Multi-loop natural circulation system flow instability model analysis method

Abstract

The invention discloses a flow instability model analysis method for a multi-loop natural circulation system, and relates to the technical field of nuclear reactor safety, and the method is characterized by comprising the following steps: establishing a multi-loop natural circulation flow system model; performing natural circulation calculation by using a self-programming program to obtain a bifurcation characteristic curve of the two-phase multi-loop natural circulation flow system; establishing a flow-heat exchange strong coupling multi-loop pressure drop-flow equation set to obtain a parameter response curve of the two-phase multi-loop natural circulation flow system; and analyzing influence characteristics of parameters in the system through the bifurcation characteristic curve and the parameter response curve. According to the method, a three-loop coupling natural circulation flow system is extracted on the basis of ocean nuclear power source design, natural circulation calculation of a two-phase multi-loop system is achieved through a self-programming program, and natural circulation flow of all loops and heat exchange influence factors between the loops are screened; the method is used for accurately analyzing influence characteristics of parameters in the nonlinear multi-loop natural circulation system.

Description

Technical Field

[0001] The invention relates to the field of nuclear reactor safety technology, and more specifically, to a flow instability model analysis method for a multi-loop natural circulation system. Background Art

[0002] The passive residual heat removal system of advanced nuclear power plants adopts single-loop natural circulation and two-loop natural circulation, and then uses a condenser circulating water system or a large-capacity water tank with forced circulation drive to remove the residual heat from the core. Because the nuclear power source is isolated from the external power grid and is unattended for a long time in the marine environment, once a power outage occurs, the three-loop condenser circulating water system will also lose its function, causing the natural circulation to fail to remove the residual heat from the core. It can be seen that compared with land-based nuclear power plants, nuclear power sources in marine environments have higher requirements for their inherent safety.

[0003] If the two-loop natural circulation is expanded to a three-loop natural circulation, and the first loop, the waste heat removal system and the condenser circulating water system are regarded as a whole to form a multi-loop coupled natural circulation system, the requirements can be met; among them, due to the strong multi-loop coupling characteristics within the three-loop coupled natural circulation system itself, and the influence of flow instability that may be caused by phase change of the waste heat removal system, the characteristics of the system will become more complicated.

[0004] In recent years, Yu Jiyang et al. proposed a passive residual heat removal system using three-loop coupled natural circulation, and used a water tank as the final heat sink. The system was calculated and analyzed using the RETRAN02 program. According to the calculation results, the passive system can effectively remove the core residual heat after selecting appropriate system parameters; Li Xiaowei et al. studied the decoupling problem in the multi-coupled natural circulation system through theoretical analysis and numerical calculation, and proposed a method that can numerically simulate the thermal-hydraulic characteristics of the multi-coupled system. However, the composition of the marine nuclear power system is complex, the construction and maintenance are difficult, and there is a large uncertainty in the cost and cycle. The current multi-loop natural circulation system is calculated based on theoretical numerical values, and there is no method specifically for modeling and analyzing multi-loop natural circulation systems in the prior art.

[0005] Therefore, the present invention aims to provide a flow instability model analysis method for a multi-loop natural circulation system to solve the above-mentioned problems. Summary of the invention

[0006] The purpose of the present invention is to provide a flow instability model analysis method for a multi-loop natural circulation system. The present invention extracts a three-loop coupled natural circulation flow system based on the design of an ocean nuclear power source, and uses a self-programming program to realize the natural circulation calculation of a two-phase multi-loop system. By screening the natural circulation flow of each loop and the heat transfer influencing factors between loops, a multi-loop pressure drop-flow equation group with strong flow-heat transfer coupling is established, which can accurately analyze the influence characteristics of parameters in a nonlinear multi-loop natural circulation system.

