Uranium nitride-silicon carbide-carbon composite pellet and preparation method thereof

Abstract

The invention discloses a uranium nitride-silicon carbide-carbon composite pellet and a preparation method thereof. The composite pellet comprises a dispersed phase, and the dispersed phase comprises uranium nitride; the continuous phase comprises silicon carbide and a carbon elementary substance, and the continuous phase is dispersed in the dispersed phase. The composite pellet has high density, heat conductivity and uranium density, low fission product release rate and high safety and economical efficiency, the preparation method is simple, and the composite pellet has wide application prospects in the field of nuclear fuel. The reactor can be applied to various advanced pressurized water reactors, fast neutron reactors and space reactors.

Description

Technical Field

[0001] The present application relates to the field of nuclear fuel technology, and in particular to a uranium nitride-silicon carbide-carbon composite pellet and a preparation method thereof. Background Art

[0002] Existing technologies use uranium dioxide as nuclear fuel, which has relatively low thermal conductivity and is prone to brittleness at higher temperatures, which limits its stability in high-temperature environments.

[0003] Uranium nitride (UN) is widely regarded as accident-tolerant fuel (ATF), fast reactor fuel and space reactor fuel because of its high thermal conductivity, high melting point, high uranium density, good irradiation stability and hard neutron spectrum. However, UN has a relatively active chemical property. Its fine powder is easily oxidized by air and spontaneously combusts in room temperature air. It is also very easy to react with water at higher temperatures, which may cause accidents such as fission product leakage in the reactor.

[0004] Therefore, the fuel pellets described above cannot meet the requirements of the reactor for higher economy and higher safety. Summary of the invention

[0005] The present application aims to solve at least one of the technical problems existing in the prior art to a certain extent. To this end, the present application provides a uranium nitride-silicon carbide-carbon composite pellet and a preparation method thereof. The composite pellet has high density, thermal conductivity, uranium density, good resistance to water and oxygen corrosion at high temperature, high safety and economy, simple preparation method, and broad application prospects in the field of nuclear fuel. It can be applied to various advanced pressurized water reactors, fast neutron reactors and space reactors. Silicon carbide (SiC) has excellent irradiation stability, chemical stability and high temperature stability, and is also resistant to corrosion by water and oxygen at high temperatures. And its thermal conductivity is also higher than that of UN.

[0006] In a first aspect of the present application, the present application proposes a composite pellet. According to an embodiment of the present application, the composite pellet comprises: a dispersed phase, the dispersed phase comprises uranium nitride; a continuous phase, the continuous phase comprises silicon carbide and carbon element and is continuously distributed, and the dispersed phase is dispersed in the continuous phase.

[0007] According to the composite pellet of the above embodiments of the present application, the dispersion phase includes uranium nitride, which has the characteristics of high melting point and high uranium density and can be used as the fuel of a nuclear reactor. Therefore, uranium nitride can meet the application requirements of nuclear fuel elements in many aspects. In addition, the continuous phase includes silicon carbide and a very small amount of carbon. The continuous three-dimensional network formed by silicon carbide can wrap the uranium nitride particles, isolate them from water and oxygen in the outside world, limit fission products, and also form a heat conduction network. The very small amount of carbon can adjust the interfacial bonding between the two phases of silicon carbide and uranium nitride. Thus, the release rate of fission products can be significantly reduced, the thermal conductivity of the composite pellet can be increased, and to a certain extent, the resistance of UN particles and the entire composite pellet to water and oxygen corrosion at high temperatures can be improved. Therefore, in the composite pellet of the present application, silicon carbide can form a "micro-encapsulation" for the uranium nitride particles wrapped therein. Each uranium nitride particle and the silicon carbide between the particles constitute a small "fuel element", and fission products can be effectively retained in the UN particles wrapped by SiC and are not easily leaked.

[0008] In addition, the composite pellet according to the above embodiments of the present application may further have the following additional technical features:

[0009] According to an embodiment of the present application, the content of uranium nitride in the composite pellet is generally not less than 50 wt%.

[0010] According to an embodiment of the present application, the content of silicon carbide in the composite pellet is generally not more than 50 wt%.

[0011] According to an embodiment of the present application, the uranium nitride is in the form of particles and is discontinuously distributed in the composite pellet.

[0012] According to an embodiment of the present application, the silicon carbide is continuously distributed in the composite pellet in a three-dimensional network shape.

