Molten salt nuclear reactor, of the fast neutron type, with a vessel filled with inert liquid salts around the reactor vessel as a system for removing residual reactor power (EPuR).

Abstract

A molten salt nuclear reactor of the fast neutron type, with a vessel filled with inert liquid salts surrounding the reactor vessel as a residual heat removal (RHR) system. The invention essentially consists of constructing a molten salt nuclear reactor of the fast neutron type, the architecture of which includes a reactor vessel free of a moderator, or at least lacking a moderator that would allow the reactor to be classified as a thermal neutron reactor. This vessel is axisymmetric in shape and surrounded by another vessel at the periphery of the reactor vessel, which defines an inter-vessel space filled with an inert liquid salt that serves as a heat transfer fluid for the removal of the reactor's residual heat by conduction through the reactor vessel. Figure for the abstract: Fig. 3

Description

Description Title of the invention: Molten salt nuclear reactor, fast neutron type, with vessel filled with inert liquid salts around the reactor vessel as a power removal system residual reactor pressure (EPuR). Technical field

[0001] — The present invention relates to the field of molten salt nuclear reactors. (acronym RSF or MSR English acronym for “Molten Salt Reactor”). More particu- lately, it concerns the field of so-called small or medium MSR reactors power or AMR in English (acronym for “Advanced Modular Reactor”),

[0002] — The main objective of the invention is therefore to find a solution for improving such reactors, particularly fast neutron reactors, to enable a evacuation of residual power from an AMR type reactor.

[0003] — By “molten salt reactor(s)” is meant here and within the scope of the invention, the meaning usual technology, namely a nuclear reactor in which the fuel nuclear is in liquid form, dissolved in molten salt, at a temperature typically between 500 and 900 °C, which acts as both a heat transfer medium and first fission containment barrier.

[0004] — Although described in relation to a primary convection circuit architecture natural, the invention applies to any molten salt(s) nuclear reactor, of the type fast neutrons whose reactor vessel is axisymmetric and includes a portion cylindrical over most of the height. Prior art

[0005] — Molten salt reactors rely on the use of a molten salt, for example lithium fluoride (LiF) and beryllium fluoride (BeF2), or chloride sodium (NaCl) and magnesium (MgCl2), serving as both heat transfer fluid and moderator as the primary fluid within the reactor vessel, which is me- metal or ceramic, such as SIC.

[0006] — The tank contains the molten salt at high temperature, typically between 600 and 900°C, generally at ambient pressure.

[0007] The fissile fuel can be uranium 235, plutonium or uranium 233, from the conversion of thorium. A molten salt reactor can itself ensure its breeding using a fertile blanket containing the fertile isotope to be irradiated.

