Isostatic pressing mold and blank preparation method for spherical fuel element

Abstract

This invention relates to the field of spherical nuclear fuel element molding technology, specifically to an isostatic pressing molding die and a method for preparing spherical fuel element blanks. The isostatic pressing molding die includes an isostatic pressing core ball mold for pressing the core ball and an isostatic pressing molding die for pressing the spherical fuel element blank. It consists of at least a die body and an inverted conical top cover for closing the opening of the die body. After mold closing, both the internal cavity and the external shape are spherical. The die material is one of nitrile rubber, silicone rubber, EPDM rubber, natural rubber, and polyurethane. When preparing spherical fuel element blanks using this die, nuclear fuel and matrix graphite powder are first filled into the die body of the core ball mold and stirred evenly. After mold closing, it is vacuum-sealed and isostatically pressed to obtain the core ball. Then, the core ball and matrix graphite powder in the fuel element blank mold are loaded, vacuum-sealed, and isostatically pressed to obtain the spherical fuel element blank. This invention provides convenient material loading, easy demolding, and reusability of the die, and also helps improve the isotropy and molding quality of the spherical fuel element blank.

Description

Technical Field

[0001] This invention relates to the field of spherical fuel element molding technology, specifically to an isostatic pressing mold for spherical fuel elements and a method for preparing spherical blanks. Background Technology

[0002] my country's high-temperature gas-cooled nuclear reactor has been successfully commercialized and is currently undergoing commercialization. The reactor uses spherical fuel elements. Spherical fuel element blanks require high levels of isotropy, density uniformity, and consistent thermal conductivity. Excellent isotropy reduces thermal and radiation stresses generated during reactor operation, contributing to reactor safety. Current fuel element production methods include quasi-isostatic pressing and dry-bag isostatic pressing. Both methods utilize the isostatic pressing concept, but neither is completely isostatic. The resulting elements still exhibit some anisotropy, leading to differences in microstructure and thermal conductivity in different directions, thus affecting the molding quality and performance of the fuel elements.

[0003] Isostatic pressing (IPC) allows pressure to be applied more evenly to the material being molded, making it an effective way to reduce the anisotropy of the preform and improve the consistency of its microstructure and properties. In IPC, the die is the key component for achieving uniform pressure transmission; the structure and material properties of the die directly affect the pressure state of the material being molded, and thus the molding quality of the preform.

[0004] Therefore, it is necessary to provide an isostatic pressing mold and a method for preparing spherical fuel elements that are easy to load, reusable, provide uniform pressure transmission, and enable the inner fuel zone and the outer non-fuel zone to form a stable integrated structure. Summary of the Invention

[0005] The purpose of this invention is to provide an isostatic pressing mold and a method for preparing spherical fuel element blanks, in order to solve the problems of large anisotropy, poor density uniformity, inconvenient mold loading, difficult demolding, poor reusability, and unstable bonding between the core ball and the outer layer in the existing spherical fuel element blanks during the molding process.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] The present invention provides an isostatic pressing mold for spherical fuel elements. The isostatic pressing mold includes a mold body for forming the shape of a spherical fuel element blank and an inverted conical top cover for closing the opening of the mold body. After the mold body and the top cover are closed by a parting surface, the internal cavity is spherical and the external shape is also spherical.

[0008] Preferably, the outer diameter of the inverted conical top cover is 0.6 to 0.7 times the diameter of the internal cavity of the mold body.

[0009] Preferably, the parting surface between the inverted conical top cover and the mold body is stepped.

[0010] Preferably, the mold material is one of nitrile rubber, silicone rubber, EPDM rubber, natural rubber, and polyurethane.

[0011] Preferably, the Shore hardness of the mold material is 30 to 70.

[0012] Furthermore, the mold includes an isostatic pressing core ball mold for pressing core balls and an isostatic pressing forming mold for pressing spherical fuel element blanks. The isostatic pressing core ball mold and the isostatic pressing forming mold for pressing spherical fuel element blanks are similar in basic structure, both including a mold body and an inverted conical top cover for closing the opening of the mold body.

[0013] Preferably, the inner cavity of the core ball mold is provided with inverted triangular grooves distributed in latitude and longitude to form an interlocking effect during the subsequent pressing process, thereby enhancing the bond between the core ball and the outer graphite matrix.

[0014] The present invention also provides a method for preparing spherical fuel element blanks using the above-mentioned isostatic pressing mold, comprising the following steps:

[0015] (1) Fill the core ball mold body with nuclear fuel and matrix graphite powder, and use a stirrer to stir the nuclear fuel and matrix graphite powder evenly.

