A method and device for numerical analysis of thermal-hydraulic parameters of a nitrogen-stabilized reactor

Abstract

The application discloses a kind of nitrogen pressure regulating reactor thermal-hydraulic numerical analysis method and equipment, through the joint analysis method of one-dimensional system level thermal-hydraulic analysis program and equipment internal three-dimensional structure level CFD program, nitrogen dissolution, migration, precipitation, accumulation and the like in nitrogen pressure regulating reactor Phenomenon and the thermal-hydraulic effect caused thereby are predicted throughout the process, and further analyze its influence on loop flow heat transfer characteristics, the working performance of key equipment.The application can realize multi-dimensional and multi-scale calculation and analysis, without consuming a large amount of computing resources, to predict the dissolution, migration, precipitation and accumulation behavior of nitrogen in the coolant and the thermal-hydraulic effect caused thereby, support the review of nitrogen pressure regulating reactor design scheme and the development of operation strategy.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of thermal-hydraulic analysis, and relates to a thermal-hydraulic numerical analysis method and equipment for a nitrogen pressurized reactor. BACKGROUND

[0002] Nitrogen is used as the working fluid in the gas space of the pressurizer in the primary loop of the nitrogen pressurized reactor. Although the solubility of nitrogen in the coolant is very low (~10 ppm) at normal temperature and pressure, the solubility of nitrogen in water can reach 5000 ppm under the working conditions of the reactor steady state operation (~15 MPa, 300 DEG C). Therefore, the nitrogen in the gas space of the pressurizer will continue to dissolve into the coolant through the gas-liquid interface until saturation. When the dissolved nitrogen concentration in the coolant is high, the nitrogen solubility will decrease greatly when the reactor has a large amplitude transient or shutdown, so that the dissolved nitrogen in the coolant will be supersaturated and precipitate. After the bubbles formed by the precipitation form two-phase flow, on the one hand, the resistance characteristics of the flow in the system will be affected, and on the other hand, the gas may accumulate in some areas of the key equipment, affecting the working performance and service life of the equipment. To accurately predict and analyze the above effects, the dissolution, migration, precipitation and accumulation of nitrogen in the primary loop need to be calculated.

[0003] However, the current thermal-hydraulic programs and related numerical analysis methods mostly focus on the steam pressurized reactor, and do not consider the above-mentioned dissolution, migration, precipitation and accumulation of nitrogen in the primary loop and the thermal-hydraulic effects caused thereby. SUMMARY

[0004] Since the prior art has the above-mentioned defects, the present application establishes a thermal-hydraulic numerical analysis method for a nitrogen pressurized reactor to predict the dissolution, migration, precipitation and accumulation of nitrogen in the coolant and the thermal-hydraulic effects caused thereby, support the checking of the design scheme of the nitrogen pressurized reactor and the development of the operation strategy, and ensure the safe operation of the nitrogen pressurized reactor.

[0005] To achieve the above-mentioned purpose, in a first aspect, the present application provides a thermal-hydraulic numerical analysis method for a nitrogen pressurized reactor, comprising the following steps:

[0006] S1, determining the geometric structure and parameters of the reactor system loop, establishing a one-dimensional simplified model of the system loop; determining the geometric structure and parameters of the key equipment, obtaining the internal three-dimensional fluid domain according to the geometric structure modeling; giving the initial conditions and boundary conditions of the system, and developing the system operation strategy;

[0007] S2, dividing the one-dimensional simplified model and the three-dimensional fluid domain into one-dimensional and three-dimensional calculation grids, and carrying out grid sensitivity analysis to balance the calculation accuracy and the consumption of calculation resources, and obtaining the grid division parameters;

