Nonlinear vibrations analysis of a fuel rod in nuclear heating reactor

Nonlinear vibrations of a single fuel rod in nuclear heating reactor with fixed-fixed ends and subjected to various excitation levels are comprehensively analyzed using explicit dynamics. The responses of empty, pellet-filled, and water-submerged rod are investigated. Pellet-cladding and pellet-pellet interactions are addressed by a penalty-based contact treatment, meanwhile the fluid-structure interactions (FSI) between the vibrating fuel rod and surrounding coolants are simulated by the Arbitrary Lagrangian-Eulerian (ALE) method, wherein the water is modeled as Eulerian elements. The results reveal that both empty and pellet-filled rods exhibit frequency hardening phenomena, where eigenfrequencies increase with vibration amplitude due to geometric nonlinearity. The damping decay resulting from fuel pellet motions is found to be positively correlated with vibration amplitude, thereby enhancing the reactor safety. For submerged rod, the frequency decreases due to added mass effects, and the overall system damping increases despite structural damping from dry friction and impacts is reduced. •Nonlinear vibrations of a single NHR200-II fuel rod are analyzed using explicit dynamics.•Both empty and pellet-filled rods exhibit frequency hardening.•Damping decay resulting from fuel pellet motion positively correlates with vibration displacement.•In water, added mass effects reduce frequency, yet overall system damping increases despite diminished structural damping.