Method for evaluating fatigue performance of fuel assembly spacer grid clamping structure

Abstract

The application discloses a kind of fuel assembly positioning grid clamping structure fatigue performance evaluation method.The method comprises: constructing the non-steady flow field load spectrum of coolant under different ice-breaking conditions of nuclear power icebreaker;Establish the dynamic finite element model of positioning grid clamping structure and fuel rod;Combining different service time correction clamping force, friction coefficient and other boundary conditions, carry out dynamic response analysis and obtain the fatigue stress results of key position;Based on multiple sets of input parameters and fatigue stress results, construct fatigue life prediction model;The working condition parameters to be evaluated are input into the model, the predicted fatigue life is output, and the fatigue failure risk is judged.The method can more accurately reflect the influence of ice-breaking conditions, irradiation relaxation and corrosion oxidation and other factors on the fatigue performance of clamping structure, and provide basis for structural optimization design and operation safety evaluation.

Description

Technical Field

[0001] This application belongs to the field of nuclear reactor technology, specifically relating to a method for evaluating the fatigue performance of a fuel assembly positioning grid clamping structure. Background Technology

[0002] Nuclear-powered icebreakers are primarily used for icebreaking navigation missions in polar regions such as the Arctic and Antarctic. The polar environment is characterized by low temperatures, high seawater salinity, and high levels of corrosive substances. During icebreaking, icebreakers also endure complex loads such as impacts, vibrations, and high pressures, which places high demands on the structural integrity and operational reliability of the ship's nuclear power plant and its key components.

[0003] The fuel assembly positioning grid is a key component of a nuclear-powered icebreaker reactor. Its main function is to maintain the stability of the fuel assemblies and prevent abnormal displacement of the fuel rods during operation. The clamping structure within the positioning grid is used to secure the fuel rods, and its clamping performance and structural integrity directly affect the stability of the fuel assemblies and the safety of reactor operation.

[0004] Nuclear-powered icebreaker fuel assemblies are subjected to complex environments such as high temperature, high pressure, corrosive water, irradiation, and flow-induced vibration for extended periods. Under long-term cyclic loading, the positioning grid clamping structure is prone to fatigue damage, which may lead to material fracture in severe cases, thereby affecting the positioning and support capabilities of the fuel assembly and the safety of reactor operation.

[0005] Existing fatigue performance evaluation methods are mostly applicable to reactor structures under relatively stable operating conditions, such as those in land-based nuclear power plants. These methods typically do not consider the influence of the flow field during fatigue performance analysis, or simplify the flow field to a steady-state flow field. For the complex icebreaking conditions that nuclear-powered icebreakers may encounter during icebreaking operations, such as unsteady flow fields, hull motion responses, core coolant flow fluctuations, and internal pressure pulsations, existing methods have insufficient applicability, and the evaluation results may deviate from actual service conditions.

[0006] Therefore, it is necessary to propose a fatigue performance evaluation method applicable to the fuel assembly positioning grid clamping structure of nuclear-powered icebreakers. This method can incorporate unsteady flow field loads under icebreaking conditions, and combine the dynamic response between the clamping structure and the fuel rods with the influence of service time to accurately evaluate the fatigue life and fatigue failure risk of the positioning grid clamping structure under different operating conditions. Summary of the Invention

[0007] The purpose of this application is to provide a method for evaluating the fatigue performance of fuel assembly positioning grid clamping structures, in order to solve the problem that traditional fatigue performance analysis methods do not fully consider the effects of unsteady flow field loads and service time under icebreaking conditions, making it difficult to accurately evaluate the fatigue performance of fuel assembly positioning grid clamping structures of nuclear-powered icebreakers.

[0008] The technical solution to achieve the purpose of this application is as follows: This application provides a method for evaluating the fatigue performance of a fuel assembly positioning grid clamping structure, comprising the following steps: S1, construct the unsteady flow field load spectrum of coolant under different icebreaking conditions of nuclear-powered icebreakers; S2, Establish the dynamic finite element model of the positioning grid clamping structure and the fuel rod; S3, Dynamic response analysis is carried out for different service durations to obtain fatigue stress results of key parts of the positioning grid clamping structure; S4. Based on multiple sets of input parameters and corresponding fatigue stress results, construct a fatigue life prediction model for the clamping structure and obtain the sensitivity results of the fatigue life prediction model for the clamping structure to the input parameters. S5. Input the parameters of the working condition to be evaluated into the fatigue life prediction model of the clamping structure, output the predicted fatigue life, and evaluate the fatigue performance of the positioning grid clamping structure under the corresponding working condition based on the predicted fatigue life.

