Nuclear fuel pellet incorporating at least one cavity, devoid of fissile material, with a cross-section in the form of at least one portion of a spiral not opening outwards to the outside of the pellet, Associated manufacturing processes.

Abstract

A nuclear fuel pellet incorporating at least one cavity, free of fissile material, with a cross-section in the form of at least one portion of a spiral not opening to the outside of the pellet. Associated manufacturing methods. The invention essentially consists of a fuel pellet (6), comprising: - a straight cylinder (60) of fissile material with central axis (X); - at least one cavity (61; 61.1 to 61.10), free of fissile material, extending over at least part of the length of the pellet, the cavity being shaped and oriented such that its cross-section, right-angled by a transverse plane perpendicular to the central axis X, has the form of at least one portion of at least one spiral, the cavity(ies) being axisymmetric with respect to the central axis X and not opening to the outside of the pellet. Figure for the abstract: Fig. 4

Description

Title of the invention: Nuclear fuel pellet incorporating at least one cavity, devoid of fissile material, with a cross-section in the form of at least one portion of a spiral not opening outwards to the outside of the pellet, Associated manufacturing processes. technical field

[0001] The present invention relates to the field of fuel elements for nuclear reactors, in particular Pressurized Water Reactors (PWR).

[0002] More specifically, it is located in the field of ceramic-type fuels made of uranium oxide or uranium and plutonium (U,Pu)O2 or MOX (acronym for "mixed oxide").

[0003] The invention essentially aims to improve the properties of these fuels, and more particularly to reduce their fracturing during power ramp-up and nominal operation.

[0004] For the purposes of this application, "nuclear reactors" means the current common meaning of the term, namely power plants that produce energy from nuclear fission reactions using fuel elements in which fissions occur that release heat power, the latter being extracted from the elements by heat exchange with a heat transfer fluid that ensures their cooling.

[0005] The term “nuclear fuel rod,” as used throughout this application, is understood to mean the official definition, for example, found in the Dictionary of Nuclear Science and Technology, namely a narrow tube of small diameter, closed at both ends, constituting the core of a nuclear reactor and containing fissile material. Thus, a “nuclear fuel needle” is a nuclear fuel rod, but the terminology is used for fast neutron reactors.

[0006] Although described with reference to Pressurized Water Reactors (PWRs), the invention relates to fuel elements that can be used in all types of reactors for power generation, heat generation, or experimental purposes, such as Boiling Water Reactors (BWRs), Fast Neutron Reactors (FNRs), including those cooled by a liquid metal, in particular by liquid sodium (Na-FNR) but also lead, lead-bismuth, etc., and all advanced 3rd and 4th generation reactors. Prior art

[0007] Nuclear reactors that use fission energy to produce heat can be classified into several different categories according to their characteristics: the form of final energy produced (electricity, heat, etc.), the type of neutron flux (fast neutrons or thermalized neutrons), the coolant used (liquid metal, water, etc.), the physical state of the coolant (liquid or gaseous), the pressure level of the coolant (for example, atmospheric for Boiling Water Reactors and high for PWRs), etc.

[0008] Fuel assemblies make it possible to produce energy by taking advantage of the nuclear fission reaction of a fuel composed partly of heavy fissile nuclei - the 235 isotope of uranium - as well as the 238 isotope of uranium which is only fertile (it will produce plutonium 239 by neutron capture).

[0009] This fuel is introduced into a cylinder with a circular cross-section, closed at each of its two ends by a plug. This cylinder is called a fuel rod and is described in more detail below. The fuel rod is sealed and constitutes the first containment barrier.

[0010] The rods are grouped into bundles and arranged in a square grid. For the French generation installation families called "levels" of 900 MWe and 1300 MWe, there are 264 rods plus 25 other tubes, which together form an assembly that also incorporates numerous structural elements (base plate, retaining grids, an instrumentation tube, 24 guide tubes in which 23 control rods slide, head plate, retaining spring, spider for handling the control rods) as shown in [Fig.4] of the aforementioned .pdf document.

