The total system design of a nuclear power plant ensures "Nuclear safety" through making practically achievable efforts to prevent and mitigate nuclear and radiological accidents. The performance based system design with "Defense in Depth (DID)" has been laid out as the key means to "preventing accidents", "controlling escalation to serious consequences", and "mitigating impact to the public". DID is extended to the management of severe accidents, and is an approach intended to provide protection against the development of a wide variety of events by means of redundant, diverse and independent protective barriers.

By combining the concepts and risk assessment discrete "Defense in Depth", it can be expressed as a risk level corresponding to the degree of each layer of the DID. A Situation that exceeds the normal operation becomes an abnormal situation in which more than even the design criteria, various AM measures are taken. The ability of the resilience to recover the function until the required function, and quantified in risk level as a safety corresponding capability of the system, are expressed as a resilience index.

By using the resilience index, safety capability of the plant system can be quantitatively grasped, that whether the safety by the AM measured is how much improvement, what is good to select which means, etc. it can be used for the decision.