WO2022218781A1

WO2022218781A1 - Fuel pellet comprising an improved metal insert

Info

Publication number: WO2022218781A1
Application number: PCT/EP2022/059153
Authority: WO
Inventors: Emmanuelle PICARD COULON
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2021-04-16
Filing date: 2022-04-06
Publication date: 2022-10-20
Also published as: FR3122027B1; FR3122027A1

Abstract

The invention relates to a fuel pellet (1) having a central axis (A) of principal extension, and comprising a fissile nuclear fuel material (10) and at least one metal insert (11) comprising a plurality of transverse plates (111). Each plate (111) has a solid volume representing more than 50% of the total volume of the plate (111) and a thickness substantially comprised between 0.2 and 1.5 mm. The insert (11) is configured to represent a proportion of between 5% and 15% of the total volume of the fuel pellet (1). Furthermore, at least one plate (111), and preferably each plate (111), has a central opening (1110) located at the centre of the plate (111). The insert is thus easy to manufacture and improves the thermal conductivity of a fuel pellet while at the same time improving its margin to meltdown.

Description

"Fuel pellet comprising an improved metal insert" TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of fuel pellets for nuclear reactor fuel rods, and more particularly fuel pellets with a metal insert. It finds a particularly advantageous application in the field of fuel pellets for fuel rods of light water nuclear reactors, such as pressurized water reactors (PWR) and boiling water reactors (BWR).

STATE OF THE ART

The core of a reactor is conventionally divided into fuel assemblies, loaded side by side in the reactor vessel. An assembly typically consists of a bundle of fuel rods. Each fuel rod typically comprises a plurality of pellets of fissile nuclear fuel material stacked inside the sheath.

The fuel pellet emits heat through fission reactions. This heat is conducted through the fuel pellet to the sheath and then transferred to the primary heat transfer fluid. The heat emitted is evacuated by the coolant from the core to the conventional island of the nuclear power plant, to transform it into electricity.

Thus, the primary function of the fuel rod is to produce, then to transmit the heat produced by the fission reactions within the combustible material to the coolant. The average linear powers in the nominal regime of a fuel pellet are typically between 150 and 300 W/cm. The low thermal conductivity of combustible materials, and in particular materials based on uranium oxide, creates a strong temperature gradient between the center and the periphery of the pellet, for example of the order of 600 to 700° C. normal conditions. In the event of a significant increase in power, the temperature at the core of the pellet rises sharply, typically to a temperature above 1,500°C, or even above 2,000°C. The contact between the pellet and the cladding increases under the effect of thermal expansion, swelling of the pellet and reduction in the diameter of the cladding due to its creep under the effect of the coolant pressure. At these temperatures, volatile fission products that are aggressive for the sheath, such as iodine, can also be released from the hotter center of the pellet. These phenomena can induce stress corrosion of the cladding, which can go as far as cladding rupture. They are commonly referred to by the term "Pellet-Sheath-Stress Corrosion Interaction", abbreviated as IPG-SCC the sheath occurs due to the absence of cooling and to the residual powers of the rod. Simulation calculations have shown that the temporal thermal consequences of this accident on the fuel rod are all the less significant as the temperatures of the fuel pellets are low under normal conditions before the accident.

To overcome these drawbacks, existing solutions consist in modifying the combustible material by modifying the chemical composition of the uranium compound to obtain a higher conductivity. These solutions have the disadvantage of inducing a different behavior in the reactor compared to the combustible materials commonly used. These solutions still require significant research and development to qualify these low-level fuels.

Pellets comprising microstructures, comprising clusters of uranium oxide surrounded by thin metal walls, are described in the literature. The thermal conductivity of the resulting fuel pellets is increased. However, in In practice, these microstructures are irregular and difficult to reproduce. The microstructures indeed include discontinuities in the metal walls, clusters of metal and poorly controlled porosity, degrading the performance of the fuel pellets.

It is also known from the document Medvedev, et al. Conductive inserts to reduce nuclear fuel temperature, Journal of Nuclear Materials, Volume 531, 2020, a fuel pellet design comprising a molybdenum insert with seven discs extending across the cross section of the pellet and having a thickness of 0.05 mm. The metal insert represents 5% of the volume of the fuel pellet. This solution allows an increase in the thermal conductivity of the pellet but it remains in practice limited in terms of performance of the pellet. Indeed, it remains difficult to manufacture and has a low resistance under irradiation. Therefore, this pellet has a low resistance to IPG-CSC and to accidents, for example a loss of primary coolant.

An object of the present invention is therefore to provide a solution improving the thermal conductivity of a fuel pellet comprising a metal insert.

The other objects, features and advantages of the present invention will become apparent from a review of the following description and the accompanying drawings. It is understood that other benefits may be incorporated.

SUMMARY OF THE INVENTION

To achieve this objective, according to a first aspect, a fuel pellet is provided having a central axis of main extension comprising:

- a fissile nuclear fuel material, and

- at least one metal insert disposed in the combustible material and comprising a plurality of plates, each plate extending in a cross section of the pellet substantially perpendicular to its central axis.

Advantageously, the insert is configured to represent a proportion substantially comprised between 5% and 15% of the total volume of the fuel pellet.

According to one example, each plate has a solid volume greater than 50% of the total volume of the plate.

According to one example, each plate has a thickness substantially between 0.2 and 1.5 mm.

At least one plate, and preferably each plate, preferably includes a central opening disposed in the center of the plate.

The metal insert is thus configured to recover the thermal energy emitted by the combustible material, allowing axial conduction between the plates and then radial conduction of this energy along the plates. The insert forms axial heat conduction paths along directions substantially parallel to the central axis of the pellet. The insert further forms continuous radial heat energy conduction paths for better heat conduction from the center to the outside of the fuel pellet. This propagation solution is therefore clearly different from the solutions implementing structures of honeycomb inserts or grids.

In addition, this range of thickness facilitates the manufacture and strength of the metal insert. In synergy with a higher volume of the insert compared to existing solutions, the pellet presents a better distribution of the insert in the pellet with plates presenting a sufficient thickness to improve the thermal conductivity of the pellet in a reliable way. The insert thus has better resistance to deformation and expansion occurring during irradiation of the pellet. The thermal conductivity of the pellet is thus improved, including when the pellet is subjected to IPG-CSC or loss of primary cooling accidents. Therefore, this pellet has improved resistance to IPG-CSC and in accident conditions compared to existing solutions, and in particular those recommending the use of thinner plates with a lower insert volume. The plate preferably comprising a central opening, the solid volume occupied by the metal insert in the fuel pellet is reduced, while allowing good radial conduction of energy in the plate.

