CN119601136A

CN119601136A - Fracture toughness transformation evaluation method for small-size specimens of RPV materials

Info

Publication number: CN119601136A
Application number: CN202411565582.8A
Authority: CN
Inventors: 陈明亚; 余伟炜; 彭群家; 赵万祥; 王水勇; 王荣山; 杨林; 郑聪勤; 遆文新; 史芳杰; 韩姚磊; 李维鹏
Original assignee: CGN Power Co Ltd; Suzhou Nuclear Power Research Institute Co Ltd
Current assignee: CGN Power Co Ltd; Suzhou Nuclear Power Research Institute Co Ltd
Priority date: 2024-11-05
Filing date: 2024-11-05
Publication date: 2025-03-11

Abstract

The method solves the problem that numerical simulation methods such as too conservative assessment of surface shallow cracks (constraint effect of crack fronts is not considered) and SIF and J integral cannot be applied to an unloading process in the prior art, can be suitable for load unloading characteristics of a primary loop of a reactor, such as working conditions of sudden drop of pressure and temperature during emergency reactor shutdown, and provides deep technical support for long-life safe operation of the RPV of a nuclear power plant.

Description

Method for evaluating fracture toughness conversion of small-size sample of RPV (reactive power transformation) material

Technical Field

The application relates to the technical field of structural integrity assessment, in particular to an energy density method-based method for assessing the fracture toughness conversion of an RPV material small-size sample.

Background

The Reactor Pressure Vessel (RPV) is a primary loop pressure-bearing device of a pressurized water reactor nuclear power plant, and belongs to a non-replaceable nuclear primary component. Mechanical property detection of RPV materials in a service stage is critical to structural integrity evaluation, and a large-size sample is required to be taken in a traditional mechanical property detection method. Currently, a nuclear power plant usually performs inspection and analysis on a supervision sample or spare parts in the same batch, but the requirement of material performance data under the condition of exceeding an initial design reference service life (such as service life extending from 40 years to 60 years or 80 years) is difficult to meet, engineering application of advanced evaluation methods such as probability statistics based on a large amount of test data of the service material is limited, and the technology for testing the fracture toughness of the small-size sample of the RPV material is generated under the background.

At present, most of nuclear power plant RPV materials running in China are 16MND5 (equivalent to A508III steel in the United states), belong to ferritic steel, and have good hardenability, high-temperature strength and good weldability. However, the toughness of the 16MND5 material is reduced under the irradiation of fast neutrons, and obvious irradiation embrittlement phenomenon is generated. The irradiation embrittlement causes the critical crack size of the structure to be reduced, the ductile-brittle transition temperature to be increased, and the probability of brittle fracture to be increased. The existing specifications measure the basic properties of the material by means of standard samples, neglecting the influence of structural and load characteristics on the fracture properties of the material. Related studies have shown that existing canonical analytical methods are too conservative and that safety margins cannot be quantitatively assessed. The nuclear power plant RPV running in China is designed and manufactured based on the French RCC-M specification and the American ASME specification, and in engineering fracture analysis, the RPV is evaluated based on Stress Intensity Factor (SIF) or J integral theory. The existing method has a plurality of inconveniences in practical application, namely 1) the fracture criteria given by the existing method are too conservative for evaluating shallow surface cracks (the restraint effect of crack front is not considered), and 2) numerical simulation methods such as SIF and J integral cannot be applied to the unloading process. In the field of nuclear industry, the probability of occurrence of large-size cracks of an RPV reactor core is extremely low, the sizes of the cracks to be evaluated are shallow cracks on the surface of the reactor core, and the working conditions such as emergency shutdown (sudden drop of pressure and temperature, with obvious unloading process) and the like are the contents of important evaluation in license update demonstration. However, there is a lack of associated fracture toughness conversion and structural integrity assessment techniques in these presently disclosed analytical systems.

Disclosure of Invention

The technical problem to be solved by the application is to provide the RPV material small-size sample fracture toughness transformation evaluation method based on the energy density method, which provides an accurate technical basis for the safety evaluation of nuclear power key equipment.

