WO2025093474A1 - Plate-type fuel element comprising a plenum for collecting gas - Google Patents

Abstract

The invention relates to a 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 fuel element being characterized in that it comprises a porous structure (4), referred to as a plenum, the porosity of which is greater than 50%, comprising a structural material delimiting a free space, the plenum being configured to collect gases generated by fission reactions in the combustible material.

Description

Description

Title: Plate-type fuel element with a plenum for gas collection

TECHNICAL FIELD

The technical field of the invention relates to fuel elements intended to be arranged in a nuclear reactor. By nuclear fuel element is meant 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 plates or cylindrical rods, 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 element takes the form of a plate, for example a flat or curved plate.

Fuel material, regardless of the reactor type, is subjected to a neutron flux and high temperatures. Exposure to high temperatures is accompanied by a release of fission gas. The free volume inside a plate-type fuel element is of the order of a few percent of the total volume of the fuel element. Free volume refers to a volume likely to be occupied by a gas in the fuel element. As the fission gases are released, they accumulate in the free volume. As the release progresses, the internal pressure increases, which can cause swelling or even loss of leaktightness of the fuel element. Excessive internal pressure can cause cracking of the cladding covering the fuel material and constituting the first safety barrier. Reducing the internal pressure increases the burnup rate while ensuring reactor safety.

In fuel rods or pins, one or more plenums are added, generally at the ends, in order to collect the fission gases.

In plate-type fuels, the porosity of the fuel is adjusted to accommodate the release of fission gases, but this degrades the performance of the fuel, in particular thermal conductivity. However, a decrease in thermal conductivity can accentuate the formation of thermal gradients in the fuel element. Such a solution is therefore not fully satisfactory.

Documents FR2143137 and W02007017503 describe plates comprising cells in which fuel elements are arranged, respectively in the form of microspheres or pellets. The volume fraction occupied by the fuel is low, of the order of 20% or 25%.

The invention described below makes it possible to obtain a fuel element whose internal pressure remains controlled, in particular under high combustion rates or at high temperatures, without having to directly adjust the porosity of the fuel.

STATEMENT OF THE INVENTION

An object of the invention is a fuel element, in the form of a plate, intended to be arranged in a nuclear reactor, the fuel element comprising a combustible material, comprising fissile or fertile material, the combustible 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, 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.

The fuel element may extend, in thickness, between a first planar end and a second planar end, the fuel element having the shape of a planar plate.

According to one possibility, the fuel 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 fuel element having the shape of a curved plate.

According to one possibility, the plenum extends from one point in the duct, at the first end, to another point in the duct, at the second end.

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

According to one possibility, the plenum comprises a plurality of adjoining cells, separated from each other by a wall, each cell delimiting a free space.

According to one possibility, the cells are distributed according to a regular mesh. Each cell can extend along a polygonal section.

According to one possibility, 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 μm. The plenum may have pillars, extending around the free space.

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.

According to one possibility:

- the combustible material is delimited by a border;

- the plenum extends along the border.

According to one possibility, the combustible material extends between four corners, with the plenum arranged along one corner.

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 can be less than 5% of the volume of the combustible material.

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

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

Figures 1A and 1B schematically illustrate examples of plate-shaped fuel elements having an integrated plenum at one corner.

Figures 2A, 2B and 2C show different configurations of a plenum whose structure forms cells.

Figure 3 shows a schematic of a fuel element in the form of a curved plate.

Figure 4 shows a schematic of a plate-type fuel element comprising a cylindrical plenum.

Figure 5 shows a cylindrical plenum, formed of pillars and plates, intended to be integrated into a fuel element. Figure 6 schematizes another rectangular plenum geometry.

PRESENTATION OF SPECIAL METHODS OF IMPLEMENTATION

Figures 1A and 1B 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 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 fuel 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. In this example, the first and second ends are flat. The thickness of the combustible material 2 is a few mm, for example 4 mm in the example of Figures 1A and 1B. Conventionally, the fuel element 2 comprises a metal sheath 3, forming a liquid-tight envelope around the combustible material. The sheath 3 is for example made of zirconium. The sheath delimits an internal space, comprising the combustible material.

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

The term "fuel material" means the material comprising the fissile or fertile material, for example UO ₂ or PuO ₂ . In this description, the term "fuel element" means the assembly formed by the fuel material, the cladding and the plenum.

The plenum 4 can extend along the entire thickness of the combustible material, from the first end 2i to the second end 2 ₂ . It is then in contact with the sheath 3 at the level of each of said ends. The plenum may extend over only part of the thickness. Along the thickness, two plenums may be superimposed on each other.

In Figure 1A, a plenum of quadrilateral cross-section is shown, running along the edge of the fuel element, at one corner of the edge. In Figure 1B, a plenum of 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 may be arranged at at least one of the four corners.

In the examples shown in Figures 1A and 1B, 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 can be arranged in the center of the combustible material.

It is estimated that for a fuel element with dimensions of 20 mm x 20 mm x 2 mm, an isosceles right triangular plenum, as shown in Figure 1B, with sides of 5.7 mm, representing 4% of the volume of the combustible material, would provide a 50% gain in free volume compared to a fuel element without a plenum. This would result in a pressure gain of 30%.

