RPV material small-size sample fracture toughness conversion evaluation method

Abstract

The invention relates to an RPV material small-size sample fracture toughness conversion evaluation method, which comprises the related steps of material fracture toughness testing based on energy method parameters, obtaining of crack initiation driving force energy parameters in a nuclear power structure integrity service state, and nuclear power structure service safety evaluation. The method provided by the invention overcomes the problems that the evaluation of the surface shallow crack is too conservative (the constraint effect of the crack front edge is not considered) and the SIF, J integral and other numerical simulation methods cannot be applied to the unloading process in the existing specification, and can be suitable for the load unloading characteristics of the primary loop of the reactor; and the deep technical support is provided for the long-life safe operation of the RPV of the nuclear power plant.

Description

Technical Field

[0001] The present application relates to the technical field of structural integrity assessment, and in particular to a method for evaluating the fracture toughness transformation of a small-size specimen of an RPV material based on an energy density method. Background Art

[0002] The Reactor Pressure Vessel (RPV) is a primary pressure-bearing device in a pressurized water reactor nuclear power plant and is a non-replaceable primary nuclear component. The mechanical property testing of RPV materials during the service phase is crucial to the assessment of their structural integrity. Traditional mechanical property testing methods require large-sized specimens. Currently, nuclear power plants usually conduct inspections and analyses on surveillance specimens or spare parts from the same batch, but this is difficult to meet the demand for material performance data under conditions exceeding the initial design benchmark life (such as extending the service life from 40 years to 60 or 80 years). This also limits the engineering application of advanced evaluation methods such as probability statistics based on a large amount of test data of service materials. It is in this context that the fracture toughness testing technology of small-sized specimens of RPV materials came into being.

[0003] At present, the RPV materials of nuclear power plants in operation in China are mostly 16MND5 (equivalent to the A508III steel in the United States), which is a ferritic steel with good hardenability, high high temperature strength and good weldability. However, the toughness of 16MND5 material decreases under the irradiation of fast neutrons, resulting in obvious radiation embrittlement. Radiation embrittlement leads to a decrease in the critical crack size of the structure, an increase in the ductile-brittle transition temperature, and an increase in the probability of brittle fracture. The existing specifications determine the basic properties of the material through standard specimens, ignoring the influence of structural characteristics and load characteristics on the fracture properties of the material. Related studies have shown that the analysis methods of the existing specifications are too conservative, and the safety margin cannot be quantitatively evaluated. Most of the RPVs of nuclear power plants in operation in China are designed and manufactured based on the French RCC-M specification and the American ASME specification. In engineering fracture analysis, most of them are evaluated based on stress intensity factor (SIF) or J integral theory. The existing methods have many inconveniences in practical application: 1) The fracture criteria given by the current methods are too conservative for the evaluation of shallow surface cracks (the constraint effect of the crack front is not considered); 2) Numerical simulation methods such as SIF and J integral cannot be applied to the unloading process. In the nuclear industry, the probability of large-scale cracks in the RPV core is extremely low. The crack sizes that need to be evaluated are shallow cracks on the core surface, and emergency shutdown (a sudden drop in pressure and temperature, accompanied by an obvious unloading process) and other working conditions are the key evaluation content in the license renewal demonstration. However, these currently disclosed analysis systems still lack relevant fracture toughness conversion and structural integrity assessment technologies. Summary of the invention

[0004] The technical problem to be solved by this application is to provide a method for evaluating the fracture toughness transformation of small-size specimens of RPV materials based on the energy density method, which provides an accurate technical basis for the safety assessment of key nuclear power equipment.

