A method of processing a finned microchannel plate-shaped fuel element

Abstract

This invention belongs to the field of nuclear fuel technology, specifically relating to a method for processing a finned microchannel plate-shaped fuel element. The processed plate-shaped fuel element employs dispersed TRISO-coated fuel particle technology. By strategically depositing TRISO-coated fuel particles in a high-temperature resistant, thermally conductive dispersed heat transfer agent and welding airfoil fins onto the fuel plate, the power distribution of the plate-shaped fuel element is flattened, the overall heat exchange area is increased, and the structural strength and safety of the fuel element are enhanced.

Description

Technical Field

[0001] This invention belongs to the field of nuclear fuel technology, specifically relating to a method for processing a finned microchannel plate-shaped fuel element. Background Technology

[0002] Nuclear fuel elements are among the most important components of a reactor core, serving as the core source of power for the reactor. Plate-shaped fuel elements have advantages such as high specific power and large heat transfer area, and are widely used in research reactors.

[0003] Early research reactors primarily used U-Al metallic plate fuel elements and UO2 ceramic plate fuel elements. Metallic plate fuel elements are easy to process and have good thermal conductivity, but they generally use highly enriched uranium fuel. Internationally, to reduce the risk of nuclear proliferation, research reactors were required to gradually phase out highly enriched uranium fuel and use low-enriched uranium fuel elements, leading to the development of UO2 ceramic plate fuel elements. Ceramic fuel has good thermal stability, a high melting point, good irradiation stability, and high compatibility with coolants. However, its disadvantages include low density, hardness and brittleness, difficulty in processing, low thermal conductivity, large temperature differences in irradiated pellets, and the risk of irradiation swelling and pellet cracking. Therefore, researchers subsequently proposed U3Si2-Al and U-Mo dispersed fuel elements. U3Si2-Al dispersed fuel elements have high uranium density, can use low-enriched uranium fuel, and have good radiation resistance, corrosion resistance, and thermal conductivity; however, their reprocessing is difficult and causes severe environmental pollution. U-Mo dispersed fuels have good anisotropic properties, but they are difficult to process and the manufacturing methods are not yet mature, so they cannot be used on a large scale at present.

[0004] As research reactors gradually move towards higher neutron flux and higher burnup, higher requirements are placed on the safety, heat transfer performance, and other characteristics of plate fuel elements. Summary of the Invention

[0005] To address the aforementioned shortcomings, the present invention aims to provide a method for processing a finned microchannel plate-shaped fuel element. The processed plate-shaped fuel element employs dispersed TRISO-coated fuel particle technology. By depositing TRISO-coated fuel particles in a dispersed heat-conducting agent with high temperature resistance and good thermal conductivity, and welding airfoil fins onto the fuel plate, the power distribution of the plate-shaped fuel element is flattened, the overall heat exchange area is increased, and the structural strength and safety of the fuel element are enhanced.

[0006] The technical solution of the present invention is as follows:

[0007] A method for fabricating a finned microchannel plate-shaped fuel element includes nine steps:

[0008] Step 1: Mix the dispersed heat conductor, material and combustion aid, and spray the mixed dispersed heat conductor powder evenly into the fuel green mold using a spraying equipment. Then, compact it tightly with a flatbed press or roller equipment to form the first layer of the fuel green.

[0009] Step 2: Use a deposition equipment to deposit TRISO-coated fuel particles onto the first layer of the fuel green. The deposition equipment is equipped with a positioning chuck, which allows the TRISO-coated fuel particles to be positioned on the dispersed thermal conductive agent to form the second layer of the fuel green.

[0010] Step 3: Repeat step 1, mix the dispersed heat-conducting powder and the combustion aid powder, and spray them onto the surface of TRISO fuel particles, so that the mixed powder fills the gaps between each TRISO fuel particle, forming the third layer of the fuel green.

[0011] Step 4, static pressure forming: the mold is placed under a certain pressure to form the fuel green blank;

[0012] Step 5, sintering: The formed fuel green blank is placed in a sintering furnace for sintering, so that the dispersed heat-conducting agent and TRISO fuel particles are integrated.

