A method and system for calculating performance degradation of a pressurized water reactor loop main pump under a small break loss of coolant accident condition

Abstract

This invention discloses a method and system for calculating the performance degradation of the main pump in the primary loop of a pressurized water reactor (PWR) under small-break loss-of-coolant (LOC) accident conditions. The method includes constructing a three-dimensional fluid computation model of the PWR primary loop system. By setting the resistance characteristics of an equivalent resistance model, the total system pressure drop under rated operating conditions is self-consistently matched with the rated head of the main pump, forming an accurate initial stable flow field. A gravity field is introduced and static initialization is completed. The main pump is smoothly started to its rated state using a preset speed control function. At a preset accident triggering time, the accident is triggered at the preset break calculation domain using a preset break opening function. Throughout the accident simulation, the main pump is kept running. By monitoring and calculating parameters such as the pump's head, torque, and internal gas content, the continuous performance degradation process is directly obtained, i.e., the different states of the main pump under different gas contents. This invention achieves a realistic coupling between the system response and the internal flow of the main pump under accident conditions.

Description

Technical Field

[0001] This invention relates to the field of nuclear power plant reactor safety analysis and main pump hydraulic performance calculation technology, specifically to a method and system for calculating the performance degradation process of reactor-loop main pumps based on three-dimensional gas-liquid two-phase flow numerical simulation under small breach loss-of-coolant accident conditions. Background Technology

[0002] The reactor main pump is a key power equipment in the pressurized water reactor nuclear power plant's loop system, and its operating status directly affects the core cooling capacity and reactor safety margin. Under normal operating conditions, the flow inside the main pump is mainly single-phase high-temperature and high-pressure liquid water, and its hydraulic performance parameters such as flow rate, head, torque, shaft power, and efficiency can be accurately obtained through steady-state tests or design calculations.

[0003] However, under accident conditions such as small-break loss-of-coolant incidents, the pressure in the primary loop system drops rapidly. The pressurized liquid is released at the break point, accompanied by a flash evaporation process, and a gas-liquid two-phase flow state gradually forms inside the system. As the accident evolves, cavitation and gas accumulation are prone to occur in the main pump inlet area, causing significant changes in the density, gas content, and flow structure of the working fluid entering the main pump, resulting in a significant degradation of the hydraulic performance of the main pump, such as head and torque.

[0004] In existing methods for primary loop system accident analysis, the two-phase performance of the main pump is typically described using empirical degradation models in a one-dimensional system program. These methods rely on pre-defined degradation curves or degradation multipliers, making it difficult to reflect the coupling relationship between the evolution of the internal flow structure and abrupt performance changes within the main pump during an accident. On the other hand, while simple three-dimensional numerical simulations can describe the complex gas-liquid two-phase flow behavior within the main pump, their calculation results are highly sensitive to boundary conditions. Under steady-state system conditions, constant flow rate or pressure can be used as boundary conditions at the main pump inlet and outlet. However, during a small-break loss-of-coolant accident, due to the combined effects of system leakage, phase change, and flow remodeling, the pressure and flow rate at the main pump inlet and outlet exhibit strong transient changes, making it difficult to accurately define them beforehand. If pre-defined or simplified boundary conditions are still used for three-dimensional calculations, it will be difficult to truly reflect the coupling response between the main pump and the primary loop system under accident conditions.

[0005] Therefore, it is necessary to develop a simulation method that can realistically reflect the dynamic coupling between the main pump and the primary loop system in a self-consistent full three-dimensional computational model, so as to achieve accurate assessment of the performance evolution of the main pump under accident conditions.