[0005] The technical solution adopted by this application to solve its technical problem is:

[0006] A method for evaluating the fracture toughness transformation of a small-size specimen of an RPV material comprises the following steps:

[0007] S1. Based on the small-size fracture toughness test specimen, the critical load P at the crack initiation front is obtained. L ;

[0008] S2. Based on the finite element numerical simulation method, the elastic energy density per unit length G corresponding to the crack front area at the time of crack initiation is obtained. PL (kJ / m 2 ), the elastic energy density G PL It refers to the result of the following formula:

[0009]

[0010] Where Ω is the arbitrary analysis area for brittle fracture assessment including the crack front, σ ep is the elastic-plastic stress tensor of the analysis area, σ e is the elastic stress tensor, Δa is the length of the elastic energy integration area at the crack front;

[0011] S3. Establish temperature T (℃) and P at different test temperatures L -G PL The corresponding relationship between elastic energy density G PC The material fracture toughness prediction equation;

[0012] S4. Based on the finite element numerical simulation software, a finite element analysis model containing the initial plane crack obtained by non-destructive testing is established for the nuclear power structure to be evaluated. The established defective finite element model is loaded with different operating conditions to obtain the elastic energy density parameter G at the crack front. P ;

[0013] S5. Comparison G P and G PC , if the energy parameter G of the crack front is obtained in the whole process P are all less than the fracture toughness G of the material at the corresponding temperature. PC , the structure is safe.

[0014] In some embodiments, in the above step S1, the small-sized fracture toughness test specimen is any one of a miniature compact tensile specimen and a three-point bending specimen, and the small-sized fracture toughness test specimen can also be a fracture toughness test specimen in other forms. In addition, the specimen can be a standard test specimen according to relevant specifications, or a non-standard specimen improved based on test requirements. The crack front is not required to meet the state requirements of plane strain during the test, that is, no requirements are made on the thickness of the specimen, etc.

[0015] In some embodiments, in the above step S3, the independent variables of the material fracture toughness prediction equation are the crack front temperature and the material ductile-brittle transition temperature (RT NDT ), the specific formula of the material fracture toughness prediction equation is:

[0016] K IC =min{39.8+3.1exp[0.36(T-RT NDT +55.5)],220}

[0017] The fracture toughness of the material The preliminary selection was made based on the existing nuclear power design specification RCC-M.

[0018] In some embodiments, in the above step S3, the elastic energy density per unit length G PC It can be converted based on the standard equation of the existing nuclear power design specifications. When -80℃≤T-RT NDT ≤60℃, G PC The prediction equation is as follows:

[0019]

[0020] In some embodiments, in the above step S3, when T-RT NDT >60℃, G PC The value is 1.11 kJ / m 2 .

[0021] In some embodiments, in the above step S4, the defective finite element analysis model refers to a three-dimensional crack analysis model established based on general finite element software, and the energy parameter G of the crack front is P The calculation is based on the elastic deformation energy (ENER_ELAS) parameter of the unit body in the post-processing model of the finite element model, wherein the elastic deformation energy (ENER_ELAS) parameter of the unit body is a general post-processing output parameter provided by existing finite element software.

[0022] The implementation of this application has at least the following beneficial effects: This application proposes a method for evaluating the fracture toughness transformation of small-size specimens of RPV materials, which combines fracture mechanics and damage mechanics to provide a basis for judging whether cracks in structures extend, and is independent of the loading type of the structure (such as compact tension, three-point bending, etc.). The calculation of small-size cracks is also more accurate, overcoming the problems that the existing specifications are too conservative in the evaluation of shallow surface cracks (not considering the restraint effect of the crack front) and that numerical simulation methods such as SIF and J integral cannot be applied to the unloading process. The method of this application can be applied to the load unloading characteristics of the reactor primary circuit, such as the sudden drop in pressure and temperature during the emergency shutdown of the reactor, and provides in-depth technical support for the safe operation of RPVs in the long life of nuclear power plants. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present application will be further described below with reference to the accompanying drawings and embodiments, in which:

[0024] Figure 1 An example of a small CT specimen for material fracture toughness prediction in this application;

[0025] Figure 2 A model is defined for the crack front damage region based on the energy criterion of the present application;

[0026] Figure 3 It is the fracture toughness test data of RPV parent material;

[0027] Figure 4 The experimental data for verifying the predicted value of fracture toughness of the material in this application;

[0028] Figure 5 A mechanical analysis model for the RPV core barrel section in the application case of this application;

[0029] Figure 6 It is the finite element analysis model of the RPV core barrel section in the application case of this application;

[0030] Figure 7 Analyze reference transient information in the application case of this application;

[0031] Figure 8 It is an explanation of the integrated output of elastic deformation energy in elastic energy density parameters in finite element software;

[0032] Fig. 9 This is the transient fracture safety evaluation result based on energy criteria in this application. DETAILED DESCRIPTION

[0033] In order to have a clearer understanding of the technical features, purposes and effects of the present application, the specific implementation methods of the present application are now described in detail with reference to the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present application, so the present application is not limited by the specific embodiments disclosed below.