During the development of the invention, it was also demonstrated that plates comprising a central opening allow an increase in the melting margin of the fuel pellets. Indeed, a combustible pellet has a radial temperature gradient and the melting temperatures of metals are generally lower than that of the combustible material. In the event of a significant increase in power, or in the event of an accident, the temperature at the center of the fuel pellet increases sharply, for example to a temperature above 1,500°C or even 2,000°C. Therefore, the temperature that can be reached inside the fuel pellet can induce melting of the metal insert and degradation of its structure.

The central opening makes it possible to move the metal away from the center of the fuel pellet and thus offer good resistance of the insert to high thermal elevations while surprisingly allowing sufficient thermal conduction of the fuel pellet. A second aspect of the invention relates to a fuel rod comprising at least one fuel pellet, and preferably a plurality of fuel pellets, according to the first aspect, surrounded by a sheath.

A third aspect of the invention relates to an assembly for a nuclear reactor comprising a bundle of fuel rods according to the second aspect.

A fourth aspect of the invention relates to a nuclear reactor comprising an assembly according to the third aspect.

BRIEF DESCRIPTION OF FIGURES

The aims, objects, as well as the characteristics and advantages of the invention will emerge better from the detailed description of an embodiment of the latter which is illustrated by the following accompanying drawings in which:

FIG. 1A represents a view in longitudinal section of a vessel of a nuclear reactor according to an exemplary embodiment of the invention.

FIG. 1B represents a view in longitudinal section of a fuel rod according to an exemplary embodiment of the invention.

FIG. 2 represents a perspective view of the fuel pellet according to an embodiment of the invention in which the insert comprises 4 plates comprising a central opening.

FIGS. 3A to 3C show a top view of a plate of the insert according to different embodiments.

FIG. 4A represents a perspective view of the fuel pellet according to an exemplary embodiment of the invention in which the insert comprises a hollow central body.

FIG. 4B represents a perspective view of the fuel pellet according to an exemplary embodiment of the invention in which the insert comprises an external body.

FIG. 4C represents a perspective view of the fuel pellet according to an exemplary embodiment of the invention in which the insert comprises a central body and an outer body. FIG. 5 represents a top view of an insert comprising four plates according to the example illustrated in FIG. 3B, according to an exemplary embodiment.

The drawings are given by way of examples and do not limit the invention. They constitute schematic representations of principle intended to facilitate understanding of the invention and are not necessarily at the scale of practical applications. In particular the relative dimensions between the length and the width of the fuel pellet, of the plates, as well as between the central body and the outer circumference of the fuel pellet, are not necessarily representative of reality.

DETAILED DESCRIPTION OF THE INVENTION

Before starting a detailed review of embodiments of the invention, optional characteristics are set out below which may optionally be used in combination or alternatively.

According to one example, the combustible material is a fissile nuclear ceramic. According to one example, the combustible material is chosen from uranium oxide, mixed uranium and plutonium oxide, mixed thorium oxide. According to one example, the plate, and preferably each plate, is in the form of a disk coaxial with the pellet.

According to one example, the plate is configured so as to form a plurality of thermal continuity paths extending radially from the central opening, towards the outer periphery of the pad. According to one example, the plate does not form a grid or a trellis, i.e. a tangle of wires or cables.

According to one example, the central axis of main extension of the fuel pellet forms an axis of revolution of the pellet.

According to one example, the external dimension of the cross-section of the fuel pellet is its external diameter.

According to one example, the insert is configured to represent only a proportion substantially less than or equal to 15%, preferably substantially less than or equal to 10%, of the total volume of the fuel pellet.

According to one example, the insert is configured to represent only a proportion substantially greater than or equal to 5%, preferably substantially greater than or equal to 7%, substantially greater than or equal to 8%, of the total volume of the fuel pellet.

According to one example, the total area of the opening(s), that is to say the area of all the openings, is less than 50%, preferably less than 30%, preferably substantially equal to 20 %, of the surface of the plate.

According to one example, the total volume of the opening(s), that is to say the volume of all the openings, is less than 50%, preferably less than 30%, preferably substantially equal to 20% , the volume of a plate.

According to one example, at least one opening, and preferably each opening, of a plate, and preferably of each plate, is of oval, oblong, ovoid, prismatic or preferably circular shape. Any other shape can be considered.

According to one example, the plate is configured so as to form a plurality of paths of thermal continuity extending radially from the center of the plate, if necessary from the central opening, towards the outer periphery of the pellet. According to one example, and preferably for each plate, the ratio between an external dimension of the cross-section of the fuel pellet, for example its diameter, and a largest dimension of the central opening, taken in the extension plane main part of the plate, for example its diameter, is between 2 and 4. During the development of the invention, it was demonstrated that these relative dimensions between the outer periphery of the pellet and the central opening make it possible to synergistically improving the extraction of thermal energy from the hottest zone of the pellet and the increase in the melting margin, by a suitable distance from the periphery of the central opening to the center of the fuel pellet. According to one example, at least one plate, and preferably each plate, comprises a plurality of openings.

According to one example, at least one plate, and preferably each plate comprises at least one peripheral opening, and preferably several peripheral openings, not arranged at the center of the plate. According to one example, the plurality of peripheral openings has a largest dimension, for example their diameter, substantially between 0.5 mm and 1.5 mm, preferably between 0.8 mm and 1.25 mm.

According to one example, the plurality of peripheral openings are distinct. According to one example, the plurality of peripheral openings and the central opening are distinct. Equivalently said openings do not touch each other.

According to one example, for at least two consecutive plates comprising at least one peripheral opening, the peripheral openings of the at least two consecutive plates are not completely and preferably not completely opposite each other. According to one example, at least one plate, and preferably each plate comprises between four and ten openings, preferably between five and ten openings.

According to one example, the plurality of apertures have rotational symmetry about the central axis.

