This term is used to identify abnormal events that are initiated from outside the nuclear station. Some examples are earthquakes, tornadoes, aircraft crashes, and floods. The plant can be protected from many of these events before the fact by careful location and investigation of the potential conse­quences of their occurrence combined with the estimated probabilities. The designer is expected to provide either passive or active defence against such events; the acceptability of these provisions is one of the major components of regulatory review for approval of a nuclear station site.

Recent events (e. g. earthquakes and tsunamis at the plants at Kashiwazaki and Daiichi in Japan) have underlined the importance of seismicity in plan­ning for location of a nuclear station, and for the facilities needed for its protection. Specifically, the Daiichi situation highlights the hazards of the operating state known as ‘station blackout’, meaning the total loss of electri­cal power for an extended time period. Other external events may prove to be equally important in different situations.