[0006] In summary, it is necessary to propose a calculation method and system that can directly solve the main pump performance degradation under small-break loss-of-coolant accident conditions based on a full three-dimensional system model, thereby providing a more reasonable and engineering-feasible technical means for reactor safety analysis. Summary of the Invention

[0007] The purpose of this invention is to provide a calculation method for the performance degradation of the primary loop main pump in a pressurized water reactor under small-break loss-of-coolant (SWD) accidents. This method addresses the problem that existing methods struggle to accurately reflect the dynamic coupling between the main pump and the primary loop system under SWD conditions, leading to insufficient accuracy in predicting the two-phase performance degradation of the main pump. This invention overcomes the lack of a mechanism in one-dimensional empirical degradation models and the distortion of boundary conditions in isolated three-dimensional simulations. By constructing a self-consistent full three-dimensional calculation model, it achieves a high-confidence direct solution for the main pump performance degradation process.

[0008] To achieve the above objectives, this invention provides a numerical simulation method for the performance degradation of the main pump in the primary loop of a pressurized water reactor (PWR) under small-break loss-of-coolant accident conditions. The method includes: constructing a fully three-dimensional closed-loop fluid model of the PWR primary loop system. This model consists of a closed flow loop formed by directly connected multiple three-dimensional fluid computational domains. The closed flow loop includes at least a three-dimensional fluid model of the main pump, a simplified three-dimensional equivalent resistance model of the pressure vessel and steam generator, and a preset break computational domain. By setting the resistance characteristics of the equivalent resistance model, the total system pressure drop under rated operating conditions is self-consistently matched with the rated head of the main pump, forming an accurate initial stable flow field. Based on this, a gravity field is introduced and static initialization is completed. The main pump is then smoothly started to its rated state using a preset speed control function. After the system flow stabilizes, at a preset accident triggering time, an accident is triggered at the preset break computational domain using a preset break opening function. Throughout the accident simulation, the main pump was kept running. By monitoring and calculating parameters such as the pump's head, torque, and gas content, the continuous degradation process of its performance was directly obtained, i.e., the different states of the main pump under different gas contents.

[0009] Preferably, both the speed control function and the break-in opening function are piecewise functions containing a smooth transition segment to achieve continuous simulation of the physical process and avoid numerical abrupt changes. Simultaneously, during the main pump startup phase, a linear or nonlinear transition time step function strategy can be employed to improve computational stability.

[0010] The other party also provides a calculation system for implementing the calculation method for the performance degradation of the main pump in the primary loop of a pressurized water reactor under the condition of a small breach loss-of-coolant accident, as described above, including: a model building module for building a three-dimensional fluid calculation model of the primary loop of the pressurized water reactor; The initialization module is used to perform system initialization and static pressure distribution settings; The main pump control module is used to execute the smooth start-up process of the main pump; The accident triggering module is used to enable the breach boundary conditions; The performance analysis module is used to calculate the main pump head, torque, and gas content, and output the main pump performance degradation results. The modules work together under the control of the processor to calculate the performance degradation of the main pump under the condition of a small breach and loss of water.

[0011] The present invention solves the problem and has the following advantages: (1) By constructing a closed one-loop full three-dimensional closed flow loop model, the boundary conditions at the main pump inlet are naturally formed by the transient response of the system, avoiding the distortion problem caused by artificially specifying boundary conditions in the isolated three-dimensional main pump simulation. This can truly reflect the coupling effect between the system dynamics and the internal flow of the main pump under a small break water loss accident.

[0012] (2). The present invention obtains the performance parameters of the main pump by directly solving the three-dimensional two-phase flow behavior inside the main pump during the accident process, without relying on the preset empirical degradation curve or degradation multiplier, which helps to reveal the intrinsic physical relationship between the main pump head, torque degradation and changes in the internal gas phase distribution flow structure.

[0013] (3) By introducing a pre-defined small-break computational domain and functionalized rotational speed, time step control and accident triggering strategy, the numerical simulation process is made closer to the actual physical reality of the accident, significantly improving the computational stability and convergence efficiency. The obtained main pump performance degradation process data can provide a more reliable engineering basis for assessing the core cooling capacity and system safety margin under accident conditions.