[0008] — The nuclear reaction is triggered by the concentration of fissile material in the com- fuel within the reactor vessel or by passing through a moderator block in graphite. A molten salt reactor can therefore be moderated by graphite, producing thermal neutrons, or without a moderator producing fast neutrons. The presence or absence of moderators thus defines the two main families of molten salt reactors, respectively thermal neutron and fast neutron. Since the 2000s, molten salt reactors have been evaluated and then selected as part of the International Generation IV Forum. They are now the subject of research with a view to deployment as fourth generation reactors, in particular as small modular reactors (SMRs), which are advanced nuclear reactors (AMR for "Advanced Nuclear Reactors"), with a power capacity of up to 300 MWe per unit: [1]. In nuclear reactors, the fundamental safety functions that must be ensured in all circumstances (normal, incidental and accidental operation) are containment, control of reactivity and evacuation of thermal power from the core. For the evacuation of residual power in accident situations, the improvement of passivity and the diversification of systems are constantly sought to ensure better overall reliability. The objective is to preserve the integrity of the structures and their geometry in all circumstances, namely the first and second (main tank) containment barriers, even in the event of a generalized electrical power failure over a long period, which corresponds to a Fukushima-type scenario. In particular, the removal of residual heat from a liquid metal reactor in a completely passive manner through the main vessel is currently being considered. While this objective appears to be unattainable in its entirety for a large reactor, due to the excessively high power, it can be realistically considered for small-power AMR reactors in order to guarantee an intrinsic improvement in the safety and residual heat removal systems, hereinafter referred to as EPuR systems, through the main vessel. Molten salt and fast neutron reactors are also affected by the removal of their residual power. Various solutions have already been considered. Reference may be made to publication [2] which proposes a solution. None of them is completely satisfactory. There is therefore a need to improve molten salt type reactors, fast neutron type reactors, particularly when they are considered as AMR reactors, in order to be able to evacuate their residual power satisfactorily. The aim of the invention is therefore to respond at least in part to this need. Statement of the invention To this end, the invention relates, in one of its aspects, to a molten salt nuclear reactor, of the fast neutron type, comprising: an axisymmetric reactor vessel around a central axis, delimiting inte- earlier a primary circuit of fuel in liquid form in in which at least one salt is melted, the interior of the tank being devoid of moderating material; another tank, arranged around the reactor tank, defining a space inter-tank (E) filled with an inert liquid salt. By "inert liquid salt" is meant a heat transfer liquid containing neither fissile nor fertile elements. According to a first embodiment, the liquid salt is chosen from NaCl, MgCl, KCl, ZnCl, PbCl, or a mixture thereof. In the context of the invention, the term "free of moderating material" means any material that allows a nuclear reactor to be classified as a thermal neutron nuclear reactor. In the usual sense, the kinetic energy of a fast neutron is greater than leV, while that of a thermal neutron is less than leV, typically of the order of 0.025eV. Reference may be made to publication [3], and in particular [Fig.4], which indicates, for several types of reactors, the thermal fraction and the fast fraction of the neutron flux. Thus, a molten salt reactor according to the invention is qualified as fast neutron. Typically, a molten salt reactor according to the invention may have a thermal neutron fraction of 0 to 0.05 and a fast fraction of 0.6 to 0.65. According to an advantageous embodiment, the reactor further comprises: at least one heat exchanger between the primary circuit of the reactor and a secondary circuit, arranged inside the reactor vessel; a first ferrule in the form of at least one hollow cylinder, with a central axis confused with that of the reactor vessel, the first shell being arranged in the reactor vessel to separate the interior of the reactor into a zone central and a peripheral zone in which the interchange is arranged heat so that in operation of the reactor, the combustible liquid at molten salt(s), circulates by natural convection in a loop from the bottom of the central zone defining the reactor core within which the reactions fission occurs, from which it rises