Fatigue analysis method for reactor pressure vessel in marine environment

Abstract

The invention discloses a reactor pressure vessel fatigue analysis method in a marine environment, which belongs to the technical field of pressure vessel analysis, and comprises the following steps: calculating a transfer function of a structure under the action of a wave load through a spectrum analysis method, and obtaining a stress component power spectrum density function of a node based on a sea condition parameter; converting the frequency domain signal into a time domain signal; calculating a stress time history under the action of the thermal-internal pressure load and carrying out superposition of stress component time histories to obtain a fatigue stress value of the structure; and calculating the fatigue damage value of each stress cycle, obtaining the fatigue damage of each group of time history, and obtaining the total fatigue damage under the working condition. According to the method, the defects of the existing fatigue analysis method can be overcome, and the fatigue damage of the reactor pressure vessel of the nuclear power station under the combined action of multiple loads in heat, internal pressure and marine environment can be accurately evaluated.

Description

Technical Field

[0001] The invention relates to the technical field of pressure vessel analysis, and in particular to a fatigue analysis method for a reactor pressure vessel in a marine environment. Background Art

[0002] The reactor pressure vessel is an important component of the primary pressure boundary of a pressurized water reactor nuclear power plant. As a nuclear safety level 1 device, it is responsible for sealing radioactive materials and preventing the escape of fission products. Its service life directly determines the overall life of the nuclear power plant. Previous studies have shown that fatigue is one of the main mechanisms of reactor pressure vessel material aging. Usually, the most susceptible parts to fatigue are geometric discontinuities such as the main bolts and the water inlet and outlet pipe sections. Since the main bolts can be replaced during service, the water inlet and outlet pipe sections become the key focus of fatigue analysis.

[0003] At present, fatigue analysis methods are mainly concentrated in reactor pressure vessels of land-based nuclear power plants and marine structures, and there is no special method for reactor pressure vessels of floating nuclear power plants. The existing fatigue analysis methods mainly include: simplified time domain analysis method, time domain analysis method and frequency domain analysis method.

[0004] Although the simplified time domain analysis method is suitable for the thermal-internal pressure loads of nuclear power plants, it cannot evaluate the high-frequency fatigue damage caused by waves, which is particularly insufficient in the floating nuclear power plant environment.

[0005] The frequency domain analysis method is highly efficient in evaluating high-frequency fatigue damage caused by waves, but it cannot handle ultra-low-cycle alternating stresses caused by heat and internal pressure, which makes it very limited in the comprehensive evaluation of fatigue damage of reactor pressure vessels of floating nuclear power plants.

[0006] Although the traditional time domain analysis method has advantages in calculation accuracy, it has a huge amount of calculation when faced with the complex marine environment conditions of floating nuclear power plants, making it difficult to apply it to evaluate the fatigue life of floating nuclear power plant reactor pressure vessels in actual engineering.

[0007] In view of this, it is necessary to provide a new technical solution to solve the above problems. Summary of the invention

[0008] In order to solve the above technical problems, the present application provides a fatigue analysis method for a reactor pressure vessel in a marine environment, which can overcome the defects of existing fatigue analysis methods and accurately evaluate the fatigue damage of a nuclear power plant reactor pressure vessel under the combined effects of heat, internal pressure and marine environment loads.

[0009] A method for fatigue analysis of a reactor pressure vessel in a marine environment, comprising: The transfer function of the structure under wave load is calculated by spectral analysis method, and the power spectrum density function of the stress component of the node is obtained based on the sea state parameters; Convert frequency domain signals into time domain signals; Calculate the stress time history under thermal-internal pressure load and superimpose the stress component time history to obtain the fatigue stress value of the structure; The fatigue damage value of each stress cycle is calculated, the fatigue damage of each group of time history is obtained, and the total fatigue damage under this working condition is obtained.