[0014] (4) This invention, through full three-dimensional self-consistent system simulation, can not only directly obtain the performance degradation data of the main pump, but also completely output the dynamic boundary parameters (pressure, flow rate, gas content, and other characteristic parameters) of the interface where the equivalent resistance model of the pressure vessel and steam generator is located. These parameters are the results of the actual coupled response of the system under accident transients, overcoming the fundamental defect of boundary condition distortion in traditional isolated component analysis. As a result, a high-fidelity boundary condition database can be formed, providing reliable input for subsequent higher-precision local safety analysis of the reactor core, fuel assemblies, etc., making it possible to analyze the influence transmission chain of high-level PIRT (Phenomena Identification and Ranking Table) phenomena such as two-phase degradation of the main pump.

[0015] Note: Phenomena Identification and Ranking Table is a core and very "official" methodological tool in the fields of nuclear engineering and safety analysis. Attached Figure Description

[0016] Figure 1 This is a flowchart of the steps of the present invention.

[0017] Figure 2 This is a schematic diagram of the overall structure of the three-dimensional calculation model of the present invention.

[0018] Figure 3 This is a schematic diagram of the equivalent structure of the break and the local computational domain of the present invention.

[0019] Figure 4 The diagram shows the steady-state establishment process before the accident; (a) is a schematic diagram of the main pump impeller speed change, and (b) is a schematic diagram of the convergence of various physical quantities of the external characteristics of the primary pump.

[0020] Figure 5 This is a schematic diagram of the function for opening the breach.

[0021] Figure 6 The curve shows the total pressure difference between the main pump outlet and inlet as a function of the main pump's gas content. x The gas content corresponding to the maximum degraded head of the main pump. This represents the total pressure difference between the inlet and outlet of the main pump when the liquid water is in single-phase state. The total pressure difference between the inlet and outlet of the main pump in two-phase operation. This represents the total pressure difference between the inlet and outlet of the main pump when the water is in single-phase gaseous state.

[0022] Figure 7 This is the curve showing the change in main pump torque as a function of the main pump's gas content. Among them, The gas content corresponding to the maximum degrading torque of the main pump. For single-phase liquid water, the main pump torque is... For two-phase operation, the main pump torque is... This refers to the main pump torque when the water is in single-phase gaseous state.

[0023] Figure 8 The curves show the changes in head reduction multiplier and torque reduction multiplier as a function of the gas content of the main pump. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Note: Unless otherwise specified, the experimental methods in the following examples are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0026] SBLOCA is an abbreviation for Small-Break Loss-of-Coolant Accident. It refers to an accident condition where a small-diameter rupture occurs at the primary circuit boundary, causing a slow but continuous release of coolant.

[0027] Local loss: The local loss parameter is used to characterize the additional resistance generated when the fluid changes its flow direction, undergoes abrupt changes in the flow cross section, or experiences flow state disturbances. Its magnitude is described by an equivalent local resistance coefficient.

[0028] Friction loss along the flow path: The friction loss parameter is used to characterize the frictional resistance generated by the fluid during the flow process in the equivalent flow channel. Its magnitude is related to the equivalent flow channel length, the equivalent hydraulic diameter, and the friction resistance coefficient.

[0029] The method provided by this invention involves constructing a three-dimensional closed-loop flow model that is self-consistently coupled with the hydraulic characteristics of the main pump and the primary loop system. This allows the inlet boundary conditions (pressure, flow rate, and gas content) of the main pump to be dynamically determined by the actual transient response of the system, rather than by pre-setting parameters, in small break loss-of-water accident (SBLOCA) simulations. To achieve engineering-feasible calculations while ensuring the core physical mechanisms (system hydraulic response and complex two-phase flow inside the main pump), this invention employs a simplified model based on the system's thermal inertia for its thermodynamic description.

[0030] Example 1 like Figure 1 As shown, a method for calculating the performance degradation of the primary loop main pump of a pressurized water reactor under a small breach loss-of-coolant accident includes: S101. Construct a three-dimensional fluid computational model of the pressurized water reactor primary loop system: The three-dimensional fluid computation model is a closed flow loop composed of multiple directly connected three-dimensional fluid computation domains. Each three-dimensional fluid computation domain includes at least a main pump, cold section piping, pressure vessel, hot section piping, steam generator, transition section piping, and reactor core, as well as pre-defined breach computation domains located on the cold section piping and transition section piping. A schematic diagram of the overall structure of the three-dimensional computation model of this invention is shown below. Figure 2 As shown.

