electrode P35a6 exposed to reactor water under low-velocity flow conditions. This will be described in more detail with reference to FIGS. 8A, 8B, and 9. Fig. 8A is a graph showing the relationship between the amount of platinum deposition and corrosion potential. Fig. 8B is a graph showing the relationship between changes in reactor water quality and corrosion potential. Fig. 9 is a graph showing the relationship between the operating time of the nuclear power plant P100 (see Fig. 1) and changes in corrosion potential when platinum desorption occurs.

[0066] In the noble metal implantation process, platinum is deposited on selected structural materials to reduce ECP through electrochemical catalysis of hydrogen. As shown in Figure 8A, when the amount of platinum deposited on the structural material is large, the corrosion potential (V) is the same as the oxidation-reduction potential of platinum. The corrosion potential (V) corresponds to the hydrogen electrode potential. Furthermore, as the amount of platinum deposited on the structural material decreases, the corrosion potential (V) increases and gradually approaches the corrosion potential (V) of alloy 82.

[0067] Furthermore, if we take the hydrogen peroxide concentration as an example of the water quality of the reactor water and show its relationship with the corrosion potential (V), as shown in Figure 8B, when the hydrogen peroxide concentration in the reactor water is sufficiently low and the hydrogen concentration in the reactor water is constant, the corrosion potential (V) of the 82 alloy with platinum attached will match the oxidation-reduction potential of platinum. Furthermore, as the hydrogen peroxide concentration in the reactor water increases, the corrosion potential (V) increases and gradually approaches the corrosion potential (V) of alloy 82. In other words, in an operating nuclear power plant P100 (see Figure 1), it is difficult to distinguish whether an increase in the corrosion potential (V) of structural materials with platinum attached is due to a change in the reactor water quality or due to platinum desorption from the structural materials.

[0068] In contrast, the corrosion potential measuring device A3 and corrosion potential measuring method of this embodiment measure the corrosion potential in the low flow velocity portion of the reactor water using the 82 alloy electrode P35a6 in the corrosion potential measuring manifold P50a (see Figure 7), and measure the corrosion potential in the high flow velocity portion of the reactor water using the 82 alloy electrode P35a5, as described above. That is, in the corrosion potential measuring device A3 and corrosion potential measuring method, as shown in Figure 9, the 82 alloy electrode P35a5, which is used to measure corrosion potential in the high flow rate section, has a larger increase gradient in the corrosion potential (V) value with respect to operating time than the 82 alloy electrode P35a6, which is used to measure corrosion potential in the low flow rate section. Incidentally, in Figure 9, the dotted line extending parallel to the axis of operating time indicates the corrosion potential (V) before platinum injection. In other words, by injecting platinum, the corrosion potential (V) decreases to the same as the oxidation-reduction potential of platinum.

[0069] 9, the area surrounded by the dotted circle indicates the region where the corrosion potential (V) measured with the 82 alloy electrode P35a6 remains low, while the corrosion potential (V) measured with the 82 alloy electrode P35a5 increases significantly. In other words, the area surrounded by the dotted circle indicates that platinum desorption is hardly occurring with the 82 alloy electrode P35a6, while platinum desorption is progressing with the 82 alloy electrode P35a5. That is, as shown in Figure 9, both the 82 alloy electrode P35a6 (corresponding to the low flow rate section) and the 82 alloy electrode P35a5 (corresponding to the high flow rate section) exhibit the oxidation-reduction potential of platinum as long as a certain amount or more of platinum is attached, even if the water quality changes. Furthermore, if platinum desorption occurs in the 82 alloy electrode P35a5 (corresponding to the high flow rate section), the corrosion potential (V) rises rapidly only in the high flow rate section. As a result, the corrosion potential measurement device A3 and corrosion potential measurement method of this embodiment can distinguish between the effects of changes in water quality and platinum desorption. In other words, by adjusting the mass transfer coefficient in the high-velocity section to be the same as that in the furnace bottom, it is possible to monitor, by ECP monitoring outside the furnace, whether or not the effect of noble metal injection is being obtained in the SCC protection target section in the furnace bottom.

