Nuclear reactor fuel assembly which has no duct and comprises a bundle of pins held laterally by grids and longitudinally by stiffening rods that extend through the grids

Abstract

The invention consists essentially of a fuel assembly (1) of hexagonal or parallelepiped cross-section, without a hexagonal duct, preferably intended for low-power-density sodium-cooled fast neutron reactors (SFR).

Description

[0001] Description

[0002] Title: Fuel assembly for nuclear reactor, without casing, comprising a bundle of needles held laterally by grids and longitudinally by stiffening rods, passing through the grids.

[0003] technical field

[0004] The present invention relates to a fuel assembly for a liquid-cooled fast neutron reactor (FNR), particularly one cooled with liquid metal, specifically liquid sodium (Na-FNR or SFR), lead, or a lead alloy, and belonging to the family of so-called Generation IV reactors. More specifically, the invention relates to a novel assembly design that eliminates the need for a hexagonal casing or tube surrounding the bundle of fuel rods or needles.

[0005] The fuel assemblies referred to in the invention can be used both in an integrated type nuclear reactor, i.e. in which the primary sodium circuit with pumping means is totally contained in a vessel also containing heat exchangers, and in a loop type reactor, i.e. in which the heat exchangers and the primary sodium pumping means are located outside the vessel.

[0006] A fuel assembly is defined as a set comprising fuel elements and loaded and / or unloaded into / from a nuclear reactor.

[0007] A fuel assembly of the type RNR-Na or SFR is a fuel assembly adapted to be irradiated in a fast neutron reactor cooled with liquid sodium, known as an RNR-Na or SFR.

[0008] By "nuclear fuel rod", we understand the official meaning defined for example, in the dictionary of Nuclear Sciences and Techniques, namely a narrow tube of small diameter, closed at both ends, constituting the core of a nuclear reactor and containing fissile material.

[0009] A "nuclear fuel needle" is a nuclear fuel rod, but the terminology is used for fast neutron reactors. For the purposes of this invention, the terms "needle" and "nuclear fuel rod" will be used interchangeably.

[0010] Previous technique

[0011] Fuel assemblies intended for use in liquid sodium-cooled fast reactors (Na-FNRs) have a particular mechanical structure in order to allow liquid sodium to pass through them.

[0012] Figures 1 to 3 show a fuel assembly 1 and its various components already used in a Na-NR nuclear reactor known as "Phoenix".

[0013] Such an assembly 1 of elongated shape along a longitudinal axis X includes first of all a tube or casing 10 with hexagonal cross-section, the upper portion 11 of which forms the gripping head of the assembly and houses an upper neutron protection device (PNS), for example of steel and the central portion 12 of which encloses fuel needles not shown.

[0014] In other words, portions 11, 12 form the same tubular envelope 10 or casing of identical hexagonal section throughout its height.

[0015] The assembly 1 finally includes a lower portion 13 forming the foot of the assembly, extending from the housing 10. The foot 13 of the assembly has a distal end 15 that is conical or rounded so that it can be inserted vertically into the supports of the reactor core. The foot 13 of the assembly has openings 16 around its periphery that open into it, allowing control of the flow of liquid sodium for heat extraction from the reactor.

[0016] Thus, in the installed configuration of a fuel assembly, i.e. in the loaded position in a reactor core, the foot 13 of an assembly 1, of male shape, is inserted into an opening in the reactor base, thus holding the assembly 1 in the latter with its longitudinal axis X vertical.

[0017] Primary sodium can circulate within the casing 10 of the assembly 1 and thus carry, by thermal conduction, the heat released by the fuel needles. The sodium is introduced through the openings 16 of the foot 13 and exits through the central opening 110 of the head 11, after passing through the bundle of fuel needles. The central portion 12 of an assembly comprises a plurality of nuclear fuel needles 100.

[0018] As illustrated in Figures 2B and 3, a spacer wire 17 is arranged in a helical winding around each of the needles 100. The spacer wire 17 makes it possible to maintain and guarantee a spacing with a regular pitch p between the needles 100 within a fuel assembly 1 and their arrangement leaves passages 18, 19 of liquid sodium, respectively within and at the periphery of the beam.

