Study On Numerical Stability Of Neutronics/Thermal Hydraulic Coupling In Core

It is necessary to couple neutronics and thermal-hydraulic in reactor core as they have a close relationship.At prerent,it's mostly external coupling which belongs to weak coupling.The numerical instability and slow convergence rate are prone to occur in the calculation process for the traditional successive iteration method.Therefore,it's important to study the numerical instability and improve the convergence performance and efficiency of neutronics and thermal-hydraulics coupling.A three-dimensional steady-state and transient neutronics/thermal-hydraulics coupling calculation program is developed by embedded the the subchannel program into the neutronics program based on Green's function nodal method steady-state program NNGFM and transient program TNGFM and subchannel thermal-hydraulic analysis program COBRA-EN.Firstly,the coupling method is studied,and the steady-state and transient neutronics/thermal-hydraulics coupling calculation processes are proposed respectively.On this basis,the multidimensional data interpolation algorithm is derived and programmed.The cross-section database for feedback is made,and the coupling feedback module,parameter transfer module and iteration control module are also developed.In order to verify the correctness of the program,the core of Qinshan Phase II reactor is calculated and analysed based on the results of SIMULATE-3.The convergence performance of neutronics and thermal-hydraulics coupling calculation is studied by numerical simulation.Relaxation factor and approximate strong coupling algorithm are proposed to improve the numerical stability of coupling calculation under steady and transient conditions respectively.Sensitivity analysis of factors that may affect the numerical stability of coupling is also carried out.At the same time,the neutronics/thermal-hydraulics coupling program is further coupled with the fine power reconstruct program for the analysis of the hottest assembly under coupling.The numerical simulation results show that the cross-section database and interpolation algorithm are effective and reliable,and the neutronics/thermal-hydraulics coupling program developed in this paper is effective and correct.In the process of coupled iteration,the numerical stability of coupled calculation can be improved by introducing relaxation factor.By adjusting the number of iterations in neutronics calculation,the convergence speed and efficiency of coupling calculation can be improved.For transient conditions,introducing approximate feedback into external iteration of neutronics can greatly reduce the number of iterations of coupling and improve the convergence performance.The power level,boron concentration and the time step also have great influence on the numerical stability and efficiency of coupling.The results of the hottest assembly can further enhance thermal margin of core when fine power reconstruction is achieved after coupling.