Warm compaction method for making curved plate fuel elements using a flexible forming die

Abstract

The application relates to a warm-pressing forming preparation method for preparing curved plate type fuel elements by using a flexible forming die, and the warm-pressing forming preparation method comprises the following steps: heating a metal-based plate type fuel element to an annealing temperature and keeping the annealing temperature for a first time length; preheating an upper die assembly and keeping the preheating temperature for a second time length; placing the annealed metal-based plate type fuel element on a flexible die body; pressurizing the upper die assembly, so that the metal-based plate type fuel element is formed into a curved plate type fuel element under the support of the flexible die body; keeping the upper die assembly pressurized for a third time length, and keeping the curved plate type fuel element warm by the afterheat of the flexible forming die during the process of keeping the upper die assembly pressurized. The lower surface of the fuel element is always completely attached to the upper surface of the flexible die body during the pressing process, so that local stress concentration is avoided. The annealing temperature is in the lower limit interval of the recrystallization temperature, so that the fuel element is effectively reduced in yield strength while avoiding a sharp drop in mechanical properties.

Description

Technical Field

[0001] This application relates to the field of reactor fuel element processing technology, and in particular to a thermoforming method for preparing curved plate-shaped fuel elements using flexible forming molds. Background Technology

[0002] To improve fuel performance and overcome the shortcomings of ceramic-type nuclear fuel in terms of thermal conductivity and ductility, high-uranium-density metal-based fuel elements have been adopted in some high-flux research reactors, high-power reactors, or long-life power reactors. These metal-based fuel elements include plate-shaped fuel elements. Plate-shaped fuel elements employ a sandwich structure of cladding + core plate + cladding, with a metallurgical bond between the cladding and core plate. This facilitates heat dissipation and reduces blistering, thus preventing fuel element failure due to cladding damage or pillow-like bulging of the cladding affecting coolant flow, thereby improving reactor safety. Plate-shaped fuel elements are divided into metal-based flat plate fuel elements and metal-based curved plate fuel elements. Among them, metal-based curved plate fuel elements, due to their high uranium loading and large heat dissipation area, are the preferred choice for ultra-high flux experimental reactors.

[0003] In related technologies, the molding and preparation of metal-based curved plate fuel elements generally involves first fully annealing a flat fuel element at a high temperature (e.g., 490°C) to reduce its yield strength. After the flat fuel element cools to ambient temperature, it is then pressed to form a curved plate fuel element. The pressing process consists of two steps: initial pressing and final pressing. Initial pressing forms the curved plate fuel element, but the resulting curved plate fuel element exhibits springback and cannot achieve the ideal size. Therefore, a final pressing is required, using a smaller final pressing head to compensate for the bending of the curved plate fuel element. This necessitates extensive experimentation to fit the bending compensation amount of the curved plate fuel element and, based on experience, determining the size of the final pressing head.

[0004] The purpose of the full annealing treatment in the above process is to reduce the yield strength of the fuel element, thereby reducing the springback. However, this also reduces the tensile strength and elongation at break of the fuel element, especially for fuel elements with high nuclear fuel content, where the reduction in mechanical properties is even greater. This has a potential impact on the operational safety of the fuel element in the reactor. In addition, in the final pressing step, a large number of experiments are required to fit the over-bending compensation of the curved plate-type fuel element, and the size of the final pressing head is determined empirically. If fuel elements of various sizes and nuclear fuel contents need to be prepared, it will result in huge time and economic costs.

[0005] In related technologies, metal-based curved plate fuel elements are generally prepared by pressing metal-based flat plate fuel elements with a forming mold. The forming mold includes an upper mold and a lower mold arranged opposite each other. The side of the upper mold facing the lower mold has a convex forming surface, and the side of the lower mold facing the upper mold has a concave forming surface. The convex and concave forming surfaces are used to press the metal-based flat plate fuel element into a curved plate fuel element.

[0006] However, when a metal-based flat fuel element is placed on the concave surface of the lower die, its two ends are supported on the concave surface, while the middle part is subjected to downward pressure from the upper die. This causes the metal-based flat fuel element to exhibit a typical three-point bending stress state during the compression process. As a result, stress concentration is likely to occur at the support points at both ends and at the contact point with the upper die, which affects the forming accuracy, damages the metallurgical bonding interface between the internal cladding and the core plate of the fuel element, and poses a potential threat to the service safety of the fuel element in the reactor. Summary of the Invention

[0007] Based on this, it is necessary to provide a warm-press molding method for preparing curved plate-shaped fuel elements using a flexible molding die, addressing at least one of the aforementioned technical problems in the related technologies.

[0008] A thermoforming method for preparing curved plate-shaped fuel elements using a flexible forming mold, wherein the flexible forming mold includes an upper mold assembly and a lower mold assembly, the lower mold assembly includes a lower mold frame and a flexible mold body installed in the lower mold frame, the flexible mold body being made of an elastic material;

[0009] The warm-press molding preparation method includes:

[0010] The metal-based flat fuel element is heated to the annealing temperature and held at the annealing temperature for a first time. During the heating process, the heating rate of the metal-based flat fuel element is 20℃ / min to 40℃ / min. The annealing temperature is set to be within the full range of recrystallization temperature of the metal material in the metal-based flat fuel element, starting from the lower limit of the full range and accounting for one-quarter of the lower limit interval.

[0011] The upper mold assembly is preheated and held at the preheated temperature for a second time, wherein the preheated temperature is lower than or equal to the annealing temperature and lower than the upper temperature resistance limit of the flexible mold.

[0012] The annealed metal-based flat fuel element is placed on the flexible mold;

[0013] Pressurize the upper mold assembly to press down on the metal-based flat fuel element, which is then shaped into a curved fuel element under the support of the flexible mold.

[0014] The upper mold assembly is kept under pressure for a third time, and the residual heat of the flexible molding die is used to keep the curved fuel element warm during the pressure-keeping process, so that the curved fuel element is kept in the molding space for a third time.

[0015] The curved fuel element is removed from the flexible molding die and cooled.

[0016] In one embodiment, the preheating temperature is 200°C to 350°C.

[0017] In one embodiment, if the annealing temperature is higher than the upper limit of the temperature resistance of the flexible mold, the annealed metal-based flat fuel element is cooled down. Once the temperature of the cooled metal-based flat fuel element is lower than the upper limit of the temperature resistance of the flexible mold, the metal-based flat fuel element is placed on the flexible mold.

[0018] In one embodiment, the temperature of the metal-based flat fuel element after cooling is equal to the preheating temperature, and then the metal-based flat fuel element is placed on the flexible mold.

[0019] In one embodiment, the temperature of the cooled metal-based flat-plate fuel element is between 250°C and 350°C.

[0020] In one embodiment, if the annealing temperature is lower than the upper limit of the temperature resistance of the flexible mold, the preheating temperature is made equal to the annealing temperature.

[0021] In one embodiment, in the step of preheating the upper mold assembly and holding it at the preheated temperature for a second time, the upper mold assembly is preheated by a first heating device, wherein the first heating device is a first heating furnace, or an electric heating rod that can be inserted into the upper mold assembly, or a covered heating shell woven from heating strips;

[0022] In the step of heating the metal-based flat fuel element to the annealing temperature and holding it at the annealing temperature for a first time, the metal-based flat fuel element is heated by a second heating device, which is a second heating furnace.

[0023] In one embodiment, the warm pressing molding preparation method further includes: spraying a release agent onto the convex molding surface before the step of preheating the upper mold assembly and holding it at the preheated temperature for a second time.

[0024] In one embodiment, the material of the flexible mold includes any one or more combinations of silicone rubber, fluororubber, perfluoroether rubber, or silicone rubber.

[0025] In one embodiment, the first duration is 20 min to 90 min; the second duration is not less than 1 h; and the third duration is 10 min to 1 h.

[0026] The aforementioned warm-press forming method for preparing curved plate-shaped fuel elements using a flexible forming mold involves placing the annealed metal-based flat fuel element on a flexible mold. When the upper mold assembly presses down, the fuel element compresses the flexible mold. Throughout the pressing process, the flexible mold undergoes elastic deformation, ensuring that the lower surface of the fuel element remains completely in contact with the upper surface of the flexible mold. This transforms the concentrated load applied by the upper mold assembly into a full-area, quasi-isostatic uniform support for the entire lower surface of the fuel element. This support mode avoids localized stress concentration caused by point or line contact in traditional hard molds, effectively preventing shell breakage or interlayer separation during the bending forming process of the fuel element's sandwich structure. Simultaneously, the uniform stress distribution promotes plastic flow and stress relaxation of the material, significantly reducing springback after forming and ensuring the dimensional accuracy and shape stability of the curved plate-shaped fuel element.