[0019] As illustrated in Figure 4, each needle is in the form of a sealed cylindrical sheath 101 inside which is stacked a column 14 of fissile fuel pellets within which the nuclear reactions that release heat take place. The sheath 101 is made of stainless steel and is closed at its ends by stainless steel plugs 102, 103 welded to the sheath.

[0020] The fuel pellets, generally made of uranium oxide and / or uranium-plutonium oxide, or uranium and / or uranium-plutonium alloy, are cylindrical, a few millimeters in diameter D and height H, most often with a central hole a few millimeters in diameter. Within the needle, or in separate needles, in the upper and / or lower part, it is possible to have a so-called fertile column or blanket 104 made of uranium-based pellets (alloy or oxide) whose physical properties allow for the regeneration of nuclear material.

[0021] The fission column 14 is held in place at its upper end by a spring 105. Finally, at the upper and lower ends of the needle are two free volumes 106 and 107, called plenums, which accommodate the fission gases (FG). A spacer 108 is arranged between the lower plenum 107 and the fertile cover 104.

[0022] All 14 columns define what is usually called the fissile zone which is approximately located halfway up an assembly 1. It is schematically represented as a black rectangle in figure 1.

[0023] The length of a needle 100 is generally between 1000 and 4000 mm for a diameter of less than 10 mm. The thickness of the cladding 101 is generally between 0.4 and 0.6 mm, typically less than 1 mm. Fast neutron reactor fuel assemblies are almost all designed as described above.

[0024] All the assemblies of the same reactor are arranged vertically on a base to form a compact hexagonal lattice core.

[0025] The assemblies in position on the bed frame are spaced from each other at their base (foot), typically a few mm between the faces opposite two adjacent hexagonal section boxes.

[0026] The solutions developed so far for manufacturing assemblies meet the design specifications of a sodium-cooled fast reactor (SFR) targeting high power densities. SFR reactors are indeed generally designed to operate at high power densities, up to 300 W / cm². 3 and more.

[0027] Thus, for high volumetric power, it is necessary to provide for heat extraction by hydraulic zoning of the reactor core and forced convection of sodium inside the hexagonal tubes 10 of the fuel assemblies 1.

[0028] Furthermore, the residence time in the core of the fuel in high-volume-power Na-NRs requires special management of the assemblies which must be replaced over the long term with an impact on the availability of the reactor and safety and security risks related to the handling operations of loading / unloading said assemblies.

[0029] The presence of a 10 hexagonal section tube for each assembly is also necessary to protect the needle bundle during these handling operations.

[0030] A hexagonal tube also has a mechanical role in maintaining the geometry of the core and limiting the occurrence and effects of compaction.

[0031] However, the presence of a hexagonal tube entails a number of disadvantages, including:

[0032] - the use of a significant quantity of steel upstream in the manufacturing process,

[0033] - a degradation of neutron performance due to the presence of a greater quantity of steel in the reactor core, - constrained thermo-hydraulic operation, which requires specific monitoring assembly by assembly, including local thermometry, cladding rupture detection by detecting gases and / or fission products released by detecting delayed neutrons, and cladding rupture localization. The devices that must be implemented for this specific monitoring are numerous, complex, highly exposed, and costly.

[0034] - a potential variation in core reactivity due to radial deformation (stacking or closure) resulting from the differential thermal loading of the faces of a hexagonal tube, which imposes deformation on the entire assembly,

[0035] - a potential blockage of the hexagonal tube, which could induce events leading to a serious reactor accident,

[0036] - a worsening of the consequences of a core meltdown with a significant amount of supernatant steel potentially reflecting neutrons from the corium bath,

[0037] - the existence of retention zones making the desodium removal operation more complex,

[0038] - more complex fuel reprocessing due to non-immediate and constrained access to the fuel material, requiring an upstream step to open the hexagonal tube and isolate the needle bundle,

[0039] - production of a significant quantity of radioactive metallic waste after reprocessing, long-lived waste which needs to be packaged into standard compacted waste packages (CSD-C).