[0010] Preferably, the calculation of the transfer function of the structure under the wave load by a spectral analysis method comprises: The hull motion amplitude at different wave directions at the center of gravity of the reactor pressure vessel structure under the action of regular waves is obtained through hydrodynamic analysis, and is calculated as the hull acceleration amplitude of each degree of freedom of the hull under different wave directions through the derivative relationship of trigonometric functions. The reactor pressure vessel structure is taken for calculation. The bottom of the reactor pressure vessel support is fixed and constrained. The reactor pressure vessel has six degrees of freedom acceleration at each frequency. The synthesized acceleration amplitude is calculated based on the phase corresponding to each acceleration amplitude and the maximum amplitude. The global acceleration is applied to perform finite element analysis. The fatigue stress amplitude of the corresponding frequency is calculated through two orthogonal phase stress values ​​at the same frequency to obtain the transfer function.

[0011] Preferably, among the hull acceleration amplitudes of each degree of freedom of the hull under different waves, there are six degrees of freedom of the hull, including surge, sway, heave, pitch, roll and bow pitch.

[0012] Preferably, in the calculation of the transfer function of the structure under the wave load by the spectral analysis method, the expression of the wave spectrum is: ; in, ; In the formula, is the circular frequency; is the acceleration due to gravity; is the peak frequency; is the peak enhancement factor; is the empirical coefficient related to the peak frequency of the wind speed and wave spectrum; is the peak shape parameter, when hour, ,when hour, .

[0013] Preferably, the expression of the stress component power spectral density function is: ; In the formula, represents the power spectral density function of stress components; is the transfer function from wave height to stress; is the wave spectral density function; is the frequency; For the wave direction; For the righteous wave high; is the spectrum peak period.

[0014] Preferably, converting the frequency domain signal into a time domain signal comprises: Extract the amplitude of each frequency from the stress component power spectral density function; Using the Monte Carlo method to randomly generate several groups of initial phases; Combining the generated groups of initial phases with the extracted amplitudes to form groups of different stress component frequency domain signals; The inverse Fourier transform is applied to each set of generated frequency domain signals to convert them into time domain signals, and several sets of different stress time histories are obtained.

[0015] Preferably, when the generated several groups of initial phases are combined with the extracted amplitudes to form several groups of different stress component frequency domain signals, the amplitude of each group of frequency domain signals is the same, but the phases are randomly different to simulate the phase combinations that may occur under actual working conditions.

[0016] Preferably, the calculating of the stress time history under the action of the thermal-internal pressure load and superposition of the stress component time history to obtain the fatigue stress value of the structure includes: Based on the historical monitoring data of the thermal-internal pressure of the reactor pressure vessel, the inflection points where each load changes significantly are determined; The structural stress at the inflection point of load change is obtained through finite element analysis; The stress time history at other time points is obtained by interpolation; The time history of the stress component caused by the wave load is superimposed with the time history of the stress component caused by the thermal-internal pressure load; The fatigue stress value of the structure is calculated according to the stress component at each moment through the stress transformation formula.

[0017] Preferably, the stress intensity is used as the fatigue stress of the reactor pressure vessel, and its calculation formula is: ; in, represent the first principal stress, the second principal stress, and the third principal stress respectively; and ; The first principal stress, the second principal stress, and the third principal stress are calculated using the following formula: ; In the formula, represents the principal stress; , , Respectively represent the normal stresses in the x, y, and z directions in the global coordinate system; , , They represent the shear stresses on the xy, xz, and yz planes in the global coordinate system, respectively.

[0018] Preferably, the calculation of the fatigue damage value of each stress cycle, obtaining the fatigue damage of each group of time history, and obtaining the total fatigue damage under the working condition includes: Rainflow counting is performed on each of the generated groups of stress time histories to extract the amplitude of each stress cycle and its corresponding number of times; According to the fatigue life curve of the structural material, the fatigue damage value of each stress cycle is calculated to obtain the fatigue damage of each group of time history; The fatigue damage values ​​of the stress-time histories in all groups are averaged to obtain the total fatigue damage under the working condition.

[0019] Compared with the prior art, this application has at least the following beneficial effects: 1. The present invention overcomes the defects of existing fatigue analysis methods by combining time domain and frequency domain analysis methods, and can accurately evaluate the fatigue damage of nuclear power plant reactor pressure vessels under the combined effects of multiple loads such as heat, internal pressure and marine environment.