[0070] [Fourth embodiment] Next, a corrosion potential measuring device according to a fourth embodiment of the present invention will be described. In the fourth embodiment, the same components as those in the first to third embodiments are denoted by the same reference numerals, and detailed description thereof will be omitted. Fig. 10 is a diagram illustrating the configuration of a nuclear power plant P100 having a boiling water reactor (BWR) to which an electrochemical corrosion potential measurement device according to a fourth embodiment of the present invention is applied. Fig. 11 is a diagram illustrating the configuration of a purification system of the nuclear power plant P100 shown in Fig. 10, in which an electrochemical corrosion potential measurement device according to the fourth embodiment of the present invention is installed. Fig. 12 is a diagram illustrating the configuration of an electrochemical corrosion potential measurement device A4 according to the fourth embodiment of the present invention.

[0071] As shown in Fig. 10, the nuclear power plant P100 of this embodiment differs from the nuclear power plant P100 of the first embodiment (see Fig. 1) in that it is provided with an oxygen injection device P43, but is otherwise configured similarly to the nuclear power plant P100 of the first embodiment (see Fig. 1). The nuclear power plant P100 of this embodiment is configured to measure ECP in order to monitor FAC in the purification system of a BWR to which noble metal injection is applied.

[0072] As shown in Fig. 10, in a nuclear power plant P100, an oxygen injection device P43 is connected to a purification system piping P20 via an oxygen injection pipe P44. Specifically, the oxygen injection pipe P44 is connected to a section of the purification system piping P20 between the pressure vessel P12 and the regenerative heat exchanger P25. The oxygen injection pipe P44 is provided with an open / close valve P45. The oxygen injection device P43 is a device for injecting oxygen and hydrogen peroxide, and injects oxygen gas and an aqueous solution of hydrogen peroxide into the cooling water extracted from the reactor P1. As shown in FIG. 10, the purification system constituting the nuclear power plant P100 in this embodiment includes a regenerative heat exchanger P25 and a non-regenerative heat exchanger P26 in the course of the purification system piping P20.

[0073] As shown in Fig. 11, the regenerative heat exchanger P25 and the non-regenerative heat exchanger P26 are each composed of multiple stages of heat exchangers. The regenerative heat exchanger P25 is composed of three stages of heat exchangers P25a, P25b, and P25c. The non-regenerative heat exchanger P26 is composed of two stages of heat exchangers P26a and P26b. The heat exchangers are connected to each other via connecting pipes P42a, P42b, P42c, P42d, P42e, and P42f. Generally, the regenerative heat exchanger P25 is composed of three stages, and the non-regenerative heat exchanger P26 is composed of two stages. However, the number of stages of the heat exchangers is not particularly limited.

[0074] Returning to FIG. 10, cooling water extracted from the reactor P1 is sent to the regenerative heat exchanger P25 through the purification system piping P20. As shown in FIG. 11, the cooling water is cooled in the heat exchangers P25a, P25b, and P25c of each stage by heat exchange with cooling water purified in the reactor water purification system P27. Then, the cooling water is introduced into the non-regenerative heat exchanger P26 through the connecting pipe P42c. The cooling water is cooled in the heat exchangers P26a and P26b of each stage by heat exchange with auxiliary cooling water from a cooling system (not shown). Then, the pressure of the cooling water is increased by the purification system pump P24 and introduced into the reactor water purification system P27.

[0075] Flow-accelerated corrosion (FAC) is electrochemical corrosion related to mass transfer resulting from the dissolution of oxide films. FAC depends on the flow rate 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 as the flow rate of the cooling water increases. It also increases as the temperature of the cooling water increases, generally reaching a maximum around 130-150°C. It also increases when the dissolved oxygen concentration of the cooling water falls below about 15 ppb.

[0076] During operation of the nuclear power plant P100 (see FIG. 10), the temperature of the cooling water extracted from the pressure vessel P12 (see FIG. 10) reaches a high temperature of approximately 280°C. The temperature of the cooling water extracted to the purification system decreases as it passes through each stage of the regenerative heat exchanger P25 and the non-regenerative heat exchanger P26 shown in FIG. 11. The temperature of the cooling water downstream of the non-regenerative heat exchanger P26 is 80°C or lower. However, the temperature is relatively high upstream and downstream of the purification system. In addition, the flow velocity in the purification system is high, at approximately several meters per second, due to the small diameter of the piping.

[0077] When hydrogen or precious metals are injected during operation of the nuclear power plant P100 (see Figure 10), the oxygen and hydrogen peroxide concentrations in the cooling water also decrease in the purification system. In particular, when precious metals are injected, excess hydrogen is likely to be generated, making it easier for the amount of hydrogen to oxygen to exceed the stoichiometric ratio in 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.

