Abstract The core of Pressurized Water Reactors (PWR) is composed by nuclear fuel assemblies, bundles of slender fuel rods containing uranium pellets, kept in position by spacer grids equipped with elastic elements for retention, and exposed to an axial flow of coolant. Fuel assemblies are subjected to fluid-induced vibrations causing fretting at the interface with spacer grids; events such as earthquakes may also constitute an external excitation resulting in large-amplitude vibrations of the fuel rods. The relationship between the excitation amplitude and the damping during large-amplitude forced vibrations of nuclear fuel assemblies in the presence of flowing fluid is not fully understood. The present experimental study investigated the vibrations of a 3 × 3 tube assembly composed by eight regularly spaced fuel rods installed around a guide tube and supported by spacer grids; tests were performed in still and flowing water. In addition to experimental modal analysis under small random excitation, stepped-sine experiments at different levels of harmonic excitation that caused large-amplitude vibrations were performed. The equivalent viscous damping ratios of the fundamental model at different excitation levels were extracted by fitting the results with a single degree-of-freedom model. The vibrations of the fuel assembly were strongly influenced by the vibrational behavior of the single rods, which constitute coupled oscillators. An increase of damping with the excitation amplitude was observed both in still and in flowing water and acted in the direction of structural safety. The water flow did not cause instabilities in the operational range; instead, the increment of flow speed increased the damping ratios in the linear (small-amplitude vibrations) and nonlinear (large-amplitude vibrations) regime.

中文翻译：

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由隔栅支撑的 3 × 3 缩小比例压水堆燃料组件的非线性振动

摘要 压水反应堆（PWR）的堆芯由核燃料组件、包含铀芯块的细长燃料棒束组成，由装有弹性元件的间隔网格保持在适当位置，并暴露在轴向流动的冷却剂中。燃料组件受到流体引起的振动，导致在与隔栅的界面处产生微动；地震等事件也可能构成导致燃料棒大幅振动的外部激励。在存在流动流体的情况下，核燃料组件在大振幅受迫振动过程中的激励振幅与阻尼之间的关系尚不完全清楚。本实验研究了一个 3 × 3 管组件的振动，该组件由安装在导管周围并由间隔网格支撑的八个规则间隔的燃料棒组成；测试在静止和流动的水中进行。除了小随机激励下的实验模态分析外，还进行了引起大振幅振动的不同谐波激励水平下的阶梯正弦实验。通过将结果与单自由度模型拟合，提取了基本模型在不同激励水平下的等效粘性阻尼比。燃料组件的振动受到构成耦合振荡器的单杆振动行为的强烈影响。在静止和流动的水中观察到阻尼随激励幅度的增加，并且作用在结构安全的方向上。水流不会造成操作范围内的不稳定；相反，流速的增加增加了线性（小振幅振动）和非线性（大振幅振动）状态下的阻尼比。