This standard presents guidelines, which will allow power reactor designers to select (1) combinations of natural hazards, (2) combinations of external man-made hazards, and (3) combinations of natural and external man-made hazards to be used in the design of power reactor structures, systems, and components. The hazards to be considered in these categories are identified. Probability level acceptance criteria are defined which will enable designers to select the combinations of hazards which are design bases for a particular site. Methods for calculating probabilities of combinations are discussed.

This standard does not address procedures for plant walkdowns immediately (within 8 hours) after an earthquake, for ensuring a safe and orderly shutdown, for long-term evaluations of the building and equipment response data, and for subsequently returning the plant to operation. These topics are addressed in ANSI/ANS-2.23-2002.