Nonlinear Vibrations of Model PWR Fuel Assemblies: Part 1 — Experimental Setup and Measurements

Nuclear fuel bundles in PWR reactors present vibrations due to coolant flow, which may result in fretting at the interface between the fuel rods and the retaining elements, named spacer grids. Seismic excitation may also occur during accidental events, such as earthquakes. In this perspective, forced vibration experiments were performed on a reduced-scale nuclear fuel bundle provided by Framatome Canada.

The presence of partially loose fuel pellets inside the fuel rods was provided in the experiments. A maximum coolant flow of 5 meters per second was reached inside a water tunnel. The dynamic effects of the periodic bundling of coupled fuel rods were also taken into account. A no-contact measurement system based on laser Doppler vibrometry and a stepped-sine excitation technique with closed-loop control of the forcing amplitude were employed.

The identification of vibration parameters was attempted in the linear regime, through modal analysis, and in the nonlinear regime, through a single-degree-of-freedom method based on harmonic balance. The value of the equivalent damping parameter was shown to increase strongly with the amplitude of the excitation and of the vibration, thus acting in the direction of safety. The fuel bundle presents moreover a peculiar softening vibration behavior in the nonlinear regime, with a marked decrease of the peak vibration frequency. The comparison with other recent experimental campaigns shows that the boundary conditions constituted by spacer grids have a predominant effect on stiffness and damping during nonlinear vibrations. At the interface between fuel rods and spacer grids, in fact, nonlinear spring and unilateral contacts, with friction and impacts, are present. Therefore, the characterization of the boundary conditions at the spacer grids was attempted by means of dedicated experiments.

Bending, shear and axial slippage modes were tested in the frequency range between DC and 50 Hertz. Tests were repeated in presence and in absence of water. The resulting force-displacement loops clearly show the presence of hysteresis and of bilinear stiffness. The decrease of stiffness with the amplitude of the imposed displacement is compatible with the transition between different support configurations and between static and dynamic friction. The decrease is also compatible with the extreme softening behavior detected during nonlinear vibration experiments. The availability of a mathematical model for the stiffness and the damping at the boundary conditions will be indispensable for the future development of reduced-order models describing the linear and nonlinear vibrations of PWR fuel bundles. Such models will eventually allow PWR designers to predict the severity of vibrations during normal operation and during accidental events.