A prediction method for mechanical properties of irradiated concrete based on ABAQUS

Abstract

The present invention discloses a method for predicting the mechanical properties of irradiated concrete based on ABAQUS, comprising the following steps: considering concrete as a composite material composed of three-phase mesoscopic materials: cement mortar, aggregate, and interface transition zone; adopting a CDP model based on field variables to consider the degradation law of the mechanical properties of the mesoscopic materials as their volume changes; establishing a finite element model of the concrete mesoscopic three-phase material in an ABAQUS static implicit solver and assigning material properties; setting an uneven temperature field and neutron fluence field based on the UTEMP subroutine and the USFLD subroutine, and controlling the volume expansion of the mesoscopic material over time based on the UEXPAN subroutine, thereby simulating the process of radiation-induced macroscopic volume change of concrete; and predicting the degradation law of the elastic properties and uniaxial compressive strength of the concrete through restart analysis.

Description

Technical Field

[0001] The present invention relates to the technical field of numerical simulation of concrete materials, and in particular to a method for predicting the residual mechanical properties of concrete affected by neutron irradiation in a nuclear power plant core. Background Art

[0002] In recent years, an increasing number of nuclear power plants built earlier around the world have reached the end of their lifespans. Because extending the lifespan of nuclear power plants offers significant economic benefits compared to new construction, the global nuclear power industry has shifted its focus from new construction to extending the service lifespan of existing plants. The core concrete biobarrier (CBS), a critical, non-replaceable component, determines the operational lifespan of a nuclear power plant.

[0003] Concrete materials exposed to nuclear radiation can swell, crack, and degrade in mechanical properties, leading to cracking and even delamination on the inner surface of concrete bioshields. Currently, experimental data on irradiation of concrete materials and their components is limited and subject to numerous constraints. To accurately and reliably assess concrete degradation, it is necessary to consider the heterogeneity of concrete at the microscopic level and the differences in the neutron irradiation response of different components. This study investigates the degradation mechanisms and time-varying patterns of concrete's physical and mechanical properties from a microscopic perspective.

[0004] Current multiscale approaches to studying the degradation of the physical and mechanical properties of irradiated concrete primarily include homogenization methods for micromechanics and micromechanics, and finite and discrete element models. Homogenization methods primarily include homogenization models based on Hashin composite spheres, multiphase and multiscale models based on generalized self-consistent schemes, and homogenization methods based on fast Fourier transforms. However, homogenization methods can only predict concrete RIVE, elastic properties, and relative internal damage, whereas microfinite and discrete element methods can predict concrete RIVE, elastic properties, crack development, and degradation in tensile and compressive strength.

[0005] Current mesoscopic finite element models generally achieve radiation-induced volume changes of mesoscopic components based on temperature changes by setting an equivalent temperature linear expansion coefficient. They cannot directly control the volume changes of mesoscopic components based on the radiation field and temperature field in concrete. At the same time, there is a lack of a general method to control the changes in the mechanical properties parameters of mesoscopic components over time. It is difficult to simulate the uneven temperature field and neutron flux field inside the concrete specimens during irradiation tests, as well as the resulting macroscopic volume expansion and mechanical property degradation of concrete.

[0006] Patent application number CN2023104790836 discloses a numerical simulation method for predicting the degradation of concrete mechanical properties under neutron irradiation. By setting an equivalent temperature linear expansion coefficient, the method indirectly measures radiation-induced volume changes of microscopic components based on temperature changes. However, this method involves complex equivalent conversions and can only set a single temperature variable, resulting in indirect and inaccurate results. Summary of the Invention

[0007] In view of this, in order to overcome the defects of the prior art, the purpose of the present invention is to provide a prediction method for the mechanical properties of irradiated concrete based on ABAQUS.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A method for predicting the mechanical properties of irradiated concrete based on ABAQUS includes the following steps:

[0010] Concrete is considered as a composite material consisting of three phases of mesoscopic materials: cement mortar, aggregate and interface transition zone;

[0011] The CDP model based on field variables is used to consider the degradation law of the mechanical properties of microscopic materials as their volume changes;

[0012] Establish a finite element model of concrete microscopic three-phase material in ABAQUS static implicit solver and assign material properties;

[0013] The non-uniform temperature field and neutron flux field are set based on the UTEMP and USFLD subroutines, and the volume expansion of the mesoscopic material with time is controlled based on the UEXPAN subroutine to simulate the process of radiation-induced macroscopic volume change of concrete.

