The IAEA requirements document divides external events into six major groups:

• Earthquakes and surface faulting

• Meteorological events

• Flooding

• Geotechnical hazards

• External human-induced events

• Other important considerations.

Earthquakes and surface faulting

The requirements clearly indicate that ‘the hazards associated with earth­quakes shall be determined by means of seismotectonic evaluation of the region with the use of the greatest possible extent of the information col­lected’, while the selected site requires an analysis of the fault capability existing there.

The earthquake and surface faulting requirements have been extended into several safety guides; there is a dedicated safety guide on seismic hazards on site evaluation (IAEA, 2010), where details on how a regional seismotectonic model and local surface faulting can be developed and how to quantify the seismic hazard by using deterministic and probabilistic approaches. These data serve to define the operating basis earthquake (OBE) and the safe shutdown earthquake (SSE) which constitute the basis of the plant’s seismic design.