A graphite mold for hot pressing of plutonium-238 fuel pellets

Abstract

The invention discloses a graphite mold for hot pressing preparation of plutonium-238 fuel pellets, all parts of the mold are made of graphite, including a pressure cap and a pressure head, a mandrel, a reinforcement sleeve and a cushion block symmetrically arranged above and below; wherein a spiral groove is arranged on the pressure head and the reinforcement sleeve, the mandrel is made of graphite with a forming porosity of 15% to 25%, and the pressure cap, the pressure head and the mandrel form a labyrinth seal structure, and the design of the above structure avoids the overflow of powder during the hot pressing process of plutonium-238 fuel pellets, reducing the harm of plutonium-238 fuel pellet production to operators; and the gas discharged from the pressing of plutonium-238 fuel powder in the mandrel is promptly and quickly exported, avoiding the potential safety hazards to operators and experimental tasks caused by the gas being entrained by the airflow and diffusing to the entire hot pressing system. The mold of the present invention can realize the one-time hot pressing precision molding of plutonium-238 fuel pellets, and improve new tools and methods for the current production of plutonium-238 fuel pellets in my country.

Description

Technical Field

[0001] The invention relates to the technical field of isotope heat source and power supply, and in particular to a graphite mould used for hot pressing preparation of plutonium-238 fuel pellets. Background Art

[0002] Isotope power supply development technology is a new technology proposed with the rapid development of the country's aerospace industry. Isotope power supplies that use plutonium-238 as nuclear fuel are widely used due to their small size, high specific power, long service life, strong environmental applicability and high reliability. They have provided electrical and thermal energy for various aerospace missions many times.

[0003] Pu-238 fuel pellets are the core components of isotope power supplies. The core parameters of power supplies such as life, electrical power, and thermal power are closely related to the plutonium-238 fuel pellets. At present, my country mainly uses cold pressing and sintering technology to develop plutonium-238 fuel pellets. Since the plutonium-238 fuel pellets prepared by cold pressing and sintering technology are prone to cracks on the surface, and have poor uniformity and stability, this seriously limits my country's current self-production of isotope power supplies and their application in various space missions.

[0004] Research has found that the plutonium-238 fuel pellets pressed by hot pressing have the advantages of being dense and stable. The pressing process is: first, the heat-treated plutonium-238 fuel powder is loaded into the graphite mold, and then the hot press is used to complete the precision molding of the pellets. Therefore, the key to the pressing of plutonium-238 fuel pellets is the graphite mold. Since plutonium-238 fuel is extremely toxic and radioactive, the existing graphite molds cannot meet the safety requirements for the pressing of plutonium-238 fuel pellets. It is necessary to develop a graphite mold suitable for hot pressing of plutonium-238 fuel pellets. Summary of the invention

[0005] In view of this, the present invention provides a graphite mold for hot pressing preparation of plutonium-238 fuel pellets. The mold is provided with a pressure cover, and a special groove-shaped guide structure is provided on the pressure head and the reinforcement sleeve, so as to avoid the overflow of powder during the hot pressing process of the plutonium-238 fuel pellets, and guide the gas discharged during the powder pressing process to be discharged in time and quickly, thereby reducing the harm of the production of plutonium-238 fuel pellets to operators.

[0006] To achieve this purpose, the present invention adopts the following technical scheme: a graphite mold for hot pressing of plutonium-238 fuel pellets, all parts of the mold are made of graphite, and the parts include a pressure cap and a pressure head, a mandrel, a reinforcement sleeve and a spacer block symmetrically arranged up and down;

[0007] The gland comprises an upper gland and a lower gland; both the upper gland and the lower gland are hollow cylinders with one end open;

[0008] The pressure head comprises an upper pressure head and a lower pressure head;

[0009] The mandrel is a hollow cylinder, and a boss bayonet is arranged on the outer side of the mandrel;

[0010] The reinforcing sleeve is a hollow cylinder, and a notch is arranged on the inner surface of the reinforcing sleeve;

[0011] The cushion block is in the shape of a semi-open cylinder;

