JP2007178157A - Method for preventive maintenance of structure in nuclear power plant - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for the preventive maintenance of structures in a nuclear power plant which restrains stress corrosion cracking (SCC) from appearing with neither high residual stress nor high hardness portions remaining in any members of the structures. <P>SOLUTION: The method for the preventive maintenance of the structures in the nuclear power plant in this invention includes processes 11 and 12 for evaluating the stress distribution and/or the strength distribution of the structures in the nuclear power plant, a process 13 for judging a region requiring any measure against the SCC in the structures on the basis of the evaluation in the processes 11 and 12 and a process 14 for removing a region tangent to a corrosion environment in the region judged to require the measure against stress corrosion cracking. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a preventive maintenance method for a nuclear plant structure.

  The reactor internal structure placed inside the reactor pressure vessel uses materials with excellent corrosion resistance and high temperature strength, such as austenitic stainless steel or high nickel alloy, but for long periods in high temperature and high pressure environments. There is a risk of material degradation due to extended operation and neutron irradiation.

  In particular, stress corrosion cracking may occur potentially at the welded portion of the furnace internal structure or in the vicinity thereof due to the sensitization of the material due to welding heat input and the influence of tensile residual stress.

  This stress corrosion cracking (hereinafter referred to as SCC) is a cracking phenomenon caused by the application of tensile stress to a metal material in a corrosive environment. There are three factors: material, environment and stress. It is thought to occur when they overlap.

  Of these three factors, the stress factor is considered to have an effect on the occurrence of SCC due to the tensile stress, strain, and slow deformation progressing slower than the corrosion rate that continuously destroy the passive film. Yes.

  In addition, regarding the SCC factors that occur in materials such as stainless steel and nickel base alloys in the primary cooling system of light water reactors, the intergranular stress corrosion cracking (Inter granular SSC) in the weld heat affected zone of ordinary stainless steel It is said that it was caused by the overlap of material sensitization due to the influence and welding residual stress.

  Furthermore, in the low carbon stainless steel, the main cause of the occurrence of SCC is that the three factors of the hardened layer by machining or the like remaining in the surface layer portion, the tensile residual stress by welding, and processing overlap.

  For this reason, as a means for preventing the occurrence of SCC, improvement of materials and corrosive environments such as improvement of corrosion resistance of materials and improvement of reactor operating water quality have been attempted.

  As for the stress factor, for example, as seen in Non-Patent Document 1, surface compression stress is applied to the target portion, or in the case of piping, construction is performed such as applying a compression force to the inner surface by high-frequency heating. It was.

  Among such technological transitions and developments, many inventions such as Patent Document 1, Patent Document 2, and Patent Document 3 are disclosed as technologies related to SCC countermeasures.

  For example, in Patent Document 1, laser beam is irradiated to the reactor internal structure to improve the residual stress, and in Patent Document 2, water jet peening is applied to the target portion to improve the residual stress. Moreover, in patent document 3, in order to correct a welding deformation after welding construction, the weld metal of martensite transformation start temperature 300-150 degreeC or 400-200 degreeC is attached to the edge part of a welding part as an additional bead. I am letting.

Thus, in the conventional structure, the construction means suitable for the event occurring in the structure itself is selected, and preventive maintenance for maintaining high strength is performed.
Japanese Patent No. 3306040 Japanese Patent No. 299545 JP 2003-103393 A Nuclear Encyclopedia URL http: // mext-atm. jst. go. jp / atomica / 020070222

  When applying the above-mentioned Patent Documents 1 to 3 or Non-Patent Document 1 as a measure for improving the stress of a structural member, although the residual stress of the structural member is improved, the high hardness portion still remains on the surface of the structural member. is doing.

  For this reason, when a new load is applied from the outside in some form such as additional repair welding, the possibility of occurrence of SCC may increase again.

  The present invention has been made based on such circumstances, and a preventive maintenance method for a nuclear plant structure that suppresses the occurrence of SCC without leaving a high residual stress or a high hardness portion after the surface treatment of the structural member. The purpose is to provide.

  In order to achieve the above object, the preventive maintenance method for a nuclear plant structure according to the present invention includes a step of evaluating one or both of stress distribution and strength distribution of the nuclear plant structure, and this evaluation. A method comprising: determining a region where stress corrosion cracking countermeasures of the structure are required based on the step; and removing a region in contact with the corrosive environment of the region where the countermeasures are determined to be necessary. is there.

  The preventive maintenance method for nuclear power plant structures according to the present invention detects, sets and removes high residual stress parts and high strength parts that cause stress corrosion cracking in advance, so that the occurrence of stress corrosion cracking is accurately suppressed. can do.

  Hereinafter, embodiments of a preventive maintenance method for a nuclear power plant structure according to the present invention will be described with reference to the drawings and the reference numerals attached to the drawings.

