A nuclear reactor fuel unit and assembly

Abstract

The present application belongs to the technical field of nuclear reactors, and particularly relates to a nuclear reactor fuel unit and assembly. The fuel unit is composed of TRISO fuel particles, a metal-coated mesh, a structural grid and a stirring grid. The TRISO fuel particles are stacked in single or multiple columns and placed in the metal-coated mesh or the perforated metal cladding. The mesh hole of the metal-coated mesh or the hole diameter of the perforated metal cladding is slightly smaller than the outer diameter of the TRISO fuel particles, so as to ensure that the fuel particles are always contained in the metal mesh. The metal-coated mesh and the internal fuel particles are fixed on the structural grid in a certain shape (such as spiral, rod, plate, grid plate, ring, porous, etc.), thereby forming the fuel unit. The stirring grid is arranged at a specific position of the structural grid of the fuel unit, so as to enhance the flow and stirring of the coolant. Multiple groups of fuel units are arranged in a certain rule, and a coolant flow channel is reserved between each area, thereby forming the fuel assembly.

Description

Technical Field

[0001] This invention belongs to the field of nuclear reactor technology, specifically relating to a nuclear reactor fuel unit and assembly. Background Technology

[0002] Traditional pressurized water reactor (PWR) nuclear power plants consist of a reactor core composed of varying numbers of fuel assemblies. Each fuel assembly comprises a different number of slender cylindrical fuel rods, guide tubes, positioning grids, and tube seats. Each fuel rod consists of nuclear fuel pellets stacked within fuel cladding tubes, which are mostly made of zirconium alloy. An air gap exists between the fuel pellets and the cladding tubes to contain fissile gases. The upper and lower parts of the fuel cladding tubes are sealed with end plugs. During normal operation of the nuclear reactor, the heat released by the fuel pellets must be transferred to the cladding through the air gap, thereby heating the coolant. The low thermal conductivity and high thermal resistance of the gas in the air gap affect the heat transfer effect of the fuel pellets, resulting in high fuel pellet temperatures. Under accident conditions, the reactor core temperature may rise significantly. At temperatures above 800°C, the zirconium alloy cladding undergoes a zirconium-water reaction, generating hydrogen gas, which may cause damage to the fuel cladding, leading to the release of radioactive materials from the fuel rods and seriously threatening the safety of the nuclear reactor.

[0003] Similarly, metal-cooled fast reactors, gas-cooled reactors, and small nuclear power reactors mostly use slender cylindrical fuel rods. Multiple fuel rods are combined according to rules to form fuel assemblies and reactor cores. The fuel rods are usually encased in a metal cladding to conduct heat and contain radioactive materials. However, because there is also an air gap between the fuel cladding and the fuel pellets, and the pre-filled gas has a low density and low thermal conductivity, the heat transfer resistance is high, which affects the heat transfer effect from the fuel pellets to the outside of the fuel cladding.

[0004] Currently, SiC fuel cladding materials are being developed. Their advantages include high thermal conductivity, resistance to high-temperature creep, corrosion resistance, and good radiation stability. However, their disadvantages include low strength in slender tubular SiC ceramic materials, making it difficult to withstand the thermal stress cycles and fuel-cladding mechanical interactions during pressurized water reactor operation; poor machinability, making it difficult to use existing conventional technologies for forming, cutting, and welding like metal materials; and the welding and sealing problems between SiC ceramic materials and structures such as metal end plugs have not yet been fully resolved.

[0005] Besides the traditional UO2 fuel pellets and cladding tubes used in fuel element rods, TRISO fuel particles have been widely used in high-temperature reactors. Their structure, from the inside out, consists of a core containing fissile material, a buffer layer of loosely pyrolytic carbon (to contain fission products), a high-density inner pyrolytic carbon layer, a SiC layer, and a high-density outer pyrolytic carbon layer. This multi-layered cladding design provides excellent fission product containment. Because the particles are spherical with relatively small diameters and high strength, and because SiC is heat-resistant, these fuel particles can maintain the integrity of the cladding and fuel for extended periods at 1600°C, preventing the release of fission products into the environment. In existing reactors, a large number of TRISO fuel particles are dispersed in graphite, SiC, or other matrices, leading to problems such as uneven particle distribution, variations in isotropic / anisotropic properties, and processing difficulties. Furthermore, the dispersed matrix increases fuel volume and weight, increases thermal resistance, and results in low uranium density (low power density).

