System for controlling the reactivity of a nuclear reactor with rotating non-fuel assemblies, said system being located on the periphery of the core

Abstract

The invention relates to a system (2) for controlling the reactivity of a fast neutron nuclear reactor comprising at least one sub-assembly comprising: - a non-fuel assembly (200) of generally cylindrical shape, comprising a plurality of pins or rods (201) made of neutron-absorbing material, and a plurality of pins or rods (202) made of neutron-reflecting material, arranged parallel to the pins or rods made of neutron-absorbing material, - a system (22) for rotating the non-fuel assembly about its axis.

Description

[0001] Description

[0002] Title: Reactivity control system of a rotating non-fueled nuclear reactor assembly, at the periphery of the core.

[0003] technical field

[0004] The present invention relates to the field of so-called fast neutron nuclear reactors, devoid of moderator material.

[0005] 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 [1], and in particular to Figure 4, which shows, for several types of reactors, the thermal fraction and the fast fraction of the neutron flux.

[0006] It relates in particular to a fast neutron nuclear reactor cooled with liquid metal, notably liquid sodium known as SFR (Sodium Fast Reactor), lead or a lead alloy, and which is part of the family of so-called fourth generation reactors (GEN IV).

[0007] The invention is preferably applicable to small or medium power reactors or SMRs (acronym for "Small Modular Reactor"), typically with an operating power between 50 and 300 MWe.

[0008] More specifically, the invention relates to a new design of such a reactor which makes it possible to overcome the disadvantages of reactivity control systems according to the state of the art.

[0009] The invention may relate to both an integrated nuclear reactor, i.e., one in which the primary circuit of liquid metal, such as sodium, with pumping means is entirely contained within a vessel also containing heat exchangers, and a loop reactor, i.e., one in which the heat exchangers and the pumping means for the primary fluid, such as sodium, are located outside the vessel. A fuel assembly is understood to be an assembly comprising fuel elements and loaded and / or unloaded into / from a nuclear reactor.

[0010] A non-combustible assembly is defined as an assembly comprising elements made of neutron-absorbing material, with, where appropriate, one or more elements made of neutron-reflecting material.

[0011] Previous technique

[0012] Conventional sodium-cooled fast reactors (SFRs) and pressurized water reactors (PWRs) have a reactivity control system that is arranged to come directly inside the reactor core.

[0013] Such a system is described as "in-core" or "Reactor In Core" (RIC) and has a number of major drawbacks which can be summarized as follows:

[0014] - it generates a strong disturbance in the axial and radial neutron flux;

[0015] - it occupies a significant amount of space;

[0016] - an accident involving an untimely retraction of the system's control bars can be very detrimental, as it occurs in the center of the heart;

[0017] - control bars can have high temperatures which cause significant expansion of materials and can alter their mechanical properties;

[0018] - it requires a high use of fissile materials;

[0019] - it can generate significant material damage (dpa) and faster consumption of neutron absorbers;

[0020] - its design and maintenance can prove complicated.

[0021] Ex-core reactivity control systems, meaning those arranged outside the reactor core, have already been proposed for certain types of reactors and they make it possible to solve at least some of the problems of the conventional systems listed above.

[0022] US patent 11,417,435 B2 discloses a reactivity control drum for a nuclear reactor, particularly a gas-cooled reactor, in the form of a double-shell casing housing a plurality of tubes comprising at least one neutron-absorbing tube and at least one neutron-diffracting tube, and at least one deflector plate. US patent 11,380,449 B2 relates to a system of reactivity control drums for nuclear reactors, particularly for space-based thermal nuclear reactors, each control drum being equipped with a drive shaft, a drum cylinder, and a planetary gear, with a single drive motor to allow simultaneous rotation of all the control drums.

[0023] US11,61O,694B2 describes a reactivity control system exclusively dedicated to gas-cooled, graphite-moderated reactors, and which consists of two coaxial cylinders forming a ring at the periphery of the core inside which are control rods.