[0007] The above technical objectives of the present invention are achieved through the following technical solutions: A method for analyzing flow instability models of a multi-loop natural circulation system, comprising the following steps:

[0008] S1. Extract and establish a multi-loop natural circulation flow system model based on marine nuclear power design;

[0009] S2. According to the multi-loop natural circulation flow system model, a natural circulation calculation is performed using a self-programming program to obtain a bifurcation characteristic curve of a two-phase multi-loop natural circulation flow system;

[0010] S3. Screen the factors affecting the natural circulation flow and heat transfer between the loops in the multi-loop natural circulation flow system model, establish a multi-loop pressure drop-flow equation group with strong coupling of flow and heat transfer, and obtain the parameter response curve of the two-phase multi-loop natural circulation flow system;

[0011] S4. Analyze the influence characteristics of system parameters through bifurcation characteristic curve and parameter response curve.

[0012] The present invention is further configured as follows: the self-programming program in step S2 is based on the compilation of the calculation program in C++ language, and the natural circulation calculation is performed by selecting a suitable thermal hydraulic model and a numerical calculation algorithm.

[0013] The present invention is further configured as follows: the thermal hydraulic model includes a natural circulation model, a flow resistance model, a heat exchange model and a net steam generation point model.

[0014] The present invention is further configured as follows: in the natural circulation model, the system model is divided into several sections according to different heating conditions of the fluid in the system model, various pressure drops in the several sections are calculated respectively, and then various pressure drops in several sections of the same circuit are added together to obtain the total pressure drop of the same circuit.

[0015] The present invention is further configured as follows: in the flow resistance model, each local pressure loss is determined in an artificially assumed manner, by pre-setting constants, and then performing an impact analysis on changes in local resistance coefficients to offset the impact of local resistance caused by the local structure of the pipeline on the flow of the fluid.

[0016] The present invention is further configured such that: the heat exchange model includes calculations of convection heat exchange between the one-way liquid and the tube wall, steam condensation heat exchange and liquid boiling heat exchange.

[0017] The present invention is further configured as follows: the liquid in the secondary circuit in the net steam generation point model is heated to a gas-liquid two-phase flow, and the dividing point between the single-phase water and the gas-liquid two-phase flow in the heat exchanger is determined by calculation using the Saha-Juber relationship.

[0018] In summary, the present invention has the following beneficial effects:

[0019] The present invention extracts a three-loop coupled natural circulation flow system from the new concept marine nuclear power source design, and uses a self-programming program based on C++ language to realize the natural circulation calculation of the two-phase multi-loop system. Through the preliminary screening of the natural circulation flow of each loop and the influencing factors of heat transfer between loops, a multi-loop pressure drop-flow equation group with strong coupling of flow-heat transfer is established, which can accurately analyze the influence characteristics of the parameters in the nonlinear multi-loop natural circulation system. The research results show that the system stability can be achieved by increasing the temperature of the three-loop cold source, reducing the three-loop pressure, increasing the second loop pressure, and reducing the second loop local resistance coefficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a flow chart of a method for analyzing a flow instability model of a multi-loop natural circulation system in an embodiment of the present invention;

[0021] Figure 2 is a simplified diagram of a three-loop coupling system in an embodiment of the present invention;

[0022] Figure 3 It is a schematic diagram of the numerical algorithm flow structure in an embodiment of the present invention;

[0023] Figure 4 is a bifurcation characteristic curve diagram of a two-phase single-loop natural circulation system in an embodiment of the present invention;

[0024] Figure 5 is a graph of the secondary loop bifurcation characteristic of a natural circulation system of a two-phase multi-loop system in an embodiment of the present invention;

[0025] Figure 6 is a relationship diagram between pressure drop and mass flow rate at various heating powers of the secondary circuit in an embodiment of the present invention;

[0026] Figure 7 is the bifurcation characteristic curve of different three-circuit cold source temperatures in the embodiment of the present invention;

[0027] Figure 8 is a bifurcation characteristic curve diagram of different three-circuit pressures in an embodiment of the present invention;