[0009] Optionally, step S1 includes: based on the motion response data of the nuclear-powered icebreaker and combined with the hydraulic characteristics of the reactor primary loop, simulating the core coolant flow fluctuations and internal pressure pulsations caused by icebreaking operations, and extracting the time history data of the fluid forces acting on the positioning grid to construct a coolant unsteady flow field load spectrum reflecting the icebreaking conditions.

[0010] Optionally, the fluid force time history data is obtained through computational fluid dynamics simulation, and the unsteady flow field load spectrum within a preset frequency range is obtained by time-domain transformation or frequency-domain transformation.

[0011] Optionally, step S2 includes: establishing a clamping fit model including grid strips, springs and fuel rods, defining the elastoplastic constitutive relation of the materials and the friction coefficient of the contact pair between the clamping structure and the fuel rod cladding, and meshing the clamping fit model.

[0012] Optionally, when performing the mesh division, the contact area between the grid strip rigid protrusions, springs, and fuel rods is refined into a finer mesh.

[0013] Optionally, step S3 includes: setting multiple characteristic time points within the service life of the positioning grid clamping structure, and correcting the boundary conditions of the dynamic response analysis according to different service times.

[0014] Optionally, the boundary conditions include the clamping force between the positioning grid clamping structure and the fuel rod, and the coefficient of friction of the contact surface between the positioning grid clamping structure and the fuel rod.

[0015] Optionally, the clamping force is corrected for attenuation over service time based on irradiation stress relaxation data, and the friction coefficient is corrected for changes in the corrosion oxide film on the material surface over service time.

[0016] Optionally, in step S3, dynamic response analysis is performed based on the unsteady flow field load spectrum of the coolant and the dynamic finite element model, and the results of the maximum stress change over time at the spring root, bending point and clamping contact area are extracted.

[0017] Optionally, step S4 includes: selecting multiple sets of input parameters for simulation calculation to obtain the corresponding maximum stress time history data, calculating the corresponding fatigue life in combination with fatigue damage theory, and establishing a nonlinear mapping relationship between input parameters and fatigue life to construct the fatigue life prediction model of the clamping structure.

[0018] Optionally, the fatigue damage theory includes Miner's linear cumulative damage theory and / or the strain-life equation; the nonlinear mapping relationship is established using a machine learning algorithm, which includes at least one of decision tree, random forest, and neural network.

[0019] Optionally, the input parameters include at least one of the following: operating temperature, system pressure, coolant density, coolant flow rate, ice-breaking load, clamping force, and friction coefficient of the clamping contact surface.

[0020] Optionally, step S5 includes: comparing the predicted fatigue life with the design life; when the predicted fatigue life is lower than the design life, determining that the positioning grid clamping structure has a risk of fatigue failure, and determining the input parameters or structural design parameters to be adjusted first based on the sensitivity results.

[0021] The beneficial technical effects of this application are as follows: The fatigue performance evaluation method for the fuel assembly positioning grid clamping structure provided in this application constructs a coolant unsteady flow field load spectrum under different icebreaking conditions, establishes a dynamic finite element model of the positioning grid clamping structure and fuel rods, conducts dynamic response analysis for different service durations to obtain fatigue stress results of key parts of the positioning grid clamping structure, and constructs a fatigue life prediction model of the clamping structure based on multiple sets of input parameters and corresponding fatigue stress results. Then, the parameters of the operating conditions to be evaluated are input into the prediction model to output the predicted fatigue life and evaluate the fatigue performance under the corresponding operating conditions. Therefore, this application can measure the coolant flow rate caused by icebreaking motion. The introduction of fluctuations, pressure pulsations, and complex flow-induced vibration characteristics into the fatigue performance evaluation process makes the fatigue stress calculation results closer to the actual operating conditions of nuclear-powered icebreakers. At the same time, it can combine different service durations to correct boundary conditions such as clamping force attenuation and changes in contact surface friction coefficient, dynamically reflecting the impact of irradiation relaxation and corrosion oxidation on fatigue failure risk. Furthermore, the fatigue life prediction model can be used to quickly obtain the fatigue performance and sensitivity results of the positioning grid clamping structure under different operating conditions, thereby providing a basis for the optimized design of the clamping structure, adjustment of sensitive parameters, and operational safety assessment, and improving the stability of fuel assemblies and the operational safety of nuclear-powered icebreaker reactors. Detailed Implementation

