According to IAEA*1,
The concept of defence in depth, which concerns the protection of both the public and workers, is fundamental to the safety of nuclear installations. As was stated hi Basic Safety Principles for Nuclear Power Plants (INSAG-3) in relation to the safety of nuclear power plants, “All safety activities, whether organizational, behavioural or equipment related, are subject to layers of overlapping provisions, so that if a failure should occur it would be compensated for or corrected without causing harm to individuals or the public at large. This idea of multiple levels of protection is the central feature of defence in depth…”
The levels of defense in depth of IAEA is defined as follows;
The conspicuous features of our DiD Risk Monitor is that the range of risk is not limited to core melt accidents but includes all kinds of negative outcome events which exceed the level 4 of the defense in depth.
*1 Defence in depth in nuclear safety, INSAG-10, IAEA, 1996