[0013] In the second aspect of the present application, the present application proposes a method for preparing the above composite pellet. According to an embodiment of the present application, the method includes: adding uranium nitride powder to an organic solvent containing a silicon carbide precursor for mixing treatment to obtain a precursor solution; drying, heat-preserving, and grinding the precursor solution to obtain a mixed powder; and sintering the mixed powder to obtain the composite pellet.

[0014] According to the method for preparing the above composite pellets according to the embodiments of the present application, by mixing uranium nitride powder with an organic solvent containing a silicon carbide precursor, the dispersed phase includes uranium nitride, and uranium nitride has the characteristics of high melting point and high uranium density and can be used as the fuel of a nuclear reactor. Thus, uranium nitride can meet the application requirements of nuclear fuel elements in multiple aspects; in addition, the continuous phase includes silicon carbide and a very small amount of carbon. The continuous three-dimensional network formed by silicon carbide can wrap the uranium nitride particles, isolate them from water and oxygen in the outside world and limit fission products, and can also form a heat conduction network. Thus, the release rate of fission products can be significantly reduced, the thermal conductivity of the composite pellets can be increased, and to a certain extent, the water and oxygen corrosion resistance of UN particles and the entire composite pellets at high temperatures can be improved. Thus, in the composite pellets of the present application, silicon carbide can form a "micro-encapsulation" for the uranium nitride particles wrapped therein. Each uranium nitride particle and the silicon carbide between the particles constitute a small fuel element, and fission products can be effectively retained in the UN particles wrapped by SiC and are not easily leaked.

[0015] In addition, the composite pellets according to the above embodiments of the present application may further have the following additional technical features:

[0016] According to the embodiments of the present application, the average particle size of the uranium nitride powder is 1 to 30 μm.

[0017] According to the embodiments of the present application, the silicon carbide precursor includes at least one of polycarbosilane, polymethylsilane, polydimethylsilane, polysilane, polysilyne, and polydimethylsiloxane.

[0018] According to the embodiments of the present application, the organic solvent includes at least one of benzene, xylene, tetrahydrofuran, n-hexane, cyclohexane, N,N-dimethylformamide, and dimethyl sulfoxide.

[0019] According to the embodiments of the present application, the mixing treatment is carried out in an inert atmosphere.

[0020] According to the embodiments of the present application, the heat preservation is carried out in an inert atmosphere or in a vacuum.

[0021] According to the embodiments of the present application, the drying treatment includes at least one of natural evaporation, heating evaporation, vacuum evaporation, vacuum rotary evaporation, freeze drying, and spray drying.

[0022] According to the embodiments of the present application, the heat preservation is carried out at 500 to 1000 °C for 0.5 to 5 h.

[0023] According to the embodiments of the present application, the sintering treatment includes spark plasma sintering, hot pressing sintering, and hot isostatic pressing sintering; preferably, spark plasma sintering.

[0024] According to an embodiment of the present application, the heating rate of the spark plasma sintering is 5 to 250 °C / min, the target temperature is 1300 to 2200 °C, the holding time is 1 to 120 min, and the pressure is 20 to 200 MPa.

[0025] According to an embodiment of the present application, the sintering treatment is carried out in an inert atmosphere or in a vacuum.

[0026] According to an embodiment of the present application, the sintering mold used in the sintering treatment contains a graphite lining.

[0027] According to an embodiment of the present application, after the sintering treatment is completed, the obtained sintered product is polished.

[0028] According to an embodiment of the present application, after the polishing treatment is completed, the polished product is subjected to isothermal heat treatment in an inert atmosphere at 1100 to 1900 °C for 1 to 100 h.

[0029] Additional aspects and advantages of the present application will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present application. Description of the Drawings

[0030] The above and / or additional aspects and advantages of the present application will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0031] Figure 1 is a schematic structural diagram of the composite pellet of the present application.

[0032] Figure 2 is an XRD pattern of uranium nitride powder before and after being coated with a silicon carbide precursor and heat-treated in Example 1 of the present application.

[0033] Figure 3 is a low-magnification SEM image of the obtained composite pellet in Example 1 of the present application.

[0034] Figure 4 is a high-magnification SEM image of the obtained composite pellet in Example 1 of the present application and its corresponding EDX image. Detailed Embodiments

[0035] Embodiments of the present application will be described in detail below. The following described embodiments are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application.

[0036] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. Further, in the description of this application, unless otherwise specified, the meaning of "plurality" is two or more.

