Fast neutron molten salt nuclear reactor comprising a safety vessel around the reactor vessel and a drain tank below the vessels

Abstract

The invention relates to a nuclear reactor (1) comprising: - at least one nuclear-reactor (1) vessel (2) filled with a bath of combustible liquid containing molten salt(s) and provided with at least one through-opening in its bottom; - another vessel (8), referred to as a safety vessel, arranged around the reactor vessel, thereby defining an inter-vessel space (E) together with the reactor vessel; - a tank (10), referred to as a drain tank, which is arranged below the vessels and is connected to the inter-vessel space by at least one pipe (11) and to the through-opening in the reactor vessel by at least one pipe (12); - a device (13) for selectively shutting off the through-opening in the reactor vessel, this being in the closed position during normal operation of the reactor and in the open position in order to drain the reactor vessel.

Description

[0001] Description

[0002] Title: Molten salt nuclear reactor, of the fast neutron type with a safety vessel around the reactor vessel and a drain tank below the vessels.

[0003] technical field

[0004] The present invention relates to the field of molten salt reactors (MSR). More particularly, it relates to the field of small or medium power MSRs, or Advanced Modular Reactors (AMR).

[0005] The main objective of the invention is therefore to provide a nuclear installation with at least one such reactor, more particularly a fast neutron reactor, which guarantees safety in the event of a leak from the reactor vessel containing the fuel liquid bath.

[0006] By "molten salt reactor(s)", we mean here and within the framework of the invention, the usual technological meaning, namely a nuclear reactor in which the nuclear fuel is in liquid form, dissolved in a molten salt, at a temperature typically between 500 and 900 °C, which acts as a heat transfer fluid.

[0007] By "pile roof", we mean here and within the framework of the invention, the usual technological meaning, namely the upper part of the internal volume of the main vessel of a molten salt reactor, located above the liquid volume of molten salt and usually filled with a neutral gas (argon or helium).

[0008] Previous technique

[0009] Molten salt reactors rely on the use of a molten salt, for example lithium fluoride (LiF) and beryllium fluoride (BeF2) or sodium chloride (NaCl) and magnesium (MgCh), serving both as a heat transfer fluid and as a moderator as the primary fluid within the reactor vessel, which is metallic or ceramic, such as SiC.

[0010] The tank contains the molten salt at high temperature, typically between 500 and 900 °C, generally at ambient pressure.

[0011] The fissile fuel can be uranium-235, plutonium, or uranium-233, the latter being produced from the conversion of thorium. A molten salt reactor can perform its own breeder reactor using a fertile blanket containing the fertile isotope to be irradiated.

[0012] The nuclear reaction is triggered by the concentration of fissile material in the fuel within the reactor vessel or by passing through a graphite moderator block.

[0013] A molten salt reactor can therefore be moderated by graphite, producing thermal neutrons, or without a moderator producing fast neutrons.

[0014] The presence or absence of moderators thus defines the two main families of molten salt reactors, respectively thermal neutron and fast neutron reactors.

[0015] From the 2000s onwards, molten salt reactors were evaluated and then selected within the framework of the Generation IV International Forum. They are now the subject of international 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"), whose power capacity can reach up to 300 MWe per unit.

[0016] Although promising in terms of safety potential, molten salt reactors may require expensive and complex systems and components.

[0017] Indeed, in a molten salt reactor, the primary fuel circuit, containing dissolved uranium or plutonium, constitutes the first safety barrier and must therefore meet very demanding design criteria in terms of leak-tightness. This primary circuit must include a core zone, in which the nuclear fission reactions take place in a chain reaction, and a heat exchange zone fluidically connected to the core, in which the heat generated in the core is transferred to a secondary circuit.

[0018] In conventional designs, the core is connected to a plurality of fluid circulation loops, each comprising a heat exchanger and a pump adapted to ensure circulation to and from the associated heat exchanger.

[0019] For example, among the programs selected for Generation IV, the homogeneous indirect-cooled reactor from the research of the LPSC laboratory in Grenoble, which is designated by the Anglo-Saxon acronym MSFR (for "Molten Salt Fast Reactor"), whose fuel is a liquid fluorinated salt with a breeder provided by Thorium, includes twelve or sixteen fluidic circulation loops. Each of the components of the loops adds complexity to the whole fluidic circuit: [1].

