Fuel element with improved thermal properties

Abstract

Fuel element (1), intended to be disposed in a nuclear reactor, the fuel element comprising a fuel material (2), comprising 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, the insert or each insert comprising a thermally conductive material, the thermal conductivity of which is greater than the thermal conductivity of the fuel material. Figure 1A.

Description

Title of the invention: Fuel element with improved thermal properties technical field

[0001] The technical field of the invention relates to combustible material elements intended to be arranged in a nuclear reactor.

[0002] A nuclear fuel element is defined as the smallest constituent of a nuclear reactor core having its own structure and containing nuclear fuel. PRIOR ART

[0003] Nuclear fuel elements generally take the form of cylindrical plates or pellets or spheres, their geometry being dependent on their purpose.

[0004] Fuel elements for power generation reactors, such as pressurized water reactors, take the form of pellets stacked one on top of the other, forming pencil-shaped assemblies. In some experimental reactors, the fuel material takes the form of flat or curved plates.

[0005] 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 induce swelling or even a loss of seal in the fuel element.

[0006] In order to limit the occurrence of high thermal gradients, it is possible to limit the operating temperature of the reactor. However, this results in a loss of power.

[0007] 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.

[0008] Furthermore, each plate can be thinned, so as to reduce the temperature gradient through the plate, but a thin plate is more difficult to manufacture.

[0009] Another option, which limits the formation of thermal gradients, is to change the nature of the fuel material, using a metallic fuel with improved thermal conductivity. Research projects include the production of fuels made of metallic alloys, such as UA1, U3Si2, UMo, and UZr.

[0010] Other forms of uranium-based fuels are also being explored, such as TRISO particles (TRI-structural ISOtropic - 3D isotropic structure) for new reactor concepts such as high-temperature reactors, or sodium-cooled fast reactors (SFRs) or microreactors.

[0011] The invention described below makes it possible to obtain a fuel element, particularly a plate type, with a lower and more homogeneous operating temperature 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 burnup rate. It also aims to increase safety margins related to the risks of fuel material meltdown or cladding rupture. Description of the invention

[0012] An object of the invention is a fuel element, intended to be disposed in a nuclear reactor, the fuel element comprising a fuel material, comprising fissile or fertile material, the fuel material extending between a first end and a second end, the fuel element comprising at least one insert, extending, through the fuel 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 fuel material.

[0013] The combustible element can extend along a plate.

[0014] The combustible element may include a sheath, enveloping the combustible material.

[0015] According to one possibility, at least one insert, or even each insert, extends from a point on the sheath, at the level of the first end, to another point on the sheath, at the level of the second end.

[0016] The thermal conductivity of the thermally conductive material may be greater than the thermal conductivity of the material forming the sheath.

[0017] According to one possibility, the first end and the second end are flat, the combustible element taking the form of a flat plate.

[0018] According to one possibility, the combustible 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 combustible element having the shape of a curved plate.

[0019] According to one possibility, the thermally conductive material comprises at least one material selected from: molybdenum, chromium, silicon carbide.

[0020] According to one possibility: - at least one insert, or even every insert, is cylindrical in shape; - the diameter or largest diagonal of said insert, or of each insert, is less than 0.5 mm.

[0021] Each insert can be a cylinder of revolution.

[0022] 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 combustible element; - the combustible element has several inserts; - the insert or each insert extends perpendicularly to the first end and at the second end.

[0023] The thickness of the combustible element may be less than 2 cm or 1 cm.

[0024] According to one possibility: - the combustible element has several inserts; - the two closest inserts to each other are spaced a distance less than the thickness of the combustible element.

[0025] When the fuel element has several inserts, the inserts can be distributed according to a regular mesh in the fuel element.

[0026] According to one possibility, the fuel element, prior to its introduction into the nuclear reactor, comprises fissile material of the Uranium 235 type and / or Plutonium 239 according to an isotopy greater than 1%.

[0027] According to one possibility, the fuel element, prior to its introduction into the nuclear reactor, comprises fertile material, of the Uranium 238 type with an isotopy greater than 99.5%.

[0028] The invention will be better understood upon reading the description of the exemplary embodiments presented later in this description, in connection with the figures listed below. FIGURES

[0029] Figure [1A] schematically represents a combustible element according to a first embodiment.

[0030] Figure [1B] shows a combustible element according to a second embodiment.

[0031] Fig. 2 represents the thermal conductivity (ordinate axis) of a UO2 type fuel element (uranium oxide) as a function of the volume fraction of conductive inserts (abscissa axis).

[0032] Figure 3 illustrates a temperature gradient in a median plane of a combustible 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.

[0033] Figure 4 represents the thermal conductivity (ordinate axis) of an element UO2 type fuel (uranium oxide) as a function of the volume fraction of conductive inserts (x-axis) taking into account different geometries of the conductive inserts.

