Tremendous growths of computer and computational fluid dynamic (CFD) technology enables the extensive analysis for the fuel and component integrity problems, the core inlet/exit flow distributions, the grid-to-rod fretting performance adjacent to the core shroud geometry, etc., by the use of large-scale computer model and resources. In this study, a full model is constructed to obtain the 3-dimensional, global and localized flow distributions of PWR reactor vessel upper plenum for Maanshan Nuclear Power Plant located in Taiwan and to characterize the important flow features based on a comprehensive flow model with extensive computational cells. Through the extensive computations, the lateral/axial flows in individual guidetube assembly, pressure and flows in the entire domain in the upper plenum region and the effect on the fuel integrity as well as the distributions of the shear stress are obtained for the upper plenum internal design improvement. The detailed flow patterns are important references to justify the patterns by subcomponent analysis or much simplified geometries due to the limitations of aged computer.