This paper presents calculation results to determine critical core configurations and a minimum number of fuel assemblies (FAs) or uranium mass of a research reactor loaded with three types of FAs such as MTR, IRT-4M and VVR-KN. The MCNP5 code and ENDF/ library were applied to estimate characteristics parameters of the fuel types and the whole core. | Calculation of critical core configurations of a research reactor using MTR, IRT-4M, VVR-KN fuel assemblies