[0078] Furthermore, the connecting pipes P42a, P42b, P42c, P42d, P42e, and P42f shown in FIG. 11 and the section of the purification system piping P20 shown in FIG. 10 outside the pressure vessel P12 (see FIG. 10) are generally made of carbon steel. These piping may have structures that affect the flow rate. Examples of structures that affect the flow rate include elbows, vents, tees, orifices, and valves.

[0079] The piping constituting the purification system, particularly the heating-side connecting pipe P42a connecting the first-stage heat exchanger P25a and the second-stage heat exchanger P25b of the regenerative heat exchanger P25 shown in Fig. 11, the heating-side connecting pipe P42b connecting the second-stage heat exchanger P25b and the third-stage heat exchanger P25c, and the cooling-side connecting pipe P42f connecting the second-stage heat exchanger P25b and the first-stage heat exchanger P25a, are made of carbon steel. Because these pipes are exposed to high flow rates and high temperatures of 150°C to 220°C, they are at high risk of FAC when the oxygen concentration of the cooling water decreases.

[0080] In the nuclear power plant P100 (see FIG. 10), piping sections with a high risk of FAC are selected as monitoring targets. As a measure to suppress FAC in the monitored section, oxygen injection or hydrogen peroxide injection can be performed using the oxygen injection device P43 (see FIG. 10). Like hydrogen injection and precious metal injection, oxygen injection and hydrogen peroxide injection can be performed while the nuclear power plant P100 (see FIG. 10) is in operation. Oxygen injection is a technique in which oxygen gas is injected into cooling water to increase the oxygen concentration in the cooling water. Hydrogen peroxide injection is a technique in which an aqueous solution of hydrogen peroxide is injected into cooling water to increase the hydrogen peroxide concentration in the cooling water.

[0081] When the oxidizer concentration in the coolant water is increased, an oxide film is formed on the surface of materials that come into contact with the coolant water. For example, in the case of carbon steel, hematite (Fe2O3) is formed. Hematite has a lower solubility than magnetite (Fe3O4), and forms a dense oxide film. When the oxidizer concentration in the coolant water is increased, the oxide film remains stable even when parameters related to mass transfer, such as flow rate, increase. This makes it possible to suppress the FAC rate of materials that come into contact with the coolant water. Whether the reactor water quality is in the hematite formation area can be easily determined by monitoring the ECP.

[0082] In the purification system shown in Fig. 11, the heat-dissipation-side connecting pipe P42b connecting the second-stage heat exchanger P25b and the third-stage heat exchanger P25c of the regenerative heat exchanger P25 reaches a temperature of approximately 170°C, posing a high risk of FAC. Therefore, in the nuclear power plant P100 of this embodiment (see Fig. 10), as shown in Fig. 11, reactor water drawn from the drain at the tube-side outlet of the heat exchanger P25b is passed through a corrosion potential measurement manifold P50b, and a corrosion potential sensor P35f is installed in this corrosion potential measurement manifold P50b. Incidentally, the reactor water flowing out of the corrosion potential measurement manifold P50b is connected to a sampling system of the reactor purification system and then discharged.

[0083] 11, the heat-receiving-side connecting pipe P42f connecting the second-stage heat exchanger P25b and the first-stage heat exchanger P25a of the regenerative heat exchanger P25 also has a high FAC risk. Therefore, in the nuclear power plant P100 of this embodiment (see FIG. 10), reactor water drawn from the drain on the shell side of the heat exchanger P25a is passed through a corrosion potential measurement manifold P50c, and a corrosion potential sensor P35g is installed in this corrosion potential measurement manifold P50c. Incidentally, the reactor water flowing out from the corrosion potential measurement manifold P50c is connected to a sampling system of the reactor purification system and then drained.

[0084] The corrosion potential sensors P35f and P35g are sensors for measuring the corrosion potential (ECP) of carbon steel that comes into contact with reactor water. The corrosion potential sensor P35f attached to the corrosion potential measurement manifold P50b and the corrosion potential sensor P35g attached to the corrosion potential measurement manifold P50c each individually constitute a corrosion potential measuring device A4 according to this embodiment. Since the corrosion potential measuring device A4 having the corrosion potential sensor P35f and the corrosion potential measuring device A4 having the corrosion potential sensor P35g have the same structure, only the corrosion potential measuring device A4 having the corrosion potential sensor P35f will be described below, and a description of the corrosion potential measuring device A4 having the corrosion potential sensor P35g will be omitted.

