WO2025093475A1 - Fuel element having improved thermal properties - Google Patents

Abstract

The invention relates to a fuel element (1) intended to be arranged in a nuclear reactor, the fuel element comprising a fuel material (2) that comprises fissile or fertile material, the fuel material extending between a first end (21) and a second end (22), the fuel element comprising at least one insert (4) extending through the fuel material between the first and second ends, wherein the insert or each insert comprises a thermally conductive material having a thermal conductivity greater than the thermal conductivity of the fuel material.

Description

Description

Title: Fuel element with improved thermal properties

TECHNICAL FIELD

The technical field of the invention relates to combustible material elements intended to be arranged in a nuclear reactor.

Nuclear fuel element means the smallest constituent of a nuclear reactor core having its own structure and containing nuclear fuel.

PREVIOUS ART

Nuclear fuel elements generally take the form of cylindrical plates or pellets or spheres, their geometry depending on their purpose.

Fuel elements for power generation reactors, such as pressurized water reactors, take the form of pellets stacked on top of each other, forming rod-shaped assemblies. In some experimental reactors, the fuel material takes the form of flat or curved plates.

The fuel material, regardless of the reactor type, is subjected to a neutron flux and high temperatures as well as significant temperature gradients. Exposure to high temperatures is accompanied by an accelerated release of fission gases and an increase in internal pressure, which can lead to swelling or even loss of leaktightness of the fuel element.

To limit the occurrence of high thermal gradients, it is possible to limit the operating temperature of the reactor. But this results in a loss of power.

When the fuel material is in the form of plates, to avoid exposing the nuclear fuel to excessively high temperatures, the air gap between two successive plates can also be increased. However, this lowers the power density of the reactor.

Furthermore, each plate can be thinned, so as to reduce the temperature gradient across the plate but a thin plate is more difficult to manufacture.

Another option to limit the formation of thermal gradients is to change the nature of the fuel material, using a metallic fuel, which has improved thermal conductivity. Research projects include the production of fuels from metal alloys, such as UAI, UaSiI, UMo, UZr. Other forms of uranium-based fuels are also being explored, such as TRISO (TRI-structural ISOtropic) particles for new reactor concepts such as high-temperature reactors, sodium-cooled fast reactors (SFRs) or microreactors.

US20210125735 describes a cylindrical fuel element comprising a metal frame. US3097152 describes a plate-shaped fuel element comprising a metal mesh. EP1913600 describes a honeycomb structure, into which pellet-shaped fuel elements are inserted. KR101383654 describes a fuel element in the form of a perforated plate, so as to allow the introduction of a thermally conductive insert.

The invention described below makes it possible to obtain a fuel element, in particular of the plate type, whose operating temperature is lower and more homogeneous than current ceramic fuel elements (UO2-PUO2), for the same power density. The objective is to improve the performance of a nuclear reactor by increasing the maximum power and/or the fuel combustion rate. It is also a question of increasing the safety margins, relating to the risks of melting of the fuel material, or of cladding rupture.

STATEMENT OF THE INVENTION

An object of the invention is a fuel element, intended to be arranged in a nuclear reactor, the fuel element comprising a combustible material, comprising fissile or fertile material, the combustible material extending between a first end and a second end, the fuel element comprising at least one insert, extending, through the combustible material, between the first and the second ends, the insert or each insert comprising a thermally conductive material, the thermal conductivity of which is greater than the thermal conductivity of the combustible material.

The fuel element can extend along a plate.

The fuel element may include a sheath, enveloping the combustible material.

According to one possibility, at least one insert, or each insert, extends from one point of the sheath, at the first end, to another point of the sheath, at the second end.

The thermal conductivity of the thermally conductive material may be greater than the thermal conductivity of the material forming the sheath. According to one possibility, the first end and the second end are planar, the fuel element taking the form of a planar plate.

According to one possibility, the fuel element extends along a thickness, between the first end and the second end, the first end and the second end being parallel, and describing, in a plane parallel to the thickness, a curved shape, the fuel element having the shape of a curved plate.