[0034] In the description of the present application, it should be understood that the terms "longitudinal", "lateral", "up", "down", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings or the orientations or positional relationships in which the products of the present application are usually placed when used. They are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0035] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of the features. In the description of this application, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0036] In this application, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0037] In the present application, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0038] Reference Figures 1 to 9Some embodiments of the present application provide a method for evaluating the fracture toughness transformation of a small-size specimen of an RPV material, comprising the following steps:

[0039] S1. Based on the small-size fracture toughness test specimen, the critical load P at the crack initiation front is obtained. L ;like Figure 1 As shown, in this embodiment, the small-sized fracture toughness test specimen is a miniature compact tensile specimen; in addition, in other embodiments, the small-sized fracture toughness test specimen may also be a three-point bending specimen or other forms of fracture toughness test specimens. In addition, the specimen may be a standard test specimen according to relevant specifications, or a non-standard specimen improved based on test requirements. The crack front is not required to meet the state requirements of plane strain during the test, that is, no requirements are made on the thickness of the specimen, etc.

[0040] S2, such as Figure 2 As shown in the figure, based on the finite element numerical simulation method, the elastic energy density G per unit length corresponding to the crack front area at the time of crack initiation is obtained. PL (kJ / m 2 ), elastic energy density G PL Refers to the result of the following formula:

[0041]

[0042] Where Ω is the arbitrary analysis area for brittle fracture assessment including the crack front, σ ep is the elastic-plastic stress tensor of the analysis area, σ e is the elastic stress tensor, Δa is the length of the elastic energy integration area at the crack front;

[0043] S3. Establish temperature T (℃) and P at different test temperatures L -G PL The corresponding relationship between elastic energy density G PC The material fracture toughness prediction equation is , where the independent variables of the material fracture toughness prediction equation are the crack front temperature and the material ductile-brittle transition temperature (RT NDT ), the specific formula of the material fracture toughness prediction equation is:

[0044] K IC =min{39.8+3.1exp[0.36(T-RT NDT +55.5)],220} (2)

[0045] like Figure 3 As shown, the fracture toughness of the material The initial selection is based on the existing nuclear power design specification RCC-M. In order to obtain a conservative evaluation result, the curve has been moved down in this formula. To envelop the test data at the bottom; at the same time, in order to avoid unnecessary conservatism, considering that high-constraint specimens are often used in the material performance test stage, for the low-constraint situation of "precise evaluation" of small and medium-sized cracks, K has been moved up in this formula IC The lower envelope curve

[0046] Elastic energy density per unit length G PC It can be converted based on the standard equation of the existing nuclear power design specifications. When -80℃≤T-RT NDT ≤60℃, G PC The prediction equation is as follows:

[0047]

[0048] According to the above formula (3), the curve of the material fracture toughness prediction equation is as follows: Figure 4 As shown; In addition, when T-RT NDT >60℃, G PC The value is 1.11kJ / m2.

[0049] S4, such as Figure 5 The RPV core barrel section structure shown in FIG. 1 is the nuclear power structure to be evaluated. For this structure, a finite element analysis model including the initial plane crack obtained by nondestructive testing is established based on finite element numerical simulation software (such as Figure 6 As shown in the figure), the size parameters of the crack position area and the crack front area in the finite element model are marked. In the figure, H is the crack front height and R is the crack front radius. The established defective finite element model is loaded with different operating conditions, and the elastic energy density parameter G at the crack front is obtained again through finite element post-processing of formula (1). P ; Among them, the energy parameter G of the crack front is P The calculation is based on the elastic deformation energy (ENER_ELAS) parameter in the unit body of the finite element model post-processing model. The elastic deformation energy (ENER_ELAS) parameter in the unit body is a common post-processing output parameter provided by existing finite element software. The output description of elastic deformation energy (ENER_ELAS) is as follows: Figure 8 As shown in the chart;