[0013] Step 6, Surface treatment: After sintering is completed, the sintered sample is surface treated, including surface inspection, grinding, polishing, etc.; the sintered sample ensures that the TRISO fuel particles are not exposed outside the dispersed heat-conducting agent, and the sample size is smaller than the alloy frame size to facilitate sample placement in the frame.

[0014] Step 7: Place the sample into the alloy frame after surface treatment;

[0015] Step 8: Chemical etching. A 1.5mm thick alloy plate is chemically etched to obtain fins and coolant channels.

[0016] Step 9: Welding. The fins are welded to the alloy frame using vacuum brazing: Brazing material is applied to the surface of the alloy frame. Under vacuum and high temperature, the brazing material melts and welds the alloy frame to the base plate of the fins. The above process is repeated multiple times to stack and process the core.

[0017] In step 1, the thickness of the first layer of fuel green is maintained between 0.5 mm and 1 mm.

[0018] In step 2, the specific distribution density and uniformity of TRISO-coated fuel particles on the dispersed heat-conducting agent are adjusted after calculations based on core physics and thermal analysis.

[0019] In step 3, the thickness of the third layer of the fuel green is maintained between 0.5 mm and 1 mm.

[0020] In step 5, the sintering process, a pressureless environment is selected. First, the sintering furnace is evacuated. After the vacuum level reaches the required level, the sintering furnace is gradually heated. After the fuel green billet is stably sintered at the sintering temperature for a period of time, the sintering furnace is filled with inert gas. Then, the fuel green billet is kept warm and cooled with the furnace, and the sintering is completed.

[0021] In step 8, the fin cross-section is airfoil-shaped, and the gaps between the fins form coolant microchannels. When the coolant flows through the microchannels, the heat generated by the fuel plate is carried away by the coolant through heat conduction, convection, and other means.

[0022] The fin distribution density and uniformity can be changed according to the power distribution of the fuel plate. The fin density is increased in local areas with high lateral power of the fuel plate and decreased in areas with low lateral power to enhance heat exchange efficiency and improve the overall heat transfer effect of the fins. Furthermore, the fin distribution density in the longitudinal height of the core can also be adjusted according to the core power distribution to maximize the overall output power of the core.

[0023] The fins are continuous semi-circular wavy, rectangular wavy, or trapezoidal wavy.

[0024] In step 9, when the plate fuel elements are stacked into a core, two plate fuel elements are connected by vacuum brazing. The upper surface of the fins of one plate fuel element is coated with brazing material and placed in a vacuum high-temperature environment. The brazing material melts and welds the heat transfer fins to the bottom of the alloy frame of the other plate fuel element.

[0025] The fuel green blank is rectangular in shape and consists of a diffused thermal conductive agent and TRISO fuel particles. The TRISO fuel particles are diffused in the diffused thermal conductive agent, and the height of the diffused thermal conductive agent is greater than the diameter of the TRISO fuel particles to ensure that the TRISO fuel particles are not exposed outside the diffused thermal conductive agent.

[0026] The beneficial effects of this invention are as follows:

[0027] Using TRISO pellet fuel and filling it with a dispersed heat-conducting agent in plate-type fuel elements can reduce the risk of irradiation swelling, breakage, and leakage of fission products that occur in other plate-type fuel elements, thereby improving the integrity and safety of plate-type fuel elements during operation.

[0028] By positioning and depositing TRISO fuel particles, not only can the risk of exposed TRISO fuel particles and high stress in particle contact be reduced in other dispersed fuels, but the distribution density of TRISO particles in the dispersed heat transfer agent can also be changed to make the lateral power distribution of the single-layer fuel plate more uniform, which can effectively improve the overall output power of the reactor.

[0029] Adding fins to plate-shaped fuel elements serves multiple purposes. The fins act as structural components, providing support for the fuel plates and enhancing the strength and rigidity of the fuel elements. They also function as turbulence and heat-conducting components in the coolant microchannels, further improving heat transfer and increasing the overall heat exchange area and efficiency. Furthermore, by varying the fin density on the plate-shaped fuel elements, the temperature distribution throughout the reactor can be made more uniform, reducing thermal stress on internal reactor components and extending material life. Attached Figure Description

[0030] Figure 1 A schematic diagram of a finned plate-shaped fuel element;

[0031] Figure 2 for Figure 1 A schematic diagram of the cross-sectional structure in the middle;

[0032] Figure 3 This is a schematic diagram of the fin structure.

