FR3150632A1 - Nuclear fuel pellet incorporating a thermally conductive insert in the form of ramified branches distributed from a hollowed-out inner cylinder towards the outside of the pellet. - Google Patents

Abstract

Nuclear fuel pellet incorporating a thermally conductive insert in the form of branched branches distributed from a hollowed-out inner cylinder toward 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, in particular metallic, in the form of branched main branches that extend from the inside of the ring toward the outside, advantageously distributed homogeneously within the ring. The insert formed by its branches and ramifications at the macrostructure 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 fuel ring. Figure for the abstract: fig.4

Description

Nuclear fuel pellet incorporating a thermally conductive insert in the form of ramified branches distributed from a hollowed-out inner cylinder towards the outside of the pellet.

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 (FNR-Na).

More specifically, it is located in the field of ceramic fuels made from uranium or uranium and plutonium oxide (U, Pu) _O2 or MOX (acronym for “mixed oxide”).

The invention essentially aims to improve the thermal properties of these fuels as well as the reduction of gaseous swelling, both in nominal operation and in incidental or accidental conditions (for example in situations of increase in power or loss of refrigerant).

By "nuclear reactors", throughout the application, is understood the usual meaning of the term to date, namely power plants for producing energy from nuclear fission reactions using fuel elements in which fissions occur which release the heat power, the latter being extracted from the elements by heat exchange with a heat transfer fluid which ensures their cooling.

By "nuclear fuel rod", throughout the application, we understand the official meaning defined for example, in the dictionary of Nuclear Sciences 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 whose terminology is used for fast neutron reactors.

Although described with reference to the application to Fast Neutron Reactors (FNR), including those cooled by a liquid metal, in particular by liquid sodium (FNR-Na) but also lead, lead-bismuth, etc., the invention relates to fuel elements which can be dedicated to all types of reactors for electrogenic, calogenic or experimental purposes, such as Boiling Water Reactors (BWR), Pressurized Water Reactors (PWR) and all advanced ^3rd and ^4th generation reactors.

Nuclear reactors that use fission energy to produce heat can be classified into several different categories depending on 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, etc.

Fuel assemblies intended for use in liquid sodium-cooled fast neutron reactors (RNR-Na) have a specific mechanical structure, in particular to allow liquid sodium to pass through them.

Figures 1 and 1A show a fuel assembly 1 conventionally used in an RNR-Na nuclear reactor.

Such an assembly 1 of elongated shape along a longitudinal axis X firstly comprises a tube or casing 10 of hexagonal 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 envelops fuel needles 100.

The portions 11, 12 form the same tubular envelope 10 or housing of identical hexagonal section over its entire height. The head 11 of the assembly has a central opening 110 opening into it and used for its handling.

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

Each needle 100 is in the form of a sealed cylindrical steel sheath tube closed at both ends by a welded cap inside which 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 usually called the fissile zone which is approximately located halfway up an assembly 1. The sheath of the needles 100 thus constitutes the first containment barrier whose integrity it is very important to preserve by protecting it from external aggressions such as mechanical shocks/constraints or excessive temperatures.

• la faible conductivité thermique de la matière fissile constitué d’une céramique oxyde qui conduit à un fort gradient thermique entre le centre et la périphérie de la pastille;
• la résistance thermique du jeu entre pastille et gaine de l’aiguille;
• la conduction thermique radiale à travers la gaine;
• la résistance due à l’échange convectif radial entre la face externe de la gaine et le fluide caloporteur.

This power is evacuated towards the cold source of the primary circuit (for example the liquid cooling sodium) by encountering a certain number of thermal resistances which can be synthesized as follows:

• the low thermal conductivity of the fissile material consisting 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 clearance between the pellet and the needle sheath;
• radial heat conduction through the sheath;
• the resistance due to radial convective exchange between the external face of the sheath and the heat transfer fluid.

There , from publication [6] gives the order of magnitude of the temperatures in a fuel pellet in nominal operating mode.

There are different ways to improve the power evacuation function of a (U,Pu)O ₂ fuel pellet or pellet stack. Since some characteristics can hardly be modified, such as the primary fluid or the cladding material, the most open innovation path is to improve the thermal conductivity of the fuel, for example (U,Pu)O ₂ .