[0008] S3, input the given initial conditions, boundary conditions and operation strategy, solve a one-dimensional thermal hydraulic program on a one-dimensional calculation grid to obtain the temperature, pressure, flow rate, gas holdup and dissolved nitrogen concentration distribution in the system loop; according to the location of the key equipment in the system loop, pass the local parameters obtained by one-dimensional calculation to the three-dimensional CFD (Computational Fluid Dynamic) program as initial boundary value conditions and solve the program; wherein, based on the temperature and pressure at each place in the system loop at the current time step, the local nitrogen solubility is calculated in real time, and whether precipitation will occur after supersaturation is judged in combination with the local dissolved nitrogen concentration; if precipitation does not occur, the equation solving of the next time step is advanced; if precipitation occurs, based on the interphase mass transfer submodel, the precipitation mass source term caused by gas-liquid interphase mass transfer is calculated, and the diameter growth of the current precipitation bubble is solved; according to the local flow field information, the critical shedding size of the bubble is solved, compared with the bubble size obtained by solving, whether the wall bubble will shed is judged, if shedding occurs, the wall bubble size is reset to the initial nucleus size, and the next bubble growth cycle begins;

[0009] S4, extract the solving results of the one-dimensional thermal hydraulic program and the three-dimensional CFD program, evaluate the current system loop design scheme and operation strategy and output the evaluation results.

[0010] The above technical scheme realizes multi-dimensional and multi-scale calculation and analysis by considering the dissolution, migration, precipitation and accumulation behaviors of nitrogen on the one-dimensional system level and the three-dimensional structure level of the equipment, and inputs the parameters obtained by one-dimensional calculation into three-dimensional calculation, so that the whole process prediction of the dissolution, migration, precipitation and accumulation behaviors of nitrogen in the coolant in the nitrogen stabilized pressure reactor, the influence of the behaviors on the flow and heat transfer characteristics of the primary loop and the working performance of the key equipment is realized without consuming a large amount of computing resources.

[0011] Further, in the step S3, the one-dimensional thermal hydraulic program solves the flow field, temperature field and dissolved nitrogen concentration field information on the system loop level, and the three-dimensional CFD program solves the fine flow field, temperature field and dissolved nitrogen concentration field distribution information inside the key equipment.

[0012] Further, in the step S3, the one-dimensional thermal hydraulic program solving method is similar to the three-dimensional CFD program solving method, the difference lies in the dimension difference, the time point of nitrogen precipitation in the system loop solved by the one-dimensional thermal hydraulic program and the amount of precipitated gas as the input of the three-dimensional CFD program, so as to improve the accuracy of the three-dimensional calculation results.

[0013] Further, the one-dimensional thermal hydraulic program solving method comprises the following steps:

[0014] S31. Set boundary conditions and initialize the initial values ​​at each node on the one-dimensional computational grid;

[0015] S32. Determine if the current time step is greater than the solution end time; if yes, end the solution; if no, continue with subsequent calculations.

[0016] S33. Discretely solve the mass, momentum, and energy equations, as well as the component transport equations of dissolved nitrogen, under a two-fluid model based on lumped parameters. The source terms in each equation are obtained by selecting an appropriate model.

[0017] S34. Based on the temperature at various points in the system loop at the current time step calculated in step S32. ,pressure The local nitrogen solubility is calculated in real time, and the local dissolved nitrogen concentration is used to determine whether supersaturation will occur and precipitation will result; the criterion for judgment is the local supersaturation. Greater than the critical supersaturation :

[0018]

[0019] In the formula, This refers to the local dissolved nitrogen concentration. The local nitrogen solubility, This refers to the surface tension coefficient under the corresponding temperature and pressure. The diameter of the bubble. This is the liquid phase pressure (system pressure) at that moment; if no precipitation occurs, the equations are solved forward to the next time step; if precipitation occurs, the precipitation mass source term caused by gas-liquid interphase mass transfer is calculated based on the interphase proton transfer model, and the diameter growth of the currently precipitated bubble is solved; whereby the bubble diameter growth is approximately described by the free growth process of bubbles in an infinitely supersaturated fluid:

[0020]

[0021] In the formula, Where is the bubble radius. The diffusion coefficient of a substance in a liquid phase. For time, The local nitrogen solubility, Let be the gas phase density; the first term on the right-hand side of the equation is the solution for the steady-state diffusion process, and the second term represents the transient change in the concentration boundary layer at the gas-liquid interface during bubble growth; the mass source term caused by gas-liquid interphase mass transfer is obtained by correspondingly increasing the bubble volume.