The technical scheme adopted for solving the technical problems is as follows:

A method for evaluating the fracture toughness conversion of a small-size sample of an RPV material comprises the following steps:

S1, testing and obtaining critical load P _L of crack front at the time of cracking based on a small-size fracture toughness test sample;

s2, obtaining elastic energy density G _PL(kJ/m² of a unit length corresponding to a crack front area during cracking based on a finite element numerical simulation method, wherein the elastic energy density G _PL is a result of calculation according to the following formula:

Wherein Ω is an arbitrary analysis region for brittle fracture assessment including a crack front, σ _ep is an elastoplastic stress tensor of the analysis region, σ _e is an elastoplastic stress tensor, and Δa is a length of an elastic energy integration region of the crack front;

S3, establishing a corresponding relation between the temperature T (degree C) and the temperature P _L-G_PL at different test temperatures, namely establishing a material fracture toughness prediction equation based on the elastic energy density G _PC;

S4, based on finite element numerical simulation software, establishing a finite element analysis model containing initial plane cracks obtained by nondestructive testing aiming at a nuclear power structure to be evaluated, loading the established finite element model containing defects under different operation conditions, and obtaining elastic energy density parameters G _P of crack fronts;

S5, comparing G _P with G _PC, and if the energy parameters G _P of the crack front obtained in the whole process are smaller than the material fracture toughness G _PC of the material at the corresponding temperature, the structure is safe.

In some embodiments, in the step S1, the small-size fracture toughness test specimen is any one of a miniature compact tensile specimen and a three-point bending specimen, and the small-size fracture toughness test specimen may also be other fracture toughness test specimens. In addition, the sample can be a standard test sample according to related specifications, and also can be a nonstandard sample improved based on test requirements, wherein the state requirement of the crack front meeting plane strain is not required in the test, namely the sample thickness and the like are not required.

In some embodiments, in the step S3, the independent variable of the fracture toughness prediction equation of the material is a difference between the crack front temperature and the ductile-brittle transition temperature (RT _NDT), and the specific formula of the fracture toughness prediction equation of the material is:

K_IC＝min{39.8+3.1exp[0.36(T-RT_NDT+55.5)],220}

wherein the material fracture toughness And performing preliminary selection based on the RCC-M specification of the existing nuclear power design specification.

In some embodiments, in the step S3, the elastic energy density G _PC per unit length can be obtained based on the conversion of the standard equation of the existing nuclear power design specification, and when the temperature is-80 ℃ and the temperature is less than or equal to T-RT _NDT and less than or equal to 60 ℃, the prediction equation of G _PC is as follows:

In some embodiments, in step S3, G _PC has a value of 1.11kJ/m ² when T-RT _NDT is >60 ℃.

In some embodiments, in the step S4, the defect-containing finite element analysis model refers to a three-dimensional crack-containing analysis model built based on general finite element software, and the energy parameter G _P of the crack front is calculated based on the elastic deformation energy (ENER _elas) parameter in the unit cell in the finite element model post-processing model, where the elastic deformation energy (ENER _elas) parameter in the unit cell is an output parameter of general post-processing provided by the general finite element software.

The method for evaluating the fracture toughness of the small-size sample of the RPV material has the advantages that the method combines fracture mechanics and damage mechanics, provides a basis for judging whether cracks in the structure are expanded or not, is independent of loading types (such as compact stretching and three-point bending) of the structure, is more accurate in calculation of the small-size cracks, and solves the problems that evaluation of the shallow surface cracks is too conservative (restraint effect of crack front edges is not considered) and numerical simulation methods such as SIF and J integral and the like cannot be applied to unloading processes in the existing specifications. The method can be suitable for load unloading characteristics of a primary loop of the reactor, such as working conditions of sudden drop of pressure and temperature during emergency shutdown of the reactor, and the like, and provides deep technical support for long-life safe operation of the RPV of the nuclear power plant.