According to one possibility, the thermally conductive material comprises at least one material chosen from: molybdenum, chromium, silicon carbide.

According to one possibility:

- at least one insert, or even each insert, is cylindrical in shape;

- the diameter or largest diagonal of said insert, or of each insert, is less than 0.5 mm.

Each insert can be a cylinder of revolution.

According to one possibility:

- the first end is flat;

- the second end is flat, parallel to the first end;

- the distance between the first end and the second end forms a thickness of the fuel element;

- the fuel element has several inserts;

- the insert or each insert extends perpendicular to the first end and to the second end.

The thickness of the fuel element can be less than 2 cm or 1 cm.

According to one possibility:

- the fuel element has several inserts;

- two inserts closest to each other are spaced apart by a distance less than the thickness of the fuel element.

When the fuel element has several inserts, the inserts can be distributed according to a regular mesh in the fuel element.

According to one possibility, the fuel element, prior to its introduction into the nuclear reactor, contains fissile material of the Uranium 235 and/or Plutonium 239 type with an isotopy greater than 1%. According to one possibility, the fuel element, prior to its introduction into the nuclear reactor, contains fertile material, of the Uranium 238 type with an isotopy greater than 99.5%.

The invention will be better understood by reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below.

FIGURES

Figure 1A shows a diagram of a fuel element according to a first embodiment.

Figure 1B shows a fuel element according to a second embodiment.

Figure 2 represents the thermal conductivity (y-axis) of a UO2 (uranium oxide) fuel element as a function of the volume fraction of conductive inserts (x-axis).

Figure 3 illustrates a temperature gradient in a median plane of a fuel element according to the invention. The X and Y axes are spatial dimensions. The vertical axis, as well as the color code, correspond to temperature levels.

Figure 4 represents the thermal conductivity (y-axis) of a UO2 (uranium oxide) fuel element as a function of the volume fraction of conductive inserts (x-axis) taking into account different geometries of the conductive inserts.

Figure 5 shows the steps involved in manufacturing a fuel element using additive manufacturing.

Figure 6 shows a schematic of a fuel element in the form of a curved plate.

PRESENTATION OF SPECIAL METHODS OF IMPLEMENTATION

Figure 1A represents a fuel element 1 according to a first embodiment. The fuel element comprises a combustible material formed from the fissile material 2, for example an enriched uranium oxide. The fissile material is then ²³⁵ U, the isotopy in ²³⁵ U being greater than a few percent or tens of percent. Alternatively, the fissile material may comprise ²³⁹ Pu. It may for example be a MOX (Mixed Oxide) type combustible material, comprising plutonium oxide and uranium oxide.

According to one variant, the fuel element 1 is intended for fast neutron reactors. The fuel material may comprise fertile nuclear material ²³⁸ U, for example in the form of depleted uranium oxide or fissile nuclear material, for example in the form of plutonium oxide. In the example shown, the fuel element 1 takes the form of a flat plate. The combustible material 2 extends between a first end 2i and a second end 2 ₂ - The distance between the first end 2i and the second end 2 ₂ forms a thickness of the combustible material 2. The thickness of the combustible material 2 is a few mm, for example 4 mm in the example of figure 1A. Conventionally, the fuel element 2 comprises a metal sheath 3, forming a sealed envelope around the combustible material. The sheath 3 is for example formed of zirconium.

The term "fuel material" refers to the material containing the fissile or fertile material, for example UO2 or PuC. The term "fuel element" refers to the assembly formed by the fuel material, the cladding and the inserts.

In order to improve the thermal conductivity within the fuel element 1, the latter comprises several inserts 4, extending between the first end 2i and the second end 22. In this example, the first and second ends are flat. The inserts 4 preferably extend perpendicular to each end. Preferably, the inserts 4 extend from the first end 2i to the second end 22. At each end, each insert 4 is preferably in contact with the sheath 3.

The inserts 4 are formed from a material, called thermally conductive material, having high thermal conductivity. By high thermal conductivity is meant a thermal conductivity greater than the thermal conductivity of the combustible material 2 and preferably greater than that of the sheath 3. The thermal conductivity of the material forming each insert is preferably 5 times or 10 times or 20 times or 30 times greater than the thermal conductivity of the combustible material 2.