[0034] Fig. 5 schematically illustrates the steps involved in manufacturing a combustible element by additive manufacturing.

[0035] Figure 6 schematically depicts a combustible element in the shape of a curved plate. DESCRIPTION OF SPECIFIC EMBODIMENTS

[0036] 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 235U, the 235U isotope being greater than a few percent or tens of percent. Alternatively, the fissile material may comprise 239Pu. It may, for example, be a MOX (Mixed Oxide) type fuel material, comprising plutonium oxide and uranium oxide.

[0037] According to one embodiment, the fuel element 1 is intended for fast neutron reactors. The fuel material may comprise fertile nuclear material 238U, for example in the form of depleted uranium oxide, or fissile nuclear material, for example in the form of plutonium oxide.

[0038] In the example shown, the fuel element 1 is in the form of a flat plate. The fuel material 2 extends between a first end 2i and a second end 22. The distance between the first end 2i and the second end 22 constitutes the thickness of the fuel material 2. The thickness of the fuel material 2 is a few millimeters, for example, 4 mm in the example of [Fig. 1A]. Conventionally, the fuel element 2 has a metallic sheath 3, forming a sealed envelope around the fuel material. The sheath 3 is, for example, made of zirconium.

[0039] The term "combustible material" refers to the material containing the fissile or fertile material, for example UO2 or PuO2. The term "combustible element" refers to the assembly formed by the combustible material, the cladding and the inserts.

[0040] To improve 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 perpendicularly from 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 cladding 3.

[0041] The inserts 4 are made of a material, called a thermally conductive material, having high thermal conductivity. High thermal conductivity means means 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.

[0042] In this example, the fuel material is made of uranium oxide, whose thermal conductivity X varies between 3 Wm⁻¹·K⁻¹ and 10 Wm⁻¹·K⁻¹ at room temperature. Each insert can be made from a molybdenum-type material (Mo⁻¹·K⁻¹ = 138 Wm⁻¹·K⁻¹ at room temperature), chromium (Cr⁻¹·K⁻¹ = 94 Wm⁻¹·K⁻¹ at room temperature), or a ceramic, for example silicon carbide (SiC⁻¹·K⁻¹ of the order of 400 Wm⁻¹·K⁻¹ at room temperature). Zirconium (Zr), forming the cladding, has a thermal conductivity of approximately 20 Wm⁻¹·K⁻¹ at room temperature.

[0043] Considering the temperature levels to which the combustible 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 2620 °C, 1910 °C, and 2800 °C, respectively. These values ​​are to be compared with the respective melting temperatures of UO2 and Zr, which are on the order of 3400 °C and 2400 °C, respectively.

[0044] Thus, the invention consists of integrating, within each fuel element 1, a thermally conductive phase corresponding to each insert 4. The inserts 4 are distributed, preferably as homogeneously as possible, within the fuel material 2. The volume fraction of the thermally conductive phase (i.e., all the inserts) can be between 0.5% and 10%. The volume fraction must be sufficiently high to allow for temperature homogenization within the fuel 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.

[0045] The objective 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 and average temperatures of the fuel material. Each insert 4 forms a thermal bridge, preferably oriented along 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 IA and IB.

[0046] 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 micrometers and a few hundred micrometers. 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 can be circular or polygonal, for example with a square or hexagonal cross-section.

[0047] The inserts are preferably distributed in a regular grid pattern within the combustible material 2. The spacing between two adjacent inserts, that is, the distance between the two closest inserts, is, for example, on the order of the thickness of the combustible material, or less than it. The grid pattern can be defined as a square, rectangular, or triangular unit cell. A square or triangular unit cell is considered optimal.

[0048] In Figures IA and IB, the inserts are shown in different shades of gray to improve readability. In [Fig. 1A], the volume fraction of the inserts is 7.7%. Each insert is a cylinder of revolution with a radius of 0.4 mm, and the spacing between the inserts is 2.3 mm. The thickness of the combustible material is 4 mm. The thickness of the cladding is 1 mm. The cladding is shown only at the first end 2i of the combustible material.

[0049] In [Fig. IB], the volume fraction of the inserts is 5.7%. Each insert is a cylinder of revolution with a radius of 0.2 mm, the spacing between the inserts being 1.4 mm, the inserts being distributed according to a regular triangular grid. The thickness of the combustible material is 4 mm. The thickness of the cladding is 1 mm.

[0050] Figure 2 represents the ideal axial thermal conductivity (i.e., along the Z-axis - ordinate axis - unit Wm*.K') (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% allows the thermal conductivity of the combustible element to be doubled; - a volume fraction of 10% allows the thermal conductivity of the combustible element to be quadrupled.

[0051] It is considered that an increase of n% in thermal conductivity reduces the amplitude of the axial thermal gradient through 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.

