The Fukushima Daiichi accident showed the need to explore scenarios where external hazards exceed the design basis. Knowledge of plant behaviour along those scenarios helps improve global safety, since the weak points can be identified and measures to limit the progression of potential accidents or to mitigate their consequences can be introduced. Based on an IAEA methodology document for Member States issued in 2011, the current publication expands the previous version by giving a more comprehensive approach and introducing the enhancements that could be identified from the research developments and practical applications in the recent years.