In this example, the fuel material is formed from uranium oxide, whose thermal conductivity λ varies between 3 Wm^.K ¹ and 10 Wm^.K ¹ at room temperature. Each insert can be formed from a material such as molybdenum (Mo - λ = 138 Wm^.K ¹ at room temperature), chromium (Cr - λ = 94 Wm^.K ¹ at room temperature), or a ceramic, for example silicon carbide (SiC - λ of the order of 400 Wm^.K ¹ at room temperature). Zirconium (Zr), forming the cladding, has a thermal conductivity of approximately 20 Wm^.K ¹ at room temperature.

Considering the temperature levels to which the fuel element is likely to be exposed, the melting temperature of the material forming the inserts is preferably above 1000 °C. The melting temperatures of Mo, Cr and SiC are respectively 2620 °C, 1910 °C and 2800 °C. These values are to be compared with the respective melting temperatures of UO2 and Zr, respectively of the order of 3400 °C and 2400 °C.

Thus, the invention consists in integrating, in each fuel element 1, a thermally conductive phase, corresponding to each insert 4. The inserts 4 are distributed, preferably as homogeneously as possible, in the combustible material 2. The volume fraction of the thermally conductive phase (i.e. all of the inserts) may be between 0.5% and 10%. The volume fraction must be sufficiently high to allow homogenization of the temperature within the combustible material, while being sufficiently low so as not to excessively reduce the power density of the fuel element compared to a fuel element without an insert.

The purpose of the thermally conductive phase is to promote temperature homogenization within the fuel element. This reduces thermal gradients in the fuel material, as well as the maximum temperature and the average temperature of the fuel material. Each insert 4 forms a thermal bridge, preferably oriented in the direction of the main heat flow, i.e. perpendicular to the plate forming the fuel element. This corresponds to the Z axis in Figures 1A and 1B.

Each insert 4 may have a cylindrical shape, for example a cylindrical shape of revolution. The diameter of each insert may be between a few tens of μm and a few hundred μm. Preferably, the diameter of each insert is less than 1 mm or 0.5 mm, and preferably less than 0.2 mm or 0.1 mm. The length of each insert is preferably equal to the thickness of the combustible material. The base of each cylindrical insert 4 may be circular or polygonal, for example with a square or hexagonal section.

The inserts are preferably distributed according to a regular mesh in the combustible material 2. The pitch between two adjacent inserts, i.e. the distance between two inserts closest to each other, is for example of the order of the thickness of the combustible material, or less than the latter. The mesh can be defined according to a square, rectangular or triangular elementary mesh. A square or triangular elementary mesh is considered optimal.

In Figures 1A and 1B, the inserts are shown in different shades of gray to facilitate readability. In Figure 1A, the volume fraction of the inserts is 7.7%. Each insert is a cylinder of revolution with a radius of 0.4 mm, with a pitch between the inserts of 2.3 mm. The thickness of the combustible material is 4 mm. The thickness of the cladding is 1 mm. The cladding has only been shown at the first end 2i of the combustible material. In Figure 1B, the volume fraction of the inserts is 5.7%. Each insert is a cylinder of revolution with a radius of 0.2 mm, the pitch between the inserts being 1.4 mm, the inserts being distributed according to a regular triangular mesh. The thickness of the combustible material is 4 mm. The thickness of the cladding is 1 mm.

Figure 2 represents the ideal axial thermal conductivity (i.e. along the Z axis - ordinate axis - unit Wm^.K ¹ ) (theoretical maximum) as a function of the volume fraction of the conductive phase (abscissa axis - %), the latter corresponding to the volume percentage of inserts 4 embedded in the combustible material 2. The prior art corresponds to a volume fraction of 0%. Compared to the prior art:

- a volume fraction of 3% makes it possible to double the thermal conductivity of the fuel element;

- a volume fraction of 10% makes it possible to quadruple the thermal conductivity of the fuel element.

