Defense in depth: a contribution to ICPE safety

The origins of the term "defense in depth" are linked to the military field, in particular with the aim of organizing fortifications and deploying troops on the territory to counter attempted aggression of all kinds. Historians report that a defense-in-depth technique was used around 2900 B.C. at Hierakonpolis in Egypt, based on a defense system involving two parallel, independent walls to reinforce the city's protection. This system was complemented by a special organization of troops, who themselves benefited from the protection afforded by the walls. The aim of such a system was to force the attacker to encounter several successive barriers and lines of defense, in order to weaken him and slow down his advance into the city. This defensive strategy was later adopted by the nuclear industry to protect operators, the population and the environment from any release of radioelements. It was initially based on the structuring of three physical barriers organized according to three levels of protection. As work in nuclear safety progressed, this strategy was extended to include organizational aspects. Today, the concept of defense in depth is based on the organization of technical, structural and organizational means of preventing risks, protecting against their consequences and safeguarding, arranged according to five levels of protection. The aim of this article is to explain how defense in depth is defined in the nuclear industry, what principles underpin it and how it is applied. Secondly, the transposition of this concept and its relevance to industrial risk management are examined, using two examples in particular.