[0031] The main pump uses a three-dimensional fluid model that includes the impeller, guide vanes and condensate chamber, while the pressure vessel (including the reactor core) and steam generator use a simplified three-dimensional equivalent resistance model.

[0032] Note: The main pump (including impeller, guide vanes, and condensate chamber), cold section piping, hot section piping, and transition section piping are actual models (i.e., the computational domain water body, not the structure), and are therefore three-dimensional fluid models. The pressure vessel (including the reactor core) and steam generator are both three-dimensional equivalent resistance models.

[0033] By setting the resistance characteristics of the three-dimensional equivalent resistance model, the total system pressure drop under rated operating conditions is matched with the main pump head in the closed flow loop formed by the direct connection of the three-dimensional fluid computation domain, thereby providing an initial flow field basis that conforms to rated operating conditions for accident simulation.

[0034] The three-dimensional equivalent resistance model is used to characterize the flow resistance characteristics of the pressure vessel, reactor core, and steam generator under rated operating conditions. These flow resistance characteristics are described by equivalent local loss parameters and friction loss parameters. These local loss parameters and friction loss parameters are preset or calibrated based on system design parameters or steady-state operating data under rated operating conditions.

[0035] The preset breach calculation domain is initially closed and is used to apply breach boundary conditions during the accident simulation phase. The equivalent breach structure and local calculation domain of the present invention are illustrated below. Figure 3 As shown.

[0036] S102. System Initialization and Steady-State Establishment: After constructing and setting the parameters of the three-dimensional equivalent drag model, a complete closed-loop calculation model is formed. Subsequently, the closed-loop calculation model is initialized, including the introduction of a gravity field and the setting of the initial pressure field and initial temperature of the system.

[0037] The system initialization process also includes setting the system's rated operating temperature. This rated operating temperature is determined based on the thermal parameters of the pressurized water reactor primary loop (a simplified pressurized water reactor primary loop includes the main pump, cold section piping, pressure vessel, hot section piping, steam generator, and transition section piping. The main pump includes impellers, guide vanes, and a pressurized water chamber; the pressure vessel includes the reactor core) under normal full-power operating conditions, and serves as the unified initial thermodynamic state input parameter for the entire three-dimensional fluid calculation model. The rated operating temperature is used to determine the coolant's density, saturation pressure, specific enthalpy, and dynamic viscosity using the IAPWS-IF97 industrial formula, and participates in subsequent pressure distribution calculations and two-phase mixing density calculations. By setting the rated operating temperature, thermodynamic consistency is ensured during the initialization phase, and a unified baseline state is provided for calculating the gas content evolution and main pump performance degradation after an accident.

[0038] Before the calculation begins, a gravitational acceleration field is activated in the closed-loop calculation model, and an initial pressure field is set for the entire computational domain based on the principles of hydrostatics to simulate the pressure distribution of the primary loop system under the weight of the coolant in a static state. Considering that gravity is a key factor affecting gas phase migration and distribution during an accident, a correct initial static pressure field is an important foundation for subsequent transient simulations.

[0039] In one specific implementation, the system's rated operating temperature for the entire loop coolant is set to be constant. The thermophysical properties of the coolant (including density, viscosity, enthalpy, and saturation pressure) are calculated according to the International Association for the Properties of Water and Steam (IAPWS-IF97) industrial formula to obtain a property description that matches the actual operating conditions.