According to one example, the combustible material does not fill the central opening and/or the peripheral openings. The central opening and/or the peripheral openings may be free of combustible material. According to one example, the combustible material fills the central opening and/or the peripheral openings, and preferably each opening. Thus, better thermal continuity between the combustible material and the insert is obtained as well as a lower cost of enriching the pellet with fissile material. According to one example, the central opening and/or the peripheral openings is (are) filled only with the combustible material. According to one example, the central opening and/or the peripheral openings is (are) completely filled with the combustible material.

According to one example, the insert does not include an outer body. Equivalently, the outer periphery of the pellet is formed by the combustible material.

According to one example, at least one plate, and preferably each plate, has a largest external dimension, taken in the main extension plane of the plate, of less than 95%, preferably 90%, preferably 80 % of a cross-sectional external dimension, eg diameter, of the fuel pellet. Thus, direct contact of the sheath and the plates is avoided, which minimizes and preferably avoids a harmful thermal gradient on the sheath.

According to one example, the metal insert comprises a number of plates comprised between 2 and 6. This number of plates makes it possible to improve the thermal conductivity and to reduce the maximum temperatures of the pellet under normal and accident conditions, in particular by axial evacuation thermal energy from the pellet, while limiting the amount of metal in the pellet. According to one example, the plates are regularly distributed along the central axis of the pellet.

According to one example, the metal insert comprises a hollow central body on which the plurality of plates is arranged. According to one example, the hollow central body is filled with the combustible material.

One advantage is to avoid enriching the rest of the fuel with U235 to deliver the same power. Thus the hollow central body forms a physical barrier around the core of the pellet, able to stop the fission products. The central body therefore allows a compartmentalization of the core of the fuel pellet and therefore a confinement of the fission products which are generated there. The risk of sheath corrosion by IPG-CSC is minimized. This also allows a delay in clad rupture in the event of an accident thanks to a drop in the pressure of the fission products in the rod. The risk of sheath rupture by fission products is therefore reduced.

According to one example, the ratio between an external dimension of the cross section of the fuel pellet and an external dimension of the cross section of the hollow central body, for example its diameter, is between 2 and 4. During the development of the invention, it was demonstrated that these relative dimensions between the outer periphery of the pellet and the hollow central body make it possible to synergistically improve the extraction of thermal energy from the zone most hot of the pellet and the increase in the melting margin, by a suitable distance from the central hollow body to the center of the fuel pellet.

According to one example, the hollow central body has a closed radial periphery delimiting an interior volume. According to one example, the plates do not extend into the interior volume of the central body. According to one example, the interior of the central body is filled only with combustible material. According to one example, the interior of the central body is completely filled with combustible material.

According to one example, the outer body has a thickness substantially greater than 5 μm. According to one example, the outer body has a thickness substantially less than 25 μm. According to one example, the outer body has a thickness substantially between 5 μm and 25 μm. According to one example, the central opening of at least one plate, and preferably of each plate, coincides with the central body.

According to one example, the cross section of the hollow central body is substantially circular.

According to one example, an external dimension of the cross section of the hollow central body is its external diameter.

According to one example, the insert is one-piece. According to one example, the hollow central body and/or the outer body connects each plate of the fuel pellet. Thus the plates are thermally connected to each other. According to an alternative example, the central body and/or the plates and/or the outer body are distinct. According to one example, the fuel pellet having a length in a direction parallel to, and preferably coinciding with, a central axis of main extension of the fuel pellet, the hollow central body extends over substantially the entire length of the fuel pellet. The insert thus allows thermal continuity of the central body over the entire length of the pad. According to one example, the metal insert comprises an outer body configured to form the outer rim of the fuel pellet. Thus, the outer body forms a physical barrier around the pellet, capable of stopping volatile fission products. The volatile fission products which are aggressive for the cladding propagate in particular at the level of radial cracks in the combustible material, going from the center of the pellet towards its outer periphery. A radial crack opening onto the sheath results in a concentration of volatile fission products favoring corrosion of the sheath. sheath. The outer body avoids this concentration of volatile fission products by confining them relative to the sheath, while improving the heat exchange between the combustible material and the sheath. The outer body is separate from the fuel rod sheath. According to one example, the metal insert comprising an outer body configured to form the outer periphery of the fuel pellet, at least one plate, and preferably each plate, extends as far as the outer body of the fuel pellet. Thus, the insert forms a path of thermal continuity to the outer body which distributes the heat evenly over the sheath. According to one example, the hollow central body and the outer body are connected by at least one plate, preferably by the plurality of plates.

The metal insert is based on or made of a metal. According to one example, the metal insert is based on or made of a metal having a thermal conductivity greater than the thermal conductivity of the combustible material. According to one example, the metal insert is based on or made of a refractory metal.

Preferably, the metal chosen from zircaloy, chromium, molybdenum and their alloys.

According to one example, the metal insert comprises platinum. According to one example, the metal insert is based on, or even consists of, platinum. Preferably, the insert comprises a surface layer based on platinum, deposited continuously or discontinuously on the surface of the insert. Preferably, the insert is based on an alloy comprising platinum.

In the present patent application, when it is indicated that two parts are distinct, it means that these parts are separate. They are: - positioned at a distance from each other, and/or

- movable relative to each other and/or

- secured to each other by being fixed by added elements, this fixing being removable or not.

A unitary one-piece part cannot therefore be made up of two separate parts.

In the following detailed description, use may be made of terms such as "longitudinal", "transverse", "upper", "lower", "top", "bottom", "front", "rear", "interior". », « outside ». These terms should be interpreted relatively in relation to the normal position of the fuel pellet in the rod. For example, the "longitudinal" direction corresponds to the main direction of extension of the fuel pellet, parallel to the central axis of extension main part of the fuel pellet.

We will also use a marker whose longitudinal direction corresponds to the z axis, the transverse direction corresponds to the y axis and the front/rear direction corresponds to the x axis. The cross section is thus the section taken perpendicular to the central axis of main extension of the fuel pellet, which is parallel to the z axis.

"Internal" designates the elements or faces facing the interior of the fuel pellet, for example facing its central axis, and "external" designates the elements or faces facing the exterior of the fuel pellet, for example in being opposite to its central axis.

A parameter “substantially equal to/greater than/less than” a given value means that this parameter is equal to/greater than/less than the given value, to within plus or minus 10%, or even within plus or minus 5%, of this value.