This path can be divided into two categories which have been explored.

The first category concerns the improvement of thermal conductivity without adding another phase within the fuel, for example (U,Pu)O ₂ .

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

- a modification of the shape of the needle, in order to increase the exchange surface between the hot source and the cold source. This modification is located by definition at the level of each rod of a fuel assembly. This solution has the major drawbacks of requiring a new design of the assembly and of having a possible impact on the manufacturability of the needle and the performance of the assembly;

- a change in the nature of the fuel by switching from a combustible oxide to a better heat-conducting fuel, for example a metallic fuel. This modification is localized by definition at the level of each fuel pellet. This solution has the major drawbacks of inducing different behavior in the reactor, requiring research and development to develop/qualify this new fuel, developing new manufacturing processes and having to redefine the downstream fuel cycle;

The second category concerns the improvement of thermal conductivity with the addition of another phase within the fuel (U,Pu)O ₂ . The addition is localized by definition at the level of each fuel pellet.

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

- a homogeneous dispersion of a material, such as diamond, graphene, etc., with a thermal conductivity higher than ceramic (U,Pu)O ₂ at the nanometer scale to the micron scale. This solution does not seem relevant to date from the point of view of its thermal behavior. The inventors believe, however, that it is difficult to decide on this solution due to a lack of information and knowledge.

- a homogeneous dispersion of a second metallic phase at the microscopic scale with the objective of increasing the equivalent thermal conductivity of the pellet: see in particular [1]. The random dispersion of this second metallic phase in the ceramic (U, Pu)O ₂ or in MOX has also been considered for consistency with the standard powder mixing process for manufacturing. It has been shown that the resulting microstructure of a fuel pellet was not effective in improving thermal properties. In addition, this solution has the major drawbacks of inducing a manufacturing difficulty and requiring the finalization and validation of existing modeling tools;

- a heterogeneous dispersion of a second metallic phase on a macroscopic scale, i.e. on a scale of a tenth of a millimeter or less, with the aim of promoting the heat flow from the (U,Pu)O ₂ pellet to the cold source. This second metallic phase is characterized by a metallic insert within the fissile material of the pellet.

Such a metal insert is described in two distinct designs in the publication [3].

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

However, the analysis carried out on this design shows that if, indeed, the choice of the structural characteristics of the fins makes it possible to achieve a significant reduction in the temperature of the pellet at the core, on the other hand this choice potentially generates several drawbacks which could have very annoying consequences on the behavior of the pellet, or even completely call into question the relevance of the solution.

• une garantie hypothétique de la fonction thermique sous irradiation et de la tenue mécanique sous irradiation des ailettes compte tenu de leur faible épaisseur;
• une tenue mécanique de la gaine non garantie, avec des gradients de température élevés et locaux sur celle-ci, typiquement de 100 à 300 °C/mm, qui sont dus aux contacts de chaque ailette avec la gaine ;
• l’absence de certitude de ne pas aggraver le phénomène d’Interaction Pastille Gaine avec une Corrosion Sous Contrainte (IPG-CSC) ;
• un niveau de corrosion susceptible d’être présent sur la gaine en cours d’irradiation ;
• une fabricabilité des ailettes métalliques dans un combustible céramique non démontrée avec notamment la possibilité du montage des ailettes.

The main potentially negative consequences are as follows in a REP reactor:

• a hypothetical guarantee of the thermal function under irradiation and of the mechanical resistance under irradiation of the fins given their low thickness;
• mechanical resistance 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 Interaction Pellet Sheath with Stress Corrosion (IPG-CSC);
• a level of corrosion likely to be present on the sheath during irradiation;
• a manufacturability of metal fins in a ceramic fuel not demonstrated, in particular with the possibility of mounting the fins.

To overcome the aforementioned drawbacks of the design shown in Figure 1a of publication [3], the applicant proposed in patent applications WO2022/223387, WO2022/223504, WO2022/223510, different forms of metal insert in substance respectively, with a four-arm cross-section, and with solid disc(s) and solid rod.

