Defense in depth principle embodied in nuclear power plant design and operation is the key principle to keep the reactor safe from mechanical failures, human errors and natural disasters by accident prevention and mitigation. The principle has been materialized based much on the system use active power. The advanced nuclear reactors are being embodied with passive and inherent nuclear safety by which the reactors are immune to failures of components, human errors and even natural hazards. To realize inherent safety in power reactors, several physical principles are used in design: physical properties of uranium fuel, gravity, pressure, and if possible the hazard itself. The nuclear reactor we are working on can be shut down no matter what the given condition is, can remove heat generated after shut down without human intervention and electric power, can limit the contamination within the physical boundary of power plant itself. Implementation of this inherent safety requires deep understanding of thermo-hydraulic phenomena, human operators’ behavior, automated actuation, high-voltage electricity transfer, and material characteristics. Balanced design using risk assessment is also required. Our research focuses on the development of innovative technologies which eliminate the possibility of any serious accident and aims to achieve inherently safe nuclear plants with longer life.