Methods of simulating large-scale rod bundle and application to a 17 × 17 fuel assembly with mixing vane spacer grid

Abstract

In the last decade, a large number of computations were presented using computational fluid dynamics (CFD) for the simulation of thermal-hydraulic problems in the nuclear industry. A common feature in almost all of those simulations is that the geometric models used in the simulations only contained a small number of fuel rods such as 3×3 and 5×5 due to the limit of the computer capacity. However, a typical fuel assembly of pressurized water reactor (PWR) consists of 17×17 rod bundle. This paper concerned on the appropriate numerical methods for CFD simulations of fluid flow and heat transfer in large-scale rod bundle such as 17×17 fuel assembly with mixing vane spacer grid at reasonable computational cost. Firstly, in order to reduce the computational amount in the research, the effectiveness of applying periodic boundary conditions to a central two-subchannel model was validated by comparing the velocity profiles to the experimental and other CFD data. Later, based on the validated two-subchannel model, the methods of domain-divided solving technique and application characteristics of polyhedral meshes for simulating a large-scale rod bundle with mixing grid were investigated. The so-called domain-divided solving technique is a method that the whole rod bundle domain is divided in several sub-domains along the axial direction and each of the sub-domains is simulated separately. The optimal subdivision method and outlet boundary condition of the sub-domain containing mixing vanes were confirmed in the present work. The application characteristics of the polyhedral meshes applied to the rod bundle with mixing grid was investigated and the polyhedral meshes show higher computational efficiency and better convergence properties for the same reasonable accuracy compared to tetrahedral meshes. Finally, a numerical simulation of a typical 17×17 fuel assembly with mixing vane spacer grid was performed by utilizing the methods investigated above. As a result of this study, we have found the appropriate methods to simulate the large-scale rod bundle with mixing grids at full axial length on a conventional computer. The CFD analysis on thermal-hydraulic problems in reactor coolant system can be conducted widely by using a real size fuel assembly in the future.