[0020] 2. The present invention also significantly improves the calculation efficiency and is particularly suitable for fatigue assessment of reactor pressure vessels of floating nuclear power plants under complex working conditions in marine environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Hereinafter, some specific embodiments of the present invention will be described in detail in an exemplary and non-limiting manner with reference to the accompanying drawings. The same reference numerals in the accompanying drawings indicate the same or similar components or parts. It should be understood by those skilled in the art that these drawings are not necessarily drawn to scale. In the accompanying drawings: Figure 1 It is a schematic diagram of the overall process of the present invention; Figure 2 This is a schematic diagram of the location of fatigue check points for the reactor pressure vessel; Figure 3Figure 2 is the acceleration amplitude diagram of the hull at the center of gravity of the structure; (a) is the acceleration amplitude diagram of the hull under the action of sway, surge and heave at a wave direction of 0 degrees; (b) is the acceleration amplitude diagram of the hull under the action of pitch, roll and bow at a wave direction of 0 degrees; (c) is the acceleration amplitude diagram of the hull under the action of sway, surge and heave at a wave direction of 45 degrees; (d) is the hull under the action of pitch, roll and bow at a wave direction of 45 degrees Acceleration amplitude diagram; (e) is the acceleration amplitude diagram of the hull under the action of surge, sway and heave at a wave direction of 90 degrees; (f) is the acceleration amplitude diagram of the hull under the action of pitch, roll and bow at a wave direction of 90 degrees; (g) is the acceleration amplitude diagram of the hull under the action of surge, sway and heave at a wave direction of 135 degrees; (h) is the acceleration amplitude diagram of the hull under the action of pitch, roll and bow at a wave direction of 135 degrees; Figure 4 It is the transfer function diagram of the stress intensity from the hull inertia force to the fatigue check point; among them, (a) is the transfer function diagram of the wave direction of 0 degrees; (b) is the transfer function diagram of the wave direction of 45 degrees; (c) is the transfer function diagram of the wave direction of 90 degrees; (d) is the transfer function diagram of the wave direction of 135 degrees; Figure 5 The power spectrum density diagram of stress intensity at the structural fatigue check point under typical sea conditions; (a) is the power spectrum density diagram of the wave direction of 0 degrees; (b) is the power spectrum density diagram of the wave direction of 45 degrees; (c) is the power spectrum density diagram of the wave direction of 90 degrees; (d) is the power spectrum density diagram of the wave direction of 135 degrees. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solution and advantages of the present application clearer, the technical solution of the present application will be clearly and completely described below in combination with the specific embodiments of the present application and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.

[0023] like Figure 1 As shown, a fatigue analysis method for a reactor pressure vessel in a marine environment comprises: S1. The transfer function of the structure under wave load is calculated by spectral analysis method, and the power spectrum density function of the stress component of the node is obtained based on the sea state parameters.

[0024] The transfer function describes the response characteristics of the structural nodes under wave loads, the relationship from the input load (such as waves) to the structural stress, and the power spectral density function reflects the distribution characteristics of the stress components in the frequency domain.

[0025] S2. Convert the frequency domain signal into a time domain signal.

[0026] Specifically, when converting the frequency domain signal into the time domain signal, the amplitude of each frequency is extracted from the power spectrum density function of the stress component. Since the power spectrum density function itself does not contain phase information, in order to reconstruct the original signal, a random phase needs to be assigned to each frequency.

[0027] The Monte Carlo method is used to randomly generate several groups of initial phases to form several groups of different stress component frequency domain signals. The amplitude of each group of frequency domain signals is the same, but the phases are randomly different to simulate the possible phase combinations under actual working conditions.

[0028] The inverse Fourier transform is applied to each set of generated frequency domain signals to convert them into time domain signals, thereby obtaining several sets of different stress time histories. These time histories are statistically consistent with the target power spectrum density and can effectively simulate the stress changes under wave loads.

[0029] S3. Calculate the stress time history under the action of thermal-internal pressure load and superimpose the stress component time history to obtain the fatigue stress value of the structure.