[0011] In [Fig. 1], a nuclear fuel rod 1 according to the state of the art is shown in its configuration for use in a PWR nuclear reactor, i.e. in a vertical position with the pellets 6 towards the lower part as specified below.

[0012] The pencil 1 consists of a sheath 2 conventionally made of Zircaloy-4 (Zr4) closed at each end by a cap 3 at the top and 4 at the bottom, respectively, which is welded to it. This sealed pencil is filled with helium at 25 bar when cold to partially counteract the effect of the external pressure of 150 bar of the heat transfer fluid.

[0013] The interior of the cladding is essentially divided into two compartments, one of which 5 in the upper part, between the top of the fissile column and the upper cap 3, constitutes a gas expansion chamber and the other houses the fissile column formed by the stacking of nuclear fuel pellets 6 which each extend along the longitudinal direction XX' of the rod 1.

[0014] The expansion chamber is a free volume intended to receive the PF in gaseous form, usually called Fission Gas (FG).

[0015] In the stack shown, each pellet 6 has substantially the same length or height H.

[0016] A helical compression spring 7, generally made of Inconel®, is housed in the expansion chamber 5 with its lower end bearing against the upper face of the highest pellet 6 in the stack of pellets and its other end bearing against the upper plug 3.

[0017] In addition to maintaining the stacking of the pellets 6 along the longitudinal axis XX' and "absorbing" the longitudinal swelling of the pellets 6 over time, the other function of this spring 7 is to prevent buckling of the sheath cross-section in its oval shape. In other words, it must prevent extreme ovalization of the sheath cross-section.

[0018] The primary function of a fuel rod is to produce, and then transmit, the heat produced by the fission reactions within the fuel.

[0019] To date, a fuel pellet as implemented in a PWR reactor consists of uranium oxide UO2 enriched in U235 to about 5%, the complement being fertile U238.

[0020] Each pellet releases energy in the form of heat by nuclear fission, which varies over time depending on fuel wear but also on the variation in altitude of the control rods.

[0021] The power thus dissipated is also a function of the position of the pellet in the pencil, the position of the pencil in the assembly and the position of the assembly in the core.

[0022] This power is dissipated towards the cold source of the primary circuit (for example, the liquid sodium coolant) by encountering a number of thermal resistances which can be summarized as follows: - a strong gradient between the center and the periphery of the pellet induced by the low thermal conductivity of UO2; - a radial thermal gradient between the pellet and the pencil cladding. Indeed, the joint between the pellet and the cladding, entirely gaseous (helium) at the beginning of irradiation, becomes completely filled at the beginning of the second cycle. However, the roughness of the pellet allows for the discontinuous presence of gas which is no longer simply helium but also includes gases: thus, the contact between the pellet and the cladding is never perfect and therefore creates, through its thermal resistance, a radial thermal gradient; - radial thermal conduction through the sheath; - the resistance due to radial convective exchange between the outer face of the duct and the heat transfer fluid.

[0023] Fig. 2, from publication 1, gives the order of magnitude of the temperatures in a fuel pellet in a PWR reactor in nominal operating mode.

[0024] The use of an energy mix with renewable energies and nuclear energy imposes new operating constraints on the latter.

[0025] In particular, nuclear energy production must be able to ramp up to power quickly after prolonged operation of a low-power reactor.

[0026] This new constraint translates for a nuclear fuel into a risk of rupture of the cladding of a fuel rod which reflects a known phenomenon of Pellet Cladding Interaction (PCI).

[0027] This risk is due to the combination of several factors which can be summarized as follows:

[0028] - the cladding of the cladding on the fuel pellet,

[0029] - the swelling of the pellet during a power increase,

[0030] - the existence of cracks or fractures in the fissile material of the pellet which generate an overconcentration of stresses in the sheath at the point where they emerge,

[0031] - the existence of migration pathways via cracks for fission products corrosives that can produce stress corrosion cracking.

[0032] It is therefore necessary to reduce the risk of cladding rupture during a fuel rod power increase.

[0033] This issue falls partly within the theme of improved fuels for safety (ATF or E-ATF).

