A method for calculating thermal-mechanical-irradiation multi-field coupling erosion of reactor fuel cladding

Abstract

This invention discloses a method for calculating the thermal-mechanical-irradiation multi-field coupled wear of reactor fuel cladding. The method first establishes a three-dimensional contact finite element model of the fuel cladding and clamping mechanism, and initializes the temperature field and neutron flux rate. During the calculation, the UMAT subroutine is called to introduce thermal expansion increments and irradiation creep deformation in real time, dynamically updating the constitutive relation and yield strength of the material under complex environments. After obtaining the normal pressure and relative slip at the contact interface, the UMESHMOTION subroutine is called to use a cladding wear model considering acceleration correction to calculate the total wear depth under thermal-mechanical-irradiation multi-field coupling. Finally, the surface mesh is reconstructed in real time using ALE adaptive meshing technology, and the wear depth is dynamically visualized through the UFIELD subroutine. This invention effectively solves the problems of traditional models being unable to consider the effects of multi-field coupling and long-life computational mesh distortion, providing a high-precision numerical simulation method for assessing the structural integrity of nuclear fuel assemblies.

Description

Technical Field

[0001] This invention belongs to the field of nuclear reactor engineering and erosion simulation technology, specifically relating to a method for calculating the multi-physics field coupled erosion of reactor fuel rod cladding and clamping mechanism under high temperature and neutron irradiation coupling. Background Technology

[0002] In the core area of ​​a pressurized water reactor (PWR), fuel assemblies are subjected to extreme environments of high temperature, high pressure, strong radiation, and coolant erosion for extended periods. Fuel rods are positioned using lattice springs and rigid protrusions. The cladding, as the first line of defense against radioactive material leakage, is structurally crucial for the stable operation of a nuclear power plant. However, under flow-induced vibration, micrometer-level relative motion occurs between the cladding tubes and the clamping mechanism—a process known as fretting wear. This long-term mechanical wear leads to thinning of the cladding wall, which in turn causes damage and failure of the nuclear fuel assemblies. In severe cases, it can trigger radioactive material leakage, seriously threatening reactor operational safety.

[0003] Currently, numerical calculations of the erosion behavior of reactor internal components face the following significant challenges: First, there is strong nonlinear coupling between physical fields. The cladding wear is not a purely mechanical process; neutron irradiation-induced material hardening and creep significantly alter the evolution logic of contact pressure; thermal expansion caused by high temperature adjusts the initial clamping force, and existing single-field or weakly coupled models are unable to accurately capture this dynamic mechanism.

[0004] Secondly, there is the imbalance between numerical simulation computation time and physical time. Fretting wear involves high-frequency mechanical cycles, while fuel assemblies have a service life of several years. Finite element analysis faces a severe computational efficiency bottleneck when handling hundreds of millions of cycles throughout the entire lifespan. Furthermore, as wear progresses, the geometry of the contact interface continuously changes, causing the mesh to be highly susceptible to severe distortion.

[0005] Therefore, developing a wear calculation method that can consider the multi-field coupling effect of heat, force and irradiation and combine adaptive mesh reconstruction technology to simulate the wear process of the cladding under complex in-reactor conditions is of great engineering value for realizing the full life-cycle reliability assessment of fuel assemblies. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention aims to provide a method for calculating the thermal-mechanical-irradiation multi-field coupled wear of reactor fuel cladding. This method can calculate the contact mechanical parameters between the cladding and the clamping mechanism under thermal-mechanical-irradiation multi-field coupling conditions, realize the coupled calling of integral point data and node data, and achieve efficient and accurate fretting wear simulation calculation by combining adaptive mesh, and enable real-time monitoring and observation of the wear state of the wear area.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for calculating the thermal-mechanical-irradiation multi-field coupled erosion of reactor fuel cladding, comprising the following steps: Step 1: Define the initial environmental field parameters, including neutron flux rate and initial temperature field; based on the thermo-mechanical-irradiation synergistic effect, and based on the Archard model, introduce the cyclic jump acceleration factor to establish a shell erosion model that considers acceleration correction. The cladding erosion model considering accelerated correction is as follows: ,in h For wear depth, As a cyclic jump acceleration factor, K The wear coefficient is... and The first i The local contact pressure and relative slip distance of the fuel cladding at position x are calculated using thermo-mechanical-irradiation multi-field coupling in an incremental step. Step 2: Use the finite element analysis software ABAQUS to establish a three-dimensional finite element model of the fuel cladding and clamping mechanism; specifically: use the ABAQUS master-slave contact algorithm to define the contact surface between the fuel cladding and the clamping mechanism. The upper surface of the clamping mechanism and the surface of the fuel rod cladding are set as master and slave surfaces, respectively, to establish a three-dimensional finite element model of the cladding and clamping mechanism. Step 3: Using the user subroutine UMAT in ABAQUS software, define the constitutive relation of the fuel cladding material under high temperature and neutron irradiation environment, calculate the thermal strain and thermal expansion increment caused by the cladding temperature field, update the overall strain deformation of the fuel cladding, calculate the yield strength change caused by irradiation hardening, and the inelastic strain increment caused by irradiation creep, and realize the thermo-mechanical-irradiation coordinated mechanical calculation of the fuel cladding. Step 4: Calculate the nonlinear mechanical response of the fuel cladding using the ABAQUS solver. After equilibrium iteration convergence, extract the mechanical parameters of the fuel cladding contact interface, including local contact pressure P and relative slip distance S. Specifically, use the UMESHMOTION subroutine to call GETVRMAVGATNODE to obtain the local contact pressure P and relative slip distance S data interpolated from the grid integration points to the nodes, thus achieving coupled access between integration point data and node data.

