The advanced fast reactors of the fourth generation should be capable to breed their own fuel from poorly fissile 238U and to recycle the actinides from their own spent fuel. However, this recycling or actually the closure of fuel cycle has negative impact on the safety parameters. The goal of this work is to develop a numerical tool, which can simulate and confirm the capability of these reactors to operate with closed fuel cycle, and which can evaluate their safety parameters. The tool is named equilibrium fuel cycle procedure for fast reactors (EQL3D) and is based on the ERANOS 2.1 code platform. Equilibrium cycle or virtually equilibrium method for considering the homogeneous recycling of actinides is a known approach; however, in EQL3D the equilibrium method is newly applied for hexagonal-z 3D and r-z 2D core geometries and typically 33 energy-group neutron-flux calculations. The utilization of hexagonal-z 3D geometry enables to characterize the equilibrium cycle for complex reloading patterns within a multi-batch scheme. Furthermore, EQL3D enables comparison of the advanced fast reactors on a common basis of their equilibrium cycle reactivity swing, fuel composition, breeding gain and safety-related parameters. The Gas-cooled Fast Reactor (GFR) was selected for verification and optimization of the EQL3D procedure. The GFR geometry was based on an international neutronics benchmark with a simple setup and potential for latter upgrade. It was used to show the impact of several EQL3D options e.g. different isotope evolution models, geometry selection, or cross-section recalculation frequency, on the equilibrium parameters. The results demonstrate the capability of the procedure to calculate the equilibrium fuel cycle for advanced fast reactors. Among others, also the ability of GFR benchmark core to be operated with closed fuel cycle is shown.