[0040] Based on the above assumptions, the initial pressure field assigns pressure values ​​at each location within the computational domain using the following relationship: in, To calculate the absolute pressure at a specific location within the domain; The reference pressure for the system can be selected at the center of the bottom of the pressure vessel at the lowest point of the loop. The system's rated operating temperature Under the given conditions, the density of the liquid coolant is determined according to the IAPWS-IF97 formula; It is the acceleration due to gravity; and These represent the vertical heights of the reference point (the center of the bottom of the pressure vessel at the lowest point of the loop) and the reference point, respectively.

[0041] Through the above initialization process, the closed flow loop at the initial moment of calculation is in an isothermal and hydrostatic equilibrium state, providing physically self-consistent initial conditions for subsequent main pump startup and transient accident simulation.

[0042] It should be noted that, in the thermodynamic description of the primary loop system, this invention uses the system's rated operating temperature. This is a simplified model for reference. This simplified model is based on the following engineering judgments and analyses: First, this invention focuses on the rapid degradation of the main pump's performance in the initial stage of an accident (typically within ten minutes). At this time, the overall system thermal inertia is large, and the main pump's performance is far more sensitive to hydraulic and two-phase flow parameters than to the overall system temperature drop. Second, the temperature change caused by SBLOCA (Small-Break Loss-of-Coolant Acciden) is spatially localized, while the working fluid at the main pump inlet mainly comes from the steam generator outlet, and its temperature is relatively stable in the initial stage of the accident.

[0043] Therefore, the system's rated operating temperature is adopted. The simplified model used for reference provides a clear and consistent thermodynamic state (saturation pressure) for the flash and cavitation processes. This ensures accurate capture of the core physical process of main pump performance degradation while avoiding the enormous computational cost and convergence problems associated with complex non-isothermal coupled calculations. This quantitative simplification is a key prerequisite for the engineering application of this method.

[0044] S103. Smooth start-up of the main pump and establishment of stable transient system conditions: After the system completes static initialization, the rotating components in the main pump's computational domain are driven to start rotating via a user-defined function (UDF). To avoid numerical instability or non-physical transient responses caused by abrupt changes in the main pump's speed during startup, this invention employs a preset speed control function to smoothly regulate the main pump's startup process.

[0045] In one specific implementation, the speed control function is a piecewise continuous function, and its acceleration phase adopts a smooth transition form based on a cosine function, defined as: in, Let be the main pump speed at time t. The rated speed of the main pump, The preset smooth start-up time is defined. The above speed control function ensures that the main pump speed starts from zero and increases continuously, and its first derivative remains continuous throughout the start-up process, thereby effectively avoiding the problems of discontinuity in flow field momentum and numerical oscillation caused by sudden changes in speed.

[0046] Furthermore, a time-varying time step control strategy can be introduced synchronously during the main pump startup phase to improve the stability and convergence of the calculation process. Specifically, the time step increases linearly from an initial small value to the normal calculation step size, and its relationship can be expressed as:

[0047] in, It is a function of the time step changing with time. It is a normal time step. It is the initial time step. This indicates the time required for the main pump impeller to rotate one revolution.

[0048] The main pump starts and runs continuously according to the aforementioned speed control function and time step strategy. Transient calculations are performed on the closed flow loop using a three-dimensional fluid solver. When key macroscopic hydraulic parameters within the system (such as the pressure difference between the main pump inlet and outlet, and the total flow rate of the loop) remain stable for a continuous period, for example, when the main pump mass flow rate remains stable for a continuous period... The relative fluctuation amplitude within a time step is less than When the flow rate of the main pump shows no continuous increase or decrease trend, the system is considered to have established a stable transient flow field operating state corresponding to the rated operating conditions. (That is, the relative fluctuation amplitude of the main pump mass flow rate over N consecutive time steps is less than...) Where N≥1000, ≤1.0. ) The pre-accident steady-state establishment process of this invention is illustrated as follows: Figure 4As shown, Figure (a) is a schematic diagram of the main pump impeller speed change, and Figure (b) is a schematic diagram of the convergence of various physical quantities of the external characteristics of the primary pump.

[0049] The stable transient flow field obtained above serves as the baseline state before the accident is triggered, and is used for numerical calculation and analysis of the main pump performance degradation process under subsequent small breach water loss accident conditions.

