The efficiency of human-machine interaction is highly dependent on the quality of information support that the power unit (PU) operators receive from the control system in different operating modes of NPP PU. The paper shows the importance of intelligent support systems (ISS) for NPP operators to ensure the reliable operation of the NPP power unit. The most significant elements of the ISS for operators in terms of the diagnostic system are the block of dynamic modeling, which anticipates the course of technological processes, and the expert system. The paper analyzes the state of the problem of creating dynamic modeling systems, singles out a "three-step" approach to creating the predictive models of a technological process, and shows its shortcomings. Relationships between dynamic modeling and operational diagnostics (OD) are presented. A group of standard diagnostic systems and classes of problems of the solvable problems of ODs are considered. The implementation of the solution to the problem of diagnosing in the intelligent support system of the NPP operator, as well as functional diagrams of operational diagnostics, is presented. The proposed implementation of the tasks of operational diagnostics is used to create intelligent support systems for NPP operators.