This publication complements existing IAEA publications and provides a methodology for evaluating the adequacy of design safety margins against external events based on the performance goals applicable to a nuclear installation. The aim is to provide procedures which can be used to assess if protection against external hazards achieved by the design corresponds to the design safety requirements established after the Fukushima Daiichi accident. It is intended for use by regulatory bodies, designers, operating organizations, vendors, research institutes, and technical support organizations working in the area of nuclear safety.