[0014] The elastic properties of concrete and the degradation law of uniaxial compressive strength are predicted by restart analysis.

[0015] According to some preferred implementation aspects of the present invention, the CDP model based on field variables considers the degradation law of the mechanical properties of mesoscopic materials as their volume changes as follows: determining the initial mechanical property parameters of the concrete mesoscopic material and the degradation law of the mechanical property parameters that depend on the radiation-induced volume strain field variables, and defining the plastic damage constitutive model of each mesoscopic material that depends on the volume strain field variables.

[0016] According to some preferred implementation aspects of the present invention, the steps for establishing a compressive constitutive model of a microscopic material are as follows:

[0017] The equations for the post-peak softening curves of mortar and ITZ are:

[0018]

[0019] Among them, σ c and ε are compressive stress and strain respectively, ε c is its intensity f c The corresponding compressive strain, α is the coefficient calculated by the following formula:

[0020]

[0021] For the compressive constitutive model of the mortar and interface transition zone, the elastic modulus E, peak stress σ c , peak strain, rising section parameter and descending section parameter;

[0022] For the compressive constitutive model of aggregate, the compressive stress-strain relationship before the peak is set to linear elastic, and the strength decreases rapidly after the peak. The elastic modulus E, peak stress σ c There are 2 parameters in total.

[0023] According to some preferred implementation aspects of the present invention, the steps for establishing the tensile constitutive model of the microscopic material are as follows:

[0024] The constitutive relation and damage evolution equation of the microscopic material in the softening stage are:

[0025]

[0026] Where, σ tu and G f are the peak tensile stress and fracture energy of the material, respectively, which are material constants and can be measured by experiments; u ck is the cracking displacement; therefore, the stress value and damage variable are both the cracking displacement u ck function;

[0027] For the tensile constitutive model of mortar and interface transition zone, the elastic modulus E, peak stress σ tu and G f Three parameters; for the tensile constitutive model of aggregate, the stress-strain relationship before the peak is set to linear elastic, and the strength after the peak decreases rapidly, requiring the elastic modulus E, peak stress σ tu and G f Three parameters.

[0028] According to some preferred embodiments of the present invention, the elastic modulus of the aggregate varies with its volume as follows:

[0029] After determining the degradation elastic modulus of the mortar under neutron flux and temperature, the compressive strength, tensile strength and peak compressive strain of the mortar are calculated according to the following formulas:

[0030]

[0031] f tm / f tm0 =0.996(f cm / f cm0 ) 1.2803

[0032] ε cm / ε cm0 =(f cm / fcm0 ) -0.83

[0033] Where, f cm is the compressive strength of the mortar (MPa), f tm is the compressive strength of the mortar (MPa), E cm is the elastic modulus of the mortar (GPa), ε cm is the peak compressive strain of the mortar.

[0034] According to some preferred embodiments of the present invention, the fracture energy of the mortar is calculated according to the following formula:

[0035]

[0036] Where, f cm is the compressive strength of the mortar (MPa), G fm is the fracture energy of the mortar (N / mm), d max is the maximum fine aggregate particle size (mm).

[0037] According to some preferred embodiments of the present invention, the mechanical properties of the interface transition zone, including its elastic modulus and compressive strength, are considered to be 0.9 times that of the mortar and are calculated using the same method as that of the mortar.

[0038] According to some preferred implementation aspects of the present invention, the finite element model of the concrete mesoscopic three-phase material uses a mapped meshing method to establish a three-phase model of cement mortar, aggregate and ITZ, and the steps are:

[0039] First, use fixed-size grid units to divide the model, then output the coordinate information of each unit and node, input the geometric position of the aggregate in the three-dimensional uniform grid, and then judge whether each unit belongs to the specific material phase in the concrete based on the unit node and aggregate shape and position parameters; assign the established material properties to the units of each phase.

