This book describes the basic concepts of risk and reliability with detailed descriptions of the different levels of probabilistic safety assessment of nuclear power plants (both internal and external). The book also maximizes readers insights into time dependent risk analysis through several case studies, whilst risk management with respect to non renewable energy sources is also explained. With several advanced reactors utilizing the concept of passive systems, the reliability estimation of these systems are explained in detail with the book providing a reliability estimation of components through mechanistic model approach. This book is useful for advanced undergraduate and post graduate students in nuclear engineering, aerospace engineering, industrial engineering, reliability and safety engineering, systems engineering and applied probability and statistics. This book is also suitable for one-semester graduate courses on risk management of non renewable energy systems in all conventional engineering branches like civil, mechanical, chemical, electrical and electronics as well as computer science. It will also be a valuable reference for practicing engineers, managers and researchers involved in reliability and safety activities of complex engineering systems.