The threat posed by external hazards is considered in the safety and design of nuclear facilities. For example, facilities are typically designed to withstand a prescribed level of earthquake or flooding. Ensuring adequate protection against earthquakes is complicated by uncertainties in the characteristics of an earthquake (frequency of occurrence, severity, etc.). For several decades, one of the main areas of activity for the Working Group on Integrity and Ageing of Components and Structures (WGIAGE) of the Committee on the Safety of Nuclear Installations (CSNI) has been seismic engineering and the response of nuclear reactor structures to earthquakes

The accident at Fukushima Daiichi demonstrated the importance of considering all possible consequences of external events, including combinations of events. The reactors withstood the earthquake but were damaged following the associated tsunami. In response, the CSNI established the Working Group on External Events (WGEV) in the 2010s to improve the understanding and treatment of external hazards. In co-ordination with the established programme of work under the WGIAGE on earthquakes, WGEV focuses on other external hazards, such as riverine flooding and combinations of hazards, such as high winds and flooding.