[0085] 12, the corrosion potential measuring device A4 of this embodiment has, in order from the upstream side of the corrosion potential measuring manifold P50b, an iron-type ECP sensor P35f1, a carbon steel electrode P35f2, and a carbon steel electrode P35f3 so as to face the interior of the corrosion potential measuring manifold P50b. Although not shown in the figure, the corrosion potential measuring device A4 also has a Pt-type ECP sensor downstream of the carbon steel electrode P35f3.

[0086] The iron-type ECP sensor P35f1 directly measures the ECP of the corrosion potential measurement manifold P50b, which is made of 300 series stainless steel (SUS316L), which is the first structural material. A Pt-type ECP sensor (not shown) is used to measure the ECP of the corrosion potential measurement manifold P50b, including calibrating the iron-type ECP sensor P35f1 as a hydrogen electrode potential. That is, the iron-type ECP sensor P35f1 constitutes a first corrosion potential measuring unit. The two carbon steel electrodes P35f2 and P35f3 (second corrosion potential measurement unit) are made of the same material as the carbon steel (second structural material) that constitutes the purification system to be protected. That is, the carbon steel electrodes P35f2 and P35f3 constitute the second corrosion potential measurement unit.

[0087] The tip of the carbon steel electrode P35f2 is set at a position farther from the corrosion potential measuring manifold P50b than the tip of the carbon steel electrode P35f3. As a result, the carbon steel electrode P35f2 is exposed to the slow-flowing reactor water that flows as a vortex from the corrosion potential measurement manifold P50b into the T-shaped pipe P56 in which the carbon steel electrode P35f2 is placed. On the other hand, the carbon steel electrode P35f3, which is arranged downstream of the carbon steel electrode P35f2, is exposed to the fast flowing reactor water flowing through the corrosion potential measuring manifold P50b. As a result, the carbon steel electrode P35f2 is less likely to cause FAC than the carbon steel electrode P35f3, and the carbon steel electrode P35f2 causes platinum detachment due to FAC later than the carbon steel electrode P35f3. That is, in the corrosion potential measuring device A4, the reactor water flow conditions in the second corrosion potential measuring section equipped with the carbon steel electrodes P35f2 and P35f3 are set to at least two different flow velocities.

[0088] In addition, in such a corrosion potential measuring device A4, the corrosion potential measured by the first corrosion potential measuring unit composed of the iron-type ECP sensor P35a1 and the second corrosion potential measuring unit composed of the carbon steel electrodes P35f2 and P35f3 is monitored to monitor the state of precious metal adhesion to the surfaces of the first structural material (300 series stainless steel: SUS316L) and the second structural material (e.g., the carbon steel forming the connecting pipes P42b and P42f) during precious metal injection, the corrosion potential value during operation of the nuclear plant P100 (see FIG. 1), and the increase in corrosion potential associated with the decrease in precious metal due to desorption, and the flow conditions of the reactor water in the second corrosion potential measuring unit are set to realize at least two mutually different reactor water flow velocities, thereby implementing a corrosion potential measuring method. That is, in the corrosion potential measuring method of this embodiment, the second structural material is carbon steel, and a first mass transfer coefficient based on the flow conditions of the reactor water facing the second structural material is set to correspond to a second mass transfer coefficient based on the flow conditions of the reactor water purification system piping.

[0089] FAC of carbon steel may become apparent when corrosion mitigation technology is applied over the long term. In particular, when precious metals are injected, platinum may adhere to the surface of carbon steel piping in the purification system, which may affect FAC. According to the corrosion potential measuring device A4 and the corrosion potential measuring method of this embodiment, it is possible to monitor the FAC rate and the adhesion and desorption of platinum in the connecting pipes P42b and P42f. Furthermore, according to the corrosion potential measuring device A4 and the corrosion potential measuring method of this embodiment, by monitoring the ECP, it is possible to monitor whether or not the ECP region where FAC occurs (a region where magnetite and iron ions are stable, which is lower than -0.3 to -0.2 vs. SHE at 150°C) is entered.

[0090] Although the first to fourth embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention. Furthermore, the elements of the first to fourth embodiments can also be appropriately combined to configure the corrosion potential measuring device and corrosion potential measurement of the present invention.