[0034] Several major categories of solutions have been proposed for this topic, which can be summarized as follows: - a decrease in fuel temperature, by increasing heat transfer between the center and the periphery of the fuel pellet; - control of the chemical activity of the pellet, with the use of ORP buffer (oxidation-reduction potential) to inhibit the formation of corrosive species; - an optimization of the shape of the pellet to reduce the stresses imposed on the sheath.

[0035] The solution according to patent application GB893742 A falls into this latter category: it consists of creating openings that form voids in the pellet along its radii, three, four, or six depending on the design. While these voids do accommodate the swelling of the pellet, they do not prevent the formation of corner(s) with an angle of at least one A piece of pellet opposite the sheath. These wedges initially exist when the openings are through-holes. Furthermore, voids such as those proposed create points of mechanical weakness that will lead to the formation of free fragments, creating a wedge effect and allowing passage for the migration of corrosive fission products.

[0036] There is therefore a need to improve the design of nuclear fuel pellets, the fissile material of which is more particularly based on (U,Pu)O2 or MOX oxides, in order to avoid or at least reduce the risk of rupture of the cladding of a rod housing the pellets, during the rise to power and nominal operating regime of the core of a reactor, in particular a Pressurized Water Reactor (PWR) core.

[0037] The aim of the invention is to meet at least part of this need. Description of the invention

[0038] To this end, the invention relates, in one of its aspects, to a nuclear fuel pellet, comprising:

[0039] - a straight cylinder of fissile material with a central axis;

[0040] - at least one cavity devoid of fissile material, which extends over at least a part of the length of the pellet, the cavity being shaped and oriented in such a way that its cross-section, right by a transverse plane perpendicular to the central axis, has the shape of at least a portion of at least one spiral, the cavity(ies) being axisymmetric with respect to the central axis and not opening onto the outside of the pellet.

[0041] Preferably, the cavity extends along the entire length of the pellet without opening onto the outside of the pellet.

[0042] In the context of the invention, the length of the pellet means its height.

[0043] By "spiral," we mean the usual meaning in plane geometry, namely a curve generated by a point that rotates around another point, moving further and further away from or closer to that center. The width of a cavity according to the invention is the distance from the space devoid of fissile material that is delimited by two parallel curves symmetrical with respect to the curve generated by the point.

[0044] The term "Archimedean spiral" refers to the common meaning, namely a spiral that starts at the origin and extends to infinity in an infinite number of regularly spaced turns. These turns do not open outwards from the outside of a pellet according to the invention.

[0045] Advantageously, the internal diameter of the portion(s) of spiral(s) is between 0 and half the radius of the right cylinder.

[0046] The width of the portion(s) of the spiral(s) can be constant or variable over its entire length.

[0047] Advantageously the width of the portion(s) of spiral(s) is less than or equal to 1 mm.

[0048] According to an advantageous embodiment, each spiral is an Archimedean spiral.

[0049] Advantageously, the pellet comprises several spirals whose center is on the central axis of the pellet and which wind outwards from the right cylinder, the spirals being distributed axisymmetrically with respect to the central axis of the right cylinder.

[0050] According to an alternative embodiment, the center of the cylinder is hollowed out, forming an internal right cylinder.

[0051] According to another advantageous embodiment, the cavity(ies) is / are filled at least partially by an insert or a porous material made of a thermally conductive material. An insert may advantageously be metallic or made of a metallic or ceramic alloy.

[0052] Advantageously, the insert material is chosen from Molybdenum (Mo), niobium (Nb) or their alloys, preferably an NbZr alloy or an NbZrC alloy.

[0053] Advantageously still, the fissile material of the straight cylinder is chosen from uranium (IV) oxide (UO2), mixed oxide (U,Pu)O2 or a mixed mixture based on uranium oxide and reprocessed plutonium oxides (MOx), or any other fissile ceramic.

[0054] According to an advantageous embodiment, the volume percentage of the cavity(ies) is between 1 and 10%.

[0055] According to another advantageous embodiment, at least one of the end faces of the straight cylinder includes at least one recess of fissile material and / or a chamfer on its periphery.