[0040] According to some preferred embodiments of the present invention, the non-uniform temperature field and neutron flux field are set based on the UTEMP subroutine and the USFLD subroutine, and the volume expansion of the mesoscopic material with time is controlled based on the UEXPAN subroutine, thereby simulating the process of radiation-induced macroscopic volume change of concrete, specifically:

[0041] The finite element software ABAQUS provides user subroutines UTEMP and USFLD to program temperature and neutron flux field functions with time step and spatial coordinates as independent variables to set non-uniform temperature and neutron flux fields. The UEXPAN subroutine also includes built-in aggregate and mortar volume expansion models based on temperature and neutron flux, enabling direct control of the volume expansion of the mesoscopic material over time. These models are then stored as field variables, which are used by the solver to call the CDP model material parameters defined previously under the corresponding field variables during the calculation process.

[0042] Since the aggregate volume expansion model and mortar volume expansion model based on temperature and neutron flux have been built into the UEXPAN subroutine, the temperature field and neutron flux field functions with time steps are input into the UTEMP and USFLD subroutines. After the calculation is submitted, the volume of each component inside the concrete specimen changes with the increase of the analysis step length, thereby generating changes in the stress-strain field and damage development, and ultimately realizing the simulation of the irradiation-induced macroscopic volume change of concrete.

[0043] According to some preferred implementation aspects of the present invention, the elastic properties of concrete and the degradation law of uniaxial compressive strength are predicted by restart analysis as follows: a restart analysis model for uniaxial compression test is established, and the restart analysis job and post-processing are submitted; the restart position of the model to be solved is set, the boundary conditions are modified, a downward displacement is applied to the top surface of the specimen, and the finite element calculation of the uniaxial compression simulation is completed to obtain the uniaxial compression stress-strain curve of the concrete specimen. Based on this curve, the elastic modulus and compressive strength of the concrete at different times during the irradiation process can be calculated.

[0044] Due to the adoption of the above technical solutions, the present invention is beneficial compared to the prior art in that: the method for predicting the mechanical properties of irradiated concrete of the present invention regards concrete as a composite material composed of three phases: cement mortar, aggregate, and interfacial transition zone (ITZ), considers the time-varying degradation of the strain and mechanical properties of the three-phase material, and predicts the degradation law of concrete volume expansion, elastic properties, and uniaxial compressive strength under neutron irradiation; in the ABAQUS finite element software, based on the UTEMP and USFLD subroutines to set non-uniform temperature fields and neutron flux fields, the aggregate volume expansion model and mortar volume expansion model based on temperature and neutron flux can be directly built in, without resorting to methods such as temperature linear expansion coefficient equivalence, and directly based on the UEXPAN subroutine to achieve the purpose of controlling the volume expansion of the microscopic material over time; in addition, in the CDP model, the mechanical property parameters that change with field variables can achieve the purpose of degrading the mechanical properties of the microscopic material over time; the method provided by the present invention has clear concepts, clear calculations, and good practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0046] Figure 1 Schematic diagram of the flow of a method for predicting mechanical properties of irradiated concrete in a preferred embodiment of the present invention; Figure 2 Graph showing the variation of elastic modulus of aggregate with its volume in a preferred embodiment of the present invention;

[0047] Figure 3 Graph showing the variation of the elastic modulus of the mortar with its volume in a preferred embodiment of the present invention;

[0048] Figure 4 Schematic diagram of establishing a three-phase model of cement mortar, aggregate and ITZ in a preferred embodiment of the present invention;

[0049] Figure 5 In the preferred embodiment of the present invention, a UEXPAN subroutine is used to build in a volume expansion diagram of aggregate based on temperature and neutron injection;

[0050] Figure 6 In the preferred embodiment of the present invention, a UEXPAN subroutine is used to build a mortar volume expansion diagram based on temperature and neutron flux;

[0051] Figure 7 is a height change curve of a concrete specimen during irradiation in a preferred embodiment of the present invention;

[0052] Figure 8 The stress-strain curve extracted at the start of calculation in the preferred embodiment of the present invention;