[0050] S5. Comparison G P and G PC , if the energy parameter G of the crack front is obtained in the whole process P are all less than the fracture toughness G of the material at the corresponding temperature. PC , the structure is safe, otherwise an in-depth assessment after crack initiation is required. Fig. 9In the case shown in the example, during the entire transient process, the fracture toughness of the material is higher than the crack growth driving force, and the RPV core structure is safe.

[0051] In the above steps, steps S1 to S3 are material fracture toughness tests based on energy method parameters, step S4 is to obtain the energy parameters of crack initiation driving force under the service state of nuclear power structure integrity, and step S5 is the service safety assessment of nuclear power structure.

[0052] Example 1

[0053] The actual structure of a RPV core barrel section is used for case analysis. The material of the RPV core barrel section is 16MND5, and its chemical composition is shown in Table 1.

[0054] Table 1 Chemical composition of a 16MND 5 material

[0055]

[0056] A fracture toughness test specimen was used for fracture toughness test. The relevant test was carried out on the mechanical testing and simulation (MTS) system, and the fracture toughness test was carried out according to the ASTM E1921 standard. Eight sets of data were tested at -65℃ and -75℃ respectively, and the critical load obtained by the test is shown in Table 2.

[0057] Table 2 Critical fracture load of standard CT specimen test

[0058]

[0059] 1) Material fracture toughness test based on energy method parameters

[0060] like Figure 4 As shown, the relationship between temperature T (℃) and P is established at different test temperatures. L -G PL The corresponding relationship is established based on the elastic energy density per unit length G PC The fracture toughness prediction equation or curve of the material.

[0061] 2) Energy parameters of crack initiation driving force under nuclear power structural integrity service status

[0062] The structure of a RPV core barrel section is as follows: Figure 5 As shown, the inner surface radius of the RPV core barrel is 2197mm, the core base material wall thickness is 220mm, and the inner surface cladding layer thickness is 7.5mm. It is assumed that the core barrel contains a circumferential buried and through-type crack, and the crack depth is 5.2mm (of which 5mm is in the RPV base material).

[0063] The three-dimensional finite element model of the RPV core cylinder with cracks is shown in Figure 2. Figure 6As shown. In order to accurately simulate the stress field in the stress concentration area at the crack front, a structured grid is divided in the crack front area. Based on the experience of literature research, the height H of the fine grid division area at the crack front is taken as 50μm, 20 layers of units are divided in the unit body for calculating energy at the crack front, and a structured grid of 200 lengths is divided in the crack extension direction (the division of the grid size is related to the structural characteristic scale of the material itself).

[0064] like Figure 7 As shown in the figure, an accident condition transient of re-pressurization of a primary circuit system is defined. At the beginning of the transient, the internal pressure is 5MPa; at the end of the transient, the internal pressure increases to 15.5MPa (normal working pressure load); at the beginning of the transient, the fluid temperature is 300℃ and gradually decreases to 20℃.

[0065] In the simulation, the outer surface of the core cylinder is set as an adiabatic boundary condition, the inner surface conducts convective heat transfer with the hot fluid, and the heat transfer coefficient is set to infinity. An internal pressure load P is applied to the inner surface, and an equivalent end surface load P is applied to the upper end surface of the core cylinder. end :

[0066]

[0067] Where Re is the outer surface radius; Ri is the inner surface radius of the core barrel section.

[0068] The integrated output of elastic deformation energy in finite element software is as follows: Figure 8 As shown, the elastic energy density parameter G at the crack front is obtained again through finite element post-processing of formula (1) P At the end of the service life, assuming that the ductile-brittle transition temperature of the RPV core material is 100°C, the safety evaluation results in the transient process are obtained as follows: Fig. 9 shown.

