Silicon carbide clad fuel element

Abstract

The application discloses a silicon carbide cladding fuel element, which comprises a SiC cladding, a first end plug and a second end plug matched at opposite ends of the SiC cladding, a pellet assembly arranged in the SiC cladding and abutting against the second end plug, and the pellet assembly comprises a plurality of fuel pellets stacked along the axial direction of the SiC cladding in sequence; a first concave-convex structure is arranged on the first end surface of each fuel pellet, and a second concave-convex structure matched with the first concave-convex structure is arranged on the second end surface opposite to the first end surface of the fuel pellet; a third concave-convex structure is arranged on the end of the second end plug facing the pellet assembly, and the fuel pellet is matched with the third concave-convex structure through the first concave-convex structure or the second concave-convex structure. The silicon carbide cladding fuel element of the application realizes the alignment and centering of the fuel pellets through the concave-convex matching of the concave-convex structures, prevents the eccentricity of the fuel pellets, and reduces the risk of failure of the SiC cladding fuel element due to the irradiation bending and the mechanical interaction between the fuel pellets and the cladding.

Description

Technical Field

[0001] This invention relates to the field of nuclear fuel technology, and more particularly to a silicon carbide clad fuel element. Background Technology

[0002] Silicon carbide (SiC) is considered a leading candidate for high-performance fuel cladding materials due to its high temperature resistance, radiation resistance, good compatibility with various coolants, and good neutron economy. SiC cladding has a multilayer structure, typically consisting of at least one composite layer and one single-element layer. The composite layer is composed of chemically vapor-infiltrated (CVI) dense SiC fiber-reinforced SiC ceramic matrix composite material, while the single-element layer is generally composed of a SiC ceramic coating prepared by chemical vapor deposition (CVD).

[0003] Currently, the common design for SiC-clad fuel rods involves directly loading solid or perforated fuel pellets into a SiC cladding, then sealing both ends with SiC ceramic end plugs. A certain gap is typically left between the fuel pellet and the SiC cladding to accommodate fuel pellet deformation and facilitate loading during manufacturing. Due to the inherent brittleness of SiC material, the SiC cladding is very susceptible to damage upon hard contact with the fuel pellet. To accommodate deformation during service, SiC-clad fuel rods tend to employ a large gap design.

[0004] A major problem with the large-gap design is that fuel pellets may become eccentric within the SiC cladding due to loading deviations and lateral impacts. That is, circumferentially, the fuel pellet may be closer to one part of the SiC cladding inner wall than to another. Unlike traditional zirconium alloy cladding, SiC cladding exhibits significant irradiation swelling within the water-cooled reactor service temperature range (270℃-350℃). This irradiation swelling is closely related to temperature; the lower the temperature, the greater the irradiation swelling. Fuel pellet eccentricity leads to a large temperature gradient in the circumferential direction of the cladding, causing the SiC clad fuel rods to bend due to the difference in circumferential irradiation swelling (this phenomenon can be found in the journal article: Lu Zhiwei, et al. Research on the Influence of Circumferential Non-uniform Heat Transfer Caused by Pellet Eccentricity on the Bending Characteristics of SiC Fuel Rods [J]. Nuclear Safety, December 2024, Vol. 23, No. 6). Therefore, the design and manufacturing of SiC-clad fuel rods require special consideration of the centering and alignment of fuel pellets.

[0005] To address the fuel pellet alignment issue in traditional zirconium alloy clad fuel rods, a proposed solution involves a central boss on one end face and a central groove on the other. Alignment is achieved through the interaction of the boss and groove. However, this solution has several drawbacks. The central portion of the fuel pellet experiences higher temperatures and greater deformation during service. The central boss exacerbates this deformation, which in turn causes adjacent fuel pellets to lift or tilt. Therefore, the central boss design is difficult to apply in practice to ensure proper fuel pellet alignment. Furthermore, it doesn't completely solve the fuel pellet alignment problem. For example, the fuel pellets may shift towards one side of the cladding, resulting in misalignment even when they are initially aligned.

[0006] In summary, current fuel pellet alignment designs cannot completely solve the centering problem of fuel pellets within fuel rods, i.e., they fail to fully address the fuel pellet eccentricity issue. Furthermore, in SiC-clad fuel rods, due to factors such as the large gap design and irradiation bending caused by circumferential irradiation swelling differences, the requirements for fuel pellet alignment and centering are far higher than for traditional zirconium alloy-clad fuel rods. Therefore, the alignment schemes for zirconium alloy-clad fuel rods are not suitable for SiC-clad fuel rods. Thus, it is necessary to propose an alignment design to address the potential eccentricity problem in SiC-clad fuel rods, thereby avoiding the risk of this eccentricity issue. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a silicon carbide-clad fuel element that enables the alignment and centering of fuel pellets within the cladding.

[0008] The technical solution adopted by this invention to solve its technical problem is as follows: A silicon carbide-clad fuel element is provided, comprising a SiC cladding, a first end plug and a second end plug fitted at opposite ends of the SiC cladding, and a fuel pellet assembly disposed inside the SiC cladding and abutting the second end plug. An annular gap is left between the fuel pellet assembly and the inner surface of the SiC cladding. The fuel pellet assembly includes a plurality of fuel pellets stacked sequentially along the axial direction of the SiC cladding. Each fuel pellet has a first concave-convex structure on its first end face and a second concave-convex structure adapted to the first concave-convex structure on its opposite second end face. In the fuel pellet assembly, the first concave-convex structure of one fuel pellet engages with the second concave-convex structure of another axially adjacent fuel pellet, thereby aligning and connecting the fuel pellets to each other. The second end plug has a third concave-convex structure on one end facing the pellet assembly. The fuel pellet abutting against the second end plug is positioned on the second end plug and is coaxial with the second end plug and the SiC cladding axis by the first concave-convex structure or the second concave-convex structure engaging with the third concave-convex structure.

