Nuclear fuel pellet incorporating a thermally conductive insert in the form of branching arms distributed from an inner hollow cylinder to the outside of the pellet.

Abstract

A nuclear fuel pellet incorporating a thermally conductive insert in the form of branching limbs extending from a hollow inner cylinder to the outside of the pellet. The invention essentially consists of a nuclear fuel pellet that incorporates, within a straight cylindrical ring of fissile material, a thermal conductor, particularly metallic, in the form of main branching limbs extending from the inside of the ring outwards, advantageously distributed homogeneously within the ring. The insert formed by its limbs and branches at the macrostructural scale reduces the maximum fuel temperature by several hundred degrees Celsius, depending on its dimensions, constituent material, and the distribution of its limbs within the straight cylindrical fuel ring. Figure for the abstract: Fig. 4

Description

Title of the invention: Nuclear fuel pellet incorporating a thermally conductive insert in the form of branched arms distributed from an inner hollow cylinder to the outside of the pellet. technical field

[0001] The present invention relates to the field of fuel elements for nuclear reactors, in particular Fast Neutron Reactors (FNRs), including those cooled by a liquid metal, in particular by liquid sodium (Na-FNR).

[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 thermal properties of these fuels as well as the reduction of gas swelling, both in nominal operation and in incident or accident regime (for example in a situation of power increase or loss of refrigerant).

[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 the application to Fast Neutron Reactors (FNRs), including those cooled by a liquid metal, in particular by liquid sodium (Na-FNR) but also lead, lead-bismuth, etc., the invention relates to fuel elements which can be dedicated to all types of reactors for power generation, heat generation or experimental purposes, such as Boiling Water Reactors (BWRs), Pressurized Water Reactors (PWRs) and all advanced 3rd and 4th generation reactors. Previous technique

[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 cooling fluid used (liquid metal, water, etc.), the physical state of the cooling fluid (liquid or gaseous), the pressure level of the cooling fluid, etc.

[0008] Fuel assemblies intended for use in liquid sodium-cooled fast neutron reactors (Na-FNRs) have a particular mechanical structure in order in particular to allow liquid sodium to pass through them.

[0009] Figures 1 and IA show a fuel assembly 1 conventionally used in a Na-NR nuclear reactor.

[0010] Such an assembly 1 of elongated shape along a longitudinal axis X comprises first of all a tube or casing 10 with hexagonal cross-section, closed and sealed around the perimeter, the upper portion 11 of which forms the gripping head of the assembly and houses an upper neutron protection device (PNS), and the central portion 12 of which encloses fuel needles 100.

[0011] The portions 11, 12 form a single tubular casing 10 or housing with identical hexagonal cross-section throughout its height. The head 11 of the assembly has a central opening 110 leading into it and used for handling.

[0012] The central portion 12 of an assembly comprises a plurality of 100 nuclear fuel needles.

[0013] Each needle 100 is in the form of a sealed cylindrical steel sheath tube, closed at both ends by a welded cap. Inside this tube is stacked a column 14 of fissile fuel pellets, within which the nuclear reactions that release heat occur. All the columns 14 define what is commonly called the fissile zone, which is located approximately halfway up an assembly 1. The sheath of the needles 100 thus constitutes the first containment barrier, the integrity of which must be preserved by protecting it from external aggressions such as mechanical shocks / stresses or excessive temperatures.

[0014] 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: - the low thermal conductivity of the fissile material made of an oxide ceramic which leads to a strong thermal gradient between the center and the periphery of the pellet; - the thermal resistance of the gap between the pad and the needle sheath; - 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.

[0015] Fig. 2, from publication [6], gives the order of magnitude of the temperatures in a fuel pellet under nominal operating conditions.

[0016] There are various ways to improve the heat dissipation function of a fuel pellet or stack of (U,Pu)O2 pellets. Since some characteristics are difficult to modify, such as the primary fluid or the cladding material, the most promising avenue for innovation is improving the thermal conductivity of the fuel, for example, (U,Pu)O2.

[0017] This route can be divided into two categories which have been explored.

[0018] The first category concerns the improvement of thermal conductivity without the addition of another phase within the fuel, for example (U,Pu)O2.