[0027] The aforementioned warm-press forming method for preparing curved-plate fuel elements using flexible forming molds sets the annealing temperature within a quarter of the lower limit range of the full range of recrystallization temperatures of the metal material. This temperature range is significantly lower than the full annealing temperature used in the prior art. This allows the metal-based flat-plate fuel element to effectively reduce its yield strength while avoiding significant deterioration in mechanical properties, thereby significantly reducing damage to the overall performance of the fuel element. Moreover, the metal-based flat-plate fuel element can be precisely formed into a curved-plate fuel element in a single pressing operation, replacing the complex process of full annealing and two-step pressing in the prior art. It also avoids the need to rely on numerous experiments to fit the over-bending compensation amount of the curved-plate fuel element, significantly improving forming efficiency and reducing research and development and manufacturing costs. Attached Figure Description

[0028] Figure 1 This is a flowchart of a warm-press molding method for preparing curved fuel elements using a flexible molding die, according to one embodiment.

[0029] Figure 2 This is a schematic diagram of the upper mold assembly in one embodiment.

[0030] Figure 3 This is a schematic diagram of the lower mold frame in one embodiment.

[0031] Figure 4 This is a schematic diagram of the centering plate according to one embodiment.

[0032] Figure 5 This is a schematic diagram of the lower mold assembly in one embodiment.

[0033] Figure 6 This is a schematic diagram of the structure of a lower mold assembly in which a metal-based flat fuel element is placed, according to one embodiment.

[0034] Figure 7 This is a schematic diagram showing the disconnection of the upper mold assembly and the lower mold assembly during mold closing, according to one embodiment.

[0035] Figure 8 for Figure 7 A schematic diagram of the upper module component after it has been unloaded.

[0036] Explanation of reference numerals in the attached figures:

[0037] ZZ', First direction; YY', Second direction; XX', Third direction;

[0038] 10. Metal-based flat-plate fuel element; 20. Curved-plate fuel element;

[0039] 100. Upper mold assembly; 110. Upper mold base; 111. Centering hole; 120. Upper forming part; 121. Convex forming surface;

[0040] 200. Lower mold assembly; 210. Lower mold base; 211. Lower mold end plate; 212. First wall block; 213. Second wall block; 214. Centering column; 201. Mold cavity; 202. Stepped groove; 203. Slide groove; 220. Flexible mold body; 221. Upper flexible mold; 222. Lower flexible mold; 230. Limiting part; 240. Centering plate; 241. Slider; 242. Strip groove; 240a. First centering plate; 240b. Second centering plate; 250. Fastener. Detailed Implementation

[0041] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0042] In the description of this application, it should be understood that if terms such as "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" appear, these terms indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0043] Furthermore, where the terms "first" and "second" appear, these terms are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, where the term "multiple" appears, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0044] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0045] In this application, unless otherwise expressly specified and limited, the use of descriptions such as "above" or "below" the second feature indicates that the first and second features are in direct contact or indirect contact via an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. Similarly, "below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0046] It should be noted that if an element is referred to as being "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. If an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intervening element. If so, the terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used in this application are for illustrative purposes only and do not represent the only possible implementation.

[0047] Please see Figure 1 This application provides a warm-press molding method for preparing curved-plate fuel elements using a flexible molding die. Please refer to [link to relevant documentation]. Figures 2 to 6 The flexible molding die includes an upper mold assembly 100 and a lower mold assembly 200. The upper mold assembly 100 includes an upper mold frame 110 and an upper forming part 120 connected to the lower side of the upper mold frame 110. The lower surface of the upper forming part 120 is a convex forming surface 121. The lower mold assembly 200 includes a lower mold frame 210 and a flexible mold body 220. The lower mold frame 210 is configured with a mold cavity 201. The flexible mold body 220 is installed in the mold cavity 201 and is made of an elastic material.

[0048] In actual use, the upper mold assembly 100 is positioned above the lower mold assembly 200, with the two facing each other, thus the convex forming surface 121 and the flexible mold body 220 are positioned opposite each other. The metal-based flat fuel element 10 is placed on the flexible mold body 220. Figures 6 to 8 By applying pressure to the flexible molding die, the upper mold assembly 100 moves downward to apply pressure to the metal-based flat fuel element 10, thereby pressing the metal-based flat fuel element 10 into a curved fuel element 20 under the elastic support of the flexible mold 220. The shape of the convex forming surface 121 matches the shape of the upper surface of the curved fuel element 20.

[0049] Please see Figure 1 The warm-press molding method for preparing curved fuel elements using flexible molding dies includes at least steps S100 to S600, and each step is described in detail below.

[0050] Step S100: Heat the metal-based flat fuel element 10 to the annealing temperature and hold it at the annealing temperature for a first time. During the heating process, the heating rate of the metal-based flat fuel element 10 is 20℃ / min to 40℃ / min. The annealing temperature is set to be within the full range of recrystallization temperature of the metal material in the metal-based flat fuel element 10, starting from the lower limit of the full range and accounting for one-quarter of the lower limit interval.

[0051] Specifically, the metal-based flat-plate fuel element 10 adopts a sandwich structure of cladding + core plate + cladding, that is, cladding is provided on both sides of the core plate, and metallurgical bonding is formed between the core plate and the cladding on both sides.

[0052] Understandably, the cladding is made of metal, while the core is made of a mixture of core material particles and metal matrix powder. The metal matrix powder in the core is made of the same metal as the cladding.

[0053] Optionally, the first duration can be 20 minutes to 90 minutes. The specific value of the first duration can be flexibly set according to actual needs.

[0054] Understandably, the metal material in the metal-based flat-plate fuel element 10 is the same metal material used for the cladding as the metal material corresponding to the metal matrix of the core plate.

[0055] The recrystallization temperature of the metal material in the metal-based flat-plate fuel element 10 is a temperature range, not a single value. In this step, the annealing temperature is set to a lower limit interval that accounts for one-quarter of the total recrystallization temperature range of the metal material in the metal-based flat-plate fuel element 10. In other words, the annealing temperature in this step can be any value within this lower limit interval.

[0056] For example, if the recrystallization temperature of the metal material in the metal-based flat plate fuel element 10 is in the full range of 250°C to 350°C, the lower limit of this full range is 250°C. Therefore, the lower limit range, which starts from the lower limit of this full range and accounts for one-quarter of the range, is 250°C to 275°C.

[0057] By annealing the metal-based flat fuel element 10 at its annealing temperature, the yield strength of the metal-based flat fuel element 10 can be reduced. In this step, during the annealing of the metal-based flat fuel element 10, by holding the metal-based flat fuel element 10 at the annealing temperature for a first time, the internal stress of the metal-based flat fuel element 10 can be effectively eliminated, the material plasticity can be improved, and the yield strength can be reduced.

[0058] Furthermore, in this step, during the heating of the metal-based plate-type fuel element 10, the heating rate of the metal-based plate-type fuel element 10 is 20℃ / min to 40℃ / min. This rapid heating rate quickly heats the metal-based plate-type fuel element 10 to the aforementioned annealing temperature, enabling it to quickly pass through the nucleation zone. Within this zone, if the heating is slow, the recrystallization nucleation rate is low while the grain growth driving force is high, easily leading to abnormal grain coarsening and grain boundary weakening. The rapid heating of 20℃ / min to 40℃ / min effectively shortens the residence time in this sensitive zone, thereby ensuring that the material is sufficiently softened to facilitate molding while maintaining a fine and uniform recrystallized structure. Ultimately, while ensuring molding accuracy, the strength, toughness, and service reliability of the fuel element are well maintained.

[0059] Step S200: Preheat the upper mold assembly 100 and keep it at the preheated temperature for a second time, wherein the preheated temperature is lower than or equal to the annealing temperature and the preheated temperature is lower than the upper temperature resistance limit of the flexible mold 220.

[0060] The specific value of the second duration can be flexibly set according to actual needs. Optionally, the second duration should not be less than 1 hour.

[0061] By preheating the upper mold assembly 100 in this step, when the upper mold assembly 100 applies pressure to the metal-based flat fuel element 10, the temperature of the upper mold assembly 100 can be approximately the same as or close to that of the metal-based flat fuel element 10. This avoids the rapid drop in temperature of the metal-based flat fuel element 10 due to an excessive temperature difference between the upper mold assembly 100 and the metal-based flat fuel element 10, which would affect the plastic state of the metal-based flat fuel element 10.

