Full scale random vibration tests have been performed on a nuclear fuel assembly to determine its vibrational properties for use in verifying analytical models that are subsequently used in earthquake and LOCA (loss-of-coolant accident) vibration analyses. The fuel assembly exhibits non-linear sinusoidal vibrational behavior. This is because the fuel rods can slip (coulomb friction) in the spacers that support the fuel rod array. As the amplitude of sinusoidal vibration is increased the resonant frequency decreases. It has been found that random vibration as a means of modal testings averages the above non-linear response of the fuel assembly. The natural frequencies and mode shapes derived from random vibration test results do not appear to be amplitude dependent and show excellent correlation with linear elastic FEM (finite element model) values. In the FEM, the non-linear effect arising from the spring and dimple type rod support is treated as an equivalent elastic connection in which the elastic constants are derived from load deflection test results. The natural frequencies and corresponding mode shapes are calculated using a two-dimensional (lateral and axial) NASTRAN FEM. The calculated results show excellent correlation with experimental results for the seven measured fuel assembly natural frequencies and mode shapes.
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