[0030] Based on the historical monitoring data of the thermal-internal pressure of the reactor pressure vessel, the inflection points where each load changes significantly are determined. Since the change of thermal-internal pressure is very slow, it is only necessary to obtain the structural stress at the inflection point of the load change through finite element analysis, and then obtain the stress time history at the remaining time points through interpolation.

[0031] The time history of the stress component caused by wave loads and the time history of the stress component caused by thermal-internal pressure loads are superimposed to consider the combined effect of multiple loads.

[0032] According to the six stress components including three normal stresses and three shear stresses at each moment, the fatigue stress value of the structure is calculated by the stress transformation formula.

[0033] S4. Calculate the fatigue damage value of each stress cycle, obtain the fatigue damage of each group of time history, and obtain the total fatigue damage under this working condition.

[0034] Specifically, rainflow counting is performed on the generated sets of stress time histories to extract the amplitude of each stress cycle and its corresponding number of times. Rainflow counting is a commonly used method in fatigue analysis, which can effectively identify stress cycles and thus quantify the fatigue effect of each cycle.

[0035] According to the fatigue life curve (SN curve) of the structural material, the fatigue damage value of each stress cycle is calculated to obtain the fatigue damage of each time history. The SN curve describes the relationship between different stress amplitudes and fatigue life and is used to predict the fatigue life under specific stress.

[0036] The fatigue damage values ​​of several groups of stress-time histories are averaged to obtain the total fatigue damage under the working condition, ensuring the reliability and accuracy of the evaluation results in a statistical sense. Multiple groups of initial phases are generated through Monte Carlo simulation, effectively reducing the impact of phase uncertainty on fatigue damage evaluation results.

[0037] As a specific embodiment of the present invention, a fatigue analysis method for a reactor pressure vessel in a marine environment is as follows: In a floating nuclear power plant, the reactor pressure vessel is not only affected by internal pressure and thermal loads, but also by ocean wave loads.

[0038] The transfer function of the structure under wave load is calculated by spectral analysis. The spectral analysis method only needs to calculate the transfer function of the structure and combine it with the wave spectrum under different sea conditions to obtain the stress spectrum of the structure and then calculate the fatigue damage.

[0039] Taking the JONSWAP wave spectrum as an example, its wave spectrum expression is: ; in, ; In the formula, is the circular frequency; is the acceleration due to gravity; is the peak frequency; is the peak enhancement factor; usually takes a value of 3.3; is the empirical coefficient related to the peak frequency of the wind speed and wave spectrum; is the peak shape parameter, when hour, ,when hour, .

[0040] Empirical coefficients related to wind speed and peak frequency of wave spectrum , which is related to the wind speed and the peak frequency of the wave spectrum, can be estimated by the significant wave height and the peak period of the spectrum. The calculation formula is as follows: ; In the formula, For the righteous wave high; is the spectrum peak period.

[0041] The stress spectrum at the structural node is obtained by multiplying the square of the hull function by the wave spectral density function. The specific formula is: ; In the formula, represents the power spectral density function of stress components; is the transfer function from wave height to stress; is the wave spectral density function; is the frequency; For the wave direction; For the righteous wave high; is the spectrum peak period.

[0042] Since the influence of the hull inertia force on the fatigue stress of the reactor pressure vessel is much greater than that of the wave pressure, in the fatigue analysis of the reactor pressure vessel, the fatigue influence of the wave pressure can be ignored, but the fatigue damage caused by the motion inertia force cannot be ignored. Figure 2 It should be noted that the attached Figure 2 The figure marks 1, 2, 3, 4, and 5 appearing in the figure respectively represent fatigue check point position 1 of the floating nuclear power plant, fatigue check point position 2 of the floating nuclear power plant, fatigue check point position 3 of the floating nuclear power plant, fatigue check point position 4 of the floating nuclear power plant, and fatigue check point position 5 of the floating nuclear power plant.

[0043] The calculation of the fatigue stress transfer function of the reactor pressure vessel under the action of the hull inertia force is as follows: (1) The hull motion amplitude at the center of gravity of the reactor pressure vessel structure under the action of regular waves is obtained through hydrodynamic analysis, and the hull acceleration amplitude is calculated through the derivative relationship of trigonometric functions, including swell X, sway Y, heave Z, pitch RX, roll RY, and bow roll RZ, as follows: Figure 3 The amplitude of the regular wave is 1, the wave direction interval is 15°, the frequency is between 0.001 Hz and 0.4 Hz, and 50 frequency points are taken at equal intervals.