[0028] Fig. 9 is a bifurcation characteristic curve diagram of different secondary circuit inlet resistance coefficients in an embodiment of the present invention;

[0029] Fig.10 is a bifurcation characteristic curve diagram of different secondary circuit outlet resistance coefficients in an embodiment of the present invention;

[0030] Fig.11 is a driving force-resistance relationship diagram for different inlet resistance coefficients in an embodiment of the present invention;

[0031] Fig.12 is a driving force-resistance relationship diagram of different outlet resistance coefficients in an embodiment of the present invention;

[0032] Fig.13 is a bifurcation characteristic curve diagram of different secondary circuit pressures in an embodiment of the present invention;

[0033] Fig.14 1 is a flow instability boundary diagram for different three-circuit cold source temperatures in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The following is combined with Figure 1-14 The present invention is described in further detail.

[0035] Embodiment: A method for analyzing a flow instability model of a multi-loop natural circulation system comprises the following steps:

[0036] S1. Extract and establish a multi-loop natural circulation flow system model based on the design of marine nuclear power sources, such as Figure 2 As shown, the system model is a simple three-circuit coupling system, in which one circuit consists of an electric heater, a voltage stabilizer, a shell-and-tube heat exchanger, a ball valve, a canned pump and a pipeline. The fluid is heated by the electric heater (or heat exchanger) in the three circuits and then flows into the heat exchanger (or cold source) through the ascending section, transferring the heat to the next circuit (or cold source). After being cooled, it returns to the electric heater (or heat exchanger) through the descending section to form a cycle; when the initial cycle established by the canned pump is established, the canned pump is turned off and the fluid will be driven by natural circulation.

[0037] The inlet and outlet fluids of the first circuit are all single-phase water, the inlet fluids of the second circuit and the third circuit are single-phase water, and the outlet fluids are gas-liquid two-phase flows; for the convenience of calculation, the situation where the outlet fluid of the second circuit is single-phase water and superheated steam is not considered in this embodiment, the heat taken away by the cooling water in the condenser of the third circuit is equal to the heat input by the electric heater, and its temperature is set constant to simulate the temperature of seawater, and the system pressure of the three circuits is controlled by the voltage stabilizer of each circuit and is always stable at the set pressure. At the same time, for the convenience of calculation, the electric heater, shell and tube heat exchanger, and condenser used in this embodiment are all ideal components, the system is an adiabatic system, and there is no heat exchange with the outside world other than the system components. The center height difference between the heat source and the cold source is 6m for the first circuit, 2m for the second circuit, and 4m for the third circuit.

[0038] S2. According to the multi-loop natural circulation flow system model, a self-programming program is used to perform natural circulation calculations to obtain the bifurcation characteristic curve of the two-phase multi-loop natural circulation flow system; wherein the self-programming program is based on the compilation of the calculation program in C++ language, and the natural circulation calculation is performed by selecting a suitable thermal-hydraulic model and a numerical calculation algorithm, and the thermal-hydraulic model includes a natural circulation model, a flow resistance model, a heat exchange model and a net steam generation point model.

[0039] The numerical calculation algorithm flow of the system in this embodiment is as follows: Figure 3 As shown, since the temperature of the cold source of the three circuits is set to be constant and known in this embodiment, this embodiment adopts the method of calculating from the three circuits to the first circuit in sequence, that is, the flow of the three circuits is first calculated (when calculating the third circuit, there is no need to run the operation of calculating the inlet temperature and the outlet temperature), and then based on the flow of the three circuits, the inlet temperature and the outlet temperature of the second circuit are calculated through the heat exchange correlation formula, and then the flow of the second circuit is calculated, and finally the flow of the first circuit is calculated.

