Chapter 2: Introduction to Nuclear Safety
Strengthened independence of levels of Defence-in-Depth and requirements on margins to prevent from cliff-edge effects
Various requirements in SSR-2/1 (Rev. 1) – including Requirements 7, 21, 22, 24, 40, 42, 46, 64, 68 – require strengthening the independence of levels of Defence-in-Depth, by keeping these levels independent as far as is practicable.
A cliff edge effect is defined in the IAEA glossary as “an instance of severely abnormal plant behaviour caused by an abrupt transition from one plant status to another following a small deviation in a plant parameter, and thus a sudden large variation in plant conditions in response to a small variation in an input”.
Requirement 7 of SSR-2/1 (Rev. 1) requires that a small deviation in a plant parameter does not lead to a cliff edge effect. The design of the plant shall provide for an adequate margin to protect items important to safety against levels of external hazards to be considered for design, derived from the hazard evaluation for the site, and to avoid cliff edge effects.
Read more:
SSR-2/1 (Rev. 1),
TECDOC-1791
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