[0056] The invention also relates to a nuclear fuel rod extending along a longitudinal direction (XX1) comprising:

[0057] - a plurality of nuclear pellets as described above, stacked one on top of the other;

[0058] - a sheath of neutron-transparent material surrounding the stack of pellets.

[0059] Advantageously, the sheath is made of zirconium alloy, in particular Zircaloy-4 (Zr4), or of M5® alloy (ZrNbO).

[0060] The invention also relates to a nuclear fuel assembly comprising a plurality of fuel rods as above and arranged among themselves in a network.

[0061] The invention also relates to the use of a nuclear fuel pellet as described above or a fuel rod as described above in a pressurized water reactor (PWR).

[0062] Thus, the invention essentially consists of a nuclear fuel pellet which incorporates, within its straight cylinder of fissile material, at least one cavity which extends over at least a part of the length of the pellet and which, in cross-section view, follows the shape of at least one portion of a spiral not opening onto the outside of the pellet.

[0063] This or these spiral-shaped cavities allow the compensation of thermal deformations of the pellet during operating cycles, in particular during a rapid power increase and normal operation of a reactor.

[0064] These non-opening spiral shapes also help to limit the transfer of potentially corrosive fission products.

[0065] Thanks to this, the pellet does not break during its operation in the reactor. The fact that it does not break makes it possible, on the one hand, to avoid the wedge effects and the over-concentration of stresses that these induce in a cladding that surrounds the pellet, and on the other hand, to limit the migration of corrosive fission products towards the inner face of the cladding.

[0066] Limiting both overconcentrations of stress and the migration of corrosive fission products makes it possible to increase the cladding's strength during a power increase and should make it possible to extend the operating range of a nuclear fuel pellet according to the invention, in order to increase its flexibility.

[0067] The manufacture of a pellet according to the invention is simple and can be carried out either by powder metallurgy as currently used for solid fuel pellets intended for PWR reactors, or by a micro-extrusion process of ceramic by additive manufacturing (“Robocasting”).

[0068] Other advantages and features of the invention will become clearer from the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0069] [Fig.1] [Fig.1] is a schematic longitudinal cross-sectional view of a nuclear fuel rod according to the state of the art, as implemented in a PWR nuclear reactor.

[0070] [Fig.2] [Fig.2] illustrates in the form of a curve the temperatures in a nuclear fuel pellet according to the state of the art, in nominal operating mode.

[0071] [Fig.3] [Fig.3] is a schematic perspective view of a basic motif M of a nuclear fuel needle whose sheath houses a fuel pellet with spiral-shaped cavity(ies), not shown, according to the invention.

[0072] [Fig.4] [Fig.4] is a schematic perspective view of a combustible pellet according to an example of the invention, with cavities in the form of two portions of Archimedean spirals whose point of creation is on the central axis of the straight cylinder of fissile material of the pellet.

[0073] [Fig.5] [Fig.5] is a schematic cross-sectional view of [Fig.4].

[0074] [Fig. 6] [Fig. 6] is a cross-sectional view of a combustible pellet according to an example of the invention, with cavities in the form of ten portions of Archimedean spirals, from a Computer-Aided Design (CAD) tool, [Fig.6] showing the stress levels in the pellet.

[0075] [Fig.7] [Fig.7] is a view analogous to [Fig.6] but for a pellet with cavities in the form of four portions of Archimedean spirals.

[0076] [Fig.8] [Fig.8] is a photographic reproduction of a pellet as shown in Figures 4 and 5, obtained by a micro-extrusion process by additive manufacturing of an alumina powder simulating a UO2 powder as a fissile material.

[0077] [Fig.9] [Fig.9] is an illustration of a temperature test showing the temperature profile generated by laser heating on a pellet made according to [Fig.8],

[0078] [Fig. 10] [Fig. 10] is a photographic reproduction of the final state of a pellet obtained according to [Fig. 8] having undergone a laser power ramp leading to a temperature gradient comparable to a case of operation in a reactor.

[0079] [Fig. 11] [Fig. 11] is a photographic reproduction of the final state of a solid pellet according to the state of the art having undergone the same laser power ramp as the pellet according to [Fig. 10].

