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In Pressurized Water Reactors (PWR) assemblies are exposed to challenging thermal, mechanical, and irradiation loads during operation. Global core and local fuel assembly flow fields coupled with seismic excitation result in fuel assembly and fuel rod vibrations. The fact that vibrations may become excessive in certain conditions has consequences on operational safety margins in fuel assemblies designs. In order to understand how the fuel assembly responds dynamically to an external excitation, it is important to identify the main characteristics of the structures. Among them, the fuel assembly system damping is a fundamental parameter that is usually identified by a number of experiments involving fluid-structure interaction. Recent studies have shown that the damping ratio increases with the excitation force when the structure is entering large-amplitude vibrations, in which case the geometric non-linearities have to be taken into account. The present paper presents an advanced identification procedure developed to identify the system characteristics from experimental non-linear response curves obtained from forced vibration tests, accounting for fluid-structure interaction, at different excitation levels. Furthermore, the numerical tool developed in this analysis is capable of working with systems presenting one-to-one internal resonance, i.e. systems with symmetry such as circular tubes and circular cylindrical shells. The method relies on a harmonic decomposition of the displacement to cope with the data usually available by vibration measurements.

In Pressurized Water Reactors (PWR) the fluid-structure interaction between the coolant and fuel assemblies is an important phenomenon that is directly associated to safety and performance issues for the nuclear industry. Fuel assemblies are formed by bundling fuel rods, long slender pressurized tubes containing uranium pellets, with hollow guide and instrumentation tubes using a number of spacer grids for support. In order to correctly simulate the response of a fuel assembly under external excitation, an investigation of the system damping is necessary. Recent study shows that the damping may grow significantly with the vibration amplitude, increasing the safety factor. In order to address this issue, the present study has developed a tool to identify the vibration characteristics of non-linear mechanical systems from experimental forced vibration data obtained at different excitation levels. In particular, this project focuses on the damping identification. A parameter identification methodology for non-linear systems, based on harmonic balance method, is used to identify the damping governing the motion of such systems. The tool has been developed by using hardening non-linear responses of two sandwich panel and a metal plate subjected to external harmonic excitation. The method has been validated by comparison with the damping identified by the full non-linear model of the two sandwich panels. In the three cases, an increase of damping with the vibration amplitude is found and discussed.

The interaction between fuel assemblies during a seismic or loss of coolant accident (LOCA) event is directly associated with safety and reliability issues for all types of nuclear reactors. This study concentrates on the modeling of a single fuel assembly represented by a cylinder subjected to external flow and an external forcing function at the base. The model investigates the response of the fuel assembly using continuum mechanics model. The general behavior of a cylinder supported at both ends and subjected to axial flow is summarized: The cylinder undergoes several flow-induced instabilities as the flow velocity increases. These instabilities start with a pitchfork bifurcation, resulting in the buckling of the cylinder. At higher flow velocities, period-doubling and torus instabilities are observed as well, eventually leading to chaotic oscillations of the cylinder. It is shown that an increased confinement results in lower critical flow velocities for the first point of instability, resulting in buckling at lower flow velocities. The cylinder response to a base excitation is also considered and it is shown that the response amplitude can change depending on the frequency of base excitation.

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