[0050] S104. Triggering and Simulation of Small Breach Water Loss Accidents: The stable transient flow field established in step S103 is used as the initial state for the accident simulation. The key macroscopic hydraulic parameters within the system no longer change with time and are consistent with the rated operating conditions.

[0051] Based on this, at the preset accident triggering time A small breach water loss accident is triggered at the breach calculation domain predefined in step S101. After the accident is triggered, a controlled breach opening function is introduced by modifying the boundary conditions of the breach calculation domain to dynamically simulate the process of the small breach transitioning from a closed state to a fully open state, thereby realizing the release of high-temperature and high-pressure coolant into the external environment. To avoid numerical instability caused by instantaneous changes in the breach boundary conditions, the breach opening process is described by a continuous and smooth time function.

[0052] In one specific implementation, the breach initiation function is used to control the pressure change over time at the boundary of the breach calculation domain. The pressure at the breach boundary... The system operating pressure corresponding to the breach at the moment the accident was triggered within a very short period of time. Smoothly descending to external environmental pressure It is represented as:

[0053] in, The characteristic time for the breach to fully open is extremely small and is used to simulate the rapid discharge process of a small breach in the early stages of an accident. The corresponding moment is the end time of the burst pressure control. The burst opening function of this invention is illustrated as follows: Figure 5 As shown.

[0054] By setting the above-mentioned accident triggering methods and breach boundary conditions, a small breach water loss accident condition can be continuously and stably introduced into the numerical calculation. This causes the coolant at the breach to flash under the drive of a sudden pressure drop, thereby forming a gas-liquid two-phase flow state within the system. This further drives a series of accident evolution processes, such as the propagation of pressure waves in the primary loop system, the reconstruction of system flow, and the gradual increase of gas content at the main pump inlet.

[0055] S105. Accident evolution process monitoring, main pump performance degradation analysis and system coupling boundary condition output: Throughout the entire transient simulation of the accident after the breach opened, the main pump was kept at its rated speed. Continuous operation. In this embodiment of the invention, the three-dimensional fluid solver is based on the finite volume method, uses a heterogeneous flow model to describe the gas-liquid two-phase flow, and couples it with a cavitation model suitable for flash evaporation (such as the ZGB model). Turbulence calculations are performed using SST. k-ω The model employs the PISO algorithm for pressure-velocity coupling. The thermophysical parameters of the working fluid are obtained in real-time during the calculation process by calling the IAPWS-IF97 industrial formula, thus ensuring a physically consistent description of the gas-liquid phase change and two-phase flow behavior.

[0056] During the accident evolution process, real-time monitoring of the main pump's performance degradation characteristics and the output of system coupled boundary conditions are implemented simultaneously. Within the core analysis segment defined in step S101, monitoring surfaces are set at the main pump inlet and outlet sections respectively, and the total pressure at the main pump inlet is continuously collected and recorded. Total export pressure and the average gas content of the inlet section Based on the above monitoring data, the calculation method described in the literature "Analysis Method of Degradation Function of Nuclear Main Pump under Two-Phase Operating Condition with Gas Shot" was used to calculate the key hydraulic performance parameters of the main pump in real time. Among them, the transient head of the main pump... It can be calculated using the following formula:

[0057] In the formula, To determine the gas content at the inlet section Calculated gas-liquid two-phase mixture density; transient torque of main pump The fluid tangential stress acting on each force-bearing surface of the main pump impeller in the three-dimensional flow field is obtained by integrating the stress. This is the acceleration due to gravity, measured in m / s². 2 By analyzing the entire process of the accident , and The temporal relationship of these changes can quantitatively describe the continuous degradation characteristics of the main pump's hydraulic performance during the gradual increase of air content under small breach water loss accident conditions.