[0006] Based on the aforementioned existing technologies, traditional fuel assemblies mostly employ slender cylindrical metal cladding to enclose UO2 fuel pellets. Due to the low heat transfer coefficient and small effective heat transfer area between the cladding and the fuel pellets, the fuel pellet temperature is high, and the core power density is limited. TRISO fuel particles possess inherent safety, inherently possessing all the functions of nuclear fuel, and have advantages such as gapless heat conduction, large effective heat transfer area, high temperature resistance, corrosion resistance, and good radiation stability, making them a novel core fuel with broad application potential. This invention redesigns the nuclear reactor fuel unit and core, proposing a metal mesh reactor fuel unit and core with partitioned enclosing of TRISO fuel particles. This design is small in size and weight, has good heat transfer performance, completely avoids zirconium-water reaction, and allows for changes in the shape and arrangement of the fuel unit and core based on different reactor types. The core structure is flexible, high-strength, and easy to process, showing great application potential. Summary of the Invention

[0007] To address the above issues, the present invention aims to provide a nuclear reactor fuel unit and assembly. The fuel unit comprises TRISO fuel particles, a metal-clad mesh, a structural grid, and a mixing grid. The TRISO fuel particles are stacked in single or multiple rows within the metal-clad mesh or a perforated metal shell. The mesh opening diameter of the metal-clad mesh or the opening diameter of the perforated metal shell is slightly smaller than the outer diameter of the TRISO fuel particles to ensure that the fuel particles are always contained within the metal mesh. The metal-clad mesh and the internal fuel particles are fixed to the structural grid in a specific shape (e.g., spiral, rod, plate, grid plate, ring, porous, etc.) to form the fuel unit. A mixing grid is installed at specific locations within the structural grid of the fuel unit to enhance coolant flow and mixing. Multiple fuel units are arranged in zones according to certain rules, with coolant flow channels reserved between zones, forming a fuel assembly. Depending on the reactor design, multiple fuel assemblies are arranged and combined in different ways to form the reactor core. Among them, guide tubes are set in specific fuel assemblies to position, guide and place functional components such as control rods, combustible poisons and neutron sources; for small reactors, a control drum area can also be reserved outside the reactor core to achieve reactivity control.

[0008] The technical solution of the present invention is as follows:

[0009] A nuclear reactor fuel unit includes TRISO fuel pellets, cladding, a positioning grid, and mixing blades. The cladding is a metal mesh or a perforated metal cladding, which forms a cladding channel. The TRISO fuel pellets are stacked in single or multiple rows within the cladding channel. The mesh diameter of the metal mesh is slightly smaller than the outer diameter of the TRISO fuel pellets. The metal mesh cladding channel covering the TRISO fuel pellets is fixed to the positioning grid, forming the fuel unit. Mixing blades are provided on the positioning grid.

[0010] The TRISO fuel pellets are used to generate heat through nuclear fission reactions. The internal fuel can be UO2 or UN, and it adopts a SiC cladding structure. The diameter of the TRISO fuel pellets is 1-5 mm.

[0011] The metal mesh is divided into many independent small units, each containing a certain amount of TRISO granules.

[0012] The mixing blades are in the form of metal sheets bent at a certain angle or cross-shaped mixing blades.

[0013] The fuel unit structure can be spiral, rod-shaped, plate-shaped, grid plate-shaped, ring-shaped, or porous, and the appropriate arrangement of fuel units can be selected according to the needs of the reactor type.

[0014] A fuel assembly comprises multiple fuel units arranged according to certain rules to form a reactor fuel assembly. The arrangement forms include: spiral fuel units arranged in sequence or interleaved, forming different assembly forms such as rectangle, circle, ring or hexagon, with coolant flow channels reserved between the fuel units.

[0015] The plate-shaped and grid-shaped fuel units are arranged in sequence, with a certain gap reserved between the fuel units for coolant to flow through. The coolant can flow directly through the interior of the metal mesh of the fuel unit.

[0016] The annular fuel units are arranged coaxially, with coolant channels reserved between different annular fuel units.

