[en] A computer model were designed to simulate the critical assembly experiment of UO₂ fuel assembly with enrichment 4.75%. Both square and hexagonal ( triangular lattices) assemblies are considered in the analysis. For square lattices fuel, pitches of 1.26 cm, 1.6 cm , 2.4 cm and 2.52 cm are tested and for hexagonal lattices, pitches of 1.35 cm , 1.72 cm and 2.26 cm are also used. The effect of water holes distributed within the assembly is considered by studding different configurations. All calculations are performed using two different neutron cross section libraries, ENDF/B-V and ENDF/B-VI. The results of the present model are compared with the experimental measurements. It was found that the calculations based on ENDF/B-VI built in library are more accurate than ENDF/B-V and both are in good agreement with the measurements