[0009] Preferably, the first concave-convex structure includes an annular boss disposed circumferentially along the first end face and close to the periphery of the first end face; the second concave-convex structure includes an annular groove disposed circumferentially along the second end face and close to the periphery of the second end face.

[0010] Preferably, the first concave-convex structure includes a plurality of protrusions, which are distributed circumferentially along the first end face and close to the periphery of the first end face. The second concave-convex structure includes a plurality of grooves, which are distributed circumferentially along the second end face and close to the periphery of the second end face; or, the second concave-convex structure includes an annular groove, which is arranged circumferentially along the second end face and close to the periphery of the second end face.

[0011] Preferably, the third concave-convex structure includes an annular protrusion, an annular groove, a plurality of protrusions, or a plurality of grooves.

[0012] Preferably, the first and second end faces of the fuel pellet each have a central end face located inside the first and second concave-convex structures, respectively; the central end face is provided with a concave dish-shaped curved surface, so that the central end face forms a dish-shaped end face; and / or, The fuel pellet has a central through hole that extends axially through the first and second end faces.

[0013] Preferably, the width of the annular gap between the chip assembly and the inner surface of the SiC cladding is 0.1 mm to 0.5 mm.

[0014] Preferably, the height of the fuel pellet is 5mm to 15mm and the diameter is 5mm to 12mm; the height or depth of the first concave-convex structure is 0.5mm to 3mm, and the height or depth of the second concave-convex structure is 0.5mm to 3mm.

[0015] Preferably, the silicon carbide-clad fuel element further includes a spring; a gap is left between the pellet assembly and the first end plug, and the spring is disposed within the gap, with its opposite ends abutting against the pellet assembly and the first end plug respectively; and / or, The silicon carbide-clad fuel element also includes an inert gas filled within the SiC cladding, wherein the pressure of the inert gas within the SiC cladding is 0.1 MPa to 2 MPa.

[0016] This invention provides another silicon carbide-clad fuel element, comprising a SiC cladding, first and second end plugs fitted at opposite ends of the SiC cladding, and a fuel cell assembly disposed inside the SiC cladding and abutting the second end plugs. An annular gap is provided between the fuel cell assembly and the inner surface of the SiC cladding. The fuel cell assembly includes a plurality of first fuel cells and a plurality of second fuel cells. The first and second end faces of the first fuel cells are respectively provided with first concave-convex structures, and the first and second end faces of the second fuel cells are respectively provided with second concave-convex structures adapted to the first concave-convex structures. The plurality of first fuel cells and the plurality of second fuel cells are alternately stacked along the axial direction of the SiC cladding. The first concave-convex structures of the first fuel cells engage with the second concave-convex structures of the axially adjacent second fuel cells, aligning and connecting the first fuel cells and the second fuel cells to each other. The second end plug has a third concave-convex structure on one end facing the pellet assembly. The first fuel pellet or the second fuel pellet abutting against the second end plug engages with the third concave-convex structure through the first concave-convex structure or the second concave-convex structure, so that the pellet assembly is positioned on the second end plug and is coaxial with the second end plug and the SiC cladding axis.

[0017] Preferably, the first concave-convex structure includes an annular boss; on the first end face of the first fuel pellet, the annular boss is arranged circumferentially along the first end face and close to the periphery of the first end face; on the second end face of the first fuel pellet, the annular boss is arranged circumferentially along the second end face and close to the periphery of the second end face. The second concave-convex structure includes an annular groove; on the first end face of the second fuel pellet, the annular boss is arranged circumferentially along the first end face and close to the periphery of the first end face; on the second end face of the second fuel pellet, the annular groove is arranged circumferentially along the second end face and close to the periphery of the second end face.

[0018] Preferably, the first concave-convex structure includes a plurality of protrusions; on the first end face of the first fuel pellet, the plurality of protrusions are distributed circumferentially at intervals along the first end face and close to the periphery of the first end face; on the second end face of the first fuel pellet, the plurality of protrusions are distributed circumferentially at intervals along the second end face and close to the periphery of the first end face. The second concave-convex structure includes a plurality of grooves or annular grooves; on the first end face of the second fuel pellet, the plurality of grooves or annular grooves are arranged circumferentially along the first end face and close to the periphery of the first end face; on the second end face of the second fuel pellet, the plurality of grooves or annular grooves are arranged circumferentially along the second end face and close to the periphery of the second end face.

[0019] Preferably, the third concave-convex structure includes an annular protrusion, an annular groove, a plurality of protrusions, or a plurality of grooves.

[0020] Preferably, the first end face and the second end face of the first fuel pellet each have a middle end face located inside the first concave-convex structure, and the first end face and the second end face of the second fuel pellet each have a middle end face located inside the second concave-convex structure; the middle end face is provided with a concave dish-shaped curved surface, so that the middle end face forms a dish-shaped end face; and / or, The first fuel pellet has a first central through hole that extends axially through its first end face and second end face, and the second fuel pellet has a second central through hole that extends axially through its first end face and second end face.

[0021] Preferably, the width of the annular gap between the chip assembly and the inner surface of the SiC cladding is 0.1 mm to 0.5 mm.

[0022] Preferably, the heights of the first fuel pellet and the second fuel pellet are 5mm to 15mm and the diameters are 5mm to 12mm, respectively; the height or depth of the first convex-concave structure is 0.5mm to 3mm, and the height or depth of the second convex-concave structure is 0.5mm to 3mm.