[0040] In the natural circulation model, the system can be divided into several sections according to the different heating conditions of the fluid in the system (adiabatic, heating, cooling), and the various pressure drops in the sections are calculated respectively. Then, the various pressure drops of each section of the same circuit are added together to obtain the total pressure drop of the circuit. If one section is taken out, the mathematical expression of the total pressure drop is:

[0041] ΔP=ΔP el +ΔP a +ΔP f +ΔP c (1)

[0042] Where ΔP el Indicates boost pressure drop, ΔP a Indicates the accelerating pressure drop, ΔP f represents the friction pressure drop, ΔP c Indicates the form resistance voltage drop.

[0043] For a closed natural circulation loop, the accelerating pressure drop ΔP in the system a is 0, the total pressure drop ΔP is 0, and formula (1) can be transformed into:

[0044] -ΔP el =ΔP f +ΔP c (2)

[0045] If ΔP d is the driving pressure head, then ΔP d =-ΔP el , if ΔP r is the flow resistance, then ΔP r =ΔP f +ΔP c , so formula (2) can be rewritten as:

[0046] ΔP d =ΔP r (3)

[0047] When the driving pressure head generated by the increased pressure drop in the system is consistent with the flow resistance of the system, the natural circulation system reaches a stable state; when the system flow is constant, if the driving pressure head is different from the flow resistance, the system flow will be adjusted until the system reaches a steady state; based on this principle, this embodiment compiles a C++ language system program for a multi-loop coupled natural circulation system to study the steady-state characteristics of the system.

[0048] In the flow resistance model, the flow of fluid in the pipeline is affected by two types of resistance: one is the resistance along the pipeline caused by friction, and the other is the local resistance caused by the local structure of the pipeline (such as variable cross-section, change in flow direction, etc.); for pipe flow, the flow resistance expression is:

[0049]

[0050] Where k is the sum of the loss coefficients caused by friction, local, turning, expansion and contraction, and V is the fluid velocity.

[0051] When the fluid flows through a straight pipe, friction will produce resistance along the way, which will in turn produce friction pressure drop. For the resistance along the way, k can be expressed as:

[0052]

[0053] Where f is the friction factor, L is the channel length, and D e Indicates equivalent diameter.

[0054] For round tubes, D e Take the cross-sectional diameter. When the cross-sectional area is not circular, De Taking the hydraulic diameter, the expression is:

[0055]

[0056] In the formula, A f Indicates the channel cross-sectional area, P W Indicates wet perimeter.

[0057] The friction factor f of the single-phase friction pressure drop is usually calculated by an empirical formula, which is a single-valued function of the Reynolds number Re. The calculation expressions of Re and f are shown in equations (7) and (8), respectively:

[0058]

[0059] Where μ represents the dynamic viscosity of the fluid.

[0060]

[0061] The two-phase friction pressure drop adopts the homogeneous flow model, which can be expressed as:

[0062] ΔP tp =Φ 2 tp ΔP tf (9)

[0063] In the formula, Φ 2 tp represents the two-phase friction factor, which can be determined by equation (10), and ΔP tf It is determined by the above single-phase formula.

[0064]

[0065] Wherein, v represents specific volume, subscript g represents saturated steam, and subscript f represents saturated water.

[0066] Among them, when the fluid flows through a boundary with a sharp change, pressure loss will occur, and then form resistance pressure drop will be generated. The key to solving its specific value is to determine the local pressure loss coefficient k. However, since this coefficient often cannot be obtained by a certain mathematical calculation formula, it needs to be obtained by experimental measurement, which brings certain difficulties to theoretical calculations. Therefore, in this embodiment, the local pressure losses are temporarily determined by artificial assumptions, by pre-setting constants, and then, the impact analysis is performed on the change of local resistance coefficient.

[0067] The heat transfer model includes the calculation of the convective heat transfer between the one-way liquid and the tube wall, the steam condensation heat transfer and the liquid boiling heat transfer. For the convective heat transfer between the single-phase liquid and the tube wall, this paper adopts the accurate Gnielinski formula, as shown in Equation 11:

[0068]

[0069] It is applicable to the case where Re=2300×106 and Pr=0.6×105; wherein Nu represents the Nusselt number, Pr represents the Prandtl number, and f represents the drag coefficient. In this embodiment, the Konakov formula is used:

[0070] f=(1.8lgRe-1.5) -2 (12)

[0071] In steam condensation heat exchange, when the fluid of the vertical heat exchanger is saturated steam under a certain pressure, it condenses into single-phase water in the pipeline. The convective heat transfer coefficient correlation formula used in this embodiment is as follows.