Regardless of the embodiment, the plenum structure is made of a structural material that is solid and resistant to the temperature levels likely to be encountered in a nuclear reactor, i.e. temperatures above 800°C or even 1000°C. Preferably, the structural material has a 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 cladding 3. The thermal conductivity of the structural material 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^ and 10 Wm^.K ¹ at room temperature. The structural material can be formed from a molybdenum-type material (Mo - λ = 138 Wm^.K ¹ at room temperature), or from 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 about 20 Wm^.K ¹ at room temperature. The melting temperatures of Mo and SiC are 2620 °C and 2800 °C respectively. These values should be compared with the melting temperatures of respective melting points of UO2 and Zr, respectively of the order of 3400 °C and 2400 °C. The structural material may include aluminum or steel.

The structural material must also be transparent to neutrons.

Alternatively, the structural material is a carbon-based material, for example pyrocarbon or depleted uranium.

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

Preferably, the structural material has a certain porosity with respect to gases, which is the case for certain SiCs. Mo is more gas-tight. In order for gases to be able 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 as an alternative, openings may be provided in the plenum, so as to allow the passage of gases. This allows the gases to diffuse through the fuel, up to 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 can diffuse through said connection.

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

Alternatively, the plenum is formed from a microporous structural material, for example a microporous ceramic.

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

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, so as to optimize the ratio of plenum volume to available free volume.

Figures 2A, 2B and 2C show examples of alveolar structures, in which the plenum structure comprises 4 _m walls parallel to the main axis Z' of the plenum. These are solid walls, _delimiting 4 _a cells. Each cell forms the free volume of the plenum. The cells may be polygonal or circular. In Figure 2A, the cells are hexagonal. In Figure 2B, the cells are triangular. In Figure 2C, the cells are diamond-shaped. Preferably, the cells are arranged in a regular mesh pattern. They may extend in a honeycomb-like configuration.

Gas may diffuse through walls 4 _m , particularly when the structural material is porous or made porous by micro-cracks resulting from manufacturing. The walls may extend to a height less than the thickness of the combustible material, in which case gas diffusion is through the remaining combustible material facing the plenum.

A honeycomb structure is considered particularly favorable in terms of mechanical resistance properties and the free volume to total volume ratio of the plenum.

In Figures 1A and 1B, the fuel element takes the form of a flat plate. However, 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 3.

Figure 4 represents another exemplary embodiment, in which the plenum is arranged in the center of the fuel element 1, the latter being of the flat plate type.

In Figure 5, the plenum 4 shown schematically in Figure 4 is shown. The structure of the plenum comprises pillars 5, arranged to form different concentric circular contours. The pillars extend between a lower face 4i and an upper face 42. The pillars are parallel to a main 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 combustible material. In Figure 5, the thickness of the fuel element extends along the Z axis. The main axis Z' of the plenum is parallel to the Z axis. The lower surface 4i is in contact with the first end 2i. The upper surface 42 is in contact with the second end 22. The lower and upper surfaces 41, 42 are preferably solid. During the manufacture of the fuel element, or during its use, the solid upper and lower surfaces 4I,4 ₂ prevent penetration of the sheath into the free volume of the plenum. In this example, the cross-section of the plenum, i.e. perpendicular to the main axis Z', is circular. According to variants, the cross-section can be polygonal, for example triangular, quadrilateral or hexagonal.

Figure 6 shows a variant in which the plenum structure comprises pillars aligned along the main axis Z'. The cross-section of the plenum is rectangular. Such an embodiment is suitable for arranging the plenum along an edge of a plate-type fuel element.

Regardless of the embodiment, the use of a structural material having a high thermal conductivity promotes heat dissipation, in particular when the plenum is in contact with the sheath 3. Limiting the temperature of the plenum makes it possible to increase the quantity of gas stored for the same pressure level. For this same reason, it is preferable for the plenum to be arranged in a “cold” part of the fuel element, that is to say a part of the fuel element in which the variation in fuel temperature is minimal.

The volume occupied by the plenum may 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.

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

Regardless of the embodiment, the addition of a plenum results in a decrease in the volume of the fuel material. This can be compensated 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.

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

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, and in that the free volume of the plenum is greater than 1% and less than 10% of the volume of the combustible material. 2. Fuel element according to claim 1, in which the combustible material (2) extends, according to a thickness, between a first flat end (2i) and a second flat end (22), the fuel element having the shape of a flat plate. 3. Fuel element according to claim 1, in which the combustible material (2) extends along a thickness, between a first end and a second end, the first end and the second end being parallel, and describes, 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 sheath, at the first end, to another point on the sheath, at the second end. 5. A fuel element according to any preceding claim, wherein the thermal conductivity of the material forming the plenum is greater than the thermal conductivity of the combustible material. 6. Fuel element according to any one of the preceding claims, in which the plenum comprises a plurality of adjoining cells (4 _a ), separated from each other by a wall (4 _m ), 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. Fuel element according to any one of claims 6 or 7, in which each cell extends in a polygonal section. 9. Fuel element according to claim 7 or claim 8, in which 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. Fuel element according to any one of the preceding claims, in which the structural material has a compressive strength greater than 100 MPa. 13. Fuel element according to any one of the preceding claims, wherein: - the combustible material is delimited by a border; - the plenum extends along the border. 14. A fuel element according to any preceding claim, wherein the combustible material extends between four corners, the plenum being arranged along one corner. 15. Fuel element according to any one of the preceding claims, wherein the free volume of the plenum is less than 5% of the volume of the combustible material. 16. Fuel element according to any one of the preceding claims, the thickness of the fuel element being less than 2 cm or 1 cm. 17. A fuel element according to any preceding claim, wherein 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%.