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Comparison of ASME and RSE-M Two Major Codes on Defect Analysis Methods in Pre-service and In-service Inspection...

2021-05-25 09:47:04
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Comparison of ASME and RSE-M Two Major Codes on Defect Analysis Methods in Pre-service and In-service Inspection of Nuclear Power Equipment


In the pre-service and in-service inspections of nuclear power plant equipment, the defects detected in the nuclear-grade welds shall be strictly analyzed and evaluated in accordance with the requirements of relevant regulations to ensure the safe operation of nuclear power plants.


At present, the pre-service and in-service inspections of domestic nuclear power units are mainly in accordance with the "Boiler and Pressure Vessel Code" issued by the American Society of Mechanical Engineers, Volume XI, "Nuclear Power Plant Components In-service Inspection Rules" (hereinafter referred to as ASME Code Volume XI) or France The requirements of the “Rules for the In-service Inspection of Pressurized Water Reactor Nuclear Island Machinery and Equipment” (hereinafter referred to as the RSE-M specification) are implemented. These two specifications have clear regulations on the processing procedures and methods of defects. So, what are the differences between the two? Let's compare it!


Defect handling process


In the pre-service and in-service inspections of nuclear-grade welds in nuclear power plants, the displayed signals found in non-destructive testing undergo a series of judgments, comparisons, analyses and evaluations to determine whether they meet the requirements of the specification and what measures should be taken to deal with defects . The defect processing flowchart is as follows:


1 Display record


The display and record principles of ASME and RSE-M codes are similar, and both are based on different testing methods and testing objects.


Ultrasonic, eddy current and other detection methods generally use the echo amplitude of the displayed signal as the basis for displaying whether to record; the detection methods such as radiation, penetration, visual inspection, and magnetic particle are generally based on the characteristic size of the displayed signal, such as length, diameter, etc. Whether to record the basis.


After the display is recorded, it is first necessary to determine whether it is a relevant display, and the non-relevant display does not require further analysis, such as false defects caused by complex structures or noise in ultrasonic testing. Only the relevant display that meets the recording standard is defined as a defect. The display recording requirements between the two specifications are compared as follows:


Ultrasound


Detect


ASME:


Display of geometric structure not less than 50% DAC


Display of uncertain nature of not less than 20% DAC


RSE-M:


Nuclear Level I: Defect display not less than 25% DAC


Nuclear Level II: Defect display not less than 50% DAC


Rays


Detect


ASME:


All defect displays are recorded


RSE-M:


Stoma of not less than 4 mm


Slag inclusion not less than 5 mm


penetration


Magnetic powder


ASME:


Defects larger than 1.5 mm are displayed


RSE-M:


Display of defects larger than 2 mm


2 Defect characterization


In Volume XI of the ASME Code, a conservative assumption is adopted in defect characterization, that is, it is assumed that all detected defects are flat or linear defects. In this way, when cracks, slag inclusions, porosity, lack of penetration, lack of fusion, delamination, and these defects appear in any combination, they can be evaluated with the least standard.


In addition, in order to simplify defect analysis, the specification reduces all irregularly shaped defects into ideal simple geometric shapes for processing. This not only simplifies the standards for defects, but also facilitates the application of the principles of fracture mechanics.


In the RSE-M specification, depending on the detection method, the qualitative defects are also different.


Penetration detection is divided into circular display and linear display, among which the linear display is considered as a dangerous defect.


Radiographic testing generally judges the nature of the defect based on the position of the defect displayed on the film and the defect characteristics in the weld. It is divided into circular display, linear display, crack, unfused, and incomplete penetration.


Manual ultrasonic testing generally classifies flat or non-planar internal defects by defect echo characteristics (EN-1713 "Non-destructive Testing of Welds").


Automatic ultrasonic testing generally judges the nature of the defect through the defect characteristics and the end point diffraction signal in the B-scan or D-scan, and divides the defect into a volume display and a flat display. The flat display is considered a dangerous defect.