  FIG. 1 is a block diagram showing a procedure of a preventive maintenance method for a nuclear power plant structure according to the present invention.

  The preventive maintenance method for a nuclear power plant structure according to the present embodiment includes a first step 11 as a stress distribution and strength distribution evaluation for evaluating either or both of a stress distribution and a strength distribution in the structure, A second step 12 as a stress / strength threshold judgment for making a judgment based on a predetermined threshold in an area where stress corrosion cracking countermeasures are required in the structure based on the evaluation, and the second step 12 The third step 13 as a countermeasure range formulation process for formulating a countermeasure range based on the determination result in step 3, and a part of the area in contact with the corrosive environment in which the countermeasure is determined to be necessary and a countermeasure area removal step to be removed The fourth step 14 is provided.

  Also, removal of high stress and high strength parts in the structure was made by preliminarily adding the dimensions of the removal to the completed shape of the structure on the premise that part of the structure will be removed in the manufacturing process of the structure. The above-described steps can also be taken for the structure.

  The removal construction may be performed after any of the factory manufacturing process of the structure, the field installation process, and the inspection process after the start of operation.

  The preventive maintenance method for a nuclear power plant structure according to the present embodiment that employs each of the above steps includes a high residual stress portion and a high strength portion that cause SCC in high-temperature pure water in a region where partial removal measures have been taken. Since it is removed, the occurrence of SCC cracks can be prevented.

  On the other hand, the specific method of the first step 11 for evaluating the stress distribution / intensity distribution during each of the above-described steps is an actual product or a production manufactured using a procedure similar to that of the product as shown in FIG. Using the same shape of the object or a part of the simulated test specimen, measure the stress based on the X-ray method, strain gauge method, photoelasticity method, etc., and perform the stress numerical analysis, or, for example, Vickers hardness The hardness is measured by tarp hardness or the like and numerical analysis is performed, or further, numerical analysis based on the geometric shape of the structure is used.

  The stress distribution evaluation means 21 is, for example, as described in “Latest Stress / Strain Measurement / Evaluation Technology (Supervision: Kozo Kawada, ETS Corp. May 1992)”, strain gauge method, photoelastic method, X-ray method. Stress measurement method, numerical analysis, magnetostriction method, holography method, speckle method, image processing method, acoustoelastic method, infrared method, acoustic tex method, stress paint film method, moire topography, holographic interference, etc. Use the technique.

  In addition, the strength (hardness) distribution evaluation means 22 uses any one of hardness distribution, residual strain distribution, and the like. In particular, any one of Vickers hardness, tarp hardness, Brinell hardness, Rockwell hardness, Shore hardness, superficial hardness, rebound hardness, and the like is used for evaluation of the hardness distribution.

  Numerical analysis is used for evaluation of the residual strain distribution.

  On the other hand, as shown in FIG. 3, the specific method of the second step 12 for performing the determination based on the threshold value predetermined for the portion in the structure where the countermeasure against the stress corrosion cracking is necessary is the first step described above. SCC generation environment determination that determines whether or not the stress threshold value and strength threshold value that are considered to exhibit SCC sensitivity are exceeded for each part during actual machine operation from the stress / intensity distribution obtained in Step 23 is provided.

  That is, this SCC occurrence environment determination step 23 is a stress determination step 24 for determining whether or not the measured stress value exceeds the stress threshold value, and whether or not the strength value exceeds the strength threshold value. A strength determination step 25 to be performed, a crack case presence / absence determination step 26 for determining whether or not the target portion in the actual machine shows SCC sensitivity from the crack case in the structure having the same shape as the structure to be evaluated, and these Of each determination step 24, 25, 26, at least two or more determination results are combined and comprehensively examined, and an SCC countermeasure necessity region determination step 27 for determining a region where the possibility of occurrence of SCC in the structure is high. It is the composition provided with.

  When the SCC countermeasure necessity area is set in the second process 12, in the third process 13 as the countermeasure range formulation process, as shown in FIG. 4, the setting result of the SCC countermeasure necessity area obtained in the second process 12 is obtained. The area for performing the removal countermeasure is set in the SCC countermeasure necessity setting determination step 28, and the SCC countermeasure item range is formulated in the countermeasure item range formulation step 29 based on the setting.

  When the SCC countermeasure item range is formulated in the countermeasure item range formulation process 29 in the third process 13, the fourth process 14 as the countermeasure required partial removal process is sequentially performed based on the formulation of the SCC countermeasure item range. Selection is performed in the countermeasure range selection step 30, selection of the countermeasure method is performed in the countermeasure method selection step 31, and determination of countermeasure implementation is performed in the countermeasure implementation determination step 32.

  Judgment of countermeasure implementation is based on the formulation of the SCC countermeasure item range in the third step 13 by formulating the countermeasure range by comparison with the design standard based on the strength calculation after construction, and examining the degree of difficulty by the selected removal processing method. The final determination of the removal processing is made from these examination results. As a result of final determination, when removal is not performed, progress monitoring is performed, and when removal processing is performed, the following measures are taken.

