The requirements consider three types of external events: aircraft crashes, chemical explosions, and other important human-induced events. The requirements consider only accidental events of that type; terrorist attacks by the same means with the only purpose of producing harm are excluded.

The currently high level of air traffic and the expected increase in the future may represent a substantial hazard to NPPs, despite the high level of safety in current and future air flights. The hazards due to the crash of a large passenger airplane include the impact itself, the explosions and large fires that may be produced. The risk can be reduced by avoiding having flight corridors close to NPPs.

The proximity of chemical installations and transportation routes also creates risks of explosions, deflagrations and large fires with the correspond­ing release of toxic substances that may affect the safety of the operating NPP; compensatory measures should be applied or the site discarded. Other important human-induced events include large fires, for instance forest fires, collisions of ships with water intake structures, and the presence of electro­magnetic waves with the potential of affecting the plant information, control and instrumentation systems.

An IAEA safety guide describes the many external human-induced events to be considered in the evaluation of a site for a NPP (IAEA, 2002a). The guide considers each one of the events, defines the associated hazards and determines how to obtain the main parameters to be used in the design basis of the plant to cope with such hazards.