A new analytical method is introduced to determine the distortion and mechanical behavior of fuel-pin bundles in a wire-wrapped fuel subassembly for liquid-metal fast breeder reactor cores. Each fuel pin is considered as an elastic beam using a model that takes wire tension effects into account. To represent the nonlinear stiffness resulting from contact between the pins and the duct, a fictitious element (the joint element) is inserted at the point of contact. This element can also represent friction effects. A substructure method and a block successive overrelaxation method are used to reduce computing time and memory requirements. This analytical method was incorporated into a three-dimensional finite element code called ÉTOILE. Sample calculations are presented that show that this code is capable of a reasonable simulation of the mechanical behavior of a fuel-pin bundle during irradiation. In particular, it has been shown that wire tension has a significant effect on the fuel-pin bundle equilibrium configuration.