[0022] S35. Based on the local flow field information, calculate the critical bubble detachment size and compare it with the bubble size obtained in step S34 to determine whether the wall bubble will detach. If detachment occurs, reset the wall bubble size to the initial gas core size and start the next bubble growth cycle. For cases with low liquid flow velocity, the bubble detachment model considers the buoyancy force on the bubble through force analysis. Surface tension on the three-phase contact line The equilibrium is used to obtain the critical bubble escape diameter:

[0023]

[0024]

[0025]

[0026] In the formula These are the advancing contact angle and the receding contact angle of the gas-liquid-solid three-phase contact line, respectively. The diameter of the three-phase contact wire. The surface tension coefficient, The density of the liquid phase is... The density is the gas phase density. It is the gravitational constant. Let be the bubble volume; however, for cases with high liquid flow velocity and strong shear near the wall, the critical bubble shear diameter is calculated using the following formula:

[0027]

[0028] in, It is the dimensionless diameter of the detached bubble. It is the Peckley number. It is the shear rate within the flow boundary layer. For local supersaturation, The gas constant is... Absolute temperature It is the Strauhall number. It is the characteristic length of the cavitation. It is the frequency of bubble detachment. The mainstream velocity is the velocity; the size of the detached bubble is positively correlated with the mainstream velocity and the shear rate near the wall.

[0029] S36. Recalculate the local dissolved nitrogen concentration based on the previous steps. With gas content And then proceed to solve the equations for the next time step.

[0030] Further, in step S33, the equations to be solved include the two-phase mass conservation equation, energy conservation equation, momentum conservation equation, group equilibrium equation, and component transport equation of dissolved nitrogen; according to the operating range, bubble size grouping is set, and the source terms in each equation are calculated by selecting, but not limited to, two-phase flow model, turbulence model, wall function, interphase interaction force model, interphase mass transfer model, bubble wall precipitation and shedding model, and bubble coalescence and breakup sub-model.

[0031] Furthermore, in step S33, the component transport equation for dissolved nitrogen considers convection, diffusion, and source terms. The convection term is calculated using local flow field information, and the diffusion term is obtained based on the local temperature gradient, dissolved nitrogen concentration gradient, and nitrogen diffusion coefficient.

[0032] Furthermore, the two-phase flow model is the Euler-Euler two-fluid model, and the bubble coalescence and breakup sub-model is the Liao model.

[0033] Furthermore, the bubble size grouping ranges from 10 micrometers to 5 millimeters.

[0034] Further, in step S34, the local nitrogen solubility is solved using a nitrogen solubility model established based on data fitting using existing table interpolation or a thermodynamic equilibrium method. The data fitting method, based on an existing nitrogen solubility database, has the advantage of requiring only simple interpolation calculations and being fast. However, its accuracy, precision, and applicability are limited by the database. The thermodynamic equilibrium method, on the other hand, is not limited by the database and has a wider range of applications, but it requires real-time calculation of the thermodynamic equations, resulting in a large computational load.

[0035] On the other hand, the present invention provides a nitrogen-pressurized reactor thermal-hydraulic numerical analysis device, including a memory storing executable program code; a processor coupled to the memory; the processor calls the executable program code stored in the memory to execute the nitrogen-pressurized reactor thermal-hydraulic numerical analysis method as described above.

[0036] Compared with the prior art, the above invention has the following advantages or beneficial effects:

[0037] (1) The nitrogen-stabilized reactor thermal-hydraulic numerical analysis method described in this invention can realize the whole process prediction of nitrogen dissolution, migration, precipitation and accumulation, and further analyze its impact on loop flow heat transfer characteristics and key equipment performance.

[0038] (2) The three-dimensional CFD solution program of the nitrogen-stabilized reactor thermal-hydraulic numerical analysis method described in this invention considers the influence of mass transfer and coalescence breakup on bubble size, and calculates the migration and aggregation behavior of bubbles of different sizes more accurately by solving the group equilibrium model, so as to more accurately evaluate the impact of two-phase flow on the working performance of key equipment.