  As described in "Latest Cutting Technology Overview Co., Ltd., Industrial Technology Service Center", the removal processing means 33 is (1) mechanical cutting (cutting with a bite cutter, saw cutting (circular saw, band) Saw), cutting with abrasive grains, cutting with grinding wheel, electrolytic cutting), (2) fusing (gas cutting, plasma cutting, wire cut electric discharge machining, electric discharge thinning), and (3) chemical cutting, Any one or more cutting methods are used.

  In addition, the work after the removal work is finished processing {roughness improvement (surface polishing (CNS, etc.))}, stress improvement (SP, LP, WJP, etc.), structure / stress improvement (SR, desensitization heat treatment, etc.) Any one of these or a combination of two or more.

  As described above, this embodiment evaluates the stress / strength of the structure in the preventive maintenance of the SCC crack of the structure, and defines the area where the stress corrosion crack countermeasure is required based on the threshold value of the stress / strength to be evaluated. After the determination and determination of the range of the determined stress corrosion cracking area, the removal of the range is performed, so that the occurrence of stress corrosion cracking can be prevented appropriately and accurately.

The block diagram which shows the procedure of the preventive maintenance method of the nuclear power plant structure which concerns on this invention. The block diagram which shows the 1st process as stress distribution and intensity distribution evaluation in FIG. The block diagram which shows the 2nd process as stress and intensity | strength threshold value determination in FIG. The block diagram which shows the 3rd process as a countermeasure range formulation process in FIG. The block diagram which shows the 4th process as a countermeasure area removal process in FIG.

Explanation of symbols

11 First Process as Stress Distribution / Strength Distribution Evaluation 12 Second Process as Stress / Strength Threshold Judgment 13 Third Process as Countermeasure Range Setting Process 14 Fourth Process 21 as Countermeasure Area Removal Process 21 Stress Distribution Evaluation Means 22 Strength (hardness) distribution evaluation means 23 SCC generation environment determination step 24 Stress determination step 25 Strength determination step 26 Crack case presence / absence determination step 27 SCC countermeasure necessity region determination step 28 SCC countermeasure necessity setting determination step 29 Measure item range Formulation process 30 Countermeasure range selection process 31 Countermeasure construction method selection process 32 Countermeasure execution determination process 33 Removal processing means

Claims (12)

A step of evaluating one or both of stress distribution and strength distribution of a nuclear power plant structure, a step of determining an area where stress corrosion cracking countermeasures of the structure are required based on the evaluation, and the countermeasure And a step of removing a region in contact with the corrosive environment determined to be necessary. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein the step of evaluating the stress distribution of the structure is performed by one of stress measurement and numerical analysis or a combination of both. Preventive maintenance method for plant structures. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein the step of evaluating the strength distribution of the structure is performed by a step of any one or a combination of hardness distribution measurement and numerical analysis. To prevent and maintain nuclear plant structures. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein an area where stress corrosion cracking countermeasures of the structure are required is determined by setting an area where the stress distribution is equal to or greater than a predetermined threshold as a countermeasure area. A preventive maintenance method for a nuclear power plant structure. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein the region where the stress corrosion cracking countermeasure of the structure is necessary is determined by setting a region where the strength distribution is equal to or greater than a predetermined threshold as a necessary countermeasure region. A preventive maintenance method for a nuclear power plant structure. 2. The nuclear power plant structure according to claim 1, wherein the determination of the area where stress corrosion cracking countermeasures of the structure are necessary sets a countermeasure area from past cracking cases. Preventive maintenance methods. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein the determination of the region where the stress corrosion cracking countermeasure of the structure is necessary is at least one of claims 4, 5, and 6. A preventive maintenance method for a nuclear power plant structure characterized by combining two or more. The nuclear power plant structure according to claim 1, wherein the step of removing the structure is performed by at least one of mechanical cutting, fusing, and chemical cutting. Preventive maintenance method of things. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein in the step of removing the structure, after the removal, surface finishing is performed by one of or both of smooth finishing and improvement of residual stress. A preventive maintenance method for nuclear plant structures. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein the determination of the area where the stress corrosion cracking countermeasure of the structure is required selects a removal range based on a design standard of the structure to be removed. A preventive maintenance method for nuclear plant structures. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein a structure having a size obtained by adding a removal amount to a completed shape in advance is manufactured on the premise that the structure is removed in a manufacturing process of the structure. A preventive maintenance method for nuclear plant structures. 2. The preventive maintenance method for a nuclear power plant structure according to claim 1, wherein the removal of the structure is performed after completion of any one of factory manufacture, field installation, and inspection after the start of operation of the structure. To prevent and maintain nuclear plant structures.

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