Method for analyzing formation and evolution characteristics of core melt debris bed based on particle method

Abstract

The method for analyzing the formation and evolution characteristics of a core melt debris bed based on a particle method mainly comprises the following steps: 1, selecting a particle size to adapt to geometric modeling and coolant modeling; 2, selecting a melt material and a coolant material, setting initial parameters, and starting calculation; 3, calculating melt column fragmentation by using a moving particle semi-implicit method basic model; 4, calculating melt particle solidification and combination by using a melting and solidification model; 5, calculating melt debris collision and settlement to form a debris bed by using a discrete element model; 6, calculating debris bed cooling or remelting by using a decay heat model and a boiling heat transfer model; 7, judging whether the end time is reached, if not, advancing by a time step, and if yes, outputting a calculation result; and the method can predict the melt fragmentation form, debris size distribution, debris bed accumulation form, debris bed coolability and debris bed remelting risk probability after a core melt accident.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of molten debris fragmentation, debris bed formation and debris bed remelting risk under the core melt of nuclear reactor, and particularly relates to a method for analyzing the formation and evolution characteristics of core molten debris bed based on particle method. BACKGROUND

[0002] Nuclear reactor severe accidents can be generally divided into two categories, A type severe accident occurs core melt, and B type severe accident does not occur core melt. There have been three severe nuclear accidents in the history of nuclear field, among which, in the first three mile island nuclear accident, due to the lack of coolant after the accident, the temperature of the primary reactor core continued to rise, and finally the core was burned out and the molten material interacted with water (Fuel and Coolant Interaction, FCI), and about 20 tons of debris bed was formed by the core molten material, the solid material which had not been melted, the fuel pellets and the cladding at the bottom of the pressure vessel.

[0003] The international community has conducted extensive and in-depth research on the molten material in-vessel retention strategy, and at present, the post-accident behavior of core melt severe accident can be divided into the following aspects: molten material and water interaction (including steam explosion stage, jet breakup), molten material debris settlement and accumulation to form debris bed, debris bed cooling, debris bed remelting, and molten pool dynamic behavior. The early research mainly focuses on the steam explosion process and the cooling process of the debris bed, while the research on the debris bed formation process and the debris bed remelting process is relatively blank.

[0004] Debris bed refers to the process that in the FCI process, the molten material will be fragmented into many irregularly shaped small particle debris, and these debris will be accumulated into a structure called debris bed in the reactor or outside the reactor. The accumulation pattern of the debris bed and the coolability of the debris bed affect the remelting process. After the formation of the debris bed, heat transfer with the coolant in the heat sink is still needed to reduce the temperature to achieve cooling effect. If the debris bed cannot be effectively cooled, the sensible heat and decay heat of the debris bed may be greater than the heat carried away by the coolant, and the debris bed continues to heat up, thereby causing the remelting of the debris bed. Subsequently, a molten pool is formed at the bottom of the pressure vessel, and the high-temperature molten material in the molten pool will heat the lower head metal in contact with it, so that the heat flux density of the molten pool to the inner wall surface is too large, which causes large area breach of the pressure vessel and failure. If the lower head of the pressure vessel is melted, the pressure shock wave generated by the interaction of the molten material and water may damage the wall structure of the pressure vessel, causing the pressure vessel to deviate from its position, and in addition, the splashes generated will cause serious damage to the surrounding structure.

[0005] Even if the core melt has formed a debris bed, timely cooling is needed to prevent it from remelting, the formation process of the debris bed has an important influence on determining the final form of the debris bed, and the research on the formation of the debris bed and the accumulation form of the debris bed provides a basic method for analyzing the formation characteristics of the debris bed, judging the coolability of the debris bed, providing data support for reducing the remelting risk of the debris bed and ensuring the integrity of the pressure vessel and the containment, and has important significance for the development of severe accident mitigation strategies of the nuclear reactor.