[0069] 3) Nuclear power structure service safety assessment

[0070] Depend on Fig. 9 It can be seen that during the entire transient process, the fracture toughness of the material is higher than the crack propagation driving force, and the RPV core structure is safe. This case is based on the safety performance analysis of small cracks actually monitored. When the crack size becomes larger, at 10,000s, the transient has a re-pressurization characteristic, which will bring significant challenges to the safety performance of the structure. Considering the effect of warm prestressing and other contents, the safety margin of the structure is improved. From the above case study, it can be seen that the use of G P The criterion can be used to systematically conduct a safety assessment of the rupture of the RPV core barrel segment.

[0071] The above embodiments only express the specific implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the patent of the present application. It should be pointed out that, for ordinary technicians in this field, without departing from the concept of the present application, the above-mentioned technical features can be freely combined, and several deformations and improvements can be made, which all belong to the protection scope of the present application. Therefore, all equivalent changes and modifications made to the scope of the claims of the present application should belong to the scope covered by the claims of the present application.

Claims

1. A method for evaluating the fracture toughness transformation of a small-size specimen of RPV material, characterized in that: The following steps are involved: S1. Based on the small-size fracture toughness test specimen, the critical load P at the crack initiation front is obtained. L ; S2. Based on the finite element numerical simulation method, the elastic energy density G corresponding to the crack front area at the time of crack initiation is obtained. PL (kJ / m 2 ), the elastic energy density G PL It refers to the result of the following formula: Where Ω is the arbitrary analysis area for brittle fracture assessment including the crack front, σ ep is the elastic-plastic stress tensor of the analysis area, σ e is the elastic stress tensor, Δa is the length of the elastic energy integration area at the crack front; S3. Establish temperature T (℃) and P at different test temperatures L -G PL The corresponding relationship between elastic energy density G PC The material fracture toughness prediction equation; S4. Based on the finite element numerical simulation software, a finite element analysis model containing the initial plane crack obtained by non-destructive testing is established for the nuclear power structure to be evaluated. The established defective finite element model is loaded with different operating conditions to obtain the elastic energy density parameter G at the crack front. P ; S5. Comparison G P and G PC , if the energy parameter G of the crack front is obtained in the whole process P are all less than the fracture toughness G of the material at the corresponding temperature. PC , the structure is safe.

2. The method for evaluating the fracture toughness transformation of a small-size specimen of RPV material according to claim 1, characterized in that: In the above step S1, the small-size fracture toughness test specimen is any one of a miniature compact tensile specimen and a three-point bending specimen.

3. The method for evaluating the fracture toughness transformation of a small-size specimen of RPV material according to claim 1, characterized in that: In the above step S3, the independent variables of the material fracture toughness prediction equation are the crack front temperature and the material ductile-brittle transition temperature (RT NDT ), the specific formula of the material fracture toughness prediction equation is: K IC =min{39.8+3.1exp[0.36(T-RT NDT +55.5)],220} The fracture toughness of the material The preliminary selection was made based on the existing nuclear power design specification RCC-M.

4. The method for evaluating the fracture toughness transformation of a small-size specimen of RPV material according to claim 3, characterized in that: In the above step S3, the elastic energy density per unit length G PC It can be converted based on the standard equation of the existing nuclear power design specifications. When -80℃≤T-RT NDT ≤60℃, G PC The prediction equation is as follows:

5. The method for evaluating the fracture toughness transformation of a small-size specimen of RPV material according to claim 3, characterized in that: In the above step S3, when T-RT NDT >60℃, G PC The value is 1.11 kJ / m 2 .

6. The method for evaluating the fracture toughness transformation of a small-size specimen of RPV material according to claim 1, characterized in that: In the above step S4, the defective finite element analysis model refers to a three-dimensional crack analysis model established based on general finite element software, and the energy parameter G of the crack front is P The calculation is based on the elastic deformation energy (ENER_ELAS) parameter of the unit body in the post-processing model of the finite element model, wherein the elastic deformation energy (ENER_ELAS) parameter of the unit body is a general post-processing output parameter provided by existing finite element software.

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