Defence in Depth
Defence in Depth : (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 materials and workers, members of the public or the environment, in operational states and, for some barriers, in accident conditions. The objectives of defence in depth are to: (1) compensate for potential human and component failures; (2) maintain the effectiveness of the barriers by averting damage to the facility and to the barriers themselves; (3) protect workers, members of the public and the environment from harm in accident conditions in the event that these barriers are not fully effective. INSAG defines five levels of defence in depth: Level 1: Prevention of abnormal operation and failures; Level 2: Control of abnormal operation and detection of failures; Level 3: Control of accidents within the design basis; Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of the consequences of severe accidents; Level 5: Mitigation of radiological consequences of significant releases of radioactive materials. The levels of defence are sometimes grouped into three safety layers: hardware, software and management control. In the context of waste disposal, the term multiple barriers is used to describe a similar concept. See: INSAG-10 for further information. (2) The application of more than one protective measure for a given safety objective, such that the objective is achieved even if one of the protective measures fails
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