A new analytical method is introduced for determining the core distortion and mechanical behavior in the fast reactor. In this method, a folded plate structural model is used to describe each single hexagonal subassembly duct. To represent the nonlinear stiffness due to the contact between neighboring surfaces, a fictitious element (the joint element) is placed on each contact surface. The element also has the ability to represent friction effects and to describe the state of partial, or angled, contact. As for the numerical procedure, a substructure method and a block successive overrelaxation method are employed to reduce computing time and storage spaces. The analytical method was implemented in a three-dimensional finite element method program named ARKAS. Some sample calculations were performed, and it was shown that the program can be an effective tool for analyzing or evaluating core mechanical performance due to thermal expansion and irradiation-induced swelling and creep.