The paper presents a mathematical model for calculating an NPP power unit's steam generator (SG) level with a VVER-1000 reactor, part of a flexible modeling system (FMS). The distributed model of an SG is designed for a more detailed description of processes in a two-phase volume of SG and modeling the readings of level gauges in non-stationary operating modes. The results of verification of the presented model as part of the FMS in the dynamic mode (disconnection of one turbopump pump from two operating ones) of the NPP power unit are presented.