[0012] The connection relationship between the above-mentioned components is: the upper pressure head and the lower pressure head are respectively sleeved on the two ends of the hollow cylinder of the core shaft, the plutonium-238 fuel to be pressed is located in the hollow cylinder of the core shaft and between the two pressure heads; the upper pressure cover and the lower pressure cover are respectively sleeved on the ends of the upper pressure head and the lower pressure head; the reinforcing sleeve is sleeved on the outside of the core shaft, and the boss snap-in on the outside of the core shaft is embedded in the notch on the inner surface of the reinforcing sleeve; the gasket is detachably inserted between the reinforcing sleeve and the lower pressure cover when the mold is filled with plutonium-238 fuel.

[0013] Preferably, the mandrel is made of graphite with an average particle size of 3um to 5um and a molding void ratio of 15% to 25%.

[0014] Preferably, the end surfaces of the upper pressing head and the lower pressing head in contact with the pressed powder are both concave arc-shaped, and the radius of the arc is 1 mm to 4 mm.

[0015] Preferably, spiral grooves are formed on the outer surfaces of the upper pressure head and the lower pressure head.

[0016] Preferably, a spiral groove is provided on the inner surface of the reinforcement sleeve.

[0017] Preferably, the cross-section of the spiral groove is triangular.

[0018] Preferably, the gland opening and the core shaft are tightly matched to form a sealed space between the gland cavity and the outer surface of the core shaft.

[0019] The beneficial effects of the present invention are as follows: the graphite mold for hot pressing preparation of plutonium-238 fuel pellets of the present invention is provided with a pressure cap, and a special groove-shaped structure is provided on the pressure head and the reinforcing sleeve, and a labyrinth sealing method is adopted between the pressure cap, the pressure head and the mandrel, so as to avoid the overflow of powder during the hot pressing process of plutonium-238 fuel pellets, and reduce the harm of plutonium-238 fuel pellet production to operators; and the gas discharged during the pressing of plutonium-238 fuel powder in the mandrel is promptly and quickly guided out, so as to avoid the powder being entrained by the air flow and diffusing to the entire hot pressing system, which brings safety hazards to operators and experimental tasks. In addition, when designing the mold, the size parameters of each component of the mandrel, the pressure head, the pressure cap and the reinforcing sleeve are precisely controlled, and the one-time precision molding of the fuel pellets is realized by the limiting method. Therefore, the graphite mold for hot pressing preparation of plutonium-238 fuel pellets of the present invention provides the possibility of realizing the hot pressing of plutonium-238 fuel pellets, and also provides support for the further development of isotope batteries in my country. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic structural diagram of a graphite mold in an unpressed state for hot-pressing preparation of plutonium-238 fuel pellets in an embodiment of the present invention;

[0021] Figure 2 It is a structural schematic diagram of a graphite mold in a pressed state for hot pressing preparation of plutonium-238 fuel pellets in an embodiment of the present invention;

[0022] In the figure: 1. upper pressure cover 2. upper pressure head 3. core shaft 4. reinforcement sleeve 5. cushion block 6. lower pressure head 7. lower pressure cover. DETAILED DESCRIPTION

[0023] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

[0024] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] A kind of Figure 1 and Figure 2 The graphite mold for hot pressing of plutonium-238 fuel pellets shown in the figure is made of graphite. All parts of the mold are made of graphite. The graphite parameters are: compressive strength 200MPa~260MPa, bending strength 60MPa~100MPa, elastic modulus 10GPa~16GPa, density 1.6g / cm 3 ~2g / cm3 , Shore hardness 60~80, thermal expansion coefficient 4E-6 / ℃~5E-6 / ℃, thermal conductivity 100W / m·k~500W / m·k;

[0026] All parts of the mold include: a pressure cover and a pressure head symmetrically arranged up and down, a core shaft 3, a reinforcing sleeve 4 and a cushion block 5;