[0016] (2) Close the top cover with the mold body and put it into a plastic bag for vacuum sealing;

[0017] (3) Place the packaged mold into an isostatic press, press it to form, take it out, and unpack it to obtain the core ball;

[0018] (4) A layer of matrix graphite powder is pre-laid in the mold body of the fuel element ball blank mold, the core ball is placed into the mold cavity of the fuel element ball blank mold, and the matrix graphite powder is continued to be filled so that the core ball is evenly wrapped by the matrix graphite powder. After the top cover is closed, vacuum sealing is performed.

[0019] (5) The encapsulated mold is placed into an isostatic press for pressing. After being pressed and formed, it is taken out and the packaging is removed to obtain a spherical fuel element blank.

[0020] Preferably, the isostatic pressing pressure of the core ball in step (3) is 2 to 10 MPa.

[0021] Preferably, when cold isostatic pressing is used in step (5), the temperature of the liquid medium is room temperature and the molding pressure is 280-300 MPa.

[0022] Preferably, when using isostatic pressing in step (5), the temperature of the liquid medium is 100-150°C and the molding pressure is 100-200 MPa.

[0023] Beneficial effects

[0024] 1. The present invention uses a spherical isostatic pressing mold composed of a mold body and an inverted conical top cover, which is beneficial to uniform pressure transmission and improves the density uniformity and isotropy of the spherical fuel element blank.

[0025] 2. The present invention, by setting an inverted conical top cover and a stepped parting surface, facilitates mold closing and positioning, vacuum sealing and demolding, and improves the stability of spherical blank forming.

[0026] 3. The present invention provides inverted triangular grooves with latitude and longitude distribution in the inner cavity of the core ball mold, which can enhance the interlocking and bonding between the core ball and the outer graphite matrix, and improve the overall structural stability of the ball blank.

[0027] 4. The mold described in this invention is made of elastic material and has the advantages of convenient loading, easy demolding, and reusability.

[0028] 5. The method of the present invention is applicable to different forming conditions such as cold isostatic pressing and warm isostatic pressing, which can improve the forming quality of spherical fuel element blanks and reduce defects such as cracks and delamination. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the fuel element blank mold after it has been filled with core balls and matrix graphite powder in this invention.

[0030] Figure 2 This is a schematic diagram of the isostatic pressing forming mold of the present invention. The structure is applicable to isostatic pressing core ball molds for pressing core balls and isostatic pressing forming molds for pressing spherical fuel element blanks.

[0031] Figure 3 This is a schematic diagram of the hydrostatic pressing principle of the present invention.

[0032] Figure 4 This is a flowchart of the method for preparing spherical fuel element blanks in this invention.

[0033] In the diagram: 1. Inverted conical top cover; 2. Core ball; 3. Mold body; 4. Parting surface; 5. Fuel zone; 6. Vacuum sealing bag; 7. Pressure transmission medium; 8. Mold. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0035] Example 1: Mold Structure Implementation

[0036] like Figure 1 , 2 As shown, this embodiment provides an isostatic pressing molding die for spherical fuel elements, including an isostatic pressing core ball mold for pressing core balls and an isostatic pressing molding die for pressing spherical fuel element blanks. Wherein, Figure 2 The structure shown can be used as an isostatic pressing mold for pressing core balls 2, and also as an isostatic pressing molding mold for pressing spherical fuel element blanks.

[0037] Both the isostatic pressing core ball mold and the isostatic pressing forming mold for pressing spherical fuel element blanks include a mold body 3 and an inverted conical top cover 1 for closing the opening of the mold body 3. After the mold body 3 and the inverted conical top cover 1 are closed by the parting surface 4, the internal cavity is spherical and the external cavity is also spherical, which is beneficial to uniformly transmit external pressure to the forming material during the isostatic pressing process, thereby improving the density uniformity and isotropy of the formed blank.

[0038] In this embodiment, the outer diameter of the inverted conical top cover 1 is 0.65 times the diameter of the internal cavity. The parting surface 4 between the inverted conical top cover 1 and the mold body 3 has a stepped structure to improve the stability of mold closing and positioning, and to facilitate vacuum sealing and subsequent demolding.

[0039] The difference between the core ball mold and the isostatic pressing mold for spherical fuel element blanks lies in the fact that the core ball mold has inverted triangular grooves distributed along the latitude and longitude of its inner cavity. These inverted triangular grooves are distributed along the inner surface of the core ball mold in a manner similar to longitude and latitude lines. After the core ball 2 is pressed, corresponding interlocking structures can be formed on its surface, thereby enhancing the bonding between the core ball 2 and the outer graphite matrix during the subsequent pressing process of the spherical fuel element blank.