[0040] Therefore, there is still a need to improve fuel assemblies for fast neutron reactors, particularly liquid-metal cooled ones, preferably with a low power density operating point, typically less than 200 W / cm². 3 , in order to avoid the presence of a hexagonal tube housing and to eliminate, or at least limit, the related disadvantages mentioned above.

[0041] The aim of the invention is to at least partially meet this need. Description of the invention

[0042] To this end, the invention relates, in one of its aspects, to a fuel assembly for a nuclear reactor, in particular for a liquid metal cooled fast neutron reactor of the Na-FNR type, comprising:

[0043] - a bundle of nuclear fuel needles, extending along a longitudinal axis (X),

[0044] - an upper part forming the head of the assembly, provided with at least one through opening for the passage of heat transfer fluid along the (X) axis,

[0045] - a lower part forming the foot of the assembly adapted to allow positioning in a reactor and the circulation of a heat transfer fluid, the foot being provided with at least one through opening for the passage of heat transfer fluid along the (X) axis and including means for retaining the needles,

[0046] - a plurality of retaining grids distributed along the beam, each grid being honeycomb-shaped, with hexagonal or parallelepiped cross-sections, with cells also of hexagonal or parallelepiped cross-section, each part of the cells of the retaining grids housing and laterally holding a nuclear fuel rod,

[0047] - a plurality of stiffening rods individually housed each in the other part of the cells of the retaining grids and fixed each at one of their longitudinal ends to the foot of the assembly and at the other of their longitudinal ends to the head of the assembly to longitudinally retain the bundle; the assembly being devoid of a casing around the bundle of needles.

[0048] In one embodiment, the foot incorporates a comb with widely spaced teeth, the spacing and shape of which are adapted to mechanically lock the lower plugs of the fuel needles. This embodiment ensures reliable and easy-to-implement axial (longitudinal) retention of the needles within the foot of the assembly.

[0049] Preferably, the head incorporates a perforated longitudinal support plate to allow the heat transfer fluid to pass through. This plate thus contributes to the robustness of the assembly.

[0050] According to another embodiment, the stiffening rods are threaded at at least one of their longitudinal ends and are fixed by a plurality of screws or nuts, each forming a mechanical tie rod. This allows for simple and reliable longitudinal support of the beam.

[0051] Advantageously, the retaining grids are arranged spaced between 100 and 600mm apart.

[0052] Even more advantageously, the height of a retaining grid along the longitudinal axis is between 250 and 450 mm.

[0053] According to an advantageous embodiment, the cells of the retaining grids incorporate on at least one of their inner walls, at least one flexible tab forming a retaining spring for a needle against which it rests.

[0054] Preferably, each inner wall of a cell incorporates at least one flexible tab.

[0055] According to an advantageous variant, the flexible tongue is shaped with its two ends formed integrally with the inner wall and forming a space with the latter.

[0056] According to an advantageous variant, the foot of the assembly includes an outgrowth forming a keying device for angular positioning in the core of a nuclear reactor.

[0057] The invention also relates to the use of a fuel assembly as described above in a liquid-metal cooled nuclear reactor, particularly a liquid sodium-cooled fast reactor (SSN-FWR), advantageously with a low power density, typically less than 200 W / cm² 3 .

[0058] The invention essentially consists of a fuel assembly with a hexagonal or parallelepiped cross-section, without a hexagonal tube casing, preferably dedicated to low volumetric power liquid sodium-cooled fast neutron reactors (Na-FNRs).

[0059] In this assembly, the bundle of fuel needles is held laterally by spaced retaining grids along the bundle. Some of the grid cells individually house and hold a needle while leaving free volumes for the passage of the heat transfer fluid, while the other part is traversed by stiffening rods that reinforce the assembly. The longitudinal ends of these rods are fixed to the base and top of the assembly, respectively. A fuel assembly according to the invention thus eliminates the need for a hexagonal tube housing and the spacer wires between fuel needles, which are present in a prior art fuel assembly.