• améliorer la conductivité thermique d’une pastille de combustible à céramique (U,Pu)O₂ou MOX, en tant que matériau fissile, et conserver des températures de fusion élevée;
• réduire la réaction combustible-gaine qui produit par la corrosion une diminution de l’épaisseur de la gaine ;
• augmenter le relâchement des gaz de fission et de l’Hélium de façon athermique lorsque la diffusion n’est pas activée par le gradient thermique (faible puissance). L’objectif d’éviter la rétention des gaz dans le combustible permet d’écarter le risque de fusion du combustible et/ou de rupture de la gaine par gonflement gazeux activé thermiquement lors d’un transitoire de puissance de type remontée intempestive de barre de commande.

If these solutions appear satisfactory for REP 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 respect the following constraints:

• to improve the thermal conductivity of a ceramic (U,Pu) _O2 or MOX fuel pellet, as a fissile material, and to maintain high fusion temperatures;
• reduce the fuel-cladding reaction which produces a reduction in the thickness of the cladding through corrosion;
• increase the release of fission gases and helium athermally when diffusion is not activated by the thermal gradient (low power). The objective of avoiding the retention of gases in the fuel makes it possible to eliminate the risk of fuel melting and/or rupture of the cladding by thermally activated gas swelling during a power transient such as an untimely control rod rise.

In other words, there is a need to improve the thermal design of nuclear fuel pellets, particularly based on oxides (U,Pu) _O2 or MOX, in order to reduce their core temperature as much as possible, in nominal operating mode, particularly for a Fast Neutron Reactor (FNR) core, while respecting the aforementioned constraints.

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

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

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

- an insert made of thermally conductive material, in the form of at least three branched main branches which extend over all or part of the length of the pellet and radially from the periphery of the hollowed-out inner straight 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 symmetrical or not branches 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.

For the purposes of the invention, the length of the pellet means its height.

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

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

The main branches with their ramifications are square, rectangular, circular or elliptical in cross section.

The branches and ramifications may 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.

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.

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

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

More preferably, the thickness of each main branch is less than 0.5 mm, preferably less than 0.1 mm.

More 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.

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

Advantageously, the fissile material of the right 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.

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

The invention also relates to a nuclear fuel needle extending in a longitudinal direction (XX') comprising:

- a plurality of nuclear pellets as described above, stacked on top of each other;

- a sheath made of neutron-transparent material surrounding the stack of pellets.

Advantageously, the sheath is made of zirconium alloy, in particular Zircaloy-4 (Zr ₄ ), or M5 ^® alloy (ZrNbO).

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

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 ramified main branches which extend from the inside of the ring towards the outside, advantageously distributed in a homogeneous manner within the ring.

The insert formed by its branches and ramifications at the macrostructure 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 fuel ring.

Typically, the macrostructure of the insert with its ramified branches allows heat to be evacuated thanks to the thermal conductivity of the chosen constituent material which can be approximately 20 to 30 times greater than that of a MOX fuel.

Thanks to its thermal properties, typically about 20 to 30 times greater than those of MOX, and the arrangement of its ramified branches in the fissile matrix, the metal insert will increase heat evacuation. A calculation of the equivalent thermal conductivity by a finite element type tool can illustrate this gain in thermal performance.

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 greater than 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 manufacturing and then during irradiation.

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

The main reaction product between Nb and O, Nb ₂ O ₅ , is known to be porous. The interface between the insert and the MOX will therefore evolve with the formation of a porous Nb ₂ O ₅ joint, which will constitute a diffusion path for fission gases and helium during irradiation, regardless of the thermal regime of the pellet (athermal diffusion).

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

This optimization should lead to specifying the geometric aspect of the macrostructure to aim for the highest performances, depending on the type of reactor envisaged.

• une diminution du gradient thermique dans la pastille qui permet de réduire sa fragmentation et la diffusion des espèces
• une diminution de la température maximale et globale de la pastille,
• une diminution du gonflement des bulles de gaz et donc de la pastille,
• une diminution de la dilatation thermique,
• une diminution de l’interaction mécanique entre la pastille et la gaine, ce qui est bénéfique pour la déformation diamétrale de la gaine et la densité d’énergie de déformation de la gaine,
• un gain de marges sur les différents critères de sûreté précités, en particulier sur la limite à fusion du combustible, ce qui augmente ainsi les marges de fonctionnement d’un réacteur nucléaire notamment afin d’être plus flexible en pilotage, et d’obtenir un gain en puissance maximale voire en burn-up maximal, en particulier afin d’améliorer la capacité de pilotage à haut burn-up.

