3D criticality assessment in complex systems using the Monte Carlo method; Assessment of the criticality of spent nuclear fuel systems combined with fuel combustion and criticality calculation aspects. Analysis of the true values of critical systems for sensitivity of cross sections in 1D / 3D systems. Determination of the maximum true values of the critical system and the minimum critical mass. 2D calculations of flow and combustion. Preparation of cross section libraries according to preselected dependencies. Estimation of neutron and photon flow in geometric regions and at point detectors placed outside the geometry limits.