[0020] For the design of a molten salt reactor, in particular of the SMR type, the inventors of the present invention initially sought to develop a design minimizing the number of pipes and components, in particular to retain the major advantage inherent in SMRs, namely the increased modularity capacity through the manufacture of components in a factory for transport to the construction site, and also to increase operational safety.

[0021] They have thus designed a molten salt nuclear reactor of the fast neutron type, described and claimed in patent application filed on December 19, 2022, under number FR2213882, entitled "Molten salt nuclear reactor of the fast neutron type, whose primary circuit is by natural convection circulation." The proposed reactor can have a reactor vessel incorporating a reduced-size primary fuel circuit, typically with a diameter of less than 2 m and an overall height of less than 4 m, making the reactor compliant with the requirements for modular AMR reactors. Therefore, a primary circuit with a reactor vessel, the inner cylindrical shell, and its primary / secondary heat exchanger, as described in this patent application, can be manufactured in a factory, transported to the site, and then used throughout the reactor's lifetime.

[0022] In theory, fast neutron molten salt reactors have the advantage of great versatility both in terms of fuel that can be used (uranium, plutonium, thorium, minor actinides) and in operating mode (burner or regenerator).

[0023] It is recalled here that a "burner" mode corresponds to a reactor operation where there is an intensive consumption of fissile isotopes with limited regeneration of fissile material.

[0024] A "breeder" mode of a nuclear reactor is an operating mode in which it produces all or part of the fissile fuel it consumes from fertile material. Thus, neutrons, generated by fission in the reactor core, are absorbed by fertile material, which in turn produces new fissile materials. In burner mode, a fast neutron molten salt reactor can use the following fissile isotopes: uranium-235, uranium-233, the fissile isotopes of plutonium, and the fissile isotopes of minor actinides.

[0025] In regenerator mode, a fast neutron molten salt reactor can use the same fissile isotopes listed above and as fertile isotopes: uranium 238, thorium 232, fertile isotopes of plutonium and fissile isotopes of minor actinides.

[0026] With the design that the inventors proposed in the aforementioned patent application FR2213882, the exploitation of the great versatility of fast neutron molten salt reactors is made possible.

[0027] In particular, a reactor according to this design can be designed to be of sufficient size to operate in iso-generator mode, a mode in which for each fission produced, a fissile nucleus is produced by fertile capture.

[0028] In a molten salt reactor, the reactor vessel, which contains the molten salt(s) as fuel liquid, constitutes the first containment barrier from a nuclear safety perspective.

[0029] Molten salts are highly corrosive products. The problem of corrosion and breaching of the first containment barrier, which could lead to a fuel leak, is therefore a major concern for reactor safety.

[0030] In the event of a leak, it must be possible to contain the leak to a delimited area, recover the fuel liquid, change the reactor vessel and restart the reactor.

[0031] A leak in the first containment barrier must therefore be accounted for in reactor operation; that is, it must not lead to the loss of the reactor or its operation. In other words, it must be treated as an operational incident from a safety perspective, and not as an accident.

[0032] Furthermore, the reactor operating procedures must allow for draining the reactor vessel so that it can be removed, a new one installed, the molten salt fuel liquid recovered and put back into the vessel to restart the reactor.

[0033] To date, a concept known as a crystallized plug exists for draining the reactor vessel in certain molten salt reactor designs: [1]. In this design, the reactor vessel has a hole in its bottom. This hole is plugged by a solid fuel salt plug that prevents the liquid fuel salt filling the vessel from flowing out during reactor operation. The fuel salt constituting the crystallized plug is solidified at this point by a dedicated cooling device. In case of a problem, the cooling can be interrupted, the solid fuel salt in the crystallized plug melts due to the heat of the liquid fuel salt above it, and all the liquid fuel salt can then flow into a drain tank.

[0034] This solution has several major drawbacks, as follows:

[0035] - it does not allow for the management of leaks in the event of unwanted holes appearing in the reactor vessel;

[0036] - the practical formation of the crystallized plug is not known;

[0037] - It is difficult to determine and predict how long it will take for the crystallized plug to melt. However, in the event of an accident and emergency draining, knowing the duration of this melting time is important, even crucial.

[0038] There is therefore a need to improve molten salt reactors, fast neutron reactors, especially when considered as AMR reactors, and more particularly those intended to operate in natural convection as envisaged in application FR2213882, in order to guarantee safety, particularly in the event of a leak of the molten salt fuel liquid.