[0080] [Fig. 12] [Fig. 12] illustrates variants of embodiment of a fuel pellet according to the invention. Detailed description

[0081] For the sake of clarity, the same element according to the state of the art and according to the invention is designated by the same numerical reference.

[0082] Figures 1 and 2 have already been discussed in the preamble. They will therefore not be detailed below.

[0083] In the examples below, the fuel pellets according to the invention are numerically simulated to evaluate their mechanical and thermal performance, by software marketed under the name "FreeFem++", version v4.7-2: https: / / freefem.org / .

[0084] A basic motif M of a nuclear fuel rod with a sheath 2 housing a fuel pellet 6 according to the invention is shown in [Fig.3].

[0085] A first example of a pellet 6 according to the invention is illustrated in figures 4 and 5.

[0086] A pellet 6 according to the invention with central axis (X) first of all comprises a straight cylinder 60 of fissile material with central axis (X) which defines the diameter (0) and the height H of the pellet 6.

[0087] According to the invention, the pellet 6 comprises at least one cavity 61, free of fissile material. Each cavity 61 is shaped and oriented such that its cross-section, right-angled by a transverse plane perpendicular to the central axis X, has the shape of at least a portion of at least one spiral, the cavity(ies) being axisymmetric with respect to the central axis X and not opening onto the outside of the pellet.

[0088] In the example illustrated in Figures 4 and 5, the pellet 6 comprises two cavities 61, 61.2, each in the form of a portion of an Archimedean spiral whose point of origin is on the central axis X of the pellet. In this example, the spirals 61.1, 61.2 do not extend to the axis but extend from a point inside the right cylinder 60.

[0089] The width of a cavity 61 is non-zero and preferably less than 1 mm.

[0090] Each of the cavities 61 devoid of fissile material can be completely emptied or at least partially filled with an insert of thermally conductive material, in particular metallic or metallic alloy, or of a porous material. This may be an insert of molybdenum (Mo), niobium (Nb) or their alloys, preferably an NbZr alloy or an NbZrC alloy.

[0091] For example, one can consider making an insert within a cavity by depositing a metallic layer, for example in Mo, by a plasma-enhanced chemical vapor deposition technique (or PECVD, for Plasma-Enhanced Chemical Vapor Deposition in English).

[0092] The dimensions of a pellet 6 according to the invention are preferably as follows:

[0093] - the diameter D of the straight cylinder 60 can be that of the pellets already used in PWR reactors, notably equal to 8.15 mm or around 11 mm for pellets already used in BWR reactors, and up to 13.5 mm for CANDU type reactors;

[0094] - the height H of the straight cylinder 60 is at least 1 mm and can go up to the height of REPs pellets;

[0095] - the outer radius of a portion of spiral 61 must be less than the outer radius of the straight cylinder 60 so as not to be open to the outside of the pellet. The difference between these two radii must be large enough so that the pellet according to the invention remains intact in all phases of manufacture;

[0096] - the internal diameter of the spiral can be zero, but preferably it should not be greater than half the radius of the right cylinder to avoid fracturing the central part of the pellet.

[0097] To define a pellet 6 according to the invention, the inventors performed simulations using FreeFem++, which demonstrated the ability of nuclear fuel pellets with cavities that are empty of material and do not open to the outside, to resist cracking. Thus, these empty cavities allow the UO2, as the fissile material of a pellet, to expand freely, and can therefore prevent pellet fracturing.

[0098] Based on knowledge of the work [2] relating to the optimization of shapes, the inventors arrived at the definition of cavities with cross-section in the form of portions of a spiral, which have the primary advantage of presenting a low gradient of mechanical stresses along the spiral, in particular for an Archimedean spiral.

[0099] Quantitative analyses then made it possible to estimate the number of spiral segments and the rolling angles sufficient to avoid cracking of the UO2. It is specified that a rolling angle is the angle between the direction of the spirals and the radial vector.

