General Illustration of Defence in Depth

The use of defence in depth in nuclear power plant design and operation involves three fundamental principles. Defence in depth provides the basic framework for most nuclear power plant safety. The concept has been refined and strengthened through years of application. All safety analysis for nuclear power plants, both deterministic and probabilistic, revolves around evaluating the performance of the plant subject to different modes of defence in depth, and the reliability of these modes [1]. 

There are many such modes that can protect people and the environment against the possibility and the effects of accidents at nuclear power plants, which vary according to the challenges to the plant generated by different abnormal events. The modes can be classified according to the severity of the challenge, measured in terms of extraordinary demands on equipment and staff performance or in terms of any resultant plant damage. This latter classification is illustrated in the second line of Table 3.1. 

The lengths of the boxes on the line labelled events are not intended to indicate any scale of probability of the events Usted in them. If a representative probability scale were shown, only normal operational events would have a probability high enough to be visible on the diagram. Nevertheless, this approach provides a simple illustration of the defence in depth required for each event. 

The other lines show how the strategy, systems, procedures and barrier integrity depend on the class of the event and its severity. The entire picture in each case is provided by the vertical axis through the event at its indicated severity. 

For instance, an accident beyond the design basis with a severity at the lower end of the range might generate damage to the reactor core that precludes reuse of the 

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General Illustration of Defence in Depth Table 3.1 Overview of defence in depth [1]... 

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