[0024] None of these documents proposes a reactivity control system truly suited to liquid-metal cooled, fast neutron nuclear reactors. Furthermore, none of them provides for effective management of the expansion of neutron absorbers, such as boron, and, in addition, management of the reaction products of boron-10 with neutrons, such as 10B(n, a)Li, 10B(n, 2a)T, and 7Li(n, na)T.

[0025] There is therefore a need to further improve so-called "ex-core" reactivity control systems, in particular so that they are adapted to fast neutron nuclear reactors, especially those cooled with liquid metal, while ensuring effective management of the expansion of neutron absorbers, such as boron, and preferably management of boron-10 reaction products.

[0026] The aim of the invention is to meet at least part of this need.

[0027] Description of the invention

[0028] To this end, the invention relates, in one of its aspects, to a fast neutron nuclear reactor comprising:

[0029] - a reactor vessel filled with a heat transfer fluid,

[0030] - a reactor core made up of fuel assemblies,

[0031] - a reactivity control system, arranged partly in the annular space between the reactor core and the reactor vessel, and comprising a plurality of cylindrical locations distributed, preferably regularly, in the annular space, each cylindrical location housing at least one non-fuel assembly or at least one non-fuel monoblock rod comprising at least one neutron-absorbing material, the at least one assembly or at least one monoblock rod being housed in a cylindrical location,

[0032] - a metallic ring called a redan, arranged inside the vessel and around the reactor core, to separate the so-called hot manifold which receives the heat transfer fluid at the outlet of the core, from the cold manifold which recovers the heat transfer fluid, the assemblies or bars of the reactivity control system being arranged in the cold manifold and through which the heat transfer fluid does not pass through the core so as to be cooled by it.

[0033] According to an advantageous variant, each cylindrical location houses a single non-combustible assembly or a plurality of monolithic bars comprising at least one neutron-absorbing material.

[0034] According to an advantageous embodiment, the reactivity control system comprises a plurality of vertical translation or rotational displacement systems around the vertical axis, of assemblies or bars, independent location by cylindrical location.

[0035] According to an advantageous embodiment, each cylindrical location is delimited by a cylindrical tube in the cold manifold forming a jacket through which the heat transfer fluid circulates.

[0036] According to another advantageous embodiment, the reactor comprises a plurality of tubes, each forming a passage through the step, each of these passages being in fluidic communication with the cold collector and housing at least part of the linkage of each system for moving the assemblies or the rods.

[0037] Advantageously, each penetration is sealed against the reactor vessel. Preferably, the inside of each penetration is connected to a liquid metal purification device (coolant), whose function is to cool it to crystallize its impurities.

[0038] Advantageously, each of the penetrations is filled in its upper part with a gas, preferably argon, forming a gas head. This gas head is preferably connected to at least one cold trap.

[0039] Preferably, the upper part of each penetration is also in fluidic communication with a gas supply device. According to an advantageous embodiment, the reactor comprises a plurality of assemblies or single-piece rods made of neutron-reflecting material, arranged in portions of the annular space not occupied by cylindrical locations. These assemblies may be in the form of baskets.

[0040] The nuclear reactor according to the invention is advantageously a liquid metal cooled reactor, in particular liquid sodium, with a loop or integrated exchanger configuration.

[0041] Thus, the invention in this first aspect essentially consists of arranging in a fast neutron nuclear reactor vessel, a reactivity control system at the periphery of the core, in a cold collector zone, between the reactor vessel and the separation step between hot and cold collector, so that the reactivity control assemblies or monobloc bars are traversed by the cold heat transfer fluid.

[0042] Thus, efficient cooling of the neutron absorbers is achieved by direct contact with the cold heat transfer fluid.

