Vibrations of PWR fuel assembly under axial coolant flow and oblique impingement of jet cross-flow from LOCA holes

Nuclear fuel bundles are exposed to localized normal impinging jet cross-flow at certain locations along the fuel rod span in a specific design of pressurized water reactors (PWRs) with a fail-safe feature for a loss-of-coolant accident (LOCA). The combined axial flow and jet cross-flow from LOCA holes can induce extensive fuel rod vibration, leading to fretting wear, particularly in rods near the LOCA holes. The dynamics of the fuel assembly depend strongly on the jet impingement angle ($\theta$) where the jet flow impinges the fuel rods. This paper investigates experimentally the effect of oblique impingement of a circular jet flow in axial flow on the vibration of a reduced scale model array of the fuel assembly. The mock-up array is tested for three jet inclination angles relative to the rod axis. In addition, the effect of axial flow on the jet-induced dynamics in the array is investigated. The tests are done for the jet centerline located symmetrically or eccentrically relative to the rod inter-column gap. The jet eccentricity is found to have a significant effect on rod bundle stability. The results show that the axial flow has a stabilizing effect on the jet-induced instability. The stability threshold of the array is significantly affected by the jet injection angle. The array becomes significantly more unstable when the jet flow is injected at $\theta =70°$ compared to the normal jet impingement case. For a jet impingement angle larger than 90 degrees, the stability behavior is more complex. While the rod bundle undergoes instability, further increasing the jet velocity did not exacerbate the vibration response, thus suggesting an apparently self-limiting instability for the non-eccentric jet.