Structures, systems and components to be protected against external events

STRUCTURES, SYSTEMS AND COMPONENTS TO BE PROTECTED AGAINST EXTERNAL EVENTS 

In order to meet the safety requirements for protection against the external events selected in the site evaluation phase, a classification of the plant items is useful to provide a rational basis for design. In addition to the safety classification, an external event (EE) classification of items may be developed so as to identify the system requirements in a DBEE (for design and maintenance), to estimate the consequences of their failure and to develop 

The external event classification, if applied, does not imply different load levels for the external event scenarios and therefore the design of items classified for external events should refer only to the extreme values of DBEEs (as identified in Refs [2-4]), or to a combination of DBEEs where at least one of them is taken at, or close to, its extreme value. However, lower load intensities could be used in the design for two main reasons  

When different levels of the same external events are considered in the same load combination, the acceptance criteria of the items classified for external events should refer to the DBEE which in the combination is assumed 

In some cases probabilistic considerations on the probabUity of the load combination and its associated risk can justify less stringent acceptance criteria 

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