Molten-salt nuclear reactor comprising a neutron reflector and rods that are made of neutron-absorbing material, arranged around the reactor vessel and movable in vertical translation

Abstract

The invention relates to a fast-neutron nuclear reactor (1) comprising: - a reactor vessel (2) of a nuclear reactor (1), the reactor vessel being filled with a pool of a molten-salt liquid fuel, the interior of the vessel being devoid of a moderator material; - a neutron reflector (8), in the form of a shell arranged around the reactor vessel; - a set of a plurality of rods (10) made of neutron-absorbing material, mounted such that they can move in vertical translation around the reactor vessel and inside the neutron reflector, between a so-called absorption position in which they absorb up to a predetermined threshold reactivity value, and a so-called disengaged position in which they absorb up to the predetermined threshold reactivity value, during the operation of the reactor.

Description

[0001] Description

[0002] Title: Molten salt nuclear reactor, with neutron reflector and neutron-absorbing material rods arranged around the reactor vessel and mobile in vertical translation.

[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 propose such a reactor, more particularly a fast neutron reactor, which guarantees management of the reactor's reactivity to allow it to operate in iso-generator mode.

[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] Previous technique

[0008] 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.

[0009] The tank contains the molten salt at high temperature, typically between 500 and 900 °C, generally at ambient pressure.

[0010] 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 function using a fertile blanket containing the fertile isotope to be irradiated. The nuclear reaction is triggered by the concentration of fissile material in the fuel within the reactor vessel or by its passage through a graphite moderator block.

[0011] A molten salt reactor can therefore be moderated by graphite, producing thermal neutrons, or without a moderator producing fast neutrons.

[0012] The presence or absence of moderators thus defines the two main families of molten salt reactors, respectively thermal neutron and fast neutron reactors.

[0013] 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.

[0014] Although promising in terms of safety potential, molten salt reactors may require expensive and complex systems and components.

[0015] 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.

[0016] 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.

[0017] For example, among the programs selected for Generation IV, the homogeneous indirect-cooled reactor developed by the LPSC laboratory in Grenoble, known by the acronym MSFR (for "Molten Salt Fast Reactor"), whose fuel is a liquid fluorinated salt with thorium-based breeder fuel, comprises twelve or sixteen fluidic circulation loops. Each component of the loops adds complexity to the overall fluidic circuit: [1]. For the design of a molten salt reactor, particularly of the SMR type, the inventors of the present invention initially sought to develop a design that minimizes the number of pipes and components, notably to preserve the major advantage inherent in SMRs, namely the increased modularity achieved by manufacturing components in a factory for transport to the construction site, and also to enhance operational safety.

[0018] 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, with a primary circuit by natural convection circulation." The proposed reactor can feature a reactor vessel incorporating a reduced-size primary fuel circuit, typically with a diameter of less than 2 meters and an overall height of less than 4 meters, 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.

[0019] 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).

[0020] 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.

[0021] A "regenerating" 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 material.

[0022] In burner mode, a fast neutron molten salt reactor can use the following as fissile isotopes: uranium-235, uranium-233, the fissile isotopes of plutonium, and the fissile isotopes of minor actinides. In breeder mode, a fast neutron molten salt reactor can use the same fissile isotopes listed above and the following as fertile isotopes: uranium-238, thorium-232, the fertile isotopes of plutonium, and the fissile isotopes of minor actinides.

[0023] Regeneration is the ability of a nuclear reactor to produce fissile material through fission in the reactor core. It is usually measured in terms of fissile atoms produced / fissile atoms consumed.

[0024] This principle is often used in liquid sodium-cooled fast neutron reactors (SFRs). Reactors like Phénix and Superphénix had a regeneration gain of around 1.1–1.2.

