Fluid-elastic instability evaluation for reactor vessel internals with structural interaction

Abstract

Provided fluid velocity reaches a specific threshold value, the vibrational amplitude of the corresponding structure increases rapidly and becomes unstable. Even though nuclear industrial concerns have been focused primarily on heat exchangers, steam generators and pipes, evaluation of flow-induced vibration is also required for safety-related components such as reactor vessel internals (RVIs). For this matter, the fluid-elastic instability of RVIs in a typical 1000 MWe nuclear power plant (NPP) was examined by enhanced methods taking into account numerical capability and uncertainties. First, computational fluid dynamics (CFD) analyses of the whole geometry were conducted to assess the flow characteristics with and without consideration of fuel assembly under various NPP conditions. Based on the resulting data, upper guide structure (UGS) was selected as the most critical among five RVI subcomponents. Second, detailed modal analyses of the UGS and attached multiple tubes were performed using both added mass and fluid-structure interaction models to find out natural frequencies and corresponding mode shapes, and to investigate effect of a flooded structure. Third, fluid-elastic instability was evaluated based upon the results from the CFD and modal analyses, and compared with a stability diagram. Thereby, it was confirmed that the RVIs considered in this study had sufficient safety margin against the fluid-elastic instability under representative conditions. Moreover, the calculated stability ratios increased when either the simple model was employed without consideration of the fuel assembly as the current common practice or fluid-structure interaction effect was incorporated.