[0023] Preferably, the silicon carbide-clad fuel element further includes a spring; a gap is left between the pellet assembly and the first end plug, and the spring is disposed within the gap, with its opposite ends abutting against the pellet assembly and the first end plug respectively; and / or, The silicon carbide-clad fuel element also includes an inert gas filled within the SiC cladding, wherein the pressure of the inert gas within the SiC cladding is 0.1 MPa to 2 MPa.

[0024] The beneficial effects of this invention are as follows: The fuel pellets are aligned and connected by a concave-convex structure, and the end plugs that abut against the fuel pellets are also positioned by a concave-convex structure, which ensures that the pellet assembly formed by stacking fuel pellets is aligned within the SiC cladding, and prevents fuel pellets from becoming eccentric. This reduces the risk of failure of SiC clad fuel elements due to irradiation bending and fuel pellet-cladding mechanical interaction (PCMI). Attached Figure Description

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings: Figure 1 This is a schematic diagram of the external structure of a silicon carbide-clad fuel element according to an embodiment of the present invention; Figure 2This is a longitudinal cross-sectional structural diagram of a silicon carbide-clad fuel element according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the structure of a fuel pellet in a silicon carbide-clad fuel element according to an embodiment of the present invention; Figure 4 yes Figure 3 A schematic diagram of the fuel pellet structure from another angle; Figure 5 This is a schematic diagram of the structure of the second end plug in a silicon carbide-clad fuel element according to an embodiment of the present invention; Figure 6 This is a schematic diagram of an embodiment of the first uneven structure on the fuel pellet in the silicon carbide-clad fuel element of the present invention; Figure 7 This is a schematic diagram of another embodiment of the first uneven structure on the fuel pellet in the silicon carbide-clad fuel element of the present invention; Figure 8 This is a schematic diagram of the assembly structure of fuel pellets in a silicon carbide-clad fuel element according to another embodiment of the present invention. Detailed Implementation

[0026] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0027] like Figure 1 , Figure 2 As shown, some embodiments of the present invention include a silicon carbide-clad fuel element comprising a SiC (silicon carbide) cladding 100, a first end plug 10, a second end plug 20, a pellet assembly, and a spring (not shown).

[0028] The SiC cladding 100 is a tube open at both ends, having a first end and a second end. A first end plug 10 and a second end plug 20 are respectively fitted onto the first and second ends of the SiC cladding 100, sealing both ends and forming a closed cladding tube. Corresponding to the SiC cladding 100, the first end plug 10 and the second end plug 20 are also made of silicon carbide material (such as silicon carbide ceramic), specifically SiC end plugs.

[0029] The fuel pellet assembly is disposed inside the SiC cladding 100 and abuts against the second end plug 20. An annular gap 110 is provided between the fuel pellet assembly and the inner surface of the SiC cladding 100 to facilitate the loading of the fuel pellet assembly and to accommodate deformation of the fuel pellet assembly. The width of the gap between the fuel pellet assembly and the SiC cladding 100 (i.e., the width of the annular gap 110) can be 0.1 mm to 0.5 mm. A larger gap helps to avoid direct contact between the fuel pellet 30 and the SiC cladding 100 during service.

[0030] Silicon carbide-clad fuel elements Figure 2 As shown, the SiC cladding 100 is placed vertically with its first end facing upwards, and the second end plug 20 is supported at the bottom of the chip assembly. A gap 120 is left between the chip assembly and the first end plug 10, and a spring is disposed within this gap 120, with the opposite ends of the spring abutting against the first end plug 10 and the chip assembly, respectively. Through the elastic extension of the spring and its abutment against the chip assembly, the chip assembly is pressed against the second end plug 20, thereby axially positioning it within the SiC cladding 100 and limiting the axial displacement of the chip assembly.

[0031] Specifically, in one embodiment, the SiC cladding 100 has a two-layer structure, with the inner layer being a chemically vapor-infiltrated (CVI) dense SiC fiber-reinforced SiC ceramic matrix composite material, and the outer layer being a SiC ceramic coating prepared by chemical vapor deposition (CVD). The SiC cladding 100 can also have a three-layer structure, with the inner and outer layers being SiC ceramic coatings prepared by chemical vapor deposition (CVD), and the middle layer being a chemically vapor-infiltrated (CVI) dense SiC fiber-reinforced SiC ceramic matrix composite material. The wall thickness of the SiC cladding 100 is 0.7 mm to 1.3 mm.

[0032] The fuel pellet assembly includes multiple fuel pellets 30, which are stacked sequentially along the axial direction of the SiC cladding 100. The number of fuel pellets 30 is determined according to the length of the SiC cladding 100 and its loading requirements. The fuel pellets 30 are made of oxides, nitrides, carbides, or silicides containing uranium or plutonium, such as at least one of UO2, MOX, UN, UC, and U3Si2. The enrichment levels of each fuel pellet 30 can be the same or different.

[0033] Each fuel pellet 30 has two opposing end faces, namely a first end face and a second end face. The first end face of the fuel pellet 30 is provided with a first concave-convex structure 40, and the second end face of the fuel pellet 30 is provided with a second concave-convex structure 50 adapted to the first concave-convex structure 40. In the pellet assembly, the first concave-convex structure 40 of one fuel pellet 30 and the second concave-convex structure 50 of another axially adjacent fuel pellet 30 are engaged to align and connect the fuel pellets 30 with each other, ensuring that the fuel pellets 30 are coaxial.

[0034] The first concave-convex structure 40 is arranged circumferentially on the first end face of the fuel pellet 30, such that the first end face of the fuel pellet 30 has a central end face located inside (inner ring) of the first concave-convex structure 40. This central end face may preferably have a concave disc-shaped curved surface, so that the central end face forms a disc-shaped end face 301. Similarly, on the second end face of the fuel pellet 30, the second concave-convex structure 50 is arranged circumferentially on the second end face, such that the second end face has a central end face located inside (inner ring) of the second concave-convex structure 50. This central end face may preferably have a concave disc-shaped curved surface, so that the central end face forms a disc-shaped end face 302.

