Plate-type combustible element with a plenum for gas collection

Abstract

Fuel element (1), intended to be placed in a nuclear reactor, the fuel element comprising a combustible material (2), comprising fissile or fertile material; the fuel element being characterized in that it comprises a porous structure (4), called a plenum, having a porosity greater than 50%, comprising a structural material delimiting a free volume, the plenum being configured to collect gases generated by fission reactions within the fuel material. Figure 1A

Description

Title of the invention: Plate-type combustible element comprising a plenum for gas collection technical field

[0001] The technical field of the invention relates to fuel elements intended to be placed in a nuclear reactor. By nuclear fuel element, we mean the smallest constituent of a nuclear reactor core having its own structure and containing nuclear fuel. PREVIOUS ART

[0002] Nuclear fuel elements generally take the form of cylindrical plates or rods, their geometry being dependent on their purpose.

[0003] Fuel elements for electricity production reactors, of the pressurized water reactor type, take the form of pellets stacked one on top of the other, forming pencil-shaped assemblies.

[0004] In some experimental reactors, the fuel element takes the form of a plate, for example a flat or curved plate.

[0005] The fuel material, regardless of the reactor type, is subjected to a neutron flux and high temperatures. Exposure to high temperatures is accompanied by the release of fission gases. The free volume inside a plate-type fuel element is on the order of a few percent of the total volume of the fuel element. Free volume refers to the volume that can be occupied by a gas within the fuel element. As fission gases are released, they accumulate in this free volume. As the gases are released, the internal pressure increases, which can induce swelling or even a loss of seal in the fuel element. Excessive internal pressure can lead to cracking of the cladding covering the fuel material, which constitutes the first safety barrier. Reducing the internal pressure makes it possible to increase the burnup rate while ensuring reactor safety.

[0006] In fuel rods or needles, one or more plenums are added, generally at the ends, so as to collect fission gases.

[0007] In plate-type fuels, the fuel porosity is adjusted to accommodate the release of fission gases, but this degrades fuel performance, particularly thermal conductivity. However, a decrease in thermal conductivity can accentuate the formation of thermal gradients within the fuel element. Therefore, such a solution is not entirely satisfactory.

[0008] The invention described below makes it possible to obtain a combustible element whose internal pressure remains under control, particularly under high combustion rates or at high temperatures, without having to directly adjust the porosity of the fuel. Description of the invention

[0009] An object of the invention is a fuel element, in the form of a plate, intended to be disposed in a nuclear reactor, the fuel element comprising a fuel material, comprising fissile or fertile material, the fuel material being enveloped in a sheath, the sheath delimiting an internal space, the fuel element being characterized in that it comprises a porous structure, called a plenum, disposed in the internal space, the porous structure comprising a structural material delimiting a free volume, the plenum being configured to collect, in the free volume, gases generated by fission reactions within the fuel material.

[0010] The combustible element can extend, according to a thickness, between a first flat end and a second flat end, the combustible element having the shape of a flat plate.

[0011] According to one possibility, the combustible element extends, along a thickness, between a first end and a 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.

[0012] According to one possibility, the plenum extends from a point in the sheath, at the level of the first end, to another point in the sheath, at the level of the second end.

[0013] Preferably, the thermal conductivity of the material forming the plenum is greater than the thermal conductivity of the combustible material.

[0014] According to one possibility, the plenum comprises a plurality of contiguous alveoli, separated from each other by a wall, each alveolus delimiting a free space.

[0015] According to one possibility, the cells are distributed according to a regular grid.

[0016] Each alveolus can extend along a polygonal section.

[0017] According to one possibility, each alveolus is delimited by different sides, the maximum dimension of a side, parallel to a section of the alveolus, being less than 1 mm or 100 pm.

[0018] The plenum may include pillars, extending around the free space.

[0019] According to one possibility, the structural material is of the silicon carbide or molybdenum or depleted uranium or pyrocarbon type and / or a microporous structural material.

[0020] According to one possibility: - the combustible material is delimited by a border; - the plenum extends along the border.

[0021] According to one possibility, the combustible material extends between four corners, the plenum being arranged along one corner.