[0027] The gland includes an upper gland 1 and a lower gland 7, both of which are hollow cylinders with one end open, and the inner diameter of the cavity opening of the upper gland 1 and the lower gland 7 is slightly larger than or equal to the outer diameter of the core shaft 3. As an embodiment, the parameters of the upper and lower glands are set as follows: the outer diameter is 50mm-55mm, the inner diameter is 30mm-40mm, the height is 45mm-55mm, and the cavity depth is 40mm-50mm;

[0028] The pressing head includes an upper pressing head 2 and a lower pressing head 6, both of which are cylindrical. As an embodiment, the parameters of the upper and lower pressing heads are set as: a diameter of 25mm~30mm, a height of 80mm~90mm, and an end face in contact with the pressed powder is a concave arc shape. This structure can ensure that the overflow of powder is reduced during powder pressing. Preferably, the arc radius is 1mm~4mm; and the outer surfaces of the upper pressing head 2 and the lower pressing head 6 are both provided with spiral grooves, and the cross-section of the spiral grooves is triangular. The purpose of providing the spiral grooves is that on the one hand, the powder overflowing during the pressing process can be fixed in the spiral groove to prevent the powder from overflowing and causing pollution. On the other hand, it is ensured that when the powder is pressed, the exhausted gas can be promptly and quickly discharged along the spiral groove. Preferably, the top width of the spiral groove is 0.1mm~1mm, and the arc radius of the knife tip at the bottom of the groove is 0.2mm~0.5mm.

[0029] The core shaft 3 is a hollow cylinder with a boss snap-on provided on the outer side of the core shaft 3. The core shaft 3 is made of graphite with an average particle size of 3um to 5um and a forming porosity of 15% to 25%. This ensures that the gas generated during the pressing process can be filtered out through the core shaft 3, while at the same time ensuring the diffusion of radioactive powder.

[0030] The reinforcing sleeve 4 is a hollow cylinder with a notch on its inner surface, which cooperates with the boss on the outer side of the core shaft 3 for positioning. A spiral groove is provided on the reinforcing sleeve 4 to establish a pressure difference between the inner and outer sides of the core shaft 3 while ensuring strength, thereby ensuring that the gas in the core shaft is quickly discharged.

[0031] The cushion block 5 is in the shape of a half-open cylinder and can be detachably inserted between the reinforcement sleeve 4 and the lower pressure cover when the mold is filled with plutonium-238 fuel.

[0032] The upper pressure head 2 and the lower pressure head 6 are respectively sleeved on the two ends of the hollow cylinder of the core shaft 3. The plutonium-238 fuel to be pressed is located in the hollow cylinder of the core shaft 3 and between the two pressure heads. The upper and lower pressure heads are used to press the plutonium-238 fuel. The upper pressure cover 1 and the lower pressure cover 7 are respectively sleeved on the ends of the upper pressure head 2 and the lower pressure head 6. The cavity opening of the pressure cover and the core shaft 3 are interference fit, so that a sealed space is formed between the pressure cover cavity and the outer surface of the core shaft 3. A labyrinth seal is formed between the pressure cover, the pressure head and the core shaft, which further prevents the radioactive substances diffused from the spiral air guide groove at the front of the pressure head from diffusing to the external space; the reinforcement sleeve 4 is sleeved on the outside of the core shaft 3, and the boss bayonet on the outside of the core shaft 3 is embedded in the notch on the inner surface of the reinforcement sleeve 4.

[0033] Example 1

[0034] This embodiment is a graphite mold required for hot pressing of 62.5W plutonium-238 fuel pellets. It is made of high-purity isostatic graphite processed by high-precision machine tools. The mold design is shown in Figure 1 and Figure 2 .

[0035] The graphite material is ATDJ-05 high-purity graphite, with an average particle size of 3um, a forming porosity of 15%, a compressive strength of 240MPa, a bending strength of 85MPa, an elastic modulus of 10GPa, and a density of 1.88g / cm 3 , Shore hardness 70, thermal expansion coefficient 4.7E-6 / ℃, thermal conductivity 100W / m·k.