[0025] When this gain is equal to 1, the reactor is said to be iso-regenerative. In theory, it produces as much fuel as it consumes. If this gain is greater than 1, the reactor is in what is called breeder mode, meaning that it produces more material than it consumes. Since the different fissile isotopes do not contribute equally to the chain reaction, iso-reactivity is also possible: a nuclear reactor is said to be "iso-reactive" if the production of fissile nuclei by regeneration is sufficient to compensate for the loss of nuclei due to fission, such that the reactor's capacity to continue the chain reaction remains unchanged.

[0026] This creation of fissile material occurs using a so-called "fertile" isotope, which captures a neutron and becomes a "fissile" isotope. For example, FU238 is fertile and, by capturing a neutron, transforms into Pu239, which is fissile. Similarly, Th232 is fertile; by capturing a neutron, it eventually transforms into U233, which is fissile.

[0027] Several problems and limitations exist to this day:

[0028] - regeneration using U238 / Pu239 can only be done in SNRs, while in moderate spectrum (or thermalized) reactors, known as thermal spectrum reactors, only regeneration using Th232 / U233 is possible;

[0029] - The fertile isotope must be placed in the reactor core. However, in solid fuel reactors, it is often placed in the form of a blanket, meaning that some parts of the core are primarily dedicated to fission, while the blankets contain other parts composed of fertile elements. This core configuration leads to the formation of pure fissile isotopes (Pu239 or U233) in the blankets, and therefore to a risk of proliferation through chemical separation of these elements; - if regeneration takes place in a blanket or a solid fuel assembly, then it is necessary to unload the fertilized portion of the core and send it to reprocessing and chemical separation units in order to incorporate the fissile material produced into new solid fuel assemblies that can then be loaded into the reactor core. This chain of operations is very long, typically between 15 and 20 years;

[0030] The concept of regeneration is rather vague from the perspective of reactor neutronics, as it only specifies the quantity of fissile material produced relative to the energy generated. A reactor can be isogenerant without necessarily producing enough fuel to ensure its operation. This is because regeneration does not incorporate concepts such as geometry, absorption, and other factors specific to the reactor itself.

[0031] There is therefore a need to design a reactor that can have long-lasting fuel for a period of 15 years or more, without having to treat fertile blankets, without having to go through a reprocessing / refabrication stage of the fuel, with a real possibility of predicting the duration of the fuel in the reactor studied, that is to say more precisely than the quantity of fissile material produced, in order to be able to control this evolution of the fuel over the long term, preferably in a simple way.

[0032] The aim of the invention is therefore to meet at least part of this need.

[0033] Description of the invention

[0034] To this end, the invention relates, in one of its aspects, to a fast neutron nuclear reactor comprising:

[0035] - a nuclear reactor vessel filled with a bath of molten salt(s) combustible liquid, the inside of the vessel being devoid of a moderator material;

[0036] - a neutron reflector, in the form of an envelope arranged around the reactor vessel;

[0037] - an assembly of a plurality of neutron-absorbing rods, mounted to move vertically around the reactor vessel and inside the neutron reflector, between an absorption position in which they absorb up to a predetermined reactivity threshold, and a clearance position in which they do not absorb up to the predetermined reactivity threshold, during reactor operation. Preferably, the reactor vessel is axisymmetric about a central axis (X), internally delimiting a primary circuit of liquid fuel in which at least one salt is fused, the reflector being axisymmetric about the reactor vessel.

[0038] According to a preferred application, each nuclear reactor in the power plant is, according to the teachings of the aforementioned patent application FR2213882.

[0039] With the design that the inventors have proposed in the aforementioned FR2213882 patent application, the exploitation of the great versatility of fast neutron molten salt reactors is made possible.

[0040] Thus, each 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.

[0041] Even more advantageously, at least one heat exchanger between the reactor's primary circuit and a secondary circuit is arranged inside the reactor vessel.

[0042] 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.

[0043] 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.

[0044] Advantageously, the neutron reflector material is chosen from lead (Pb) or lead monoxide (PbO). These materials have the advantages of being weak neutron absorbers while only slightly moderating neutrons.

