This thesis presents a preliminary analysis of the accuracy of ECCO deterministic calculations applied to LFR (Lead-cooled Fast Reactor) fuel assemblies, by comparing them with reference Monte Carlo simulations. The primary objective is to quantify and understand the discrepancies introduced by deterministic modeling when applied to fast reactor systems, particularly in view of ECCO's computational efficiency compared to Monte Carlo methods. Verification was carried out using the OpenMC Monte Carlo code, which is widely accepted as a reliable and validated tool in the nuclear community. Experimental data were not available at this stage, so the comparison is limited to numerical benchmarks.