Nuclear reactor operation is the process of initiating, controlling, and sustaining a controlled nuclear fission chain reaction within a designed system to generate thermal energy for subsequent conversion. As a research concept and field, it investigates the physics, engineering, and safety principles governing the achievement and maintenance of criticality, thermal management, fuel cycle dynamics, transient behavior, and the implementation of safety systems. Key characteristics involve precise control of reactivity, efficient heat removal, robust system integrity, and comprehensive operational procedures, with its significance rooted in enabling large-scale, reliable energy generation and advancing nuclear technology and safety protocols.