Definition, objectives, levels and barriers

The decision to create DID in the plants was taken at the start of nuclear energy development which indicates a remarkable farsightedness, as subsequent history has demonstrated that it has been the best defence against the uncertainties of the technology and the mistakes initially made (see, for example, the Three Mile Island accident). Obviously, in the first period of nuclear energy, many protests were made against this waste of resources which consisted in the construction of costly barriers (e.g. the containment) without, according to some, a real need for them. 

Accordingly, in the most recent documents, the DID is based on four principal barriers against the external release of radioactive products (fuel matrix, fuel cladding, reactor cooling circuit pressure boundary, and containment system), and on five defence levels in order to best use these barriers (illustrated in Table 9-1). 

The actual implementation of DID needs the support of some base requirements which apply to all of the five quoted levels. These requirements descend from the technical principles of nuclear safety which lead to the specific measures ariss 

As it can be observed, practically all the issues of nuclear safety can be viewed as an implementation 

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