[0006] At present, for the processes of molten material fragmentation, debris bed settlement and accumulation, debris bed cooling, and debris bed remelting, many programs have been developed at home and abroad for numerical simulation, such as the development of IFCI program and MELCOR program for simulation of molten material fragmentation, the development of CFD-DEM method and MPS-DEM method for simulation of debris bed formation, the development of porous medium model for simulation of debris bed cooling, and the development of MPS method and SPH method for simulation of molten pool formation. However, there is no method that can directly simulate the series of processes of fragmentation, settlement, accumulation, cooling, and remelting at present, and it is difficult to obtain the comprehensive characteristics of the formation and remelting process of the core molten debris bed. SUMMARY

[0007] In order to study the fragmentation of molten material after core melt accident, the solidification phase change phenomenon, obtain the shape characteristics of the debris bed formed by the settlement and accumulation of debris, obtain the overall temperature distribution, temperature change, and temperature peak of the debris bed under the action of decay heat, and judge the remelting risk of the debris bed, the present application provides a particle method-based analysis method for the formation and evolution characteristics of the core molten material debris bed under the severe accident of the nuclear reactor core melt, which can calculate the fragmentation process of the molten material column, simulate the solidification and combination of the molten material to form debris, obtain the shape characteristics of the accumulated debris bed, judge the remelting risk of the debris bed, has the simulation function of multiple phases, multiple components, and multiple phases, has the functions of simple calculation setting, high precision, and direct simulation of multiple accident processes, can obtain the parameters such as the fragmentation characteristics of the molten material column, the shape and size distribution of the molten material debris, the shape characteristics of the debris bed, the temperature distribution of the debris bed, and the remelting time of the debris bed, and through the above data, the whole sequence of direct simulation of the severe accident phenomenon of the nuclear reactor core melt can be carried out, the key phenomenon characteristics can be obtained, and the safety evaluation of the pressure vessel under the severe accident is facilitated.

[0008] In order to achieve the above purpose, the present application adopts the following technical solutions to be implemented:

[0009] The particle method-based analysis method for the formation and evolution characteristics of the core molten material debris bed comprises the following steps:

[0010] Step 1: Establishing a container geometry model for a nuclear reactor core meltdown accident, designing numerical simulation parameters, selecting an appropriate particle size for particle modeling of the geometric container, the molten liquid column and the coolant;

[0011] Step 2: Setting the density, viscosity, surface tension coefficient, specific heat capacity, thermal conductivity of each type of particle according to the materials of the geometric container, the molten liquid column and the coolant, setting the initial velocity of the molten liquid column, the temperature distribution of the molten liquid column according to the numerical simulation conditions, setting the adiabatic wall or constant temperature wall according to the cooling conditions of the geometric container, setting the optimal time step according to the particle size, and setting the decay heat model and the boiling heat transfer model to be enabled when the change rate of the debris bed height and width is less than 0.1%;

[0012] Step 3: Starting the calculation, the molten liquid column is injected into the coolant at a certain speed, the molten liquid column undergoes instability fragmentation under the hydrodynamic action of the coolant, the basic model of the moving particle semi-implicit method is used to calculate the interaction between different components, and the position, velocity and temperature of all particles at the next time are calculated;

[0013] Step 4: The temperature of the liquid phase molten particle decreases after being cooled by the coolant, until the solidification phase change occurs when the enthalpy is lower than the saturation enthalpy of the liquid phase molten material, the liquid phase molten particle is introduced into the solid phase rate α to describe the phase state, and the energy conservation equation and the melting and solidification model are used to calculate the solidification phase change of the liquid phase molten particle and the boiling phase change of the liquid particle, and the liquid particle disappears when the enthalpy reaches the saturation enthalpy of the liquid phase;

[0014] Step 5: The solid phase molten particle and the liquid phase molten particle settle in the coolant under the action of gravity, the force of the liquid phase molten particle is still calculated by formula (2), the force of the solid phase molten particle is calculated by the discrete element model and the fluid-structure interaction force model, and the PMS model is used to calculate the overall translation speed and angular velocity of the combination of the solid phase molten particle and the liquid phase molten particle after the force calculation;

