The dissertation is devoted to developing safe operation conditions and ensuring energy production controllability and stability in nuclear power units (NPU) during transient processes in the reactor by improving mathematical models and methods for optimizing automated control systems of NPU with VVER-1000, using boundaries of representation of internal disturbances of the core.
The first section, “Modeling of automated process control systems for energy facilities: Energy Challenges and Trends”, analyzes the state of nuclear energy globally, examines NPP control system problems, automation trends, safe operation technologies, risks of emergencies, and options for autonomous NPP control, particularly neural networks and deep learning, algorithms and IT for parametric optimization, and models and methods for automated NPU control.
The second section, “Mathematical modeling of NPU with a pressurized water reactor as a control object”, presents the development of a three-dimensional mathematical model of the core. This model enables automated control in real time, considering homogeneous and heterogeneous neutron absorber characteristics, which helps maintain electrical power and axial offset. The NPU mathematical model includes models of the reactor, steam generator, turbogenerator, and other systems. The reactor model is considered as a distributed multi-zone model, where the control actions are boric acid concentration and control rods position. The kinetics mathematical model considers the fission reactions of 235U and 239Pu nuclei, providing an accurate reproduction of reactor dynamics. The energy release model considers nuclear fission energy, and the heat transfer model details thermal processes. Reactivity effects models consider the control group influence, boric acid concentration, power, and temperature changes, allowing reactivity disturbances to be analyzed and controlled. The steam generator model describes heat, steam formation, and relationships between parameters such as feedwater volume, steam mass and volume, temperature, thermal effects, steam pressure, and flow rate. The turbogenerator model covers unit dynamics, considering changes in generator power, steam pressure, and turbine rotor speed. The coolant lag model considers coolant movement speed and its impact on thermal processes. The presented models are essential tools for research and NPU control system improvement.
The third section, “Simulation modeling of NPU control with VVER-1000 under internal and external disturbances”, presents a simplified reactor core model, divided into zones by altitude layers, sectors by 60° symmetry segments, and fuel assembly sections within the sector by operation term. A complex simulation model of NPU as a control object includes models of the reactor, steam generator, turbogenerator, and coolant lag in pipelines. Static programs for NPU power control were considered, systematized, and analyzed, with results presented in tables, aiding in choosing a control strategy. The automated control method of planned NPU power changes, which has been further developed, consists of three control loops: one maintains reactor power change through the equilibrium model of boric acid concentration in the coolant, the second maintains axial offset by adjusting control rods, and the third controls coolant temperature by adjusting turbogenerator valves.
In the fourth section, “Improved automatic power control system of NPU with VVER-1000”, the research goal was achieved. Different approaches to reactor control in maneuvering mode are considered, indicating the effectiveness of axial offset control, xenon transient reduction, and water exchange minimization. An automated control system structural diagram for cyclic loading has been developed, considering the physical-mathematical and approximation models of the object for three static control programs. This made it possible to identify effective control strategies, ensuring core stability and an optimal control system structure. The improved computer system for NPU automation ensures stable and controlled energy release by reactor core volume, minimizing external and internal disturbances. The boundaries of using physical-mathematical and approximation models for simulation modeling of the automated control system for changing power were identified. Increasing permissible deviation from calculated reactivity values contributes to the entry of reactivity values obtained with the approximation model into the deviation corridor, which is the object of studying boundaries of using core internal disturbances to ensure balance between accuracy of the simulated values and time of process modeling. These results can be used in future research and development to enhance NPU efficiency and reliability.
