Progressive deformation of nuclear fuel assemblies that occurs during a succession of irradiation cycles within PWR is at the core of numerous interactions. Progresses in the study of this phenomenon resulted from more and more complex models. To date, in the more advanced models, a mechanical model is combined to a hydraulic model of the core to address the fluid-structure interaction. A complete mechanical model of the core is generally built with a simplified finite element representation of each assembly using beam or simpler elements. The simplifying approach is required due to the high complexity of the multi-body multi-physics calculations. In order to improve the representativeness of the core model without penalizing the computational cost of the coupled simulations, a new method of model order reduction is proposed. It is specially adapted to this context and borrows the concepts of NTFA non-linear homogenization. The theoretical foundations and the application to a slender structure are presented. First, the method is validated on a simple case with spatially homogeneous characteristics. Finally, the case of the creep response of a fuel assembly under realistic heterogeneous loads is demonstrated.