Definition

1. A hierarchical deployment of different levels of diverse equipment and procedures to prevent the escalation of anticipated operational occurrences and to maintain the effectiveness of physical barriers placed between a radiation source or radioactive material and workers, members of the public or the environment, in operational states and, for some barriers, in accident conditions.

Scope notes

a. The objectives of defence in depth are:
- (a) To compensate for human induced events and component failures;
- (b) To maintain the effectiveness of the barriers by averting damage to the facility and to the barriers themselves;
- (c) To protect workers, members of the public and the environment from harm in accident conditions in the event that these barriers are not fully effective
c. Five levels of defence in depth are discussed in SSR-2/1 (Rev. 1) (See [INTERNATIONAL ATOMIC ENERGY AGENCY, Safety of Nuclear Power Plants: Design, IAEA Safety Standards Series No. SSR 2/1 (Rev. 1), IAEA, Vienna (2016).] for further information):
- (a) The purpose of the first level of defence is to prevent deviations from normal operation and the failure of items important to safety.
- (b) The purpose of the second level of defence is to detect and control deviations from normal operation in order to prevent anticipated operational occurrences from escalating to accident conditions.
- (c) The purpose of the third level of defence is to prevent damage to the reactor core and releases of radioactive material requiring off-site protective actions and to return the plant to a safe state by means of inherent and/or engineered safety features, safety systems and procedures.
- (d) The purpose of the fourth level of defence is to prevent the progress of, and to mitigate the consequences of, accidents that result from failure of the third level of defence by preventing accident sequences that lead to large release of radioactive material or early release of radioactive material from occurring.
- (e) The purpose of the fifth and final level of defence is to mitigate radiological consequences of a large release of radioactive material or an early release of radioactive material that could potentially result from an accident
d. The International Nuclear Safety Group (INSAG) defined five levels of defence in depth (See [INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP, Defence in Depth in Nuclear Safety, INSAG Series No. 10, IAEA, Vienna (1996).] for further information):
- (a) Level 1: Prevention of abnormal operation and failures.
- (b) Level 2: Control of abnormal operation and detection of failures.
- (c) Level 3: Control of accidents within the design basis.
- (d) Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of the consequences of severe accidents.
- (e) Level 5: Mitigation of radiological consequences of significant releases of radioactive material.
- e. The levels of defence are sometimes grouped into three safety layers: hardware, software and management control.
- f. In the context of waste disposal, the term multiple barriers is used to describe a similar concept.
- g. Note that defence in depth is used with a different meaning in the IAEA Nuclear Security Series in the context of nuclear security.
- b. The Fundamental Safety Principles (IAEA Safety Fundamentals) [EUROPEAN ATOMIC ENERGY COMMUNITY, INTERNATIONAL ATOMIC ENERGY AGENCY, FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, INTERNATIONAL LABOUR ORGANIZATION, INTERNATIONAL MARITIME ORGANIZATION, OECD NUCLEAR ENERGY AGENCY, PAN AMERICAN HEALTH ORGANIZATION, UNITED NATIONS ENVIRONMENT PROGRAMME, WORLD HEALTH ORGANIZATION, Fundamental Safety Principles, IAEA Safety Standards Series No. SF-1, IAEA, Vienna (2006).] (para. 3.31) states that: “Defence in depth is implemented primarily through the combination of a number of consecutive and independent levels of protection that would have to fail before harmful effects could be caused to people or to the environment. If one level of protection or barrier were to fail, the subsequent level or barrier would be available. When properly implemented, defence in depth ensures that no single technical, human or organizational failure could lead to harmful effects, and that the combinations of failures that could give rise to significant harmful effects are of very low probability. The independent effectiveness of the different levels of defence is a necessary element of defence in depth”.

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