By a material “based” on an element A, we mean that this material comprises the element or the species A, possibly supplemented by other elements or species.

In the context of the present invention, the thicknesses of an element or of a part are taken along a direction normal to the tangent to the surface of the element or of the part.

By a cylinder is meant a solid delimited by a surface of substantially parallel generating lines and two substantially parallel planes, preferably perpendicular to the generating lines. The cross section of a cylinder is preferably circular but not necessarily: it can be oval, oblong, ovoid or prismatic. Note that the term "cylinder" includes a substantially cylindrical shape, that is to say an imperfect cylindrical shape, for example in connection with manufacturing tolerances.

By solid volume, which can also be designated occupied volume, is meant the volume occupied by the material or materials constituting the plate, as opposed to openings forming a free volume, in the total apparent volume of the plate. The total apparent volume of the plate corresponds to the volume defined by the external envelope of the plate with these openings, or equivalently of a solid plate without opening. The apparent total volume is more particularly formed by the surfaces of the plate, extending substantially perpendicularly to the central axis A, and its radial periphery without the openings. The sum of the solid volume and the free volume gives 100% of the total volume of the plate. The various aspects of the invention are now described according to several exemplary embodiments. Generally, a nuclear reactor 4 comprises a vessel 40 in which a heart 41 is arranged, as illustrated for example in FIG. 1A. The nuclear fuel of the fuel pellets 1 is included in a fuel rod 2. A bundle of fuel rods 2 forms an assembly 3. These rods 2 are typically linked by a rigid structure, for example consisting of guide tubes and grids. Assemblies 3 are arranged side by side in vessel 40 of reactor 4. By way of example, there are 157 assemblies 3 in the core for a reactor with a power of 900 MWe in France. There are 3 different types of assemblies in the core depending on the main function they are intended to perform. The fuel pellet 1 which will be described in detail below is preferably carried by a fuel rod 2 in a fuel assembly 3.

A heat transfer fluid circulates in the tank 40 to extract the thermal energy produced by the fuel rods, as for example illustrated by the arrows in the tank 40 in FIG. 1A. The heat transfer fluid is a liquid. According to one example, reactor 4 is a light water nuclear reactor (abbreviated REL or LWR for English light water reactor). An REL is a nuclear reactor whose fuel coolant is water, also called light water in distinction to heavy water. The most common RELs are pressurized water reactors (abbreviated REP, or PWR for English pressurized water reactor) and boiling water reactors (abbreviated REB, or BWR for English boiling water reactor). In the following, reference is made to the non-limiting example in which the reactor is an REL. In a PWR, the water is typically at a pressure of approximately 150 bar and at a temperature of approximately 320°C. By way of example, we can cite the 900 MWe or 1300 MWe PWRs of the French sector. In a BWR, the water is typically at a pressure of approximately 70 bar and at a temperature of approximately 215°C.

A fuel rod 2 generally comprises a plurality of fuel pellets 1 stacked in a longitudinal direction, and surrounded by a sheath 20, commonly made of Zircaloy, as illustrated for example in FIG. 1B. The sheath 20 is closed and sealed and constitutes the first confinement barrier. It can have a diameter of between 8 and 13 mm. The fuel pellets are typically in the form of cylinders with a diameter substantially between 7 and 9 mm, for example substantially equal to 8.2 mm when cold, that is to say without irradiation of the pellet, and of length L1 substantially between 10 and 15 mm, for example substantially equal to 13.6 mm, as for example illustrated by FIG. 2. The hot diameter, that is to say under irradiation, of a fuel pellet can be substantially equal to the internal diameter of the sheath 20, for example substantially 8.36 mm. In Indeed, the joint between the fuel pellets 1 and the sheath 20, entirely gaseous in helium at the start of irradiation, becomes completely filled at the start of the second irradiation cycle.

A fuel pellet 1 comprises a nuclear fuel material 10, fissile under irradiation in a nuclear reactor. This material 10 is shown in dotted lines in the figures representing an overall view of the pellet 1. The combustible material 10 can be a nuclear fuel ceramic. The combustible material 10 can for example be based on a uranium oxide, for example uranium dioxide of formula U02. Uranium dioxide can be enriched with uranium U235, for example to about 5% the balance being fertile U238. The combustible material 10 can for example be based on a mixed oxide (abbreviated MOX, English Mixed OXide) of uranium and plutonium (U, Pu) 02, for example with a Pu content of between 5 and 10% by mass relative to the total mass of U+Pu and depleted uranium, and/or a mixed thorium oxide of formula (U, Th)02. These ceramic combustible materials are generally poor thermal conductors. During operation of the reactor, this induces strong radial thermal temperature gradients in the fuel pellet, and therefore strong thermomechanical stresses in the fuel pellet. In nominal operation, the temperature of a fuel pellet is around 1,000°C in the center and 400 to 500°C at its outer edge.

In the event of a significant increase in power, the temperature at the core of the pellet rises sharply, for example up to 1,500°C, or even beyond 2,000°C in some cases.

With the thermal expansions and the swelling of the pellet 1, there is a reduction in the diameter of the sheath 20 due to its creep under the effect of the pressure of the coolant. The constraints of pad 1 on sheath 20 increase.

At these temperatures, fission products (abbreviated PF below) can be released. Among these fission products, a distinction is made between gaseous fission products and volatile fission products. The gaseous fission products are typically rare gases, and in particular xenon and/or krypton. These gases create thermomechanical stresses in the pellet 1 and exert a pressure on the sheath 20, which can lead to its rupture under accidental conditions. Volatile fission products, such as tellurium or iodine, are corrosive to the sheath. These fission products are notably released during strong power transients. These volatile FPs propagate in particular at the level of radial cracks in the combustible material, going from the center of the pellet towards its outer periphery. These cracks result in particular from deformations of the combustible material under the effect of thermomechanical stresses. A radial crack induces a concentration of PFs near the cladding, which can induce, with the simultaneous stresses on the cladding, corrosion of the cladding, or even its rupture. In addition, in the event of an accident, it is important to be able to reduce the temperatures of the fuel pellets in order to reduce the temporal thermal consequences and improve the safety of the rods.