[0040] In this embodiment, both the core ball mold and the isostatic pressing mold for the spherical fuel element blank are made of nitrile rubber with a Shore hardness of 50. This material has good elasticity, pressure resistance, and resilience, facilitating loading and demolding, and enabling relatively uniform transmission of external pressure during isostatic pressing. It should be noted that in other embodiments, the mold material can also be silicone rubber, EPDM rubber, natural rubber, or polyurethane, and the Shore hardness can be between 30 and 70.

[0041] Example 2: Core Ball Preparation Implementation Method

[0042] This embodiment provides a method for preparing core ball 2 using the core ball mold described in Embodiment 1.

[0043] First, nuclear fuel and matrix graphite powder are filled into the mold body 3 of the core ball mold, and a stirrer is inserted into the powder to fully mix the nuclear fuel and matrix graphite powder to improve the uniformity of the internal composition distribution of the core ball 2.

[0044] Subsequently, the inverted conical top cover 1 of the core ball mold is closed with the mold body 3 and placed into a vacuum sealing bag 6 for vacuum sealing to remove residual gas in the mold cavity and between powder particles.

[0045] After vacuum sealing, the sealed mold 8 is placed in an isostatic press for pressing and shaping. In this embodiment, the isostatic pressing pressure of the core ball 2 is 6 MPa. After pressing, the mold 8 is removed, the vacuum sealing bag 6 is opened, and the core ball 2 is obtained.

[0046] Because the inner cavity of the core ball mold has inverted triangular grooves distributed with latitude and longitude, the surface of the resulting core ball 2 forms a corresponding interlocking structure. This structure facilitates a more stable mechanical interlocking between the outer matrix graphite powder and the core ball 2 during subsequent ball blank pressing, thereby improving the bonding strength between the inner and outer layers.

[0047] Example 3: Implementation of Cold Isostatic Pressing Method for Preparing Spherical Fuel Element Blanks

[0048] This embodiment provides a method for preparing spherical fuel element blanks using the isostatic pressing mold described in Embodiment 1.

[0049] First, a layer of matrix graphite powder is pre-laid inside the mold body 3 of the spherical fuel element blank mold; then, the core ball 2 obtained in Example 2 is placed in the middle of the spherical mold cavity of the mold body 3; then, matrix graphite powder is continued to be filled so that the core ball 2 is evenly wrapped by the matrix graphite powder, thereby forming the loading structure of the spherical fuel element blank, wherein the inner layer is the fuel zone 5.

[0050] After the material is loaded, the inverted conical top cover 1 is placed on the mold body 3, so that the mold body 3 and the inverted conical top cover 1 are closed through the stepped parting surface 4 to form an integral mold 8 with a spherical interior and exterior. Then, the closed mold 8 is placed into a vacuum sealing bag 6 for vacuum sealing.

[0051] After vacuum sealing, the sealed mold 8 is placed into the pressure-transmitting medium 7 in an isostatic press for pressing, such as... Figure 3 As shown. This embodiment uses a cold isostatic pressing process, with the liquid medium temperature at room temperature and the molding pressure at 290 MPa. After pressing, the mold 8 is removed, the vacuum sealing bag 6 is opened, and the material is demolded to obtain a spherical fuel element blank.

[0052] In this embodiment, because the surface of the core ball 2 has an interlocking structure formed by the inverted triangular grooves, the outer matrix graphite powder can form a stable bond with the surface of the core ball 2 during the pressing process, thereby forming a stable integrated structure between the inner fuel zone 5 and the outer matrix graphite layer. Meanwhile, the overall shape of the mold 8 is spherical, which facilitates more uniform application of external pressure to the molding material during isostatic pressing, improving the density uniformity and isotropy of the spherical blank.

[0053] Example 4: Warm isostatic pressing method for preparing spherical fuel element blanks

[0054] This embodiment is basically the same as embodiment 3, except that the final pressing adopts a warm isostatic pressing process.

[0055] First, a layer of matrix graphite powder is pre-laid inside the mold body 3 of the spherical fuel element blank mold. The core ball 2 obtained in Example 2 is placed into the mold cavity of the mold body 3, and then the matrix graphite powder is filled in to make the core ball 2 evenly coated. Then, the inverted conical top cover 1 is covered, so that the inverted conical top cover 1 and the mold body 3 are closed through the stepped parting surface 4, and the entire mold 8 is placed into the vacuum sealing bag 6 for vacuum sealing.

