[en] Highlights: • Neutronic calculations for fast spectrum molten salt reactor. • Evaluation of the fissile matter to be used in such reactor as initial fissile load. • Capabilities to transmute transuranic elements. • Deployment scenarios of the Thorium fuel cycle. • Waste management optimization with molten salt fast reactor. - Abstract: There is currently a renewed interest in molten salt reactors, due to recent conceptual developments on fast neutron spectrum molten salt reactors (MSFRs) using fluoride salts. It has been recognized as a long term alternative to solid-fueled fast neutron systems with a unique potential (large negative temperature and void coefficients, lower fissile inventory, no initial criticality reserve, simplified fuel cycle, wastes reduction etc.) and is thus one of the reference reactors of the Generation IV International Forum. In the MSFR, the liquid fuel processing is part of the reactor where a small side stream of the molten salt is processed for fission product removal and then returned to the reactor. Because of this characteristic, the MSFR can operate with widely varying fuel compositions, so that the MSFR concept may use as initial fissile load, ²³³U or enriched uranium or also the transuranic elements currently produced by light water reactors. This paper addresses the characteristics of these different launching modes of the MSFR and the Thorium fuel cycle, in terms of safety, proliferation, breeding, and deployment capacities of these reactor configurations. To illustrate the deployment capacities of the MSFR concept, a French nuclear deployment scenario is finally presented, demonstrating that launching the Thorium fuel cycle is easily feasible while closing the current fuel cycle and optimizing the long-term waste management via stockpile incineration in MSRs

[en] The French National Center for Scientific Research (CNRS) is carrying-out design studies for a Nuclear Electric Propulsion (NEP) engine based on a Molten Salt Reactor (MSR). A NEP engine based on a liquid nuclear fuel could allow developing a core design with relatively high power densities and temperatures while using simple reactivity control systems and keeping low pressure and temperature gradients in the fuel. Nevertheless, the design work of such engine poses significant technical challenges and requires advance numerical simulation tools. Different configurations of a MSR for space are currently being studied. In this work, a reactor concept using a fast neutron spectrum is investigated using a multi-physic tool based on a numerical coupling between the OpenFOAM (CFD) and Serpent 2 (Monte Carlo neutronics) codes. The analysis of this paper is focused mainly on the reactor core coupled thermal-hydraulics and neutron-transport aspects. The results show that the proposed core layout and materials allows the chosen reactor design to obtain satisfactory temperatures and temperature gradients without a large penalization on the reactor operating temperatures. (authors)

[en] Molten Salt Reactors have attracted increased attention in recent years because of the design and safety possibilities offered by the use of a liquid fuel. The European project SAMOFAR1 (2015-2019) is currently coordinating the research efforts on the Molten Fast Salt Reactor (MSFR) concept in Europe. The MSFR is a fast-spectrum breeder reactor with a large negative power coefficient that can be operated in a Thorium fuel cycle. The fast spectrum allows the reduction of the reprocessing requirements and a better reactor breeding ratio. Other advantages of the MSFR design are the possibility for actinide burning and extending fuel resources, on-line fuel loading and reprocessing and the use of novel passive safety systems such as the fuel salt draining system. The reference MSFR concept is a 3000 MW(th) reactor with three different circuits: the fuel circuit, the intermediate circuit and the power conversion system. The main components of the fuel circuit are the fuel salt which serves as fuel and coolant, the core cavity, the inlet and outlet pipes, the gas injection system (not shown in the figure), the salt-bubble separators, the fuel heat exchangers and the pumps. A lithium fluoride salt has been selected as the fuel matrix. The initial composition (non-irradiated) of the MSFR fuel salt is a mixture of a lithium fluoride, thorium fluoride salts and actinides fluoride (LiF-ThF₄-²³³UF₄ or LiF-ThF₄- ^enrUF₄-(Pu-MA)F₃), with the proportion of LiF fixed at about Development of accurate numerical models for the MSFR can only be accomplished by the use of a multiphysics approach which allows taking into account the coupling between relevant neutronics, thermal hydraulics and thermomechanics phenomena. As it is discussed in the next section, such a reactor model requires further progress on the knowledge of molten salt thermal hydraulics, in particular regarding the accuracy of the fuel salt Computational Fluid Dynamics (CFD) models. The Salt at Wall: Thermal Exchanges (SWATH) experiment is being designed to contribute to this particular point. SWATH is one of the experimental activities of the European H2020 SAMOFAR project and includes both experimental and numerical developments. The main objectives of SWATH are to: (i) Improve molten salt numerical models used for design and safety studies and (ii) Demonstrate the working principles of the cold plug device. The cold plug device is a key safety component of the salt draining system of the MSFR. The cold plug is intended to passively melt during accidental conditions thus allowing the flow of the nuclear fuel from the core cavity to the fuel salt draining tanks by gravity forces. The fuel salt draining tanks are designed to ensure that adequate margins are obtained regarding the fuel cooling capabilities and the reactivity control. The next sections provide an overview of the current challenges in molten salt thermal hydraulics modeling and the SWATH experiment design. (authors)

[en] There is currently a renewed interest in molten salt reactors, due to recent conceptual developments on fast neutron spectrum molten salt reactors (MSFRs) using fluoride salts. This concept, operated in the Thorium fuel cycle, may be started either with ²³³U, enriched U and/or TRU elements as initial fissile load. It has been recognized as a long term alternative to solid fuelled fast neutron systems with a unique potential (such as large negative temperature and void coefficients, lower fissile inventory, no initial criticality reserve, simplified fuel cycle, wastes reduction…) and is thus one of the reference reactors of the Generation IV International Forum. This paper will focus on recommendations to define a demonstrator representing the key points of the reference MSFR power reactor (3000 MWth, fuel salt volume of 18 m³). The MSFR demonstrator is designed to assess the technological choices of this innovative system (fuel salt, structural materials, fuel heat exchangers…). It seems finally possible to slightly modify such a demonstrator which could then be a self-breeder modular reactor. (author)