[0019] This first category can itself be subdivided into three subcategories as follows:

[0020] - a modification of the shape of the needle, in order to increase the exchange surface between hot source and cold source. This modification is localized by definition at the level of each fuel rod in a fuel assembly. The major drawbacks of this solution are that it requires a new design of the assembly and may have an impact on the manufacturability of the needle and the performance of the assembly;

[0021] - a change in the nature of the fuel by switching from a combustible oxide to a A better heat conductor fuel, for example a metallic fuel. This modification is by definition localized at the level of each fuel pellet. The major drawbacks of this solution are that it induces different behavior in the reactor, requires research and development to develop / qualify this new fuel, necessitates the development of new manufacturing processes, and requires redefining the downstream fuel cycle;

[0022] The second category concerns the improvement of thermal conductivity with the addition of another phase within the (U,Pu)O2 fuel. The addition is localized by definition at the level of each fuel pellet.

[0023] This second category can itself be subdivided into three subcategories as follows:

[0024] - a homogeneous dispersion of a material, such as diamond, graphene..., of Thermal conductivity superior to (U,Pu)O2 ceramic at the nanometer scale down to the micron scale. This solution does not appear relevant at present from the point of view of its thermal behavior. The inventors believe that it is, however It is difficult to make a determination regarding this solution due to a lack of information and knowledge.

[0025] - a homogeneous dispersion of a second metallic phase at the microscale scopic analysis with the aim of increasing the equivalent thermal conductivity of the pellet: see in particular [1]. Random dispersion of this second metallic phase in (U,Pu)O2 ceramic or in MOX was also considered for consistency with the standard powder mixing process for manufacturing. It was shown that the resulting fuel pellet microstructure was not effective in improving thermal properties. Furthermore, this solution has the major drawbacks of introducing manufacturing difficulties and requiring the finalization and validation of existing modeling tools;

[0026] - a heterogeneous dispersion of a second metallic phase on a macro scale scopic, that is, at the scale of a tenth of a millimeter or less, with the objective of promoting the heat flow of the (U,Pu)O2 pellet towards the cold source. This second metallic phase is characterized by a metallic insert within the fissile material of the pellet.

[0027] Such a metallic insert is described under two distinct designs in publication [3].

[0028] One such design, shown in Figure 1a of this publication [3], consists of a set of six thin fins, uniformly distributed according to an angular pattern like the spokes of a bicycle wheel. This design effectively improves the thermal conductivity of the fuel pellet at the center.

[0029] However, the analysis carried out on this design shows that while the choice of structural characteristics of the fins does indeed allow for a significant reduction in the core temperature of the pellet, this choice also potentially generates several disadvantages that could have very troublesome consequences on the behavior of the pellet, or even completely call into question the relevance of the solution.

[0030] The main potentially negative consequences in a PWR reactor are as follows: - a hypothetical guarantee of the thermal function under irradiation and of the mechanical strength under irradiation of the fins given their small thickness; - mechanical strength of the sheath not guaranteed, with high and local temperature gradients on it, typically from 100 to 300 °C / mm, which are due to the contacts of each fin with the sheath; - the lack of certainty of not aggravating the phenomenon of Pellet Sheath Interaction with Stress Corrosion (IPG-CSC); - a level of corrosion likely to be present on the sheath during the process irradiation; - the manufacturability of metallic fins in a ceramic fuel has not been demonstrated, including the possibility of mounting the fins.

[0031] To overcome the aforementioned drawbacks of the design shown in figure la of publication [3], the applicant proposed in patent applications WO2022 / 223387, WO2022 / 223504, WO2022 / 223510, different forms of metallic insert in substance respectively, with cross section in four branches, and with solid disc(s) and solid rod.

[0032] If these solutions appear satisfactory for PWR reactors, the inventors of the present invention have set themselves the objective of proposing a fuel pellet specifically designed for fast neutron reactors, and therefore to meet the following constraints: - improve the thermal conductivity of a ceramic (U,Pu)O2 or MOX fuel pellet, as a fissile material, and maintain high melting temperatures; - reduce the fuel-cladding reaction which produces a decrease in cladding thickness through corrosion; - to increase the release of fission gases and helium in an athermal manner when diffusion is not activated by the thermal gradient (low power). The objective of preventing gas retention in the fuel eliminates the risk of fuel meltdown and / or cladding rupture due to thermally activated gas expansion during a power transient such as an unintended control rod lift.