[0037] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0038] In this document, the terms "include" or "comprising" are open expressions, that is, including the contents specified in the present application but not excluding other contents.

[0039] As used herein, the terms "optionally", "optional" or "optionally" generally mean that the subsequently described event or circumstance may but need not occur, and that the description includes instances where the event or circumstance occurs and instances where it does not.

[0040] The present application proposes a composite core block and a preparation method thereof, which will be described in detail below.

[0041] Composite pellets

[0042] In a first aspect of the present application, the present application proposes a composite pellet. According to an embodiment of the present application, the composite pellet comprises: a dispersed phase, the dispersed phase comprises uranium nitride; a continuous phase, the continuous phase comprises silicon carbide and carbon, and the continuous phase is dispersed in the dispersed phase. According to an embodiment of the present application, Figure 1 The composite core block includes uranium nitride, silicon carbide and carbon.

[0043] According to the method for preparing the above-mentioned composite pellet according to the embodiments of the present application, by mixing uranium nitride powder with an organic solvent containing a silicon carbide precursor, the dispersed phase thereof includes uranium nitride. Uranium nitride has the characteristics of high melting point and high uranium density and can be used as the fuel of a nuclear reactor. Thus, uranium nitride can meet the application requirements of nuclear fuel elements in multiple aspects. In addition, the continuous phase includes silicon carbide and a very small amount of carbon. The continuous three-dimensional network formed by silicon carbide can wrap the uranium nitride particles, isolate them from water and oxygen in the outside world, limit fission products, and also form a heat conduction network. Thus, the release rate of fission products can be significantly reduced, the thermal conductivity of the composite pellet can be increased, and to a certain extent, the resistance of UN particles and the entire composite pellet to water and oxygen corrosion at high temperatures can be improved. Thus, silicon carbide in the composite pellet of the present application can form "micro-encapsulation" for the uranium nitride particles wrapped therein. Each uranium nitride particle and the silicon carbide between the particles constitute a small fuel element, and fission products can be effectively retained in the UN particles wrapped by SiC and are not easily leaked.

[0044] In addition, the composite pellet according to the above embodiments of the present application may further have the following additional technical features:

[0045] According to the embodiments of the present application, the uranium nitride content in the composite pellet is not less than 50 wt%. Thus, the uranium density of the composite pellet can be further increased, and the content ratio can be flexibly adjusted.

[0046] According to the embodiments of the present application, the silicon carbide content in the composite pellet is not greater than 50 wt%. Thus, it is ensured that the composite pellet has a high thermal conductivity and a low fission product release rate, while improving its corrosion resistance and high-temperature resistance, and the content ratio can be flexibly adjusted.

[0047] According to the embodiments of the present application, the uranium nitride is in the form of dispersed particles and is discontinuously distributed in the composite pellet. Thus, an efficient micro-encapsulation structure is formed, ensuring the high uranium density of the composite pellet and the efficiency and performance of nuclear fuel.

[0048] According to the embodiments of the present application, the silicon carbide is continuously distributed in the composite pellet in a three-dimensional network form. Thus, an efficient micro-encapsulation structure is formed, thereby increasing the thermal conductivity of the composite pellet at high temperatures and helping to limit the spillage of fission products.

[0049] Method for preparing a uranium nitride-silicon carbide-carbon composite pellet

[0050] In the second aspect of the present application, the present application proposes a method for preparing the above-mentioned composite pellet. According to an embodiment of the present application, the method comprises: adding uranium nitride powder to an organic solvent containing a silicon carbide precursor for mixing to obtain a precursor solution; drying the precursor solution, keeping it warm, and grinding it to obtain a mixed powder; and sintering the mixed powder to obtain the composite pellet.

[0051] According to the method for preparing the above-mentioned composite pellets in the embodiment of the present application, uranium nitride powder is mixed with an organic solvent containing a silicon carbide precursor. Uranium nitride has the characteristics of high thermal conductivity, high melting point, and high uranium density, and can be used as a fuel for a nuclear reactor. Therefore, uranium nitride can meet the application requirements of nuclear fuel elements in many aspects; in addition, the continuous phase includes silicon carbide and carbon element, and silicon carbide can wrap uranium nitride particles, limit the leakage of fission products, and isolate it from external water and oxygen, and limit the fission products, and can also form a heat conduction network to improve the thermal conductivity of the composite pellets; the precursor solution is dried, and the silicon carbide precursor is precipitated and coated on the surface of the uranium nitride powder, and the grain growth of silicon carbide and uranium nitride is promoted by heat preservation, and a mixed powder is obtained after grinding; the mixed powder is sintered to obtain a formed composite pellet, and the content ratio of uranium nitride and silicon carbide in the composite pellet and the size of the pellet can be flexibly adjusted. Therefore, the silicon carbide in the composite pellet prepared by this method can form a "micro-encapsulation" for the uranium nitride particles wrapped therein, and each uranium nitride particle and the silicon carbide between the particles constitute a small fuel element. The fission products can be effectively retained in the SiC-wrapped UN particles and are not easy to leak.