[0043] Furthermore, by adding a jacket to contain the gases and other products resulting from the interaction between the heat transfer fluid and neutron absorbers, efficient collection and potential treatment of radioactive gases can be ensured. Indeed, particularly with liquid sodium, when it passes either inside the non-combustible assemblies or in contact with the solid fuel rods, it carries away helium as well as tritium, which is a product of the reactions of boron-10 with neutrons, such as 10B(n, a)Li, 10B(n, 2a)T, and 7Li(n, na)T. Tritium is problematic because it is volatile and radioactive.

[0044] Once collected, these products can be purified by cold traps that fix them.

[0045] The removal of helium also helps to prevent excessive expansion of neutron absorbers, such as boron carbide (B4C).

[0046] In other words, the nuclear reactivity control system as arranged in a fast neutron nuclear reactor makes it possible to control the expansion of neutron absorbers, by efficient cooling and continuous evacuation and treatment of the gases generated by the interaction with a liquid metal.

[0047] Ultimately, the arrangement of an "ex-core" reactivity control system according to the invention offers numerous advantages, including: - efficient management of the expansion of neutron absorbers, with the possibility of extracting gases, particularly tritium, produced by the reaction of neutrons with the neutron absorber, such as B4C,

[0048] - less disturbance in the neutron flux, which allows for better fuel consumption and a flatter power profile (i.e., fewer temperature peaks),

[0049] - a reduction in size, as it occupies a limited space in the cold collector of a nuclear reactor,

[0050] - a lower local power increase in the event of degraded rod operation (for example, the unexpected retraction of rods), because the neutron flux outside the core is reduced,

[0051] - Low operating temperatures of non-combustible assemblies or bars, which reduce material expansion and therefore deformation of the assemblies or bars, thereby improving safety.

[0052] - a lower penalty on the amount of fissile material to be used. The lower importance of neutrons produced at the periphery of the core means that their absorption, due to the presence of absorbers, must be compensated by a smaller quantity of fissile material than with absorbers positioned at the center of the core, which capture more significant neutrons.

[0053] - a lower neutron flux on the control rods due to their distance from the core. As a result, material damage, typically expressed in displacements per atom (dpa), is reduced, as neutron absorbers consume more slowly and neutron activation is significantly decreased. This simplifies system design and maintenance.

[0054] - improved system diversification, due to the space saved compared to an "in-core" system, which allows for the addition of more control bars and the use of multiple motors in parallel to ensure proper bar movement,

[0055] - a reduction in thermal shock in the event of an AAR (Automatic Engine Shutdown),

[0056] - Bypassing the coolant in the control system is less problematic due to its location outside the core. The invention also relates, in another aspect, to a reactivity control system for a fast neutron nuclear reactor, comprising at least one sub-assembly including:

[0057] - a non-combustible assembly of generally cylindrical shape, comprising a plurality of needles or pencils of neutron-absorbing material, and a plurality of needles or pencils of neutron-reflecting material, arranged parallel to the needles or pencils of neutron-absorbing material,

[0058] - a system for rotating the assembly around its axis.

[0059] According to an advantageous embodiment, the system comprises:

[0060] - a plurality of non-combustible assemblies,

[0061] - a plurality of rotational displacement systems around the central axis of the non-combustible assemblies, each displacement system of an assembly being independent of the others.

[0062] According to an advantageous embodiment, each non-combustible assembly comprises:

[0063] - a plurality of neutron-absorbing needles or pencils grouped together in parallel with each other,

[0064] - a plurality of needles or pencils made of neutron reflecting material grouped together in parallel with each other and adjacent to the plurality of needles or pencils made of neutron absorbing material.

[0065] Advantageously, the height of the needles or pencils made of neutron absorbing material and that of the needles or pencils made of neutron reflecting material is equal to or greater than the height of a nuclear reactor core including fuel assemblies.

[0066] Needles or pencils made of neutron-reflecting material preferably occupy a larger volume within each non-combustible assembly than needles or pencils made of neutron-absorbing material.