To remedy this, the fuel pellet 1 further comprises a metal insert 11. The insert 11 is a metallic structure or in an equivalent way a metallic structural element arranged in the combustible material 10. The insert 11 is configured to increase the thermal conductivity of the pellet 1 by forming a highly conductive structure of the heat evacuating radially and axially the thermal energy produced from the center towards the periphery of the pellets.

For this, the metal insert 11 comprises a plurality of plates 111 arranged substantially perpendicular to its central axis A of main extension. According to the example illustrated in FIG. 2, the plates 111 extend in a plane parallel to the plane (x, y), perpendicular to the z axis. In the following, it is considered, without limitation, that the plates 111 are circular and that the fuel pellet 1 is cylindrical and are coaxial. The number of plates 111 can for example be between 2 and 6.

The thickness of the different plates 111a, 111b, 111c, 111d can be different or substantially equal. The thickness in a direction parallel to the z axis of each plate 111a, 111b, 111c is substantially between 0.2 mm and 1.5 mm, preferably substantially between 0.23 mm and 1.37 mm. Thus, the plates allow radial and axial conduction of thermal energy while having a thickness allowing better resistance to thermomechanical stresses, and in particular to IPG-CSC.

Furthermore, the insert 11 is preferably configured to represent only a proportion substantially less than or equal to 15%, preferably substantially less than or equal to 10%, of the volume of the fuel pellet 1. The insert is preferably configured to represent a proportion substantially greater than or equal to 5% of the total volume of the fuel pellet. For this, the thickness of the plates 111 can in particular be adapted according to their solid volume and the number of plates 111 in the insert 11, as will be described later through several embodiment examples. According to one example, the thickness of the plates is all the lower as the number of plates is high, for example for a volume total metal of 10% relative to the pellet (see Table 3 given later).

This volume range makes it possible to limit the volume occupied by the insert 11 in place of the combustible material 10 in the pellet. Also, some metals show slight neutron absorption. In order to avoid an additional enrichment in U235 of the pellet to reach the same power generated as in a fuel pellet 1 without insert 11, it is preferable to limit the quantity of metal present in the fuel pellet 1. These proportions therefore minimize the use to an increase in the enrichment in uranium 235 of the uranium of the combustible material 10. Preferably, as for example illustrated by FIG. 2, the plates 111 are regularly distributed along the axis A to better distribute the insert 11 in the height Li of the fuel pellet 1 taken in a direction parallel to the direction z. The thermal conduction paths are thus distributed axially along the patch 1. Preferably, the plates 111a, 111b, 111c, 111 d are consecutively arranged at equidistance from each other along the patch 1 in a direction parallel to the z axis. Preferably, each plate is separated from the adjacent plate or plates by a portion of combustible material 10 of identical height, in a direction parallel to the z axis. According to one example, the height of the portions of combustible material 10 can be substantially between 1.7 and 4.2 mm. Preferably, each plate 111 is circular with a diameter substantially equal to

90% of the diameter of pellet 1 and has a thickness between 0.69 mm to 0.23 mm varying with the number of plates ranging from 2 to 6.

As for example illustrated by FIG. 2, the plates 111 can extend as far as the outer periphery 1a of the fuel pellet 1. According to another example, in particular when the insert 11 does not comprise an outer body 112 described later, in at least one plate 111, and preferably each plate 111, extends transversely as far as a region located at the edge of the outer circumference 1a of the fuel pellet 1. The region located at the edge, with respect to the outer circumference 1a of the fuel pellet 1, is located at a distance substantially less than 20%, preferably substantially less than 10%, and substantially less than

5%, of the outer circumference 1a of the fuel pellet 1, in a direction normal to the tangent to the circumference 1a. Thus, the insert 11 forms radial paths of thermal continuity of sufficient length for the extraction of the thermal energy, while avoiding generating a radial thermal gradient at the level of the sheath. According to an example, in a direction normal to the tangent to the circumference 1a, the largest external dimension of a plate 111, for example its diameter Dm represented in FIGS. 3A to 3C, is less than 95%, preferably 90%, preferably 80% of an external dimension of the cross section, for example the diameter D1, of the fuel pellet 1.

In the following, the characteristics are described relative to at least one plate 111 and preferably apply to each plate 111.

As for example illustrated in FIGS. 2 and 3A to 3C, the plate 111 comprises at least one central opening 1110. The central opening 1110 is more particularly arranged at the center of the plate 111, for example substantially centered on the central axis A of the pellet 1. The relative dimensions between the central opening 1110 and the outer circumference

1a of pellet 1 are preferably optimized to maximize the thermal conductivity of pellet 1 and increase its melting margin. Thus, the extraction of thermal energy can be improved for higher operating temperatures than in existing solutions. The core of the fuel pellet 1 can therefore rise to temperatures above the melting temperature of the metal of the insert without risking deterioration of the insert 11.

For this, the patch 1 has a dimension Di taken between two opposite points of its cross-section relative to its central axis A, and the central opening 110 can have a dimension Dmo taken between two opposite points of its cross-section relative to the central axis A. The dimension Di can more particularly be the diameter of the pellet 1. The dimension Dmo can more particularly be the diameter of the central opening 1110. The ratio between Di and D o can be substantially between 2 and 4, preferably between 2.5 and 3.5, more preferably still substantially equal to 3.2. Note that when the insert 11 comprises an outer body 112 forming the outer circumference 1a of the patch, as described in more detail later, the diameter Di of the patch 1 includes the outer body 122.

The occupied volume of the plate 111 is substantially greater than or equal to 50%, preferably substantially greater than or equal to 70%, and more preferably still substantially equal to 80% of the total volume of the plate 111. Equivalently, the plate 111 is delimited between two parallel planes, the opening surface, taken in projection on one of the two parallel planes, is substantially less than or equal to 50%, preferably substantially less than or equal to 30%, and more preferably still substantially equal to 20 % of the surface of the plate 111, taken in projection on one of the two parallel planes.

Various embodiments of the plates 111 are now described in reference to Figures 3A to 3C.

In the following, it is considered, without limitation, that the openings 1110, 1111 are preferably circular. Note that any other shape can be considered. The opening 1110, 1111 is more particularly through on either side of the plate 111, in a direction substantially parallel to the direction z.

As for example illustrated in FIG. 3B, the plate 111 can only comprise a central opening 1110.

