The article presents the use of the new Monte Carlo Serpent code for 3D modeling of the WWER-1000 reactor core. Core models for the first loading of RNPP4 and the 28th loading of SUNPP3, the fuel assemblies’ models of different manufacturers were developed and presented. Considerable attention was paid to the detailed modeling of the upper, lower and side reflectors. Validation calculations of the Monte Carlo Serpent code for the WWER-1000 reactor were performed on the basis of the first RNPP4 loading. For the 28th loading of SUNPP3, albedo coefficients for radial and axial reflectors were obtained.