[0067] According to a preferred configuration, the neutron absorber needles or pencils are distributed in each assembly such that in a given nominal position, a plurality of assemblies define a cylindrical surface concentric to the cylindrical part of a reactor vessel, around its core.

[0068] Advantageously, each non-combustible assembly comprises a plurality of retaining plates or grids distributed along the height of the assembly. These grids or plates ensure correct positioning of the needles or pencils and provide vibration resistance for the non-combustible assembly.

[0069] In an advantageous embodiment, each drive system comprises a rotating drive motor coupled to a right-angle gearbox, which is itself coupled to a linkage whose free end is attached to a non-combustible assembly. The rotation of this non-combustible assembly can thus be achieved by an electric motor that rotates the linkage via an electromagnetic device capable of step-by-step operation.

[0070] According to this method, the system includes at least one sealing element mounted around the angle drive, forming a sealed interface with the outside of a nuclear reactor vessel.

[0071] The invention also relates to the use of a reactivity control system as described above in a fast neutron nuclear reactor, of the liquid metal cooled type, in particular liquid sodium, with loop or integrated exchanger configuration.

[0072] Thus, the invention in this second aspect consists of a nuclear reactivity control system with non-fuel assemblies, generally cylindrical in shape, comprising at least one neutron-absorbing material which can be rotated around their central axis, in order to control the nuclear reactivity, in particular of a fast neutron nuclear reactor.

[0073] Advantageously, each non-combustible assembly comprises needles or rods made of absorbing material, and others made of neutron reflecting material. Such a mixed neutron reflector / absorbing assembly allows the same area of ​​the nuclear reactor to be dedicated to performing two different, complementary functions.

[0074] In conclusion, compared to vertical displacement reactivity control systems within a nuclear reactor core, a reactivity control system with rotational displacement around the axis of control assemblies according to the invention offers numerous advantages, including:

[0075] - better use of space in a nuclear reactor because rotational movement requires less space to function correctly, i.e., to control reactivity effectively,

[0076] - an improvement in system safety, in particular by eliminating a known serious accident involving the unexpected retraction of control bars,

[0077] - the possibility of incorporating a neutron reflector in the same assembly as the neutron absorber, which makes it possible to reduce the size of a reactor core, instead of providing two different spaces as in conventional reactors, particularly those of the Na-NR type,

[0078] - the possibility of having a useful reactivity control height (height of the neutron absorber) which can be greater than the fissile height of the core, thus allowing the control function to be maintained even in the event of a physical fall of all or part of the core fuel assemblies,

[0079] - Optimized fuel consumption, thanks to a more homogeneous neutron flux. Indeed, with the rotational movement of the neutron-absorbing needles or rods concentrically around the core, the core's neutron flux is symmetrically affected by the presence of the absorber, unlike conventional reactors where the top-mounted control rods promote a more inhomogeneous power output towards the bottom.

[0080] - a simplification of simplified maintenance, because with an adapted control of the rotational movement system, it is possible to replace a single needle or neutron absorber pencil per non-combustible assembly, in a single step, instead of necessarily replacing a complete control rod assembly as in conventional systems.

[0081] Other advantages and features of the invention will become clearer upon reading the detailed description of examples of implementations of the invention given by way of illustration and not limitation with reference to the following figures.

[0082] Brief description of the drawings

[0083] [Fig. 1] Figure 1 is a longitudinal cross-sectional view of a liquid sodium-cooled nuclear reactor (NSR-Na) equipped with a nuclear reactivity control system according to the invention. [Fig. 2] Figure 2 is a cross-sectional view of the reactor as shown in Figure 1.

[0084] [Fig 3] Figure 3 is a perspective view of a nuclear reactivity control system according to an advantageous embodiment of the invention, with rotational displacement of assemblies.

[0085] [Fig 4], [Fig 4A], [Fig 4B] Figures 4, 4A and 4B are schematic longitudinal section views, respectively, along AA and BB, of the lower part of a reactor and a subset of a reactivity control system according to the invention.