Drawings

The application will be further described with reference to the accompanying drawings and examples, in which:

FIG. 1 is an example of a small CT specimen for fracture toughness prediction of the materials of the present application;

FIG. 2 is a model of the crack front damage region definition based on the energy criteria of the present application;

FIG. 3 is the RPV base material fracture toughness test data;

FIG. 4 is experimental data for validating a predicted value of fracture toughness of a material in accordance with the present application;

FIG. 5 is a mechanical analysis model of an RPV core barrel section in an application case of the application;

FIG. 6 is a finite element analysis model of an RPV core barrel section in an application case of the application;

FIG. 7 is a diagram showing analysis of reference transient information in an application case of the present application;

FIG. 8 is an integrated output illustration of elastic deformation energy in elastic energy density parameters in finite element software;

fig. 9 is a transient fracture safety evaluation result based on the energy criterion of the present application.

Detailed Description

For a clearer understanding of technical features, objects and effects of the present application, a detailed description of embodiments of the present application will be made with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. The present application may be embodied in many other forms than described herein and similarly modified by those skilled in the art without departing from the spirit of the application, whereby the application is not limited to the specific embodiments disclosed below.

In the description of the present application, it should be understood that the terms "longitudinal," "transverse," "upper," "lower," "top," "bottom," "inner," "outer," and the like indicate orientations or positional relationships based on the orientation or positional relationships shown in the drawings or those conventionally placed in use of the product of the present application, are merely for convenience in describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and therefore should not be construed as limiting the present application.

Furthermore, the terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature. In the description of the present application, the meaning of "plurality" means at least two, for example, two, three, etc., unless specifically defined otherwise.

In the present application, unless explicitly specified and limited otherwise, the terms "mounted," "connected," "secured," and the like are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally formed, mechanically connected, electrically connected, directly connected, indirectly connected through an intervening medium, or in communication between two elements or in an interaction relationship between two elements, unless otherwise explicitly specified. The specific meaning of the above terms in the present application can be understood by those of ordinary skill in the art according to the specific circumstances.

In the present application, unless expressly stated or limited otherwise, a first feature "up" or "down" a second feature may be the first and second features in direct contact, or the first and second features in indirect contact via an intervening medium. Moreover, a first feature being "above" a second feature may be that the first feature is directly above or obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. The first feature being "under" the second feature may be the first feature being directly under or obliquely under the second feature, or simply indicating that the first feature is level less than the second feature.

Referring to fig. 1-9, some embodiments of the present application provide a method for evaluating the fracture toughness conversion of small-sized samples of RPV material, comprising the steps of:

S1, based on a small-size fracture toughness test sample, a critical load P _L when a crack is initiated is obtained by testing, as shown in FIG. 1, in the embodiment, the small-size fracture toughness test sample is a miniature compact tensile sample, in addition, in other embodiments, the small-size fracture toughness test sample can also be a three-point bending sample or other fracture toughness test sample, in addition, the sample can be a standard test sample according to related specifications or a nonstandard sample improved based on test requirements, and the state requirement of the crack front meeting plane strain is not required in the test, namely, the thickness and the like of the sample are not required;

S2, as shown in FIG. 2, based on a finite element numerical simulation method, obtaining elastic energy density G _PL(kJ/m² of a unit length corresponding to a crack front area during cracking, wherein the elastic energy density G _PL is a result of calculation according to the following formula:

S3, establishing a corresponding relation between the temperature T (degree C) and the temperature P _L-G_PL at different test temperatures, namely establishing a material fracture toughness prediction equation based on elastic energy density G _PC, wherein the independent variable of the material fracture toughness prediction equation is the difference between the crack front temperature and the material ductile-brittle transition temperature (RT _NDT), and the specific formula of the material fracture toughness prediction equation is as follows:

K_IC＝min{39.8+3.1exp[0.36(T-RT_NDT+55.5)],220} (2)

as shown in fig. 3, the fracture toughness of the material therein Based on the prior nuclear power design specification RCC-M specification, the whole curve is moved downwards in the formula to obtain the estimation result which is deviated from conservationIn order to avoid unnecessary conservation degree, the test method considers that the test stage of material performance adopts a high-restraint-degree sample, and aims at the condition of low restraint degree of 'accurately evaluating' medium-small-size cracks, the lower envelope curve of K _IC is shifted upwards in the formula

