Abstract: A computer-assisted method for determining an optimal core-loading pattern for a nuclear reactor core. Positions of nuclear fuel assemblies are tested to assign optimal positions and to load the reactor. The reactor core includes cells positioned symmetrically to axes of symmetry and a standard assembly for insertion into each cell. Standard assemblies are distributed by the number of previous production cycles. Groups of cell positions symmetric to the axes of symmetry are identified, and symmetric positions are counted. Families of standard assemblies having similar burnups are formed, wherein the standard assemblies correspond to positions in a group. The loading pattern of standard assemblies in initial positions is tested by numerical simulation, then the positions are swapped while maintaining the previously formed families of assemblies. The swapped positions loading pattern is tested by numerical simulation. This is repeated until at least one candidate pattern for loading the reactor is obtained.