[0033] In the diagram: 1. Fuel plate, 2. Fins, 3. Coolant microchannels, 101 alloy frame, 102 TRISO coated fuel particles, 103 dispersed heat transfer agent. Detailed Implementation

[0034] 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.

[0035] 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.

[0036] The plate-shaped fuel element consists of a fuel plate and fins. The fuel plate comprises an alloy frame, a dispersed thermally conductive agent, and TRISO fuel particles. The alloy frame is made of a high-temperature resistant and thermally conductive metallic material, such as niobium-based alloys or molybdenum-based alloys. The dispersed thermally conductive agent is a high-temperature resistant and thermally conductive powder material, such as zirconium hydride, graphite, or yttrium hydride. The TRISO particles have a spherical structure, consisting of, from the inside out, a fuel core, a loose pyrolytic carbon layer, an inner dense pyrolytic carbon layer, a dense silicon carbide layer, and an outer dense pyrolytic carbon layer. The core composition can be UN, UO2, PuO2, etc.

[0037] The fins are obtained by chemical etching of an alloy plate. The alloy plate material is consistent with the alloy frame material of the fuel plate so that the fins and the fuel plate can be connected by vacuum brazing.

[0038] like Figure 1 , Figure 2 , Figure 3As shown, the plate-shaped fuel element consists of a fuel plate 1 and fins 2. The fuel plate 1 consists of an alloy frame 101, TRISO-coated fuel particles 102, and a dispersed thermal conductive agent 103. The alloy frame 101 is made of a high-temperature resistant and thermally conductive metallic material, such as niobium-based alloys or molybdenum-based alloys. The dispersed thermal conductive agent 103 is a high-temperature resistant and thermally conductive powder material, such as zirconium hydride, graphite, or yttrium hydride. The TRISO-coated fuel particles 102 have a spherical structure, consisting of a fuel core, a loose pyrolytic carbon layer, an inner dense pyrolytic carbon layer, a dense silicon carbide layer, and an outer dense pyrolytic carbon layer from the inside out. The core composition can be UN, UO2, PuO2, etc.

[0039] The fin 2 is obtained by chemical etching of an alloy plate. The alloy plate material is the same as the alloy frame 101 material of the fuel plate 1, so that the fin 2 and the fuel plate 1 can be connected by vacuum brazing.

[0040] The fuel element manufacturing process is as follows:

[0041] Step 1: Mix the dispersed thermal conductive agent 103 with the combustion aid (e.g., ball milling). Spray the mixed dispersed thermal conductive agent powder evenly onto the fuel green mold using a spraying device. Then, compact it tightly using a flatbed press or roller equipment to form the first layer of the fuel green. The thickness of the first layer should be maintained between 0.5mm and 1mm.

[0042] Step 2: Using a deposition equipment, TRISO-coated fuel particles 102 are deposited onto the first layer of the fuel green stock. The deposition equipment is equipped with a positioning chuck, which allows the TRISO-coated fuel particles 102 to be positioned on the thermally conductive dispersed agent 103, forming the second layer of the fuel green stock. The specific distribution density and uniformity of the TRISO-coated fuel particles 102 on the thermally conductive dispersed agent 103 can be determined after calculations based on core physics and thermal analysis. By depositing TRISO-coated fuel particles 102 in a targeted manner, the power distribution of the fuel plates 1 in the lateral direction can be made more uniform. Furthermore, for a reactor core composed of stacked plate-shaped fuel elements, by changing the distribution density of TRISO-coated fuel particles 102 in each layer of fuel plates—for example, sparse distribution density in the fuel plates in the middle of the core and dense distribution density in the upper and lower parts of the core—the overall power distribution of the reactor core can be effectively flattened, making the reactor operation more stable.

