This lecture covers the fundamental concepts of nuclear engineering, including reactor technology, nuclear physics, fuel cycle, reactor physics, reactor kinetics, thermal hydraulics, radiation protection, nuclear safety principles, design basis accidents, and advanced reactor systems. It also discusses the structure of light water reactors, fission reactions, fission product composition, delayed neutrons, reactivity variations, and safety measures in nuclear power plants. The instructor presents the physical barriers, decay heat, and the overall safety concept in nuclear reactors, emphasizing the defense-in-depth approach and the containment system.