[0035] The formation of dished end faces 301 and 302 on the first and second end faces of the fuel pellet 30 can compensate for axial thermal expansion and irradiation swelling. The dished end faces 301 and 302 can also provide additional cavities to accommodate fission gas, buffer gas pressure, and reduce pressure load inside the cladding.

[0036] In some embodiments, reference Figure 3 and Figure 4 The first concave-convex structure 40 includes an annular boss 41. The annular boss 41 is arranged circumferentially along the first end face of the fuel pellet 30 and close to the periphery of the first end face, so that the first end face of the fuel pellet 30 has an annular end face located on the outer periphery of the annular boss 41. Because the annular boss 41 is close to the periphery of the first end face, the annular boss 41 has a lower temperature during service, and the area where the annular boss 41 is located has less deformation compared with the area where the middle end face of the first end face is located, which helps the fuel pellet 30 to maintain alignment and centering. The top periphery of the annular boss 41 and the periphery of the annular end face are both chamfered to prevent chipping and chipping due to impacts.

[0037] Corresponding to the annular boss 41, the second concave-convex structure 50 includes an annular groove 51. The annular groove 51 is arranged circumferentially along the second end face of the fuel pellet 30 and close to the periphery of the second end face, so that an annular end face located on the outer periphery of the annular groove 51 is left on the second end face of the fuel pellet 30. Because the annular groove 51 is close to the periphery of the second end face, the annular groove 51 has a lower temperature during service, and the area where the annular groove 51 is located has less deformation compared to the area where the central end face of the second end face is located, which is more conducive to maintaining the alignment and centering of the fuel pellet 30. The periphery of the annular end face is chamfered to prevent chipping and chipping due to impacts or other factors.

[0038] The multiple fuel pellets 30 of the pellet assembly are connected to each other by the interlocking of a first convex-concave structure 40 and a second convex-concave structure 50. Within the SiC casing 100, the fuel pellets 30 may have their first end face and first convex-concave structure 40 facing the first end plug 10, and their second end face and second convex-concave structure 50 facing the second end plug 20; or, the fuel pellets 30 may have their second end face and second convex-concave structure 50 facing the first end plug 10, and their first end face and first convex-concave structure 40 facing the second end plug 20.

[0039] The enrichment levels of the first fuel pellet 31 and the second fuel pellet 32 ​​can be the same or different. In embodiments with different enrichment levels, the first convex-concave structure 40 and the second convex-concave structure 50 help to mark the first fuel pellet 31 and the second fuel pellet 32 ​​with different enrichment levels, which helps to distinguish them.

[0040] refer to Figure 2 and Figure 5 To ensure the alignment and assembly of the fuel pellet assembly within the SiC cladding 100 and to prevent radial displacement of the fuel pellet assembly within the SiC cladding 100 that could cause eccentricity, a third concave-convex structure 60 is provided on the end of the second end plug 20 facing the fuel pellet assembly. The fuel pellet 30, which abuts against the second end plug 20, engages with the third concave-convex structure 60 through the first concave-convex structure 40 or the second concave-convex structure 50, thereby positioning the fuel pellet assembly on the second end plug 20 and making it coaxial with the axis of the second end plug 20 and the SiC cladding 100.

[0041] To accommodate the first concave-convex structure 40 or the second concave-convex structure 50, the third concave-convex structure 60 may include an annular protrusion or an annular groove.

[0042] Combination Figures 3-5 In an embodiment where the fuel pellet 30 faces the first end face and the first convex-concave structure 40 toward the first end plug 10, and faces the second end face and the second convex-concave structure 50 toward the second end plug 20, the third convex-concave structure 60 on the second end plug 20 is adapted to fit the second convex-concave structure 50. When the second convex-concave structure 50 includes an annular groove 51, the third convex-concave structure 60 includes an annular protrusion. The annular protrusion is embedded in the annular groove 51, thereby positioning the fuel pellet 30 on the second end plug 20, and thus positioning the entire pellet assembly on the second end plug 20. The annular protrusion of the third convex-concave structure 60 can be replaced by several spaced protrusions, which can also be embedded in the annular groove 51 to position the fuel pellet 30 on the second end plug 20. The protrusions can be columnar or arc-shaped in length.

[0043] In the embodiment where the fuel pellet 30 faces the first end plug 10 with its second end face and second convex-concave structure 50, and faces the second end plug 20 with its first end face and first convex-concave structure 40, the third convex-concave structure 60 on the second end plug 20 is adapted to fit the first convex-concave structure 40. When the first convex-concave structure 40 includes an annular boss 41, the third convex-concave structure 60 includes an annular groove, and the annular boss 41 is embedded in the annular groove, thereby positioning the fuel pellet 30 on the second end plug 20, and thus positioning the entire pellet assembly on the second end plug 20.

[0044] In some embodiments, reference Figures 6-7 The first uneven structure 40 includes a plurality of protrusions 42. Unlike annular protrusions arranged around the entire circumference of the first end face, the plurality of protrusions 42 are distributed dispersedly. Specifically, the plurality of protrusions 42 are spaced apart circumferentially along the first end face of the fuel pellet 30 and are close to the periphery of the first end face. The plurality of protrusions 42 may be two or more. The shape of the protrusions 42 is not limited; for example, in… Figure 6 In the illustrated embodiment, protrusion 42 is an arc-shaped protrusion in length; in Figure 7 In the embodiment shown, the protrusion 42 is a columnar protrusion, specifically a cylindrical protrusion.