A ceramic fuel pellet with a higher equivalent thermal conductivity has many advantages, including:

• a reduction in the thermal gradient in the pellet which reduces its fragmentation and the diffusion of species
• a decrease in the maximum and overall temperature of the pellet,
• a reduction in the swelling of gas bubbles and therefore of the pellet,
• a decrease in thermal expansion,
• a decrease in the mechanical interaction between the pellet and the cladding, which is beneficial for the cladding diametrical deformation and the cladding strain energy density,
• a gain in margins on the various safety criteria mentioned above, in particular on the fuel fusion limit, which thus 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 burn-up, in particular in order to improve the control capacity at high burn-up.

The industrial applications mainly targeted for the invention are all 4th generation reactors with fast neutron spectrum.

Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given for illustrative and non-limiting purposes with reference to the following figures.

there is a schematic view in longitudinal section of a nuclear fuel rod according to the state of the art, as implemented in a PWR type nuclear reactor.

there is a cross-sectional view of the .

there 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.

there is a schematic perspective view of a basic M pattern of a nuclear fuel needle whose sheath houses a fuel pellet with a metal insert with branched branches, not shown, according to the invention.

there is a schematic cross-sectional view of a fuel pellet according to an example of the invention, with a metal insert with six main branches branched three times.

there is a partial cross-sectional view of the pellet according to the , resulting from a Computer Aided Design (CAD) tool, the showing the geometric parameters of branching.

there resumes the as it results from a digital simulation.

there takes up part of the with a representation of the digital mesh.

there resumes the with a representation of the different temperatures within the pellet.

there is a schematic cross-sectional view of a fuel pellet according to another example of the invention, with a metal insert with six main branches branched only once.

Detailed description

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.

Figures 1 and 2 have already been commented on in the preamble. They will therefore not be detailed below.

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

It was represented in , a basic pattern M of a nuclear fuel rod with a cladding 2 housing a fuel pellet 6 according to the invention.

A first example of a pellet 6 according to the invention is illustrated in .

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

The inner straight cylinder 61 is hollowed out and defines the inner diameter (Øint) of the pellet 6, while the outer straight cylinder 62 defines the outer diameter (Øext) of the pellet 6.

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 the : 7.1, 7.2, 7.3, 7.4, 7.5, 7.6) branched which extend over the entire length of the pellet and radially from the periphery of the hollowed-out inner straight cylinder 61 towards the outside of the pellet, preferably up to the outer diameter 62.

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.

The number n of branches 70, 71, 72, per main branch and per branch is equal to n, n being greater than or equal to 2.

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 distant from each other.

Additive manufacturing processes can be implemented to achieve this type of branched insert geometry.

The example of the is a pellet 6 whose insert 7 is made up of six rectilinear main branches 7.1 to 7.6 distributed at 60° from each other, each main branch being branched by two symmetrical ramifications 70 which are also rectilinear.

The number of straight symmetrical branches 70 to 72 from each main branch is equal to 3.

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

The reference dimensions, shown in are shown in Table 1 below.

Dimensional parameters of the pellet 6 and its insert 7 inner diameter (Øint = 2*Rint) 1.0 mm outer diameter (Øext) 7.14 mm Branch and ramification width ( >L ) 0.032 mm R1 0.35 mm θ1 (theta1) 27° R2 0.75 mm θ2 (theta 2 ) 27° R3 0.5 mm θ3 (theta3 ) 15° R4 1.1 mm θ4 (theta4 ) 20° R5 0.5 mm

An analysis of the expected performance of a 6-point insert 7-point insert with branched branches as illustrated in and 8 compared to those of a MOX fuel pellet without insert according to the state of the art was made by the inventors in Table 2.

The following Table 2 summarizes these performances.

