Three-dimensional finite element models of a fast breeder reactor’s above-core structures have been incorporated in a finite element program SAFE/RAS (Safety Analysis by Finite Elements/Reactor Analysis and Safety Division). Both material nonlinearities and geometric nonlinearities are included. Arbitrarily large rotations are treated by defining the orientations of nodes by unit vectors and the deformable elements are treated by a co-rotational formulation where the coordinate system is embedded in the elements. The time integration is carried out by the central difference method. The code agrees well with semi-analytical results for elastic beam buckling. Comparison of code predictions for the magnitude of upward displacement and lateral deformation of the above-core structures support columns with scaled experiments involving significant plastic buckling also shows good agreement, which indicates the suitability of this model for coupled structure hydrodynamics computations of core disruptive accidents.