[0045] Corresponding to the plurality of protrusions 42, the second concave-convex structure 50 may include an annular groove 51, such as Figure 4 As shown. The second concave-convex structure 50 may also include a plurality of grooves (not shown), which are distributed circumferentially along the second end face and close to the periphery of the second end face. The plurality of grooves are provided in a one-to-one correspondence with a plurality of protrusions 42, and each protrusion 42 is embedded in a corresponding groove.

[0046] In an embodiment where the fuel pellet 30 faces the second end plug 20 with a plurality of protrusions 42, a third concave-convex structure 60 on the second end plug 20 is adapted to fit the plurality of protrusions 42, including a plurality of grooves or annular grooves, specifically as provided with the second concave-convex structure 50. In an embodiment where the fuel pellet 30 faces the second end plug 20 with a plurality of grooves or annular grooves, the third concave-convex structure 60 on the second end plug 20 is adapted to fit the plurality of grooves or annular grooves, specifically including a plurality of protrusions.

[0047] In some embodiments, the fuel pellet 30 has a height of 5mm to 15mm and a diameter of 5mm to 12mm. The height or depth of the first convex-concave structure 40 is 0.5mm to 3mm (when the first convex-concave structure 40 includes annular protrusions or several protrusions, the corresponding height is considered). The height or depth of the second convex-concave structure 50 is 0.5mm to 3mm (when the second convex-concave structure 50 includes annular grooves or several grooves, the corresponding depth is considered).

[0048] When the height of the first convex-concave structure 40 is greater than the depth of the second convex-concave structure 50, the fuel pellets 30 contact each other through the annular protrusions 41 (several protrusions) of the first convex-concave structure 40 and the annular grooves (several grooves) of the second convex-concave structure 50. When the height of the first convex-concave structure 40 is less than the depth of the second convex-concave structure 50, the fuel pellets 30 contact each other through the shoulder (i.e., the annular end face) around the annular protrusions 41 (several protrusions) of the first convex-concave structure 40. When the height of the first convex-concave structure 40 is comparable to the depth of the second convex-concave structure 50, the fuel pellets 30 contact each other through the annular protrusions 41 (several protrusions) and the shoulder (i.e., the annular end face) around them.

[0049] The width of the second concave-convex structure 50 (annular groove / recess) is appropriately larger than the width of the first concave-convex structure 40 (annular boss 41 / protrusion) to accommodate manufacturing tolerances and facilitate the insertion of the annular boss 41 / protrusion into the annular groove / recess. In one embodiment, the width r1 of the annular boss 41 / protrusion is 0.5mm to 3mm, and the width r2 of the annular groove / recess is 0.53mm to 3.1mm, where r2 > r1. r2 - r1 can be equal to 0.03mm.

[0050] like Figure 8 As shown and combined Figure 1 and Figure 2 Other embodiments of the present invention include a silicon carbide-clad fuel element comprising a SiC (silicon carbide) cladding 100, a first end plug 10, a second end plug 20, a pellet assembly, and a spring (not shown).

[0051] The SiC cladding 100 is a tube open at both ends, having a first end and a second end. A first end plug 10 and a second end plug 20 are respectively fitted onto the first and second ends of the SiC cladding 100, sealing both ends and forming a closed cladding tube. Corresponding to the SiC cladding 100, the first end plug 10 and the second end plug 20 are also made of silicon carbide material (such as silicon carbide ceramic), specifically SiC end plugs.

[0052] The fuel pellet assembly is disposed inside the SiC cladding 100 and abuts against the second end plug 20. An annular gap 110 is provided between the fuel pellet assembly and the inner surface of the SiC cladding 100 to facilitate the loading of the fuel pellet assembly and to accommodate deformation of the fuel pellet assembly. The width of the gap between the fuel pellet assembly and the SiC cladding 100 (i.e., the width of the annular gap 110) can be 0.1 mm to 0.5 mm. A larger gap helps to avoid direct contact between the fuel pellet 30 and the SiC cladding 100 during service.

[0053] Silicon carbide-clad fuel elements Figure 2As shown, the SiC cladding 100 is placed vertically with its first end facing upwards, and the second end plug 20 is supported at the bottom of the chip assembly. A gap 120 is left between the chip assembly and the first end plug 10, and a spring is disposed within this gap 120, with the opposite ends of the spring abutting against the first end plug 10 and the chip assembly, respectively. Through the elastic extension of the spring and its abutment against the chip assembly, the chip assembly is pressed against the second end plug 20, thereby axially positioning it within the SiC cladding 100 and limiting the axial displacement of the chip assembly.

[0054] In one embodiment, the SiC cladding 100 has a two-layer structure, with the inner layer being a chemically vapor-infiltrated (CVI) dense SiC fiber-reinforced SiC ceramic matrix composite material, and the outer layer being a SiC ceramic coating prepared by chemical vapor deposition (CVD). The SiC cladding 100 can also have a three-layer structure, with the inner and outer layers being SiC ceramic coatings prepared by chemical vapor deposition (CVD), and the middle layer being a chemically vapor-infiltrated (CVI) dense SiC fiber-reinforced SiC ceramic matrix composite material. The wall thickness of the SiC cladding 100 is 0.7 mm to 1.3 mm.

[0055] The fuel pellet assembly includes a plurality of first fuel pellets 31 and a plurality of second fuel pellets 32, which are alternately stacked along the axial direction of the SiC cladding 100. The number of first fuel pellets 31 and second fuel pellets 32 is set according to the length of the SiC cladding 100 and its loading requirements. The fuel pellet assembly also includes a plurality of fuel pellets 30, which are sequentially stacked along the axial direction of the SiC cladding 100. The number of fuel pellets 30 is set according to the length of the SiC cladding 100 and its loading requirements.

