9-1 Definition, objectives, levels and barriers

As already discussed in Section 1-2, the Defence in Depth (DID) concept in nuclear safety consists in providing multiple independent protections against the occurrence of accidents and their progression, in such a way that, should one of them fail, at least another is present whose failure is independent from the operation of the first. It has to be said, however, that the object of independent barriers in totality is only an objective, and it is not always possible in reality in every conceivable accident sequence. The definition of DID has to be understood as a general defence principle, to be implemented to the maximum technically feasible degree. DID is implemented through design and operation provisions in a way to provide a graded protection against a vast variety of transients, abnormal events and accidents, including the malfunction of components, human errors in the plant and events initiated outside it. ^{AR177,AR178,AR185}

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...