[0022] To enable those skilled in the art to better understand this application, specific embodiments of this application are clearly and completely described below. Obviously, the following embodiments are only some embodiments of this application and are not intended to limit the scope of protection of this application. Equivalent substitutions, modifications, or improvements made by those skilled in the art based on the content described in this application without departing from the concept of this application should all fall within the scope of protection of this application.

[0023] This embodiment provides a method for evaluating the fatigue performance of a fuel assembly positioning grid clamping structure for a nuclear-powered icebreaker, which includes the following steps.

[0024] S1. Construct the unsteady flow field load spectrum of coolant under different icebreaking conditions. Specifically, based on the motion response data of nuclear-powered icebreakers and combined with the hydraulic characteristics of the reactor primary loop, simulate the core coolant flow fluctuations and internal pressure pulsations caused by icebreaking operations, extract the time history data of fluid forces acting on the positioning grid, and construct the unsteady flow field load spectrum of coolant that can reflect the icebreaking conditions.

[0025] In a specific example, based on motion monitoring data of a nuclear-powered icebreaker under the condition of "continuous ice ridge crossing," a fluctuation component with an amplitude of ±20% and a dominant frequency of 5Hz was superimposed on the average flow rate at the reactor core inlet. Computational fluid dynamics software was used to simulate the flow field around the positioning lattice under this flow rate condition. The time-history data of fluid resistance acting on individual lattice strips were extracted through simulation, and the unsteady flow field load spectrum in the range of 0Hz to 2000Hz was obtained through Fourier transform, serving as the load input for subsequent dynamic response analysis.

[0026] S2. Establish a dynamic finite element model of the positioning grid clamping structure and fuel rods. Specifically, establish a clamping fit model including the grid strips, springs, and fuel rods, define the elastoplastic constitutive relations of the materials and the friction coefficients of the contact pairs between the clamping structure and the fuel rod cladding, and mesh the clamping fit model. During meshing, the mesh can be refined in the contact areas between the rigid protrusions of the grid strips, the springs, and the fuel rods to improve the accuracy of stress calculation in the contact areas.

[0027] In one specific example, a clamping fit model is established between the grid strips and spring clamping structure made of Zr-4 alloy and the fuel rods. The friction coefficient of the contact pair between the clamping structure and the fuel rod cladding is initially set to 0.2. The clamping structure can be discretized using shell elements and meshed using hexahedral elements. The mesh is refined in the contact area between the strip protrusions, the springs, and the fuel rods, with the mesh size controlled within 0.2 mm.

[0028] S3. Dynamic response analysis is conducted for different service durations to obtain fatigue stress results for key components of the positioning grid clamping structure. Specifically, multiple characteristic time points are set within the service life of the positioning grid clamping structure, and the boundary conditions for the dynamic response analysis are modified according to different service times. The boundary conditions may include the clamping force between the positioning grid clamping structure and the fuel rod, as well as the coefficient of friction of the contact surface between the positioning grid clamping structure and the fuel rod.

[0029] In one specific example, three typical time points within the service life of the positioning grid clamping structure were selected: EFPD=0 at the beginning of the service life, EFPD=200 during the middle of the service life, and EFPD=400 at the end of the service life. Based on the irradiation relaxation test data of the clamping springs, the spring clamping force was set to 50N at the beginning of the service life, decreasing to 30N during the middle of the service life, and decreasing to 15N at the end of the service life. Simultaneously, the coefficient of friction was gradually increased from 0.2 at the beginning of the service life to 0.35 at the end of the service life to simulate the impact of oxide film thickening caused by corrosion and other factors during the in-core service of the clamping structure.