[0017] The fuel unit is arranged in a porous form with fuel particles dispersed within the porous area enclosed by a metal mesh.

[0018] The beneficial effects of this invention are as follows:

[0019] (1) Compared with the SiC cladding tube, the present invention uses TRISO fuel particles with SiC cladding directly as nuclear fuel, which retains the advantages of SiC cladding such as high temperature resistance, corrosion resistance and good radiation stability. It does not require additional cutting, welding, sealing, etc., and overcomes the difficulties of SiC cladding tube processing and the adverse effects of air gap, irradiation and interaction between the core and SiC cladding tube under high temperature conditions.

[0020] (2) Compared with TRISO particles dispersed into cores of graphite, ceramic or high-temperature metal, the present invention uses TRISO fuel particles with SiC cladding directly as nuclear fuel. The fuel is small in volume, light in weight, and has high power density. It eliminates the problems of uniformity of dispersed particles, isotropic / anisotropic variation, and low uranium density (low power density), and is easy to process.

[0021] (3) In order to solve the technical problems that may occur under the accident conditions of traditional pressurized water reactors, such as zirconium water reaction and release of radioactive materials due to fuel cladding damage, the present invention adopts the method of direct contact heat exchange between TRISO fuel particles and coolant. Its SiC cladding will not undergo zirconium water reaction, and can maintain the integrity of cladding and fuel at a high temperature of 1600℃, effectively preventing the release of fission products into the environment and greatly improving reactor safety.

[0022] (4) Design a metal mesh or perforated metal shell-type covering structure. TRISO fuel particles are stacked and arranged in the metal mesh to form fuel units. At the same time, coolant channels are reserved between the metal meshes so that the coolant can flow directly through the surface of the fuel element, which significantly increases the direct contact area between the coolant and the fuel, eliminates the influence of the air gap thermal resistance of the traditional rod-shaped fuel element, and significantly enhances the heat transfer effect.

[0023] (5) The metal mesh or perforated metal cladding structure allows coolant to flow. The metal mesh or perforated metal cladding itself does not bear the internal and external pressure and pressure difference of the fuel unit, but only bears the impact of external coolant flow. It has high strength, is easy to process, and is suitable for different reactor types and different operating pressures.

[0024] (6) Different types of reactors require fuel assemblies with specific structures and shapes. The metal mesh or perforated metal cladding structure of the fuel unit in this invention can be designed into different shapes as needed, including rod-shaped, plate-shaped, grid plate-shaped, ring-shaped, porous, etc., which are suitable for modular production, flexible in structure, and applicable to different types of reactor cores.

[0025] (7) Different numbers of fuel units are arranged in a partitioned manner to form the reactor core. On the one hand, regular fuel units and coolant channels can be constructed, and on the other hand, accidents caused by local damage to the metal mesh, which are extremely rare, can be prevented, such as large-scale fuel particle leakage and blockage of the flow channel.

[0026] (8) The fuel unit and core structure invented can be applied to different types of reactors such as pressurized water reactors, boiling water reactors, gas-cooled fast reactors, and metal-cooled fast reactors, and have a wide range of applications. Attached Figure Description

[0027] Figure 1 This is a partial structural diagram of the single-group metal mesh fuel unit described in this invention;

[0028] Figure 2 This is a schematic diagram of the single-group metal mesh fuel unit structure described in this invention;

[0029] Figure 3 This is a schematic diagram of the reactor core configuration consisting of multiple sets of metal mesh fuel units as described in this invention.

[0030] Figure 4 This is a partial structural diagram of the single-group perforated metal-clad fuel unit described in this invention;

[0031] Figure 5 This is a schematic diagram of the single-group perforated metal-clad fuel unit structure described in this invention;

[0032] Figure 6 This is a schematic diagram of the reactor core configuration consisting of multiple sets of perforated metal cladding fuel units as described in this invention.

[0033] Figure 7 This is a schematic diagram of the core structure described in this invention;

[0034] Figure 8 This is a schematic diagram of the grid plate-shaped core structure described in this invention;

[0035] Figure 9This is a schematic diagram of the annular core structure described in this invention.

