Digital modeling technologies’ development and integration into modern engineering solutions play a key role in improving the efficiency and reliability of complex technical systems, such as nuclear power plants (NPP). The paper is devoted to the urgent task of creating digital twins of units of NPPs with a VVER-1000 reactor based on flexible modeling complexes. Digital twins have the potential to substantially improve operational monitoring, diagnostics, control, and the forecasting of process system states, which is especially important in addressing the growing requirements for safety and efficiency in nuclear energy production. The paper examines the current state of digital modeling technologies, highlighting their growing role in the nuclear energy sector, and discusses the primary advantages and disadvantages of existing solutions. The main emphasis is placed on the concept of a flexible modeling complex, which serves as a fundamental element in the development of digital twins. It is emphasized that the flexibility of the architecture of this complex enables it to adapt to various operating modes of units and changes in their performance characteristics. The paper describes the structure and principles of operation of a flexible modeling complex, including control and information flows implemented in its architecture. The validation and verification of the modeling complex, based on experimental data, confirmed its high accuracy and applicability in solving digital modeling problems.