[0030] Based on the unsteady flow field load spectrum of the coolant constructed in step S1 and the dynamic finite element model established in step S2, dynamic response calculations and analyses were performed at different characteristic time points, and the fatigue stress changes over time at the spring root, bending point, and / or clamping contact area were extracted. In a specific example, the maximum stress in the contact area between the clamping structure and the fuel rod was extracted to be 600 MPa at the beginning of the service life, 700 MPa during the service life, and increased to 780 MPa at the end of the service life.

[0031] S4. Based on multiple sets of input parameters and corresponding fatigue stress results, a fatigue life prediction model for the clamping structure is constructed, and the sensitivity results of the clamping structure fatigue life prediction model to the input parameters are obtained. Specifically, multiple sets of input parameters are selected for simulation calculation to obtain the corresponding fatigue stress time history data. Combined with fatigue damage theory, the corresponding fatigue life is calculated, and a nonlinear mapping relationship between input parameters and fatigue life is established to construct the fatigue life prediction model for the clamping structure.

[0032] In one specific example, 50 sets of calculation samples are designed. The input parameters include operating temperature, system pressure, and coolant flow rate. The operating temperature can range from 200℃ to 350℃, the system pressure can range from 8MPa to 12MPa, and the coolant flow rate can range from 1.0m / s to 1.5m / s. The fatigue stress time history results of each set of samples are obtained through simulation calculations. The fatigue life corresponding to each set of samples is calculated by combining Miner's linear cumulative damage theory and the modified strain-life equation. The fatigue life can be expressed as the number of cycles or the equivalent number of full-power days.

[0033] Furthermore, multiple sets of input data can be used as training features, and the corresponding calculated fatigue life can be used as the target label. These are then input into a machine learning-based model for training to establish a nonlinear mapping relationship between the input parameters and fatigue life. The machine learning algorithm can be at least one of decision trees, random forests, and neural networks. In a specific example, a neural network model is used for training, and the most significant sensitive parameter affecting fatigue life is identified by quantifying the contribution rate of each input parameter to the variance of the output variable.

[0034] S5. Input the parameters of the working condition to be evaluated into the fatigue life prediction model of the clamping structure, output the predicted fatigue life, and evaluate the fatigue performance of the positioning grid clamping structure under the corresponding working condition based on the predicted fatigue life. Specifically, compare the predicted fatigue life with the design life. When the predicted fatigue life is lower than the design life, it is determined that the positioning grid clamping structure has a risk of fatigue failure, and the input parameters or structural design parameters to be adjusted first are determined based on the sensitivity results. When the predicted fatigue life is not lower than the design life, it can be determined that the fatigue performance of the positioning grid clamping structure under the corresponding working condition meets the design requirements.

[0035] In a specific example, the prediction model established in step S4 is used to predict the fatigue performance under extreme icebreaking conditions. These extreme icebreaking conditions may include an operating temperature of 340℃, an operating pressure of 10MPa, and an instantaneous flow velocity of 1.5m / s. The prediction results show that the predicted lifespan under this condition is 800 days of equivalent full-power operation. Compared with the design requirement of 400 days of equivalent full-power operation, it can be determined that the fatigue failure risk of this structural design is low and meets the design requirements.

[0036] Using the above method, this application can introduce the unsteady flow field load under the icebreaking conditions unique to nuclear-powered icebreakers into the fatigue performance evaluation of the positioning grid clamping structure. Combined with the influence of service time on boundary conditions and fatigue life prediction model, it can achieve rapid prediction and evaluation of the fatigue failure tendency of the clamping structure under different operating conditions, thereby providing support for the design improvement, sensitive parameter adjustment and operation safety assessment of the positioning grid clamping structure.

[0037] The above embodiments illustrate this application, but this application is not limited to the above embodiments. Structures, modeling methods, simulation methods, data processing methods, and evaluation methods not described in detail in this application can all be implemented using existing technologies in the field.