[0056] After vacuum sealing, the sealed mold 8 is placed in an isostatic press and pressed within the pressure-transmitting medium 7. In this embodiment, the liquid medium temperature is 120°C and the molding pressure is 150 MPa. After pressing, the mold 8 is removed, the vacuum sealing bag 6 is opened, and the mold is demolded to obtain a spherical fuel element blank.

[0057] In this embodiment, isostatic pressing conditions are beneficial for improving the compaction effect of the molding material and reducing the breakage rate of the spherical blank during pressing and demolding. Due to the interlocking structure on the surface of the core ball 2 and the overall spherical compression characteristics of the mold 8, the resulting spherical fuel element blank also has good overall bonding, density uniformity and isotropy.

[0058] Example 5 Performance Comparison Experiment

[0059] To verify the improvement effect of the isostatic pressing method of the present invention on the isotropy of the graphite spheres, several graphite spheres were pressed using a quasi-isostatic pressing method, a dry-bag isostatic pressing method, and the isostatic pressing method described in Example 3 of the present invention. After carbonization, turning, and purification, graphite spheres were obtained. The drop strength and thermal expansion anisotropy were tested respectively, and the results are as follows:

[0060] Table 1. Comparison of ball drop intensity (ball drop height 5m, unit: times)

[0061]

[0062] Table 2. Comparison of anisotropy in thermal expansion (unit: K)-1 )

[0063]

[0064] The test results show that the samples prepared using the quasi-isostatic pressing method, the dry-bag isostatic pressing method, and the method of this invention all meet the technical requirements. Further comparison reveals that the sample prepared by the method of this invention has a thermal expansion coefficient ratio closer to 1 when parallel to the pressing direction and perpendicular to the pressing direction, indicating that the method of this invention can effectively reduce the anisotropy of the pellets and improve their isotropy. Good isotropy is beneficial to improving the stability and safety of fuel elements during in-reactor operation.

Claims

1. A spherical fuel element isostatic pressing mold, characterized in that, The isostatic pressing mold includes a mold body for forming the shape of a spherical fuel element blank and an inverted conical top cover for closing the opening of the mold body. After the mold body and the top cover are closed by the parting surface, the internal cavity is spherical and the external shape is also spherical.

2. The isostatic pressing mold for spherical fuel elements according to claim 1, characterized in that, The outer diameter of the inverted conical top cover is 0.6 to 0.7 times the diameter of the internal cavity of the mold body.

3. The isostatic pressing mold for spherical fuel elements according to claim 1, characterized in that, The parting surface between the inverted conical top cover and the mold body is stepped.

4. The isostatic pressing mold for spherical fuel elements according to any one of claims 1-3, characterized in that, The mold is made of one of the following materials: nitrile rubber, silicone rubber, EPDM rubber, natural rubber, or polyurethane.

5. The isostatic pressing mold for spherical fuel elements according to claim 4, characterized in that, The Shore hardness of the material is 30 to 70.

6. The isostatic pressing mold for spherical fuel elements according to any one of claims 1-5, comprising an isostatic pressing mold for pressing core balls and an isostatic pressing mold for pressing spherical fuel element blanks.

7. The core ball mold according to claim 6, characterized in that, The mold cavity has inverted triangular grooves distributed with latitude and longitude.

8. A method for preparing spherical fuel element blanks using the isostatic pressing mold for spherical fuel elements as described in any one of claims 1 to 7, characterized in that, It includes the following steps.

9. (1) Fill the core ball mold body with nuclear fuel and matrix graphite powder, and use a stirrer to stir the nuclear fuel and matrix graphite powder evenly. (2) Close the top cover with the mold body and put it into a plastic bag for vacuum sealing; (3) Place the packaged mold into an isostatic press, press it to form, take it out, and unpack it to obtain the core ball; (4) A layer of matrix graphite powder is pre-laid in the mold body of the fuel element ball blank mold, the core ball is placed into the mold cavity of the fuel element ball blank mold, and the matrix graphite powder is continued to be filled so that the core ball is evenly wrapped by the matrix graphite powder. After the top cover is closed, vacuum sealing is performed. (5) The encapsulated mold is placed into an isostatic press for pressing. After being pressed and formed, it is taken out and the packaging is removed to obtain a spherical fuel element blank.

10. The method according to claim 8, characterized in that, In step (3), the isostatic pressing pressure of the core ball is 2 to 10 MPa.

11. The method according to claim 8, characterized in that, When pressing with an isostatic press in step (5), the temperature of the liquid medium is room temperature and the molding pressure is 280-300 MPa.

12. The method according to claim 8, characterized in that, When pressing in step (5) using an isostatic press, the temperature of the liquid medium is 80-150℃ and the molding pressure is 100-200MPa.

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