[0062] Meanwhile, after the upper mold assembly 100 is preheated and kept at a certain temperature for a second time, the convex forming surface 121 can be stably maintained near the preheating temperature. In this way, in conjunction with the subsequent pressurization operation in step S400, the thermal creep of the material can be fully guaranteed, the stress can be effectively relaxed, and the springback of the component after molding can be reduced.

[0063] It should be noted that, in step S100, the heating rate of the metal-based flat fuel element 10 is relatively fast, while in step S200, the heating rate of the upper mold assembly 100 is slower than that of the metal-based flat fuel element 10. Therefore, the upper mold assembly 100 can be heated in advance, and after the upper mold assembly 100 has reached a certain temperature, the metal-based flat fuel element 10 can be heated further.

[0064] In this step, the preheating temperature should be lower than the upper temperature limit of the flexible mold 220. For example, if the upper temperature limit of the flexible mold 220 is 300°C, the preheating temperature should be lower than 300°C to prevent the flexible mold 220 from overheating and failing during pressing in step S400.

[0065] Since the flexible mold 220 is made of elastic materials (mostly polymeric materials, such as rubber), it is less resistant to prolonged high temperatures compared to rigid metal materials. Therefore, in this step, only the upper mold assembly 100 is preheated, and the lower mold assembly 200 is not preheated. After preheating, the upper mold assembly 100 (rigid metal material) has sufficient heat to maintain the temperature of the fuel element through heat conduction during pressing and holding. At the same time, the material used in the flexible mold 220 has low thermal conductivity, which also provides good insulation for the fuel element.

[0066] Step S300: Place the annealed metal-based flat fuel element 10 onto the flexible mold 220.

[0067] Step S400: Pressurize the upper mold assembly so that the upper mold assembly 100 presses down on the metal-based flat fuel element 10, thereby forming the metal-based flat fuel element 10 into a curved plate fuel element 20 under the elastic support of the flexible mold 220.

[0068] After step S100, the yield strength of the metal-based flat fuel element 10 has decreased. At this time, through steps S300 and S400, the annealed metal-based flat fuel element 10 is placed on the flexible mold 220. The upper mold assembly 100 applies pressure to the metal-based flat fuel element 10, forcing the metal-based flat fuel element 10 to undergo plastic deformation, thereby pressing the metal-based flat fuel element 10 into a curved plate fuel element 20.

[0069] like Figure 5 As shown, in its natural state (without the metal-based flat fuel element 10 placed), the upper surface of the flexible mold 220 is a plane perpendicular to the first direction ZZ'. Therefore, when the metal-based flat fuel element 10 is placed on the upper surface of the flexible mold 220, the lower surface of the metal-based flat fuel element 10 can completely fit against the upper surface of the flexible mold 220. Because the flexible mold 220 can elastically deform, the shape of the upper surface of the flexible mold 220 changes with the shape of the fuel element throughout the pressing process, thereby ensuring that the lower surface of the fuel element always remains completely fitted against the upper surface of the flexible mold 220.

[0070] Because the lower surface of the fuel element remains in complete contact with the upper surface of the flexible mold 220 throughout the entire pressing process, the concentrated load applied by the upper mold assembly 100 is transformed into a full-area, quasi-isostatic uniform support for the entire lower surface of the fuel element. This support mode avoids the local stress concentration caused by point or line contact in traditional hard molds, effectively preventing shell breakage or interlayer separation of the fuel element sandwich structure during bending and forming. At the same time, the uniform force distribution promotes the plastic flow and stress relaxation of the material, significantly reducing springback after forming and ensuring the dimensional accuracy and shape stability of the curved plate fuel element 20.

[0071] Step S500: The upper mold assembly 100 is kept under pressure for a third time, and the residual heat of the flexible molding die is used to keep the curved plate fuel element 20 warm during the pressure-keeping process, so that the curved plate fuel element 20 is kept in the molding space for a third time.

[0072] Optionally, the third duration is 10 minutes to 1 hour, and the specific value of the third duration can be flexibly set according to actual needs.

[0073] The upper mold assembly 100 is kept under pressure for a third duration, thereby allowing the upper mold assembly 100 and the flexible mold 220 to jointly confine the curved fuel element 20 within the forming space (the space between the convex forming surface 121 and the upper surface of the flexible mold 220) for a third duration. During this pressure-holding stage, the residual heat stored in the upper mold assembly 100 is continuously transferred to the curved fuel element 20 through the convex forming surface 121. At the same time, the flexible mold 220, due to its poor thermal conductivity, plays a role in heat preservation, together forming an environment that utilizes residual heat for heat preservation. This heat preservation process promotes creep and stress relaxation within the material of the curved fuel element 20, allowing the material more time for plastic rheology and internal stress release. The continuous elastic support of the flexible mold 220 during this period maintains the stability of the shape of the curved fuel element 20, thereby significantly reducing the springback of the curved fuel element 20 after the upper mold assembly 100 is unloaded, ensuring high precision and high stability of the final dimensions of the curved fuel element 20.

[0074] Step S600: Remove the curved fuel element 20 from the flexible molding die and cool it.

[0075] The upper mold assembly 100 can be removed from above the lower mold assembly 200, thereby removing the formed curved plate fuel element 20 and cooling it to finally obtain the desired curved plate fuel element 20. Optionally, step S600 involves air cooling the curved plate fuel element 20 after removing it from the flexible molding die.

[0076] The above-described warm-press molding method for preparing curved plate-shaped fuel elements using flexible molding dies involves annealing the metal-based flat plate-shaped fuel element 10 within the lower limit of its recrystallization temperature range, allowing it to retain higher mechanical properties while achieving the necessary plasticity. The upper mold assembly 100 is preheated to a preheating temperature not exceeding the annealing temperature and lower than the upper temperature resistance limit of the flexible mold 220, and held at this temperature for a second duration. This stabilizes the temperature of the convex forming surface 121, preventing excessive temperature difference between the convex forming surface 121 and the fuel element during pressing, which could lead to deterioration of its plasticity, while simultaneously protecting the flexible mold 220 from high-temperature damage. During pressing, the flexible mold 220 undergoes elastic deformation, providing uniform support across the entire surface of the fuel element's lower surface, thus allowing the upper mold to... The concentrated load of component 100 is transformed into a quasi-isostatic distribution, which effectively prevents the shell of the fuel element sandwich structure from breaking or separating due to local stress concentration, and promotes the plastic flow of materials to reduce springback. By maintaining pressure on the upper mold component 100 for a third time, the curved fuel element 20 is kept in the forming space for a third time. At the same time, the residual heat of the upper mold component 100 is used to transfer heat through the convex forming surface 121 and the heat insulation of the flexible mold 220 to keep it warm, promote the internal creep and stress relaxation of the fuel element. Combined with the continuous elastic support of the flexible mold 220, the internal stress is further released and the shape is stabilized, significantly reducing the amount of springback and ensuring the high precision and stability of the dimensions of the curved fuel element 20.

[0077] The above-described warm-press forming method for preparing curved plate-shaped fuel elements using flexible forming molds involves placing the annealed metal-based flat plate-shaped fuel element 10 on a flexible mold 220. When the upper mold assembly 100 presses down, the fuel element compresses the flexible mold 220. Throughout the pressing process, the flexible mold 220 undergoes elastic deformation, ensuring that the lower surface of the fuel element remains completely in contact with the upper surface of the flexible mold 220. This transforms the concentrated load applied by the upper mold assembly 100 into a full-area, quasi-isostatic uniform support for the entire lower surface of the fuel element. This support mode avoids localized stress concentration caused by point or line contact in traditional hard molds, effectively preventing shell breakage or interlayer separation during the bending forming process of the fuel element sandwich structure. Simultaneously, the uniform stress distribution promotes plastic flow and stress relaxation of the material, significantly reducing springback after forming and ensuring the dimensional accuracy and shape stability of the curved plate-shaped fuel element 20.

[0078] The aforementioned warm-press forming method for preparing curved-plate fuel elements using flexible forming molds sets the annealing temperature within a quarter of the lower limit range of the full range of recrystallization temperatures of the metal material. This temperature range is significantly lower than the full annealing temperature used in the prior art. This allows the metal-based flat-plate fuel element 10 to effectively reduce its yield strength while avoiding significant deterioration of its mechanical properties, thereby significantly reducing damage to the overall performance of the fuel element. Moreover, the metal-based flat-plate fuel element 10 can be precisely formed into a curved-plate fuel element 20 in a single pressing operation, replacing the complex process of full annealing and two-step pressing in the prior art. It also avoids the need to rely on numerous experiments to fit the over-bending compensation amount of the curved-plate fuel element, significantly improving forming efficiency and reducing research and development and manufacturing costs.