[0039] (3) The numerical analysis method of nitrogen-stabilized reactor thermal-hydraulic system described in this invention realizes multi-dimensional and multi-scale analysis and calculation through one-dimensional thermal-hydraulic solution part and three-dimensional CFD solution part, which well balances the calculation accuracy and computing power consumption. Attached Figure Description

[0040] The invention, its features and advantages will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings.

[0041] Figure 1 This is a flowchart of the numerical analysis method for the thermal-hydraulic system of a nitrogen-stabilized reactor in this invention;

[0042] Figure 2 This is a flowchart of the one-dimensional thermal hydraulic program solution part in the numerical analysis method for nitrogen-pressurized reactor thermal hydraulics of the present invention. Detailed Implementation

[0043] The structure of the present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the present invention.

[0044] In the following detailed description, numerous specific details are set forth to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that well-known algorithms and models are not shown in detail to avoid obscuring the gist of the invention; and that the techniques not detailed in the following effect examples are readily available prior art.

[0045] Example

[0046] Please see Figure 1 This embodiment provides a numerical analysis method for the thermal-hydraulic dynamics of a nitrogen-stabilized reactor, including the following steps:

[0047] S1. Determine the geometry and parameters of the reactor system loop, and establish a one-dimensional simplified model of the system loop; determine the geometry and parameters of key equipment, and obtain its internal three-dimensional fluid domain based on the geometric model; given the initial and boundary conditions of the system, formulate the system operation strategy. The key equipment here refers to the equipment of practical interest, such as the reactor system's main pumps, exhaust pumps, primary side of the steam generator, primary side of the heat exchanger, core sub-channels, ion exchangers, etc.

[0048] S2. Based on the one-dimensional simplified model of the system loop and the three-dimensional fluid domain inside the key equipment, one-dimensional and three-dimensional computational grids are divided, and grid sensitivity analysis is carried out to balance computational accuracy and computational resource consumption to obtain appropriate grid division parameters.

[0049] S3. Using the given initial conditions, boundary conditions, and operating strategy as input, a one-dimensional thermal-hydraulic program is solved on a one-dimensional computational grid to obtain the temperature, pressure, flow velocity, gas content, and dissolved nitrogen concentration distribution within the system loop. Based on the location of the key equipment in the system loop, the local parameters obtained from the one-dimensional calculation are passed as initial and boundary conditions to the three-dimensional CFD program for solution. More specifically, the one-dimensional thermal-hydraulic program solves for the flow field, temperature field, and dissolved nitrogen concentration field information at the system loop level, while the three-dimensional CFD program solves for the detailed flow field, temperature field, and dissolved nitrogen concentration field distribution information inside the key equipment. Specifically, based on the temperature and pressure at various points in the system loop at the current time step, the local nitrogen solubility is calculated in real time, and the local dissolved nitrogen concentration is used to determine whether supersaturation will occur and precipitation will result. If precipitation does not occur, the equations for the next time step are solved. If precipitation occurs, the precipitation mass source term caused by gas-liquid phase-to-phase mass transfer is calculated based on the interphase proton transfer model, and the diameter growth of the current precipitated bubble is solved. Based on the local flow field information, the critical bubble detachment size is solved, and compared with the solved bubble size, it is determined whether the wall bubble will detach. If detachment occurs, the wall bubble size is reset to the initial gas core size, and the next bubble growth cycle begins.

[0050] As an example, see Figure 2 The one-dimensional thermal-hydraulic program solution method includes the following steps:

[0051] S31. Set boundary conditions and initialize the initial values ​​at each node on the one-dimensional computational grid.

[0052] S32. Determine if the current time step is greater than the solution end time; if yes, end the solution; if no, continue with subsequent calculations.

[0053] S33. Discretely solve the mass, momentum, and energy equations, as well as the component transport equations of dissolved nitrogen, under a two-fluid model based on lumped parameters. The source terms in each equation are obtained by selecting an appropriate model.

