Development Of Moving Particle Semiimplicit Models For Simulation Of Key Phenomena In Molten Corium Pool Under Severe Accident

During a postulated severe accident in a light water reactor,the fuel pins and supporting structures may be melted due to the loss of the coolant.The melt migrates and falls down to the low head of reactor pressure vessel（RPV）in the end.This process covers multi-scale and multi-physical phenomena including eutectic reaction and density stratification.Eutectic reaction forms low-melting-point alloys at the interface of two or more materials which significantly affects core degradation process and molten pool behavior.It leads to not only the early failure of fuel cladding,but also precipitation of the heavy metal which affects the stratification layers in the molten pool.For example,the eutectic reaction between uranium-base fuels and iron-base claddings leads to the early dissolution of the cladding in fast reactors with metallic fuel.The precipitation of heavy metal because of eutectic reaction results in the inverse stratification in the formation of molten pool in UO₂-fuel reactors.The configuration of the molten pool determines the heat flux distribution on the outer surface of the pressure vessel and affects the intactness of the RPV.It is a big challenge for the computational fluid dynamics（CFD）method based on the Eulerian description to deal with the complex interface variation.So the Moving Particle Semi-implicit（MPS）method which is a kind of Lagranigian method is utilized to simulate the multi-phase and multi-component phenomena due to its excellence at handling the large interface deformation problems.For MPS method,the domain is full of a series of particles,these particles carry physical information such as mass,momentum and energy.The interaction between particles is calculated by kernel function integration and the gradient model and Laplace model are used to discrete governing equations.By tracking each particle,the information of the entire solution domain can be obtained.The initial MPS method is improved to ensure the accuracy and stability of the pressure calculation,such as the kernel function,gradient model and pressure source term.The simulations of hydrostatic pressure,dam break and inverse stratification are conducted to verify the modified models.The results show that the improved models own more accurate and stable pressure calculation.The phase change model is developed to simulate the melting process in the molten pool,including the enthalpy-based model and phase diagram-based model.Both models are verified by the one-dimensional alloy melting,and the results are consistent with the theoretical solutions.The eutectic reaction model is developed to simulate the eutectic reaction phenomenon.The coupling of mass diffusion and phase diagram is applied to calculate the eutectic reaction.In order to verify the eutectic reaction model,the eutectic reaction of uranium and iron in two semi-infinite domains is simulated,the predicted interface position agrees well with the theoretical solution.The viscosity of the materials changes sharply over the liquid fraction and affect the motion of liquid phase during the phase change process.Hence the viscosity model is applied to ensure the stability and efficiency of the calculation.The passively moving solid（PMS）model is brought up to realize the interaction of the solid and fluid.Finally,the Wood melting experiment and TREAT experiment are calculated to validate the accuracy of the improved program,the results shows great consistency with the experimental data.The density stratification occurs when the melt falls down to the lower head of RPV.For the sake of accurate prediction of interface change,the potential-force surface tension model is adopted and extended to calculate the interface tension.The surface tension model is verified by the square droplet oscallation and contact angle of the solid-liquid phase,and the results are agreeable with the theoretical solutions.The interface tension model is verified by the simulation of a floating droplet,and the results show accurate prediction of the contact angle between immiscible fluids.In order to demonstrate the precision of the interface tension model at the solid surface,the wall contact angle with immiscible fluids is simulated and the predicted value agrees well with the theoretical value.In order to further validate the modified MPS method,the two-dimensional dam-break stratification experiment is simulated.The validation shows that the interface deformation can be accurately reproduced by the interface tension model.Finally,three-dimensional injection stratification is simulated to prove the effectiveness of implemented modification.The transient formation process of molten pool is simulated with the improved MPS method.The sensitivity analysis of some parameters of the melt is carried out including the injection sequence,injection position and the uranium concentration in the molten material.Furthermore,the melting and stratification process of fuel elements and reactor structures in the molten pool are also calculated.In conclusion,this paper developes an integrated numerical simulation method for the coupling behavior of multi-physical and multi-scale phenomena of fuel eutectic melting and molten pool transient stratification based on the MPS method.The program is capable to study the mechanism of eutectic reaction between multi-component materials in the reactor,and explore the formation mechanism and evolution law of molten pool.