[0052] In addition, the composite core block according to the above embodiment of the present application may also have the following additional technical features:

[0053] According to an embodiment of the present application, the average particle size of the uranium nitride powder is 1 to 30 μm. Thus, this particle size range facilitates the bonding between the uranium nitride powder and the silicon carbide precursor, ensuring a suitable contact area and reaction activity during the sintering process, so that the uranium nitride is granular and discontinuously distributed in the composite pellet.

[0054] According to an embodiment of the present application, the silicon carbide precursor includes at least one of polycarbosilane, polymethylsilane, polydimethylsilane, polysilane, polysilane, polysilyne, and polydimethylsiloxane. Thus, the selected silicon carbide precursor can be coated on the surface of the uranium nitride powder, so that the silicon carbide is continuously distributed in the composite core block in a three-dimensional network.

[0055] According to an embodiment of the present application, the organic solvent includes at least one of benzene, xylene, tetrahydrofuran, n-hexane, cyclohexane, N,N-dimethylformamide, and dimethyl sulfoxide, so that the precursor can be uniformly dissolved and dispersed to form a uniform coating layer on the surface of the UN powder.

[0056] According to an embodiment of the present application, the mixing process is performed in an inert atmosphere, thereby protecting the uranium nitride powder from oxidation, avoiding the formation of oxides, and ensuring the purity of the powder and the nuclear purity level requirements.

[0057] According to an embodiment of the present application, the heat preservation is performed in an inert atmosphere or vacuum, thereby protecting the uranium nitride powder from oxidation, avoiding the formation of oxides, and ensuring the purity of the powder and the nuclear purity level requirements.

[0058] According to an embodiment of the present application, the drying process includes at least one of natural evaporation, heating evaporation, reduced pressure evaporation, reduced pressure rotary evaporation, freeze drying, and spray drying, thereby causing the silicon carbide precursor to precipitate and evenly coat the surface of the uranium nitride powder.

[0059] According to an embodiment of the present application, the heat preservation is to stand at 500-1000° C. for 0.5-5 hours. Thus, the silicon carbide precursor is fully solidified on the surface of the uranium nitride powder, promoting the grain growth of silicon carbide and uranium nitride and the graphitization of carbon.

[0060] According to an embodiment of the present application, the sintering process includes spark plasma sintering, hot pressing sintering, hot isostatic pressing sintering; preferably, spark plasma sintering. Thus, the sintering technology is used to achieve rapid densification of the powder at a lower temperature and in a shorter time.

[0061] According to the embodiment of the present application, the heating rate of the spark plasma sintering is 5-250℃ / min, the target temperature is 1300-2200℃, the holding time is 1-120min, and the pressure is 20-200MPa, preferably: the heating rate is 200℃ / min, the target temperature is 1300-2200℃, the holding time is 1-120min, and the pressure is 50MPa. Thus, the appropriate heating rate can quickly reach the high temperature required for sintering, while avoiding the destruction of the material structure caused by too fast heating; the appropriate target temperature ensures that the material can be densified at an appropriate temperature; the appropriate holding time ensures that the material is fully sintered to form a tight bond between particles; the appropriate pressure promotes the close arrangement of particles and the increase of density.

[0062] According to an embodiment of the present application, the sintering process is performed in an inert atmosphere or vacuum, thereby protecting the uranium nitride powder from oxidation, avoiding the formation of oxides, and ensuring the purity of the powder and the nuclear purity level requirements.

[0063] According to an embodiment of the present application, the sintering die used in the sintering treatment contains a graphite paper or graphite foil lining. Thereby, heat can be rapidly and uniformly transferred during the sintering process, promoting the densification of the composite pellet.