by heating to the top of the central zone where it is deflected to the top of the peripheral zone to cross the interchange then descends towards the bottom of the peripheral zone where it is diverted towards the core of the reactor; a neutron reflector, arranged on the periphery of the core against the tank reactor, to ensure the maintenance of the neutron flux in the core. According to a first alternative, - the molten salt(s) fuel liquid of the primary circuit being a mixture of NaCI-UCI3, preferably in proportions of 25 to 30 mol% for UCI3, and PuCI3, preferably in proportions of 5 to 30 mol%, as salts, with depleted uranium. - the part of the first shell arranged above the exchanger(s) is a cylindrical ring closed on itself while the part arranged below the exchanger(s) is a hollow cylinder, - the neutron reflector is made of silicon carbide (SiC). - the molten salt fuel liquid of the primary circuit being a mixture of NaCI-UCI3, preferably at 34% mol, as salt with enriched uranium U235 (HALEU), preferably in proportions of 5 to 20%. - the first ferrule is a cylindrical ring closed on itself, - the neutron reflector is graphite. Thanks to this inert salt reservoir, this allows the residual power to be evacuated from the reactor passively, only by conduction through the reactor vessel, which is of small dimensions, of the SMR modular type, which typically corresponds to powers below 300 MWth. According to another advantageous embodiment, the nuclear reactor comprises a second shell arranged concentrically inside the first shell, so as to guide the combustible liquid which rises between the two zones where it is diverted. According to this embodiment, the interior of the second shell advantageously defines a space in which nuclear reaction control and / or safety rods extend. In other words, this second shell forms an emerging column arranged coaxially inside the first shell and in the center of the reactor vessel. This central column advantageously makes it possible to direct the rising fuel liquid thus guided into the annular space between the first and second shells, and to provide a location for control and / or safety rods. Preferably, the outside diameter of the second ferrule is between 5 and 30% of the inside diameter of the first ferrule. More preferably, the first ferrule and, if applicable, the second ferrule is (are) fixed by suspension to the cover plug closing the reactor vessel. The first ferrule and, if applicable, the second ferrule is (are) preferably made of stainless steel or a nickel-based alloy. Within the scope of the invention, the core cover plug may be supported or formed integrally with the reactor closure slab forming the upper portion. from the tank well. According to another advantageous embodiment, the reactor comprises at least one deflector, preferably in the form of a torus portion, arranged below and / or above the first shell so as to distribute the flow rate of the deflected molten salt(s) combustible liquid. In other words, this (these) toroidal deflector(s) makes it possible to optimize the distribution of the flow rate of the combustible liquid within the reactor vessel. The deflector(s) is (are) preferably made of stainless steel or a nickel-based alloy. According to another advantageous embodiment, the reactor vessel comprises a sky, usually called a cell sky, filled with an inert gas, such as argon, helium above the molten salt fuel liquid(s). This sky makes it possible to absorb the thermal expansion of the fuel salt liquid within the reactor vessel, when it undergoes a level variation. According to an advantageous construction variant, the heat exchanger(s) comprises a bundle of tubes, of the bayonet tube type, with hollow tubes each opening inside a blind tube, defining the exchange part with the secondary circuit, immersed substantially vertically at least partially in the combustible liquid with molten salt(s), the hollow tubes opening being connected to an inlet manifold and the blind tubes being connected to an outlet manifold for the secondary fluid. As an exchanger between primary and secondary circuits, structures other than bayonet tubes can be considered. For example, other U-shaped tubes, helical (plate) tubes can be provided, provided that the inlets and outlets are towards the top of the reactor and that there are low pressure losses. Advantageously, the secondary fluid inlet and outlet collectors are arranged in the stack canopy. With such an arrangement, direct contact between these collectors and the salt in the primary circuit is avoided, which increases their service life as well as the operating safety of the exchangers, since the only submerged part remains a part of the height of the tube bundle. The nuclear reactor can have one or other of the following dimensional characteristics