[0045] Preferably, the thickness of the reflector around the reactor vessel is between 50 and 70cm.

[0046] Even more advantageously, the neutron-absorbing material is chosen from enriched or unenriched boron carbide (B4C), hafnium (Hf), a ternary silver-indium-cadmium alloy (AIC).

[0047] According to a preferred configuration, the absorption position corresponds to a height approximately equal to half the reactor core.

[0048] Preferably, the reactivity threshold value is less than or equal to 1000 pcm.

[0049] According to an advantageous embodiment, the neutron-absorbing material bars are mounted to move in vertical translation from the top of the reactor vessel.

[0050] According to an advantageous embodiment, the reactor includes at least one vertical descent / ascent mechanism for each of the bars or common to all the bars.

[0051] According to this mode, the mechanism is preferably adapted to move the bars at a translation speed of less than lem / s, preferably less than 0.5cm / s.

[0052] Advantageously, the number of bars arranged around the reactor vessel is between 2 and 12. The cross-section of the bars can be cylindrical or rectangular.

[0053] Even more advantageously, the distance between the rods and the reactor vessel can be between 1 and 20 cm.

[0054] A nuclear reactor may have one or both of the following dimensional characteristics for a power output typically between 250 and 600 MWth:

[0055] - the diameter of the reactor vessel is between 2 and 2.4 m;

[0056] - The height of the primary circuit inside the reactor vessel is between 4 and 5 m. Preferably, the molten salt fuel 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.

[0057] During reactor operation, the temperature of the molten salt(s) fuel liquid in the primary circuit can be between 500 and 750°C.

[0058] The power of a nuclear reactor is advantageously between 250 and 600 MWth.

[0059] Thus, the invention essentially consists of creating a nuclear reactor whose vessel is surrounded by a neutron reflector inside which bars of neutron-absorbing material can be inserted by vertical translation into the reflector to control the reactivity in the reactor core.

[0060] Reactivity, measured in pcm (parts per 100,000), reflects the multiplier by which the number of fissions increases or decreases in the reactor core with each new generation of neutrons. When a reactor's reactivity is 1, the reactor is in a critical (stable) state.

[0061] When it is negative, the reactor is subcritical (the reactor's power decreases, or even stops)

[0062] When it is greater than 1, the reactor is supercritical (its power is increasing).

[0063] Reactivity is a neutronic quantity involving the quantities of fertile and fissile materials, but also the geometry of the reactor, the components, etc. It is therefore a quantity representative of a reactor.

[0064] Iso-reactivity measures the ability of a reactor to produce reactivity as it consumes it.

[0065] Neutron absorption rods, by moving along the reactor vessel, will allow for the long-term control of fuel evolution within the reactor. Advantageously, it is anticipated that from a midpoint, halfway up the reactor core (the reactor's initial critical state), all the rods should be able to absorb a predetermined threshold reactivity value during descent and release this threshold value during ascent. Typically, this threshold value can be 1000 pcm.

[0066] In conclusion, a molten salt fast neutron nuclear reactor according to the invention offers numerous advantages, including:

[0067] - a reactor capable of having a long-lasting fuel supply for a period of 15 years;

[0068] - active control of fuel evolution over the long term with a system that is simple to implement;

[0069] - a small footprint for reactivity control because the moving bar control system can be completely integrated around the reactor vessel.

[0070] 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.

[0071] Brief description of the drawings

[0072] [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 power plant according to the invention.

[0073] [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 according to Figure 1.

[0074] [Fig 3] Figure 3 is a schematic longitudinal cross-sectional view of the reactor vessel with a neutron reflector and reactivity control rods arranged around it according to the invention.

[0075] Detailed description

[0076] Throughout this application, the terms "vertical," "lower," "upper," "bottom," "top," "below," and "above" are to be understood with reference to a fast neutron molten salt nuclear reactor, as envisaged in its vertical operating configuration according to the invention. It is specified that the various temperatures, power outputs, volumes, flow rates, etc., indicated are for illustrative 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 for the intended application.