[0060] In conclusion, a combustible assembly with a hexagonal cross-section according to the invention, which is without a hexagonal tube housing around the bundle of combustible needles, offers numerous major advantages compared to, among which we can mention:

[0061] - minimizing the amount of metal needed for manufacturing and the metallic waste produced after fuel reprocessing, and therefore the environmental impact;

[0062] - a lesser constraint on the thermo-hydraulics of the core, approaching that of a pressurized water reactor (PWR), due to the absence of the need for hydraulic zoning;

[0063] - an increase in the ratio between the amount of nuclear fuel, preferably a mixture of uranium and plutonium, and the steel in a reactor core;

[0064] - the elimination of one of the initiators of serious accidents, which is the blocking of the hexagonal tube (assembly housing according to the state of the art);

[0065] - a reduction in the occurrence of a return or maintenance of criticality in the molten corium bath in the event of core meltdown, by promoting neutron leakage due to the amount of supernatant steel being minimized;

[0066] - a desodizing operation at the end of its life after cooling, made more efficient by minimizing retention areas;

[0067] - easier access to the material for head process operations during fuel reprocessing, by direct shearing of the assembly during reprocessing.

[0068] 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.

[0069] Brief description of the drawings

[0070] [Fig. 1] Figure 1 is an external perspective view of a state-of-the-art fuel assembly already used in a high-power-volume sodium-cooled fast nuclear reactor (SFR-Na). [Fig. 2] Figure 2 is a perspective view of a state-of-the-art fuel assembly already used in a high-power-volume sodium-cooled fast nuclear reactor (SFR-Na).

[0071] [Fig 2A] Figure 2A is a longitudinal cross-sectional view of a combustible assembly as shown in Figure 1 or 2.

[0072] [Fig 2B] Figure 2B is a cross-sectional view of a combustible assembly as shown in Figure 1 or 2.

[0073] [Fig 3] Figure 3 is a schematic perspective view of a nuclear fuel needle around which a spacer wire is helixed within a state-of-the-art fuel assembly, as illustrated in Figures 1 to 2B.

[0074] [Fig 4] Figure 4 is a longitudinal cross-sectional view of a fuel needle from a state-of-the-art fuel assembly bundle, as illustrated in Figures 1 to 2B.

[0075] [Fig 5] Figure 5 is a perspective view of a fuel assembly according to the invention, intended for use in a low volumetric power of a liquid sodium-cooled nuclear reactor (NSFR-Na).

[0076] [Fig 6] Figure 6 is a side view of the fuel assembly according to Figure 5.

[0077] [Fig 7] Figure 7 is a detailed perspective view of the fuel assembly according to Figures 5 and 6 showing the head of the assembly.

[0078] [Fig 8] Figure 8 is a detailed longitudinal section view of the fuel assembly according to Figures 5 and 6 showing the head of the assembly.

[0079] [Fig 9] Figure 9 is a detailed perspective and longitudinal section view of the fuel assembly according to Figures 5 and 6 showing the base of the assembly.

[0080] [Fig 10] Figure 10 is a detailed perspective view of the combustible assembly according to figures 5 and 6 showing the base of the assembly.

[0081] [Fig 11] Figure 11 is a side detail view showing an example of the foot of the assembly.

[0082] [Fig 11 A] Figure 11A is a detailed cross-sectional view of the foot of the assembly shown in Figure 11, illustrating the attachment and retention of the profile to a pin and the retaining plate of the assembly foot, respectively. [Fig 12] Figure 12 is a perspective view showing the interior of the assembly foot, to which assembly stiffening rods are attached.

[0083] [Fig 13] Figure 13 is a perspective view showing a grid for retaining the needle bundle of a combustible assembly according to the invention.

[0084] [Fig 13 A] Figure 13A is a perspective detail view of Figure 13.

[0085] [Fig 14] Figure 14 is a cross-sectional detail view showing part of a needle bundle retaining grid of a combustible assembly according to the invention.

[0086] [Fig 15] Figure 15 is a longitudinal cross-sectional view of a fuel needle from a fuel assembly bundle according to the invention.