[0091] In addition, in the above embodiment, the corrosion potential measuring devices A1, A2, A3, and A4 and the corrosion potential measuring method applied to the nuclear plant P100 using a boiling water reactor (BWR) have been described, but the corrosion potential measuring device of the present invention can also be applied to nuclear plants using an advanced boiling water reactor (ABWR) or a pressurized water reactor (PWR).

[0092] FIG. 13 is a diagram illustrating the configuration of a nuclear power plant having an advanced boiling water reactor (ABWR) to which the corrosion potential measuring device of the present invention is applied. As shown in FIG. 13, a nuclear power plant P100 using an ABWR is configured to measure ECP in reactor water above a downcomer P17. The corrosion potential measuring device in the nuclear plant P100 is configured to include an corrosion potential sensor P35e. The corrosion potential sensor P35e is connected above the downcomer P17 and is installed on a flange P36 of a pipe P20a connected to the purification system pipe P20.

[0093] Furthermore, the electrochemical corrosion potential measuring device in the nuclear plant P100 may also be configured to include an electrochemical corrosion potential sensor P35a, as shown in FIG. The corrosion potential sensor P35a can be installed in a corrosion potential measurement manifold P50a provided in a branch line P34a of the bottom drain line P34. The corrosion potential sensor P35a measures the ECP at the reactor bottom of the nuclear power plant P100 that uses ABWR, and is important as a countermeasure against SCC of the shroud support of the ABWR. In general, in nuclear power plants, when hydrogen injection is not performed during startup, shutdown, hydrogen injection trip, etc., the reactor water may become oxidized for a short period of time, which may lead to platinum desorption from the structural materials. By using electrodes made of the same material as these structural materials, the nuclear power plant P100 can perform ECP monitoring that improves the reliability of the nuclear power plant P100.

[0094] In the first embodiment, as shown in FIG. 2, the corrosion potential sensor P35a is described in which an 82 alloy electrode P35a5 (first corrosion potential measurement unit) is installed downstream of the iron-type ECP sensors P35a1 and P35a2 (first corrosion potential measurement units). FIG. 14 is a structural explanatory diagram showing a modified example of the electrochemical corrosion potential sensor P35a (see FIG. 2) in the first embodiment. 14, in the modified electrochemical corrosion potential sensor P35a, a T-pipe P56 in which an iron-type ECP sensor P35a1 is installed and an electrochemical corrosion potential measurement manifold P50a are formed at a welded portion P55 made of alloy 82. Such a welded portion P55 made of alloy 82 is exposed to reactor water.

[0095] The iron-type ECP sensor P35a1 of the electrochemical corrosion potential sensor P35a according to this modification measures the ECP of the electrochemical corrosion potential measurement manifold P50a (including the T-shaped pipe P56) closest to the iron-type ECP sensor P35a1 at its tip. That is, the iron-type ECP sensor P35a1 measures the ECP of the weld P55 made of 82 alloy in the electrochemical corrosion potential measurement manifold P50a. An iron-type ECP sensor P35a1 having such a weld P55 made of 82 alloy can be used as a substitute for the 82 alloy electrode P35a5 (see Figure 2) in the first embodiment by being positioned downstream of the iron-type ECP sensor P35a1 (see Figure 2) in the first embodiment. In addition, the iron-type ECP sensor P35a1 (see Figure 14) of the modified corrosion potential sensor P35a can be configured to have a corrosion potential measuring manifold P50a in which only the part where the iron-type ECP sensor P35a1 is joined is partially formed of alloy 82, instead of providing a welded part P55. [Explanation of symbols]

[0096] A1 Corrosion potential measuring device A2 Corrosion potential measuring device A3 Corrosion potential measuring device A4 Corrosion potential measuring device P35a1 Iron-type ECP sensor (first corrosion potential measurement section) P35a2 Iron-type ECP sensor (first corrosion potential measurement section) P35a3 SUS316L electrode (first corrosion potential measurement part) P35a4 Pt-type ECP sensor (first corrosion potential measurement section) P35a5 82 alloy electrode (second corrosion potential measurement part) P35a6 82 alloy electrode (second corrosion potential measurement part) P35f1 Iron-type ECP sensor (first corrosion potential measurement section) P35f2 Carbon steel electrode (second corrosion potential measurement part) P35f3 Carbon steel electrode (second corrosion potential measurement part) P50a Corrosion Potential Measurement Manifold (Piping) P50b Corrosion Potential Measurement Manifold (Piping) P50c Corrosion Potential Measurement Manifold (Piping) P34 Bottom drain line (piping) P53 Mass transfer coefficient adjustment section P53a Reactor water flow path narrowing part P100 Nuclear Plant

