Requirements for defence in depth

In the design process, defence in depth shall be incorporated as described in Section 2. The design therefore  

To ensure that the overall safety concept of defence in depth is maintained, the design shall be such as to prevent as far as practicable  

The design shall be such that the first, or at most the second, level of defence is capable of preventing escalation to accident conditions for all but the most improbable PIEs. 

The design shall take into account the fact that the existence of multiple levels of defence is not a sufficient basis for continued power operation in the absence of one level of defence. All levels of defence shall be available at all times, although some relaxations may be specified for the various operational modes other than power operation. 

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