[0052] Simulations have shown that, considering a UO2 plate with a thickness of 4 mm, a volumetric power of 600W / cm3 within the combustible element induces: - an axial thermal gradient whose maximum amplitude is 400°C; - and an average temperature in the combustible material that is 270°C higher than the cladding temperature. The average temperature is determined in the median plane of the combustible material, that is, in a plane parallel to the plate, passing through the mid-thickness.

[0053] To perform these simulations, it was assumed that the respective thermal conductivities of the combustible material and the inserts are 3 Wm*.K1 and 100 Wm*.K

[0054] 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 the 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 the 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 the insert). The average temperature, in the median plane of the combustible material, is 100°C higher than the cladding temperature (compared to 270°C without the insert).

[0055] Figure 3 represents a local temperature field in the median plane of a combustible material considering inserts with a radius of 0.2 mm, a volume fraction of 3.5%, and a power density of 600 W / cm³. The maximum temperature is 386 °C. The average combustible temperature in the plate is approximately 300 °C, compared to 400 °C for a configuration without inserts.

[0056] Figure 4 shows the thermal conductivity of a combustible material (ordinate axis - unit Wm⁻².K⁻¹) as a function of the insert volume fraction (abscissa axis - %), considering cylindrical inserts of revolution with radii of 0.4 mm (curve a) and 0.2 mm (curve b). Curve c corresponds to an ideal homogeneous distribution of the inserts within the volume fraction, as described in relation to Figure 2. The fact that the inserts are arranged discontinuously within the combustible material leads to a difference between the performance expected in the ideal homogeneous configuration (see Figure 2 or curve c in Figure 4) and realistic configurations, taking into account the dimensions of the inserts (curves a and b in Figure 4). It can be observed that by decreasing the radius from 0.4 mm to 0.2 mm, a 20% improvement in thermal conductivity is achieved.

[0057] It is understood that for each geometry of fuel element (shape, section, thickness), simulations will make it possible to define optimal insert geometries in terms of thermal conductivity gain, with regard to ease of manufacture and power released.

[0058] Figure 5 schematically illustrates the main steps of a process for manufacturing a fuel element as described above. In this example, the process is an additive manufacturing process, in which layers are successively built up, one on top of the other. Each layer can be produced by depositing a powder and solidifying it. The process comprises the following steps:

[0059] Step 100: supply of combustible material in the form of a powder;

[0060] Step 110: Solidification of the combustible material powder by exposure to a heat source, usually a laser beam. The solidification of the powder can be achieved by scanning the laser beam;

[0061] Step 120: supply of the material forming the inserts in the form of a powder of said material;

[0062] Step 130: solidification of the powder of said material by exposure to the heat source.

[0063] Steps 100 to 130 are implemented to form a first layer, then reiterated so as to form, at each iteration, a layer superimposed on the layer resulting from the previous iteration.

[0064] The steps can be carried out in the chronological order shown in [Fig. 5], or in other chronological orders: for example, step 100 can be carried out, then step 120, with solidification steps 110 and 130 being combined into a single solidification step. Alternatively, steps 120 and 130 can be carried out, followed by steps 100 and 110.

[0065] 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 decrease in the maximum temperature; - on the other hand, by reducing the internal pressure due to the decrease in 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%.

[0066] Improved thermal conductivity allows for the design of thicker fuel elements, thereby reducing the number of plates in a single assembly. The amount of nuclear material in the same assembly volume can be increased compared to the prior art.

[0067] Improving thermal conductivity allows the use of a combustible material with higher porosity than in the prior art, thereby increasing the free volume, i.e., the volume available to be occupied by gases resulting from fission. This reduces the internal pressure within the combustible element.

[0068] Homogenizing the temperature within the fuel element also helps to limit 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. In one possibility, the inserts extend on either side around a median plane, the median plane passing through the mid-thickness of the plate. The plate has a central portion, extending around the median plane, without inserts.

[0069] 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 some experimental reactors. The fuel material is contained between two curved parallel faces, forming respectively two ends of the fuel element. In a plane parallel to the thickness of the plate, the two parallel faces describe respectively two parallel curves, or curves that can be considered parallel. Such a plate is schematically illustrated in [Fig. 6].

Claims

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 (2i) and a second end (22), 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.

2. The fuel element of claim 1, wherein said fuel element extends in a plate.

3. A 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 cladding.

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. A fuel element according to claim 9, wherein 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 wherein: - the first end is planar; - the second end is planar, 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 comprises 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 claims 11 or 12, wherein: - the fuel element comprises several inserts; - two inserts closest to each other are spaced apart by a distance less than the thickness of the fuel element.

14. A fuel element according to any preceding claim, wherein the fuel element comprises a plurality of inserts, the inserts being distributed in a regular mesh pattern within 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 14, in which the fuel element, prior to its introduction into the nuclear reactor, comprises fertile material, of the type Uranium 238 with an isotopy greater than 99.5%.

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