[0054] S34. Based on the temperature at various points in the system loop at the current time step calculated in step S32. ,pressure The local nitrogen solubility is calculated in real time, and the local dissolved nitrogen concentration is used to determine whether supersaturation will occur and precipitation will result; the criterion for judgment is the local supersaturation. Greater than the critical supersaturation :

[0055]

[0056] In the formula, This refers to the local dissolved nitrogen concentration. The local nitrogen solubility, This refers to the surface tension coefficient under the corresponding temperature and pressure. The diameter of the bubble. This is the liquid phase pressure (system pressure) at that moment; if no precipitation occurs, the equations are solved forward to the next time step. The local nitrogen solubility can be solved using a nitrogen solubility model established based on data fitting from existing tables or thermodynamic equilibrium methods. If precipitation occurs, the precipitation mass source term caused by gas-liquid mass transfer is calculated based on the interphase proton transfer model, and the diameter growth of the current precipitated bubble is solved; the bubble diameter growth can be approximated by the free growth process of bubbles in an infinitely supersaturated fluid.

[0057]

[0058] In the formula, Where is the bubble radius. The diffusion coefficient of a substance in a liquid phase. For time, The local nitrogen solubility, Let be the gas phase density; the first term on the right side of the equation is the solution for the steady-state diffusion process, and the second term represents the transient change in the concentration boundary layer at the gas-liquid interface during bubble growth; the mass source term caused by gas-liquid mass transfer is obtained by corresponding to the increase in bubble volume.

[0059] S35. Based on the local flow field information, calculate the critical bubble detachment size and compare it with the bubble size obtained in step S34 to determine whether the wall bubble will detach. If detachment occurs, reset the wall bubble size to the initial gas core size and start the next bubble growth cycle. For cases with low liquid flow rates, the bubble detachment model considers the buoyancy force on the bubble through force analysis. Surface tension on the three-phase contact line The equilibrium is used to obtain the critical bubble escape diameter:

[0060]

[0061]

[0062]

[0063] In the formula These are the advancing contact angle and the receding contact angle of the gas-liquid-solid three-phase contact line, respectively. The diameter of the three-phase contact wire. The surface tension coefficient, The density of the liquid phase is... For gas phase density, It is the gravitational constant. Let be the bubble volume; however, for cases with high liquid flow velocity and strong shear near the wall, the critical bubble shear diameter is calculated using the following formula:

[0064]

[0065] in, It is the dimensionless diameter of the detached bubble. It is the Peckley number. It is the shear rate within the flow boundary layer. For local supersaturation, The gas constant is Absolute temperature It is the Strauhall number. It is the characteristic length of the cavitation. It is the frequency of bubble detachment. The mainstream velocity is the velocity that escapes the bubble; the size of the bubble is positively correlated with the mainstream velocity and the shear rate near the wall.

[0066] S36. Recalculate the local dissolved nitrogen concentration based on the previous steps. With gas content And then proceed to solve the equations for the next time step.

[0067] In this embodiment, the diameter growth of the precipitated bubbles is obtained by solving the RP equation. The critical bubble detachment size is related to the flow shear near the wall and is calculated using the shear rate. This scheme is derived from the analysis of actual physical processes, has practical physical significance, and is highly operable.

[0068] The solution method for three-dimensional CFD programs is similar to that for one-dimensional thermal-hydraulic programs. The difference lies in the dimensionality, the timing of nitrogen evolution in the system loop obtained from the one-dimensional thermal-hydraulic program, and the amount of evolved gas, which are used as inputs for the three-dimensional CFD program.

[0069] S4. Extract the solution results of the one-dimensional thermal hydraulic program and the three-dimensional CFD program, evaluate the current system loop design scheme and operation strategy, and output the evaluation results.