[0058] Simultaneously, dynamic parameter time-series data at the outlet position of the steam generator equivalent resistance model (i.e., the inlet of the core analysis section) and the inlet position of the pressure vessel equivalent resistance model (i.e., the outlet of the core analysis section) are monitored and fully recorded. These parameters include at least pressure, mass flow rate, and gas content. This time-series data constitutes a dynamic boundary condition set directly obtained from the three-dimensional coupled calculation of the entire system, accurately reflecting the internal pressure propagation, flow redistribution, and phase change coupling effects during the evolution of a small breach accident. This boundary condition set can serve as a high-fidelity input to drive subsequent independent safety analyses of local areas such as the reactor core and fuel assemblies with higher spatial resolution, thereby supporting the assessment of key safety indicators such as cladding peak temperature at the system level.

[0059] Simulation Examples The breach area is located 1D before the main pump and in the same direction as the main pump outlet. Only one breach area is opened, and a small breach water loss accident simulation is performed according to the above steps.

[0060] Figure 6 The curve shows the total pressure difference between the main pump outlet and inlet as a function of the main pump's gas content. x The gas content corresponding to the maximum degraded head of the main pump. This represents the total pressure difference between the inlet and outlet of the main pump when the liquid water is in single-phase state. The total pressure difference between the inlet and outlet of the main pump in two-phase operation. This represents the total pressure difference between the inlet and outlet of the main pump when the water is in single-phase gaseous state. For example... Figure 6 As shown, when the gas content in the main pump begins to rise, the pressure difference between the main pump outlet and inlet begins to decrease; when the gas content rises to... x When the value is %, the corresponding main pump head degradation multiplier is 1. The head at this time is the maximum degradation head of the main pump, and the gas content at this time is the gas content under the maximum degradation head of the main pump. Figure 7 This is the curve showing the change in main pump torque as a function of the main pump's gas content. Among them, The gas content corresponding to the maximum degrading torque of the main pump. For single-phase liquid water, the main pump torque is... For two-phase operation, the main pump torque is... This refers to the main pump torque when the water is in single-phase gaseous state. For example... Figure 7 As shown, similarly, when the gas content begins to rise, the main pump torque begins to decrease; when the gas content rises to... y At a certain percentage, the corresponding main pump torque degradation multiplier is 1. The head at this point is the maximum degradation torque of the main pump, and the gas content at this point is the gas content under the maximum degradation torque of the main pump. The curves showing the changes in head degradation multiplier and torque degradation multiplier with the main pump gas content are as follows: Figure 8 As shown.

[0061] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A method for calculating the performance degradation of the primary loop main pump of a pressurized water reactor under small breach loss-of-hydraulic accident conditions, characterized in that, Include: Step 1. Construct a three-dimensional fluid computational model of the pressurized water reactor primary loop system: The three-dimensional fluid calculation model is a closed flow loop composed of multiple three-dimensional fluid calculation domains directly connected. The three-dimensional fluid calculation model includes a three-dimensional fluid model, a three-dimensional equivalent resistance model, and a preset break calculation domain. Step 2. System initialization and steady-state establishment: The three-dimensional fluid calculation model described in step 1 is initialized by introducing a gravity field and setting an initial pressure distribution for the entire calculation domain based on the hydrostatic relationship, so that the pressurized water reactor primary loop system is in a state of hydrostatic equilibrium at the initial moment of calculation. Step 3. Smooth start-up of the main pump and establishment of stable transient state of the system: The main pump is driven to smoothly increase from a stationary state to the rated speed through a preset speed control function, and then operates under the rated speed conditions until the flow state in the closed flow loop reaches stability. Step 4. Triggering and Simulation of Small Breach Water Loss Accidents: At a preset accident triggering time after the flow state reaches stability, the breach boundary condition is activated at the preset breach calculation domain described in step 1 to simulate the occurrence of a small breach water loss accident and the discharge process. Step 5. Monitoring the accident evolution process, analyzing the performance degradation of the main pump, and outputting system coupling boundary conditions: During the accident simulation, the main pump is kept running at its rated speed, and the fluid parameters at the inlet and outlet of the main pump are monitored. Based on the fluid parameters, the gas content, head and torque of the main pump at different stages of the accident evolution are calculated, thereby obtaining the continuous degradation process of the main pump performance as the gas content increases under the condition of a small breach and water loss accident.