[0039] The aim of the invention is therefore to meet at least part of this need.

[0040] Description of the invention

[0041] To this end, the invention relates, in one of its aspects, to a nuclear reactor comprising:

[0042] - at least one nuclear reactor vessel filled with a bath of molten salt(s) combustible liquid and equipped with at least one opening in its bottom;

[0043] - another tank, called the safety tank, arranged around the reactor tank by defining an inter-tank space (E) with the reactor tank;

[0044] - a reservoir, called a drain reservoir, arranged below the tanks, and connected to the inter-tank space by at least one pipe and to the outlet of the reactor tank by at least one pipe; - a selective closing device for the outlet of the reactor tank, in the closed position during normal reactor operation and in the open position to carry out the draining of the reactor tank.

[0045] Preferably, the nuclear reactor is of the fast neutron type, the reactor vessel is axisymmetric around a central axis, internally delimiting a primary circuit of a fuel in liquid form in which at least one salt is melted, the interior of the vessel being devoid of a moderator material.

[0046] According to a preferred application, the nuclear reactor is according to the teaching of the aforementioned patent application FR2213882.

[0047] Thus, the nuclear reactor advantageously includes a shell in the form of at least one hollow cylinder, with the central axis coinciding with that of the reactor vessel, the shell being arranged in the reactor vessel to separate the interior of the latter into a central zone and a peripheral zone in which the heat exchanger is arranged so that in reactor operation, the molten salt(s) fuel liquid circulates by natural convection in a loop from the bottom of the central zone defining the reactor core (C) in which the fission reactions occur, 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 pass through the exchanger (ZE) and then descends to the bottom of the peripheral zone where it is deflected to the reactor core.

[0048] Even more advantageously, at least one heat exchanger between the reactor's primary circuit and a secondary circuit is arranged inside the reactor vessel.

[0049] For the purposes of this invention, "free of moderator material" means any material that allows a nuclear reactor to be classified as a thermal neutron 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 on the order of 0.025 eV. Reference may be made to publication [2], and in particular to Figure 4, which shows, for several types of reactors, the thermal fraction and the fast fraction of the neutron flux.

[0050] Thus, a molten salt reactor according to the invention can be classified as a fast neutron reactor. Typically, a molten salt reactor according to the invention can exhibit a thermal neutron fraction of 0 to 0.05 and a fast neutron fraction of 0.6 to 0.65.

[0051] According to an advantageous embodiment, comprising a neutron reflector in the form of a shell arranged around the reactor vessel.

[0052] Advantageously, the neutron reflector material is chosen from lead (Pb) or lead monoxide (PbO). These materials have the advantage of being weak neutron absorbers and moderating neutrons very little.

[0053] Preferably, the thickness of the reflector around the reactor vessel should be between 50 and 70cm.

[0054] According to an advantageous embodiment, the pipes and / or the drain tank is / are double-walled. These double walls ensure continuity to the second containment barrier.

[0055] Preferably, the pipes and / or the drain tank should be made of stainless steel or nickel-based alloy.

[0056] A nuclear reactor may have one or both of the following dimensional characteristics for a typical power output of 150MWth:

[0057] - the diameter of the reactor vessel is between 1.5 and 2m;

[0058] - the height of the primary circuit inside the reactor vessel is between 2.5 and 4m.

[0059] Preferably, the molten salt fuel(s) of the primary circuit is selected from a mixture of NaCl-UCh, preferably in proportions of 25 to 30 mol% for UCh, and PuCh, preferably in proportions of 5 to 36 mol%, as salts, with depleted uranium U235, preferably less than 0.3%, atomic, or a mixture of NaCl-UCh, preferably at 34 mol%, as salt with enriched uranium U235 (HALEU), preferably in proportions of 5 to 20%. The molten salt(s) may also contain ThC14.

[0060] During reactor operation, the temperature of the molten salt(s) fuel liquid in the primary circuit can be between 500 and 750°C.

[0061] The power output of a nuclear reactor is advantageously between 10 and 500 MWth, which corresponds to a power range sought for AMR-type reactors. The invention also relates to a method of operating a nuclear reactor comprising the following step, at the end of a normal operating fuel cycle of the nuclear reactor: i / placing the selective shut-off device in the open position so as to cause the drain tank to be filled by gravity draining of the molten salt(s) fuel liquid contained in the reactor vessel.