[0070] In this embodiment, the component transport equations for dissolved nitrogen mainly consider convection, diffusion, and source terms. The convection term is calculated using local flow field information, while the diffusion term is obtained based on the local temperature gradient, dissolved nitrogen concentration gradient, and nitrogen diffusion coefficient. In this embodiment, the 3D CFD program uses FLUENT or CFX software, with some sub-models embedded by user-defined functions. The equations solved by the 3D CFD program include the two-phase mass conservation equation, energy conservation equation, momentum conservation equation, group equilibrium equation, and dissolved nitrogen component transport equation. Appropriate bubble size groupings are set according to the operating conditions, and suitable two-phase flow models, turbulence models, wall functions, interphase interaction force models, interphase mass transfer models, and bubble coalescence / breakdown sub-models are selected. In the reactor system loop, the bubble size detaching from the wall into the main flow is on the order of tens of micrometers, while bubbles larger than 5 mm are difficult to maintain stable existence for long periods under reactor flow conditions. In this embodiment, the bubble size grouping ranges from 10 micrometers to 5 millimeters to ensure coverage of the vast majority of precipitated bubble sizes in the reactor system.

[0071] In this embodiment, the Euler-Euler two-fluid model is used for the two-phase flow model, and the Liao model is used for the bubble coalescence and breakup sub-model. Compared to the Euler-Lagrange model, the Euler-Euler model consumes fewer computational resources when there are many bubbles. Compared to the Mixture model, the Euler-Euler model solves the conservation equations for the gas and liquid phases separately, resulting in higher accuracy. The Liao model considers a more comprehensive bubble coalescence and breakup mechanism, and according to the literature, its accuracy in predicting bubble size distribution is higher than other models in many scenarios.

[0072] Compared with existing reactor thermal-hydraulic analysis methods, this invention considers phenomena such as nitrogen dissolution, migration, precipitation, and accumulation, as well as the resulting thermal-hydraulic effects, from a one-dimensional simplified model at the system level to a three-dimensional fluid model at the equipment level. This provides a more effective numerical analysis method for predicting the dissolution, migration, precipitation, and accumulation behavior of nitrogen in the coolant in nitrogen-pressurized reactors and analyzing its impact on the primary circuit thermal-hydraulic system.

[0073] The aforementioned numerical analysis method for the thermal-hydraulic structure of a nitrogen-pressurized reactor can be executed using a numerical analysis device for the thermal-hydraulic structure of a nitrogen-pressurized reactor. The device includes a memory storing executable program code; a processor coupled to the memory; the processor calls the executable program code stored in the memory to execute the aforementioned numerical analysis method for the thermal-hydraulic structure of a nitrogen-pressurized reactor.

[0074] Optionally, the memory may include, but is not limited to, high-speed random access memory and non-volatile memory. For example, one or more disk storage devices, flash memory devices, or other non-volatile solid-state storage devices; the processor may include, but is not limited to, a central processing unit (CPU), a graphics processing unit (GPU), a network processor (NP), etc.; it may also be a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other programmable logic devices, discrete gate or transistor logic devices, or discrete hardware components.

[0075] In summary, this invention provides a numerical analysis method and apparatus for the thermal-hydraulic properties of nitrogen-pressurized reactors. Through a combined analysis method using a one-dimensional system-level thermal-hydraulic analysis program and a three-dimensional CFD program covering the internal structure of the equipment, it predicts the entire process of nitrogen dissolution, migration, precipitation, and accumulation in nitrogen-pressurized reactors and the resulting thermal-hydraulic effects. Furthermore, it analyzes the impact on loop flow and heat transfer characteristics and the performance of key equipment. This invention enables multi-dimensional and multi-scale computational analysis, predicting the dissolution, migration, precipitation, and accumulation behavior of nitrogen in the coolant and the resulting thermal-hydraulic effects without consuming excessive computing resources. This supports the verification of nitrogen-pressurized reactor design schemes and the formulation of operational strategies.

[0076] Those skilled in the art should understand that variations can be implemented by combining existing technology with the above embodiments, which will not be elaborated here. Such variations do not affect the essence of the present invention, and will not be elaborated here either.

[0077] The preferred embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and the devices and structures not described in detail should be understood as being implemented in a conventional manner in the art. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the scope of the present invention. This does not affect the essential content of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention's technical solutions still fall within the protection scope of the present invention.