for a power typically of 150MWth: the internal diameter of the reactor vessel is between 1.5 and 2m; the height of the primary circuit inside the reactor vessel is included between 2.5 and 4m. During reactor operation, the temperature of the molten salt fuel liquid in the primary circuit can be between 600 and 750°C. Preferably, the secondary fluid circulating in the exchanger(s) is based on a mixture of molten salts NaCl-MgCl, NaCI-MgCl-KCI or NaCI-MgCl-KCI-ZnCl. Advantageously, the temperature of the secondary fluid at the inlet of the exchanger(s) is of the order of 550°C while its temperature at the outlet of the exchanger(s) is of the order of 600°C. The external diameter of the other tank, filled with inert liquid salt, can be between 2.8 and 3.2 m. The power of the nuclear reactor is advantageously less than 300 MWth, which corresponds to a power range sought for AMR type reactors. Thus, the invention essentially consists of producing a molten salt nuclear reactor of the fast neutron type, the architecture of which has a reactor vessel free of moderator or at least of a moderator making it possible to qualify a reactor as a thermal neutron reactor, of axisymmetric shape surrounded by another vessel at the periphery of the reactor vessel which delimits an inter-vessel space filled with an inert liquid salt which serves as a heat transfer fluid for the evacuation of the residual power of the reactor by conduction through the reactor vessel. In operation to remove the residual power from the reactor, the inert liquid salt reservoir on the periphery of the reactor vessel performs two functions permanently, immediate and completely passive thermal storage, i.e. a capacity thermal inertia; an evacuation system via exchanger(s) immersed in the inter-tank space permanently, allowing permanent evacuation between 2 and 5% of the thermal power in nominal operation. Ultimately, a molten salt, fast neutron nuclear reactor according to the invention has numerous advantages, including: the implementation of a tank around the reactor vessel to fill the space inter-tanks by an inert liquid salt in order to be able to evacuate the power residual reactor: the possibility of circulation only by natural convection for the primary circuit with the possibility of implementing a single exchanger between primary and secondary circuits; the possibility of a simplification of fluid circuits compared to state-of-the-art solutions, including the removal of all pipes and pumps required in the MSFR concepts cited in the preamble; the possibility of sizing a reactor vessel integrating a circuit reduced size fuel primary, typically with a smaller diameter at 2m, and an overall height of less than 4m, which makes the reactor meets the requirements of SMR modular reactors. Thus, a circuit primary with a reactor vessel, the inner cylindrical shell and its primary / secondary exchanger according to the invention can be manufactured in the factory, transported to the site and then used throughout the lifetime of the reactor. During the de- mantling, such a circuit could be fully loaded in a castle transport to be processed in a suitable factory. In other words, the invention makes it possible to drastically simplify manufacturing and deployment and the dismantling of a molten salt nuclear reactor of the type fast neutrons, low and medium power; the possibility of using the same primary circuit architecture with a identical reactor vessel, only the shells and neutron reflectors being adapt, for different combustible liquids with molten salt(s), and therefore the pos- ability to take advantage of the versatility offered by molten salt technology fast neutron. Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings [Fig.1] [Fig.1] is a view from a simulation coupling computational fluid dynamics (CFD) and 3D neutronics, showing the circulation of the primary fluid with the temperature field within a molten salt nuclear reactor, of the fast neutron type according to the invention. [Fig.2] [Fig.2] is a schematic view in longitudinal section illustrating a bayonet tube exchanger, with its inlet and outlet collectors, as it can be arranged in a reactor according to the invention. [Fig.3] [Fig.3] is a view from a numerical simulation as in [Fig.1] for a reactor simulation case according to the invention in which the molten salt fuel liquid of the primary circuit is a mixture of NaCl, 25% UCl3, 9% PuCl3 in molar proportions as salts, with depleted uranium U235 at 0.7%. [Fig.4] [Fig.4] is a view from a numerical simulation as in [Fig.1] for a reactor simulation case according to the invention in which the molten salt fuel liquid of the primary circuit is a mixture of NaCl, 34% UCI3 in molar proportions, as salts with enriched uranium U235 (HALEU), in proportion 20%. [Fig.5] [Fig.5] is