[0087] Detailed description

[0088] Throughout this application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a fuel assembly as it is in a vertical configuration in a nuclear reactor.

[0089] Figures 1 to 3 relating to the state of the art have already been detailed in the preamble, so they will not be commented on below.

[0090] For the sake of clarity, the same element according to the invention and according to the state of the art is designated by the same numerical reference in all of Figures 1 to 14.

[0091] To avoid a fuel assembly configuration with a hexagonal tube housing 10 and the presence of spacer wires 17 between the fuel needles, the inventors designed a fuel assembly 1, without a housing, as illustrated in figures 4 to 10.

[0092] Such a fuel assembly 1 includes first of all a bundle of nuclear fuel needles 100 which extends along a longitudinal axis (X).

[0093] An upper part of this assembly forms the head of assembly 11, which is provided with at least one through opening 110 for the passage of the heat transfer fluid (liquid sodium) along the X axis.

[0094] A lower portion forms the foot of assembly 13, adapted to allow positioning within a reactor and circulation of the heat transfer fluid. The foot 13 is provided with at least one through-hole 130 for the passage of heat transfer fluid along the (X) axis. The foot 13 also includes individual retaining means 131, 132 for each needle 100, detailed below.

[0095] The head 11 and the foot 13 can advantageously be made of either austenitic steel of type AISI 316LN, or of type 15-15Ti AIM1, or of ferrito-martensitic steel of type EM10 (9% Cr and 1% Mo), or another ferritic grade.

[0096] According to the invention, a plurality of retaining grids 2 are distributed along the beam. Preferably, the distance h separating two adjacent grids 2 is constant and can be between 250 and 450 mm, for example equal to 300 mm.

[0097] Preferably, the height h of a retaining grid along the longitudinal axis is between 50 and 100 mm.

[0098] Each support grid 2 is honeycomb or with parallelepiped cells, with a hexagonal cross-section.

[0099] The retaining grids 2 can advantageously be made either of austenitic steel of type AIM1 or 15-15Ti work-hardened, or of ferrito-martensitic steel of type EM10 (9% Cr and 1% Mo), or even in a Nickel base alloy (see what the grids of PWR assemblies are made of).

[0100] As illustrated in figures 12 to 13, the cells 20 of a retaining grid 2 are also of hexagonal cross-section.

[0101] Each of the 20 cells houses and laterally holds a nuclear fuel needle 100.

[0102] To ensure the rigidity and longitudinal support of the needle bundle 100 and thus guarantee the solidity of the fuel assembly 1, a plurality of stiffening rods 3 are individually housed in the other part of the alveoli 20 of the retaining grids.

[0103] Each of the stiffening rods 3 is fixed at one of their longitudinal ends to the foot 13 and at the other of their longitudinal ends to the head 11 of the assembly.

[0104] The rods 3 can advantageously be made of either austenitic steel, or ferrito-martensitic steel of type EM10 (9% Cr and 1% Mo), or nickel-based alloy. As detailed below, this fastening is preferably achieved by means of screws that are individually screwed into each of the longitudinal ends of each hollow rod 3. Bolting with nuts is also possible.

[0105] As illustrated in detail in figures 7 and 8, the head 11 advantageously incorporates a longitudinal support plate 111 for the needles 100, which is perforated to allow the heat transfer fluid to pass through.

[0106] The head 11 may also include a hollow cylindrical tip 112 which delimits, at its upper end, the through opening 110.

[0107] The head 11 may include a stiffening lattice 113 attached to the tip 112 and which bears against the periphery of the longitudinal retaining plate 111.

[0108] The nozzle 112 can accommodate a plate 114 with regularly spaced open holes 115 for the outlet of the heat transfer fluid.

[0109] A central screw 116 can secure this plate 114 to a portion 117 in the upper extension of the retaining plate 111.

[0110] As illustrated in detail in figures 9 to 12, the foot 13 advantageously incorporates a retaining plate 131 for the needles 100. This retaining plate 131 is perforated to allow the heat transfer fluid from the opening 130 to pass through.