[0056] The materials used to prepare the first fuel pellet 31 and the second fuel pellet 32 ​​are each oxides, nitrides, carbides or silicides containing uranium or plutonium, such as at least one of UO2, MOX, UN, UC and U3Si2.

[0057] Specifically, the first end face and the second end face of the first fuel pellet 31 are respectively provided with a first concave-convex structure 40, and the first end face and the second end face of the second fuel pellet 32 ​​are respectively provided with a second concave-convex structure 50 adapted to the first concave-convex structure 40. A plurality of first fuel pellets 31 and a plurality of second fuel pellets 32 are alternately stacked along the axial direction of the SiC cladding 100. The first concave-convex structure 40 of the first fuel pellet 31 and the second concave-convex structure 50 of the axially adjacent second fuel pellet 32 ​​are in concave-convex fit, so that the first fuel pellets 31 and the second fuel pellets 32 are aligned and connected to each other, ensuring that the first fuel pellets 31 and the second fuel pellets 32 are coaxial.

[0058] The first concave-convex structure 40 is arranged along the circumference of the first fuel pellet 31 on both the first and second end faces, such that the first and second end faces of the first fuel pellet 31 each have a central end face located inside (inner ring) of the first concave-convex structure 40. This central end face may preferably have a concave, dish-shaped curved surface, forming a dish-shaped end face. Similarly, on the first and second end faces of the second fuel pellet 32, the second concave-convex structure 50 is arranged along the circumference of the second fuel pellet 32, such that the first and second end faces of the second fuel pellet 32 ​​each have a central end face located inside (inner ring) of the second concave-convex structure 50. This central end face may preferably have a concave, dish-shaped curved surface, forming a dish-shaped end face.

[0059] The first uneven structure 40 on the first fuel pellet 31 can be referenced. Figure 3 The first protrusion-contact structure 40 on the fuel pellet 30 shown can include an annular boss, or refer to [other configurations]. Figure 6 and Figure 7 The diagram shows several protrusions. On the first end face of the first fuel pellet 31, an annular boss or several protrusions are arranged circumferentially along the first end face and close to its periphery; on the second end face of the first fuel pellet 31, an annular boss or several protrusions are arranged circumferentially along the second end face and close to its periphery. Because the annular boss or protrusions are arranged close to their peripheries on the first and second end faces respectively, the annular boss or protrusions have lower temperatures during service, and the area where the annular boss or protrusions are located has less deformation compared to the area where the middle end face is located, which is more conducive to maintaining alignment and centering between fuel pellets. The top periphery of the annular boss and the periphery of the annular end face located on the outer periphery of the annular boss or several protrusions are chamfered to prevent chipping and chipping due to impacts.

[0060] The second protrusion / contour structure 50 on the second fuel pellet 32 ​​can be referenced. Figure 4 The second protrusion-contour structure 50 on the fuel pellet 30 shown can include an annular groove; the second protrusion-contour structure 50 can also include several grooves corresponding to several protrusions. On the first end face of the second fuel pellet 32, the annular groove or several grooves are arranged circumferentially along the first end face and close to the periphery of the first end face; on the second end face of the second fuel pellet 32, the annular groove or several grooves are arranged circumferentially along the second end face and close to the periphery of the second end face. Since the annular groove or several grooves are arranged close to the periphery on the first end face and the second end face respectively, the annular groove or several grooves have a lower temperature during service, and the area where the annular groove or several grooves are located has less deformation compared to the area where the middle end face is located, which is more conducive to maintaining alignment and centering between fuel pellets.

[0061] When the first fuel pellet 31 and the second fuel pellet 32 ​​are stacked, the annular boss (or several protrusions) on the first fuel pellet 31 is embedded in the annular groove (or several grooves) of the second fuel pellet 32, so that the first fuel pellet 31 and the second fuel pellet 32 ​​are connected by a concave-convex fit.

[0062] Within the SiC cladding 100, the fuel pellet assembly faces the first end plug 10 with either the first fuel pellet 31 or the second fuel pellet 32. Depending on the number of fuel pellets, the fuel pellet assembly faces and abuts the second end plug 20 with either the second fuel pellet 32 ​​or the first fuel pellet 31.

[0063] refer to Figure 2 and Figure 5 To ensure the alignment and assembly of the fuel pellet assembly within the SiC cladding 100 and to prevent radial displacement of the fuel pellet assembly within the SiC cladding 100 that could cause eccentricity, a third concave-convex structure 60 is provided on the end of the second end plug 20 facing the fuel pellet assembly. This structure 60 is used to engage with the first concave-convex structure 40 of the first fuel pellet 31 or the second concave-convex structure 50 of the second fuel pellet 32, thereby positioning the fuel pellet assembly on the second end plug 20 and making it coaxial with the axis of the second end plug 20 and the SiC cladding 100.

[0064] In an embodiment where the first fuel pellet 31 and the first convex-concave structure 40 face and abut against the second end plug 20, the third convex-concave structure 60 on the second end plug 20 is adapted to fit the first convex-concave structure 40. When the first convex-concave structure 40 includes an annular boss or several protrusions, the third convex-concave structure 60 includes an annular groove, and the annular boss or several protrusions are embedded in the annular groove, so that the first fuel pellet 31 is positioned on the second end plug 20, and thus the entire pellet assembly is positioned on the second end plug 20.

[0065] In an embodiment where the fuel pellet assembly faces and abuts the second end plug 20 with the second fuel pellet 32 ​​and the second convex-concave structure 50, the third convex-concave structure 60 on the second end plug 20 is adapted to fit the second convex-concave structure 50. When the second convex-concave structure 50 includes an annular groove, the third convex-concave structure 60 includes an annular protrusion or several protrusions, which are embedded in the annular groove to position the second fuel pellet 32 ​​on the second end plug 20, thereby positioning the entire fuel pellet assembly on the second end plug 20. When the second convex-concave structure 50 includes several grooves, the third convex-concave structure 60 includes several protrusions, each protrusion being embedded in a corresponding groove to position the second fuel pellet 32 ​​on the second end plug 20.