3 Defect quantification


Defect quantification, that is, the measurement of defect size and quantity. The size of the defect mainly includes the length, height and area of the defect. Different types of defects require different information to be measured. During the pre-service and in-service inspections, the size of the defect is the main basis for judging whether it is acceptable.


4 Defect acceptance


Defect acceptance mainly refers to the process of comparing the acceptance criteria (usually in the form of a table) specified in the specification with the nature of the detected defects and the size of the measured defects to determine whether the defects are acceptable. For some flat defects or hypothetical flat defects in the pre-service and in-service inspections, the size normalization process and correlation analysis must be performed before the comparison of the defect acceptance table.


(1) Acceptance criteria


The ASME code stipulates that if the detected defect meets the non-destructive testing acceptance standard (acceptance standard in component manufacturing) in Volume III of the ASME code during the pre-service and in-service inspection, and there is a certification document in the quality assurance record, or detection If the defects meet the acceptance criteria in Volume XI of the ASME Code, the inspected parts can be accepted.


During the in-service inspection, if the detected defect exceeds the acceptance criteria in Volume XI of the ASME Code, but the analysis and evaluation conducted in accordance with ASME's requirements meet the acceptance criteria, the inspected component can still be accepted.


Among them, the acceptance standard of Volume XI of the ASME code divides the defects into flat, delamination (can be considered as a type of flat defects) and linear defects. According to the nuclear level and material of the inspected object, each type of defect is targeted. It specifies the acceptance form for evaluation and acceptance. After the defects are normalized, the size of the allowable defects is determined according to the thickness of the inspected object.


During the pre-service inspection specified in the RSE-M specification, if the detected defect meets the French "Rules for the In-service Inspection of Nuclear Island Machinery and Equipment" (hereinafter referred to as the RCC-M specification) acceptance criteria (acceptance criteria in component manufacturing), it shall be inspected The parts can be accepted.


During the in-service inspection, if the detected defect meets the acceptance criteria of RCC-M specification, or meets the acceptance criteria of RSE-M Appendix 5.2, or is analyzed and evaluated according to the requirements, the safety margin meets the requirements (defects are analyzed and evaluated in The entire operating life of a nuclear power plant will not affect the safe operation of the nuclear power plant due to defect size expansion or other reasons), then the component can be accepted.


In the RSE-M specification, only a special acceptance standard is specified for flat defects, and the provisions in the RCC-M specification are generally quoted for other types of defects.


(2) Normalized size of plane defect


The so-called normalized size is to process the measured defect size according to a certain conversion criterion, so that it is convenient to quantitatively describe the shape of the defect according to a unified principle. The acceptance standards of ASME code and RSE-M code both stipulate the method of normalizing the size of flat defects.


(3) Defect correlation analysis


Taking into account that adjacent defects may expand and be connected together, the specification stipulates the correlation judgment principle for adjacent defects, and two or more related defects should be merged into one defect. And use the combined size to evaluate whether the defect is acceptable. The ASME code and the RSE-M code are basically the same in the overall idea of judging whether adjacent defects are related, and there are only some differences in the evaluation indicators. Whether adjacent defects are related depends on the normalized size (such as length or height) of each defect size and the distance between the defects.


The main difference in defect handling


1The RSE-M specification requires the use of inspection methods for defect characterization in the pre-service stage (such as plane or volume, circular or linear);


The ASME code conservatively believes that all defects found before and during service are flat or linear and other hazardous defects.


2The defect tracked in the RSE-M code in-service inspection needs to be compared with the previous inspection results of the defect, and it is determined whether the defect has changed significantly; ASME does not have this requirement.


3The defect acceptance table of the RSE-M code cannot be applied to defects of all sizes (length-to-height ratio); ASME can judge the acceptability of defects of any size.


4The acceptance criteria of RSE-M, the principle of merging adjacent defects, etc., are separately stipulated according to the different equipment functions (such as pressure vessels, stabilizers or steam generators, etc.);


ASME's acceptance standards are specified separately according to the size of the equipment, operating conditions, etc. (such as weld thickness, stress factor, etc.).