[0022] According to one possibility, the free volume of the plenum is greater than 1% and less than 10% of the volume of the combustible material. The free volume of the plenum may be less than 5% of the volume of the combustible material.

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

[0024] 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%.

[0025] 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%.

[0026] The invention will be better understood upon reading the description of the exemplary embodiments presented later in this description, in conjunction with the figures listed below. FIGURES

[0027] Figures IA and IB schematically illustrate examples of plate-shaped fuel elements having an integrated plenum at one corner.

[0028] Figures 2A, 2B and 2C show different configurations of a plenum whose structure forms alveoli.

[0029] Figure 3 schematically represents a combustible element in the shape of a curved plate.

[0030] Figure 4 schematically illustrates a plate-type combustible element comprising a plenum cylindrical in shape.

[0031] Fig. 5 shows a cylindrical plenum, formed of pillars and plates, intended to be integrated into a combustible element.

[0032] Fig. 6 schematically represents another rectangular plenum geometry. PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0033] Figures IA and IB represent a fuel element 1 according to a first embodiment. The fuel element comprises a fuel material 2 formed of fissile material, 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.

[0034] 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.

[0035] 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. In this example, the first and second ends are flat. The thickness of the fuel material 2 is a few millimeters, for example, 4 mm in the example shown in Figures IA and IB. Conventionally, the fuel element 2 has a metallic sheath 3, forming a liquid-tight enclosure around the fuel material. The sheath 3 is, for example, made of zirconium. The sheath delimits an internal space containing the fuel material.

[0036] The internal space of the fuel element comprises a structure 4, also referred to as the "plenum". Structure 4 is a porous, or hollow, structure, advantageously with a porosity greater than 30%, or even 50%, and advantageously greater than 75%. Porosity is defined as the ratio of the free volume of structure 4 to the total volume of said structure. Free volume is defined as a volume initially filled with air, or another gas, and intended to collect gases resulting from the fission of the fuel material.

[0037] The term "combustible material" refers to the material comprising the fissile or fertile material, for example UO2 or PuO2. In this description, the term "combustible element" refers to the assembly formed by the combustible material, the sheath and the plenum.

[0038] The plenum 4 can extend along the entire thickness of the combustible material, from the first end 2i to the second end 22. It is then in contact with the cladding 3 at each of said ends. The plenum can extend over only a portion of the thickness. Along the thickness, two plenums can be superimposed one on top of the other.

[0039] In [Fig. IA], a plenum with a quadrilateral cross-section is shown running along the edge of the fuel element at one corner of the edge. In [Fig. 1B], a plenum with a triangular cross-section, in the shape of an isosceles right triangle, is shown running along the edge of the fuel element at one corner of the edge. When the fuel element is delimited by four corners, a plenum can be located at at least one of the four corners.

[0040] In the examples shown in Figures IA and IB, the plenum is placed at the periphery of the combustible material 1. According to other configurations, described in connection with Figures 4 or 5, the plenum may be placed at the center of the combustible material.

[0041] It is estimated that for a combustible element with dimensions of 20 mm x 20 mm x 2 mm, a right-angled isosceles triangular plenum, as shown in [Fig. 1B], with sides of 5.7 mm, representing 4% of the volume of the combustible material, would allow for a 50% increase in free volume compared to a combustible element without a plenum. would result in a 30% pressure increase.

[0042] Regardless of the embodiment, the plenum structure is made of a solid structural material resistant to the temperature levels likely to be encountered in a nuclear reactor, i.e., temperatures exceeding 800°C or even 1000°C. Preferably, the structural material has high thermal conductivity. High thermal conductivity means a thermal conductivity greater than that of the fuel material 2 and preferably greater than that of the cladding 3. The thermal conductivity of the structural material is preferably 5, 10, 20, or 30 times greater than that of the fuel material 2.