[0077] With reference to Figure 1, a molten salt fast neutron nuclear reactor 1 is described, according to a primary circuit configuration as described and claimed in patent application FR2213882. This Figure 1 is a numerical simulation view obtained by coupling Computational Fluid Dynamics (CFD) and 3D neutronics, as explained below.

[0078] The reactor 1 with central axis X includes a tank 2 with a metal jacket preferably made of stainless steel or nickel-based alloy, with a thickness of approximately 10 to 20 mm, and formed of a hemispherical tank bottom and a vertical cylinder.

[0079] 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.

[0080] A single annular heat exchanger 3 between the reactor's primary circuit and a secondary circuit is arranged inside the reactor vessel 2.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] The ferrules 4 and 5 can be made of stainless steel or nickel-based alloy. Advantageously, the ferrules 4 and 5 are suspended from the cap-lid closing the reactor vessel 2.

[0085] 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.

[0086] 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.

[0087] 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 and then descends towards the bottom of the peripheral zone where it is deflected by the deflector 7 towards the core of reactor C.

[0088] 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.

[0089] The deflectors 6, 7, by their shapes and arrangement, each allow the flow of the diverted molten salt(s) combustible liquid to be distributed.

[0090] 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.

[0091] The dimensional, temperature and power characteristics of the molten salt fuel liquid obtained are as follows:

[0092] - dimensions: tank diameter 2 between 1.5 and 2m, primary circuit height between 2.5 and 4m;

[0093] - power between 10 and 600 MWth;

[0094] - primary circuit operating temperature between 450 and 750 °C;

[0095] - molten salt fuel liquid of the primary circuit to be chosen from a mixture of NaCl-UCh 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%, where appropriate including ThC14.

[0096] Advantageously, elements such as MgCh, minor actinide chlorides or other elements from the periodic table of elements can be added in varying proportions.

[0097] As 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.

[0098] Each bayonet tube comprises a hollow tube 30 opening into the inside of a blind tube 31.

[0099] Each tube 30, 31 is immersed substantially vertically in the molten salt(s) combustible liquid with a partial immersion height Hi.

[0100] 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.

[0101] The inlet manifolds 32 and outlet manifolds 33 of the secondary fluid are advantageously arranged in the stack crown 20.

[0102] A nuclear reactor 1 as just described, operates with molten salt(s) whose temperature varies between 450°C and 750°C.

[0103] According to the invention, a neutron reflector 8, in the form of an envelope, is arranged around the reactor vessel 2. The reflector 8 is preferably axisymmetric around the reactor vessel 2. This reflector is advantageously made of Pb and has a thickness between 50 and 70 cm.

[0104] A plurality of bars 10 made of neutron-absorbing material are mounted to move vertically around the reactor vessel 2 and inside the neutron reflector 8, between an absorption position, in which they absorb up to a predetermined reactivity threshold, and a clearance position, in which they absorb up to the predetermined reactivity threshold, during reactor operation. These bars 10 may be cylindrical or have a rectangular cross-section. Their composition is chosen according to their neutron absorption capacity. Typically, they may be enriched or unenriched boron carbide, hafnium, AIC, or any other neutron-absorbing material.

[0105] The reactivity threshold value is preferably predetermined at 1000 pcm.

[0106] Each of the 10 rods can be moved vertically by means of at least one displacement mechanism, which may be specific to each rod or common to all rods. Since the evolution of reactivity in the reactor core is slow, the mechanism can be adapted to give the rods a relatively slow displacement speed. Typically, a speed of 0.1 cm / s may be sufficient.

[0107] We now describe the operation of reactor 1 which has just been described.

[0108] It is specified that initially, the composition of the molten salt(s) fuel liquid is adjusted so that the reactor is initially critical, with 10 bars positioned at mid-stroke of displacement and to guarantee F iso-reactivity of the reactor over a period of normal operation, desired between 10 and 20 years.