[0079] In one embodiment, the annealing temperature is lower than the temperature at which the metal matrix and nuclear fuel particles undergo an interfacial reaction, thereby preventing changes in core properties, particularly mechanical properties.

[0080] In one embodiment, the material of the flexible mold 220 includes any one or more combinations of silicone rubber, fluororubber, perfluoroether rubber, or silicone rubber.

[0081] The flexible mold 220 can be made of a variety of high-temperature resistant elastic materials, such as silicone rubber with a temperature resistance of about 300°C, fluororubber (such as FKM) with a temperature resistance of about 320°C, perfluoroether rubber (such as FFKM) with a temperature resistance of up to 330°C or higher, and silicone rubber with similar temperature resistance after special formulation optimization; for high-temperature pressing molding scenarios, special engineering elastomers such as polyimide with a temperature resistance exceeding 360°C can also be selected.

[0082] These elastic materials all maintain stable chemical properties and elastic modulus within a preheating and molding temperature range of 200°C to 350°C. By using these materials individually or in combination, the flexible mold 220 can continuously provide uniform and reliable full-area support during thermoforming, while ensuring no contamination or adhesion when in contact with the fuel element, thus guaranteeing the molding accuracy and surface quality of the curved fuel element 20.

[0083] In one embodiment, the preheating temperature is 200°C to 350°C.

[0084] The upper temperature resistance limit of commonly used materials for flexible molds (such as high-temperature resistant silicone rubber, fluororubber, etc.) is usually between 250℃ and 360℃.

[0085] In this embodiment, the preheating temperature is set to 200°C to 350°C. This ensures that the convex forming surface 121 of the upper mold assembly 100 has sufficient heat to effectively maintain the plastic forming conditions of the metal-based flat fuel element 10 during the pressing and holding process. It also avoids the risk of exceeding the upper temperature limit of the flexible mold 220, preventing permanent damage and elastic failure due to overheating. Thus, while ensuring the temperature requirements of the molding process, it ensures that the flexible mold 220 can stably and reliably provide elastic support function over the entire area.

[0086] In one embodiment, if the annealing temperature is higher than the upper limit of the temperature resistance of the flexible mold 220, the annealed metal-based flat fuel element 10 is cooled down. Once the temperature of the cooled metal-based flat fuel element 10 is lower than the upper limit of the temperature resistance of the flexible mold 220, the metal-based flat fuel element 10 is placed on the flexible mold 220.

[0087] In this embodiment, if the annealing temperature of the metal-based flat fuel element 10 is higher than the upper temperature resistance limit of the flexible mold 220, the annealed metal-based flat fuel element 10 is cooled to a temperature lower than the upper temperature resistance limit of the flexible mold 220. This prevents the overheated metal-based flat fuel element 10 from directly contacting and damaging the flexible mold 220, which has limited temperature resistance. Although the temperature of the metal-based flat fuel element 10 is reduced, its yield strength is significantly reduced due to the annealing process, and it still maintains good plasticity. This ensures both good deformation capability of the fuel element material during pressing and safe use of the flexible mold 220.

[0088] In one embodiment, the temperature of the cooled metal-based flat fuel element 10 can be the same as the preheating temperature of the upper mold assembly 100 before placing the metal-based flat fuel element 10 on the flexible mold 220. Alternatively, the temperature of the cooled metal-based flat fuel element 10 can be close to the preheating temperature of the upper mold assembly 100 (e.g., within 30°C higher or lower than the temperature of the upper mold assembly 100) before placing the metal-based flat fuel element 10 on the flexible mold 220.

[0089] In this embodiment, the temperature of the cooled metal-based flat fuel element 10 is the same as or close to the preheating temperature of the upper mold assembly 100, so that the temperature of the metal-based flat fuel element 10 and the preheated convex surface 121 is the same as or close, avoiding a large temperature difference between the two during pressing.

[0090] In one embodiment, when the temperature of the cooled metal-based flat fuel element 10 is in the temperature range of 240°C to 350°C, the metal-based flat fuel element 10 is then placed on the flexible mold 220.

[0091] In this embodiment, the metal-based flat fuel element 10 is cooled to a temperature range of 240°C to 350°C, which is the same as or close to the temperature of the preheated convex surface 121 to avoid excessive temperature difference between the two, and is below the upper limit of the temperature resistance of the flexible mold 220 to avoid damage to the flexible mold 220. At the same time, the metal-based flat fuel element 10 is in a temperature range of 240°C to 350°C, which is still within the temperature range suitable for plastic forming.

[0092] In other embodiments, the annealing temperature of the metal-based flat fuel element 10 is lower than the upper temperature limit of the flexible mold 220. Therefore, the preheating temperature of the upper mold assembly 100 is the same as the annealing temperature of the metal-based flat fuel element 10 (or the difference between the preheating temperature and the annealing temperature is within 30°C). This eliminates the need to cool the metal-based flat fuel element 10. After annealing and holding at that temperature for a first time, the metal-based flat fuel element 10 can be directly placed on the flexible mold 220. This avoids damage to the flexible mold 220, and ensures that the temperature of the metal-based flat fuel element 10 is the same as or close to that of the preheated convex surface 121, while also placing the metal-based flat fuel element 10 within a suitable temperature range for plastic forming.

[0093] In one embodiment, the arc length of the convex surface 121 is greater than the arc length of the designed arc-shaped plate fuel element, and the difference between the two is 10-50 mm.

[0094] In one embodiment, the axial length of the convex surface 121 is greater than the axial length of the designed arc-shaped plate fuel element, and the difference between the two is 20-100 mm.

[0095] In one embodiment, the length of the flexible phantom 220 is greater than the axial length of the designed arc-shaped plate-shaped fuel element, and the difference between the two is 10-100 mm.

[0096] In one embodiment, the width of the flexible mold 220 is greater than the width of the metal-based flat fuel element corresponding to the designed arc-shaped plate fuel element. Among all the arc-shaped plate fuel elements to be formed, the width of the metal-based flat fuel element corresponding to the arc-shaped plate fuel element with the largest width is 20-50 mm smaller than the width of the flexible mold 220.

[0097] In one embodiment, in step S200, the upper mold assembly 100 is preheated by a first heating device.

[0098] The first heating device can be a first heating furnace.

[0099] The first heating device can be an electric heating rod that can be inserted into the upper mold assembly 100. For example, the upper mold assembly 100 is provided with a socket, and the electric heating rod is inserted into the socket.

[0100] The first heating device can be a covered heating shell made of woven heating strips, which covers the upper mold assembly 100.

[0101] In one embodiment, in step S100, the metal-based flat-plate fuel element 10 is heated by a second heating device, which is a second heating furnace.

[0102] In this embodiment, the metal-based flat fuel element 10 is heated by a second heating device, and the upper mold assembly 100 is preheated by a first heating device. That is, the metal-based flat fuel element 10 and the upper mold assembly 100 are heated by different heating devices.

[0103] In one embodiment, the warm pressing molding preparation method further includes: spraying a release agent onto the convex forming surface 121 before step S200.

[0104] By spraying a release agent onto the convex surface 121, direct adhesion between the convex surface 121 and the metal-based flat fuel element 10 can be prevented, as well as adhesion between the convex surface 121 and the curved fuel element 20 can be prevented. This makes it easy to remove the curved fuel element 20 after it has been formed.

[0105] In one embodiment, the annealing process for the metal-based flat-plate fuel element 10 can be carried out in air or in a vacuum. If it must be carried out in a vacuum, the vacuum level must be no less than 10. -2 Pa.

[0106] Please see Figures 1 to 5 This application provides a flexible molding die, which includes an upper mold assembly 100 and a lower mold assembly 200 disposed opposite to each other along a first direction ZZ'.

[0107] The upper mold assembly 100 includes an upper mold base 110 and an upper forming part 120. The upper forming part 120 is connected to the lower side of the upper mold base 110. The lower surface of the upper forming part 120 is a convex forming surface 121. The length direction of the upper forming part 120 is the second direction YY', and the width direction of the upper forming part 120 is the third direction XX'. The second direction YY', the third direction XX', and the first direction ZZ' are perpendicular to each other.