[0044] from Figure 3 From the spectrum of the acceleration amplitude of the hull under different waves shown in the figure, it can be seen that rolling, pitching and heaving motions can cause the hull to produce a larger acceleration amplitude.

[0045] (2) Simplify the structural model and only take the reactor pressure vessel structure for calculation. The bottom of the reactor pressure vessel support is fixed and constrained. The reactor pressure vessel has six degrees of freedom acceleration at each frequency. The synthesized acceleration amplitude is calculated based on the phase corresponding to each acceleration amplitude and each maximum amplitude. The global acceleration is applied and finite element analysis is performed.

[0046] (3) By using two orthogonal phase stress values ​​at the same frequency, the fatigue stress amplitude at the corresponding frequency is calculated to obtain the transfer function, as shown in Figure 4 shown.

[0047] pass Figure 4 The transfer functions of the reactor pressure vessel nodes under different wave directions and frequencies shown in Figure 2 provide a more intuitive understanding of the impact of waves on the structure.

[0048] The typical sea condition is a short-term sea condition with a significant wave height of 5m and a spectrum peak period of 9s, and it is assumed that the occurrence time of different wave directions in this short-term sea condition is the same within ten days. The wave spectrum uses the JONSWAP wave spectrum, and the power spectrum density is calculated as follows: Figure 5 shown.

[0049] The frequency domain amplitude is calculated from the power spectral density of each stress component, and a random phase is assigned to each frequency component to form a frequency domain signal. This process is repeated 1000 times to obtain 1000 different frequency domain signals for each stress component.

[0050] These frequency domain signals are converted into time domain signals using inverse Fourier transform, resulting in 1000 different time histories of stress components.

[0051] Based on the historical monitoring data of the thermal-internal pressure of the reactor pressure vessel, the inflection points where each load changes significantly are determined. Since the change of thermal-internal pressure is very slow, it is only necessary to obtain the structural stress at the inflection point of the load change through finite element analysis, and then obtain the stress time history at the remaining time points through interpolation.

[0052] The time history of the stress component caused by wave loads and the time history of the stress component caused by thermal-internal pressure loads are superimposed to consider the combined effect of multiple loads.

[0053] The fatigue stress value is calculated based on the six stress components at each moment. In the ASME and RCC-M specifications, stress intensity (SI) is used as the fatigue stress of the reactor pressure vessel, and its calculation formula is as follows: ; In the formula, represents the principal stress; , , Respectively represent the normal stresses in the x, y, and z directions in the global coordinate system; , , They represent the shear stresses on the xy, xz, and yz planes in the global coordinate system, respectively.

[0054] Principal stress Three values ​​can be solved and arranged in order of magnitude: ; In the formula, They represent the first principal stress, the second principal stress, and the third principal stress respectively.

[0055] The calculation formula for the fatigue stress of the reactor pressure vessel is: ; Among them, stress intensity is taken as the fatigue stress of the reactor pressure vessel, and SI in the formula is stress intensity.

[0056] The fatigue stress time history was statistically analyzed using the rain flow counting method to extract the stress cycle amplitude and the corresponding number of occurrences, and the fatigue damage value of each stress cycle was calculated based on the S-N curve. The fatigue damage values ​​of 1000 groups of stress time histories were averaged to obtain the total fatigue damage under this working condition.

[0057] Compared with the technical solution of the present invention, the existing analysis methods each have their own applicable fields and limitations, and they cannot realize fatigue analysis of reactor pressure vessels in marine environments, as follows: In land-based nuclear power plants, fatigue analysis of reactor pressure vessels usually adopts simplified time domain analysis methods. This type of method is recommended by the French RCCM code and the American ASME code. By performing stress analysis on the transient combination of heat and internal pressure under reactor operating conditions, the fatigue cumulative use factor of the structure is calculated. With the extension of the service life of nuclear power plants in service and the emergence of the third generation of nuclear power plants, the fatigue assessment of reactor pressure vessels faces higher requirements.