[0043] Step 3: Repeat Step 1, mixing the dispersed thermal conductive agent 103 powder with the combustion aid powder and spraying it onto the surface of the TRISO-coated fuel particles 102. The mixed powder fills the gaps between the TRISO-coated fuel particles 102, forming the third layer of the fuel green. The thickness of the third layer is maintained between 0.5mm and 1mm. The fuel green is generally rectangular in shape, composed of the dispersed thermal conductive agent 103 and the TRISO-coated fuel particles 102. The TRISO-coated fuel particles 102 are dispersed within the dispersed thermal conductive agent 103, with the height of the dispersed thermal conductive agent 103 greater than the diameter of the TRISO-coated fuel particles 102, ensuring that the TRISO-coated fuel particles 102 are not exposed outside the dispersed thermal conductive agent 103.

[0044] Step 4: Static pressing. The mold is placed under a certain pressure to form the fuel green body.

[0045] Step 5: Sintering. The formed fuel green billet is placed in a sintering furnace for sintering, so that the dispersed thermal conductive agent 103 and the TRISO-coated fuel particles 102 are integrated. The sintering process can be carried out in a pressureless environment. First, the sintering furnace is evacuated. After the vacuum level is lower than a certain value (such as below 10 Pa), the sintering furnace is gradually heated. After the fuel green billet is stably sintered at the sintering temperature for a period of time, the sintering furnace is filled with an inert gas (to ensure that the fuel green billet is not oxidized), such as argon. Then the fuel green billet is kept warm and cooled in the furnace, and sintering is completed.

[0046] Step 6: Surface treatment. After sintering, the sintered sample needs to undergo surface treatment, including surface inspection, grinding, polishing, etc. The sintered sample must ensure that the TRISO-coated fuel particles 102 are not exposed outside the dispersed thermal conductive agent 103, and the sample size should be slightly smaller than the alloy frame size to facilitate sample placement in the frame.

[0047] Step 7: Placement. Place the surface-treated sample into the alloy frame 101.

[0048] Step 8: Chemical Etching. A 1.5mm thick alloy plate is chemically etched to obtain fins 2. Fins 2 have an airfoil-shaped cross-section, and the gaps between fins 2 form coolant microchannels 103. When coolant flows through the microchannels 103, the heat generated by the fuel plate is carried away by the coolant through conduction and convection. The distribution density and uniformity of fins 2 can be changed according to the power distribution of the fuel plate 1. For example, the density of fins 2 can be increased in local areas of high lateral power on the fuel plate 1, and decreased in areas of low lateral power to enhance heat transfer efficiency and improve the overall heat transfer effect of fins 2. Furthermore, the distribution density of fins 2 along the longitudinal height of the core can also be adjusted according to the core power distribution to maximize the overall output power of the core. Fins 2 can be discontinuous airfoils, or continuous semi-circular wavy, rectangular wavy, trapezoidal wavy, etc.

[0049] Step 9: Welding. The fins 2 and alloy frame 101 are welded together using vacuum brazing: brazing material is applied to the surface of alloy frame 101, and under vacuum and high temperature conditions, the brazing material melts, firmly welding the alloy frame 101 to the base plate of fin 2. Furthermore, when plate-shaped fuel elements are stacked to form a reactor core, the same vacuum brazing method is used to connect two plate-shaped fuel elements. Brazing material is applied to the upper surface of the fins 2 of one plate-shaped fuel element and placed in a vacuum and high temperature environment. The brazing material melts, firmly welding the heat transfer fin to the bottom of the alloy frame 101 of the other plate-shaped fuel element. By repeating the above process multiple times, the core can be stacked and processed.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] The accompanying drawings of the embodiments disclosed in this invention only involve the methods involved in the embodiments of this disclosure. Other methods can be referred to the general design. In the absence of conflict, the same embodiment and different embodiments of this invention can be combined with each other.