[0108] The lower mold assembly 200 includes a lower mold frame 210 and a flexible mold body 220. The lower mold frame 210 includes a lower mold end plate 211 and a first wall block 212 and a second wall block 213 disposed on the lower mold end plate 211 and spaced apart along the third direction XX'. A mold cavity 201 is constructed between the first wall block 212 and the second wall block 213. The flexible mold body 220 is installed in the mold cavity 201 and is disposed opposite to the convex forming surface 121. The flexible mold body 220 is made of elastic material and is used to provide support for the fuel element plate.

[0109] The upper forming part 120 and the upper die holder 110 can be connected by bolts or integrally formed.

[0110] The first wall block 212, the second wall block 213 and the lower die end plate 211 can be an integrally formed structure.

[0111] The length direction of the flexible die body 220 is the second direction YY', and the width direction of the flexible die body 220 is the third direction XX'. The thickness direction of the flexible die body 220 is along the first direction ZZ'. The length direction of the die cavity 201 is the second direction YY', and the width direction of the die cavity 201 is the third direction XX'.

[0112] In the above flexible forming die, by arranging a flexible die body 220 made of an elastic material in the die cavity 201 of the lower die holder 210, it is oppositely arranged with the convex forming surface 121 of the upper die assembly 100. When the upper die assembly 100 presses down on the metal-based flat fuel element 10, the flexible die body 220 can elastically deform, so that the shape of its upper surface always fits the lower surface of the fuel element, thereby converting the concentrated load applied by the upper die assembly 100 into a full-area, quasi-isostatic uniform support for the entire lower surface of the fuel element. This support mode enables the fuel element to avoid local stress concentration caused by point or line contact in the traditional rigid die during the bending forming process, effectively preventing the cladding from being damaged or layer separation occurring in the sandwich structure of the fuel element. At the same time, the uniform force distribution promotes the plastic flow and stress relaxation of the material, significantly reducing the springback after forming, and ensuring the dimensional accuracy and shape stability of the curved plate-shaped fuel element 20.

[0113] In one embodiment, the flexible die body 220 includes an upper flexible die 221 and a lower flexible die 222 arranged in a stacked manner. The thickness of the upper flexible die 221 is less than the thickness of the lower flexible die 222, and the upper flexible die 221 is detachably installed in the die cavity 201.

[0114] The upper flexible die 221 and the lower flexible die 222 do not need to be strongly fixed to the die cavity 201 through any connection structures (such as screws, adhesives, etc.), and only need to be placed in the die cavity 201. The flexible die body 220 and the die cavity 201 can be in a clearance fit. There is no need to fix the upper flexible die 221 and the lower flexible die 222 through any connection structures either. The upper flexible die 221 can be stacked on the lower flexible die 222.

[0115] In the above embodiments, the flexible die body 220 is designed to include an upper flexible die 221 and a lower flexible die 222 which are stacked, the thickness of the upper flexible die 221 is made smaller than that of the lower flexible die 222, and at the same time, the upper flexible die 221 is detachably installed in the die cavity 201. This structural arrangement enables the upper flexible die 221 that directly contacts the fuel element and bears the main wear and deformation to be replaced separately after reaching the service life, without replacing the entire flexible die body 220. The lower flexible die 222 serves as the main elastic support matrix, providing a stable and sufficient elastic deformation space and restoring force through its larger thickness, thereby significantly reducing the long-term use and maintenance costs of the die while ensuring the durability and stability of the overall support performance of the flexible die body 220.

[0116] Please refer to Figure 7 , in an embodiment, the width of the upper flexible die 221 is greater than the width of the lower flexible die 222, and the shape of the die cavity 201 matches the shape of the flexible die body 220.

[0117] In the above embodiments, by setting the width of the upper flexible die 221 to be greater than the width of the lower flexible die 222, the flexible die body 220 forms a structure with a wider upper part and a narrower lower part in cross-section, and at the same time, the shape of the die cavity 201 matches the shape of the flexible die body 220. This structure enables the acting force to be transmitted to the wider two sides of the upper flexible die 221 when the upper die assembly 100 presses down on the fuel element, and is restricted by the side walls of the die cavity 201, thereby effectively converting the downward pressure into a lateral pressing force on the entire flexible die body 220, preventing it from shifting or tilting in the die cavity 201. This structure with a wider upper part and a narrower lower part in combination with the matching die cavity 201 can ensure that the flexible die body 220 always maintains a stable installation position and shape when承受较大压力和发生复杂变形时,始终保持稳定的安装位置和形态,从而为燃料元件提供持续、均匀且可靠的全面积弹性支撑,避免了因支撑体失稳而导致的成型缺陷。

[0118] Please refer to Figure 7 , in an embodiment, the side surfaces of both sides of the flexible die body 220 along the third direction XX' are inclined surfaces, so that the width of the flexible die body 220 gradually becomes wider from bottom to top. Among them, the side surfaces of both sides of the upper flexible die 221 along the third direction XX' are inclined surfaces, the side surfaces of both sides of the lower flexible die 222 along the third direction XX' are inclined surfaces, and the side surfaces of both sides of the upper flexible die 221 along the third direction XX' and the side surfaces of both sides of the lower flexible die 222 along the third direction XX' are coplanar.

[0119] In the above embodiments, by setting both side surfaces of the upper flexible mold 221 and the lower flexible mold 222 along both sides in the third direction XX' to be coplanar inclined surfaces, a continuous inclined surface structure with a gradually increasing width from bottom to top is formed as a whole for the flexible mold body 220. This continuous inclined surface structure cooperates with the mold cavity 201 with a matching shape, so that when the flexible mold body 220 is pressed down by the upper mold assembly 100, its lateral deformation is uniformly restricted by the inclined inner wall of the mold cavity 201, effectively converting the vertical downward pressure into a lateral pressing force on the whole flexible mold body 220, avoiding stress concentration or relative dislocation at the interlayer joint between the upper flexible mold 221 and the lower flexible mold 222; at the same time, this continuous inclined surface structure ensures that the upper flexible mold 221 and the lower flexible mold 222 can work together as a whole when受力变形, enhancing the structural stability and overall stiffness of the flexible mold body 220 under complex loads, thereby providing a more uniform, stable and reliable full-area elastic support for the fuel element and ensuring the accuracy and repeatability of the forming process.

[0120] In some other embodiments, it may also be that the width of the upper flexible mold 221 remains uniform from bottom to top, and the width of the lower flexible mold 222 remains uniform from bottom to top.

[0121] Optionally, the width of the upper flexible mold 221 is 10 - 30 mm larger than the width of the lower flexible mold 222.

[0122] In the above embodiments, by setting the widths of the upper flexible mold 221 and the lower flexible mold 222 to remain uniform from bottom to top, a stepped structure is formed at the joint between the upper flexible mold 221 and the lower flexible mold 222, and thus a stepped structure is also formed in the mold cavity 201. This stepped structure provides a clear positioning reference for the upper flexible mold 221, ensuring that it can be quickly and accurately reset during repeated disassembly and assembly.

[0123] Please refer to Figure 3 , in an embodiment, two stepped grooves 202 are further constructed between the first wall block 212 and the second wall block 213, and the two stepped grooves 202 are respectively located at both ends of the mold cavity 201 along the second direction YY'.

[0124] Combined with Figure 5 and Figure 6 , the flexible forming die further includes two limiting parts 230, and the two limiting parts 230 correspond to the two stepped grooves 202 one by one. The shape of the limiting part 230 is adapted to the shape of the stepped groove 202, and the limiting part 230 is detachably connected to the first wall block 212 and the second wall block 213. The two limiting parts 230 are used to limit both ends of the flexible mold body 220 along the second direction YY'.

[0125] In the above embodiments, stepped grooves 202 are provided at both ends of the mold cavity 201 along the second direction YY', and detachable limiting parts 230 with adapted shapes are configured. When the flexible mold body 220 is placed in the mold cavity 201, both ends thereof are respectively constrained by the limiting parts 230. Thus, when the flexible mold body 220 undergoes elastic deformation under the downward pressure of the upper mold assembly 100, the displacements of both ends thereof along the second direction YY' are mechanically restricted, effectively preventing excessive expansion or slippage of the flexible mold body 220 in the length direction. At the same time, the detachable limiting parts 230 facilitate the installation and disassembly operations during the installation or replacement of the flexible mold body 220, and provide an accurate positioning reference for the flexible mold body 220 in the length direction. This end limiting structure ensures that the flexible mold body 220 always maintains a stable working position when repeatedly subjected to pressing loads, thereby providing continuous and stable full-area support for the fuel element, and ensuring the repeated accuracy and reliability of the forming process.