Performances State-of-the-art fuel pellet Fuel pellet 6 according to the invention THERMAL

• margin for merger

2 W/mK at 2400K

~300 K (for a linear power of 500W/cm)
30 W/mK maximum

>1200 K THERMOCHEMISTRY

• risk of sheath rupture due to primary stress and creep

200 µm (max burn rate) with 50% reduction in sheath thickness

high hence the need for suitable needle sizing to manage this risk
0 µm


reduced DIFFUSION OF FISSION GASES
- release of fission gases
- during power transients, risk of fuel melting due to closing of the central hole due to gaseous swelling
- risk of mechanical interaction between pellet and sheath at high combustion rates or during a transient

70 to 80% in full on the tablet

proven and dimensioning



proven and dimensioning

>95%


none



none

• d’augmenter la puissance ;
• d’éviter la fusion du combustible dans la quasi-totalité des transitoires accidentels ;
• d’atteindre des taux de combustion très élevés ;
• de réduire les épaisseurs de gaine et donc des coûts et des déchets de structure ;
• d’améliorer de manière significative la sûreté ;
• de ne pas pénaliser la hauteur d’aiguille car les volumes libres sont déjà dimensionnés pour 100% de relâchement.

It is clear from this table that the insert with branched branches according to the invention allows, compared to a pellet without a MOX fuel insert according to the state of the art:

• to increase power;
• to avoid fuel fusion 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% release.

To quantify the expected improvement in performance, the inventors performed a 2D thermal calculation by imposing a heat source in the fissile material and imposing the temperature of the external edge 62 of the pellet 6. This results in a heat flow from the edge of the hollowed-out center 61 to the edge 62 of the pellet 6. The calculations were performed on a pellet 6 whose fissile material is of formula (U1-y,Puy)Ox and the insert 7 is made of NbZr or NbZrC alloy.

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

Physical properties Fissile material Metal insert 7 material (U _1-y ,Pu _y )Ox Nb Zr or NbZrC stoichiometry (2-x) 1.98 - value of y 0.3 - thermal conductivity (W/m.K) Equation 1
Equation 2
melting temperature (K) T _solidus =416y ² – 521y + 3143 T _solidus =2673

Equation 1 is written as:

λ: thermal conductivity in W/m.K

T: temperature in K,

x: the deviation from stochiometry is x=O/M-2.0,

z ₁ : the americium content Am (Am/(U+Pu+Np+Am)),

z ₂ : the neptunium content Np (Np/(U+Pu+Np+Am)).

Equation 2 is written as:

k: thermal conductivity in W/m.K

T: temperature in K.

2D thermal calculations solve the following equation 3:

with Dirichlet boundary conditions on the edge of the pellet.

Conductivity is a function of the material, as is the source term , which is uniform within the fissile material and zero in the metallic insert (we neglect the few capture reactions).

The calculation conditions are shown in Table 4 below.

Thermal power linear heat rate (W/cm) 548 radial profile at the beginning of life flat Temperature outside temperature of the pellet in 62 1144.1 maximum temperature of the pellet in 61 to be determined

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 well.

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

There illustrates another example of pellets 6 with six main straight branches 7.1 to 7.6, each being branched only once into two symmetrical branches 70 which are also straight.

Table 5 below 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 conditions and operating conditions.

Fuel tablet State-of-the-art pellet without insert, type (U,Pu)O2 Pastille according to the example in figure 4 Pastille according to the example in figure 9 Geometry (mm) Φext/Φint equals 7.14/2.01 or 3.55 Φext/Φint equals 7.14/2.01 or 3.55
Width L branches and ramifications equal to 0.032 Φext/Φint equals 7.14/2.01 or 3.55
Width L branches and ramifications equal to 0.088 Applied linear power (W/cm) 548 548 548 Metal share/total 0%Vol 10.1% vol 10.0% Tmax fuel (K) 2573 1655 1622 Fuel Tmin (K) 1144 1144 1144 ΔT fuel (K) 1430 515 478

• un gain de près de 915K dans les températures maximum de pastille est obtenu avec une géométrie d’insert métallique selon la , ce qui corrobore la performance thermique attendue ;
• à fraction volumique de métal donnée, un gain de température sensiblement équivalent est obtenu pour une géométrie selon la , avec un seul niveau de ramifications.