a schematic longitudinal sectional view of a fast neutron molten salt nuclear reactor according to the invention showing the primary, secondary and tertiary circuits. Detailed description Throughout this application, the terms "vertical", "lower", " upper”, “lower”, “high”, “below” and “above” are to be understood by reference to a fast neutron molten salt nuclear reactor, as provided in the vertical operating configuration according to the invention. By "primary fluid", "secondary fluid", "tertiary fluid" we mean the fluid which constitutes respectively the primary, secondary and tertiary circuit. Please note that the different temperatures, powers, volumes, flow rates, etc. indicated are for information purposes only. For example, other temperatures may be considered depending on the configurations, in particular the power of the molten salt reactor, the volume of molten salt fuel liquid, the power requirement for the intended application, etc. [Fig. 1] describes a fast neutron type molten salt nuclear reactor 1, according to a primary circuit configuration according to an embodiment of the invention. This [Fig. 1] is a digital simulation view obtained by coupling computational fluid dynamics (CFD) and 3D neutronics, as explained below. The reactor 1 with central axis X comprises a tank 2 with a metal casing, preferably made of stainless steel, with a thickness of around 10 to 20 mm, and formed of a hemispherical tank bottom and a vertical cylinder. This reactor vessel 2 internally delimits a primary circuit of fuel in liquid form in which at least one salt is melted. The interior of the vessel 2 is devoid of moderating material. In other words, the combustible liquid with molten salt(s) fills and circulates inside the vessel without being moderated. A single heat exchanger 3 between the primary circuit of the reactor and a secondary circuit is arranged inside the reactor vessel 2. A first shell 4, with a central axis coincident with that of the reactor vessel, is arranged in the reactor vessel 2 to separate the interior of the latter into a central zone and a peripheral zone in which the heat exchanger 3 is arranged. In this configuration of [Fig.1], the shell 4 is made up of an upper part 40 facing the exchange zone ZE which is in the form of a cylindrical ring closed on itself, and a lower part 41 facing the core C which is in the form of a hollow cylinder. For example, for a total height H equal to 2.5 m, the height H1 of the lower part 41 of the shell 4 is equal to Im. A second shell 5 is arranged concentrically inside the first shell 4. The interior of the second shell 5 defines a space in which nuclear reaction control and / or safety rods can extend. Ferrules 4, 5 can be made of stainless steel or nickel-based alloy. The ferrules 4, 5 are advantageously fixed by suspension to the cover plug closing the reactor vessel 2. At the bottom of the reactor vessel 2, below the first shell 4, a first deflector 6, in the form of a portion of a torus. At the top of the reactor vessel 2, above the first shell 4, a second deflector 7, also in the form of a portion of a torus. As symbolized by the arrows in [Fig.1], with the shells 4, 5 and the deflectors 6, 7 as arranged, in operation of the reactor, the molten salt(s) fuel liquid circulates solely by natural convection in a loop from the bottom of the central zone defining the reactor core C within which the fission reactions occur, from which it rises by heating to the top of the central zone between the shells 4 and 5 where it is deflected by the deflector 7 towards the top of the peripheral zone to pass through the exchanger 3 then descends towards the bottom of the peripheral zone where it is deflected by the deflector 7 towards the core of the reactor C. The shell 5 guides the combustible liquid which rises between the two zones where it is diverted, that is to say in the central zone of the reactor from the zone of diversion by the deflector 6 passing through the core C to the zone of diversion by the deflector 7. The deflectors 6, 7, by their shapes and their arrangement, each make it possible to distribute the flow of the combustible liquid with deflected molten salt(s). A neutron reflector 21 made of silicon carbide is arranged on the periphery of core C against reactor vessel 2. The configuration of [Fig.1] is dedicated to a molten salt(s) fuel liquid of the primary circuit being a mixture of NaCI-UCI3, preferably in proportions of 25 to 30 mol%, and PuC13, preferably in proportions of 9 to 11 mol%, as salts, with depleted uranium. As illustrated in [Fig.2], the reactor vessel 2 comprises a sky, usually called a cell sky 20, filled with an inert gas, such as argon, above the molten salt(s) combustible liquid. According to the invention, as shown in Figures 3 and 4, an external