An increase of n% in thermal conductivity is considered to reduce the amplitude of the axial thermal gradient across the combustible material by n%. The amplitude of the axial thermal gradient corresponds to the maximum temperature difference in the combustible material along the Z axis.

Simulations have shown that considering a 4 mm thick UO ₂ plate, a power density of 600W/cm ³ within the fuel element induces:

- an axial thermal gradient with a maximum amplitude of 400°C; and an average temperature in the combustible material 270°C higher than the temperature of the sheath. The average temperature is determined in the median plane of the combustible material, i.e. in a plane parallel to the plate, passing through the mid-thickness.

To carry out these simulations, it was assumed that the respective thermal conductivities of the combustible material and the inserts are 3 Wm^.K ¹ and 100 Wm^.K ¹ .

Using cylindrical inserts of revolution (round section), with a radius of 0.2 mm:

- when the volume fraction of the inserts is 3.5%, the maximum amplitude of the axial thermal gradient is 260°C (compared to 400°C without insert). The average temperature, in the median plane of the combustible material, is 170°C higher than the temperature of the cladding (compared to 270°C without insert); - when the volume fraction of the inserts is 10%, the maximum amplitude of the axial thermal gradient is 140°C (compared to 400°C without insert). The average temperature, in the median plane of the combustible material, is 100°C higher than the temperature of the cladding (compared to 270°C without insert).

Figure 3 represents a local temperature field in the median plane of a combustible material considering inserts of radius 0.2 mm, whose volume fraction is 3.5%, for a volume power of 600W/cm ³ . The maximum temperature is equal to 386 °C. The average combustible temperature in the plate is approximately 300°C, compared to 400°C for a configuration without insert.

Figure 4 shows the thermal conductivity of a combustible material (y-axis - unit Wm^.K ¹ ) as a function of the insert volume fraction (x-axis - %), taking into account cylindrical inserts of revolution with radii of 0.4 mm (curve a), 0.2 mm (curve b). Curve c corresponds to an ideal homogeneous distribution of the inserts in the volume fraction, as described in connection with Figure 2. The fact that the inserts are arranged discontinuously in the combustible material induces a difference between the expected performances in the ideal homogeneous configuration (see Figure 2 or curve c of Figure 4), and realistic configurations, taking into account the dimensions of the inserts (curves a and b of Figure 4). It is observed that by reducing the radius from 0.4 mm to 0.2 mm, a gain of 20% on the thermal conductivity is obtained.

It is understood that for each fuel element geometry (shape, section, thickness), simulations will make it possible to define optimal insert geometries in terms of gain in thermal conductivity, with regard to ease of manufacture and the power released.

Figure 5 shows the main steps of a fuel element manufacturing process as previously described. In this example, the process is an additive manufacturing process, in which layers are successively manufactured, superimposed on each other. Each layer can be made by depositing a powder and solidifying it. The process involves the following steps:

Step 100: supply of combustible material in the form of a powder;

Step 110: Solidification of the combustible material powder by exposure to a heat source, usually a laser beam. Solidification of the powder can be achieved by scanning the laser beam; Step 120: supply of the material forming the inserts in the form of a powder of said material;

Step 130: solidification of the powder of said material by exposure to the heat source.

Steps 100 to 130 are implemented to form a first layer, then repeated so as to form, during each iteration, a layer superimposed on the layer resulting from the previous iteration.

The steps may be carried out in the chronological order shown in Figure 5, or in other chronological orders: for example, step 100 may be carried out, then step 120, with solidification steps 110 and 130 being combined into a single solidification step. Steps 120 and 130 may be carried out, then 100 and 110.

The invention makes it possible to obtain a significant reduction in the internal pressure in the fuel element:

- on the one hand by releasing a smaller quantity of moles of fission gas into the fuel element, due to a reduction in the maximum temperature;

- on the other hand by reducing the internal pressure due to the decrease in the average temperature. It is estimated that by lowering the average temperature by 200°C in a combustible material heated to 1000°C, the pressure reduction is 20%.

Improved thermal conductivity allows for thicker fuel elements, which reduces the number of plates in a single assembly. The amount of nuclear material in a single assembly volume can be increased compared to the prior art.

