This PhD work is part of the study of the dynamics of fuel assemblies subjected to axial flow and external mechanical excitation, particularly of seismic type. The objective is to develop an innovative numerical approach capable of accurately predicting the three-dimensional dynamic response of one or several assemblies, while accounting for the coupled effects between the fluid flow and mechanical loads. This problem is particularly complex due to the need to consider large displacements, potential contacts between structures, and strong interactions with the surrounding fluid.