The article discusses the approach conformity with which the flexible software package (FMSP) is the kernel of the digital twin of an NPP power unit. The models that are part of the FMSP cover the main and a number of auxiliary technological systems of the power unit, automatic control systems. The developed flexible modeling software package is designed to simulate dynamic processes occurring in NPP power units with VVER-1000 reactors. The FMSP allows considering various types of violations in the operation of safety systems and energy imbalances in the reactor core. The results of verification of a flexible modeling complex based on stationary neutron-physical calculations of the VVER-1000 reactor core using the BIPR-7 program and NPP operational data are presented.