[0072] When Re < 1800, the liquid film is in a laminar state, and McAdams recommends using the correlation formula:

[0073]

[0074] Where r represents the latent heat of vaporization, λ represents the thermal conductivity, and t s represents the saturation temperature, t w Represents the wall temperature.

[0075] When Re>1800, the liquid film is in a turbulent state. This embodiment adopts the Kirkbride formula, as shown in formula (14):

[0076]

[0077] After the liquid is heated to a saturated state in the pipe, it undergoes saturated boiling heat exchange with the wall. In this embodiment, the convective heat transfer coefficient of the heat exchange process is calculated by the Jens-Lottes equation:

[0078]

[0079] In the formula, Δθ J represents the film temperature difference, is the heat flux, Q is the heating power, L s represents the length of the heating section, and Δθ is calculated by formula (15): J Then, by substituting it into formula (16), the convective heat transfer coefficient h can be obtained:

[0080]

[0081] In the net steam generation point model, the liquid in the secondary circuit may be heated to a gas-liquid two-phase flow. Therefore, it is necessary to determine the dividing point between the single-phase water and the gas-liquid two-phase flow in the heat exchanger, that is, the net steam generation point. The result is calculated in this paper using the Saha-Juber relationship.

[0082] When Pe < 70000,

[0083]

[0084] When Pe > 70000,

[0085]

[0086] where represents the Peclet number, M represents the mass flow rate, represents the mass velocity, represents the cross-sectional area of the pipe, represents the heating power per unit area, Q represents the heating power, and t in represents the inlet temperature.

[0087] In a two-phase natural circulation system under given structural conditions and heating power, the mass flow rate of the system is jointly determined by the driving head and the flow resistance. However, at the same time, there is a strong mutual influence between the two and the system flow rate. When and only when there is a flow rate such that the driving head is equal to the flow resistance, the three reach equilibrium. At this time, the flow rate is the mass flow rate under the steady-state condition of the system.

[0088] Figure 4 The bifurcation characteristic curve of a two-phase single-loop natural circulation system is given. When the heating power Q < Q1, the system is in single-phase natural circulation, that is, the outlet fluid is still single-phase water; when Q > Q1, the system is in two-phase natural circulation, that is, the outlet fluid is gas-liquid two-phase flow.

[0089] From Figure 4 it can be seen that when Q < Q2 or Q > Q3, there is only one equilibrium solution for the mass flow rate, and there is a one-to-one correspondence between the heating power and the mass flow rate. However, when Q2 < Q < Q3, there are three different solutions for the mass flow rate under the same heating power, that is, the system can operate at three operating points at this time. Which operating point it operates at is determined by the historical conditions and disturbance situation of the system. Therefore, when Q2 < Q < Q3, the system exhibits a bifurcation phenomenon.

[0090] Figure 5 The bifurcation characteristic curve of a two-phase multi-loop natural circulation system is given. It can be found that the two-phase system is similar to the single-phase system and both exhibit bifurcation phenomena. Figure 6 shows that at a heating power of 30 kW, there is only one intersection point between the driving force and the resistance, corresponding to Figure 6 the situation of the driving force and the resistance in the region with only one operating point on the left. The 34KW system corresponds to the situation with only one operating point on the right. Both are stable operating conditions, consistent with the two-phase single-loop system. The 32KW system corresponds to Figure 6There are three operating points in the region of driving force and resistance. This region, like the two-phase single-loop system, can be stable or unstable, depending on the system conditions.