vessel 22 is arranged around the reactor vessel 2, defining an inter-vessel space E filled with a neutron-inert liquid salt. The salt contained in the inter-tank space can be NaCl or MgCl. The single heat exchanger 3 comprising a bundle of bayonet tubes defining the exchange part with the secondary circuit. As illustrated in [Fig.2], each bayonet tube comprises a hollow tube 30 each opening inside a blind tube 31. Each tube 30, 31 is immersed substantially vertically in the combustible liquid. with molten salt(s) according to a partial immersion height Hi. Each hollow tube 30 is connected to an inlet manifold 32 while each blind tube is connected to an outlet manifold 33 of the secondary fluid. The inlet 32 ​​and outlet 33 collectors of the secondary fluid are advantageously arranged in the stack ceiling 20. The inventors carried out simulations of the dimensioning of a reactor 1, such as the one shown in [Fig.1]. For a given salt composition, the inventors adapted the design methodology of this type of primary circuit as follows: Step / / : Determination of the minimum and maximum acceptable temperatures in the molten salt of the combustible liquid. The minimum temperature is generally imposed to limit the risk of solidification of the salt, while the maximum temperature is imposed by the maximum temperatures admissible by the materials. Step ii / : From the thermal power targeted for reactor 1, determination of the flow rate of the primary circuit necessary to respect the temperature difference imposed in step i / . Step iii / : For a targeted primary circuit height, imposed for example by transportability constraints of the reactor vessel 2, determination of the horizontal passage sections at the level of the core C and the exchanger 3 which are necessary to obtain the flow rate of the primary circuit targeted in step ii / . Step iv / : The horizontal section of the core C being known, determination of the height of the core C necessary to guarantee its criticality. For high power cores, i.e. with a large section, a reduced height may prove sufficient. Conversely, a low power requirement may lead to a more elongated core. In practice, the inventors carried out pre-dimensioning calculations on the primary circuit of a reactor as shown in [Fig.1] using numerical simulation tools coupling CFD thermohydraulics, in order to determine the local velocity and temperature fields and 3D neutronics, in order to determine the achievement of criticality and the spatial distribution of power. The CFD numerical simulation software may be that known under the name TrioCFD. This TrioCFD code was developed by the Applicant and validated to effectively deal with various physical problems, such as turbulent flows, fluid / solid coupling, multiphase flows or flows in porous media: [4]. The 3D neutron simulation software can be the one known as ERANOS. This ERANOS software has been developed and validated with the aim of providing a suitable basis for reliable neutron calculations of existing or future advanced fast neutron reactor cores: [5]. This ERANOS software has been developed in 1970s and validated with the aim of providing a suitable scientific computing tool for reliable neutron calculations of sodium-cooled fast reactor cores: [5]. Publication [6] is an example of dimensioning by coupling thermo-hydraulic CFD with 3D neutronics. Repeated iterations are necessary to achieve an optimal design. The dimensional, temperature and power characteristics, and the molten salt combustible liquid obtained are as follows: power of 150 MWth; primary circuit operating temperature between 600 and 750°C; the molten salt combustible liquid of the primary circuit to be chosen from a mixture of NaCI-UCI; 25 to 30 mol%-PuCl; 9 to 11 mol% with 0.7% depleted uranium, or a mixture of NaCl-UCI; at 34% mol with uranium enriched to 20% U235; atmospheric pressure inside reactor vessel 2; as shown in figures 3 and 4, presence of another tank 22, arranged around reactor vessel 2 by defining an inter-vessel space (E) filled of a neutronically inert liquid salt. [Fig.3] illustrates a case of reactor 1 obtained by the aforementioned numerical coupling simulation, for a fuel liquid with a mixture of NaCI- 25%UCI3-9%PuC13 salts with 0.7% depleted uranium. The neutron reflector is made of SiC. For this case, the data obtained are as follows: - internal diameter of reactor vessel 2 equal to 1.78m, - external diameter of reactor vessel 2 equal to 1.8m, - height of reactor vessel 2 equal to 2.5m, - height of the core C equal to 0.8 m, - height of the ZE exchange zone equal to 1.5 m, - height of the transition zone ZT, between the exchange zone ZE and the core C equal to 0.2m, - internal diameter of ring 40 of ferrule 4 equal to 0.89 m, - external diameter of ring 40 of ferrule 4 equal