Study On Transient Nodal Green’s Function Method And Coupled Method With Thermal-Hydraulic

It is a tendency to utilize the coupled three-dimensional neutronics andthermal-hydraulics simulation when the feedback effects are significant and powerdistribution varies severely. The temporal discretization scheme in conventionalmodularize coupling simulation is of order1, and the calculation is time-consuming.The dissertation mainly focuses on the application of high order and self-adaptivetemporal discretization schemes on neutron spatial kinetics problem and the modularizecoupling numerical method.Firstly, Nodal Green’s Function Method (NGFM) based on the second boundarycondition is used for solving transient neutron diffusion equations. Diagonally ImplicitRunge-Kutta (DIRK) methods are used for temporal discretization to improve accuracy.At each stage of DIRK, the spatial kinetics problem is transferred into the fixed sourceproblem (FSP). The FSP is solved using the source iteration with coarse meshrebalancing to accelerate convergence. Four Strongly S-stable DIRK formulas of orderfrom1to4are selected in the dissertation. Automatic time step size control is achievedthrough Richardson Extrapolation or embedded lower order Runge-Kutta formulas.Hermite interpolation is used for dense output on time variable. NGFMN-K code isdeveloped accordingly. The numerical results agree well with reference solutions.Comparison on results of DIRK formulas show that the2-stage, second order DIRKformula embedded first order, called DIRK(2,2)-E, is the most efficient. In addition,DIRK formulas are more accurate and efficient than Backward Euler scheme, especiallyfor severely varied transient problem.Secondly, based on solving diffusion equation in a node approximately, the pinpower reconstruction methods are studied，which are categorized by different selectionof hyperbolic functions, boundary conditions and the order of Legendre polynomials.Three methods are proved to be more accurate than rest through numerical tests.Thirdly, DIRK formulas are applied on temporal discretization of the modularizecoupling simulation to improve accuracy. The COBRA-Ⅳ sub-channel analysis code iscoupled to the NGFMN-K code for core transient analysis. The nonlinear equationscoupled neutronics and thermal-hydraulics are solved using block iterative method.Finally, the reasonable temporal coupling and synchronization approach is proposed, in which the RELAP5code is coupled using the parallel coupling way.Furtherly, through integrating the COBRA-Ⅳ code for sub-channel calculation on hotassembly, the PWR transient analysis code system RNCC is developed. The time stepsizes of core coupled module and system module are controlled independently.Accuracy of the temporal coupling and synchronization approach in RNCC is studied.The numerical results show that results of RNCC agree well with reference solutions,and DIRK(2,2)-E is the most efficient.The dissertation improves the temporal coupling and synchronization approach ofmodularize coupling simulation, achieves high accuracy and efficiency, and finallyconstructs an advanced multi-level coupled model which is capable of simulating theplant system and evaluating reliable safety parameters.