A nuclear or chemically hazardous facility located near an airport or near an air corridor may need to undergo a probabilistic risk assessment for an accidental aircraft crash onto the facility. Such a risk assessment involves calculating theprobability of an aircraft impacting a given facility and the adverse consequences of such an impact to the workers as well as to the offsite public. Over the last three decades, primarily in connection with the safety evaluation of nuclear power plants,some standardized methods have evolved for calculating the probability of an aircraft impact onto a given facility. For many facilities, the evaluation of risks from accidental aircraft impact ends with this probability calculation when statistically itcan be shown that such an impact is not credible (typically if the probability of impact is less than 10{sup}-7 per year).However, for facilities near airports or Air Force bases, it is not always feasible to demonstrate that accidental aircraft impact is incredible. For such cases, the primary method of demonstrating the facility safety is to show that the buildingstructure is an effective barrier against an aircraft impacting a safety related structure, system or component (e.g., containment or confinement system) inside the building. If the building structure can withstand the impact safely, a series of costlyand cascading consequence analyses can be avoided. But, since a building structure can typically get impacted by aircrafts with varying physical characteristics and velocities, the impact analysis is typically performed assuming an ultraconservativecombination of aircraft weight and velocity. As a result, impact analyses often conclude that the structure is not an adequate barrier against an aircraft impact. To avoid such unrealistic analyses, this paper proposes a simple probabilistic method ofdetermining the design basis hazard. The method uses site-specific aircraft types and their relative flight frequencies, but generic velocity and impact angle distributions, to develop a site-specific aircraft impact hazard curve or table. Guidelines forselecting design basis aircraft missile(s) are also provided.
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