[0100] Figures 6 and 7 illustrate examples of modeling a pellet with cavities having an Archimedean spiral cross-section, numbering ten and four respectively, with a parameter a = 0.2, the models having been made with maximum stress criteria. The parameter a corresponds to the distance separating the loop of a spiral after a rotation, divided by the radius of the pellet. Thus, the smaller the parameter a, the faster the spiral winds.

[0101] Different types of manufacturing processes for a pellet 6 according to the invention can be envisaged.

[0102] A powder sintering metallurgy process can be implemented. This process can incorporate all the steps of an industrial process for manufacturing solid UO2-based fuel pellets already used in PWR reactors, with the exception of the pressing step, in which a pressing mold in the form of a hollow cylinder is used, but rather a hollow cylinder containing one or more inserts whose cross-section incorporates spiral-shaped portions.

[0103] Thus, once the pellet is removed from the sintering mold, cavities empty of material are formed at the location of the insert(s) of the pressing mold.

[0104] Additive manufacturing processes (3D printing) can also be implemented to produce pellets with spiral cavities 61.1 to 61.10.

[0105] A micro-extrusion process using additive manufacturing (“robocasting”) of a UO2 powder dispersed in an organic gel can thus be envisaged. The inventors have therefore carried out manufacturing tests according to this process with an alumina powder. simulating UO2 powder and they showed that it was possible to achieve the desired geometry with spiral portions.

[0106] An example of a two-cavity pellet 6 61.1, 61.2 in the shape of a portion of a spiral, produced according to these manufacturing tests is shown in [Fig.8].

[0107] The inventors carried out tests with alumina pellets 6, produced by robocasting, during which they were subjected to a temperature gradient similar to that created in a PWR reactor. These tests showed the good mechanical behavior of the pellets 6, more particularly with their ability not to break.

[0108] More specifically, these tests were carried out with two pellets approximately 1 mm thick which were manufactured according to the same manufacturing process, namely from the same initial alumina powder and the same organic binder and according to the same drying / sintering sequence.

[0109] One of these two pellets was a pellet 6 according to the invention, i.e. with a straight cylinder of alumina incorporating empty cavities in the shape of spirals and the other of these two pellets, by way of comparative example, consisted of a solid straight cylinder, like the pellets usually used in PWR reactors.

[0110] The two pellets were subjected to the same heat treatment obtained by a laser. The temperature field created by this laser exhibited a temperature gradient with a central temperature that gradually increased up to 1800°C. This temperature profile generated by the laser is illustrated in [Fig. 9].

[0111] The inventors then observed that: - the pellet according to the invention did not break at a core temperature of 1800°C; - the solid pellet according to the comparative example broke for a core temperature below 1000°C.

[0112] The final state, after this heat treatment test of the pellet according to the invention and according to the comparative example, is shown respectively in figures 10 and 11.

[0113] This heat treatment test demonstrates that a pellet according to the invention with empty cavities in cross-section in the shape of spiral portions can withstand stronger temperature gradients than a solid pellet, before being damaged.

[0114] Now, the temperature of 1800°C is of the order of magnitude of the maximum temperature at the center of a conventional fuel pellet in a PWR reactor subjected to a power ramp up to 420 W / cm.

[0115] It can therefore be deduced that a pellet according to the invention should not break during its stay in a nuclear reactor, because the maximum power that this pellet can withstand under normal or transient conditions does not exceed 420 W / cm².

[0116] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0117] Other variants and improvements may be envisaged without departing from the scope of the invention.

[0118] As illustrated in [Fig. 12], the straight cylinder 60 of fissile material 60 may include on one and / or the other of its end faces a recess of material 62, preferably axisymmetric with respect to the central axis. One and / or the other of its end faces may be chamfered by a chamfer 63, preferably at 45°. By way of indicative example, using the usual dimensions of a PWR pellet, for a straight cylinder 60 with a diameter 0 of 8.19 + / - 0.012 mm and a height H of 11.50 + / - 1.15 mm, the recess 62 may have a width L of 5.0 + / - 0.3 mm and the chamfer 63 a height h of 0.28 + / - 0.03 mm.