[0053] Figure 9 This is a graph showing changes in elastic modulus during irradiation in a preferred embodiment of the present invention;

[0054] Figure 10 This is a graph showing changes in compressive strength during irradiation in a preferred embodiment of the present invention; DETAILED DESCRIPTION

[0055] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0056] The present invention provides an ABAQUS-based method for predicting the mechanical properties of irradiated concrete, which considers concrete as a composite material consisting of three phases: cement mortar, aggregate, and interfacial transition zone (ITZ). First, the degradation of the mechanical properties of the three-phase material as its volume changes is considered based on the CDP model using field variables. Then, a microscopic three-phase finite element model of concrete is established and material properties are assigned in the ABAQUS static implicit solver. Then, the non-uniform temperature field and neutron flux field are set based on the UTEMP and USFLD subroutines. The volume expansion of the mesoscopic material is controlled over time based on the UEXPAN subroutine, thereby simulating the process of radiation-induced macroscopic volume change of concrete. Finally, a restart analysis can be used to conveniently and accurately predict the degradation law of concrete elastic properties and uniaxial compressive strength. The present invention utilizes the user subroutines UTEMP, USFLD, and UEXPAN provided by the finite element software ABAQUS, and through specific steps and methods, achieves the purpose of directly controlling the volume and mechanical properties of the mesoscopic components as they change with field variables based on the radiation field and temperature field within the concrete.

[0057] The present invention is based on ABAQUS neutron irradiation induced concrete mechanical property degradation prediction method, the prediction method process is as follows Figure 1 As shown, the specific steps include:

[0058] Step (1) Consider the degradation of the mechanical properties of the three-phase material with its volume change based on the CDP model using field variables

[0059] Determine the initial mechanical properties of concrete mesoscopic materials and the degradation laws of mechanical properties parameters that depend on radiation-induced volumetric strain field variables, and define the plastic damage (CDP) constitutive model of each mesoscopic material that depends on volumetric strain field variables. The details are as follows:

[0060] The equations for the post-peak softening curves of mortar and ITZ are:

[0061]

[0062] Among them, σ c and ε are compressive stress and strain respectively, ε c is its intensity f c The corresponding compressive strain, α is a coefficient calculated by the following formula:

[0063]

[0064] For the compressive constitutive model of mortar and ITZ, the elastic modulus E, peak stress σ c , peak strain ε c , ascending segment parameters and descending segment parameters, a total of 5 parameters.

[0065] For the compressive constitutive model of aggregate, the compressive stress-strain relationship before the peak is assumed to be linear elastic, and the strength decreases rapidly after the peak. The elastic modulus E, peak stress σ c There are 2 parameters in total.

[0066] The tensile plastic damage constitutive relation of the microscopic material is described by the stress-displacement relation based on the fracture energy cracking criterion to solve the mesh sensitivity problem. The constitutive relation and damage evolution equation of the material in the softening stage are:

[0067]

[0068] Where, σ tu and G f are the peak tensile stress and fracture energy of the material, respectively, which are material constants and can be measured by experiments; u ck is the cracking displacement. Therefore, the stress value and damage variable are both the cracking displacement u ck function.

[0069] For the tensile constitutive model of mortar and ITZ, the elastic modulus E, peak stress σ tu and G f Three parameters. For the tensile constitutive model of aggregate, the stress-strain relationship before the peak is assumed to be linear elastic, and the strength decreases rapidly after the peak. The elastic modulus E, peak stress σ tu and G f Three parameters.

[0070] The variation law of the parameters of the mechanical properties of mesoscopic materials with their volume can be obtained through irradiation tests and relevant literature. Then, based on the above mechanical property parameters when the mesoscopic materials have different volume changes induced by radiation, the CDP constitutive parameters of each mesoscopic material that depend on the volume strain field variables can be defined.

[0071] This embodiment provides a method for determining the elastic modulus of aggregate and mortar as a function of their volume. Figure 2 and Figure 3 In the figure, GA represents coarse aggregate and Sand represents fine aggregate. Aggregate strength degradation is not considered. The strength, peak strain, and fracture energy of the mortar are all based on the degraded elastic modulus.

