Sodium-cooled fast reactors are very sensitive to core deformation because of their hard spectrum. Fast reactor core is subjected to deformations caused by assembly bowing due to thermal or irradiation-related stresses or also by seismic events. Core assemblies are usually supported by lower grid plates ; their interactions in the upper part are controlled by the restraint core system. This system has to be accurately design in order to avoid uncontrolled compaction and consequent positive reactivity injection, but also negative reactivity effects, caused by radial flowering of the core. In this work, we will analyze different deformed configurations of the core of an axially heterogeneous SFR in order to evaluate the corresponding reactivity changes evaluated with a Montecarlo code and a mechanical code using diffusion to solve neutron balance equation.