2. The method according to claim 1, characterized in that, Step 1 also includes: the closed flow loop further includes a main pump, cold section pipeline, hot section pipeline, transition section pipeline, steam generator and core, and the preset break calculation domain is set on the cold section pipeline and the transition section pipeline; The main pump adopts a three-dimensional fluid model including impeller, guide vanes and condensate chamber, while the pressure vessel, core and steam generator adopt a simplified three-dimensional equivalent resistance model. The resistance characteristics of the three-dimensional equivalent resistance model are set so that the total system pressure drop under rated operating conditions in the closed flow loop matches the head of the main pump. The preset breach calculation domain is initially closed and is used to apply breach boundary conditions during the accident simulation phase.

3. The method according to claim 1, characterized in that, Step 2, the system initialization process, also includes setting the rated operating temperature. The rated operating temperature is determined based on the thermal parameters of the pressurized water reactor primary loop under normal full-power operation conditions and serves as the unified initial thermodynamic state input parameter for the entire three-dimensional fluid calculation model.

4. The method according to claim 1, characterized in that, The pressure distribution described in step 2 is based on the hydrostatic formula: in, To calculate the absolute pressure at a specific location within the domain; This is the system reference pressure; The system's rated operating temperature; The system's rated operating temperature Liquid coolant density determined according to the IAPWS-IF97 formula under the specified conditions; It is the acceleration due to gravity; and These represent the vertical heights of the reference point and the reference point, respectively, where the reference point is the center of the bottom of the pressure vessel at the lowest point of the loop.

5. The method according to claim 1, characterized in that, The speed control function mentioned in step 3 is a smooth function that continuously changes the main pump speed from zero to the rated speed within a preset acceleration time. The speed control function is a piecewise continuous function, and its acceleration phase adopts a smooth transition form based on a cosine function, defined as follows: in, Let be the main pump speed at time t. The rated speed of the main pump, This is the preset smooth startup time.

6. The method according to claim 1 or 5, characterized in that, It also includes the main pump start-up stage in step 3, which introduces a time step control strategy that varies with time. The time step increases linearly from an initial small value to a normal calculation step size, and the relationship between the two is expressed as follows: in, It is a function of the time step changing with time. It is a normal time step. It is the initial time step. This indicates the time required for the main pump impeller to rotate one revolution.

7. The method according to claim 1, characterized in that, The breach boundary condition described in step 4 is applied through a preset breach opening function. This breach opening function controls the pressure change over time at the boundary of the breach calculation domain. The pressure at the breach boundary... The system operating pressure corresponding to the breach at the moment the accident was triggered within a very short period of time. Smoothly descending to external environmental pressure It is represented as: in, The characteristic time for the break to fully open. The preset accident trigger time, This refers to the moment when the bevel comes into contact with the external environment and experiences pressure.

8. The method according to claim 1, characterized in that, Step 5: Transient head of the main pump Calculate using the following formula: In the formula, To determine the gas content based on the inlet section The calculated gas-liquid two-phase mixture density and the total pressure at the main pump outlet are... Main pump inlet total pressure Main pump transient torque The fluid tangential stress acting on each force-bearing surface of the main pump impeller in the three-dimensional flow field is obtained by integrating the stress.

9. A computing system for implementing the method according to any one of claims 1-8, characterized in that, include: The model building module is used to build a three-dimensional fluid computation model of the primary loop of a pressurized water reactor. The initialization module is used to perform system initialization and static pressure distribution settings; The main pump control module is used to execute the smooth start-up process of the main pump; The accident triggering module is used to enable the breach boundary conditions; The performance analysis module is used to calculate the main pump head, torque, and gas content, and output the main pump performance degradation results. The modules work together under the control of the processor to calculate the performance degradation of the main pump under the condition of a small breach and loss of water.

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