Claims

1. A numerical analysis method for the thermal-hydraulic dynamics of a nitrogen-pressurized reactor, characterized in that: Includes the following steps: S1. Determine the geometry and parameters of the reactor system loop and establish a one-dimensional simplified model of the system loop; determine the geometry and parameters of key equipment and obtain its internal three-dimensional fluid domain based on the geometry model. Given the initial and boundary conditions of the system, formulate the system operation strategy; S2. Divide the one-dimensional simplified model and the three-dimensional fluid domain into one-dimensional and three-dimensional computational grids, and conduct grid sensitivity analysis to balance computational accuracy and computational resource consumption, and obtain grid partitioning parameters. S3. Using the given initial conditions, boundary conditions, and operating strategy as input, solve the one-dimensional thermal-hydraulic program on a one-dimensional computational grid to obtain the temperature, pressure, flow velocity, gas content, and dissolved nitrogen concentration distribution within the system loop; the one-dimensional thermal-hydraulic program solution method includes the following steps: S31. Set boundary conditions and initialize the initial values ​​at each node on the one-dimensional computational grid; S32. Determine if the current time step is greater than the solution end time; if yes, end the solution; if no, continue with subsequent calculations. S33. Discretely solve the mass, momentum, and energy equations, as well as the component transport equations of dissolved nitrogen, under a two-fluid model based on lumped parameters. The source terms in each equation are obtained by selecting an appropriate model. The equations to be solved include the mass conservation equation, energy conservation equation, momentum conservation equation, group equilibrium equation, and component transport equation of dissolved nitrogen for the two phases. According to the operating range, bubble size grouping is set, and the source terms in each equation are calculated by selecting models including two-phase flow model, turbulence model, wall function, interphase interaction force model, interphase mass transfer model, bubble wall precipitation and shedding model, and bubble coalescence and breakup model. S34. Based on the temperature and pressure at various points in the system loop at the current time step calculated in step S32, calculate the local nitrogen solubility in real time, and determine whether precipitation will occur after supersaturation is reached by combining the local dissolved nitrogen concentration. S35. Based on the local flow field information, calculate the critical bubble detachment size and compare it with the bubble size obtained in step S34 to determine whether the wall bubble will detach. If it detaches, reset the wall bubble size to the initial gas core size and start the next bubble growth cycle. S36. Recalculate the local dissolved nitrogen concentration and gas content based on the previous steps, and advance the equation solution to the next time step. Based on the location of the key equipment in the system loop, the local parameters obtained from one-dimensional calculation are passed as initial boundary conditions to the three-dimensional CFD program for solution. Specifically, based on the temperature and pressure at various points in the system loop at the current time step, the local nitrogen solubility is calculated in real time. Combined with the local dissolved nitrogen concentration, it is determined whether supersaturation will occur and precipitation will result. If precipitation does not occur, the equations for the next time step are solved. If precipitation occurs, based on the interphase proton transfer model, the precipitation mass source term caused by gas-liquid interphase mass transfer is calculated, and the diameter growth of the current precipitated bubble is solved. Based on the local flow field information, the critical bubble detachment size is solved. Comparing this with the solved bubble size, it is determined whether the wall bubble will detach. If detachment occurs, the wall bubble size is reset to the initial gas core size, and the next bubble growth cycle begins. S4. Extract the solution results of the one-dimensional thermal hydraulic program and the three-dimensional CFD program, evaluate the current system loop design scheme and operation strategy, and output the evaluation results.

2. The numerical analysis method for the thermal-hydraulic dynamics of a nitrogen-stabilized reactor according to claim 1, characterized in that, In step S3, the one-dimensional thermal-hydraulic program solves for the flow field, temperature field, and dissolved nitrogen concentration field information at the system loop level, while the three-dimensional CFD program solves for the detailed flow field, temperature field, and dissolved nitrogen concentration field distribution information inside the key equipment.

3. The numerical analysis method for the thermal-hydraulic dynamics of a nitrogen-pressurized reactor according to claim 1 or 2, characterized in that, In step S3, the one-dimensional thermal-hydraulic program solution method is similar to the three-dimensional CFD program solution method, the difference being the dimensionality difference, the time point of nitrogen evolution in the system loop obtained by the one-dimensional thermal-hydraulic program solution, and the amount of evolved gas as input to the three-dimensional CFD program.