[0033] In other words, there is a need to improve the thermal design of nuclear fuel pellets, more particularly based on (U,Pu)O2 or MOX oxides, in order to reduce their core temperature as much as possible, in nominal operating conditions in particular for a Fast Neutron Reactor (FNR) core while respecting the aforementioned constraints.

[0034] The aim of the invention is to meet at least part of this need. Description of the invention

[0035] To this end, the invention relates, in one of its aspects, to a nuclear fuel pellet, comprising:

[0036] - a straight cylindrical ring of fissile material with a central axis delimited by a cylinder an inner right cylinder and an outer right cylinder whose length defines the length of the pellet, the inner right cylinder being hollow and defining the inner diameter, the outer right cylinder defining the outer diameter of the pellet;

[0037] - an insert made of thermally conductive material, in the form of at least three main branches extending over all or part of the length of the pellet and radially from the periphery of the hollow inner right cylinder to the outside of the pellet, each of the main branches being symmetrically or not branched at least p times, p being greater than or equal to 1; the number n of symmetrical or not branchings per main branch and per branching being equal to n, n being greater than or equal to 2; the main branches with their branchings being regularly distributed in the volume of the ring and spaced apart from each other.

[0038] In the context of the invention, the length of the pellet means its height.

[0039] Advantageously, the number p is between 1 and 7, the number n is equal to 2.

[0040] The main branches with their ramifications may be straight or curved. An insert can advantageously be metallic or made of a metallic alloy or ceramic.

[0041] The main branches with their ramifications are of square, rectangular, circular or elliptical cross-section.

[0042] The branches and ramifications can extend along the entire length of the pellet or be distributed along planes transverse to the central axis, being separated from each other by an axial distance, preferably constant over the length / height of the pellet.

[0043] Throughout the description, the branch or ramification section referred to is that calculated from the diameter of the outer right cylinder or from the thickness of the vertical plane.

[0044] According to an advantageous embodiment, each of the main branches and its ramifications extends radially to the outer diameter of the pellet.

[0045] Preferably, the ratio between the cross-section of each branch and that of each main branch is less than or equal to 1.

[0046] Preferably, the thickness of each main branch is less than 0.5 mm, preferably less than 0.1 mm.

[0047] Preferably, the ratio between section and length of each of the main branches and its ramifications being less than 0.5, preferably less than 0.1.

[0048] Advantageously, the material for the main branches and their ramifications is chosen from molybdenum (Mo), niobium (Nb), or their alloys, preferably an NbZr or NbZrC alloy. An NbZr or NbZrC alloy has a melting point of 2742 K, which is very close to that of a fissile MOX material, which is between 2700 and 3000 K depending on the Pu content and the O / M ratio. Nb, as well as the NbZr or NbZrC alloy, traps oxygen and is transparent to neutrons in the fast spectrum.

[0049] Advantageously, the fissile material of the straight cylinder is chosen from uranium (IV) oxide (UO2), mixed oxide (U,Pu)O2 or a mixed mixture based of reprocessed uranium oxide and plutonium oxides (MOx), or any other fissile ceramic.

[0050] Preferably, the volume percentage of the main branches with their ramifications is between 1 and 20%.

[0051] The invention also relates to a nuclear fuel needle extending along a longitudinal direction (XX1) comprising:

[0052] - a plurality of nuclear pellets as described above, stacked one on top of the other;

[0053] - a sheath of neutron-transparent material surrounding the stack of pellets.

[0054] Advantageously, the sheath is made of zirconium alloy, in particular Zircaloy-4 (Zr4), or of M5® alloy (ZrNbO).

[0055] The invention also relates to the use of a nuclear fuel pellet as described above or a fuel needle as described above in a fast neutron reactor (FNR), in particular cooled by liquid metal, such as liquid sodium (Na-FNR), a pressurized water reactor (PWR), a boiling water reactor (BWR).

[0056] Thus, the invention essentially consists of a nuclear fuel pellet which integrates, within a straight cylindrical ring of fissile material, a thermal conductor, in particular metallic, in the form of main branched branches which extend from the inside of the ring to the outside, advantageously distributed homogeneously within the ring.