[0111] The foot 13 may also include a hollow end piece 132 which delimits, at its upper end, the opening 110. The external shape of this hollow end piece 132 is with a cylindrical portion extended by a conical portion itself extended by a hexagonal portion.

[0112] In the glossy example, the retaining plate 131 is integrated into the hexagonal portion of the tip 132.

[0113] The retention between the plate 131 and the needles 100 is ensured by retaining profiles 133. More specifically, as illustrated in Figure 1 IA, a profile 133 comprises an upper portion 134 which is fixed by complementary shapes in the lower plug 103 of a needle 100, and a lower portion 135 which is straight and which is inserted into a complementary groove 136 in the retaining plate 131.

[0114] Thus, for assembly, one can proceed by individual sliding insertion of the needles grouped in rows on a fixing profile 133. Preferably, each stiffening rod 3 is fixed longitudinally by a profile 133 which passes through it laterally.

[0115] The foot 13 can accommodate a screw plate 137 in the form of an annular flange, through which the lower longitudinal ends of the rods 3 are screwed. The hollowed-out interior of this annular flange 133 allows the heat transfer fluid to pass from the opening 130 in the foot 13.

[0116] The foot 13 may advantageously include an outgrowth 138 forming a key for angular positioning in the core of a nuclear reactor.

[0117] An example of an advantageous implementation of the lateral support grids 2 is illustrated in figures 12 to 14.

[0118] For optimal support of the fuel needles 100, each cell of a grid 2 is provided on at least one of its inner walls with a flexible tab 21 forming a spring to hold a needle against which it rests.

[0119] Preferably, each inner wall of a cell 20 is provided with a flexible tab 21 both for improved retention and for optimal centering of a needle 100. Thus, as shown in Figure 13, a cylindrical fuel needle of diameter 0 is perfectly centered and held by the six flexible tabs 21 distributed evenly within each cell 20.

[0120] The free spaces 22 between a needle 100 and the inner walls of the alveoli define the circulation channels of the heat transfer fluid.

[0121] Thus, the bundle of combustible needles 100 is held laterally by the flexible tabs 21 of the honeycomb retaining grids 2 which are advantageously regularly spaced over the height of the bundle and of which part of the cells 20 is crossed by stiffening rods 3 which are screwed and thus assemble the head 11 and the foot 13 with the sub-assembly made up of the bundle of needles 100 and the grids 2.

[0122] To allow a small longitudinal displacement, due to thermal expansion, the fixing of the rods 3 with the head 11 can be achieved with helical springs 31 arranged each around a rod to allow a spacing or coming together between retaining plate 111 and the tip 112, as illustrated in figure 8. In other words, the retaining plate 111 axially maintains the needle bundle while allowing a small axial displacement, along the longitudinal axis, thanks to the assembly with the springs 31, in order to compensate for thermal expansion.

[0123] An advantageous example of a combustible needle 100 suitable for the realization of a combustible assembly 1 as just described is shown in Figure 15.

[0124] This fuel needle 100 has a diameter (0) preferably between 10 and 15 mm and a length of 2 to 4000 mm. This large diameter of the needle maximizes the proportion of nuclear material per unit volume of the core and is therefore particularly suitable for long cycles.

[0125] This needle 100 is in the form of a sealed cylindrical sheath 101 inside which is stacked a column 14 of fissile fuel pellets within which the nuclear reactions that release heat take place. The sheath 101 is made of stainless steel and is closed at its ends by stainless steel plugs 102, 103 welded to the sheath. The thickness of the sheath 101 is between 0.4 and 0.6 mm, typically less than 1 mm.

[0126] The fuel pellets, usually of uranium oxide and / or uranium-plutonium, or of uranium and / or uranium-plutonium alloy, are cylindrical and with a central hole of a few mm.

[0127] The fissile column 14 is held in its upper part by a spring 105.

[0128] In the upper and lower part of the needle, there are two free volumes 106, 107, called plenums, which allow the fission gases (GF) to be accommodated.

[0129] A spacer 108 is arranged between the lower plenum 107 and the fissile column 14.