[0054] 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 method for processing a finned microchannel plate-shaped fuel element, comprising nine steps, characterized in that: Step 1: Mix the dispersed heat conductor, material and combustion aid, and spray the mixed dispersed heat conductor powder evenly into the fuel green mold using a spraying equipment. Then, compact it tightly with a flatbed press or roller equipment to form the first layer of the fuel green. Step 2: The TRISO-coated fuel particles are deposited on the first layer of the fuel green using a deposition equipment. The deposition equipment is equipped with a positioning chuck, which allows the TRISO-coated fuel particles to be positioned on the heat-conducting disperser to form the second layer of the fuel green. The distribution density and uniformity of the TRISO-coated fuel particles on the heat-conducting disperser are adjusted after calculations based on core physics and thermal analysis. Step 3: Mix the dispersed heat-conducting powder and the combustion aid powder and spray them onto the surface of TRISO-coated fuel particles, and fill the gaps between each TRISO-coated fuel particle with the mixed powder to form the third layer of the fuel green. Step 4: Static pressing, the mold is placed under a certain pressure to form the fuel green blank; Step 5: Sintering. The formed fuel green blank is placed in a sintering furnace for sintering, so that the dispersed heat-conducting agent and TRISO-coated fuel particles are integrated into one. Step 6: Surface treatment. After sintering is completed, the sintered sample is subjected to surface treatment, including surface inspection, grinding, polishing, etc. Sintered samples ensure that TRISO-coated fuel particles are not exposed outside the dispersed thermal conductive agent, and the sample size is smaller than the alloy frame size to facilitate sample placement within the frame. Step 7: Place the sample into the alloy frame after surface treatment; Step 8: Chemical etching. A 1.5mm thick alloy plate is chemically etched to obtain fins. Step 9: Welding. The fins are welded to the alloy frame using vacuum brazing: Brazing material is applied to the surface of the alloy frame. Under vacuum and high temperature, the brazing material melts and welds the alloy frame to the base plate of the fins. The above process is repeated multiple times to stack and process the core.

2. The method for processing a finned microchannel plate-shaped fuel element as described in claim 1, characterized in that: In step 1, the thickness of the first layer of fuel green is maintained between 0.5 mm and 1 mm.

3. The method for processing a finned microchannel plate-shaped fuel element as described in claim 1, characterized in that: In step 3, the thickness of the third layer of the fuel green is maintained between 0.5 mm and 1 mm.

4. The method for processing a finned microchannel plate-shaped fuel element as described in claim 1, characterized in that: In step 5, the sintering process selects a pressureless environment. First, the sintering furnace is evacuated. After the vacuum level reaches the required level, the sintering furnace is gradually heated. After the fuel green billet is stably sintered at the sintering temperature for a period of time, the sintering furnace is filled with inert gas. Then, the fuel green billet is kept warm and cooled with the furnace, and the sintering is completed.

5. The method for processing a finned microchannel plate-shaped fuel element as described in claim 1, characterized in that: In step 8, the fin cross-section is airfoil-shaped, and the gaps between the fins form coolant microchannels. When the coolant flows through the microchannels, the heat generated by the fuel plate is carried away by the coolant through heat conduction, convection, and other means.

6. The method for processing a finned microchannel plate-shaped fuel element as described in claim 5, characterized in that: The fin distribution density and uniformity can be changed according to the power distribution of the fuel plate. The fin density is increased in local areas with high lateral power of the fuel plate and decreased in areas with low lateral power to enhance heat exchange efficiency and improve the overall heat transfer effect of the fins. Furthermore, the fin distribution density in the longitudinal height of the core can also be adjusted according to the core power distribution to maximize the overall output power of the core.

7. A method for processing a finned microchannel plate-shaped fuel element as described in any one of claims 1, 5, or 6, characterized in that: The fins are continuous semi-circular wavy, rectangular wavy, or trapezoidal wavy.

8. The method for processing a finned microchannel plate-shaped fuel element as described in claim 1, characterized in that: In step 9, when the plate fuel elements are stacked into a core, two plate fuel elements are connected by vacuum brazing. The upper surface of the fins of one plate fuel element is coated with brazing material and placed in a vacuum high-temperature environment. The brazing material melts and welds the heat transfer fins to the bottom of the alloy frame of the other plate fuel element.

9. A method for processing a finned microchannel plate-shaped fuel element as described in any one of claims 1-6, characterized in that: The fuel green blank is rectangular in shape and consists of a dispersed thermal conductive agent and TRISO-coated fuel particles. The TRISO-coated fuel particles are dispersed in the dispersed thermal conductive agent, and the height of the dispersed thermal conductive agent is greater than the diameter of the TRISO-coated fuel particles to ensure that the TRISO-coated fuel particles are not exposed outside the dispersed thermal conductive agent.

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