[0043] 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. The structural material can be made from a molybdenum-type material (Mo⁻¹·X⁻¹ = 138 Wm⁻¹·K⁻¹ at room temperature) or silicon carbide (SiC⁻¹·X⁻¹ of approximately 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. The melting points of Mo and SiC⁻¹ are 2620 °C and 2800 °C, respectively. These values ​​can be compared with the melting points of UO₂ and Zr, which are approximately 3400 °C and 2400 °C, respectively. The structural material can include aluminum or steel.

[0044] The structural material must also be transparent to neutrons.

[0045] According to one possibility, the structural material is a carbon-based material, for example pyrocarbon or depleted uranium.

[0046] The structural material must possess mechanical strength properties to withstand the manufacturing stages or the stresses encountered during irradiation in a nuclear reactor core. The compressive strength is preferably greater than 100 MPa. SiC exhibits a compressive strength ranging from 3000 to 5000 MPa. Mo exhibits a compressive strength ranging from 200 to 2000 MPa.

[0047] Preferably, the structural material exhibits some porosity with respect to gases, as is the case with certain SiCs. Mo is more gas-tight. In order for gases to accumulate in the free volume inside the plenum, when the structural material is gas-tight, the plenum height may be less than the thickness of the fuel material. In addition or alternatively, openings may be provided in the plenum to allow the passage of gases. This allows the gases to diffuse through the fuel into the free volume delimited by the plenum. According to one possibility, the connection between the plenum 4 and the cladding 3 is not gas-tight. Fission gases may diffuse through this connection.

[0048] The plenum may have been made porous during the manufacturing process, by the appearance of micro-cracks in the structural material.

[0049] According to one possibility, the plenum is formed of a material with a microporous structure, for example a microporous ceramic.

[0050] According to one possibility, the combustible element comprises a plurality of plenums, in the periphery and / or in the central part of the combustible material.

[0051] The plenum acts as a collector for the fission gases released during fission reactions induced by neutron irradiation of the fuel. Its porosity is as high as possible, in order to optimize the ratio of plenum volume to available free volume.

[0052] Figures 2A, 2B, and 2C show examples of honeycomb structures in which the plenum structure comprises walls 4m parallel to the principal axis Z' of the plenum. These are solid walls, delimiting cells 4a. Each cell forms the free volume of the plenum. The cells can be polygonal or circular. In [Fig. 2A], the cells are hexagonal. In [Fig. 2B], the cells are triangular. In [Fig. 2C], the cells are rhombic. Preferably, the cells are arranged in a regular grid. They can extend in a honeycomb-like configuration.

[0053] The gas can diffuse through the walls, particularly when the structural material is porous or made porous by micro-cracking resulting from manufacturing. The walls may extend to a height less than the thickness of the combustible material, in which case gas diffusion occurs through the remaining combustible material facing the plenum.

[0054] An alveolar structure is considered particularly favorable with respect to mechanical resistance properties and the free volume to total volume ratio of the plenum.

[0055] In Figures IA and IB, the fuel element takes the form of a flat plate. However, the invention applies to fuel elements with other geometries. For example, the invention applies to fuel elements in the form of curved plates. This type of fuel element is found in some experimental reactors. The fuel material is contained between two curved parallel faces, forming the two ends of the fuel element. In a plane parallel to the thickness of the plate, the two parallel faces describe two parallel curves, or curves that can be considered parallel. Such a plate is shown schematically in [Fig. 3].

[0056] Fig. 4 represents another example of an embodiment, in which the plenum is arranged in the center of the fuel element 1, the latter being of the flat plate type.

[0057] In [Fig. 5], the plenum 4 is shown schematically in [Fig. 4]. The structure of the The plenum comprises five pillars arranged to form various concentric circular contours. The pillars extend between a lower face 4i and an upper face 42. The pillars are parallel to a principal axis Z'. The space between the pillars forms the free space. When the plenum is integrated into the fuel element, each pillar is oriented perpendicular to the flat ends delimiting the fuel material. In [Fig. 5], the thickness of the fuel element extends along the Z-axis. The principal axis Z' of the plenum is parallel to the Z-axis. The lower surface 4i is in contact with the first end 2b. The upper surface 42 is in contact with the second end 22. The lower and upper surfaces 4b42 are preferably solid. During the manufacture of the combustible element, or during its use, the upper and lower surfaces 4i,42pleines prevent penetration of the cladding into the free volume of the plenum.