[0126] Please refer to Figures 3 to 6 , in an embodiment, sliding grooves 203 extending along the third direction XX' are respectively provided at the tops of the first wall block 212 and the second wall block 213. The flexible forming die further includes two centering plates 240, which are respectively arranged at the tops of the first wall block 212 and the second wall block 213. Sliders 241 are respectively provided at the bottoms of the two centering plates 240, and the sliders 241 are in sliding fit with the sliding grooves 203 along the third direction XX' to adjust the distance between the two centering plates 240.

[0127] [[ID=⑧]]In the above embodiments, by respectively providing sliding grooves 203 extending along the third direction XX' at the tops of the first wall block 212 and the second wall block 213, and respectively providing sliders 241 at the bottoms of the two centering plates 240 that are in sliding fit with the sliding grooves 203, the two centering plates 240 can accurately and smoothly adjust the distance between them along the sliding grooves 203 according to the width dimension of the metal-based flat fuel element 10. This structure ensures that the centering plates 240 maintain linear motion without deviation during the movement. When the fuel element is placed on the flexible mold body 220, by adjusting the centering plates 240 to contact both sides of the fuel element, the fuel element can be quickly and accurately positioned at the center position in the width direction of the lower mold assembly 200, thereby ensuring symmetric and uniform force on the fuel element during the pressing process, effectively avoiding defects such as distortion of the formed part, uneven wall thickness, or stress concentration caused by placement deviation, and being conducive to obtaining high-quality and dimensionally accurate curved plate-shaped fuel elements 20.

[0128] In an embodiment, the edges of the two centering plates 240 close to each other along the third direction XX' both extend above the upper surface of the flexible mold body 220 to limit the two edges of the upper surface of the flexible mold body 220.

[0129] In the above embodiments, by extending the side edges of the two centering plates 240 that are close to each other in the third direction XX' above the upper surface of the flexible mold body 220, this structure enables the centering plates 240 to form reliable limits on the two side edges of the upper surface of the flexible mold body 220 after completing the centering in the width direction of the fuel element. When the upper mold assembly 100 presses down on the fuel element, during the elastic deformation process of the flexible mold body 220, the upward bulging trend of its two side edges is rigidly constrained by the centering plates 240, preventing uneven distribution of the supporting force caused by excessive expansion in the edge area. At the same time, this limiting effect cooperates with the constraint of the limiting portion 230 at the stepped groove in the length direction to jointly constitute a complete constraint system for the three-dimensional deformation of the flexible mold body 220, ensuring that it always maintains a regular deformation form during the pressing process, thereby providing a full-area elastic support with clear boundaries, stable control for the fuel element, and effectively guaranteeing the forming accuracy and dimensional consistency of the curved plate type fuel element 20.

[0130] It should be noted that when the two centering plates 240 center the metal-based flat fuel element 10 in the width direction, the sides of the two centering plates 240 that are close to each other are respectively abutted against the two sides of the metal-based flat fuel element 10 in the width direction. After the two centering plates 240 complete the centering of the metal-based flat fuel element 10 in the width direction, the two centering plates 240 need to be appropriately moved a certain avoidance distance in the direction away from each other along the third direction XX' to ensure that there is no interference between the upper mold assembly 100 and the two centering plates 240 when pressing down. At the same time, please refer to Figure 7 , after the two centering plates 240 are moved this avoidance distance in the direction away from each other along the third direction XX', the side edges of the two centering plates 240 that are close to each other along the third direction XX' are still located above the upper surface of the flexible mold body 220, so as to be able to form reliable limits on the two side edges of the upper surface of the flexible mold body 220 during the pressing process of the upper mold assembly 100 and inhibit the edge bulging of the flexible mold body 220.

[0131] Refer to Figure 7 , in an embodiment, the centering plate 240 at the top of the first wall block 212 is the first centering plate 240a, and the centering plate 240 at the top of the second wall block 213 is the second centering plate 240b.

[0132] After the two centering plates 240 complete the centering of the metal-based flat fuel element 10 in the width direction and are appropriately moved a certain avoidance distance in the direction away from each other along the third direction XX', the side of the first centering plate 240a away from the second centering plate 240b along the third direction XX' is aligned with the edge of the first wall block 212, and the side of the second centering plate 240b away from the first centering plate 240a along the third direction XX' is aligned with the edge of the second wall block 213.

[0133] In the above embodiments, by setting the avoidance positions of the first pair of middle plates 240a and the second pair of middle plates 240b after centering to be aligned with the edges of the first wall block 212 and the second wall block 213 respectively, this design establishes a clear and fixed mechanical positioning reference. When the operator moves the centering plates, there is no need to use measuring tools for precise distance measurement. Only by visually observing whether the outer sides of the centering plates are flush with the edges of the wall blocks can the positioning operation be quickly and accurately completed. This visual alignment method not only significantly simplifies the operation process and improves production efficiency, but also completely eliminates the risk of insufficient or excessive avoidance distance caused by human measurement errors, ensuring that the two centering plates 240 can stay stably in the established safe positions. It not only provides a reliable avoidance space for the downward movement of the upper die assembly 100, but also continues to maintain the effective limit function for the edge of the flexible die body 220.

[0134] Combined with Figures 4 to 5 , in an embodiment, both of the two centering plates 240 are provided with strip-shaped grooves 242 extending along the third direction XX'. Fasteners 250 are provided at the tops of both the first wall block 212 and the second wall block 213.

[0135] The fastener 250 at the top of the first wall block 212 is used to pass through the corresponding strip-shaped groove 242 to lock the centering plate 240 with the first wall block 212. The fastener 250 at the top of the second wall block 213 is used to pass through the corresponding strip-shaped groove 242 to lock the centering plate 240 with the second wall block 213.

[0136] The position of the fastener 250 at the top of the first wall block 212 / second wall block 213 can be fixed. For example, threaded holes can be provided at the tops of the first wall block 212 / second wall block 213, and the fastener 250 is a bolt. By screwing the fastener 250, it can be mated with the threaded hole or removed from the threaded hole.

[0137] In the above embodiments, by providing strip-shaped grooves 242 extending along the third direction XX' on the two centering plates 240 and corresponding fasteners 250 at the tops of the first wall block 212 and the second wall block 213, the centering plate 240 can be reliably fixed through the cooperation of the fastener 250 and the strip-shaped groove 242 after the position adjustment is completed. This structural design enables the operator to slide the centering plate 240 along the sliding groove 203 to a suitable position according to fuel elements of different widths, and then firmly lock the centering plate 240 at the predetermined position by screwing the fastener 250. The extension length of the strip-shaped groove 242 provides a sufficient stroke range for the position adjustment of the centering plate 240, ensuring that it can meet the centering requirements of fuel elements of various specifications. This adjustable and lockable positioning mechanism not only ensures the flexibility of the centering operation, but also ensures the stability of the position of the centering plate 240 during the pressing process, enabling it to continuously provide a reliable edge limit function for the flexible die body 220.

[0138] It should be noted that after the centering is completed, the two centering plates 240 should be appropriately moved away from each other along the third direction XX' by an avoidance distance to ensure that there is no interference between the upper die assembly 100 and the two centering plates 240 when the upper die assembly 100 presses down, and then the two centering plates 240 are locked.

[0139] In one embodiment, centering columns 214 are respectively provided at both ends of the lower die end plate 211 along the second direction YY'. Centering holes 111 are provided on the upper die holder 110, and the centering columns 214 and the centering holes 111 are arranged in one-to-one correspondence. The centering columns 214 and the corresponding centering holes 111 are inserted and fitted along the first direction ZZ'.

[0140] In the above embodiment, by providing the centering columns 214 at both ends of the lower die end plate 211 along the second direction YY' and providing the centering holes 111 that cooperate with them at the corresponding positions on the upper die holder 110, during the downward pressing process of the upper die assembly 100, the centering columns 214 can accurately guide the centering holes 111 to complete the insertion and fitting along the first direction ZZ'. This structure ensures that the convex forming surface 121 of the upper die assembly 100 and the mold cavity 201 of the lower die assembly 200 always maintain an accurate coaxial centering relationship during the mold closing process, effectively preventing problems such as unilateral compression of the fuel element, uneven wall thickness, or local stress concentration caused by mold misalignment; at the same time, this rigid centering mechanism provides a stable guiding reference for the entire forming system, ensuring that the pressure applied by the upper die assembly 100 can be evenly transmitted to the entire supporting surface of the flexible mold body 220, enabling the fuel element to obtain a symmetric stress environment during the pressing process, thereby significantly improving the forming accuracy, dimensional consistency, and structural integrity of the curved plate type fuel element 20.

