Lattice Boltzmann Modeling For The Coupling Of Neutron Transport And Thermal Hydraulics

The detailed neutron transport-thermal-hydraulics(NTH)coupling calculation of nuclear reactors is one of the main researches of the advanced reactors numerical simulation,involving the intersection of neutron physics,fluid mechanics,and heat transfer.Due to the complexity of the neutron transport simulation and the differences between different physical processes,the detailed simulation of the coupled NTH process inside the reactor core still needs to be studied in depth.This work,based on the lattice Boltzmann(LB)method with the advantage of simple implementation,strong parallel characteristics,and multi-physics coupling,develops the LB numerical calculation methods for neutron transport simulations,establishes their adaptive mesh refinement,unstructured mesh,and large-amount CPU parallel accelerated techniques.Based on this foundation,a unified LB framework for coupled NTH simulation is established.The neutron transport high-accuracy LB models are established and the calculation programs are compiled.Aiming at the three widely used neutron transport approximations,including the neutron transport S_N equation,the SP₃ equation,and the neutron diffusion equation,three high-accuracy neutron transport LB models are established,respectively.By using the high-order Chapman-Enskog multi-scale expansion,the high-accuracy neutron diffusion LB model(NDLBM)is established,which can effectively improve the calculation accuracy without obviously increasing complication.By using the double-distribution-function LB model and high-order Chapman-Enskog multi-scale expansion,the high-accuracy neutron transport SP₃ LB(SP3LBM)model is established,which can effectively improve the calculation accuracy by maintaining all the advantages of standard LBM.From the discrete velocity Boltzmann equation(DVBE),the finite-differential LB model for solving the neutron transport S_N equation(SNFDLBM)is established,which improves the accuracy and stability by using the adjustable temporal-spatial discretization.Numerical results show that the above high-accuracy LB models have higher accuracy and stability than the standard LBM,and have higher accuracy and flexibility of multi-dimensional heterogeneous reactor core and space-time kinetics.The neutron transport LB models are developed to adaptive and unstructured meshes.The adaptive-mesh and unstructured neutron transport LB models are established,and the calculation programs are compiled.Aiming at the complicated neutron distribution,the streaming-based block-structured adaptive-mesh-refinement(SSAMR)neutron transport LB model which can adaptive adjusts mesh construction with a clear mesh relationship is developed.The complicated tree-typed data structure is eliminated and the inflexibility of the multi-block(MB)mesh technique is overcome.To improve the adaptability to complex reactor core geometries,the unstructured mesh finite-volume neutron transport LB model is developed,which can flexibly simulate the neutron transport problems under complex geometries.The simulation results of benchmark problems show that the SSAMR based neutron transport LB model can accurately simulate the multi-group neutron transport problem,and can flexibly and simply adjust the mesh structure adaptively;the unstructured-mesh neutron transport LB model can accurately and flexibly adapt to the reactor core structure under different geometric conditions.The parallel accelerated techniques are studied in the neutron transport LB models.In view of the time-consuming characteristics of detailed reactor numerical simulation,a GPU parallel accelerated LB model for neutron transport is developed.Because of the simplicity and locality of neutron transport LBM,it is very suitable for GPU multithreading parallel computing.Aiming at the angular discretization of the neutron transport S_N equation,a space-angle two-level parallel LB model for accelerating the neutron transport S_N equation in GPU is developed.The results show that the GPU parallel accelerated LB model can effectively improve the calculation efficiency,and the space-angle two-stage parallel acceleration can further improve the calculation speed of the neutron transport S_N equation LB model.On the basis of the above research,the LB calculation framework of neutron transport-thermal-hydraulics coupling is established and the LB calculation program of multi physics coupling is compiled.Based on the neutron transport LBM,a unified LB framework lbm NTH is established to solve the coupling process of neutronics-thermal-hydraulics in reactor.Three common neutron transport governing equations,including S_N,SP₃ and diffusion equations,heat conduction and convective heat transfer,and fluid flow equations,such as Navier-Stokes and LES equations,are unified into LB framework to solve,and their coupling relationship is considered under the unified data structure and discrete format.Based on the finite Boltzmann form,an LB model for the conservation of delayed neutron precursors in liquid fuel is developed.The numerical results show that the lbm NTH framework can simulate the coupled NTH process flexibly and accurately;the SP₃ approximation can simulate the neutron transport process more accurately than the neutron diffusion approximation under small-scale conditions;the temperature feedback has a strong effect under high-temperature conditions;increasing the moderator flow rate can effectively improve the heat transfer and flatten the temperature distribution,which is conducive to the safe and stable operation of the core.In conclusion,to realize the coupling solution of neutron transport,heat transfer and fluid flow processes in nuclear reactor,a high-precision LB numerical simulation method of neutron transport process is established,and the coupling simulation of neutron transport,heat transfer and fluid flow processes is realized under the unified LB framework.The work in this paper is an effective application and development of engineering thermophysics theory in the field of nuclear reactor engineering,and can provide a new idea for the research of reactor multiphysics coupling and large-scale engineering applications.