Fuel Assembly Nonlinear Dynamic Model

Fuel assembly finite-element dynamic models are developed to perform the core seismic analysis. The fuel assembly is modeld by a single vertical beam, which represents the cross-sectional inertias of the fuel rods, guide thimbles and instrument thimble, and a series of rotational springs. The rotational springs are located at the intermediate spacer grid locations. Benchmarking to fuel assembly natural frequencies determined by testing is accomplished by adjusting the moment of inertia and the grid rotational stiffness to find their effective values. Most often these models are linear and are appropriate for the small amplitude stiffness representation of the fuel assembly. Large deflection problems are approximated by choosing a fuel assebly stiffness value appropriate to the average deflection range. Some loss of accuracy will naturally result from this approach. This paper presents a nonlinear model to approximate the hysterisis and free vibration response for large amplitudes fuel assembly motion. The force required to impose the initial displacement (pluck) and the free vibration responses are used to compare the nonlinear model’s behavior with the test data. This model correctly predicts fuel assembly deflection shapes as a function of axial position for various lateral loads for several fuel assembly designs. Displacement hysterisis is primarily due to fuel assembly to grid slippage, which is a strong function of grid preload. “Tight” and “relaxed” prototypes were tested to account for grid preload effect. The model correctly analyzes the grid preload effect. A nonlinear fuel assembly model provides better matching of grid impact loads determined by fuel assembly lateral impact testing and also prvides better matching of all of the initial conditions (initial deflection, initial force, initial energy and impact velocity). In this test, the fuel assembly impacts a test wall to determine grid internal stiffness. This value is used in the core model for seismic analyses.