[0036] The parameters of the upper and lower covers of the mold are: outer diameter 55mm, inner diameter 38mm, height 51.8mm, and inner hole depth 45.8mm.

[0037] The parameters of the upper and lower rams of the mold are: diameter 27.5mm, height 81mm, the end surface in contact with the pressed powder is arc-shaped, and the arc radius is 2mm. A through triangular groove is embedded on the outer surface of the ram, the width of the top of the groove is 0.1mm, the arc radius of the tool tip at the bottom of the groove is 0.2mm, and the lead is 0.3mm.

[0038] The parameters of the mold mandrel are: outer diameter 38mm, inner diameter 27.5mm, height 179mm; the outer surface of the mandrel is embedded with a protruding edge, the width of the edge is 3mm, it protrudes 3mm horizontally outward, and the distance from the bottom of the mandrel is 40mm.

[0039] The parameters of the mold pad are as follows: the bottom is a cuboid with a length of 45mm, a width of 45mm, and a height of 10mm. The top is a cylindrical through hole with an outer diameter of 45mm, a height of 30mm, and an internal through hole radius of 38mm.

[0040] The mold can complete the hot pressing preparation of plutonium-238 simulated fuel pellets by applying pressure in both directions, and the obtained pellets have performance parameters shown in Table 1. It can be seen that the hot pressing mold provided by the present invention can be used to press out dense and stable plutonium-238 simulated fuel pellets.

[0041] Table 1 Performance parameters of plutonium-238 simulated fuel pellets (cerium oxide)

[0042]

Claims

1. A graphite mold for hot pressing of plutonium-238 fuel pellets, characterized in that: All parts of the mold are made of graphite, including a gland and a pressure head symmetrically arranged up and down, a mandrel, a reinforcing sleeve and a cushion block; The gland comprises an upper gland and a lower gland; both the upper gland and the lower gland are hollow cylinders with one end open; The pressure head comprises an upper pressure head and a lower pressure head; The mandrel is a hollow cylinder, and a boss bayonet is arranged on the outer side of the mandrel; The reinforcing sleeve is a hollow cylinder, and a notch is arranged on the inner surface of the reinforcing sleeve; The cushion block is in the shape of a semi-open cylinder; The connection relationship between the above-mentioned components is: the upper pressure head and the lower pressure head are respectively sleeved on the two ends of the hollow cylinder of the core shaft, the plutonium-238 fuel to be pressed is located in the hollow cylinder of the core shaft and between the two pressure heads; the upper pressure cover and the lower pressure cover are respectively sleeved on the ends of the upper pressure head and the lower pressure head; the reinforcing sleeve is sleeved on the outside of the core shaft, and the boss snap-in on the outside of the core shaft is embedded in the notch on the inner surface of the reinforcing sleeve; the gasket is detachably inserted between the reinforcing sleeve and the lower pressure cover when the mold is filled with plutonium-238 fuel.

2. The graphite mold for hot pressing of plutonium-238 fuel pellets according to claim 1, characterized in that: The core shaft is made of graphite with an average particle size of 3um to 5um and a forming void ratio of 15% to 25%.

3. The graphite mold for hot pressing of plutonium-238 fuel pellets according to claim 1, characterized in that: The end surfaces of the upper pressing head and the lower pressing head in contact with the pressed powder are both concave arc-shaped, and the arc radius is 1mm to 4mm.

4. The graphite mold for hot pressing of plutonium-238 fuel pellets according to claim 1, characterized in that: The outer surfaces of the upper pressing head and the lower pressing head are provided with spiral grooves.

5. The graphite mold for hot pressing of plutonium-238 fuel pellets according to claim 1, characterized in that: The inner surface of the reinforcement sleeve is provided with a spiral groove.

6. The graphite mold for hot pressing of plutonium-238 fuel pellets according to claim 4 or 5, characterized in that: The cross section of the spiral groove is a triangle.

7. The graphite mold for hot pressing of plutonium-238 fuel pellets according to claim 1, characterized in that: The inner surface of the gland opening is interference fit with the outer surface of the core shaft.

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