[0109] The reactor's reactivity will evolve from an initial critical state and evolve within a range between -1000 pcm and +1000 pcm for a period between 10 and 20 years, indicating that the reactor operates at iso-reactivity to within + / - 1000 pcm.

[0110] The natural operation of the molten salt reactor will be to compensate for this variation in reactivity by playing on its temperature to maintain the reactor in a critical state.

[0111] To do this, the molten fuel(s) salt(s) will intrinsically either increase in operating temperature if the reactivity increases, or decrease in operating temperature if the reactivity decreases: an increase in reactivity of 1 pcm will result in a rise in the average temperature in the core area of ​​the order of 0.1°C.

[0112] The neutron-absorbing material rods 10 allow the molten fuel salt(s) (and the reactor) to be maintained at a fixed operating temperature.

[0113] The movement of these bars 10 is done like the adjustment or recalibration of a setpoint in the operation of the reactor: if the reactivity increases, the bars 10 are lowered, so as to maintain the temperature of the molten salt(s) fuel(s) at its / their normal operating point planned for the reactor; conversely, if the temperature decreases, the bars 10 are raised so as to maintain the temperature of the molten salt(s) fuel(s) at its / their normal operating point planned for the reactor.

[0114] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0115] Other variants and embodiments may be considered without departing from the scope of the invention.

[0116] We can consider different variants for the rod system 10 Thus, it is possible to increase the size and number of rods depending on the power of the molten salt reactor(s), typically from 250 to 600 MWth.

[0117] List of cited references

[0118] [1]: E. Merle-Lucotte, M. Allibert, M. Brovchenko, D. Heuer, V. Ghetta, A. Laureau, P. Rubiolo, Chapter “Introduction to the Physics of Thorium Molten Salt Fast Reactor (MSFR) Concepts'”, Thorium Energy for the World, Springer International Publishing, Switzerland (2016).

[0119] [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. A fast neutron nuclear reactor (1) comprising: - a nuclear reactor vessel (2) (1) filled with a bath of molten salt(s) fuel liquid, the inside of the vessel being devoid of a moderator material; - a neutron reflector (8), in the form of an envelope arranged around the reactor vessel; - a set of a plurality of bars (10) made of neutron-absorbing material, mounted movably in vertical translation around the reactor vessel and inside the neutron reflector, between a so-called absorption position in which they absorb up to a predetermined threshold value of reactivity, and a so-called clearance position in which they do not absorb up to the predetermined threshold value of reactivity, during the operation of the reactor.

2. Nuclear reactor according to claim 1, 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 reflector being axisymmetric about the reactor vessel.

3. Nuclear reactor according to claim 1 or 2, the nuclear reactor (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 according to any one of the preceding claims, the neutron reflector material being selected from lead (Pb), lead monoxide (PbO).

5. Nuclear reactor according to one of the preceding claims, the thickness of the reflector around the reactor vessel being between 50 and 70cm.

6. Nuclear reactor according to any one of the preceding claims, the neutron-absorbing material being selected from enriched or unenriched boron carbide (B4C), hafnium (Hf), a ternary silver-indium-cadmium alloy (AIC).

7. Nuclear reactor according to any one of the preceding claims, the absorption position corresponding to a height approximately equal to half of the reactor core.

8. Nuclear reactor according to any one of the preceding claims, the threshold reactivity value being less than or equal to 1000 pcm.

9. Nuclear reactor according to any one of the preceding claims, the neutron-absorbing material rods being mounted movable in vertical translation from the top of the reactor vessel.

10. Nuclear reactor according to any one of the preceding claims, comprising at least one vertical descent / ascent mechanism for each of the rods or common to all the rods.

11. Nuclear reactor according to claim 9, the mechanism being adapted to move the rods at a translation speed of less than lem / s, preferably less than 0.5cm / s.

12. Nuclear reactor according to any one of the preceding claims, the number of rods arranged around the reactor vessel being between 2 and 12.

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