It therefore emerges from this table 5 that:

• A gain of nearly 915K in maximum pellet temperatures is obtained with a metal insert geometry according to the , which corroborates the expected thermal performance;
• at a given volume fraction of metal, a substantially equivalent temperature gain is obtained for a geometry according to the , with only one level of branching.

Furthermore, the inventors believe that the branched geometry of the insert according to the has the advantage of having a more uniform temperature at the edge of the pellet than that of the .

The invention is not limited to the examples which have just been described; it is possible in particular to combine characteristics of the examples illustrated within non-illustrated variants.

Other variations and improvements may be envisaged without departing from the scope of the invention.

Depending on the applications envisaged (mainly fast neutron reactors), different parameters of a pellet with a metal insert with branched branches can be optimized. Among the parameters to be optimized, we can cite:

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

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

- the choice of a fissile material other than MOX;

- an arrangement and dimensions of the branches different from those of figures 4 and 9;

- a dimensioning of the internal and external diameters of the pellet different from that of the calculations above.

List of cited references

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

[2]: Y. Guerin, “2.21 - Fuel Performance of Fast Spectrum Oxide Fuel”, in Comprehensive Nuclear Materials , RJM Konings, Ed., Oxford: Elsevier, 2012, p. 547-578. doi:10.1016/B978-0-08-056033-5.00043-4.

[3]: Medvedev PG & Mariani RD – “ Conductive inserts to reduce nuclear fuel ”, JNM 531 (2020) 151966.

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

[5]: DJ Senor, JK Thomas, KL Peddicord, Journal of Nuclear Materials 173 (1990) page 261-273 and page 274-283.

Claims (14)

Nuclear fuel pellet (6), comprising:

• a straight cylindrical ring (60) of fissile material with a 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 hollowed out and defining the inner diameter (Ø _int ), the outer straight cylinder defining the outer diameter (Ø _ext ) of the pellet;

• an insert (7) made of thermally conductive material, in the form of at least three branched main branches (7.1 to 7.6) which extend over all or part of the length of the pellet and radially from the periphery of the hollowed-out inner straight 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 symmetrical or not branches (70, 71, 72, 73) 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.

Pastille (6) according to claim 1, the number p being between 1 and 7, the number n being equal to 2. Pastille (6) according to one of claims 1 or 2, the main branches with their ramifications being straight or curved. Pastille (6) according to one of the preceding claims, the main branches with their ramifications being of square, rectangular, circular or elliptical cross section. Pellet (6) according to one of the preceding claims, each of the main branches and its ramifications extending radially to the outer diameter of the pellet. Pastille (6) according to one of the preceding claims, the ratio between the section of each branch and that of each main branch being less than 1. Pellet (6) according to claim 6, the section of each main branch being less than 0.5 mm², preferably less than 0.1 mm² . Pastille (6) according to 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. Pellet (6) according to one of the preceding claims, the material of the main branches and its ramifications being chosen from Molybdenum (Mo), niobium (Nb) or their alloys, preferably an NbZr alloy or an NbZrC alloy. Pellet (6) according to one of the preceding claims, the fissile material of the right cylinder being chosen from uranium (IV) oxide (UO ₂ ), mixed oxide (U, Pu)O ₂ or a mixed mixture based on uranium oxide and reprocessed plutonium oxides (MOx). Pastille (6) according to one of the preceding claims, the volume percentage of the main branches with their ramifications being between 1 and 20%. Nuclear fuel needle (1) extending in a longitudinal direction (XX') comprising:
a plurality of nuclear pellets (6) according to one of the preceding claims, stacked on top of each other;
a sheath (2) made of neutron-transparent material surrounding the stack of pellets. Needle (1) according to claim 12, the sheath being made of zirconium alloy, in particular Zircaloy-4 (Zr ₄ ), or M5® alloy (ZrNbO). Use of a nuclear fuel pellet (6) according to one of claims 1 to 11 or of a nuclear fuel needle (1) according to claim 12 or 13 in a fast neutron reactor (RNR), in particular cooled by liquid metal, such as liquid sodium (RNR-Na), a pressurized water reactor (PWR), a boiling water reactor (BWR).