to 1.22 m, - thickness of the hollow cylinder 41 of the ferrule 4 equal to 0.05 m, - internal diameter of exchanger 3 equal to 1.22 m, - external diameter of exchanger 3 equal to 1.78 m, - thickness of the neutron reflector equal to 0.145m, - external diameter of the ferrule 5 equal to 0.32m, - external diameter of the peripheral tank 22 equal to 3m, - height of inert salt in the inter-tank space E equal to 3m, - flow rate of combustible liquid within the core equal to 1856 kg / s with a temperature difference between the inlet and outlet of the core C equal to 140°C; - volume of dissolved salt equal to 3.55 m* with 901 kg of fissile Pu; - effective neutron multiplication factor keff, which expresses the factor by which the number of fissions is multiplied from one generation of neutrons to the next equal to 1.005 + / - 0.2. [Fig.4] illustrates a case of reactor 1 obtained by the aforementioned numerical coupling simulation, for a fuel liquid with a mixture of NaCI-UCI, at 34% mol with uranium U235 enriched to 20%. The neutron reflector is made of graphite. For this case, the data obtained are as follows: - internal diameter of reactor vessel 2 equal to 1.78m, - external diameter of reactor vessel 2 equal to 1.8m, - height of reactor vessel 2 equal to 2.5m, - height of the core C equal to 0.8 m, - height of the ZE exchange zone equal to 1.5 m, - height of the transition zone ZT, between the exchange zone ZE and the core C equal to 02m, - internal diameter of ring 40 of ferrule 4 equal to 0.89 m, - external diameter of ring 40 of ferrule 4 equal to 1.22 m, - internal diameter of exchanger 3 equal to 1.22 m, - external diameter of exchanger 3 equal to 1.78 m, - thickness of the neutron reflector equal to 0.145m, - external diameter of the ferrule 5 equal to 0.32m, - external diameter of the peripheral tank 22 equal to 3m, - height of inert salt in the inter-tank space E equal to 3m, - flow rate of combustible liquid within the core equal to 1750 kg / s with a temperature difference between the inlet and outlet of the core C equal to 150°C; - volume of dissolved salt equal to 2.95 m* with 997 kg of fissile Pu; - effective neutron multiplication factor keff, equal to 1.008 + / - 0.2. All components (tanks 2, 22, shells 4, 5, exchanger 3) are made of stainless steel for the configurations in figures 3 and 4. [Fig.5] illustrates an example of integration of reactor 1 which has just been described within a reactor building 10 and the secondary and tertiary circuits. The secondary fluid consists of a mixture of NaCl-MgCl salts; which enters by forced convection into exchanger 3 at 550°C and leaves at 600°C. Exchanger 11 between the secondary and tertiary circuits is within reactor building 10. The tertiary fluid consists of a mixture of NaCI-ZnCI salts; which enters by forced convection into exchanger 11 at 500°C and leaves at 550°C. The invention is not limited to the examples which have just been described; in particular, it is possible to combine characteristics of the illustrated examples within non-illustrated variants. Other variants and embodiments may be envisaged without departing from the scope of the invention. As an exchanger between primary and secondary circuits, structures other than bayonet tubes can be considered. For example, U-shaped tubes or helical (plate) tubes can be used, provided that the inlets and outlets are at the top of the reactor and that the pressure drops are low. Other neutral liquid salts than NaCl or MgCl may be considered within the scope of the invention. List of cited references [1]: E. Merle-Lucotte, M. Allibert, M. Brovchenko, D. Heuer, V. Ghetta, A. Laureau, P.Rubiolo, Chapitre “Introduction to the Physics of Thorium Molten Salt Fast Reactor (MSFR) Concepts”, Thorium Energy for the World, Springer International Publishing, Switzerland (2016). [2]: https: / / www.irsn.fr / en / newsroom / news / documents / irsn_report-geniv_04-2015.pdf [3]: Jiri Krepel et al. “Self-Sustaining Breeding in Advanced Reactors: Characte- rization of Selected Reactors”, Encyclopedia of Nuclear Energy 2021, Pages 801-819. https: / / www.sciencedirect.com / science / article / pii / B9780128197257001239?via%3Dih ub [4]: Pierre-Emmanuel Angeli et al. “OVERVIEW OF THE TRIOCFD CODE: MAIN FEATURES, V&V PROCEDURES AND TYPICAL APPLICATIONS TO NUCLEAR ENGINEERING”. NURETH-16, Chicago, IL, August 30-September 4, 2015. [5]: G.Rimpault,et al. “The ERANOS Code and data system for fast reactor neutronic analyses.” PHYSOR2002-International Conference on the New Frontiers of Nuclear Technology: Reactor Physics, Safety and High-Performance Computing, Oct 2002, Seoul, SouthKorea. [6]: Marco Tiberga et al. “Results from a multi-physics numerical benchmark for codes dedicated to molten salt fast reactors”, Annals of Nuclear Energy 142 (2020) 107428.

