The Defense in Depth (DiD) is a classical defensive concept currently applied to a variety of technical fields, including nuclear (where this concept is widely applied) and chemical industry, Information and Communication Technology (ICT), transport, and many others. It deals with slowdown of the progression of an “attack” against a “target” by using multiple and independent levels of protection (or lines of defense), designed to compensate for the failure of one or more defenses, ensuring that the risks are kept acceptable. Concerning the current practices for the DiD implementation and the rationale for its evolution, there is a shared recognition that the reinforcement of DiD is the key to improve the safety of future installations for all types of technologies and industries. Within this context, the results of Probabilistic Safety Assessment (PSA) play a key role in the demonstration of both the robustness of the design and safety, supporting the verification that the DiD principles are correctly implemented. A key issue, still open, is related to the link that must be put in place to provide the DiD probabilistic success criteria through PSA insights. After an analysis of DiD evolution in time and DiD application to different industrial fields, this paper deals with the key issue, still open, relevant to the link that must be put in place to provide the DiD probabilistic success criteria through PSA insights. Practical proposals outlined point out the open questions.
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