The objective this thesis is the application of genetic algorithm techniques to optimize the few-group energy grid for the multiphysics simulation of Lead-cooled Fast Reactors (LFR). The identification of a proper energy group structure to collapse the group constants to be used in full-core simulations is a crucial step: such energy structure has to be knowledgeably chosen as it may strongly affect the accuracy and efficiency of the multigroup calculation results. The energy grid structure to be used must be selected with the intention of preserving the physics of the problem, which is at the basis of accurate results, and to minimize as much as possible the computational effort involved in the simulation.