The elastic energy density G _PC in unit length can be obtained based on the conversion of the standard equation of the existing nuclear power design specification, and when the temperature is less than or equal to-80 ℃ and less than or equal to 60 ℃ and is less than or equal to T-RT _NDT, the prediction equation of G _PC is as follows:

According to the formula (3), the curve of the fracture toughness prediction equation of the material is shown in FIG. 4, and in addition, when T-RT _NDT is higher than 60 ℃, the value of G _PC is 1.11kJ/m2.

S4, establishing a finite element analysis model (shown in FIG. 6) containing initial plane cracks obtained by nondestructive testing based on finite element numerical simulation software aiming at the structure, marking size parameters of crack position areas and crack front areas in the finite element model, wherein H is the height of the crack front and R is the radius of the crack front, loading different operation working conditions of the established finite element model containing defects, and obtaining elastic energy density parameters G _P of the crack front through finite element post-processing of a formula (1), wherein the energy parameters G _P of the crack front are calculated based on elastic deformation energy (ENER _ELAS) parameters in unit bodies in the finite element model post-processing model, and the elastic deformation energy (ENER _ELAS) parameters in the unit bodies are output parameters of general post-processing provided by the existing finite element software, and the output of the elastic deformation energy (ENER _ELAS) is illustrated as shown in a graph of FIG. 8;

S5, comparing the G _P with the G _PC, if the energy parameters G _P of the crack front obtained in the whole process are smaller than the material fracture toughness G _PC of the material at the corresponding temperature, the structure is safe, otherwise, the deep evaluation after the crack is initiated is needed. In the case shown in the case of fig. 9, the fracture toughness of the material is higher than the crack propagation driving force during the whole transient process, and the RPV core structure is safe.

In the steps, the steps S1 to S3 are material fracture toughness tests based on energy method parameters, the step S4 is to obtain crack initiation driving force energy parameters under the nuclear power structure integrity service state, and the step S5 is to evaluate the nuclear power structure service safety.

Example 1

Case analysis was performed with the actual structure of a certain RPV core barrel section, the RPV core barrel section material being 16MND5, the chemical composition of which is shown in table 1.

TABLE 1 chemical composition of certain 16MND 5 materials

Fracture toughness testing was performed using a test specimen for fracture toughness, and the related tests were performed on a Mechanical Test and Simulation (MTS) system, and fracture toughness testing was performed according to ASTM E1921. Eight sets of data were tested at-65 ℃ and-75 ℃ and the critical loads obtained for the tests are shown in table 2.

Table 2 critical breaking load for standard CT specimen testing

1) Material fracture toughness test based on energy method parameters

As shown in fig. 4, the correspondence between temperature T (° C) and P _L-G_PL is established at different test temperatures, and a material fracture toughness prediction equation or curve based on elastic energy density per unit length G _PC is established.

2) Crack initiation driving force energy parameter under nuclear power structure integrity service state

As shown in FIG. 5, the diameter of the inner surface of the RPV reactor core barrel is 2197mm, the wall thickness of the reactor core base metal is 220mm, and the thickness of the inner surface surfacing layer is 7.5mm. The core barrel was assumed to contain circumferential buried, penetrating cracks with a crack depth dimension of 5.2mm (where 5mm depth is in the RPV parent material).

The three-dimensional finite element model of the RPV core barrel section containing the crack is shown in FIG. 6. In order to accurately simulate the stress field of the crack front stress concentration area, structured meshing is performed on the crack front area. Based on literature study experience, the height H of the fine grid-divided region at the crack front was taken to be 50 μm, 20 layers of cells were divided in the cell body of the crack front calculation energy, and 200 lengths of structured grids were divided in the crack propagation direction (division of the grid size was related to the tissue feature scale of the material itself).

