Nuclear safety criteria are based upon the concept that plant situations that are expected to have a high frequency of occurrence must not pose a danger to the public, and that plant situations that pose the greatest danger to the public must be limited to situations that have a very low expected frequency of occurrence. This concept is implemented by grouping plant situations (or events) into categories that are defined according to their expected frequencies of occurrence. It is important not to allow the boundaries of these categories to be crossed. Plant designs cannot allow low-consequence events of high expected frequency to develop into events of more serious, high-consequence categories.

The development of this system of frequency-based categorization is discussed, followed by an evaluation of various methods that have been proposed and applied to show how minor events could be prevented from becoming major events.

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