Claims

1. 1. A method for measuring corrosion potential in a nuclear power plant, the method comprising: adding hydrogen and a noble metal to reactor water; and the method comprising: adding a first structural material made of 300 series stainless steel to reactor water; and adding a noble metal to reactor water; a first corrosion potential measuring unit for measuring the corrosion potential of the first structural material is installed in a pipe through which reactor water flows, and a second corrosion potential measuring unit for measuring the corrosion potential of the second structural material is installed downstream of the first corrosion potential measuring unit or in parallel with the first corrosion potential measuring unit; a corrosion potential measuring method for measuring a corrosion potential of a reactor, the corrosion potential measuring unit being configured to measure a first mass transfer coefficient based on a flow condition of reactor water in the second corrosion potential measuring unit, so as to correspond to a second mass transfer coefficient based on a flow condition of reactor water facing the second structural material that forms a portion to be protected against stress corrosion cracking within the reactor;

2. the piping is a bottom drain line through which reactor water flows out from the reactor bottom, the second structural material is Alloy 82; By monitoring the corrosion potentials measured by the first corrosion potential measurement unit and the second corrosion potential measurement unit, the state of adhesion of the precious metal to the surfaces of the first structural material and the second structural material at the time of the precious metal injection, the value of the corrosion potential during operation of the nuclear power plant, and the increase in the corrosion potential due to the reduction of the precious metal due to desorption are monitored; 2. The corrosion potential measuring method according to claim 1, wherein the flow conditions of the reactor water in the second corrosion potential measuring section are set to realize at least two different reactor water flow velocities.

3. 3. The corrosion potential measuring method according to claim 1, wherein a reactor water flow rate in the piping, a representative diameter of the piping, and a reactor water temperature are set so that a first mass transfer coefficient at at least one portion in the second corrosion potential measuring unit is equal to a second mass transfer coefficient at a portion in the reactor that is to be protected against stress corrosion cracking.

4. 2. The corrosion potential measuring method according to claim 1, wherein the second structural material is a nickel-based alloy having a portion where the nickel / iron composition ratio is 10 or more and the iron composition is 7.5% or less, and a portion where the nickel / iron composition ratio is 10 or more and the iron composition is 7% or less due to dilution by welding.

5. 2. The corrosion potential measuring method according to claim 1, wherein the second structural material is carbon steel, and a first mass transfer coefficient based on a flow condition of reactor water facing the second structural material is set to correspond to a second mass transfer coefficient based on a flow condition of reactor water purification system piping.

6. 1. A corrosion potential measuring device for use in a nuclear power plant, comprising: a first structural material made of 300 series stainless steel; and a second structural material that undergoes flow-accelerated corrosion, and in which a noble metal is provided so as to come into contact with reactor water containing hydrogen, a first corrosion potential measuring unit that is installed in a pipe through which reactor water flows and that measures the corrosion potential of the first structural material; a second corrosion potential measuring unit that is installed downstream of the first corrosion potential measuring unit or in parallel with the first corrosion potential measuring unit and that measures the corrosion potential of the second structural material; and a corrosion potential measuring device in which a first mass transfer coefficient based on the flow conditions of reactor water in the second corrosion potential measuring section is set to correspond to a second mass transfer coefficient based on the flow conditions of reactor water facing the second structural material that forms a portion to be protected against stress corrosion cracking within the reactor.

7. 7. The corrosion potential measuring device according to claim 6, wherein the second corrosion potential measuring unit has a mass transfer coefficient adjusting unit that adjusts the first mass transfer coefficient in the second corrosion potential measuring unit so as to simulate a second mass transfer coefficient in a part to be protected against stress corrosion cracking in a reactor.

8. 8. The corrosion potential measuring device according to claim 7, wherein the mass transfer coefficient adjusting unit is a reactor water flow path narrowing unit that makes the flow velocity of the reactor water in the second corrosion potential measuring unit faster than the flow velocity of the reactor water in the first corrosion potential measuring unit.

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