[0091] S3. Screen the factors affecting the natural circulation flow and heat transfer between the loops in the multi-loop natural circulation flow system model, establish a multi-loop pressure drop-flow equation group with strong coupling of flow and heat transfer, and obtain the parameter response curve of the two-phase multi-loop natural circulation flow system;

[0092] Figure 7 , Figure 8 They respectively represent the influence of the two-phase multi-circuit natural circulation system on the bifurcation characteristic curve of the two-circuit system when the three-circuit cold source temperature and the three-circuit pressure are different; Figure 7 It can be seen that when the temperature of the three-circuit cold source increases, the area where the bifurcation phenomenon occurs becomes smaller, which is more conducive to the stability of the system. This is because when the temperature of the three-circuit cold source increases, the temperature at the inlet of the second circuit will also increase accordingly, that is, the underheating at the inlet of the second circuit decreases. When other system conditions remain unchanged, the fluid only needs to absorb less heat to reach the two-phase state, resulting in a decrease in the power of flow drift, thereby increasing the stable area.

[0093] from Figure 8 It can also be seen that the change of the third-circuit pressure has little effect on the second-circuit bifurcation characteristic curve, but a certain trend can still be seen from the figure, that is, the reduction of the third-circuit pressure can slightly reduce the unstable area, thereby improving the stability of the system. This is because the increase of the third-circuit pressure will lead to an increase in the heat transfer coefficient of the third circuit, which will reduce the inlet temperature of the second circuit. From the above, it can be seen that the lower the inlet temperature of the second circuit, the more unstable the system is.

[0094] Fig. 9 , Fig.10 It shows the influence of the change of the secondary circuit inlet resistance coefficient and outlet resistance coefficient of the two-phase multi-circuit natural circulation system on the bifurcation characteristic curve of the secondary circuit system; Fig. 9 , Fig.10 It can be seen that the increase of the resistance coefficient at the secondary circuit inlet is conducive to system stability, while the increase of the resistance coefficient at the secondary circuit outlet is not conducive to system stability.

[0095] The reason is that the fluid at the inlet of the secondary circuit is single-phase water. Increasing the resistance coefficient at the inlet of the secondary circuit is equivalent to increasing the single-phase resistance coefficient, which increases the resistance of the single-phase part, while the resistance of the two-phase part remains unchanged. In general, it reduces the influence of the two-phase area. Fig.11 The performance is that the second half of the resistance curve is "raised", while the first half remains unchanged, which makes the curvature of the resistance curve smaller, resulting in a decrease in the probability of the resistance curve and the driving force curve having three intersection points, thereby improving the stability of the system.

[0096] Increasing the resistance coefficient at the secondary circuit outlet will increase the resistance of the two-phase region at the same flow rate, while the single-phase region remains almost unchanged, which will increase the impact of the two-phase region. Fig.12 The performance is that the resistance curve rises as a whole, but finally gathers at the same point, which causes the curvature of the resistance curve to become larger. The resistance curve is more likely to have three intersections with the driving force curve, which increases the instability of the system.

[0097] Fig.13 It shows the influence of the change of the secondary circuit pressure of the two-phase multi-circuit natural circulation system on the bifurcation characteristic curve of the secondary circuit system. It can be seen from the figure that the higher the pressure of the secondary circuit system, the smaller the unstable area of ​​the system and the better the stability of the system. The reason is that as the system pressure increases, the gas-liquid density difference of steam and water will decrease accordingly, which will reduce the influence of the two-phase area of ​​the resistance curve in the driving force-relationship diagram, making the system more stable.

[0098] S4. Analyze the influence characteristics of system parameters through bifurcation characteristic curve and parameter response curve.

[0099] By changing the temperature of the cold source of the three circuits and calculating the phase change number of the second circuit and supercooling number Derive N sub and N pch is the flow instability boundary diagram of the horizontal and vertical axes, such as Fig.14 As shown. The square data is approximated from left to right Figure 6 The data obtained from the unstable region in Figure 6 The circular data is obtained from right to left, and its corresponding Figure 6 Point c in .

