The paper addresses the description of computer model for the spent fuel assemblies storage system in SCALE and MCNP codes, as well as the results in selection of conservative assumptions made to justify the nuclear safety of moving, transport and storage operations with the VVER-1000 spent nuclear fuel (SNF) in designed Centralized Spent Fuel Storage Facility (CSFCF). When justifying the nuclear safety, it is necessary to confirm that the maximum value of the effective multiplication coefficient K eff in SNF storage systems is kept below specified limit of 0.95 in any design-basis operation mode. The paper presents calculation results and analysis on the selection of the most conservative conditions of neutron multiplication leading to the maximum value of Keff.