[0066] In some embodiments, the height of the first fuel pellet 31 and the second fuel pellet are each 5mm to 15mm, and the diameter is 5mm to 12mm. The height or depth of the first convex-concave structure 40 is 0.5mm to 3mm (when the first convex-concave structure 40 includes annular protrusions or several protrusions, the corresponding height is considered). The height or depth of the second convex-concave structure 50 is 0.5mm to 3mm (when the second convex-concave structure 50 includes annular grooves or several grooves, the corresponding depth is considered).

[0067] When the height of the first convex-concave structure 40 is greater than the depth of the second convex-concave structure 50, the fuel pellets (first fuel pellet 31 and second fuel pellet 32) contact each other through the annular bosses (several protrusions) of the first convex-concave structure 40 and the annular grooves (several grooves) of the second convex-concave structure 50. When the height of the first convex-concave structure 40 is less than the depth of the second convex-concave structure 50, the fuel pellets contact each other through the shoulders (i.e., annular end faces) around the annular bosses (several protrusions) of the first convex-concave structure 40. When the height of the first convex-concave structure 40 is comparable to the depth of the second convex-concave structure 50, the fuel pellets 30 contact each other through the annular bosses (several protrusions) and their surrounding shoulders (i.e., annular end faces).

[0068] The width of the second concave-convex structure 50 (annular groove / recess) is appropriately larger than the width of the first concave-convex structure 40 (annular protrusion / raise) to accommodate manufacturing tolerances and facilitate the insertion of the annular boss / protrusion into the annular groove / recess. In one embodiment, the width r1 of the annular boss / protrusion is 0.5mm to 3mm, and the width r2 of the annular groove / recess is 0.53mm to 3.1mm, where r2 > r1. r2 - r1 can be equal to 0.03mm.

[0069] Understandably, the end of the first end plug 10 facing the pellet assembly may also be provided with a concave-convex structure, at least for the end that positions the spring. The first end plug 10 may also have the same structure as the second end plug 20 through the setting of the concave-convex structure, so that the first end plug 10 and the second end plug 20 are end plugs with the same structure. During the assembly of the fuel element, there is no need to distinguish the structures for assembly, which improves the convenience and efficiency of assembly.

[0070] Furthermore, in all the above embodiments of silicon carbide-clad fuel elements, at least one of the fuel pellet 30, the first fuel pellet 31, and the second fuel pellet 32 ​​may also be provided with a central through hole extending axially through the first end face and the second end face, so that the fuel pellet 30 (the first fuel pellet 31 or the second fuel pellet 32) forms a central hole pellet.

[0071] Furthermore, the silicon carbide-clad fuel element of the present invention may also include an inert gas filled within a sealed SiC cladding 100. The inert gas pressure within the SiC cladding 100 is 0.1 MPa to 2 MPa. The filling of the inert gas helps to improve inter-gap heat transfer.

[0072] In one embodiment, the sealed SiC cladding 100 is filled with helium or a helium-xenon mixture.

[0073] In one application, the silicon carbide-clad fuel element of the present invention is a silicon carbide-clad fuel rod.

[0074] In summary, in the silicon carbide-clad fuel element of the present invention, the fuel pellets are aligned and stacked by means of a concave-convex structure. Furthermore, the end plugs and adjacent fuel pellets are also aligned and positioned within the cladding by means of a concave-convex structure, thus preventing the fuel pellets from becoming eccentric within the cladding and reducing the risk of irradiation bending and fuel pellet-cladding mechanical interaction (PCMI) in the fuel element.

[0075] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A silicon carbide-clad fuel element, comprising a SiC cladding, a first end plug and a second end plug fitted at opposite ends of the SiC cladding, and a fuel pellet assembly disposed inside the SiC cladding and abutting the second end plug, wherein an annular gap is provided between the fuel pellet assembly and the inner surface of the SiC cladding, and the fuel pellet assembly comprises a plurality of fuel pellets stacked sequentially along the axial direction of the SiC cladding; characterized in that, Each fuel pellet has a first concave-convex structure on its first end face and a second concave-convex structure on its opposite second end face that is adapted to the first concave-convex structure. In the pellet assembly, the first concave-convex structure of one fuel pellet engages with the second concave-convex structure of another axially adjacent fuel pellet, so that the fuel pellets are aligned and connected to each other. The second end plug has a third concave-convex structure on one end facing the pellet assembly. The fuel pellet abutting against the second end plug is positioned on the second end plug and is coaxial with the second end plug and the SiC cladding axis by the first concave-convex structure or the second concave-convex structure engaging with the third concave-convex structure.

2. The silicon carbide-clad fuel element according to claim 1, characterized in that, The first concave-convex structure includes an annular boss disposed circumferentially along the first end face and close to the periphery of the first end face; the second concave-convex structure includes an annular groove disposed circumferentially along the second end face and close to the periphery of the second end face.

3. The silicon carbide-clad fuel element according to claim 1, characterized in that, The first concave-convex structure includes a plurality of protrusions, which are distributed at intervals along the circumference of the first end face and close to the periphery of the first end face. The second concave-convex structure includes a plurality of grooves, which are distributed circumferentially along the second end face and close to the periphery of the second end face; or, the second concave-convex structure includes an annular groove, which is arranged circumferentially along the second end face and close to the periphery of the second end face.

4. The silicon carbide-clad fuel element according to claim 1, characterized in that, The third concave-convex structure includes an annular protrusion, an annular groove, a plurality of protrusions, or a plurality of grooves.

