A full scale test program was designed to study the slip joint leakage flow instability. A number of cases were tested with different operating conditions and slip joint modifications (simulated wear). A Slip Joint Extension (SJE) design, developed by Westinghouse (WEC) that stabilizes the leakage flow was designed and installed. The design is applicable to a BWR/4 configuration. It is found that the vibration level with the WEC SJE installed is much lower than the baseline case and the high level vibration frequency seen in the baseline data is not presented with the fix installed.

A 3D CFD model is used to study the flow phenomena in the narrow gaps of slip joint. The result shows that leakage mass flow decreases and turbulence kinetic energy decrease significantly with the SJE design. This means that the extension of gap flow passage can effectively decrease the flow fluctuations to suppress the leakage-flow-induced vibration. The experimental and CFD results confirmed that the SJE offers a low-cost, low-risk effective solution that eliminates jet pump flow-induced vibration (FIV) by changing the flow across the slip joint from divergent (unstable) to convergent (stable).