[0015] Step 6: After all the molten particles are calculated in step 5, a debris bed is formed at the bottom of the geometric container, and the results of the debris bed height and width are calculated in real time, and when the change rates of the debris bed height and width are both less than 0.1%, it indicates that the change rate of the debris bed accumulation form characteristics is not large, at this time the melting and solidification model is closed and the decay heat model and the boiling heat transfer model are enabled to calculate the temperature distribution of the debris bed;

[0016] Step 7: The overall temperature distribution and peak temperature of the debris bed are calculated, and when the external cooling cannot balance the temperature rise caused by the decay heat, the debris bed will undergo remelting, the remelting time point of the debris bed is calculated, and the process of the debris bed remelting and forming a molten pool is calculated by the melting and solidification model;

[0017] Step 8: repeating steps 3-5 within the set calculation time to obtain the processes of particle fragmentation, solidification, combination, fluid-structure coupling, collision and accumulation of the melt particles, and to obtain the position, velocity, pressure and temperature distribution of the melt particles and liquid particles at different times; when the change rate of the fragment bed accumulation form characteristics is not large, i.e. the change rates of the fragment bed height and width are less than 0.1%, repeating step 6 can obtain the temperature distribution of the fragment bed under the action of decay heat, and the remelting risk of the fragment bed is judged, and the pool formation characteristics can be obtained if remelting occurs.

[0018] Compared with the prior art, the present application has the following advantages:

[0019] The analysis method of the present application can complete the full series simulation of the core melt accident, from the fragmentation of the melt to the formation of the pool, including the key phenomenon models of fragmentation, solidification phase change, fragment formation, fragment collision, fragment bed formation, fragment bed cooling and fragment bed remelting, etc., while considering the energy conservation equation, decay heat model, boiling heat transfer model and PMS model in the above processes, so that the simulation of the severe accident process of the nuclear reactor is more realistic. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The method flowchart of the present application is shown.

[0021] Figure 2 The numerical simulation schematic diagram of the core melt accident condition is shown.

[0022] Figure 3 The fragment bed simulation result diagram of the core melt accident condition is shown. DETAILED DESCRIPTION

[0023] The present application will be further described in detail below in combination with the drawings and specific embodiments.

[0024] Step 1: establishing a container geometric model for the core melt accident of the nuclear reactor, designing numerical simulation parameters, and selecting suitable particle sizes for particle modeling of the geometric container, the melt column and the coolant;

[0025] Step 2: setting the density, viscosity, surface tension coefficient, specific heat capacity and thermal conductivity of each type of particle in the input file according to the materials of the geometric container, the melt column and the coolant, setting the initial velocity of the melt column and the temperature distribution of the melt column in the particle modeling file according to the numerical simulation condition, setting the adiabatic wall or constant temperature wall according to the cooling condition of the geometric container, setting the optimal time step in the input file according to the particle size, and setting the enabling criterion of the decay heat model and the boiling heat transfer model as the change rate of the fragment bed accumulation height and width being less than 0.1%;

[0026] Step 3: Start calculation, the melt liquid column is injected into the coolant at a certain speed, the melt liquid column is broken under the hydraulic action of the coolant, the basic model of moving particle semi-implicit method is used to calculate the interaction between different components, the position, velocity and temperature of all particles at the next time are calculated, the mass conservation equation used in the basic model is formula (1), the momentum conservation equation is formula (2), and the energy conservation equation is formula (3):

[0027]

[0028] In the formula, ρ is density / kg·m -3 ; t is time / s; is velocity vector / m·s -1 ; p is particle pressure / Pa; μ is viscosity / Pa·s; is the external force on the particle / N, including surface tension / N, fluid-structure coupling force / N; g is gravity acceleration / m·s -2 ; h is specific enthalpy / J·kg -1 ; k is thermal conductivity / W·m -1 ·K -1 ; Q is initial decay heat source / W·m -3 ; T is temperature / K;