In addition, the plate 111 can comprise at least one and preferably several peripheral openings 1111. The peripheral openings 1111 are preferably eccentric with respect to the center of the plate 111. According to an example, as illustrated by FIGS. peripheral openings 1111 are distributed in the plate 111 around the central opening 1110.

According to one example, at least one plate, and preferably each plate comprises between four and ten openings, preferably between five and ten openings as illustrated by way of example by FIGS. 3B and 3C respectively.

According to one example, the plurality of openings has a rotational symmetry around the central axis A. The radial profile of conduction of the thermal energy in the plate 111 is thus rendered more homogeneous. According to one example, the peripheral openings 1111 are arranged around the central axis A while being separated by the same angular interval, for example the angular interval is substantially equal to 360° divided by the number of peripheral openings 1111.

According to one example, the peripheral openings have a largest dimension, for example their diameter Dnn, substantially between 0.5 mm and 1.5 mm, and preferably between 0.8 mm and 1.25 mm, and more preferably between 0.82 and 1.23 mm. This dimension can in particular be adapted according to the number of peripheral openings 1111, as will be described later for example embodiments.

Preferably, the combustible material fills each opening. The presence of at least one opening filled with the combustible material 10 makes it possible to minimize the volume of the insert 11 with respect to the volume of combustible material 10, and thus reduce the enrichment in U235 to be made to achieve the same dissipated power as a combustible pellet without an insert. In addition, the opening or openings 1110, 1111 make it possible to increase the contact surface between the combustible material 10 and the plate 111, to promote the extraction of thermal energy from the combustible material 10. Thus, and synergistically with the aperture number 1110, 1111 of the plate

111, better thermal continuity between the combustible material and the insert is obtained.

In addition, the insert 11 may comprise a hollow central body 110, as illustrated for example by FIG. 4A. According to this example, the central body 110 extends along a principal direction substantially parallel, and preferably coincident, with the central axis A of principal extension of the fuel pellet 1, this axis being substantially parallel to the direction z. According to a preferred example, the central body 110 and the pellet form a coaxial structure. In the following, it is considered, without limitation, that the central body 110 and the patch are cylindrical and are coaxial.

The plates 111 can be separate from the central body 110. According to another example, the plates 111 are arranged on the central body 110. For example, the central opening 1110 of the plates 111 coincides with the central body 110. According to one example, the plates 111 and the central body 110 form a one-piece assembly.

According to one example, the central body 110 has an interior volume not filled with the combustible material 10. The interior volume can be completely free of combustible material. However, this requires expensive U235 over-enrichment to compensate for the less volume of combustible material in the core of the pellet and deliver the same power.

According to an alternative and preferred example, the central body 110 is filled, and preferably filled only, with the combustible material 10. This insert 11 is thus configured to confine the FPs at least in the center of the combustible pellet 1 in order to limit their propagation. towards the outer periphery of the pellet 1 and therefore towards the sheath of the rod and to confine the FPs generated in situ to the core of the pellet 1. Preferably, the central body 110 is completely filled with the combustible material 10. Thus, the contact heat between the combustible material 10 and the insert 11 is favored. As with the central openings 1110, the relative dimensions between the central body 110 and the outer circumference 1a of the pellet 1 can be adapted to maximize the thermal conductivity of the pellet and increase its melting margin.

For this, the patch 1 has a dimension Di taken between two opposite points of its cross-section relative to its central axis A, and the central body 110 has a dimension Duo taken between two opposite points of its cross-section relative to the central axis A. The dimension Di may more particularly be the diameter of the pellet 1. The dimension Duo may more particularly be the external diameter of the hollow central body 110 . The ratio between Di and Duo can be substantially between 2 and 4, preferably between 2.5 and 3.5, more preferably still substantially equal to 3.2. Note that when the insert 11 comprises an outer body 112 forming the outer circumference 1a of the pellet, as described in more detail later, the diameter Di of the pellet 1 includes the outer body 122.

As for example illustrated by FIG. 4A, the central body can extend over a distance substantially greater than or equal to 80%, preferably substantially greater than or equal to 90%, preferably substantially greater than or equal to 95% of the length Li of the pellet 1. Preferably, the central body 110 extends over substantially the entire length Li of the fuel pellet 1. The central body 110 thus forms a thermal continuity path over the entire length of the pellet 1. The central body 110 further forms an element conferring good mechanical strength on the insert 11, in particular when the plates 111 are mounted on the central body 110. The central body 110 may have at least one end 110a flush with the surface of the combustible material 10. Preferably , each end 110a is flush with the surface of the combustible material 10. According to an alternative example not shown, the central body 110 may only extend over only a fraction of the length Li , either intermittently or continuously.

According to one example, the central body 110 is formed by a wall with a thickness of between approximately 250 μm and 450 μm. This range of thicknesses allows good manufacturability of the insert and sufficient rigidity of the central body 110. The amount of metal in the pellet is also limited.

As for example illustrated by FIG. 2, the outer circumference 1a of the pellet 1 can be formed by the combustible material 10, the combustible material 10 being shown in dotted lines.

As for example illustrated by FIG. 4B, the metal insert 11 can comprise an outer body 112 configured to form the outer circumference 1a of the fuel pellet 1, at least for its radial circumference. Thus, the outer body 112 allows a compartmentalization of the combustible material 10 to confine the FPs. When the insert further comprises a central body 110, for example as illustrated in FIG. 4C, the outer body 112 improves the compartmentalization of the combustible material 10 to confine the FPs. Preferably, the outer body 112 and the central body 110 form a coaxial structure. The outer body may more particularly be cylindrical. As for the central body 110, the outer body 112 can extend over a distance substantially greater than or equal to 80%, preferably substantially greater than or equal to 90%, preferably substantially greater than or equal to 95% of the length Li of the pellet 1. Preferably, the outer body 112 extends over substantially the entire length Li of the fuel pellet 1. The body external body 112 thus forms a path in thermal continuity over the entire length of the pad 1. The external body 112 can also form an element conferring good mechanical strength on the insert 11. The external body 112 can only extend over only a fraction of the length Li, either fractionally or continuously.

