[en] An arbitrary Lagrangian-Eulerian (ALE) finite element method for analyzing the transient, nonlinear fluid-structure interaction in fast-reactor core subassemblies is presented. The method combines the basic attributes of the finite element technique -namely, ease in modeling complex geometries and in mixing fluid elements with structural elements- and the flexibility in moving the fluid mesh offered by the ALE description. The result of this combination is a very versatile modeling technique which permits accommodation of large fluid and structure displacements and logically simple, but accurate fluid-structure coupling. Numerical results are presented to illustrate the effectiveness of the proposed method. These include the response of a single hexagonal duct to internal, static or dynamic, pressure loading for which numerical predictions are compared to experimental data, and applications to clustered hexcans