Specification of the provisions

The safety principles described in Ret [2] are commonly shared safety concepts that indicate how to achieve safety objectives at different levels of defence in depth. The safety principles do not guarantee that plants will be absolutely free of risk. Nonetheless, INSAG [2] has stated that, if the principles are adequately applied, the nuclear power plants should be very safe. It is therefore considered that the safety principles provide a useful basis for the comprehensiveness of the provisions. 

The first step for assignment is shown in Fig. 3. A prehminary assignment is done as a horizontal band selected from the safety principles in Fig. 3, located within the boundaries of the different levels of defence. Of course, the complex nature of some of the principles cannot be fuUy reflected by a one dimensional projection of this kind. Furthermore, the boundaries of the levels are not so clear and some overlapping between levels exists. 

The second step for assignment is shown in Fig. 4, which is reproduced from Ret [2], showing the physical barriers and levels of protection in defence in depth. The message conveyed by Fig. 4 is that any violation of general safety principles such as in design management, quality assurance or safety culture can adversely affect several levels of defence at the same time. Specific safety principles that usually address the performance of various hardware components are typically assigned to different levels of defence. 

The third step for assignment of safety principles to individual levels of defence is provided by the explanatory text on the safety principles themselves in Ref. [2] and the derived requirements for design and operation in the IAEA Safety Standards [4,5]. 

It can be seen from Table 2 that many principles have a bearing on more than one level of defence. For example achievement of quality (safety principle 249 (SP (249)) has an impact across Levels 1-4, since it affects the reliability of all the engineering provisions that are in place to provide the defences at those levels. 

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