[0072] Based on the similarity in the degradation mechanisms of the mechanical properties of mortar and concrete, the relevant formulas for calculating the performance parameters of mortar, except for volume expansion and elastic modulus, refer to the discrete element simulation conclusions of irradiated concrete by Sasano et al., and the degraded elastic modulus, tensile strength, and peak strain of concrete are calculated based on the relationship between the physical properties of irradiated concrete.

[0073] After determining the degraded elastic modulus of the mortar at a certain neutron flux and temperature, the corresponding compressive strength, tensile strength and peak compressive strain can be calculated according to the following formula:

[0074]

[0075] f tm / f tm0 =0.996(f cm / f cm0 ) 1.2803

[0076] ε cm / ε cm0 =(f cm / f cm0 ) -0.83

[0077] Where, f cm is the compressive strength of the mortar (MPa), f tm is the compressive strength of the mortar (MPa), E cm is the elastic modulus of the mortar (GPa), ε cm is the peak compressive strain of the mortar.

[0078] The fracture energy of mortar is calculated according to the following formula:

[0079]

[0080] Where, f cm is the compressive strength of the mortar (MPa), G fm is the fracture energy of the mortar (N / mm), d max is the maximum fine aggregate particle size (mm).

[0081] For the mechanical properties of the interface transition zone (ITZ), its elastic modulus and compressive strength can be regarded as 0.9 times that of the mortar and calculated using the same method as the mortar.

[0082] Based on the above parameters, the parameters of the CDP model under different field variables can be easily calculated and input into the ABAQUS finite element software.

[0083] Step (2): Establish a concrete microscopic three-phase finite element model

[0084] This embodiment uses the mapping grid division method to establish the cement mortar, aggregate and ITZ three-phase model, as shown in Figure 4 .

[0085] The steps of the mapped meshing method are as follows: first, the model is divided into fixed-size grid cells, then the coordinate information of each cell and node is output, the geometric position of the aggregate is input into the three-dimensional uniform grid, and then the specific material phase of the concrete is determined based on the shape and position parameters of the cell nodes and aggregates. Finally, the material properties established in step (1) are assigned to the cells of each phase.

[0086] Step (3): Directly control the volume expansion and boundary conditions of microscopic materials based on the UEXPAN subroutine

[0087] Through the user subroutines UTEMP and USFLD provided by the finite element software ABAQUS, the temperature field and neutron flux field functions with time step and space coordinates as independent variables are written according to the actual situation to set the non-uniform temperature field and neutron flux field, and the UEXPAN subroutine is used to build in the aggregate volume expansion model and mortar volume expansion model based on temperature and neutron flux (respectively as shown in Figure 2). Figure 5 and Figure 6 As shown, Figure 5 The curves for coarse aggregate GA and fine aggregate Sand at two temperatures are present in the , achieving the purpose of directly controlling the volume expansion of the mesoscopic material over time and storing it as a field variable. This field variable is used by the solver to call the CDP model material parameters corresponding to the field variable defined in step (1) during the calculation process. Only the rigid body displacement of the concrete specimen is restricted, allowing it to expand freely.

[0088] Step (4): Submit analysis job and post-processing

[0089] correspond Figure 1 The concrete specimen volume in step 4 is free to expand. Since the aggregate volume expansion model and mortar volume expansion model based on temperature and neutron injection have been built into the UEXPAN subroutine, the temperature field and neutron injection field functions with time steps are input into the UTEMP and USFLD subroutines. After the calculation is submitted, the internal components of the concrete specimen will change in volume as the analysis step length increases, thereby generating changes in the stress-strain field and damage development, and ultimately achieving the simulation of the irradiation-induced macroscopic volume change of concrete. Create an analysis job in the software program and submit the analysis. After the finite element calculation is completed, post-processing is performed (extracting the specimen top surface displacement-time curve), and the height change curve of the concrete specimen during the irradiation process can be obtained, such as Figure 7 shown.