[en] This paper discusses a physical evaluation of the driving procedures of the MSFR (Molten Salt Fast Reactor), to assess the flexibility of the concept during normal operation. The reference MSFR design is a 3000 MWth reactor with a total fuel salt volume of 18 m³, operated at a mean fuel temperature of around 1000 K. The reactor includes 3 different circuits: the fuel circuit, the intermediate circuit and the power conversion circuit. The MSFR, with a fast neutron spectrum and operated in the Thorium fuel cycle, may be started either with ²³³U, enriched U and/or TRU elements as initial fissile load. These studies focus on the whole fuel circuit behavior in interaction with the intermediate circuit during the start-up and the load following phases. The latter relies on advanced point-kinetics modeling and simplified thermal evaluations. Regarding the start-up procedure, the studies deal with the determination of the reactivity during the filling of the core and its uncertainties compared to the reactivity margins available (injection of fissile matter, operating temperature variation...)

[en] reactor called the Molten Salt Fast Reactor (MSFR). The reference MSFR design is a 3000 MWth reactor with a total fuel salt volume of 18 m3, operated at a mean fuel temperature of 750 deg. C. The first confinement barrier of the reactor includes a salt draining system. In case of a planned reactor shut down or in case of accidents leading to an excessive increase of the temperature in the fuel circuit, the fuel configuration may be changed passively by gravitational draining of the fuel salt in dedicated draining tank located under the reactor and designed to provide adequate reactivity margins while insuring a passive cooling of the fuel salt to extract the residual heat from the short to the long term. The present preliminary assessment of this sub-critical draining system has been performed to identify the physical constraints and to give some orders of magnitude of characteristic time periods (authors)

[en] Experimental studies have been developed on a new freeze plug concept for safety valves in facilities using molten salt. They are designed to allow the closure of an upstream circuit by solidifying the molten salt in a section of the device and to passively melt in case of a loss of electric power, thus releasing the upper fluid. The working principle of these cold plug designs relies on the control of the heat transfer balance inside the device, which determines whether the salt inside the cold plug solidifies or melts. The device is mainly composed of steel masses that are dimensioned to provide sufficient thermal heat storage to melt the salt and thus open the cold plug after the electric power is stopped. The final goal of the work is to provide useful recommendations and guidelines for the design of a cold plug for the emergency draining system of a molten salt reactor. Some numerical thermal simulations were performed with ANSYS mechanical (Finite Element Method) to be compared with results of the experiments and to make extrapolations for a new component to be used in a reactor. (authors)

[en] Demonstration and Demonstrator of MSFR: - Sizing of the facilities: Small size: ~1liter - chemistry and corrosion – off-line processing; Pyrochemistry: basic chemical data, processing, monitoring. Medium size: ~100 liters – hydrodynamics, noble FP extraction, heat exchanges; Process analysis, modeling, technology tests. Full size experiment: ~1 m3 salt / loop – validation at loop scale Validation of technology integration and hydrodynamics models. - 3 levels of radio protection: → Inactive simulant salt ⇒ Standard laboratory; Hydrodynamics, material, measurements, model validation; → Low activity level (Th, depleted U) ⇒ Standard lab + radio protect; Pyrochemistry, corrosion, chemical monitoring. → High activity level (^enrichedU, ²³³U, Pu, MA) ⇒ Nuclear facility; Fuel salt processing: Pyrochemistry, Actinides recycling

[en] In case of a loss of coolant accident occurring in a spent fuel pool (SFP), not only the spent fuel assemblies' integrity can be compromised due to its inadequate decay power cooling, but also in some hypothetical scenarios serious questions could arise about the maintaining of adequate criticality margins. In the SFP the spent fuel assemblies are usually arranged in racks and immersed in borated water, which serves both as coolant and reactivity control. The distance between the racks is a key parameter in reactivity control. In this paper, a tool using multiphysics coupling was presented to asses this type of accidents. This new tool has been developed using OpenFOAM code to solve the continuum mechanics equations (e.g. Thermal-hydraulics equations) and the Monte Carlo code Serpent for the neutronics aspects. The model was then used to perform a sensitivity analysis on various spent fuel pool (SFP) parameters using the SFP of Fukushima Daiichi NPP Unit 4 as a reference. It was observed from this study that criticality is never achieved even though some parts of the assembly lacks modelling (i.e. spacers, grids). The sensitivity analysis let us nevertheless identifying the mechanisms that could eventually lead to an increase of the SFP reactivity. Moreover, it was found that under certain conditions the margin to criticality could be significantly reduced: certain levels of burnup, vapor or void fraction as the fuel pool drains, decrease of soluble boron, among others. Finally, the multiphysics approach has been used to perform a first calculation of an accident without reaching criticality.

[en] Recent conceptual developments on the design of fast neutron spectrum molten salt reactors (Molten Salt Fast Reactor or MSFR) using fluoride salts open promising possibilities to exploit the ²³²Th-²³³U cycle. On the other hand, the MSFR concept can also contribute to significantly diminish the radiotoxic inventory from present-reactors spent fuels in particular by lowering the masses of transuranium elements (TRU). This paper provides an overview of the current status of the MSFR design, the ongoing research activities at the National Centre for Scientific Research (CNRS, France) and the future perspectives of the concept. (author)