[0086] [Fig 5A], [Fig 5B] Figures 5A and 5B are schematic cross-sectional views along AA and BB respectively of the upper part of a reactor and a subset of a reactivity control system according to the invention.

[0087] [Fig 6] Figure 6 is a perspective view of one of the subsets of a reactivity control system, according to one embodiment.

[0088] [Fig 7] Figure 7 is a detail view of a subassembly according to Figure 6 showing the motorization part and part of the linkage connecting the motorization to a non-combustible assembly of the subassembly.

[0089] [Fig 8], [Fig 9], [Fig 10] Figures 8, 9 and 10 are schematic views in global and detail perspective respectively of a subset of a reactivity control system, with needle assemblies according to a first variant of embodiment.

[0090] [Fig 11], [Fig 11A] Figures 11 and 11A are schematic views respectively in perspective and in cross-section of a subset of a reactivity control system, with needle assemblies according to a second variant of embodiment.

[0091] [Fig 12] Figure 12 is a schematic view showing the arrangement of a reactivity control system in the cold collector of a Na-NR nuclear reactor with monoblock control rods.

[0092] Detailed description

[0093] Throughout this application, the terms "horizontal," "vertical," "lower," "upper," "below," and "above" are to be understood with reference to a reactor vessel arranged vertically and its position relative to the cold or hot zone. Similarly, throughout this application, the terms "upstream" and "downstream" are to be understood with reference to the direction of sodium flow.

[0094] Figures 1 and 2 show a loop-type SNR-Na nuclear reactor, globally designated by reference 1, comprising a reactivity control system 2 according to the invention.

[0095] Reactor 1 includes first of all a reactor vessel 10 with central axis X, inside of which is arranged a core 11 in which the heat is released following the nuclear reactions.

[0096] The vessel 10 is closed by a plug 12, commonly called the "core lid plug," which includes the instrumentation necessary for the control and proper functioning of the nuclear reactions.

[0097] The core 11 consists of fuel assemblies 13. Advantageously, these assemblies are devoid of a box or hexagonal tube around the needles which constitute them, and made as described and claimed in the patent application filed by the applicant on November 19, 2024 under number FR2412649 and entitled "Fuel assembly for nuclear reactor, of type RNR-Na, devoid of a box, comprising a bundle of fuel needles held laterally by grids with cells and longitudinally by stiffening rods, passing through the grids".

[0098] The core 11 may advantageously include in its center an assembly or rod 14 of an autonomous system that ensures the automatic shutdown of the reactor. The control motor for this system is housed in the plug 12, the control linkage extending from the plug to the assembly or rod 14.

[0099] The core 11 is supported by a base 15 into which are embedded the feet of the assemblies 13 constituting the core and which has fluidic passages, this base 15 being supported by a non-watertight platform 16 resting on the bottom of the tank 10.

[0100] A so-called hot collector 17 is separated from the so-called cold collector 18 below by a suitable separation device 19, called a step, in the form of a metal ring. As illustrated, this ring 19 has a frustoconical shape in its upper part, which is extended by a cylindrical shape in its lower part, down to the level of the core 15. Spaces 110, free of fuel assemblies 11, may be provided between the core 11 and the step 19.

[0101] According to the invention, the reactivity control system 2 is arranged partly in the annular space between the core 11 and the reactor vessel 10, and it comprises a plurality of cylindrical locations distributed, preferably regularly, in the annular space.

[0102] Each cylindrical location houses a part of a subset 20 of system 2 comprising at least one non-combustible assembly or at least one non-combustible monobloc bar, generally cylindrical in shape, comprising at least one neutron-absorbing material.

[0103] The assemblies or control bars 200 of each sub-assembly 20 of the reactivity control system 2 are arranged in the cold manifold 18 to be traversed by the heat transfer fluid not passing through the core 11 so as to be cooled by it, as detailed below.