[0029] Step 4: After the liquid phase melt particles are cooled by the coolant, their temperature decreases until the melting point occurs, the liquid phase melt particles with enthalpy lower than the saturation enthalpy of the melt liquid phase are introduced into the solid phase rate α to describe the phase state, the energy conservation equation and the melting and solidification model are used to calculate the solidification phase change of the liquid phase melt particles and the boiling phase change of the liquid particles, when the enthalpy of the liquid particles reaches the saturation enthalpy of the liquid phase, the liquid particles vaporize and disappear, the melting and solidification model is formula (4) and formula (5); for two melt particles with solid phase rate in the range of 0<α≤1 and particle spacing less than 1.2l0, the two melt particles are combined to form a fragment, see formula (6):

[0030]

[0031] In the formula, T s is melting point / K; h s0 is specific enthalpy of the solidus of the melt material / J·kg -1 ; C ps is specific heat capacity of the solid phase melt material / J·kg -1 ·K -1 ; h s1 is specific enthalpy of the liquidus of the melt material / J·kg -1 ; C pl is specific heat capacity of the liquid phase melt material / J·kg -1 ·K -1 ; α iis the solid phase rate of i particles; a j is the solid phase rate of j particles; r ij is the distance between i particles and j particles; l0is the particle diameter / m -1 ;

[0032] Step 5: The solid-phase melt particles and the liquid-phase melt particles are settled in the coolant under the action of gravity, the force on the liquid-phase melt particles is still calculated by formula (2), the force on the solid-phase melt particles is calculated by the discrete element model and the fluid-structure interaction force model, after the force is calculated, the PMS model is used to calculate the overall translational velocity and angular velocity of the combination formed by the solid-phase melt particles and the liquid-phase melt particles, the discrete element model is shown in formula (7), the fluid-structure interaction force model is shown in formula (8), and the PMS model is shown in formula (9):

[0033]

[0034] In the formula: F X (t) is the normal force on the particle / N; e n (t) is the normal elastic force / N; d n (t) is the normal damping force / N; e n (t-△t DEM ) is the normal elastic force of the previous time step; k n is the normal elastic coefficient / N·m -1 ; △X is the overlap in the normal direction / m; c n is the normal damping coefficient / m; △t DEM is the time step calculated by the DEM model / s; F Y(Z) (t) is the tangential force on the particle; e s (t) is the tangential elastic force / N; d s (t) is the tangential damping force / N; e s (t-△t DEM ) is the tangential elastic force of the previous time step; k s is the tangential elastic coefficient / N·m -1 ; △Y(Z) is the overlap in the tangential direction / m; c s is the tangential damping coefficient / m; is the fluid-structure interaction force / N; is the force on the solid by the fluid pressure / N; is the force caused by the viscosity of the fluid / N; is the velocity of the combination ii in which the particle i is located / m·s -1 ; N is the number of particles of the combination ii; is the velocity of the particle i / m·s -1 ; is the angular velocity of the combination / rad·s -1 ; Iii Moment of inertia of the assembly / kg m 2 ; m i Mass of particle i / kg Centroid position of particle i Centroid position of assembly ii

[0035] Step 6: After the calculation of all melt particles by step 5, the debris bed is formed at the bottom of the geometric container, and the height and width of the debris bed are calculated in real time. When the change rates of the height and width of the debris bed are both less than 0.1%, it is indicated that the change rate of the debris bed accumulation form characteristics is not large, at which time the melting and solidification model is closed and the decay heat model and the boiling heat transfer model are started to calculate the temperature distribution of the debris bed. The decay heat model is shown in formula (10), and the boiling heat transfer model is shown in formula (11):

[0036]

[0037] (1) T m -T sat <5K: natural convection zone, heat transfer is calculated by formula (3)

[0038] (2) 5K < T m -T sat <30K: nucleate boiling zone

[0039]

[0040] (3) 30K < T m -T sat <120K: transition boiling zone, the boundary value between the nucleate boiling zone and the transition boiling zone is interpolated

[0041] (4) 120K < T m -T sat : film boiling zone (11)

