Vibrations and damping identification of non-linear systems under periodic excitation for nuclear applications

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A significant technical issue identified by AREVA's fluid-structure interaction technical team is related to the vibrations and fluid-structure interaction of fuel rods, found in fuel assemblies, subjected to seismic excitations. Fuel rods are pressurized, long slender metal tubes containing pellets of fissionable material that are assembled into bundles, or fuel assemblies supported by metallic grid structures (spacer grids). During the irradiation cycle the connection between the rods and the spacer grids relaxes allowing for oscillations and subsequent impacts between the two structures. Excessive vibrations caused by seismic excitations, large pipe breaks, etc., could lead to fretting between the rods and the spacer grids that could eventually fail the safety barrier between nuclear fission material and the coolant. Therefore, recognizing the mechanisms that dissipate energy in fuel assemblies and fuel rods is of paramount importance for nuclear engineers in order to make sure that fuel assembly components can withstand and remain functional during and after the worst earthquake condition scenarios. Studies performed by Professor Amabili's team demonstrated a substantial increase of the structural damping ratio as the amplitude of the oscillations increases. The proposed research plan continues our previous collaboration with AREVA Canada (projects EG 462720-14 and EGP 476586-14) consisting in designing a numerical procedure to identify the non-linear behavior of structural damping embedded in complex systems. This research project will (i) investigate and extend the non-linear damping identification methodology by performing experiments and advancing the theoretical formulation of the methodology to include different types of geometrical non-linear vibrational responses. (ii) The results from these experiments will be used to validate the ability of the enhanced parameter identification methodology to describe the evolution of non-linear damping and stiffness of fuel assembly components. This will assist in the validation and performance evaluation of these components under seismic or loss-of-coolant conditions increasing their safety and improving competitiveness of the final nuclear fuel product in the Canadian nuclear market.