[0008] Step 5: In the UMESHMOTION subroutine, use the cladding erosion model considering accelerated correction established in Step 1, and call the local contact pressure P and relative slip distance S obtained in Step 4 to calculate the total wear depth of the fuel cladding under the combined effects of heat, force and irradiation.

[0009] Step 6: Using ALE adaptive meshing technology, the calculated total wear depth is applied to the normal direction displacement of the fuel cladding contact surface nodes, and a volume smoothing algorithm is executed to eliminate mesh distortion caused by fuel cladding material loss; Step 7: Use the UFIELD subroutine to display the calculated total wear depth in real time as a cloud map, and use the previous calculation result as the initial condition for the next iteration at the end of each incremental step, until the analysis time is reached.

[0010] Compared with the prior art, the present invention has the following advantages: 1. By using the UMAT subroutine to achieve multi-field coupling, the limitations of the traditional pure mechanical wear model in single field are overcome. Thermal expansion, radiation hardening, and radiation creep are incorporated into the wear calculation, which can truly reflect the wear law of fuel cladding in extreme reactor environment. 2. The coupling and calling of integral point stress data with local contact pressure and relative slip distance data of node surfaces were realized, thus ensuring the accuracy and real-time performance of the data called for wear calculation; 3. Through the coupling of multiple subroutines, the calculated wear depth can be displayed in real time in the form of a cloud map. Attached Figure Description

[0011] Figure 1 This is a flowchart of the thermal-mechanical-irradiation multi-field coupled abrasion calculation method.

[0012] Figure 2 It is a three-dimensional geometric model of the fuel cladding-clamping mechanism (rigid convex) friction pair structure.

[0013] Figure 3 It is a wear contour map calculated numerically. Detailed Implementation

[0014] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0015] This invention provides a method for calculating the thermal-mechanical-irradiation multi-field coupled abrasion of reactor fuel cladding. This method can calculate the contact mechanical parameters between the cladding and the clamping mechanism under thermal-mechanical-irradiation multi-field coupled conditions, enabling coupled access to integral point data and nodal data. Combined with an adaptive mesh, it achieves efficient and accurate fretting wear simulation calculations. The overall calculation process is shown in the attached figure. Figure 1 As shown; the three-dimensional geometric model of the friction pair structure of the casing and clamping mechanism is attached. Figure 2 As shown; the final abrasion numerical calculation contour map is attached. Figure 3 As shown. The main implementation steps are as follows: Step 1: Initialize and define the initial environmental field parameters, including a neutron fluence rate of 2×10⁻⁶. 14The initial temperature is 573.15 K. Based on the Archard model, a cyclic jump acceleration factor N=2000 is introduced to establish a mathematical model of cladding erosion considering acceleration correction. The cladding erosion model considering accelerated correction is established as follows: ,in h For wear depth, This is the cyclic jump acceleration factor. Its value is 2000. K The wear coefficient is... and The first i Each incremental step at position x The local contact pressure and relative slip distance of the fuel cladding were calculated based on the multi-field coupling of thermo-mechanical-irradiation. Step 2: Use the finite element analysis software ABAQUS to establish a multi-field coupled three-dimensional wear finite element model of the fuel cladding and clamping mechanism. For example... Figure 2 As shown, it includes two components: a fuel cladding and a clamping mechanism. The contact definition between the cladding and the clamping mechanism is defined, with the rigid convex surface as the principal surface and the cladding tube as the secondary surface. The motion constraints of the model are defined, reciprocating micro-motion is applied to the fuel cladding, and the finite element model is meshed. Step 3: Call the user subroutine UMAT in the ABAQUS software to complete the multi-field coupled stress calculation. In UMAT, update the thermal strain and thermal expansion increment based on the constitutive model of the fuel cladding material under high temperature irradiation, consider the change in yield strength caused by irradiation hardening, and the inelastic strain increment caused by irradiation creep, and calculate the stress-strain data. Step 4: In the UMESHMOTION subroutine, call GETVRMAVGATNODE to obtain the local contact pressure P and relative sliding distance S of the contact interface node; Step 5: In the UMESHMOTION subroutine, use the cladding erosion model considering accelerated correction established in Step 1, call the local contact pressure P and relative slip distance S obtained in Step 4 to calculate the total wear depth of the fuel cladding under thermo-mechanical-irradiation synergy, and complete the erosion calculation of thermo-mechanical-irradiation multi-field coupling. Step 6: Using ALE adaptive meshing technology, the calculated wear depth is applied to the normal direction displacement of the fuel cladding contact surface nodes, and a volume smoothing algorithm is executed to eliminate mesh distortion caused by fuel cladding material loss. Step 7: Use the UFIELD subroutine to display the calculated wear depth in real time as a cloud map. The wear calculation results for the fuel liner and clamping mechanism are as follows: Figure 3 As shown, the wear profile corresponds to the wear cloud map, and at the end of each incremental step, the previous calculation result is used as the initial condition for the next iteration until the analysis time is reached.