[0057] The insert formed by its branches and ramifications at the macro-structure scale makes it possible to reduce the maximum temperature of the fuel by several hundred degrees Celsius depending on its dimensions, its constituent material and the distribution of its branches in the straight cylindrical ring of fuel.

[0058] Typically, the macrostructure of the insert with its branched arms allows heat to be evacuated thanks to the thermal conductivity of the chosen constituent material which can be about 20 to 30 times greater than that of a MOX fuel.

[0059] Thanks to its thermal properties, typically about 20 to 30 times greater than those of MOX, and the arrangement of its branched filaments in the fissile matrix, the metallic insert will increase heat dissipation. A calculation of the equivalent thermal conductivity using a finite element method can illustrate this improvement in thermal performance.

[0060] The advantageously chosen metal, NbZr(C), being refractory, the melting temperature of the insert is almost as high as that of the fissile phase, typically above 2700 K. The differential thermal expansion between the fissile material, in particular MOX and the metal of the insert must be taken into account during the manufacture and then during the irradiation.

[0061] When the insert is made of Nb or NbZr or NbZrC, the niobium will trap the oxygen atoms released following the fission reactions and thus reduce or even eliminate the corrosion of the sheath of a needle.

[0062] The main reaction product between Nb and O, Nb2O5, is known to be porous. The interface between the insert and the MOX will therefore evolve with the formation of a porous Nb2O5 seal, which will constitute a diffusion path for fission gases and helium during irradiation, regardless of the thermal regime of the pellet (athermal diffusion).

[0063] To optimize the macrostructure of the insert in the pellet, the thermal properties of each of the materials and the constraints in terms of combustible element design will have to be taken into account.

[0064] This optimization should lead to specifying the geometric aspect of the macrostructure in order to aim for the highest performance, depending on the type of reactor envisaged.

[0065] A ceramic fuel pellet having a higher equivalent thermal conductivity offers numerous advantages, including: - a reduction in the thermal gradient within the pellet, which helps to reduce its fragmentation and the spread of species - a decrease in the maximum and overall temperature of the pellet, - a decrease in the swelling of the gas bubbles and therefore of the pellet, - a decrease in thermal expansion, - a decrease in the mechanical interaction between the pellet and the sheath, which is beneficial for the diametral deformation of the sheath and the deformation energy density of the sheath, - an increase in margins on the various safety criteria mentioned above, in particular on the fuel meltdown limit, which increases the operating margins of a nuclear reactor in particular in order to be more flexible in control, and to obtain a gain in maximum power or even in maximum bum-up, in particular in order to improve the control capacity at high burn-up.

[0066] The main industrial applications targeted for the invention are all 4th generation fast neutron spectrum reactors.

[0067] 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

[0068] [Fig-1] [Fig.1] is a schematic longitudinal cross-sectional view of a pencil nuclear fuel according to the state of the art, as implemented in a PWR type nuclear reactor.

[0069] [Fig.1A] [Fig.1A] is a cross-sectional view of [Fig. 1].

[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 a branched metallic insert, not shown, according to the invention.

[0072] [Fig.4] [Fig.4] is a schematic cross-sectional view of a pellet combustible according to an example of the invention, with a metallic insert with six main branches branched three times.

[0073] [Fig. 5] [Fig. 5] is a partial cross-sectional view of the pellet according to the [Fig.4], from a Computer-Aided Design (CAD) tool, [Fig.5] showing the geometric parameters of branch ramification.

[0074] [Fig.6] [Fig.6] reproduces [Fig.4] as it comes from a numerical simulation.

[0075] [Fig.7] [Fig.7] reproduces part of [Fig.6] with a representation of the mesh digital.

[0076] [Fig.8] [Fig.8] reproduces [Fig.6] with a representation of the different elements temperatures within the lozenge.

[0077] [Fig.9] [Fig.9] is a schematic cross-sectional view of a pellet combustible according to another example of the invention, with a metallic insert having six main branches branched only once. Detailed description

[0078] 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.

[0079] Figures 1 and 2 have already been discussed in the preamble. They will therefore not be detailed below.

