Thermomechanical effects, irradiation, and structural restrictions result in very tangled behavior of assemblies in sodium-cooled fast reactors (SFRs). Reactivity feedback caused by the assembly behavior (deformation or distortion) is one of the key parameters in the inherent safety analysis of fast reactor systems. However, to date there has been no accurate and efficient deterministic way to compute directly the reactivity changes by actual local perturbation. This paper evaluates the feasibility of applying the Galerkin finite element method (GFEM) based on linear shape functions to estimate reactivity changes due to local core deformations in SFRs. Assessment of reactivity changes is conducted for six types of deformation scenarios of the two-dimensional prototype Gen-IV SFR. Uniform expansions and local deformations are included in the scenarios. The results from the multigroup diffusion equation based on the GFEM are compared with references calculated by MCNP5. The study shows that diffusion analysis based on the GFEM with linear shape functions can properly estimate reactivity changes by core deformation in the fast reactor with ~13% relative error of Δρ.