4. The numerical analysis method for the thermal-hydraulic system of a nitrogen-pressurized reactor according to claim 3, characterized in that, In step S34, the temperature at each point in the system loop at the current time step is calculated based on the temperature obtained in step S32. ,pressure The local nitrogen solubility is calculated in real time, and the local dissolved nitrogen concentration is used to determine whether supersaturation will occur and precipitation will result; the criterion for judgment is the local supersaturation. greater than critical supersaturation : ， In the formula, This refers to the local dissolved nitrogen concentration. The local nitrogen solubility, This refers to the surface tension coefficient under the corresponding temperature and pressure. The diameter of the bubble. It is the liquid phase pressure at that moment, i.e., the system pressure; if no precipitation occurs, then proceed to the next time step to solve the equations. If precipitation occurs, the precipitation mass source term caused by gas-liquid interphase mass transfer is calculated based on the interphase proton transfer model, and the diameter growth of the current precipitated bubble is solved; whereby the bubble diameter growth is approximately described by the free growth process of bubbles in an infinitely supersaturated fluid: ， In the formula, Where is the bubble radius. The diffusion coefficient of a substance in a liquid phase. For time, The local nitrogen solubility, Let be the gas phase density; the first term on the right-hand side of the equation is the solution for the steady-state diffusion process, and the second term represents the transient change in the concentration boundary layer at the gas-liquid interface during bubble growth; the mass source term caused by gas-liquid interphase mass transfer is obtained by correspondingly increasing the bubble volume. In step S35, for cases with low liquid flow rates, the bubble detachment model considers the buoyancy force acting on the bubbles through force analysis. Surface tension on the three-phase contact line The equilibrium is used to obtain the critical bubble escape diameter: ， ， ， In the formula These are the advancing contact angle and the receding contact angle of the gas-liquid-solid three-phase contact line, respectively. The diameter of the three-phase contact wire. The surface tension coefficient, The density of the liquid phase is... For gas phase density, It is the gravitational constant. Let be the bubble volume; however, for cases with high liquid flow velocity and strong shear near the wall, the critical bubble shear diameter is calculated using the following formula: ， in, It is the dimensionless diameter of the detached bubble. It is the Peckley number. It is the shear rate within the flow boundary layer. For local supersaturation, The gas constant is... Absolute temperature It is the Strauhall number. It is the characteristic length of the cavitation. It is the frequency of bubble detachment. The mainstream velocity is the velocity; the size of the detached bubble is positively correlated with the mainstream velocity and the shear rate near the wall.

5. The numerical analysis method for the thermal-hydraulic dynamics of a nitrogen-stabilized reactor according to claim 4, characterized in that, In step S33, the component transport equation of dissolved nitrogen considers convection, diffusion and source terms. The convection term is calculated using local flow field information, and the diffusion term is obtained based on the local temperature gradient, dissolved nitrogen concentration gradient and nitrogen diffusion coefficient.

6. The numerical analysis method for the thermal-hydraulic dynamics of a nitrogen-pressurized reactor according to claim 4, characterized in that, The two-phase flow model is the Euler-Euler two-fluid model, and the bubble coalescence and breakup sub-model is the Liao model.

7. The numerical analysis method for the thermal-hydraulic dynamics of a nitrogen-pressurized reactor according to claim 4, characterized in that, Bubble size groups range from 10 micrometers to 5 millimeters.

8. The numerical analysis method for the thermal-hydraulic system of a nitrogen-pressurized reactor according to claim 4, characterized in that, In step S34, the local nitrogen solubility is solved by a nitrogen solubility model established based on data fitting of existing table interpolation or thermodynamic equilibrium methods.

9. A numerical analysis device for the thermal-hydraulic system of a nitrogen-pressurized reactor, characterized in that, It includes a memory storing executable program code; a processor coupled to the memory; the processor calls the executable program code stored in the memory to execute the numerical analysis method for thermal-hydraulic reactors according to any one of claims 1 to 8.

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