[0062] Advantageously, the process includes, once step i / is completed, the following steps: ii / placing the selective shut-off device in the closed position; iii / filling the reactor vessel with fresh molten salt(s) fuel liquid or emptying the reactor vessel.

[0063] Thus, the invention essentially consists of producing a molten salt nuclear reactor whose fuel liquid is contained in a tank, itself lined with a safety tank arranged around the reactor tank, this safety tank therefore constituting a second containment barrier from a safety point of view.

[0064] Each of the two tanks, reactor and safety, is connected by a pipe to a drain tank.

[0065] The solution according to the invention allows for management of:

[0066] - normal operating procedures for emptying the reactor vessel;

[0067] - leaks that may occur through the reactor vessel, as operational incidents from a safety perspective, and not as accidents;

[0068] - fuel liquid for the purpose of filling another reactor vessel.

[0069] In conclusion, a molten salt nuclear reactor of the fast neutron type, according to the invention, offers numerous advantages, including:

[0070] - increased safety for both in-service draining operations and recovery of fuel liquid leaks as operational incidents;

[0071] - a small footprint for the various components (safety tank, drain tank, connecting pipes) because they are completely integrated within the reactor building.

[0072] - Increased simplicity of maintenance operations: after disconnecting the pipes linking the tanks to the drain tank, these can be replaced by lifting them with a handling bridge, thus limiting to a minimum the operations to be carried out in a radioactive environment.

[0073] Other advantages and features of the invention will become clearer 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.

[0074] Brief description of the drawings

[0075] [Fig 1] Figure 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 installed within a reactor building of a nuclear power plant.

[0076] [Fig 2] Figure 2 is a schematic longitudinal cross-sectional view illustrating a bayonet tube heat exchanger, with its inlet and outlet manifolds, as it may be arranged in a reactor of the power plant according to Figure 1.

[0077] [Fig 3] Figure 3 is a schematic view of a molten salt reactor, of the fast neutron type with a safety vessel and a molten salt drain tank according to the invention.

[0078] Detailed description

[0079] Throughout this application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a fast neutron molten salt nuclear reactor, as provided for in the vertical operating configuration according to the invention.

[0080] It should be noted that the various temperatures, power outputs, volumes, flow rates, etc., indicated are for guidance purposes only. For example, other temperatures may be considered depending on the configuration, particularly the power output of the molten salt reactor(s), the volume of molten salt fuel(s), and the power requirements of the intended application.

[0081] With reference to Figure 1, a molten salt fast neutron nuclear reactor 1 is described, with a primary circuit configuration as described and claimed in patent application FR2213882. Figure 1 is a numerical simulation view obtained by coupling computational fluid dynamics (CFD) and 3D neutronics, as explained below. The reactor 1, with its central axis X, comprises a vessel 2 with a metallic shell made preferably of stainless steel or nickel-based alloy, approximately 10 to 20 mm thick, and consisting of a hemispherical bottom and a vertical cylinder.

[0082] This reactor vessel 2 internally delimits a primary circuit of liquid fuel containing at least one molten salt. The interior of vessel 2 is devoid of moderator material. In other words, the molten salt fuel fills and circulates inside the vessel without being moderated.

[0083] A single annular heat exchanger 3 between the reactor primary circuit and a secondary circuit is arranged inside the reactor vessel 2.

[0084] A first shell 4 in the form of at least one hollow cylinder, with its central axis coinciding 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.

[0085] The thickness of the bottom of the ferrule 4, in the core area C, can be reduced compared to that of the top of the ferrule 4. As an example, for a total height H equal to 2.5m, the reduced height H1 of the bottom of the ferrule 4 is equal to 1m.

[0086] A second ferrule 5 is arranged concentrically inside the first ferrule 4. The interior of the second ferrule 5 defines a space in which control and / or safety bars for nuclear reactions can extend.

[0087] Ferrules 4, 5 can be made of stainless steel or nickel-based alloy.

[0088] The ferrules 4, 5 are advantageously fixed by suspension to the cap-lid closing the reactor vessel 2.

[0089] At the bottom of reactor vessel 2, below the first shell 4, a first deflector 6, in the form of a portion of a torus.

[0090] At the top of reactor vessel 2, above the first shell 4, a second deflector 7, also in the form of a portion of a torus.

[0091] As symbolized by the arrows in Figure 1, with the shells 4, 5 and the deflectors 6, 7 as arranged, in reactor operation, the molten salt(s) fuel liquid circulates only by natural convection in a loop from the bottom of the central zone defining the reactor core C in 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 reactor C.