[0090] Step (5): Establish the restart analysis model for uniaxial compression test, submit the restart analysis job and post-processing

[0091] Set the restart position of the model to be solved, modify the boundary conditions, apply a downward displacement to the top surface of the specimen, and complete the finite element calculation of the uniaxial compression simulation to obtain the uniaxial compression stress-strain curve of the concrete specimen. Figure 8 The extracted stress-strain curve at the beginning of the calculation is shown.

[0092] By evenly selecting 8 moments in the total time step of the calculation process for restart analysis, 8 gradually oxidized stress-strain curves can be extracted. Based on these curves, the elastic modulus and compressive strength at different moments in the irradiation process can be further obtained, as shown in the following example: Figure 9 and Figure 10 , to achieve the purpose of predicting the degradation of concrete mechanical properties.

[0093] The restart model is a model that captures all the state data, including the stress and strain fields, at any selected analysis moment during the four-step calculation process. This model can be easily configured within the ABAQUS software. Post-processing involves extracting the specimen's load-displacement curve and converting it into a stress-strain curve.

[0094] For the convenience of description and easy understanding, the steps are distinguished and numbered above, but this does not limit the meaning. During specific implementation, the above steps can be performed simultaneously or in no particular order.

[0095] The ABAQUS-based prediction method for the degradation of concrete mechanical properties induced by neutron irradiation in this application can regard concrete as a composite material composed of three phases: cement mortar, aggregate, and interfacial transition zone (ITZ). Considering the time-varying degradation of the strain and mechanical properties of the three-phase material, the degradation law of concrete volume expansion, elastic properties, and uniaxial compressive strength under neutron irradiation is predicted. In the ABAQUS finite element software, based on the UTEMP and USFLD subroutines to set the non-uniform temperature field and neutron flux field, the aggregate volume expansion model and mortar volume expansion model based on temperature and neutron flux can be directly built in. Without the need to use methods such as temperature linear expansion coefficient equivalence, the purpose of controlling the volume expansion of microscopic materials over time can be achieved directly based on the UEXPAN subroutine. In addition, in the CDP model, the mechanical property parameters that change with field variables can achieve the purpose of degrading the mechanical properties of microscopic materials over time. The prediction method of this application does not involve complex equivalent conversions, but is directly set according to the expansion function, which is simpler and more direct. Moreover, it can be set based on two field variables, neutron flux and temperature, with clear concepts and clear calculations, and good practicality.

[0096] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A prediction method for mechanical properties of irradiated concrete based on ABAQUS, characterized in that: The steps include: Concrete is considered as a composite material consisting of three phases of mesoscopic materials: cement mortar, aggregate and interface transition zone; The CDP model based on field variables is used to consider the degradation law of the mechanical properties of microscopic materials as their volume changes; Establish a finite element model of concrete microscopic three-phase material in ABAQUS static implicit solver and assign material properties; The non-uniform temperature field and neutron flux field are set based on the UTEMP and USFLD subroutines, and the volume expansion of the mesoscopic material with time is controlled based on the UEXPAN subroutine to simulate the process of radiation-induced macroscopic volume change of concrete. Predicting the elastic properties of concrete and degradation of uniaxial compressive strength through restart analysis; The finite element model of concrete mesoscopic three-phase materials uses the mapped meshing method to establish a three-phase model of cement mortar, aggregate, and ITZ. The steps are as follows: First, the model is divided into fixed-size grid units, and then the coordinate information of each unit and node is output. The geometric position of the aggregate is input into the three-dimensional uniform grid. Then, based on the unit node and the shape and position parameters of the aggregate, each unit is determined to belong to a specific material phase in the concrete. The established material properties are assigned to the units of each phase. The non-uniform temperature field and neutron flux field are set based on the UTEMP and USFLD subroutines, and the volume expansion of the mesoscopic material with time is controlled based on the UEXPAN subroutine to simulate the process of radiation-induced macroscopic volume change of concrete. Specifically: The finite element software ABAQUS provides user subroutines UTEMP and USFLD to write temperature field and neutron flux field functions with time step and spatial coordinates as independent variables to set non-uniform temperature and neutron flux fields. The UEXPAN subroutine also includes built-in aggregate volume expansion models and mortar volume expansion models based on temperature and neutron flux, achieving the purpose of directly controlling the volume expansion of the mesoscopic material over time and storing it as field variables. This field variable is used to calculate the CDP model material parameters under the corresponding field variable defined before the solver is called; Since the aggregate volume expansion model and mortar volume expansion model based on temperature and neutron flux have been built into the UEXPAN subroutine, the temperature field and neutron flux field functions with time steps are input into the UTEMP and USFLD subroutines. After the calculation is submitted, the volume of each component inside the concrete specimen changes with the increase of the analysis step length, thereby generating changes in the stress-strain field and damage development, and ultimately realizing the simulation of the irradiation-induced macroscopic volume change of concrete.