[0100] The area between the circular data and the square data in the figure is the unstable area of ​​the system. When the steady-state operating point of the system is in this area, the system is unstable and will change to other steady-state operating points after being disturbed.

[0101] Can be obtained from Fig.14 It can be seen that with the increase of the temperature of the cold source of the three circuits, the unstable area of ​​the system gradually decreases. When the temperature of the cold source increases to a certain level, the unstable area of ​​the system is completely eliminated.

[0102] Conclusion: The present invention aims at the two-phase multi-circuit natural circulation system under marine conditions. The steady-state flow rate under different heating powers is calculated through a self-programming program, the bifurcation characteristic curve of the two-phase natural circulation system is given, and its cause is analyzed. The parameter response curve of the system is drawn by changing the parameters of the system, and it is analyzed whether the system parameters are conducive to the stability of the system. Finally, the calculation results show that increasing the temperature of the cold source of the third circuit, reducing the pressure of the third circuit, increasing the pressure of the second circuit, and reducing the local resistance coefficient of the second circuit are beneficial to the stability of the two-phase multi-circuit natural circulation system.

[0103] This specific embodiment is merely an explanation of the present invention and is not a limitation of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed. However, as long as they are within the scope of the claims of the present invention, they are protected by the patent law.

Claims

1. A method for analyzing flow instability model of multi-loop natural circulation system, characterized by: The following steps are involved: S1. Extract and establish a multi-loop natural circulation flow system model based on marine nuclear power design; S2. According to the multi-loop natural circulation flow system model, a natural circulation calculation is performed using a self-programming program to obtain a bifurcation characteristic curve of a two-phase multi-loop natural circulation flow system; S3. Screen the factors affecting the natural circulation flow and heat transfer between the loops in the multi-loop natural circulation flow system model, establish a multi-loop pressure drop-flow equation group with strong coupling of flow and heat transfer, and obtain the parameter response curve of the two-phase multi-loop natural circulation flow system; S4. Analyze the influence characteristics of system parameters through bifurcation characteristic curve and parameter response curve.

2. The method for analyzing flow instability model of a multi-loop natural circulation system according to claim 1 is characterized by: The self-programming program in step S2 is compiled based on the C++ language to complete the calculation program, and the natural circulation calculation is performed by selecting a suitable thermal hydraulic model and a numerical calculation algorithm.

3. The method for analyzing flow instability model of a multi-loop natural circulation system according to claim 2 is characterized by: The thermal hydraulic model includes a natural circulation model, a flow resistance model, a heat exchange model and a net steam generation point model.

4. The method for analyzing flow instability model of a multi-loop natural circulation system according to claim 3 is characterized by: In the natural circulation model, the system model is divided into several sections according to the different heating conditions of the fluid in the system model, and various pressure drops in several sections are calculated respectively. Then, various pressure drops in several sections of the same circuit are added together to obtain the total pressure drop of the same circuit.

5. The method for analyzing flow instability model of a multi-loop natural circulation system according to claim 3 is characterized by: The flow resistance model uses artificial assumptions to determine local pressure losses, by pre-setting constants and then performing an impact analysis on changes in local resistance coefficients to offset the impact of local resistance on fluid flow caused by the local structure of the pipeline.

6. The method for analyzing flow instability model of a multi-loop natural circulation system according to claim 3 is characterized by: The heat transfer model includes calculations of convection heat transfer between the one-way liquid and the tube wall, steam condensation heat transfer and liquid boiling heat transfer.

7. The method for analyzing flow instability model of a multi-loop natural circulation system according to claim 3 is characterized by: In the net steam generation point model, the liquid in the secondary circuit is heated to a gas-liquid two-phase flow, and the Saha-Juber relationship is used to calculate to determine the boundary point between the single-phase water and the gas-liquid two-phase flow in the heat exchanger.

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