Mechanism and risk assessment of baffle-jetting-induced vibrations in fuel rods

Abstract

As nuclear reactor cores approach the end of their lifespans, it is a crucial aspect to ensure that the structural integrity of the reactor internals is not affected. The baffle-former bolts are subjected to significant mechanical stress and neutron irradiation from the reactor core during the plant operation. These circumstances can cause the bolts to degrade and lose some of their load-bearing capacity over the reactor lifespan. The degraded baffle-former bolts can lead to separation/openings between the baffle plates which cause water jet cross-flow injecting through these openings. This phenomenon is called baffle jetting. This jet flow can induce the vibration of fuel rods located near the baffle gaps, resulting in grid-to-rod fretting and, in extreme cases, fuel rod failures.This paper investigates the risk of baffle jetting-induced fluidelastic instability in fuel assemblies. A 6 × 5 axisymmetric flexible rod bundle having a prototypical nuclear fuel assembly is designed and fabricated to simulate a part of a fuel assembly under combined axial flow and baffle jetting. A special displacement mechanism is used to vary the position of jet flow relative to the array (i.e. jet eccentricity). Two cases, the jet-gap aligned case, and the jet-rod aligned case, are investigated to determine the variation of critical jet velocity with the jet eccentricity. In addition, three axial flow velocity cases, VAxial = 1.0 m/s, 1.5 m/s, and 2.0 m/s are tested in order to determine the stability effect of axial flow velocity on jet-induced instability. The experiments show that the mock-up array response resembles fluidelastic instability behavior. The stability threshold of the mock-up array depends strongly on the relative position of the jet flow with respect to the array centerline. In the jet-rod aligned case, the axial flow shows a strong effect on the combined flow-induced instability.