[0042]

[0043] In the formula: Q dec Decay power / MW; T m Temperature of solid melt particles / K; T sat Saturation temperature of liquid particles / K; h mw Heat transfer coefficient between solid melt and liquid / W m -3 K -1 ; k w Thermal conductivity of liquid / W m -1 K -1 ; D m Diameter of solid melt particles / m; p w Density of liquid / kg m-3 ; v mw is the relative velocity / m·s -1 ; a v is the volume fraction of liquid vaporization; m w is the dynamic viscosity of liquid / Pa·s; Pr w is the Prandtl number of liquid; F is a calculation coefficient; T w is the temperature of liquid / K; Su is a calculation coefficient; Cp w is the specific heat capacity of liquid / J·kg -1 ·K -1 ; s is the surface tension coefficient N / m; p v is the density of liquid after vaporization / kg·m -3 ; P w is the particle pressure of liquid / Pa; P is the particle pressure of solid melt / Pa; h is the heat transfer coefficient between solid melt and liquid / W·m -3 ·K -1 ; s' is the Boltzmann constant / W·m -2 ·K -4 ; H vw is the latent heat of vaporization / J·kg -1 ; k v is the thermal conductivity of liquid after vaporization / W·m -1 ·K -1 ; m v is the dynamic viscosity of liquid after vaporization / Pa·s; b is the thermal expansion coefficient / K -1 ; Re is the Reynolds number;

[0044] Step 7: the overall temperature distribution of the debris bed and the peak temperature are calculated, and when the external cooling cannot reach a balance state with the heating of the decay heat, the debris bed will be remelted, and the remelt time point of the debris bed can be calculated, and the process of remelting and forming a molten pool of the debris bed can be calculated through a melting and solidification model;

[0045] Step 8: steps 3-5 are repeated in the set calculation time to obtain the processes of fragmentation, solidification, combination, fluid-structure coupling and collision accumulation of the melt particles, and the positions, velocities, pressures and temperature distributions of the melt particles and liquid particles at different times are obtained; when the change of the debris bed accumulation form characteristics is not severe, that is, the change rates of the height and width of the debris bed are both less than 0.1%, step 6 is repeated to obtain the temperature distribution of the debris bed under the action of the decay heat, and the remelting risk of the debris bed can be judged, and the formation characteristics of the molten pool can be obtained when remelting occurs.

[0046] The effects of the present application will be described below in combination with specific calculation objects. Figure 2The numerical simulation is carried out by using the method of the application with the object of the shown core melt accident condition. Firstly, the equipment size of the coolant container, the coolant amount and the molten mass are obtained, and the particle size such as 0.001 m is selected to carry out the particle modeling of the molten material, the coolant and the geometric container. The material of the geometric container is set as low alloy steel, the material of the molten material is set as stainless steel, the coolant is set as liquid water, the initial molten material speed is 0 m / s, the coolant is static, the time step is set as 1e-6 s, the coolant temperature is set as 25 ℃, and the molten material temperature is set as 1400 ℃. After starting the calculation, the molten material particle flows into the coolant pool from the gap, the steps 3-5 are repeated to obtain the processes of the molten material particle fragmentation, solidification, combination, fluid-structure coupling, collision and accumulation, and the position, speed, pressure and temperature distribution of the molten material particle and the liquid particle at different times are obtained; when the change of the shape characteristics of the debris bed is not sharp, the step 6 is repeated to obtain the temperature distribution of the debris bed under the action of the decay heat, and finally the output result is obtained, that is, the numerical simulation result of the whole sequence process after the core melt accident is obtained, as shown in the formula (1). Figure 3 The method of the application is helpful for the analysis of the special phenomenon of the core melt accident process and the evaluation of the containment integrity after the core melt accident.