[0141] In one embodiment, the thickness design of the flexible mold body 220 along the first direction ZZ' satisfies that the maximum elastic deformation amount that the flexible mold body 220 can allow along the first direction ZZ' is greater than the maximum chord height of all the to-be-formed arc-shaped fuel element plates.

[0142] Optionally, the maximum elastic deformation amount that the flexible mold body 220 can allow along the first direction ZZ' should be 20%-30% larger than the maximum chord height of all the to-be-formed arc-shaped fuel element plates. The maximum elastic deformation amount that the flexible mold body 220 can allow along the first direction ZZ' should be 2 mm - 10 mm larger than the maximum chord height of all the to-be-formed arc-shaped fuel element plates.

[0143] Optionally, the maximum elastic deformation amount that the flexible mold body 220 can allow along the first direction ZZ' does not exceed 25% of the total thickness of the flexible mold body 220.

[0144] In the above embodiments, by specifically designing the thickness of the flexible die body 220 along the first direction ZZ', the maximum allowable elastic deformation amount thereof is made greater than the maximum chord height of all the to-be-formed arc-shaped fuel element plates. This design criterion ensures that the flexible die body 220 has sufficient elastic deformation space. When the upper die assembly 100 presses the fuel element to the maximum designed chord height, the flexible die body 220 can provide continuous and stable full-area support for its lower surface on the premise of maintaining a completely elastic deformation state, effectively avoiding support failure or local stress concentration caused by insufficient deformation amount. At the same time, this design ensures that the flexible die body 220 can completely recover to its initial shape after experiencing the maximum compressive deformation, preventing permanent deformation, thereby ensuring stable support performance and forming accuracy of the die during long-term repeated use, and providing a reliable elastic support guarantee for the high-quality forming of fuel elements of different specifications with curved plates.

[0145] In one embodiment, the convex forming surface 121 is a convex arc surface, and its outer diameter is equal to or slightly smaller than the inner diameter of the designed arc-shaped fuel element plate.

[0146] In the above embodiments, by designing the convex forming surface 121 as a convex arc surface and making its outer diameter equal to or slightly smaller than the inner diameter of the designed arc-shaped fuel element plate, this dimensional design enables the upper die assembly 100 to perform overbending compensation on the fuel element during the pressing process. When the convex forming surface 121 presses down with a size slightly smaller than the target curvature radius, the fuel element is forced to bend to a greater degree than the final designed curvature under the elastic support of the flexible die body 220. After the pressure is removed at the end of the pressure holding stage, the springback amount of the fuel element due to the elasticity of the material just makes it recover to the designed precise curvature. This preset overbending compensation amount combined with the uniform support provided by the flexible die body 220 effectively offsets the dimensional deviation caused by the elastic recovery of the material, ensuring that the arc-shaped fuel element 20 can stably reach the designed curvature radius and geometric accuracy after forming, and achieving the technical effect of obtaining a formed part with high dimensional accuracy through a single pressing.

[0147] In other embodiments, the arc-shaped fuel element 20 can also be an involute-shaped plate fuel element, or other plate fuel elements with curved shapes. Correspondingly, the convex forming surface 121 can also be an involute-shaped convex surface.

[0148] In one embodiment, the material used for the upper die assembly 100 is a high-temperature stable chromium-based or nickel-based alloy steel or an austenitic stainless steel subjected to high-temperature stabilization treatment, specifically including ferritic stainless steels 430, 441, 409L, nickel-based alloy steels GH600, GH3030, GH4169, austenitic stainless steels S310, 321, etc. The surface roughness of the convex forming surface 121 is not higher than 1.8 μm. The material used for the lower die holder 210 may be the same as that of the upper die assembly 100 or an ordinary stainless steel material. The material used for the centering plate 240 may be the same as that of the upper die assembly 100.

[0149] In one embodiment, the above warm compaction forming preparation method and the flexible forming die can be used for the preparation of Al-based, Zr-based dispersion fuels or UMo x Metal-based arc plate type fuel elements with Zr coatings coated on alloy thin plates.

[0150] Some more specific embodiments are introduced below:

[0151] First embodiment: The curved plate type fuel element is an arc-shaped plate U3Si2-Al-based dispersion fuel element. The warm compaction forming preparation method of the arc-shaped plate U3Si2-Al-based dispersion fuel element includes the following steps:

[0152] (1) Install the upper die assembly 100 on the upper working table of the press, install the lower die holder 210 on the lower working table of the press, and evenly spray a release agent on the convex forming surface 121.

[0153] (2) Place two layers of silicone rubber molds (the upper flexible mold and the lower flexible mold respectively) in the mold cavity 201.

[0154] (3) Adjust the distance between the two centering plates 240 according to the width of the U3Si2-Al-based flat dispersion fuel element.

[0155] (4) Place the upper die assembly 100 in the first heating furnace for preheating and heat preservation. The preheating temperature is 300 ± 5 °C, and the heat preservation time is not less than 1 h. The first heating furnace is a horizontally split heating furnace.

[0156] (5) Place the cold-rolled U3Si2-Al-based flat fuel element in the second heating furnace for annealing. The annealing temperature is 350 ± 5 °C, and the heat preservation time is 20 min to 1.5 h.

[0157] (6) Use a fixture to place the annealed U3Si2-Al-based flat fuel element on the upper silicone rubber mold and ensure centering.

[0158] (7) Move the two centering plates 240 away from each other in the third direction so that the two centering plates 240 are respectively aligned with the edges at the tops of the corresponding wall blocks (the first wall block 212 / the second wall block 213), and then fix them using fasteners.

[0159] (8) Open the first heating furnace and position the heating component inside the first heating furnace at a position that does not affect the up and down movement of the upper die assembly 100.

[0160] (9) Start the press to lower the upper die assembly 100. After the convex forming surface 121 contacts the U3Si2-Al-based flat fuel element, it descends slowly (5 mm / min) until the upper die assembly reaches the end position of the predetermined stroke; continue to maintain pressure on the upper die assembly 100 for a holding pressure time of not less than 30 min.

[0161] (10) Start the press to lift the upper die assembly 100 so that the upper die assembly 100 does not affect the subsequent unloading operation of the fuel element.

[0162] (11) Use a fixture to transfer the obtained arc-shaped plate-like U3Si2-Al-based dispersion fuel element to the product rack and let it cool naturally to obtain the U3Si2-Al-based dispersion fuel element with the designed dimensions.

[0163] In this embodiment, the target dimensions of the arc-shaped plate-like U3Si2-Al-based dispersion fuel element are: thickness 2.0 mm, inner diameter radius 110 mm, arc length 60 mm, and length 500 mm. The radius of the convex forming surface 121 can be designed as 105 mm, arc length 100 mm, and length 600 mm. The material of the upper die assembly 100 can be ferritic stainless steels such as 430, 441, 409L, etc.

[0164] Second embodiment: The curved plate-shaped fuel element is an arc-shaped plate-like UO2-Zr-based dispersion fuel element. The warm compaction forming preparation method of the arc-shaped plate-like UO2-Zr-based dispersion fuel element includes the following steps:

[0165] (1) Install the upper die assembly 100 on the upper working table of the press, install the lower die holder 210 on the lower working table of the press, and evenly spray a release agent on the convex forming surface 121.

[0166] (2) Place two layers of perfluoroether rubber molds (the upper flexible mold and the lower flexible mold respectively) in the mold cavity 201.

[0167] (3) Adjust the distance between the two centering plates 240 according to the width of the UO2-Zr-based flat dispersion fuel element.

[0168] (4) Wrap the upper die assembly 100 with a covered heating shell woven with heating tapes for preheating and heat preservation of the upper die assembly 100. The preheating temperature is 300 ± 5 °C, and the heat preservation time is not less than 1 h.

[0169] (5) Place the cold-rolled UO2-Zr-based flat fuel element in the second heating furnace for annealing. The annealing temperature is 475 ± 5 °C, and the heat preservation time is 20 min to 1.5 h.

[0170] (6) Take out the annealed UO2-Zr-based flat fuel element from the second heating furnace, place it on the product rack and cool it in the air to 300 °C, then use a fixture to place the UO2-Zr-based flat fuel element on the upper perfluoroether rubber mold and ensure alignment.

[0171] (7) Move the two alignment plates 240 away from each other along the third direction so that the two alignment plates 240 are respectively aligned with the edges at the tops of the corresponding wall blocks (the first wall block 212 / the second wall block 213), and then fix them with fasteners.

