Methodological program techniques for performing neutron-physical calculations of fast-reactor cores with a description of the main codes TRIGEX, JARFR, GEFEST, MMKKENO, and ModExSys are described. The results of verification and assessment of the methodological errors on the basis of a test model of BN-600 are presented. Transport effects and mesh errors in the reaction rate distributions in the core, lateral screen, and in-reactor storage area are evaluated. An integral assessment of the computational accuracy is performed by comparing with the experimental data obtained by γ-scans of BN-600 fuel assemblies. It is shown that calculations describe the experimental data over the core with an error no worse than 5%. In the lateral screen and the in-reactor storage area, the discrepancy between the calculations and experiment does not exceed 20-30%.Computational modeling of the neutron field is the main method of determining the energy release in a fast-reactor core, where, as a rule, there is no apparatus for tracking the power density. Correspondingly, computational accuracy is one of the key elements of the operational reliability of fuel elements and assemblies. Another aspect of the problem is substantiating the conservatism of fast-reactor core design -design limits or margins affecting the technical-economic characteristics. Experience in operating BN-600 makes it possible to evaluate the accuracy of the design and operational programs under real conditions of a commercial medium-power fast reactor and to take it into account in subsequent development work.Such an analysis is based on the measurement of the power density in BN-600 by means of γ-scanning. Eleven such experiments were completed in the time period from physical startup of the reactor in 1980 and up to 2008. The present article analyzes the last experiments performed in 2003-2006 in the course of transitioning from a 01M1 core to a 01M2 core with maximum burnup 11.1% [1]. This upgrade was an important next step in making use of the serviceability margin of reactor fuel assemblies, which required greater attention to monitoring the characteristics of the core [2].Program and Constants System for Neutron-Physical Calculations of Fast Reactors. A consistent system for performing neutron-physical calculations of fast reactors is now available. It includes the program systems TRIGEX [3], JARFR [4], GEFEST [5], and MMKKENO [6] as well as the BNAB constants library [7,8]. It should be noted that the base