[0080] In the examples below, the fuel pellets according to the invention are numerically simulated to evaluate their thermal performance, by software marketed under the name "FreeFem++", version v4.7-2: https: / / freefem.org / .

[0081] 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].

[0082] A first example of a pellet 6 according to the invention is illustrated in [Fig.4].

[0083] A pellet 6 according to the invention with central axis (X) firstly comprises a straight cylindrical ring 60 of fissile material with central axis (X) delimited by an inner straight cylinder 61 and an outer straight cylinder 62 whose length defines the length (H) of the lozenge.

[0084] The internal right cylinder 61 is hollowed out and defines the internal diameter (0int) of the disc 6, while the external right cylinder 62 defines the external diameter (0ext) of the disc 6.

[0085] According to the invention, the pellet 6 comprises an insert 7 made of thermally conductive material, in the form of at least three main branches (6 main branches on [Fig.4]: 7.1, 7.2, 7.3, 7.4, 7.5, 7.6) branching which extend over the entire length of the pellet and radially from the periphery of the hollow inner straight cylinder 61 to the outside of the pellet, preferably to the outer diameter 62.

[0086] Each of the main branches 7.1 to 7.6 is branched symmetrically or not at least p times, p being greater than or equal to 1.

[0087] The number n of ramifications 70, 71, 72, per main branch and per ramification is equal to n, n being greater than or equal to 2.

[0088] The main branches 7.1 to 7.6 with their ramifications 70 to 72 are distributed regularly or not in the volume of the ring 60 and are distant from each other.

[0089] Additive manufacturing processes can be implemented to produce this type of branched-branch insert geometries.

[0090] The example in [Fig.4] is a pellet 6 whose insert 7 consists of six main straight branches 7.1 to 7.6 distributed at 60° to each other, each main branch being branched by two symmetrical straight ramifications 70.

[0091] The number of symmetrical rectilinear branches 70 to 72 from each main branch is equal to 3.

[0092] The inventors have produced a precise example of dimensioning of this first example of a pellet with six main branches 7.1 to 7.6 branched.

[0093] The reference dimensions, shown in [Fig.5] are indicated in Table 1 below.

[0094] [Tables 1] Dimensional parameters of the pellet 6 and its insert 7: inner diameter (0int = 2*Rint) 1.0 mm, outer diameter (0ext) 7.14 mm, branch and ramification width (>L) 0.032 mm, RI 0.35 mm, 01 (theta) 27° R2 0.75 mm, 02 (theta2) 27° R3 0.5 mm, 03 (theta3) 15° R4 1.1 mm, 04 (theta4) 20° R5 0.5 mm

[0095] An analysis of the expected performance of a pellet 6 with an insert 7 with branched branches as illustrated in [Fig.7] and 8 compared to that of a MOX fuel pellet without an insert according to the state of the art was carried out by the inventors in Table 2.

[0096] The following table 2 summarizes these performances.

[0097] [Tables2] Fuel Pellet Performance according to the state of the art Fuel Pellet 6 according to the invention THERMAL - melting margin 2 W / mK at 2400K -300 K (for a linear power of 500W / cm) 30 W / mK maximum >1200 K THERMOCHEMISTRY - risk of cladding rupture due to primary stress and thinning 200 pm (maximum combustion rate) with a 50% reduction in cladding thickness high, hence the need for appropriate needle sizing to manage this risk 0 pm reduced FISSION GAS DIFFUSION - release of fission gases - during power transients, risk of fuel melting due to closure of the central hole caused by gas expansion - risk of mechanical interaction between the pellet and cladding at high combustion rates or during a transient 70 to 80% integral on the pellet proven and sizing proven and sizing >95% none none

[0098] It appears from this table that the branched insert according to the invention allows, compared to a pellet without an insert using MOX fuel according to the state of the art: - to increase the power; - to avoid fuel meltdown in almost all accidental transients; - to achieve very high combustion rates; - to reduce sheath thicknesses and therefore costs and structural waste; - to significantly improve safety; - not to penalize the needle height because the free volumes are already sized for 100% relaxation.

