Other Naturally-Occurring External Events

Initiating events are typically divided into two broad groups. Internal events include equipment failures and human errors occurring within the plant such as pipe breaks, stuck valves, damaged pumps, instrument failures, and operator errors. External events include natural and human-caused events outside the plant such as earthquakes, tornadoes and other severe weather, floods caused by heavy precipitation or dam failure, aircraft crashes, and volcanic activity. There are sometimes exceptions to the use of the plant boundary to distinguish internal from external events. For example, fires internal to the plant have traditionally been classified as external events (although many analysts now agree they should be classified as internal events). 

A DBEE should not be considered in combination with events that may occur independently, such as other external human induced events, natural phenomena, equipment failures and operator errors, unless a combination of these events is shown to have a sufficiently high probability of occurrence. In this assessment, the possibility of a causal relationship should be evaluated, according to Ref. [16]. 

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