[0130] The central fissile column 14 is surmounted by a column 109 made of reflective material, for example of MgO pellets or steel.

[0131] A combustible assembly 1 with its bundle of combustible needles 100, which has just been described, allows for:

[0132] - reduce the power output of a nuclear reactor core, - reduce the percentage of fissile material in the core and therefore the enrichment in fissile isotopes to promote the fertilization (internal breeding) of the core matrix and move towards isogeneration,

[0133] - to have a high fuel density in the reactor core. Thus, the invention makes it possible to achieve very long operating cycles of a nuclear reactor, without having to refuel with new fuel, gain in availability and minimize the number of fuel loading / unloading operations.

[0134] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0135] While the rods in the illustrated example are hollow and internally tapped at their longitudinal ends for screwing, other designs are possible for stiffening and fastening. For example, solid rods could be used along at least part of their length, externally tapped at at least one of their longitudinal ends to allow nuts to be tightened around them.

[0136] Other variants and embodiments may be considered without departing from the scope of the invention.

Claims

Demands 1. Fuel assembly (1) for a nuclear reactor, in particular for a liquid metal cooled fast neutron reactor of the Na-FNR type, comprising: - a bundle of nuclear fuel needles (100), extending along a longitudinal axis (X), - an upper part forming the head of the assembly (11), provided with at least one through opening (110) for the passage of a heat transfer fluid along the axis (X), - a lower part forming the foot of the assembly (13) adapted to allow positioning in a reactor and circulation of the heat transfer fluid, the foot being provided with at least one through opening (130) for the passage of heat transfer fluid along the axis (X) and including means for retaining (131, 132) the needles, - a plurality of support grids (2) distributed along the beam, each grid being honeycomb-shaped, with hexagonal or parallelepiped cross-section, with cells (20) also with hexagonal or parallelepiped cross-section, each part of the cells of the support grids housing and laterally holding a nuclear fuel needle, the free spaces (22) between a needle (100) and the inner walls of the cells define the circulation channels of the heat transfer fluid, - a plurality of stiffening rods (3) individually housed each in the other part of the cells of the retaining grids and fixed each at one of their longitudinal ends to the foot of the assembly and at the other of their longitudinal ends to the head of the assembly to retain the bundle longitudinally; the assembly being devoid of a casing around the bundle of needles.

2. Fuel assembly (1) according to claim 1, the fuel needles comprising lower plugs, the foot incorporating a comb with teeth spaced apart, the spacing between teeth and their shape being adapted to mechanically block the lower plugs.

3. Fuel assembly (1) according to claim 1 or 2, the head incorporating a longitudinal retaining plate (131) perforated to allow the heat transfer fluid to pass through.

4. Combustible assembly (1) according to any one of the preceding claims, the stiffening rods being threaded at at least one of its longitudinal ends and being fixed by a plurality of screws or nuts, each forming a mechanical tie rod.

5. Fuel assembly (1) according to any one of the preceding claims, the retaining grids being arranged apart from each other by a distance d between 100 and 600mm.

6. Fuel assembly (1) according to one of the preceding claims, the height h of a retaining grid along the longitudinal axis being between 50 and 100mm.

7. Combustible assembly (1) according to any one of the preceding claims, the cells (20) of the retaining grids integrating on at least one of their inner walls, at least one flexible tab (21) forming a retaining spring for a needle against which it rests.

8. Combustible assembly (1) according to claim 6, each inner wall of a cell incorporating at least one flexible tab.

9. Combustible assembly according to claim 6 or 7, the flexible tab being formed with its two ends integrally formed with the inner wall and forming a space with the latter.

10. Fuel assembly (1) according to any one of the preceding claims, the foot of the assembly comprising an outgrowth (138) forming an angular positioning key in the core of a nuclear reactor.

11. Use of a fuel assembly (1) according to any one of the preceding claims, in a liquid-metal cooled nuclear reactor, in particular a liquid sodium-cooled fast reactor (SFR), advantageously of low power density, typically less than 200 W / cm² 3 .

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