2. The prediction method according to claim 1, characterized in that The CDP model based on field variables considers the degradation law of the mechanical properties of mesoscopic materials as their volume changes as follows: determining the initial mechanical property parameters of the concrete mesoscopic material and the degradation law of the mechanical property parameters that depend on the radiation-induced volume strain field variables, and defining the plastic damage constitutive model of each mesoscopic material that depends on the volume strain field variables.

3. The prediction method according to claim 2, characterized in that The steps to establish the compressive constitutive model of microscopic materials are as follows: The equation of the post-peak softening curve of cement mortar and interface transition zone is: Among them, σ c and ε are compressive stress and strain respectively, ε c is its intensity f c The corresponding compressive strain, α is the coefficient calculated by the following formula: For the compressive constitutive model of the mortar and interface transition zone, the elastic modulus E, peak stress σ c , peak strain, rising section parameter and descending section parameter; For the compressive constitutive model of aggregate, the compressive stress-strain relationship before the peak is set to linear elastic, and the strength decreases rapidly after the peak. The elastic modulus E, peak stress σ c There are 2 parameters in total.

4. The prediction method according to claim 2, characterized in that The steps to establish the tensile constitutive model of microscopic materials are as follows: The constitutive relation and damage evolution equation of the microscopic material in the softening stage are: Where σ tu and G f are the peak tensile stress and fracture energy of the material, respectively, which are material constants and can be measured by experiments; u ck is the cracking displacement; therefore, the stress value and damage variable are both the cracking displacement u ck function; For the tensile constitutive model of mortar and interface transition zone, the elastic modulus E, peak stress σ tu and G f Three parameters; for the tensile constitutive model of aggregate, the stress-strain relationship before the peak is set to linear elastic, and the strength after the peak decreases rapidly, requiring the elastic modulus E, peak stress σ tu and G f Three parameters.

5. The prediction method according to claim 1, wherein: The elastic modulus of aggregate changes with its volume as follows: After determining the degradation elastic modulus of the mortar under neutron flux and temperature, the compressive strength, tensile strength and peak compressive strain of the mortar are calculated according to the following formulas: f tm / f tm0 =0.996(f cm / f cm0 ) 1.2803 e cm / e cm0 =(f cm / f cm0 ) -0.83 Where, f cm is the compressive strength of the mortar (MPa), f tm is the compressive strength of the mortar (MPa), E cm is the elastic modulus of the mortar (GPa), ε cm is the peak compressive strain of the mortar.

6. The prediction method according to claim 5, characterized in that The fracture energy of mortar can be calculated according to the following formula: Where, f cm is the compressive strength of the mortar (MPa), G fm is the fracture energy of the mortar (N / mm), d max is the maximum fine aggregate particle size (mm).

7. The prediction method according to claim 5, characterized in that The mechanical properties of the interface transition zone are as follows: its elastic modulus and compressive strength are regarded as 0.9 times that of the mortar, and are calculated using the same method as that of the mortar.

8. The prediction method according to any one of claims 1 to 7, characterized in that: The restart analysis method for predicting the elastic properties of concrete and the degradation law of uniaxial compressive strength is as follows: establish a restart analysis model for uniaxial compression testing, submit the restart analysis job and post-processing; set the restart position of the model to be solved, modify the boundary conditions, apply a downward displacement to the top surface of the specimen, complete the finite element calculation of the uniaxial compression simulation, and obtain the uniaxial compression stress-strain curve of the concrete specimen. Based on this curve, the elastic modulus and compressive strength of the concrete at different times during the irradiation process can be calculated.

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