[0058] Existing studies have shown that low-cycle thermomechanical fatigue is one of the key factors affecting the life of reactor pressure vessels. Fekete et al. studied the behavior of the VVER-440 reactor pressure vessel under low-cycle thermomechanical fatigue conditions and verified the accuracy of the commonly used fatigue life prediction model. These studies have important reference value for the life assessment and life extension of reactor pressure vessels.

[0059] In order to improve the reliability of fatigue analysis, Niu et al. proposed a fatigue reliability design method based on stress-strength and load-life conversion models. Compared with the traditional safety factor design, this method can optimize the performance and cost of the reactor pressure vessel while ensuring safety, especially providing new design ideas in terms of material properties and load dispersion.

[0060] Muscat et al. studied the creep-fatigue behavior of reactor pressure vessels under high temperature conditions and found that the combined effects of long-term high temperature and cyclic loads significantly reduced the fatigue life of the equipment, which provided a theoretical basis for the design improvement of reactor pressure vessels.

[0061] Although the above methods have achieved good results in land-based nuclear power plants, their applicability is limited in complex marine environments. Although the simplified time domain analysis method has high accuracy in dealing with thermal and internal pressure loads, it is computationally intensive and inefficient when dealing with the complex working conditions of floating nuclear power plants.

[0062] In addition, the working environment of floating nuclear power plants in the marine environment is more complicated. Its reactor pressure vessel is not only subjected to thermal and internal pressure loads, but also affected by waves in the marine environment. The high cyclic stress process caused by these waves makes it impossible to effectively evaluate fatigue damage using the simplified time domain analysis method used in land-based nuclear power plants.

[0063] In the field of marine engineering, fatigue assessment of hull structures usually uses time domain analysis or frequency domain analysis. The time domain analysis method is recognized as the most accurate fatigue analysis method, but because the marine environment contains thousands of different fatigue conditions, the nonlinear time domain analysis process is very time-consuming, so it is usually only used at the end of the design or verification stage, or to verify the accuracy of the frequency domain method.

[0064] In order to improve the efficiency of fatigue analysis, the frequency domain analysis method has been widely used. This method calculates the stress amplitude power spectrum density function of the fatigue check point through spectral analysis, and then calculates the fatigue damage based on the known stress range distribution. Frequency domain analysis is suitable for hull structures because its stress response time history is difficult to obtain directly, and the frequency domain method can effectively calculate through spectrum data.

[0065] However, the stress of the reactor pressure vessel caused by thermal and internal pressure loads is an ultra-low cycle alternation, and stress spectrum analysis is not applicable to this ultra-low cycle stress process. Therefore, the existing frequency domain analysis method shows limitations in the fatigue assessment of the reactor pressure vessel of the floating nuclear power plant, and cannot accurately assess the fatigue damage under this stress process.

[0066] This scheme combines the advantages of time domain and frequency domain, combines time domain and frequency domain conversion, and superimposes stress components, comprehensively considering the combined effects of temperature, internal pressure and wave loads to obtain accurate fatigue stress and calculate fatigue damage of the structure. Compared with the traditional time domain analysis method, this method not only improves the accuracy of fatigue assessment, but also significantly improves the calculation efficiency. It is particularly suitable for fatigue assessment of reactor pressure vessels of floating nuclear power plants under complex working conditions in marine environments.

[0067] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0068] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for fatigue analysis of reactor pressure vessels in marine environment, characterized in that: include: The transfer function of the structure under wave load is calculated by spectral analysis method, and the power spectrum density function of the stress component of the node is obtained based on the sea state parameters; Convert frequency domain signals into time domain signals; Calculate the stress time history under thermal-internal pressure load and superimpose the stress component time history to obtain the fatigue stress value of the structure; The fatigue damage value of each stress cycle is calculated, the fatigue damage of each group of time history is obtained, and the total fatigue damage under the corresponding working conditions is obtained.

2. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 1, characterized in that: The method of calculating the transfer function of the structure under the wave load by using the spectral analysis method comprises: The hull motion amplitude at different wave directions at the center of gravity of the reactor pressure vessel structure under the action of regular waves is obtained through hydrodynamic analysis, and is calculated as the hull acceleration amplitude of each degree of freedom of the hull under different wave directions through the derivative relationship of trigonometric functions. The reactor pressure vessel structure is taken for calculation. The bottom of the reactor pressure vessel support is fixed and constrained. The reactor pressure vessel has six degrees of freedom acceleration at each frequency. The synthesized acceleration amplitude is calculated based on the phase corresponding to each acceleration amplitude and the maximum amplitude. The global acceleration is applied to perform finite element analysis. The fatigue stress amplitude of the corresponding frequency is calculated through two orthogonal phase stress values ​​at the same frequency to obtain the transfer function.

3. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 2, characterized in that: Among the hull acceleration amplitudes of each degree of freedom of the hull under different waves, the hull has six degrees of freedom, including surge, sway, heave, pitch, roll and bow pitch.

4. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 3, characterized in that: In the calculation of the transfer function of the structure under the wave load by the spectrum analysis method, the expression of the wave spectrum is: ； in, ; In the formula, is the circular frequency; is the acceleration due to gravity; is the peak frequency; is the peak enhancement factor; is the empirical coefficient related to the peak frequency of the wind speed and wave spectrum; is the peak shape parameter, when hour, ,when hour, .

5. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 4, characterized in that: The expression of the stress component power spectrum density function is: ； In the formula, represents the power spectral density function of stress components; is the transfer function from wave height to stress; is the wave spectral density function; is the frequency; For the wave direction; For the righteous wave high; is the spectrum peak period.

6. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 5, characterized in that: The converting the frequency domain signal into the time domain signal comprises: Extract the amplitude of each frequency from the stress component power spectral density function; Using the Monte Carlo method to randomly generate several groups of initial phases; Combining the generated groups of initial phases with the extracted amplitudes to form groups of different stress component frequency domain signals; The inverse Fourier transform is applied to each set of generated frequency domain signals to convert them into time domain signals, and several sets of different stress time histories are obtained.

7. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 6, characterized in that: When the generated several groups of initial phases are combined with the extracted amplitudes to form several groups of different stress component frequency domain signals, the amplitudes of each group of frequency domain signals are the same, but the phases are randomly different to simulate the phase combinations that occur under actual working conditions.

8. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 7, characterized in that: The method of calculating the stress time history under the action of thermal-internal pressure load and superimposing the stress component time history to obtain the fatigue stress value of the structure includes: Based on the historical monitoring data of the thermal-internal pressure of the reactor pressure vessel, the inflection points where each load changes significantly are determined; The structural stress at the inflection point of load change is obtained through finite element analysis; The stress time history at other time points is obtained by interpolation; The time history of the stress component caused by the wave load is superimposed with the time history of the stress component caused by the thermal-internal pressure load; The fatigue stress value of the structure is calculated according to the stress component at each moment through the stress transformation formula.

9. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 8, characterized in that: The stress intensity is used as the fatigue stress of the reactor pressure vessel, and its calculation formula is as follows: ； in, represent the first principal stress, the second principal stress, and the third principal stress respectively; and ; The first principal stress, the second principal stress, and the third principal stress are calculated using the following formula: ； In the formula, represents the principal stress; , , Respectively represent the normal stresses in the x, y, and z directions in the global coordinate system; , , They represent the shear stresses on the xy, xz, and yz planes in the global coordinate system, respectively.

10. The method for fatigue analysis of a reactor pressure vessel in a marine environment according to claim 9, characterized in that: The fatigue damage value of each stress cycle is calculated to obtain the fatigue damage of each time history and the total fatigue damage under the working condition, including: Rainflow counting is performed on each of the generated groups of stress time histories to extract the amplitude of each stress cycle and its corresponding number of times; According to the fatigue life curve of the structural material, the fatigue damage value of each stress cycle is calculated to obtain the fatigue damage of each group of time history; The fatigue damage values ​​of the stress-time histories in all groups are averaged to obtain the total fatigue damage under the working condition.

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