[en] Defense in depth is widely acknowledged as a cornerstone of nuclear safety. However practical implementation of this concept may be a challenge in the early stage of a new reactor design, in particular when it comes to the assessment of the sufficiency of the features implemented. In France, the Line of Defense concept (LoD) has been successfully used for a long time in the design and licensing of research reactors and advanced reactors like sodium fast reactors (SFR). It is now used in the design of new reactors, like ASTRID SFR or molten salt fast reactor that is investigated in the frame of the European SAMOSAFER R&D project. The implementation of this concept at an early stage of the design guarantees that it meets the safety objectives related to defense in depth. Basically the principle is that unacceptable consequences of an accident situation, like large or early releases, shall always be prevented by at least three LoDs that are independent, diversified and reliable enough. This concept shall include, as appropriate, specific regulatory requirements related to the prevention and mitigation of Severe Plant Conditions, like fuel melting if applicable. It is also used to achieve the demonstration of practical elimination of severe plant conditions that are not reasonably manageable. This methodology is applicable to any kind of technology and provides a robust deterministic assessment of the plant safety. This paper describes the nature of the LoDs, the general requirements associated to their design and provides example of application in current advanced reactor projects. (author)

[en] Liquid-fuelled molten salt reactors (MSRs) may be different in terms of design, operation and safety issues compared to large water-cooled nuclear power plants (NPPs). This paper presents in a first part an analysis of the safety paradigm for such systems, based on the identification of safety-related innovations of such MSRs. The second part of the article is dedicated to an application to the Molten Salt Fast Reactor (MSFR) concept developed in France by CNRS to illustrate the safety principles application on a specific design representative of similar liquid-fuelled MSRs.