[0104] As illustrated in Figure 2, in order to occupy the maximum space in the cold collector 18, the identical subassemblies 20 can be arranged so that they are joined to each other.

[0105] The cold collector 18 can also house assemblies of pencils or monobloc bars of neutron reflector material 180, preferably assembled in the form of baskets, in the spaces not occupied by the sub-assemblies 20 of the reactivity control system 2.

[0106] The heat dissipation circuit followed by sodium (S) in normal operation of core 11 is schematically represented by the dashed arrows in Figure 1:

[0107] - the cold sodium arriving in the cold collector 18 descends between the tank 10 and the step 19 but without passing into the assemblies or bars 200 which are housed in a cylindrical tube, 210, sealed on its periphery except for its lower end, forming a jacket which therefore isolates the assemblies or bars 200 from the cold sodium circulating in this space between cold collector 18 and tank 2,

[0108] - the cold sodium reverses direction at the level of the bed 15 so that a major flow reaches the core 11 after passing through the deck 16 and the bed 15 and the remaining flow reaches the assemblies or control bars 200, - the major flow of sodium becomes hot as it passes through the core 11 and rises to be evacuated from the vessel 10 from the hot collector 17,

[0109] - The remaining flow of cold sodium is cooled by direct contact with all the jackets 210 and the assemblies or bars 200 through which it flows, thanks to the presence of an inlet plug 207 at its lower end and a vent plug 207 at its upper end, allowing the passage of sodium. As illustrated in Figure 9, a vent plug 207 can be made from a block of porous material.

[0110] Figure 3 shows an advantageous example of a reactivity control system 2 according to the invention with a plurality of identical subassemblies 20, each of whose non-combustible assemblies 200 is intended to occupy a cylindrical location in the cold collector. The portions of all the non-combustible assemblies 200 that affect the reactivity of the core 11 extend parallel to and along the entire height of the core.

[0111] Each subsystem 20 comprises a displacement system 22 rotating around the vertical axis of a non-combustible assembly 200 which is specific to each subassembly 2. In other words, the displacement systems 22 are independent of each other and therefore location by location cylindrical.

[0112] As illustrated in figures 6, 6A, 6B and 7A and 7B, the nuclear reactor 1 comprises a plurality of tubes 3, each forming a crossing of the redan.

[0113] Each of these crossings 3, sealed with respect to the reactor vessel 10, is in fluidic communication with the cold collector 18 and houses at least part of the linkage 220 of each assembly movement system 22 or rods 200.

[0114] Each of the through-holes 3 is filled in its upper part with a gas, preferably argon, forming a gas headspace 30. The gas headspace 30 is fluidically connected to a cold trap 32 via a pipe 31, outside the reactor vessel 10. This cold trap 32 can be connected to a gas pressurization device 33. Such a cold trap 32 makes it possible to trap the tritium and helium that are removed by the cold sodium passing through the assemblies 200 and therefore from inside the sealed through-holes 3. The removal of tritium, which is a volatile and radioactive gas, can thus be carried out continuously, as can the removal of helium, which also prevents excessive expansion of the neutron-absorbing materials, such as B4C, of ​​the assemblies 200.In fact, the pressure that the primary pumps exert on the heat transfer fluid, such as sodium, makes it possible to take a small amount of this heat transfer fluid from within the non-combustible assemblies and to extract the gases using the cold trap 32.

[0115] In addition, the cold sodium level S inside each sealed penetration 3 is intended to be discharged to a sodium purification device, not shown, via another tube 34, also outside the reactor vessel 10.

[0116] The entire set of crossings 3 is fixed to a suitable support itself fixed to the reactor vessel 10, in particular to a part surrounding the plug 12, as illustrated in Figure 5B.