The outer body can be configured to form a surface substantially greater than or equal to 80%, substantially greater than or equal to 90%, preferably substantially greater than or equal to 95% of the outer surface of the fuel pellet 1. The outer body 112 can comprise at least one, and preferably two surfaces 112a configured to cover the longitudinal ends of the pad 1 as for example illustrated in FIG. 4C by the two surfaces 112a each disposed in a plane parallel to the plane (x,y).

In order to minimize the quantity of metal in the fuel pellet 1, the outer body 112 potentially extending over a large surface of the pellet, its thickness can be limited. For example, the thickness of the outer body 112 can be substantially between 5 μm and 25 μm. This range of thickness makes it possible in particular to form a barrier to PFs by minimizing the quantity of metal in pellet 1.

The plates 111 can be separate from the outer body 112. According to another example, the plates 111 and the central body 112, possibly with the central body 110, form a one-piece assembly.

According to one example, the central body 110 and the outer body 112 are connected by at least one plate 111, preferably by each plate 111. The heat exchange with the sheath is further improved by a continuous thermal path from the central body 110 to 'to the outer body 112. In addition, the robustness of the insert 11 is improved.

According to one example, between different plates 111, the plates 111 are aligned with each other so that, in a projection along a plane perpendicular to the central axis A, the openings 1110, 1111, and in particular the peripheral openings 1111, are coincident.

According to the example illustrated in FIG. 5, between different plates 111, the openings, and in particular the peripheral openings 1111, of consecutive plates may not be entirely opposite. Preferably these openings are completely non-facing. Equivalently, the openings of at least two consecutive plates may not be located opposite each other in a direction parallel to the central axis A. According to the example illustrated, the peripheral openings 1111 between different plates can be consecutively separated by the same angular interval, by example substantially equal to 360° divided by the sum of the number of openings 1111 of the plates 111. Thus, the thermal conduction paths are distributed in three dimensions in the volume of the patch 1.

The insert 11 is based or made of a metal, preferably based or made of a refractory metal. For example, the metal is chosen from zircaloy, chromium or molybdenum. Some properties of these metals are shown in Table 1. These metals are refractory metals with relatively low thermal neutron capture cross sections and have a thermal conductivity greater than GIIO2, substantially between 2 and 4 W.nr ¹ .K ¹ . Molybdenum and chromium are preferred for their higher melting temperatures. Zircaloy has the advantage of having good transparency to neutrons. On the other hand, its melting temperature and thermal conductivity are lower than the other two metals.

Table 1:

The combustible material 10 can be doped with at least one additive based on an alkaline earth metal, for example barium Ba, and/or based on an element of the platinum group, also referred to as a platinoid. The alkaline-earth metal is capable of trapping FPs. This trap can be an alkaline earth metal (for example barium and/or strontium and/or calcium) or an alkaline earth metal oxide (for example BaO, and/or SrO or CaO, or BaÜ2, or SrÜ2 or CaÜ2), as well as mixtures and combinations of the aforementioned species. The combustible material 10 may comprise, for example at the surface or near its surface, at least one anticorrosion material of the sheath comprising at least one alkaline-earth metal. The anti-corrosion material can form an internal coating of the sheath. According to one example, the anticorrosion material has a thickness substantially between 50 μm and 70 μm, thus making it possible to prevent corrosion up to a combustion rate of 10 at%.

The combustible material 10 and/or the insert 11 may comprise, for example on the surface or close to its surface, at least one anticorrosion element of the sheath belonging to the platinum group, which may be palladium or ruthenium or rhodium or osmium or iridium or platinum, so as to form tellurium and/or iodide compounds with this element of the platinum group and the generated FPs. This doping allows both an increase in the thermal conductivity of the pellet 1 and allows better retention of the corrosive FPs. The anticorrosion element can form an internal coating of the sheath. According to one example, the quantity of element of the platinum group is substantially greater than or equal to the total quantity of tellurium and/or iodine, produced by the fission at the target maximum combustion rate of the combustible material 10. The numbers of moles of tellurium and/or iodine, of iodine created in the fuel are close to 1.15276.10 ^e moles of tellurium/at%/g of oxide fuel and 6.69344.10 ⁷ moles of iodine/at%/g of oxide fuel. Thus, the calculated quantity of palladium to form a compound of the PdTe type for a combustion rate of 20 at%, is 1 g of palladium which represents approximately 2 μm of palladium distributed over the entire external surface of the pellets or the entire surface inside the sheath at the level of the fissile column. According to an alternative or complementary example, the insert 11 comprises an element of the platinum group, which can be palladium or ruthenium or rhodium or osmium or iridium or preferably platinum. According to one example, the metal insert is based on, or even consists of, platinum. Preferably, the insert 11 comprises a surface layer based on platinum, deposited continuously or discontinuously on the surface of the insert. The surface layer may have a thickness substantially less than or equal to 5 μm, preferably less than or equal to 2 μm. Preferably, the insert 11 is based on an alloy comprising platinum. Thus, the insert 11 is capable of forming tellurium and/or iodide compounds and thus improving the retention of the FPs. The present description also provides for combining the characteristics detailed in the previous examples. For example, provision can be made for a fuel pellet 1 to comprise a plurality of plates 111, each plate being mounted on a separate central body 110, the central bodies 110 not being continuous along the pellet 1. The fuel pellet 1 can be made by additive manufacturing. The insert 11 and the combustible material 10 can be formed layer by layer to obtain the pellet 1. According to another example, the insert can be formed by extrusion or by molding. Combustible material 10 can be added to insert 11, for example by adding combustible material 10 in powder form and then by sintering. The outer body 112 can be made by additive manufacturing. According to another example, the outer body 112 is formed by physical vapor deposition to limit its thickness.

Specific examples of realization

Simulations were carried out to study the different embodiments described. The pellets are created using CAD software marketed under the name "SolidWork" and modeled by mesh. Three-dimensional 3D thermal conduction calculations are carried out on these pads using its "Professional Simulation" module in its 2019 version. The gain in thermal conductivity and maximum temperatures of the pad with insert is evaluated by comparison with the maximum temperature of the U02 combustible material of a standard pellet from a 900 MWe PWR of the French sector.

For the following examples, the linear powers of the pellets are taken as equal to 300 W/cm, the normal operating power of PWR rods, and to 500 W/cm relating more to the accidental domain.