[0099] To quantify the expected performance improvement, the inventors performed a 2D thermal calculation by imposing a heat source in the fissile material and imposing the temperature of the outer edge 62 of the pellet 6. This results in a heat flux from the edge of the hollow center 61 to the edge 62 of the pellet 6. The calculations were performed on a pellet 6 whose fissile material has the formula (Ul-y,Puy)Ox and the insert 7 is made of NbZr or NbZrC alloy.

[0100] The properties of these materials, extracted respectively from references [4] and [5], are given in Table 3 below.

[0101] [Tables3] Physical properties Fissile material Metallic insert 7 material (Ui y,Puy)Ox Nb Zr or NbZrC stoichiometry (2-x) 1.98 - value of y 0.3 - thermal conductivity (W / mK) Equation 1 Equation 2 Melting point (K) TsoiiduS=416y2 - 521 y + 3143 TsoiiduS=2673

[0102] Equation 1 can be written as:

[0103] X: thermal conductivity in W / mK

[0104] T: temperature in K,

[0105] x: the deviation from stoichiometry, i.e., x=0 / M-2.0,

[0106] Zi: the americium content Am (Am / (U+Pu+Np+Am)),

[0107] z2: the neptunium content Np (Np / (U+Pu+Np+Am)).

[0108] Equation 2 can be written as: , 4 HW) W / ml

[0109] k: thermal conductivity in W / mK

[0110] T: temperature in K.

[0111] The 2D thermal calculations solve the following equation 3:

[0112] -Æv(Xx)Vr) =f(x)

[0113] with Dirichlet boundary conditions T = 1144 K at the edge of the pellet.

[0114] The conductivity 2 is a function of the material, as is the source term / , which is uniform within the fissile material and zero in the metallic insert (the few capture reactions are neglected).

[0115] The conditions for the calculations are indicated in Table 4 below.

[0116] [Tables4] Thermal power linear heat rate (W / cm) 548 radial profile at start of life flat temperature external temperature of the pellet at 62 1144.1 maximum temperature of the pellet at 61 to be determined

[0117] The calculation was carried out on a mesh composed of 182004 then 728016 triangles with a variation from one to the other of 1K in terms of maximum temperature, which allows us to conclude that the calculations have converged.

[0118] The results obtained for an example of the geometry of pellet 6 as according to [Fig.4] are illustrated in Figures 6 to 8. It should be noted that in [Fig.8], the scale is distorted to visualize the variations in the local thermal conductivity of pellet 6.

[0119] Fig. 9 illustrates another example of 6-pronged pellets with six main branches 7.1 to 7.6 straight, each being branched only once into two symmetrical branches 70 also straight.

[0120] The following table 5 presents the results of the thermal calculations on a pellet according to the invention with insert 7 according to figures 4 and 9 in comparison with a MOX fuel pellet without insert according to the state of the art with the same geometry and operating conditions.

[0121]

[0122]

[0123]

[0124]

[0125]

[0126] [Tables 5] Fuel pellet State-of-the-art pellet without insert, type (U,Pu)O2 Pellet as shown in Figure 4 Pellet as shown in Figure 9 Geometry (mm) Oext / Oint equals 7.14 / 2.01 or 3.55 Oext / Oint equals 7.14 / 2.01 or 3.55 Width L branches and ramifications equals 0.032 Oext / Oint equals 7.14 / 2.01 or 3.55 Width L branches and ramifications equals 0.088 Applied linear power (W / cm) 548 548 548 Metal / total content 0% Vol 10.1% vol 10.0% Tmax fuel (K) 2573 1655 1622 Tmin fuel (K) 1144 1144 1144 AT fuel (K) 1430 515 478 It therefore appears from Table 5 that: - a gain of nearly 915K in the maximum pellet temperatures is obtained with a metallic insert geometry according to [Fig.4], which corroborates the expected thermal performance; - for a given volume fraction of metal, a substantially equivalent temperature gain is obtained for a geometry according to [Fig.9], with a single level of branching. In addition, the inventors believe that the branched geometry of the insert according to [Fig.4] has the advantage of having a more uniform edge temperature than that of [Fig.9]. The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants. Other variations and improvements can be considered without going outside the scope of the invention. Depending on the intended applications (primarily fast neutron reactors), different parameters of a branched-branch metallic insert pellet can be optimized. Some of the parameters that can be optimized include:

[0127] - the choice of a refractory metal other than NbZr or NbZrC;

[0128] - a metal / ceramic volume ratio different from that of the calculations above;

[0129] - the choice of a fissile material other than MOX;

[0130] - an arrangement and dimensions of the ramifications different from those of figures 4 and 9;

[0131] - a dimensioning of the inner and outer diameters of the pellet different from that of the calculations above. List of cited references

[0132] [1]: Kim DJ - Rhee YW & al - “Fabrication of Micro-Cell_UO2-Mo with enhanced thermal conductivity". JNM 462 (2015) 289-295.