Claims

Claims

1. Nuclear reactor (1) with molten scl(s), of the fast neutron type, including: a reactor vessel (2) axisymmetric around an axis central, internally delimiting a primary circuit of com- fuel in liquid form in which at least one substance is melted a salt, the interior of the tank being devoid of mo- material derator; another tank (22), arranged around the reactor tank in defining an inter-tank space (E) filled with a liquid salt inert.

2. Molten salt nuclear reactor {1) according to claim 1, the salt inert liquid being chosen from NaCl, MgCl, KCI, ZnCI2, PbCI2 or a mixture of these.

3. Molten salt nuclear reactor according to one of the preceding claims- preceding, further comprising: at least one heat exchanger (3) between the primary circuit of the reactor and a secondary circuit, arranged inside the reactor vessel; a first ferrule (4) in the form of at least one cylinder hollow (40, 41), with a central axis merged with that of the tank of reactor, the first shell being arranged in the tank of reactor to separate the interior of it into a zone central and a peripheral area in which is arranged the heat exchanger so that in operation of the reactor, the combustible liquid with molten salt(s), circulates through natural convection in a loop from the bottom of the zone central defining the reactor core (C) within which the fission reactions occur, from which it rises by warming up to the top of the central zone where it is deflected up the peripheral zone to cross the interchange (ZE) then descends to the bottom of the peripheral zone where it is diverted towards the reactor core; a neutron reflector (21), arranged on the periphery of the core against the reactor vessel, to ensure that the flow is maintained neutron in the heart.

4. Molten salt nuclear reactor according to claim 3, in which : - the combustible liquid with molten salt(s) of the primary circuit being a mixture of NaCI-UC13, preferably in proportions of 25 to 30% mol, and PuCl3, preferably in proportions of 5 to 30 mol%, as salts, with depleted uranium, - the part of the first ferrule arranged above the exchanger(s) is a cylindrical ring closed on itself while the part arranged below the exchanger(s) is a hollow cylinder, - the neutron reflector is made of silicon carbide (SIC).

5. Molten salt nuclear reactor according to claim 3, in which - the combustible liquid with molten salt(s) of the primary circuit being a mixture of NaCI-UCI;, preferably at 34% mol, as salt with enriched uranium U235 (HALEU), preferably in proportions of 5 at 20%, - the first ferrule is a cylindrical ring (40) closed on itself, - the neutron reflector is graphite

6. Molten salt nuclear reactor according to one of claims 3 to 5, comprising a second ferrule (5) arranged concentrically to inside the first ferrule, so as to guide the combustible liquid which rises between the two zones where it is deflected

7. Molten salt nuclear reactor according to claim 6, the interior of the second ferrule defining a space in which extend control and / or safety rods for nuclear reactions.

8. Molten salt nuclear reactor according to one of the preceding claims preceding, the external diameter of the other tank, filled with liquid salt inert being between 2.8 and 3.2m.

9. Molten salt nuclear reactor according to one of the preceding claims- preceding, the internal diameter of the reactor vessel being between 1.5 and 2m.

10. Molten salt nuclear reactor according to one of the preceding claims- preceding, the height of the primary circuit inside the tank of reactor being between 2.5 and 4m

11. Molten salt nuclear reactor according to one of the preceding claims- previous, in operation of the reactor, the temperature of the liquid com- molten salt fuel(s) of the primary circuit being between 600 and 750°C.

12. Molten salt nuclear reactor according to one of the preceding claims- preceding, the secondary fluid circulating in the exchanger(s) being at base of a mixture of molten salts of a mixture of molten salts NaCI- MgCl2 or NaCI-MgCl2-KCI or NaCI-MgCl2-KCI-ZnCI2.

13. Molten salt nuclear reactor according to claim 12, the tem- temperature of the secondary fluid at the inlet of the exchanger(s) being of the order of 550°C while its temperature at the outlet of the exchanger(s) being the order of 600°C

14. Molten salt nuclear reactor according to one of the preceding claims- previous ones, whose power is less than 300 MWth.