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[en] The safety of fuel loading of VVER reactors is justified by calculations of the neutronic characteristics of the forthcoming campaign. These calculations are based on the design parameters of fuel assemblies (FA) — fuel enrichment, materials, design features, etc. However, during operation, some parameters change in an uncontrolled manner. In particular, FA can deform — bend or twist, this leads to the appearance of increased gaps between the fuel assemblies. These regions filled with a moderator lead to an increase in comparison with the calculations for the generation of thermal neutrons and, as a result, to a surge in the power of the fuel rods surrounding these regions. Safety requirements limit the power of fuel rods. Therefore, design capacities are increased by means of the so-called engineering margin factor to account for random outbursts. The deviation of the size of the water gaps between the fuel assemblies from the design ones should be known to calculate this coefficient, for example, the size distribution function of the, gaps. This information is most often obtained by modeling the mechanical state of fuel assemblies in the, reactor core. Other approaches are based on experimental data. Measurements in the core during discharging campaign are not possible. Therefore, the geometric parameters of the fuel assembly after the the, from the core are measured. The presented paper uses the data of such measurements obtained after 24th fuel campaign of ZNPP unit mechanical It is assumed that the fuel assemblies tend to retain the form they have in the “free” state, and analyzed., interaction with neighboring fuel assemblies leads to a certain equilibrium state that can be easily proposed In contrast to similar calculations, the elastic energy functional of interacting fuel assemblies is were, whose minimum gives the required size distribution function of the gaps. 24 and 25 campaigns studied., modelled; the role of inter-sector FA shuffling was the, The distribution of the gaps between the fuel assemblies in VVER-1000 core is calculated based on of, measured deformations of the fuel assemblies discharged from the core and the elastic characteristics not, the fuel assemblies. It was demonstrated that 95 % of gaps in the cores both with FA-A and FA-WR do the, exceed 7.6 mm. The results can be used to calculate the engineering margin factor in determining release., peaking factors of energy release.