Fuel assembly damping for accident studies: An analytical approach

The mechanical accident studies call for the use of assembly mechanical models, which must be fitted to reactor conditions. In such model, the assembly damping is of a first importance for assembly lateral response under seismic conditions. Different tests have shown that this damping is mainly due to fluid structure interaction forces, which are added to the mechanical damping of the structure. The purpose of this paper is to present an analytical approach which helps understanding the different phenomena involved in the hydraulic damping. Calculations were performed for harmonic movement of the rod bundle with parameterization on frequency and amplitude X = X0 · sin(ω · t) in a range covering the assembly movement during seismic event. These calculations were performed with and without axial flow. The results show that the fluid structure interaction forces can be expressed by: F = F0 ·cos(ω·t + φ), from which can be deduced the water coupling mass and the water damping. (author)