[0058] In this example, the cross-section of the plenum, i.e., perpendicular to the principal axis Z', is circular. According to variants, the cross-section can be polygonal, for example triangular, quadrilateral or hexagonal.

[0059] Figure 6 shows a variant in which the plenum structure comprises pillars aligned along the principal axis Z'. The plenum has a rectangular cross-section. This embodiment is suitable for a plenum arrangement along the edge of a plate-type combustible element.

[0060] Regardless of the embodiment, the use of a structural material with high thermal conductivity promotes heat dissipation, particularly when the plenum is in contact with the cladding 3. Limiting the plenum temperature allows for an increased quantity of gas stored at the same pressure level. For this same reason, it is preferable for the plenum to be located in a "cold" part of the fuel element, that is, a part of the fuel element where the temperature variation of the fuel is minimal.

[0061] The volume occupied by the plenum can be between 1 and 10% of the volume of the combustible material. Preferably, the volume occupied by the plenum is less than 5% of the volume of the combustible material.

[0062] Several manufacturing techniques can be used to obtain a plenum: additive manufacturing, molding, laser ablation.

[0063] Regardless of the embodiment, the addition of a plenum results in a decrease in the volume of the fuel material. This can be compensated for by increasing the enrichment of the fissile material contained in the fuel material, so as not to affect the power density of the fuel element.

[0064] The invention makes it possible to obtain a significant reduction in the internal pressure in the fuel element, by increasing the free volume inside the fuel element.

Claims

Claims

1. Fuel element (1), in the form of a plate, intended to be arranged in a nuclear reactor, the fuel element comprising a combustible material (2), comprising fissile or fertile material, the combustible material being enveloped in a sheath (3), the sheath delimiting an internal space, the fuel element being characterized in that it comprises a porous structure (4), called a plenum, arranged in the internal space, the porous structure comprising a structural material delimiting a free volume, the plenum being configured to collect, in the free volume, gases generated by fission reactions within the combustible material.

2. A fuel element according to claim 1, extending, in a thickness, between a first planar end (2J and a second planar end (22), the fuel element having the shape of a planar plate.

3. Fuel element according to claim 1, extending along a thickness, between a first end and a 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.

4. A fuel element according to any preceding claim, wherein the plenum extends from one point on the cladding, at the first end, to another point on the cladding, at the second end.

5. A fuel element according to any preceding claim, wherein the thermal conductivity of the plenum-forming material is greater than the thermal conductivity of the fuel material.

6. Fuel element according to any one of the preceding claims, in which the plenum comprises a plurality of adjoining cells (4a), separated from each other by a wall (4m), each cell delimiting a free space.

7. Fuel element according to claim 6, in which the cells are distributed according to a regular mesh.

8. A fuel element according to any one of claims 6 or 7, wherein each cell extends in a polygonal section.

9. A fuel element according to claim 7 or claim 8, wherein each cell is delimited by different sides, the maximum dimension of a side, parallel to a section of the cell, being less than 1 mm or 100 pm.

10. A fuel element according to any one of claims 1 to 5, wherein the plenum comprises pillars, extending around the free space.

11. Fuel element according to any one of the preceding claims in which the structural material is of the silicon carbide or molybdenum or depleted uranium or pyrocarbon type and / or a microporous structural material.

12. A fuel element according to any preceding claim, wherein: - the combustible material is delimited by a border; - the plenum extends along the border.

13. A fuel element according to any preceding claim, wherein the combustible material extends between four corners, the plenum being arranged along one corner.

14. A fuel element according to any preceding claim, wherein the free volume of the plenum is greater than 1% and less than 10% of the volume of the combustible material.

15. A fuel element according to claim 14, wherein the free volume of the plenum is less than 5% of the volume of the combustible material.

16. A fuel element according to any preceding claim, wherein the thickness of the fuel element is less than 2 cm or 1 cm.

17. Fuel element according to any preceding claim, 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%.

18. Fuel element according to any one of claims 1 to 16, 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%.

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