Claims

1. A method for analyzing the formation and evolution characteristics of a core melt fragment bed based on the particle method, characterized in that: The method includes the following steps: Step 1: Establish a container geometric model for a nuclear reactor core meltdown accident, design numerical simulation parameters, and select appropriate particle sizes to perform particle modeling of the geometric container, molten liquid column, and coolant. Step 2: Set the density, viscosity, surface tension coefficient, specific heat capacity, and thermal conductivity of various particles according to the materials of the geometric container, molten liquid column, and coolant. Set the initial velocity and temperature distribution of the molten liquid column according to the numerical simulation conditions. Set the adiabatic wall or isothermal wall according to the cooling conditions of the geometric container. Set the optimal time step according to the particle size. Set the activation criteria for the decay heat model and boiling heat transfer model as: the rate of change of the height and width of the debris bed is less than 0.1%. Step 3: Start the calculation. The molten liquid column is injected into the coolant at a certain speed. The molten liquid column undergoes unstable fragmentation under the hydraulic action of the coolant. The basic model of the moving particle semi-implicit method is used to calculate the interaction between different components and to calculate the position, velocity and temperature of all particles at the next moment. Step 4: After being cooled by the coolant, the temperature of the liquid-phase molten particles decreases until they reach the melting point and undergo a solidification phase transition. For liquid-phase molten particles with an enthalpy value lower than the liquid phase saturation enthalpy, a solid fraction α is introduced to describe the phase state. The solidification phase transition of the liquid-phase molten particles and the boiling phase transition of the liquid particles are calculated using the energy conservation equation and the melting-solidification model. When the enthalpy value of the liquid particles reaches the liquid phase saturation enthalpy, they vaporize and disappear. Step 5: Solid melt particles and liquid melt particles settle in the coolant under the action of gravity. The force on the liquid melt particles is still calculated by formula (2). The force on the solid melt particles is calculated by discrete element model and fluid-structure interaction force model. After calculating the force, the PMS model is used to calculate the overall translational velocity and angular velocity of the combination of solid melt particles and liquid melt particles. Step 6: After calculating all molten particles in Step 5, a fragment bed is formed at the bottom of the geometric container. The height and width of the fragment bed are statistically analyzed in real time. When the change rate of the height and width of the fragment bed is less than 0.1%, it indicates that the change rate of the fragment bed's morphological characteristics is not large. At this time, the melting and solidification model is turned off and the decay heat model and boiling heat transfer model are enabled to calculate the temperature distribution of the fragment bed. Step 7: Calculate the overall temperature distribution and peak temperature of the fragment bed. When the external cooling cannot reach an equilibrium with the decay heat rise, the fragment bed will remelt. Calculate the remelting time point of the fragment bed and calculate the process of the fragment bed remelting and forming a molten pool using the melting and solidification model. Step 8: Repeat steps 3-5 within the set calculation time to obtain the process of molten particles fragmentation, solidification, combination, fluid-structure interaction, and collision accumulation. Obtain the position, velocity, pressure, and temperature distribution of molten particles and liquid particles at different times. When the rate of change of the morphological characteristics of the fragment bed is not large, that is, the rate of change of the height and width of the fragment bed is less than 0.1%, repeat step 6 to obtain the temperature distribution of the fragment bed under the action of decay heat, judge the risk of remelting of the fragment bed, and if remelting occurs, the characteristics of molten pool formation can also be obtained.

2. The method for analyzing the formation and evolution characteristics of a core melt debris bed based on the particle method according to claim 1, characterized in that: In step 3, the basic model uses the following equations: mass conservation equation (1), momentum conservation equation (2), and energy conservation equation (3). In the formula: ρ is density / kg·m -3 t represents time in seconds. Velocity vector / m·s -1 p is particle pressure / Pa; μ is viscosity / Pa·s; The external force acting on the particle is N, including surface tension and fluid-structure interaction forces; g is the acceleration due to gravity (m·s). -2 h represents specific enthalpy / J·kg -1 k is the thermal conductivity / W·m -1 ·K -1 Q represents the initial decay heat source (W·m). -3 T represents temperature in K.