[0092] The shell 5 helps to guide the fuel liquid which rises between the two areas where it is diverted, that is to say in the central area of ​​the reactor from the diversion area by the deflector 6 through the core C to the diversion area by the deflector 7.

[0093] The deflectors 6, 7, by their shapes and arrangement, each allow the flow of the diverted molten salt(s) combustible liquid to be distributed.

[0094] As shown in Figure 1, the thickness of the part of the first shell, arranged above the exchanger 4, can be greater than that of its part arranged below the exchanger, i.e. at the level of the core C.

[0095] The dimensional, temperature and power characteristics of the molten salt fuel liquid obtained are as follows:

[0096] - dimensions: tank diameter 2 between 1.5 and 2m, primary circuit height between 2.5 and 4m;

[0097] - power between 10 and 500 MWth;

[0098] - primary circuit operating temperature between 500 and 750 °C;

[0099] - molten salt fuel liquid of the primary circuit to be chosen from a mixture of NaCl-UCL of 25 to 30% mol-PuCh of 9 to 11% mol with depleted uranium U235 at 0.7%, or a mixture of NaCl-UCL at 34% mol with natural uranium U235 enriched to 20%.

[0100] Advantageously, elements such as MgCh, minor actinide chlorides or other elements from the periodic table of elements can be added in varying proportions.

[0101] The reactor vessel 2 includes a head, usually called the fuel cell head, filled with an inert gas, such as argon or helium, above the molten salt fuel(s). Depending on the power of reactor 1, the free volume of the fuel cell head can typically be between 1 and 4 m³ 3As illustrated in Figure 2, the heat exchanger(s) 3 may include a bundle of bayonet tubes defining the exchange portion with the secondary circuit. The secondary fluid circulating in the heat exchanger(s) 3 may be based on a mixture of molten salts such as NaCl-MgCl2, NaCl-MgCl2-KCl, or NaCl-MgCl2-KCl-ZnCl2.

[0102] Each bayonet tube comprises a hollow tube 30 opening into the inside of a blind tube 31.

[0103] Each tube 30, 31 is immersed substantially vertically in the molten salt(s) combustible liquid according to a partial immersion height Hi.

[0104] Each open 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.

[0105] The inlet 32 ​​and outlet 33 manifolds of the secondary fluid are advantageously arranged in the stack crown.

[0106] A nuclear reactor 1 as just described, operates with molten salt(s) whose temperature varies between 450°C and 750°C.

[0107] To ensure the containment of the combustible liquid, fluid connections and tanks must be separated from the external environment by several containment barriers.

[0108] Thus, the inventors chose an architecture with another tank 8, called a safety tank, arranged around the reactor tank and inside the tank well, not shown, by defining an inter-tank space (E) with the reactor tank 2.

[0109] The inter-tank space E is filled with argon or another inert gas, preferably at a slight overpressure relative to the pressure in the reactor tank 2.

[0110] A neutron reflector 9 in the form of an envelope is arranged around reactor vessel 2.

[0111] The reflector 9 is preferably axisymmetric around the reactor vessel 2. This reflector is advantageously made of Pb and has a thickness between 50 and 70 cm.

[0112] A drain tank 10 is arranged below the reactor vessel, and is connected to the inter-vessel space (E) by at least one conduit 11 and to an outlet 20 in the bottom of the reactor vessel by at least one conduit 12. The drain tank for the molten salt(s) fuel liquid is designed to be in a subcritical configuration from the point of view of reactivity and to allow the evacuation of the residual power of the fuel.

[0113] A selective closure device 13 for the outlet opening 20 is in the closed position during normal operation of the reactor and in the open position to carry out the emptying of the reactor vessel 2.

[0114] Pipes 11, 12 and drain tank 10 are double-walled to form a continuity with the second containment barrier, which is the safety tank 8.

[0115] Pipes 11, 12 and / or drain tank 10 may preferably be made of stainless steel or nickel-based alloy.

[0116] At the end of a normal operating fuel cycle of the nuclear reactor, that is, when there is no longer enough fissile material to sustain the chain reaction in the fuel salt(s) contained in reactor vessel 2, the selective shut-off device 13 is opened. This causes the drain tank 10 to be filled by gravity with the molten fuel salt(s) contained in reactor vessel 2.

