This paper describes the calculation algorithm, software package and visualization of the three- dimensional neutron field deformation according to the archive of the RBMK (high-power pressure-tube reactor) reactor operational parameters. The determination of the field deformation is based on finding "natural" functions for approximating the readings of in- reactor control discretely located sensors. This paper considers the use of generally accepted harmonic functions and "natural" functions for describing the spatial deformation of the neutron field. It is shown that to describe the deformation of the neutron field, it is sufficient to use only a few "natural" functions, in contrast to the use of all harmonic functions from a given set. This, in turn, opens up new possibilities, in particular, for solving the problems of predictive diagnostics of in-reactor control sensors.