The improvement in thermal conductivity makes it possible to use a fuel material with a higher porosity than in the prior art, so as to increase the free volume, i.e. the volume intended to be occupied by gases resulting from fission. This makes it possible to reduce the internal pressure in the fuel element.

Homogenizing the temperature within the fuel element also limits mechanical gradients induced by differential thermal expansion, as well as local thermal gradients. This reduces the risk of delamination at the fuel material/cladding interface. According to one possibility, the inserts extend on both sides around a median plane, the median plane passing through the mid-thickness of the plate. The plate has a middle part, extending around the median plane, without an insert. Although described in connection with a fuel element in the form of a flat plate, the invention applies to fuel elements having other geometries. For example, the invention applies to fuel elements in the form of curved plates. This type of fuel element is present in certain experimental reactors. The combustible material is contained between two curved parallel faces, respectively forming two ends of the fuel element. In a plane parallel to the thickness of the plate, the two parallel faces respectively describe two parallel curves, or which can be considered as parallel. Such a plate is shown diagrammatically in Figure 6.

Claims

1. Fuel element (1), intended to be arranged in a nuclear reactor, the fuel element comprising a combustible material (2), comprising fissile or fertile material, the combustible material extending between a first end (2 and a second end (2 ₂ ), the fuel element comprising at least one insert (4), extending, through the combustible material, between the first and the second ends, the insert or each insert comprising a thermally conductive material, the thermal conductivity of which is greater than the thermal conductivity of the combustible material, the fuel element being characterized in that: • the fuel element has several inserts; • two inserts closest to each other are spaced a distance less than the thickness of the fuel element. 2. The fuel element of claim 1, wherein said fuel element extends in a plate. 3. Fuel element according to claim 1 or claim 2, comprising a sheath (3), enveloping the combustible material. 4. Fuel element according to claim 3, and in which at least one insert, or even each insert, extends from one point of the sheath, at the first end, to another point of the sheath, at the second end. 5. A fuel element according to claim 3 or claim 4, wherein the thermal conductivity of the thermally conductive material is greater than the thermal conductivity of the material forming the sheath. 6. A fuel element according to any preceding claim, wherein the first end and the second end are planar, the fuel element taking the form of a planar plate. 7. Fuel element according to any one of claims 1 to 5, extending along a thickness, between the first end and the second end, the first end and the second end being parallel, and describing, in a plane parallel to the thickness, a curved shape, the fuel element having the shape of a curved plate. 8. Fuel element according to any one of the preceding claims, in which the thermally conductive material comprises at least one material chosen from: molybdenum, chromium, silicon carbide. 9. Fuel element according to any one of the preceding claims, wherein: - at least one insert is cylindrical in shape; - the diameter or largest diagonal of said insert, or of each insert, is less than 0.5 mm. 10. Fuel element according to claim 9, in which the or each insert is a cylinder of revolution. 11. Fuel element according to any one of claims 1 to 6 or 8 to 10 in which: - the first end of the combustible material is flat; - the second end of the combustible material is flat, parallel to the first end; - the distance between the first end and the second end forms a thickness of the fuel element; - the fuel element has several inserts; - the insert or each insert extends perpendicular to the first end and to the second end. 12. The fuel element of claim 11, wherein the thickness of the fuel element is less than 2 cm or 1 cm. 13. Fuel element according to any one of the preceding claims, in which the volume fraction of the thermally conductive material is between 0.5% and 10%. 14. Fuel element according to any one of the preceding claims, in which the fuel element comprises several inserts, the inserts being distributed in a regular mesh in the fuel element. 15. Fuel element according to any one of the preceding claims, in which the fuel element, prior to its introduction into the nuclear reactor, comprises fissile material of the Uranium 235 and/or Plutonium 239 type according to an isotopy greater than 1%. 16. Fuel element according to any one of claims 1 to 13, in which the fuel element, prior to its introduction into the nuclear reactor, comprises fertile material, of the Uranium 238 type according to an isotopy greater than 99.5%.