Characterization of Fuel Rods Supported by Dimple‐Less Spacer Grids and Subjected to Nonlinear Vibrations

ABSTRACT An experimental campaign was undertaken to demonstrate the applicability of the Bouc–Wen hysteretic model to describe the hysteretic system of the recent Framatome dimple‐less spacer grid design for Pressurized Water Reactors. The experimental setup subjected a reduced‐length fuel rod, supported at both ends by dimple‐less grids, to forced sinusoidal excitation while immersed in water. The resulting frequency–amplitude curves, taken at different excitation amplitudes, exhibited an initial softening and then hardening behavior. This behavior is consistent with recent literature on reduced‐length rods supported by traditional grids that feature both springs and dimples. Additionally, there was a nonlinear damping increase observed around the fundamental resonance frequency. To simulate the response to forced vibrations, a numerical model based on a single‐degree‐of‐freedom nonlinear equation was employed. The model includes a Bouc–Wen stiffness term, quadratic and cubic stiffness terms, and an equivalent viscous damping which is a function of the vibration amplitude. Notably, when dimple‐less grids were used, the resulting fuel rod vibrations exhibited greater damping compared with traditional grids. These findings contribute to our understanding of fuel rod behavior and provide insights for grid design. The interactions between fuel rods and spacer grids were also investigated using an experimental setup capable of measuring the force exchange and sliding relative motion at the interface of the fuel rods and spacer grids. The main conclusions from these tests indicate that the maximum amplitude of force and slippage at the boundary is directly related to the maximum amplitude of excitation and displacement experienced by rods. When dimple‐less grids are used, lower amplitudes of interaction and sliding are reached. This reduction may be beneficial in mitigating fretting‐related issues. Furthermore, in the time domain, evidence of detrimental phenomena—such as impacts and losses of contact—is also weaker with the novel Framatome dimple‐less grid design.