[0133] [2]: Y. Guérin, “2.21 - Fuel Performance of Fast Spectrum Oxide Fuel”, in Com prehensile Nuclear Materials, RJM Konings, Ed., Oxford: Elsevier, 2012, p. 547-578. doi:10.1016 / 6978-0-08-056033-5.00043-4.

[0134] [3]: Medvedev P.G. & Mariani R.D. - ‘‘Conductive inserts to reduce nuclear fuel”, JNM 531 (2020) 151966.

[0135] [4]: M. Kato, K. Maeda, T. Ozawa, M. Kashimura and Y. Kihara, “Physical Properties and Irradiation Behavior Analysis ofNp- and Am-Bearing MOX Fuels”, J. Nucl. Sci. Technol. 48 (2011) 646.

[0136] [5]: D.J. Senor, J.K. Thomas, K.L. Peddicord, Journal of Nuclear Materials 173 (1990) page 261-273 and page 274-283.

Claims

1.

2.

3.

4.

5.

6.

7. Demands Nuclear fuel pellet (6), comprising: - a straight cylindrical ring (60) of fissile material with central axis (X) delimited by an inner straight cylinder (61) and an outer straight cylinder (62) whose length defines the length (H) of the pellet, the inner straight cylinder being hollow and defining the inner diameter (0int), the outer straight cylinder defining the outer diameter (0ext) of the pellet; - an insert (7) of thermally conductive material, in the form of at least three main branches (7.1 to 7.6) branching which extend over all or part of the length of the pellet and radially from the periphery of the hollow inner right cylinder towards the outside of the pellet, each of the main branches being branched symmetrically or not at least p times, p being greater than or equal to 1; the number n of branches (70, 71, 72, 73) symmetrical or not per main branch and per branch being equal to n, n being greater than or equal to 2; the main branches with their branches being regularly distributed in the volume of the ring and spaced apart from each other. Pellet (6) according to claim 1, the number p being between 1 and 7, the number n being equal to 2. A disc (6) according to claim 1 or 2, the main branches with their ramifications being straight or curved. A disc (6) according to any one of the preceding claims, the main branches with their ramifications having a square, rectangular, circular, or elliptical cross-section. Pellet (6) according to any one of the preceding claims, each of the main branches and its ramifications extending radially to the outer diameter of the pellet. Pellet (6) according to any one of the preceding claims, the ratio between the cross-section of each branch and that of each main branch being less than 1. Pellet (6) according to claim 6, the cross-section of each branch main one being less than 0.5 mm2, preferably less than 0.1 mm2

8. Pellet (6) according to any one of the preceding claims, the ratio between section and length of each of the main branches and its ramifications being less than 0.5 mm, preferably less than 0.1 mm.

9. Pellet (6) according to any one of the preceding claims, the material of the main branches and its ramifications being selected from Molybdenum (Mo), niobium (Nb) or their alloys, preferably an NbZr alloy or an NbZrC alloy.

10. 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).

11. Pellet (6) according to any one of the preceding claims, the volume percentage of the main branches with their ramifications being between 1 and 20%.

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

13. Needle (1) according to claim 12, the sheath being made of zirconium alloy, in particular Zircaloy-4 (Zr4), or of M5® alloy (ZrNbO).

14. Use of a nuclear fuel pellet (6) according to any one of claims 1 to 11 or of a nuclear fuel needle (1) according to claim 12 or 13 in a fast neutron reactor (FNR), in particular cooled by liquid metal, such as liquid sodium (Na-FNR), a pressurized water reactor (PWR), a boiling water reactor (BWR).

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