[0172] (8) Remove the covered heating shell woven with heating tapes that wraps the upper die assembly 100.

[0173] (9) Start the press to lower the upper die assembly 100. After the convex forming surface 121 contacts the UO2-Zr-based flat fuel element, it slowly descends (5 mm / min) until the upper die assembly reaches the end position of the predetermined stroke; keep the pressure on the upper die assembly 100, and the pressure holding time is not less than 30 min. Among them, the deformation of the flexible die body 220 along the first direction ZZ' caused by the predetermined stroke of the upper die assembly 100 is between 15 and 20 mm.

[0174] (10) Start the press to lift the upper die assembly 100 so that the upper die assembly 100 does not affect the subsequent unloading operation of the fuel element.

[0175] (11) Use a fixture to transfer the obtained arc-shaped plate-like UO2-Zr-based dispersion fuel element to the product rack and cool it naturally to obtain the UO2-Zr-based dispersion fuel element with the designed dimensions.

[0176] In this embodiment, the target dimensions of the arc-shaped plate-like UO2-Zr-based dispersion fuel element are: thickness 2.0 mm, inner diameter radius 85 mm, arc length 120 mm, length 500 mm. The radius of the convex forming surface 121 can be designed as 85 mm, arc length 200 mm, length 600 mm. The surface roughness of the convex forming surface 121 is not higher than 1.8 μm.

[0177] The materials of the upper die assembly 100 and the lower die holder 210 are austenitic stainless steel 321 treated by high-temperature stabilization. The material of the alignment plate is stainless steel.

[0178] Third Embodiment: The curved plate type fuel element is arc-shaped plate-like UMo x -Al based dispersion fuel element, the arc-shaped plate-like UMo x -The warm compaction forming preparation method of the Al based dispersion fuel element comprises the following steps:

[0179] The curved plate type fuel element is arc-shaped plate-like UMo x -Al based dispersion fuel element, the arc-shaped plate-like UMo x -The warm compaction forming preparation method of the Al based dispersion fuel element comprises the following steps:

[0180] [[]] (1) Install the upper die assembly 100 on the upper working table of the press, install the lower die holder 210 on the lower working table of the press, and evenly spray a release agent on the convex forming surface 121.

[0181] (2) Place two layers of silicone rubber molds (the upper flexible mold and the lower flexible mold respectively) in the mold cavity 201.

[0182] (3) Adjust the distance between the two centering plates 240 according to the width of the UMo x -Al based flat plate type dispersion fuel element.

[0183] (4) Preheat and keep warm the upper die assembly 100 by using the first heating furnace, the preheating temperature is 250±5°C, the holding time is not less than 1 h, and the first heating furnace has a lower push-pull type furnace door.

[0184] (5) Place the cold-rolled UMo x -Al based flat plate type fuel element in the second heating furnace for annealing, the annealing temperature is 420±5°C, and the holding time is 20 min to 1.5 h.

[0185] (6) Take out the annealed UMo x -Al based flat plate type fuel element from the second heating furnace, place it on the product rack and cool it in the air to 270°C, and then use a fixture to place the UMo x -Al based flat plate type fuel element on the upper silicone rubber mold and ensure centering.

[0186] (7) Move the two centering plates 240 away from each other along the third direction so that the two centering plates 240 are respectively aligned with the edges at the tops of the corresponding wall blocks (the first wall block 212 / the second wall block 213), and then fix them by using fasteners.

[0187] (8) Push and pull the furnace door of the first heating furnace to an area that does not affect the up and down movement of the upper die assembly 100.

[0188] (9) Start the press to lower the upper die assembly 100, and after the convex forming surface 121 contacts the UMo x -Al-based flat fuel element, it descends slowly (5 mm / min) until the upper die assembly reaches the end position of the predetermined stroke; keep the pressure on the upper die assembly 100 for a pressure-holding time of not less than 30 min. Among them, the deformation of the flexible die body 220 in the first direction ZZ' caused by this predetermined stroke of the upper die assembly 100 is between 15 and 20 mm.

[0189] (10) Start the press to lift the upper die assembly 100 so that the upper die assembly 100 does not affect the subsequent unloading operation of the fuel element.

[0190] (11) Use a fixture to transfer the obtained arc-shaped UMo x -Al-based dispersion fuel element to the product rack and let it cool naturally to obtain a UMo x -Al-based dispersion fuel element with the designed dimensions.

[0191] In this embodiment, the target dimensions of the arc-shaped UO2-Zr-based dispersion fuel element are: thickness 2.0 mm, inner diameter radius 85 mm, arc length 120 mm, and length 500 mm. The radius of the convex forming surface 121 can be designed to be 85 mm, arc length 200 mm, and length 600 mm. The surface roughness of the convex forming surface 121 is not higher than 1.8 μm.

[0192] The materials of the upper die assembly 100 and the lower die holder 210 are chromium-based or nickel-based alloy steel. The material of the centering plate is stainless steel.

[0193] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0194] The above-described embodiments only represent several implementation manners of the present application. Their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the patent application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A warm-press molding method for preparing curved-plate fuel elements using a flexible molding die, characterized in that, The flexible molding die includes an upper mold assembly and a lower mold assembly. The lower mold assembly includes a lower mold frame and a flexible mold body installed in the lower mold frame. The flexible mold body is made of an elastic material. The warm-press molding preparation method includes: The metal-based flat fuel element is heated to the annealing temperature and held at the annealing temperature for a first time. During the heating process, the heating rate of the metal-based flat fuel element is 20℃ / min to 40℃ / min. The annealing temperature is set to be within the full range of recrystallization temperature of the metal material in the metal-based flat fuel element, starting from the lower limit of the full range and accounting for one-quarter of the lower limit interval. The upper mold assembly is preheated and held at the preheated temperature for a second time, wherein the preheated temperature is lower than or equal to the annealing temperature and lower than the upper temperature resistance limit of the flexible mold. The annealed metal-based flat fuel element is placed on the flexible mold; Pressurize the upper mold assembly to press down on the metal-based flat fuel element, which is then shaped into a curved fuel element under the support of the flexible mold. The upper mold assembly is kept under pressure for a third time, and the residual heat of the flexible molding die is used to keep the curved fuel element warm during the pressure-keeping process, so that the curved fuel element is kept in the molding space for a third time. The curved fuel element is removed from the flexible molding die and cooled.

2. The warm pressing molding preparation method according to claim 1, characterized in that, The preheating temperature is 200℃~350℃.

3. The warm pressing molding preparation method according to claim 1, characterized in that, If the annealing temperature is higher than the upper limit of the temperature resistance of the flexible mold, the annealed metal-based flat fuel element is cooled down. Once the temperature of the cooled metal-based flat fuel element is lower than the upper limit of the temperature resistance of the flexible mold, the metal-based flat fuel element is placed on the flexible mold.

4. The warm pressing molding preparation method according to claim 3, characterized in that, After the metal-based flat fuel element has cooled down to the same temperature as the preheating temperature, it is then placed on the flexible mold.

5. The warm pressing molding preparation method according to claim 3, characterized in that, The temperature of the cooled metal-based flat-plate fuel element is between 250°C and 350°C.

6. The warm pressing molding preparation method according to claim 1, characterized in that, If the annealing temperature is lower than the upper limit of the temperature resistance of the flexible mold, then the preheating temperature is made equal to the annealing temperature.

7. The warm pressing molding preparation method according to claim 1, characterized in that, In the step of preheating the upper mold assembly and maintaining it at the preheated temperature for a second time, the upper mold assembly is preheated by a first heating device, wherein the first heating device is a first heating furnace, or an electric heating rod that can be inserted into the upper mold assembly, or a covered heating shell woven from heating strips; In the step of heating the metal-based flat fuel element to the annealing temperature and holding it at the annealing temperature for a first time, the metal-based flat fuel element is heated by a second heating device, which is a second heating furnace.

8. The warm pressing molding preparation method according to claim 1, characterized in that, Also includes: Before the step of preheating the upper mold assembly and holding it at the preheated temperature for a second time, a release agent is sprayed onto the convex surface of the upper mold assembly.

9. The warm pressing molding preparation method according to claim 1, characterized in that, The material of the flexible mold includes any one or a combination of silicone rubber, fluororubber, and perfluoroether rubber.

10. The warm pressing molding preparation method according to claim 1, characterized in that, The first duration is 20 min to 90 min; the second duration is not less than 1 h; and the third duration is 10 min to 1 h.

Images