Experimental Characterization of the Nonlinear Boundary Conditions Applied by Two Different Designs of Spacer Grids on PWR Fuel Rods

Spacer grids in Pressurized Water Reactors (PWRs) hold fuel rods in place by means of springs and rigid stops during the operation of the reactor core; in addition, they are used to improve the mixing of the coolant, improving heat transfer. The constraint exerted by spacer grids influences the amplitude of the vibrations reached by fuel rods under the action of the turbulent coolant flow; in turn, these vibrations may result in Grid-To-Rod Fretting (GTRF), the main cause of failure for nuclear fuel rods. As a result, considerable effort is dedicated to the design of spacer grids. In this perspective, the boundary condition exerted by two prototypical spacer grids on fuel rods was characterized experimentally. First, a traditional spacer grid, employing both compliant and stiffer elements (springs and dimples) to retain fuel rods was tested; afterwards, an innovative spacer grid, employing springs only, was tested. The rotational constraint on the bending of fuel rods was measured imposing angular displacements varying sinusoidally in time to rigid tubes inserted in the spacer grids. The spacer grids were immersed in water and the direction of the excitation was varied with respect to the spacer grids. The displacements were measured by means of Laser Doppler Vibrometers (LDV), while the resulting alternating compressive forces were measured through a load cell installed on the electrodynamic exciter that applied the time-varying displacements. Force-displacement loops revealed in both cases a hysteretic behavior described well by nonlinear hysteretic models such as Caughey’s bilinear model. The behavior of either spacer grid is not affected by the frequency of the sinusoidal excitation, but it is affected strongly by the amplitude of the latter. In particular, the hysteresis area increases substantially with the amplitude of displacement for both spacer grids, while the terminal stiffness decreases for spacer grid 1. Such nonlinear conditions may be related to the strongly softening behavior shown by the large-amplitude vibrations of fuel rods supported by spacer grids, which happen at lower resonant frequencies and with larger damping values for increasing vibration amplitudes. The inclusion of hysteretic boundary conditions improves substantially the simulation of the forced vibrations of fuel rods supported by spacer grids. The vibrations of fuel rods supported by dimple-less spacer grids result much more damped than those if fuel rods supported by traditional spacer grids. Additional damping acts in the sense of safety and may be related to a larger hysteretic dissipation at the boundary conditions when dimple-less spacer grids are used.