For the following examples, the dimensions of the pellet 1 and of the insert 11 are indicated in tables 2 and 3. According to this example, the pellet 1 and the plates 111 are cylindrical and of circular section.

Table 2:

Table 3:

The thermal gain of this concept is evaluated by comparison with the maximum temperature of the oxide fuel of a standard pellet of a 900 MWe PWR of the French sector. The linear powers of the pellets chosen for the calculations amount to 300 W/cm corresponding to the standard operating conditions of the fuel rod and 500 W/cm higher power falling more within the accident domain.

The maximum temperatures of the pellets are calculated and compared with those of a standard ceramic fuel in table 4 for several examples of embodiments with 2, 4, 6 plates 111 of solid zirconium, that is to say without an opening of thickness indicated in Table 3. The insert 11 represents a volume of 10% relative to the total volume of the pellet. Perfect contact is assumed between the metallic 111 plates and the UÜ2 as well as between the pad and the sheath. A decrease in maximum pellet temperatures between 43 and

183°C is calculated respectively for 2 to 6 zircaloy 111 plates in a pellet 1.

This decrease increases favorably from 111°C to 301°C respectively for 2 to 6 zircaloy plates in a pellet for a higher power of 500 W/cm. With identical molybdenum or chromium 111 plates, the calculated temperatures are lowered favorably by approximately 10 to 15% compared to those obtained for zircaloy plates.

Table 4:

With a central opening 1110, the metal is moved away from the center of the pad, the hottest point of the pad. The temperatures calculated at 500 W/cm are lowered to 801°C with an insert comprising a central opening, instead of 970°C in the center of the pellet, which makes it possible to gain approximately 169°C on the melting margins of a insert comprising plates with a central opening versus without a central opening. It has also been observed that these radial temperature differences increase with power.

Temperature calculations made previously have been extrapolated for plates comprising a central opening 1110 and peripheral openings 1111, filled with UC>2. The dimensions of the openings are shown in Table 5. With 80% by volume occupied of molybdenum or chromium per plate and 20% opening filled with UC>2, for 8% by volume of the insert 11 relative to the volume of the pellet, the conductivities of the plates with an opening remain much higher than that of the solid zircaloy plates.

Thus, the gains in maximum temperatures of the pellets with perforated plates 111 correspond to those indicated in table 4 with a volume of the insert 11 reduced to 8% of the volume of the fuel pellet 1.

Table 5:

In view of the foregoing description, it clearly appears that the invention improves the thermal conductivity and reduces the maximum temperatures of a fuel pellet comprising a metal insert.

The invention is not limited to the embodiments previously described and extends to all the embodiments covered by the invention. The present invention is not limited to the examples described above. Many other variant embodiments are possible, for example by combining features previously described, without departing from the scope of the invention. Furthermore, features described in relation to one aspect of the invention may be combined with another aspect of the invention.

The examples detailed in the description are with reference to a cylindrical geometry of the pellet and of the metal insert. The characteristics described are however applicable to other geometries of the insert and/or the pellet, for example a rectangular geometry.

Claims

1. Fuel pellet (1) having a central axis (A) of main extension comprising: “a fissile nuclear combustible material (10), and

• at least one metal insert (11) disposed in the combustible material (10) and comprising a plurality of plates (111), each plate (111) extending in a cross section of the fuel pellet (1) perpendicular to its axis center (A), o each plate (111) has a solid volume greater than 50% of the total volume of the plate (111), and o at least one plate (111), and preferably each plate (111), comprises a central opening (1110) arranged in the center of the plate (111), the pellet being characterized in that:

• each plate (111) has a thickness substantially between 0.2 and 1.5 mm,

• the insert (11) is configured to represent a proportion of between 5% and 15% of the total volume of the fuel pellet (1), and · at least one plate (111), and preferably each plate (111), comprises at least one peripheral opening (1111) not placed in the center of the plate,

• for at least two plates (111) arranged consecutively along the central axis (A) and comprising at least one peripheral opening (1111), the peripheral openings (1111) of the at least two consecutive plates (111) are not completely in look between them.

2. Fuel pellet (1) according to the preceding claim, in which the ratio between an outer dimension (Di) of the cross section of the fuel pellet (1) and a largest dimension (Dmo) of the central opening (1110 ) is between 2 and 4.

3. Fuel pellet (1) according to any one of the preceding claims, in which the at least one peripheral opening (1111) has a largest dimension (Dnn) substantially between 0.5 mm and 1.5 mm .

4. Fuel pellet (1) according to any one of the preceding claims, in which at least one plate (111), and preferably each plate (111), comprises between four and ten openings (1110, 1111).

5. Fuel pellet (1) according to any one of the preceding claims, at least one plate (111), and preferably each plate (111), comprising a plurality of openings (1110, 1111), the plurality of openings has rotational symmetry around the central axis (A).

6. Fuel pellet (1) according to any one of the preceding claims, in which at least one plate (111), and preferably each plate (111), has a largest external dimension (Dm), less than 95% , preferably 90%, preferably 80% of an outer dimension (Di) of the cross section of the fuel pellet (1).

7. Fuel pellet (1) according to any one of the preceding claims, in which the metal insert (11) comprises a number of plates comprised between 2 and 6.

8. Fuel pellet (1) according to any one of the preceding claims, in which the metal insert (11) comprises a hollow central body (110) on which the plurality of plates (111) is arranged.

9. Fuel pellet (1) according to any one of the preceding claims, in which the metal insert (11) comprises an outer body (112) configured to form the outer periphery (1a) of the fuel pellet (1).

10. Fuel pellet (1) according to any one of the preceding claims, in which the metal insert (11) is based on a metal having a thermal conductivity greater than the thermal conductivity of the combustible material (10).

11. Fuel pellet (1) according to any one of the preceding claims, in which the metal insert (11) is based on a refractory metal, preferably chosen from zircaloy, chromium, molybdenum and their alloys.

12. Fuel pellet (1) according to any one of the preceding claims, in which the metal insert (11) comprises platinum.

13. Fuel rod (2) comprising at least one fuel pellet (1) according to any one of the preceding claims, surrounded by a sheath.

14. Assembly (3) for a nuclear reactor (4) comprising a bundle of fuel rods (2) according to the preceding claim.

15. Nuclear reactor (4) comprising at least one assembly (3) according to the preceding claim.