[0036] In the diagram: 1. TRISO fuel pellets; 2. Metal mesh; 3. Positioning grid; 4. Mixing vanes; 5. Fuel unit; 6. Reactor fuel assembly. Detailed Implementation

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0038] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] Example 1:

[0040] A nuclear reactor fuel unit includes TRISO fuel pellets 1, a metal mesh 2, a positioning grid 3, and mixing blades 4. The metal mesh 2 forms a covering channel, and the TRISO fuel pellets 1 are stacked in single or multiple rows within the covering channel. The mesh diameter of the metal mesh 2 is slightly smaller than the outer diameter of the TRISO fuel pellets 1. The covering channel of the metal mesh 2 covering the TRISO fuel pellets 1 is fixed on the positioning grid 3 to form a fuel unit 5, and mixing blades 4 are provided on the positioning grid.

[0041] TRISO fuel pellets 1 are used to initiate nuclear fission reactions and generate heat. The internal fuel can be UO2, UN, etc., with a fuel enrichment of up to 9%. They adopt a SiC cladding structure to contain radioactive fission products. The diameter of TRISO fuel pellets is 1-5 mm.

[0042] The metal mesh 2 can be divided into many independent small units, each of which contains a certain amount of TRISO pellets 1. This can prevent a large amount of fuel particles from leaking out due to local damage to the metal mesh in the extremely low probability. Even if a small amount of fuel particles leak out, it can be detected in time, and there will be no large-scale fuel particle leakage or blockage of the flow channel.

[0043] Mixing vanes 4 are installed on the positioning grid to enhance the mixing of coolant flowing over the surface of TRISO fuel pellets 1, thereby strengthening heat transfer. Optional forms of mixing vanes include: metal sheets bent at a certain angle, cross-type mixing vanes, etc.

[0044] The structural shapes of fuel unit 5 include spiral, rod, plate, grid plate, ring, and porous shapes, and the appropriate arrangement of fuel unit 5 can be selected according to the needs of the reactor type.

[0045] Multiple fuel units 5 are arranged according to certain rules to form reactor fuel assemblies 6. The arrangement forms include: spiral fuel units arranged in sequence or interlocked to form different assembly forms such as rectangles, circles, rings, and hexagons, with coolant flow channels reserved between fuel units; plate-shaped and grid-shaped fuel units are arranged in sequence, with certain gaps reserved between fuel units for coolant to flow through, and coolant can also flow directly through the interior of the metal mesh of the fuel unit; ring-shaped fuel units are arranged coaxially, with coolant channels reserved between different ring-shaped fuel units; in addition, fuel units can also be arranged in a porous form in sections, with fuel particles dispersed inside the porous area surrounded by the metal mesh.

[0046] Multiple sets of guide tubes are installed at specific fuel assembly locations in the reactor core for the positioning, guidance, and placement of functional components such as control rods, combustible poisons, and neutron sources. The number of guide tubes can be designed according to different fuel types, enrichment levels, and arrangement methods to achieve reactor control. For small reactors, a control drum or other reactivity control device operating structure can be reserved outside the core composed of fuel assemblies for reactivity control.

[0047] Example 2:

[0048] A nuclear reactor fuel unit includes TRISO fuel pellets 1, fuel cladding, positioning grid 3, and mixing blades 4. The fuel cladding has a perforated mesh structure and forms a covering channel. The TRISO fuel pellets 1 are stacked in single or multiple rows within the covering channel. The mesh diameter of the metal mesh 2 is slightly smaller than the outer diameter of the TRISO fuel pellets 1. The metal mesh 2 covering the TRISO fuel pellets 1 and the covering channel are fixed on the positioning grid 3 to form a fuel unit 5. The mixing blades 4 are provided on the positioning grid.

[0049] TRISO fuel pellets 1 are used to initiate nuclear fission reactions and generate heat. The internal fuel can be UO2, UN, etc., and it adopts a SiC shell structure to contain radioactive fission products. The diameter of TRISO fuel pellets is 1-5mm.

[0050] The perforated structure of the metal mesh cladding can be divided into many independent small units, each containing a certain amount of TRISO pellets. This can prevent a large amount of fuel particles from leaking out due to local damage to the metal mesh in the extremely low probability. Even if a small amount of fuel particles leaks out, it can be detected in time, preventing large-scale fuel particle leakage and blockage of the flow channel.