[0119] Depending on the intended applications (primarily pressurized water reactors), various parameters of a cavity pellet in the shape of a portion(s) of a spiral can be optimized. Among the parameters to be optimized are:

[0120] - an arrangement and dimensions of the spirals different from those illustrated;

[0121] - a different cavity / fuel ceramic volume ratio;

[0122] - the choice of a fissile material other than UO2 or MOX;

[0123] - the choice of a refractory insert metal within a different cavity;

[0124] - dimensioning the diameters of the straight cylinder and the spirals of the pellet different from that of the calculations above. List of cited references

[0125] [1]: Kim DJ - Rhee YW & al - “Fabrication of Micro-Cell_UO2-Mo with enhanced thermal conductivity". JNM 462 (2015) 289-295.

[0126] [2]: G.Allaire “Shape Optimization by the Homogenization Method” Appl. Mech. Rev. Mar 2003, 56(2): B26-B27. https: / / doi.org / 10.1115 / L1553443

Claims

Demands

1. Nuclear fuel pellet (6), comprising: - a straight cylinder (60) of fissile material with central axis (X); - at least one cavity (61; 61.1 to 61.10), devoid of fissile material, extending over at least part of the length of the pellet, the cavity being shaped and oriented such that its cross-section, right by a transverse plane perpendicular to the central axis X, has the form of at least a portion of at least one spiral, the cavity(ies) being axisymmetric with respect to the central axis X and not opening onto the outside of the pellet.

2. Pellet (6) according to claim 1, the internal diameter of the portion(s) of spiral(s) being between 0 and half the radius of the right cylinder.

3. Nuclear fuel pellet (6) according to claim 1 or 2, the width of the portion(s) of spiral(s) being constant or variable over its entire length.

4. Nuclear fuel pellet (6) according to any one of the preceding claims, the width of the portion(s) of spiral(s) being less than or equal to 1 mm.

5. A nuclear fuel pellet (6) according to any one of the preceding claims, each spiral being an Archimedean spiral.

6. A nuclear fuel pellet (6) according to any one of the preceding claims, comprising several spirals whose center is on the central axis of the pellet and which wind outwards from the straight cylinder, the spirals being distributed axisymmetrically with respect to the central axis of the straight cylinder.

7. Nuclear fuel pellet (6) according to any one of the preceding claims, the center of the cylinder being hollowed out to form an internal right cylinder (62).

8. Nuclear fuel pellet (6) according to any one of the preceding claims, the cavity(ies) being filled at least partially by an insert or porous material, made of thermally conductive material.

9. Pellet (6) according to claim 8, the insert being metallic or made of a metallic alloy or ceramic

10. Pellet (6) according to claim 9, the insert material being selected from molybdenum (Mo), niobium (Nb) or their alloys, preferably an NbZr alloy or an NbZrC alloy.

11. Pellet (6) according to any one of the preceding claims, the fissile material of the straight cylinder being selected from uranium (IV) oxide (UO2), mixed oxide (U,Pu)O2 or a mixed mixture based on uranium oxide and reprocessed plutonium oxides (MOx).

12. Pellet (6) according to any one of the preceding claims, the volume percentage of the cavity(ies) being between 1 and 10%.

13. Pellet (6) according to any one of the preceding claims, at least of the end faces of the straight cylinder comprising at least one recess of fissile material and / or a chamfer on its periphery.

14. Nuclear fuel rod (1) extending along a longitudinal direction (XX1) comprising: - a plurality of nuclear pellets (6) according to one of the preceding claims, stacked one on top of the other; - a cladding (2) of neutron-transparent material surrounding the stack of pellets.

15. Pencil (1) according to claim 14, the sheath being made of zirconium alloy, in particular Zircaloy-4 (Zr4), or of M5® alloy (ZrNbO).

16. Nuclear fuel assembly comprising a plurality of fuel rods according to any one of claims 14 or 15 and arranged together in a network.

17. Use of a nuclear fuel pellet (6) according to any one of claims 1 to 13 or of a nuclear fuel rod (1) according to claim 14 or 15 in a pressurized water reactor (PWR).

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