[0117] A subset 20 is illustrated in Figure 6. In this subset 20, a non-combustible assembly 200 comprises a plurality of one-piece, cylindrical-shaped needles or pencils of neutron-absorbing material 201, typically B4C, and a plurality of one-piece, cylindrical-shaped needles or pencils of neutron-reflecting material 202.

[0118] The upper end of the non-combustible assembly 200 is fixed to a linkage 220 in the form of a single bar, of the displacement system 22.

[0119] As shown in detail in Figure 7, the upper end of this linkage 220 is fixed to a right-angle gearbox 221 coupled to a rotary drive motor 222. The motor 222 is preferably

[0120] Thus, the activation of motor 222 rotates the non-fuel assembly 200 around its vertical axis within its cylindrical location. This rotation allows for adjustment of the number of needles or rods made of neutron-absorbing material 201 that move closer to or further from the core 11, thereby controlling the reactor's reactivity.

[0121] In other words, a sub-assembly 20 with a displacement system 22 rotating the assembly 200 allows the reactivity to be controlled by rotating it around their axis, vertically in the reactor.

[0122] As illustrated in Figure 2, in an assembly 200, the neutron-reflecting needles or rods 202 occupy a larger volume than the neutron-absorbing needles or rods 201. With this arrangement, rotating an assembly 200 around its central axis vertically allows the neutron-absorbing needles or rods 201 or those made of neutron-reflecting material to be exposed to varying degrees to the reactor core 11, depending on the requirements. The effect of reduced core reactivity when the neutron absorber 201 is more oriented towards the core is related both to the absorber 201 itself and to the physical removal of the neutron reflector 202, which is located further from the core 11.

[0123] As illustrated in Figure 2, an advantageous distribution of the neutron absorber needles or pencils 201 in the assemblies 200 is such that in a given nominal position, they define a cylindrical surface concentric to the cylindrical part of the step 19 and to that of the reactor vessel 10, around the reactor core 11. This nominal position preferably corresponds to a just critical state of the reactor 1 which is in normal operation, i.e. the reactivity of the reactor is zero at every instant.

[0124] In other words, this concentric cylindrical surface must be formed by a distribution of neutron-absorbing needles or rods 201 and neutron-reflecting needles or rods 202 that ensures a continuously critical state of the reactor throughout its operation. For example, for a total of 50 non-fuel assemblies 200, a proportion of 10 needles or rods 201 and 40 needles or rods 202, respectively, distributed to form the concentric cylindrical surface, guarantees the reactor's critical state.

[0125] Since the critical state of a reactor evolves over time, a rotation of the non-fuel assemblies 200 allows the amount of neutron reflector / absorber facing the reactor core to be adapted so that it constantly corresponds to the critical state that has evolved.

[0126] Thus, thanks to the rotation of the fuel assemblies 200 planned with a judicious distribution between needles or rods 201 and needles or rods 202, the critical state of the reactor can always be guaranteed, even if it evolves over time.

[0127] Figure 7 shows in detail the drive system 22, which comprises a rotating drive motor 220 coupled to a right-angle gearbox 221, itself coupled to a linkage 222, which may be in the form of a single bar. The lower end of this linkage 222 is attached to a non-combustible assembly 200. To ensure a seal against the outside, a sealing block 223 can be mounted around the right-angle gearbox 221. To further enhance the seal, an additional sealing bell 224 can also be mounted around the right-angle gearbox 221, forming the sealed interface with the outside of the tank 10.

[0128] A first embodiment of a non-fuel assembly 200, generally cylindrical in shape, is shown in Figures 8 to 10. The neutron-absorbing needles or rods 201, grouped parallel to each other, are arranged adjacent to the neutron-reflecting needles or rods 202, themselves grouped parallel to each other. The number and distribution of these two groups 201 and 202 can vary according to the configuration of the reactor core 11 and advantageously according to the distribution described above.

