In the design and operation of nuclear reactors, a concept, called defence-in-depth, is applied to establish a series of levels of defence. The general objective of defence-in-depth is to ensure that a single failure at one level of defence would not propagate to jeopardize defence-in-depth at subsequent levels. There are five levels of defence-in-depth in the concept of nuclear power plant (NPP): 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.