Claims

1. A method for evaluating fatigue performance of a fuel assembly spacer grid clamping structure, characterized by, Includes the following steps: S1, construct the unsteady flow field load spectrum of coolant under different icebreaking conditions of nuclear-powered icebreakers; S2, Establish the dynamic finite element model of the positioning grid clamping structure and the fuel rod; S3, Dynamic response analysis is carried out for different service durations to obtain fatigue stress results of key parts of the positioning grid clamping structure; S4. Based on multiple sets of input parameters and corresponding fatigue stress results, construct a fatigue life prediction model for the clamping structure and obtain the sensitivity results of the fatigue life prediction model for the clamping structure to the input parameters. S5. Input the parameters of the working condition to be evaluated into the fatigue life prediction model of the clamping structure, output the predicted fatigue life, and evaluate the fatigue performance of the positioning grid clamping structure under the corresponding working condition based on the predicted fatigue life.

2. The method of claim 1, wherein the method is characterized by: Step S1 includes: based on the motion response data of the nuclear-powered icebreaker and combined with the hydraulic characteristics of the reactor primary loop, simulating the core coolant flow fluctuations and internal pressure pulsations caused by icebreaking operations, and extracting the time history data of the fluid forces acting on the positioning grid to construct a coolant unsteady flow field load spectrum reflecting the icebreaking conditions.

3. The method of claim 2, wherein the method is characterized by: The fluid force time history data is obtained through computational fluid dynamics simulation and then transformed in the time domain or frequency domain to obtain the unsteady flow field load spectrum within a preset frequency range.

4. The method of claim 1, wherein Step S2 includes: establishing a clamping fit model containing grid strips, springs and fuel rods, defining the elastoplastic constitutive relation of the materials and the friction coefficient of the contact pair between the clamping structure and the fuel rod cladding, and meshing the clamping fit model.

5. The method of claim 4, wherein the method further comprises: determining the fatigue performance of the fuel assembly spacer grid clip structure based on the measured strain and the measured stress. When performing the mesh division, the contact area between the grid strip rigid protrusions, springs, and fuel rods is refined into a finer mesh.

6. The method of claim 1, wherein Step S3 includes: setting multiple characteristic time points within the service life of the positioning grid clamping structure, and correcting the boundary conditions of the dynamic response analysis according to different service times.

7. The fatigue performance evaluation method for the fuel assembly positioning grid clamping structure according to claim 6, characterized in that, The boundary conditions include the clamping force between the positioning grid clamping structure and the fuel rod, and the coefficient of friction of the contact surface between the positioning grid clamping structure and the fuel rod.

8. The fatigue performance evaluation method for the fuel assembly positioning grid clamping structure according to claim 7, characterized in that, The clamping force is adjusted based on the irradiation stress relaxation data over service time, and the friction coefficient is adjusted based on the changes in the corrosion oxide film on the material surface over service time.

9. The fatigue performance evaluation method for the fuel assembly positioning grid clamping structure according to claim 1, characterized in that, In step S3, dynamic response analysis is carried out based on the unsteady flow field load spectrum of the coolant and the dynamic finite element model, and the results of the maximum stress change over time at the spring root, bending point and clamping contact area are extracted.

10. The fatigue performance evaluation method for the fuel assembly positioning grid clamping structure according to claim 1, characterized in that, Step S4 includes: selecting multiple sets of input parameters for simulation calculation, obtaining the corresponding maximum stress time history data, calculating the corresponding fatigue life in combination with fatigue damage theory, and establishing a nonlinear mapping relationship between input parameters and fatigue life to construct the fatigue life prediction model of the clamping structure.

11. The fatigue performance evaluation method for the fuel assembly positioning grid clamping structure according to claim 10, characterized in that, The fatigue damage theory includes Miner's linear cumulative damage theory and / or strain-life equation; the nonlinear mapping relationship is established using a machine learning algorithm, which includes at least one of decision tree, random forest and neural network.

12. The fatigue performance evaluation method for the fuel assembly positioning grid clamping structure according to claim 10, characterized in that, The input parameters include at least one of the following: operating temperature, system pressure, coolant density, coolant flow rate, ice-breaking load, clamping force, and friction coefficient of the clamping contact surface.

13. The fatigue performance evaluation method for the fuel assembly positioning grid clamping structure according to claim 1, characterized in that, Step S5 includes: comparing the predicted fatigue life with the design life; when the predicted fatigue life is lower than the design life, determining that the positioning grid clamping structure has a risk of fatigue failure, and determining the input parameters or structural design parameters to be adjusted first based on the sensitivity results.