[0129] The two groups 201, 202 are held together by parallel retaining plates 203, fixed, for example, by a mechanical tie rod 204 at the center of the assembly. These retaining plates 203 are perforated both to accommodate the various needles or rods 201, 202 and to allow the heat transfer fluid to circulate within the assembly by contact with each of said needles or rods 201, 202. The upper retaining plate 203 can be directly attached to the linkage 220 of the movement system 22. At least one retaining grid 205 can be provided between the two retaining plates 203, typically midway between them, to further enhance the mechanical support of the assembly 200.

[0130] Figures 11 and 11A show another embodiment of a non-combustible assembly 200. Here, the needles or pencils 201, 202 are held by retaining grids 205, distributed over the height of the assembly and a head 206 in the form of a lattice which can be directly attached to the linkage 202 or made monobloc with it.

[0131] Regardless of the assembly method used to achieve the non-combustible assembly 200, the retaining grids ensure correct positioning of all needles 201, 202 and resistance to vibration.

[0132] Figure 12 shows another embodiment of the invention in which each cylindrical location in the cold collector 18 of the reactor 1 is occupied by a single cylindrical monoblock bar 200 which extends vertically over the entire height of the core 11. Each monoblock bar 200 is here made up of a part of neutron absorbing material 201 and a part of neutron reflecting material 202 over a larger volume.

[0133] Each monobloc bar 200 can be rotated around its axis. The invention is not limited to the examples just described; features of the illustrated examples can, in particular, be combined in unillustrated variants.

[0134] Other variations and improvements may be considered without departing from the scope of the invention. List of cited references:

[0135] [1]: 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. System (2) for controlling the reactivity of a fast neutron nuclear reactor comprising at least one subassembly comprising: a non-fuel assembly (200) of generally cylindrical shape, comprising a plurality of needles or pencils (201) of neutron-absorbing material, and a plurality of needles or pencils (202) of neutron-reflecting material, arranged parallel to the needles or pencils of neutron-absorbing material, a system (22) for rotating the non-fuel assembly about its axis.

2. A reactivity control system according to claim 1, comprising: - a plurality of non-combustible assemblies, - a plurality of rotational displacement systems around the central axis of the non-combustible assemblies, each displacement system of an assembly being independent of the others.

3. Reactivity control system according to claim 1 or 2, each non-combustible assembly (200) comprising: - a plurality of neutron-absorbing needles or pencils (201) grouped together in parallel with each other, - a plurality of needles or pencils of neutron reflecting material (202) grouped together being parallel to each other and adjacent to the plurality of needles or pencils of neutron absorbing material (201).

4. Reactivity control system according to claim 3, the height of the needles or pencils in neutron absorbing material (201) and of the needles or pencils in neutron reflecting material (202) being equal to or greater than the height of a nuclear reactor core comprising fuel assemblies (13).

5. Reactivity control system according to claim or 4, the needles or pencils (202) of neutron reflecting material occupying, within each non-combustible assembly (200), a larger volume than the needles or pencils (201) of neutron absorbing material.

6. Reactivity control system according to claim 5, the needles or pencils (201) of neutron absorber being distributed in each assembly (200) such that in a given nominal position, a plurality of assemblies define a cylindrical surface concentric to the cylindrical part of a reactor vessel (10), around its core (11).

7. Reactivity control system according to any one of the preceding claims, each non-combustible assembly comprising a plurality of retaining plates or grids (203, 205) distributed over the height of the assembly.

8. Reactivity control system according to any one of the preceding claims, each displacement system (22) comprising a rotating drive motor (220) coupled to a right-angle gearbox (221) itself coupled to a linkage (222) the free end of which is fixed to a non-combustible assembly (200).

9. Reactivity control system according to claim 8, comprising at least one sealing element (223, 224) mounted around the angle drive (221), forming a sealed interface with the outside of a nuclear reactor vessel (10).

10. Use of a reactivity control system according to any one of the preceding claims in a fast neutron nuclear reactor, of the liquid metal cooled type, in particular liquid sodium, with loop exchanger configuration or integrated.

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