[0117] Once the reactor 2 tank is emptied, this allows operational procedures such as changing the reactor 2 tank, which can be considered irradiated.

[0118] Drain tank 10 allows for the independent management of maintenance or replacement of reactor vessel 2 and of the molten salt fuel(s). When the molten salt fuel is in drain tank 10, it is in a safe state with regard to criticality and thermal power dissipation.

[0119] Once the emptying step is completed, the following steps can be carried out: ii / putting the selective sealing device 13 into the closed position; iii / filling the reactor vessel 2 with new molten salt(s) fuel liquid or emptying the reactor vessel.

[0120] Furthermore, line 11 allows for the gravity flow of molten salt fuel(s) in the event of leaks from reactor vessel 10. Therefore, leaks from reactor vessel 2 can be considered operational incidents from a safety perspective, rather than accidents. The gas overpressure in the inter-vessel space does not impede gravity flow.

[0121] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0122] Other variants and embodiments may be considered without departing from the scope of the invention.

[0123] Several variants can be considered for the drain tank, with scaling depending on the reactor size. For example, it is possible to increase the size of the drain tank to allow for gravity drainage of the molten salt fuel at the end of the cycle for a 1000 MW molten salt reactor, or to reduce the volume for a 50 MW reactor.

[0124] If in the illustrated examples the nuclear reactor operates with a fuel salt based on Plutonium chloride, uranium and minor actinides, the invention can be implemented for a fuel liquid from fluoride, iodide and bromide salts.

[0125] List of cited references

[0126] [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).

[0127] [2]: Jiri Krepel et al. “ Self-Sustaining Breeding in Advanced Reactors: Characterization of Selected Reactors”, Encyclopedia of Nuclear Energy 2021, Pages 801-819. https: / / www.sciencedirect.com / science / article / pii / B97801281972570012397via%3Dihub

Claims

Demands 1. Nuclear reactor (1) comprising: - at least one nuclear reactor vessel (2) (1) filled with a bath of molten salt(s) fuel liquid and equipped with at least one opening in its bottom; - another tank (8), called the safety tank, arranged around the reactor tank by defining an inter-tank space (E) with the reactor tank; - a tank (10), called a drain tank, arranged below the tanks, and connected to the inter-tank space by at least one pipe (11) and to the outlet of the reactor tank by at least one pipe (12); - a selective closure device (13) for the outlet of the reactor vessel, in the closed position during normal reactor operation and in the open position to carry out the emptying of the reactor vessel.

2. Nuclear reactor (1) according to claim 1, of the fast neutron type, the reactor vessel being axisymmetric about a central axis (X), internally delimiting a primary circuit of a fuel in liquid form in which at least one salt is melted, the interior of the vessel being devoid of a moderator material.

3. Nuclear reactor (1) according to claim 1, comprising a shell (4) in the form of at least one hollow cylinder, with the central axis coinciding with that of the reactor vessel, the shell being arranged in the reactor vessel to separate the interior thereof into a central zone and a peripheral zone in which the heat exchanger is arranged so that in reactor operation, the molten salt(s) fuel liquid circulates 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 where it is deflected to the top of the peripheral zone to pass through the exchanger (ZE) and then descends to the bottom of the peripheral zone where it is deflected to the reactor core.

4. Nuclear reactor (1) according to any one of the preceding claims, comprising a neutron reflector (9), in the form of a shell arranged around the reactor vessel.

5. Nuclear reactor according to claim 4, the neutron reflector material being selected from lead (Pb), lead monoxide (PbO).

6. A nuclear reactor according to claim 4 or 5, the thickness of the reflector around the reactor vessel being between 50 and 70 cm.

7. A nuclear reactor (1) according to any one of the preceding claims, the piping and / or the drain tank being double-walled.

8. Nuclear reactor (1) according to any one of the preceding claims, the pipes and / or drain tank being preferably made of stainless steel or nickel-based alloy.

9. A method for operating a nuclear reactor (1) according to any one of the preceding claims, comprising the following step, at the end of a normal operating fuel cycle of the nuclear reactor: i / opening the selective shut-off device so as to cause the drain tank to be filled by gravity draining of the molten salt fuel contained in the reactor vessel.

10. A method according to claim 9, comprising, once step i / has been completed, the following steps: ii / closing the selective shut-off device; iii / filling the reactor vessel with fresh molten salt fuel or draining the reactor vessel.

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