[0051] Mixing vanes 4 are installed on the positioning grid to enhance the mixing of coolant flowing over the surface of TRISO fuel pellets 1, thereby strengthening heat transfer. Optional forms of mixing vanes include: metal sheets bent at a certain angle, cross-type mixing vanes, etc.

[0052] The structural shapes of fuel unit 5 include spiral, rod, plate, grid plate, ring, and porous shapes, and the appropriate arrangement of fuel unit 5 can be selected according to the needs of the reactor type.

[0053] Multiple fuel units 5 are arranged according to certain rules to form reactor fuel assemblies 6. The arrangement forms include: spiral fuel units arranged in sequence or interlocked to form different assembly forms such as rectangles, circles, rings, and hexagons, with coolant flow channels reserved between fuel units; plate-shaped and grid-shaped fuel units are arranged in sequence, with certain gaps reserved between fuel units for coolant to flow through, and coolant can also flow directly through the interior of the metal mesh of the fuel unit; ring-shaped fuel units are arranged coaxially, with coolant channels reserved between different ring-shaped fuel units; in addition, fuel units can also be arranged in a porous form in sections, with fuel particles dispersed inside the porous area surrounded by the metal mesh.

[0054] Multiple sets of guide tubes are installed at specific fuel assembly locations in the reactor core for the positioning, guidance, and placement of functional components such as control rods, combustible poisons, and neutron sources. The number of guide tubes can be designed according to different fuel types, enrichment levels, and arrangement methods to achieve reactor control. For small reactors, a control drum or other reactivity control device operating structure can be reserved outside the core composed of fuel assemblies for reactivity control.

[0055] It should be noted that in the description of this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means at least two.

[0056] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0057] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

[0058] The accompanying drawings of the embodiments disclosed in this invention only involve the methods involved in the embodiments of this disclosure. Other methods can refer to the general design. In the absence of conflict, the same embodiment and different embodiments of this invention can be combined with each other.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A nuclear reactor fuel element comprising TRISO fuel particle spheres, a cladding, a spacer grid, a mixing vane, characterized in that: The cladding is a metal mesh or a perforated metal cladding, which forms a cladding channel, the TRISO fuel particle spheres are arranged in single or multiple columns in the cladding channel, the mesh hole diameter of the metal mesh is slightly smaller than the outer diameter of the TRISO fuel particle, the metal mesh cladding channel cladding the TRISO fuel particle spheres is fixed on the positioning grid to form a fuel unit, and a stirring vane is arranged on the positioning grid.

2. A nuclear reactor fuel unit according to claim 1, characterized in that: The TRISO fuel particle spheres are used for nuclear fission reaction and heat generation, the internal fuel is selected from UO2 and UN, and a SiC cladding structure is adopted, and the diameter of the TRISO fuel particle spheres is 1-5 mm.

3. A nuclear reactor fuel element according to claim 1, wherein: The metal mesh is divided into many independent small units, and each unit is internally provided with a certain amount of TRISO particle spheres.

4. A nuclear reactor fuel element according to claim 1, wherein: The stirring vane is in the form of a bent metal sheet with a certain angle or a cross-type stirring vane.

5. A nuclear reactor fuel element according to claim 1, wherein: The fuel unit structure is in the form of a spiral, a rod, a plate, a grid plate, a ring or a porous body, and a suitable arrangement form of the fuel unit is selected according to the needs of the reactor type.

6. A fuel assembly using the fuel assembly of claim 1, characterized in that: A plurality of groups of fuel units are arranged according to certain rules to form a reactor fuel assembly, and the arrangement forms include: spiral fuel units arranged in order and in a plug-in manner to form a rectangular, circular, annular or hexagonal assembly form, and a coolant flow channel is reserved between the fuel units.

7. The fuel assembly of claim 6, wherein: The plate-shaped and grid plate-shaped fuel units are arranged in order, a certain gap is reserved between the fuel units for the coolant to flow through, and the coolant directly flows through the interior of the metal mesh of the fuel unit.

8. The fuel assembly of claim 6, wherein: The fuel unit is in the form of a porous body and is arranged in a partitioned manner, and the fuel particles are dispersed in the interior of the porous region surrounded by the metal mesh.

Images