5The ASME code separately stipulates acceptance criteria for all defects that may appear during pre-service and in-service inspections;


The RSE-M specification only specifies the acceptance criteria for flat defects, and the acceptance criteria for other types of defects need to quote the relevant requirements of the RCC-M specification.


Case Analysis


A power plant implemented an in-service inspection. The thickness of the welding seam between the pressure stabilizer cylinder and the bottom head of the plant was 84 mm, and a 7 mm stainless steel corrosion-resistant layer was built-up on the inner surface.


In an in-service inspection, the weld was ultrasonically inspected from the outer surface according to the requirements of the specification. During the inspection, two adjacent circumferential defects were found showing D04 and D05. The specific defect information on the weld is shown in the following table:


It is analyzed and evaluated according to RSE-M and ASME respectively. The defect size is shown in the figure below:


Analysis based on Volume Ⅺ of ASME Code


(1) Defects show complex


The distance between D04 and D05 defects and the center line of the weld is 14-15 mm, and the distance S is less than 13 mm. The two defects can be considered as coplanar plane defects. The distance S between the two defects is 4.5 mm, which is greater than 4 mm, so it is displayed as two independent defects.


(2) Defect acceptance analysis


The shape ratio of D04 (the ratio of the normalized height and the length of the defect) a/l=2/21.5=0.093, the proximity factor of the defect and the surface Y=19.8/2, greater than 1, take Y=1, the following table shows the ASME code stability Defect display acceptance criteria for compressor (excerpt):


Using the linear difference method to calculate the wall thickness of 84 mm, the allowable a/t value for defects with a shape ratio of 0.093 is 3.27%, that is, a must not be greater than 2.75 mm. Therefore, the defect D04 is acceptable. In the same way, the defect D05 is also acceptable.


According to RSE-M standard analysis


 (1) Analysis of the geometric envelope size of the defect display


The distance between D04 and D05 defects and the center line of the weld is in the range of 14~15 mm, and the analysis should be carried out according to the condition of mutual uncorrelation of coplanar defects.


For D04 and D05 defect display, the distance of the nearest end point is 4.5 mm. Establish correlation rectangles for D04 and D05 respectively, and the defect display correlation diagram is shown in the following figure:


These two interrelated rectangles intersect (d=2.4 mm, and the overlap distance is 0.3 mm), so the two defects are shown to be related. According to the RSE-M standard, these two defects should be combined into one defect for analysis.


 (2) Defect acceptance analysis


After D04 and D05 are combined, the height of the defect display is h = 9 mm, the length l = 47.5 mm, and the defect display is A/L = 0.19 (A is the normalized height of the defect, L is the normalized length of the defect), which can be known by using the defect acceptance table , The allowable defect height H'm=8.4 mm, the defect display height after D04 and D05 is 9 mm, does not meet the defect acceptance criteria in Appendix 5.2 of the RSE-M specification, it is not acceptable, the RSE-M specification voltage stabilizer shows a defect The acceptance criteria are shown in the following table (Hi is the allowable value for the height of the inner surface defect; He is the allowable value for the height of the outer surface defect; H′m is the allowable value for the height of the buried defect):


The defect display is shown in the following figure after merging:


It can be seen from the analysis results that when the same size defects are accepted and analyzed according to the ASME and RSE-M codes, the acceptance results are also different due to the different judgment basis of the adopted codes. When defects are found in pre-service and in-service inspections, reasonable analysis should be carried out in accordance with the requirements of the specification to ensure the safe operation of nuclear power equipment.


in conclusion


Both the ASME code and the RSE-M code clearly stipulate the processing procedures and acceptance requirements for defects found in pre-service and in-service inspections.


Due to the different normative systems, the requirements for handling and acceptance of related defects in the two norms are different, but generally speaking, both norms are based on conservative principles to analyze and deal with defects, and are mainly aimed at scalable and harmful defects.


Generally speaking, a nuclear power plant designed with a certain standard system should deal with defects in accordance with the defect handling procedures and acceptance criteria specified in the series of specifications to avoid the mixed use of specifications.


The author of this article: Ge Liang, an engineer, is mainly engaged in non-destructive testing technology research and engineering applications at the Institute of Nuclear Power Operations.


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