[en] CNRS has focused R&D efforts on the development of a new reactor concept called the Molten Salt Fast Reactor (MSFR), characterized by a circulating liquid fuel and a fast neutron spectrum. This paper presents the new design proposed for the fuel circuit in the frame of the SAMOFAR European project and focuses on the design studies of the emergency draining system of the MSFR. These new designs result from physical and preliminary safety studies such as optimizing the use of the molten salt both as fuel and coolant, defining the operating procedures or minimizing the fuel leakage risks. Additional requirements are considered for the emergency draining system to be able to confine the fuel and to evacuate the residual heat over very long time periods (months) with no human intervention and to guaranty that under no circumstance the salt may reach criticality in this area. (author)

[en] CNRS has focused R&D efforts on the development of a new reactor concept called the Molten Salt Fast Reactor (MSFR), characterized by a circulating liquid fuel and a fast neutron spectrum. This paper presents the new design proposed for the fuel circuit in the frame of the SAMOFAR European project and focuses on the design studies of the emergency draining system of the MSFR. These new designs result from physical and preliminary safety studies such as optimizing the use of the molten salt both as fuel and coolant, defining the operating procedures or minimizing the fuel leakage risks. Additional requirements are considered for the emergency draining system to be able to confine the fuel and to evacuate the residual heat over very long time periods (months) with no human intervention and to guaranty that under no circumstance the salt may reach criticality in this area. (author)

[en] Molten salt reactors are liquid-fueled reactors so that they are flexible in terms of operation (load-following capabilities...) or design (core geometry, fuel composition, specific power level...) choices, but they are very different in terms of design and safety approach compared to solid-fueled reactors. Such reactors call for a new definition of their operating procedures. Dedicated studies are performed in the frame of the European SAMOFAR (Safety Assessment of Molten Salt Fast Reactors) project of Horizon 2020. This paper focuses on the behavior of the MSFR fuel circuit in interaction with the intermediate circuit. It is devoted to the start-up procedure of the MSFR, including the description of the proposed procedure, a presentation of the approach to criticality and the reactivity measurement during the filling of the core, and finally studies of accident scenarios (overcooling, reactivity insertion) at low power during the divergence step of the MSFR, highlighting the very good behavior of the reactor to such abnormal transients. (author)

No abstract available

[en] The molten salt reactor designs, where fissile and fertile materials are dissolved in molten salts, under consideration in the framework of the Generation IV International Forum, present some unusual characteristics in terms of design, operation, safety and also proliferation resistance issues. This paper has the main objective of presenting some proliferation challenges for the reference version of the Molten Salt Fast Reactor (MSFR), a large power reactor based on the thorium fuel cycle. Preliminary studies of proliferation resistance are presented here, dedicated to the threat of nuclear material diversion in the MSFR, considering both the reactor system itself and the processing units located onsite. The present study focuses on a specific threat: the diversion of ²³³Pa by the host state, exfiltrating it from the site, and processing it in a concealed independent installation, in view of producing nuclear weapons. Our main hypothesis is that the 2.6 MeV gamma radiation emitted by the decay from ²⁰⁸Tl to ²⁰⁸Pb allows the detection of any illegitimate handling of nuclear materials. With this hypothesis, we conclude that it would be impossible to divert nuclear materials directly from the salt circulating in the reactor but that it would be possible to do so by misusing the salt cleaning facility

[en] The Molten Salt Fast Reactor (MSFR) with its liquid circulating fuel and its fast neutron spectrum calls for a new safety approach and adaptation of the analysis tools. In the frame of the Horizon2020 program SAMOFAR (Safety Assessment of the Molten Salt Fast Reactor), a safety approach suitable for Molten Salt Reactors has been developed and is now applied to the MSFR. For this purpose, the Lines of Defence (LoD) method is selected to drive the design consistently with the Defence in Depth principle. The main objective of the LoD method is to ensure that every accidental evolution of the reactor state is always prevented by a minimum set of homogenous (in number and quality) safety provisions - called Lines of Defence - before a given situation may arise. This paper presents the main characteristics of the method, along with some practical guidelines to apply it to the specific case of the MSFR; moreover, some initiating events are analyzed through the implementation of the LoD tool. The outcomes of this analysis drive the design evolution

[en] Highlights: • A new passive decay heat removal system has been invented for molten salt reactor emergency draining tank system. • This system is based on the loop heat pipe concept. • It includes an evaporating tank (emergency draining tank), and where the liquid fuel is mixed with a working fluid (sodium) and to vaporizes the working fluid. • The resulting vapor pressure drives the vapor through the pipes to the condensers, where the vapor condenses, releasing its latent heat of vaporization to a provided heat sink. • The condensers are located above the emergency draining tank at a higher elevation than the vaporization tank, so the condensed working fluid can flow back to the emergency draining tank by gravitational force. • Therefore, the system can continuously and passively transport the latent heat of vaporization from the emergency draining tank to the condenser section. - Abstract: A passive decay heat removal system is proposed that is actuated by the phase change heat transfer method (latent heat of fusion and evaporation) for an emergency draining tank system of molten salt reactors. The emergency draining tank serves an evaporating tank, where the liquid fuel is mixed with an immiscible working fluid and later to vaporize this working fluid. The top header of the vaporizing tank is connected by pipes to a condenser. The resulting vapor pressure of the working fluid drives the vapor through these pipes to condensers, The condensers are located above the reactor, where the vapor condenses, releasing its latent heat of vaporization to a provided heat sink (air heat exchanger). The condensed working fluid flows at first in a Pythagorean cup type collector, which is located just below the condensers, the collected working fluid flows back, in batch, to the emergency draining tank by gravitational force. In this batch manner the working fluid can be strongly mixed with the liquid fuel to enhance the heat transfer between liquid fuel and working fluid without any mechanical or electrical assistance, i.e. a completely passive heat removal process. Therefore the system can passively transport the latent heat of vaporization of the working fluid from the emergency draining tank to the condenser section. This proposed decay heat removal system is described in more detail in this paper. The passive cooling of the liquid fuel to extract the residual heat has been studied.