[0016] The parts of this invention not described in detail are common knowledge to those skilled in the art.

Claims

1. A method for calculating the thermal-mechanical-irradiation multi-field coupled erosion of reactor fuel cladding, characterized in that: Includes the following steps: Step 1: Define the initial environmental field parameters, including neutron flux rate and initial temperature field; Based on the synergistic effect of heat, force and irradiation, and based on the Archard model, a cyclic jump acceleration factor is introduced to establish a shell erosion model that considers acceleration correction. The cladding erosion model considering accelerated correction is as follows: ,in h For wear depth, As a cyclic jump acceleration factor, K The wear coefficient is... and The first i Each incremental step at position x The local contact pressure and relative slip distance of the fuel cladding at the location were calculated based on the multi-field coupling of thermo-mechanical-irradiation. Step 2: Use the finite element analysis software ABAQUS to create a three-dimensional finite element model of the fuel cladding and clamping mechanism; Step 3: Using the user subroutine UMAT in ABAQUS software, define the constitutive relation of the fuel cladding material under high temperature and neutron irradiation environment, perform fuel cladding stress calculation under multi-field coupling, calculate the thermal strain and thermal expansion increment caused by the cladding temperature field, and update the overall strain and deformation of the fuel cladding. The change in yield strength caused by irradiation hardening and the increment of inelastic strain caused by irradiation creep are calculated to achieve the thermo-mechanical-irradiation coordinated mechanical calculation of fuel cladding. Step 4: Calculate the nonlinear mechanical response of the fuel cladding using the ABAQUS solver. After the equilibrium iteration converges, extract the mechanical parameters of the fuel cladding contact interface, including the local contact pressure P and the relative slip distance S. Step 5: In the UMESHMOTION subroutine, use the cladding erosion model considering accelerated correction established in Step 1, and call the local contact pressure P and relative slip distance S obtained in Step 4 to calculate the total wear depth of the fuel cladding under the combined effects of heat, force and irradiation. Step 6: Using ALE adaptive meshing technology, the calculated total wear depth is applied to the normal direction displacement of the fuel cladding contact surface nodes, and a volume smoothing algorithm is executed to eliminate mesh distortion caused by fuel cladding material loss; Step 7: Use the UFIELD subroutine to display the calculated total wear depth in real time as a cloud map, and use the previous calculation result as the initial condition for the next iteration at the end of each incremental step, until the analysis time is reached.

2. The method for calculating the thermal-mechanical-irradiation multi-field coupled erosion of reactor fuel cladding according to claim 1, characterized in that, Step 2 specifically involves: using the ABAQUS master-slave contact algorithm in the finite element analysis software to define the contact surface between the fuel cladding and the clamping mechanism. The upper surface of the clamping mechanism and the surface of the fuel rod cladding are respectively set as master and slave surfaces, and a three-dimensional finite element model of the cladding and the clamping mechanism is established.

3. The method for calculating the thermal-mechanical-irradiation multi-field coupled erosion of reactor fuel cladding according to claim 1, characterized in that: The extraction of mechanical parameters of the fuel cladding contact interface described in step 4 is achieved by using the UMESHMOTION subroutine to call GETVRMAVGATNODE to obtain the local contact pressure P and relative slip distance S data interpolated from the grid integration points to the nodes, thus realizing the coupled calling of integration point data and node data.

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