3. The method for analyzing the formation and evolution characteristics of a core melt debris bed based on the particle method according to claim 1, characterized in that: In step 4, the melting and solidification model is as shown in equations (4) and (5); for two molten particles with a solid fraction in the range of 0 < α ≤ 1 and a particle spacing of less than 1.2l0, they will combine to form fragments, as shown in equation (6): In the formula: T s Melting point / K; h s0 Specific enthalpy of the solidus of the molten material / J·kg -1 C ps Specific heat capacity of solid-phase molten material / J·kg -1 ·K -1 h s1 Specific enthalpy of the liquidus of the molten material / J·kg -1 C pl Specific heat capacity of liquid phase molten material / J·kg -1 ·K -1 ;α i Let α be the solid fraction of particle i; j Let r be the solid fraction of particle j; ij l is the distance between particles i and j; l0 is the particle diameter in meters. -1 .

4. The method for analyzing the formation and evolution characteristics of a core melt debris bed based on the particle method according to claim 1, characterized in that: In step 5, the discrete element model is shown in equation (7), the fluid-structure interaction force model is shown in equation (8), and the PMS model is shown in equation (9): In the formula: F X (t) represents the normal force on the particle in N; e n (t) represents the normal elastic force in N; d n (t) represents the normal damping force in N; e n (t-△t DEM ) represents the normal elastic force at the previous time step; k n Normal elastic modulus / N·m -1 ; △X is the overlap in the normal direction / m; c n Normal damping coefficient / m; △t DEM Time step for DEM model calculation / s; F Y(Z) (t) represents the tangential force on the particle; e s (t) represents the tangential elastic force in N; d s (t) represents the tangential damping force in N; e s (t-△t DEM ) represents the tangential elastic force at the previous time step; k s Tangential elastic modulus / N·m -1 ; △Y(Z) is the overlap in the tangential direction / m; c s The tangential damping coefficient is given in meters (m). The force is the fluid-structure interaction force in N. Force exerted by fluid pressure on a solid / N; Force caused by fluid viscosity / N; The velocity of the composite body ii containing particle i is given in m·s. -1 N is the number of particles in composite ii; The velocity of particle i is given in m·s. -1 ; Angular velocity of the combined body / rad·s -1 ; I ii Moment of inertia of the composite body / kg·m 2 ; m i The mass of particle i is given in kg. Let be the position of the center of mass of particle i; Let be the position of the center of mass of assembly ii.

5. The method for analyzing the formation and evolution characteristics of a core melt debris bed based on the particle method according to claim 1, characterized in that: In step 6, the decay heat model is shown in equation (10), and the boiling heat transfer model is shown in equation (11): In the formula: Q dec Decay power / MW; T m Temperature of solid-phase melt particles in K; T sat The saturation temperature of the liquid particles is given in K; h mw The heat transfer coefficient between the solid melt and the liquid is given by W·m. -3 ·K -1 ;;k w Thermal conductivity of the liquid / W·m -1 ·K -1 ; D m ρ is the diameter of the solid melt particles in meters (m); w Liquid density / kg·m -3 ;v mw Relative velocity / m·s -1 ;α v The volume fraction of liquid phase vaporization; μ w Pr is the dynamic viscosity of the liquid in Pa·s. w The liquid Prandtl number is represented by F; the calculated coefficient is T. w Liquid temperature in K; Su is a calculation coefficient; Cp w Specific heat capacity of liquid / J·kg -1 ·K -1 ; σ is the surface tension coefficient in N / m; ρ v Density of the vaporized liquid (kg·m³) -3 ;P w Liquid particle pressure / Pa; P is the particle pressure of the solid melt / Pa; h is the heat transfer coefficient between the solid melt and the liquid / W·m -3 ·K -1 σ' is the Boltzmann constant / W·m -2 ·K -4 ;;H vw Latent heat of vaporization / J·kg -1 ;k v Thermal conductivity of the vaporized liquid / W·m -1 ·K -1 μ v β is the dynamic viscosity of the liquid after vaporization (Pa·s); β is the coefficient of thermal expansion (K). -1 Re is the Reynolds number.

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