In this paper, the fluid-solid interaction of a jet pump of a boiling water reactor type 5 (BWR/5), with its riser subjected to a leakage flow through its slip joint, is reported. This is a fluid-elastic instability problem. A methodology is proposed for the evaluation of the velocity of the fluid at the slip joint with and without a labyrinth seal. It is calculated with computational fluid dynamics. The results show that such a seal reduces the velocity of the fluid and produces a stable and linear behavior between the inlet and the outlet fluid velocities at the slip joint. Then the first five natural frequencies of the jet pump assembly are evaluated. The range is between 24.74 Hz and 60.21 Hz. The mass of water inside and outside of such an assembly is considered. With these data and the dimensions of the slip joint, a finite element mesh is developed and the time step (∆t = 0.001 s) is determined. The fluid and structure mesh are coupled. The fluid flow through the slip joint without a labyrinth seal is evaluated with a two-way fluid-structure interaction under normal conditions of operation. Accelerations up to 8 g can be developed at the bottom of the mixer.

The fluid flow is estimated during the first 0.25 s. Flow-induced vibration can be exacerbated in resonance conditions. These values are similar to those obtained in the experimental analyses reported in the open literature. One of the excitation frequencies caused by the interaction between the fluid and the structure was close to the third natural frequency of this assembly (46.99 Hz). If the integrity of the labyrinth seal is maintained, the jet pump will not present high-amplitude oscillations. Therefore, an adequate management of seal degradation is required and failures of the jet pump can be avoided.