Risk Importance Measures in the Design and Operation of Nuclear Power Plants
3. Deterministic Principles of Safety Importance and Need for Consideration of Risk
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Traditional view on “importance” of a particular SSC was associated with its role (function) in ensuring appropriate safety margins (and, hence, low undesirable consequences) in the case of anticipated or credible occurrences. The “importance” of an SSC was related to its quality requirements resulting in a set of deterministic rules and principles for the classification of SSCs into quality groups such as “A”, “B”, “C” and “D” (“Class 1”, “Class 2”, “Class 3” and “Non-Class”). For example, the highest importance with regard to ensuring all safety margins concerning the cooling of the reactor core was given to the integrity of the primary system. Hence, the components of the reactor coolant pressure boundary were assigned Quality Group “A” (or “Class 1”). In the case of primary system integrity failure, ensuring the safety margins relies on the systems with functions such as emergency core cooling and containment heat removal. Thus, those systems were assigned Group “B” (or “Class 2”); supporting systems, such as cooling water systems, to the systems of Group “B” were assigned Group “C” (or “Class 3”), etc.