In the pressure vessel of a PWR, the core is filled of up to 241 fuel assemblies. Under operation, they are subjected to irradiation growth, creeping, tightening relaxation and rod slipping. Combined with the hydraulic forces induced by the coolant flow, these phenomena lead to assembly lateral deformation. Between two operating cycles, when changing the assembly position in the core, this deformation may sometime create difficulties with regard to handling operations. In order to prevent this situation, it would be useful to simulate the thermomechanical behaviour of the core, taking into account the previously mentioned phenomena. For that purpose, we present a 3D modelling approach in which each fuel assembly is represented by a reduced model that uses shape functions for kinematics as well as internal variables. The constitutive equations are then expressed in the reduced basis by defining and coupling the modal thermodynamic forces. We finally analyse the accuracy and performance of that new method through a simple case.