Levels of defence

The hypothetic formula of ideal fatty foods was elaborated in which, along with the content of fatty acids, were taken into account the atherogenic properties, the level of defence against the peroxide oxygenation of the lipids and the concentration of biologically active substances. 

As table 6.5 shows, Britain devoted a higher proportion of her national income to defence than any other European country, except France at the height of the Algerian war. It was not difficult to establish in the minds of ministers a correlation between a relatively high level of defence expenditure, on the one hand, and a falling share of world exports of manufactures, a lower growth rate than other Western... 

Antioxidant protection can be viewed as consisting of three sequential levels of defensive activity which are most clearly understood in the mechanism of prevention of lipid peroxidation, and these can be explained in terms of the five mechanisms outlined above. The first level of defence, which is largely enzymatic, involves enzymes the activity of which depends principally on trace amounts of the minerals Mn, Cu, Zn and Se it is concerned with the control... 

Requirements and concepts for the safe design of nuclear power plants are developed in Ref [1], in which PIEs are defined. PIEs can challenge any level of defence in depth and have to be considered in the design process. The PIEs to be considered will include internal hazards. PIEs are defined in appendix I of Ret [1]. 

In order of preference, the best design approach is to practically eliminate the PIE (i.e. to make P acceptably small) the next best approach is to separate SSCs from sources (i.e. to make P2 acceptably small) there is also the option of making the consequences acceptable (i.e. to make P3 acceptably small). However, to the extent possible, defence in depth should be maintained by ensuring that the second level of defence and, if necessary, the third level of defence are effective. It may also be necessary in some cases to use a combination of all three levels. 

The design should have provisions to detect the failure or bypass of each level of defence as far as applicable. The requested levels of defence should be specified for each operational mode (for example, an open containment may be allowed in certain shutdown modes, and the specified levels of defence should be available at all times when in that mode). 

Sufficient defence in depth has been provided and the levels of defence are preserved in that potential accident sequences are arrested as early as possible. 

Acceptance criteria should be developed for events and conditions within the design basis as set forth in Ref. [ 1 ]. These criteria should ensure that an adequate level of defence in depth is maintained by preventing damage to barriers against the release of radioactive material and preventing unacceptable radiological releases. 

The first two levels of defence carry the primary burden of safety ... 

Within each level of defence, none of the safety systems is disproportionately more significant than the others. 

The selection of items under item (c) above is related to the defence in depth approach in the event of an earthquake of level SL-2, all levels of defence are required to be available at all times (Ref. [1 ], para. 4.4). The physical barriers designed to protect the plant from external events other than seismic events should maintain their integrity and functionality during an earthquake. 

Although the IAEA safety standard NS-R-1 [15] provides a consensus definition of the defence in depth levels, the definitions suggested in INSAG-10 [17] may better suit for NPPs with innovative reactors. For the future reactors, reference [17] envisages the following trends of the different levels of defence in depth ... 

Some major highlights of the PFPWR50 design, structured in accordance with the various levels of defence in depth are brought out below ... 

The IRIS design provides for multiple levels of defence for accident mitigation (in-depth-in-depth (DID)), resulting in extremely low core damage probabilities. In addition to the traditional DID levels (barriers, redundancy, diversity, etc.) IRIS introduces a very basic level of DID, i.e., elimination by design of accident initiators or reduction of their consequences/probability. This is implemented through the safety-by-design IRIS philosophy, which was briefly presented in the introduction. 

Application of the concept of defence in depth in the design of the research reactor provides a series of levels of defence (inherent features, equipment and procedures) which are aimed at preventing accidents and ensuring appropriate protection in the event that prevention fails. However, defence in depth shall be applied with account taken of the graded approach as mentioned in Section 1 and of the fact that many low power research reactors do not qualify for the fifth level of defence or even for the fourth level. 

The aim of the first level of defence is to prevent deviations from normal operation and to prevent system failures. This leads to the requirement... 

The fifth and final level of defence is aimed at mitigation of the radiological consequences of potential releases of radioactive material that may result from accident conditions. This requires the provision of an adequately equipped emergency control centre and plans for the on-site and off-site emergency response. 

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. ... 

External events may challenge defence in depth at many levels. The basic plant protection should be addressed in the first level of defence either by... 

For these reasons, the design of a full scope protecting barrier may be unreliable and in some cases even unfeasible and a challenge to one level of defence in depth may be envisaged. 

Lower safety margins or reduced acceptance criteria compared with other DBEEs may be specified for items pertaining to the third or fourth level of defence in depth (mitigation of design basis accidents and of severe accidents) ... 

If such a challenge to a level of defence in depth is envisaged, dedicated operational procedures should be put in place with reference to limits and conditions for normal operation, supported by adequate warning systems (where possible) and monitoring (see the following subsections). Moreover, a dedicated probabilistic evaluation should be made of the consequences of these special assumptions. 

The concept of defence in depth has been developed from the original idea of placing multiple barriers between radioactive materials and the environment. At present the concept includes a more general structure of multiple physical barriers and complementary means to protect the barriers themselves, the so-called levels of defence. It ensures that a high level of safety is reliably achieved with sufficient margins to compensate for equipment failures and human errors. 

The five levels of defence in depth are covered in the present review. For given objectives at each level of defence, a set of challenges is identified, and several root mechanisms leading to the challenges are specified. Finally, to the extent possible, a comprehensive list of safety provisions, which contribute to preventing these mechanisms from occurring, is provided. A broad spectrum of provisions, which encompass the inherent safety features, equipment, procedures, staff availabiUty, staff training and safety culture aspects, is considered. 

For easier and more user friendly applicability, the method that is reviewed in this publication, including the overview of all challenges, mechanisms and provisions for all levels of defence, is illustrated in the form of objective trees . ... 

With a view to providing a clear interpretation of the term defence in depth and a better understanding of the completeness of this concept, the present publication contains a comprehensive review of the provisions for all levels of defence. However, this publication does not provide any guidance for evaluating the safety significance of omissions or for the prioritization of the defence in depth provisions. 

Section 2 addresses the strategy of defence in depth and the importance of fulfilling the safety functions (SFs) to achieve the objectives for the different levels of defence. Section 3 provides a detailed description of the approach for making an inventory of the defence in depth capabilities of a plant. Section 4 discusses the applications of the approach and how to use the approach for assessing defence in depth. Section 5 presents conclusions. A discussion of the SFs is presented in Appendix I. In Appendix II, the objective trees graphically represent how, for each relevant safety principle , the safety objectives of the different levels of defence can be achieved by establishing defence in depth provisions at different stages of the lifetime of the plant. A test application of the approach is summarized in Appendix III. Definitions are provided at the end of the book. 

Safety principle a commonly shared safety concept stating how to achieve safety objectives at different levels of defence in depth. 

Levels of defence in depth Objective Essential means for achieving the objective... 

The levels are intended to be independent to the extent practicable. The general objective of defence in depth is to ensure that a single failure, whether an equipment failure or a human failure, at one level of defence, and even a combination of failures at more than one level of defence, does not propagate to jeopardize defence in depth at subsequent levels. The independence of different levels of defence is crucial to meeting this objective. 

---