5. The silicon carbide-clad fuel element according to any one of claims 1-4, characterized in that, The first and second end faces of the fuel pellet each have a central end face located inside the first and second concave-convex structures, respectively; the central end face has a concave dish-shaped curved surface, thus forming a dish-shaped end face; and / or, The fuel pellet has a central through hole that extends axially through the first and second end faces.

6. The silicon carbide-clad fuel element according to any one of claims 1-4, characterized in that, The width of the annular gap between the chip assembly and the inner surface of the SiC cladding is 0.1 mm to 0.5 mm.

7. The silicon carbide-clad fuel element according to any one of claims 1-4, characterized in that, The fuel pellet has a height of 5mm to 15mm and a diameter of 5mm to 12mm; the height or depth of the first concave-convex structure is 0.5mm to 3mm, and the height or depth of the second concave-convex structure is 0.5mm to 3mm.

8. The silicon carbide-clad fuel element according to any one of claims 1-4, characterized in that, The silicon carbide-clad fuel element further includes a spring; a gap exists between the fuel cell assembly and the first end plug, and the spring is disposed within the gap, with its opposite ends abutting against the fuel cell assembly and the first end plug, respectively; and / or The silicon carbide-clad fuel element also includes an inert gas filled within the SiC cladding, wherein the pressure of the inert gas within the SiC cladding is 0.1 MPa to 2 MPa.

9. A silicon carbide-clad fuel element, comprising a SiC cladding, a first end plug and a second end plug fitted at opposite ends of the SiC cladding, and a die assembly disposed inside the SiC cladding and abutting the second end plug, wherein an annular gap is provided between the die assembly and the inner surface of the SiC cladding; characterized in that, The fuel cell assembly includes a plurality of first fuel cells and a plurality of second fuel cells. The first fuel cells have a first concave-convex structure on their opposite first end face and second end face, and the second fuel cells have a second concave-convex structure on their opposite first end face and second end face, respectively, that matches the first concave-convex structure. The plurality of first fuel cells and the plurality of second fuel cells are alternately stacked along the axial direction of the SiC cladding. The first concave-convex structure of the first fuel cells engages with the second concave-convex structure of the axially adjacent second fuel cells, so that the first fuel cells and the second fuel cells are aligned and connected to each other. The second end plug has a third concave-convex structure on one end facing the pellet assembly. The first fuel pellet or the second fuel pellet abutting against the second end plug engages with the third concave-convex structure through the first concave-convex structure or the second concave-convex structure, so that the pellet assembly is positioned on the second end plug and is coaxial with the second end plug and the SiC cladding axis.

10. The silicon carbide-clad fuel element according to claim 9, characterized in that, The first concave-convex structure includes an annular boss; on the first end face of the first fuel pellet, the annular boss is arranged circumferentially along the first end face and close to the periphery of the first end face; on the second end face of the first fuel pellet, the annular boss is arranged circumferentially along the second end face and close to the periphery of the second end face. The second concave-convex structure includes an annular groove; on the first end face of the second fuel pellet, the annular boss is arranged circumferentially along the first end face and close to the periphery of the first end face; on the second end face of the second fuel pellet, the annular groove is arranged circumferentially along the second end face and close to the periphery of the second end face.

11. The silicon carbide-clad fuel element according to claim 9, characterized in that, The first concave-convex structure includes a plurality of protrusions; on the first end face of the first fuel pellet, the plurality of protrusions are distributed circumferentially at intervals along the first end face and close to the periphery of the first end face; on the second end face of the first fuel pellet, the plurality of protrusions are distributed circumferentially at intervals along the second end face and close to the periphery of the first end face. The second concave-convex structure includes a plurality of grooves or annular grooves; on the first end face of the second fuel pellet, the plurality of grooves or annular grooves are arranged circumferentially along the first end face and close to the periphery of the first end face; on the second end face of the second fuel pellet, the plurality of grooves or annular grooves are arranged circumferentially along the second end face and close to the periphery of the second end face.

12. The silicon carbide-clad fuel element according to claim 9, characterized in that, The third concave-convex structure includes an annular protrusion, an annular groove, a plurality of protrusions, or a plurality of grooves.

13. The silicon carbide-clad fuel element according to any one of claims 9-12, characterized in that, The first end face and the second end face of the first fuel pellet each have a middle end face located inside the first concave-convex structure; the first end face and the second end face of the second fuel pellet each have a middle end face located inside the second concave-convex structure; the middle end face is provided with a concave dish-shaped curved surface, so that the middle end face forms a dish-shaped end face; and / or, The first fuel pellet has a first central through hole that extends axially through its first end face and second end face, and the second fuel pellet has a second central through hole that extends axially through its first end face and second end face.

14. The silicon carbide-clad fuel element according to any one of claims 9-12, characterized in that, The width of the annular gap between the chip assembly and the inner surface of the SiC cladding is 0.1 mm to 0.5 mm.

15. The silicon carbide-clad fuel element according to any one of claims 9-12, characterized in that, The heights of the first fuel pellet and the second fuel pellet are 5mm to 15mm and the diameters are 5mm to 12mm, respectively; the height or depth of the first convex-concave structure is 0.5mm to 3mm, and the height or depth of the second convex-concave structure is 0.5mm to 3mm.

16. The silicon carbide-clad fuel element according to any one of claims 9-12, characterized in that, The silicon carbide-clad fuel element further includes a spring; a gap exists between the fuel cell assembly and the first end plug, and the spring is disposed within the gap, with its opposite ends abutting against the fuel cell assembly and the first end plug, respectively; and / or The silicon carbide-clad fuel element also includes an inert gas filled within the SiC cladding, wherein the pressure of the inert gas within the SiC cladding is 0.1 MPa to 2 MPa.

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