As shown in FIG. 7, an accident condition transient is defined in which the one-circuit system is re-pressurized. The internal pressure is 5MPa at the time of transient initiation, the internal pressure is increased to 15.5MPa (normal working pressure load) at the time of transient termination, and the fluid temperature is 300 ℃ at the time of transient initiation and gradually reduced to 20 ℃.

In the simulation, the outer surface of the reactor core barrel is set as an adiabatic boundary condition, the inner surface and the hot fluid perform convection heat exchange, and the heat exchange coefficient is set to be infinite. An internal pressure load P is applied to the inner surface, and an equivalent end face load P _end is applied to the upper end face of the core barrel:

re-the radius of the outer surface and Ri-the radius of the inner surface of the cylinder section of the reactor core.

The integrated output description of the elastic deformation energy in the finite element software is shown in fig. 8, and the elastic energy density parameter G _P of the crack front is obtained through the finite element post-processing of the formula (1). At the end of the life, assuming that the ductile-brittle transition temperature of the RPV core material is 100 ℃, the safety evaluation result in the transient process is obtained as shown in fig. 9.

3) Nuclear power structure service safety assessment

As can be seen from fig. 9, the fracture toughness of the material is higher than the crack propagation driving force throughout the transient, and the RPV core structure is safe. The case is based on the fact that small cracks monitored are subjected to safety performance analysis, when the size of the cracks is increased, the transient state has the characteristic of re-pressurization at 10000 seconds, the safety performance of the structure is obviously challenged, and the safety margin of the evaluation of the structure is improved by considering the effects of temperature prestress and the like. From the case study, the fracture safety evaluation of the RPV reactor core barrel section can be systematically carried out by adopting the G _P criterion.

The foregoing examples merely illustrate specific embodiments of the application, which are described in greater detail and are not to be construed as limiting the scope of the application; it should be noted that, for those skilled in the art, the above technical features may be freely combined and several variations and modifications can be made without departing from the spirit of the application, and these variations and modifications are included in the scope of the application, therefore, all equivalent changes and modifications as come within the scope of the claims shall fall within the scope of the application.

Claims

1. The method for evaluating the fracture toughness conversion of the small-size sample of the RPV material is characterized by comprising the following steps of:

S2, based on a finite element numerical simulation method, obtaining elastic energy density G _PL(kJ/m² corresponding to a crack front area during cracking, wherein the elastic energy density G _PL is a result calculated by the following formula:

2. The method for evaluating the fracture toughness conversion of a small-sized sample of an RPV material according to claim 1, wherein in the step S1, the small-sized fracture toughness test sample is any one of a miniature compact tensile sample and a three-point bending sample.

3. The method for evaluating the fracture toughness conversion of a small-sized sample of an RPV material according to claim 1, wherein in the step S3, the independent variable of the fracture toughness prediction equation of the material is a difference between a crack front temperature and a ductile-brittle transition temperature (RT _NDT), and the specific formula of the fracture toughness prediction equation of the material is:

K_IC＝min{39.8+3.1exp[0.36(T-RT_NDT+55.5)],220}

4. The method for evaluating the fracture toughness conversion of small-sized samples of RPV material according to claim 3, wherein in the step S3, the elastic energy density G _PC per unit length is obtained based on the conversion of the standard equation of the existing nuclear power design specification, and when the temperature is-80 ℃ and T-RT _NDT and 60 ℃, the prediction equation of G _PC is as follows:

5. The method for evaluating the fracture toughness of small-sized samples of RPV material according to claim 3, wherein in said step S3, G _PC has a value of 1.11kJ/m ² when T-RT _NDT is higher than 60 ℃.

6. The method according to claim 1, wherein in the step S4, the defect-containing finite element analysis model is a three-dimensional crack-containing analysis model built based on general finite element software, and the energy parameter G _P of the crack front is calculated based on an elastic deformation energy (ENER _elas) parameter in a unit body in a finite element model post-processing model, wherein the elastic deformation energy (ENER _elas) parameter in the unit body is an output parameter of general post-processing provided by the general finite element software.