[0064] According to an embodiment of the present application, after the sintering treatment is completed, the obtained sintered product is subjected to a grinding treatment. Thereby, the graphite lining on the surface of the composite pellet and possible surface defects are removed, ensuring that the surface of the composite pellet is flat and smooth, meeting the appearance and dimensional requirements of the nuclear fuel element.

[0065] According to an embodiment of the present application, the method further includes: subjecting the ground product to isothermal heat treatment in an inert atmosphere at 1100 - 1900 °C for 1 - 100 h. Thereby, the grain growth of silicon carbide and uranium nitride is promoted, further adjusting the thermal conductivity of the composite pellet, thereby improving the overall quality and reliability of the composite pellet.

[0066] The solution of the present application will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. For those not specified in the embodiments regarding specific techniques or conditions, they shall be carried out according to the techniques or conditions described in the literature in the field or according to the product specifications. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0067] Example 1

[0068] (1) In a glove box with a high-purity argon atmosphere, 3 g of high-purity UN powder with an average particle size of 3 μm was weighed into a 250 mL round-bottom flask and sealed with a rubber stopper. 0.6 g of nuclear-grade polycarbosilane precursor was completely dissolved in 5 mL of xylene. After it was completely dissolved, it was drawn into a 10 mL syringe with a needle. After expelling the air bubbles in the syringe, the needle was inserted into the rubber stopper, and the xylene solution of polycarbosilane was injected into the round-bottom flask through the syringe. The round-bottom flask was operated according to the operation unit of "rotary oscillation for 10 minutes + ultrasonic treatment for 20 minutes" and repeated 3 times.

[0069] (2) The round-bottom flask was connected to a rotary evaporator and subjected to reduced-pressure rotary evaporation. After the xylene solvent was completely evaporated, polycarbosilane precipitated from the xylene and coated the surface of the UN powder. The UN powder coated with the precursor was transferred to a furnace, evacuated to below 1 Pa, heated to 850 °C and held for 1 hour, and then cooled with the furnace. The UN powder coated with the precursor was ground in the glove box for 10 minutes.

[0070] (3) Cut a high-purity graphite paper with a thickness of 0.1 mm into two circular wafers with a diameter of 1 cm and one circular paper tube with a diameter of 1 cm and a height of 4 cm. Select a set of isostatic pressing graphite molds with high strength and high purity. The inner diameter of the cylindrical sleeve of the mold is 1.02 cm, the outer diameter is 4 cm, and the height is 4 cm. The upper and lower plugs of the mold are graphite cylinders with a diameter of 1 cm and a height of 3 cm. Insert the graphite paper tube into the mold sleeve and ensure it is flat. Place a graphite paper wafer on the top of the lower plug and insert it about 1.5 cm from the lower port of the mold sleeve. Pour the precursor-coated UN powder prepared in the previous step into the mold from the upper end of the mold sleeve and vibrate it to compact. Place a graphite paper wafer on the top of the upper plug and insert and press it from the upper port of the mold sleeve. After connecting the upper and lower plugs to the upper and lower bases, the sample and the mold are assembled.

[0071] (4) Sintering of the composite pellets. Transfer the assembled mold and the sample therein into an SPS sintering furnace, evacuate to below 5 Pa and maintain the vacuum state. Heat the mold and the sample to 1700 °C at a heating rate of 200 °C / min, and simultaneously slowly apply pressure to 55 MPa. Hold at 1700 °C and 55 MPa for 15 minutes. Cool down to room temperature with the furnace and then break the vacuum. After sintering, the sample and the mold are cooled to room temperature with the furnace. After breaking the vacuum, take out the mold and the sample therein.

[0072] (5) Remove the sample and the mold after sintering in the previous step. Remove the upper and lower bases and plugs of the mold, and take out the composite pellet wrapped with graphite paper on the outside. Polish the composite pellet successively with emery papers of 800 mesh and 2000 mesh until the black graphite paper wrapped on its surface is completely removed and the silver-white, metallic-luster composite pellet is exposed. What is obtained is the uranium nitride / silicon carbide composite pellet.

[0073] Example 2

[0074] (1) In a glove box with a high-purity argon atmosphere, weigh 3 g of high-purity UN powder with an average particle size of 3 μm into a 250 mL round-bottom flask and seal it with a rubber stopper. Completely dissolve 1.5 g of nuclear pure grade polycarbosilane precursor in 5 mL of xylene. After it is completely dissolved, aspirate it into a 10 mL syringe with a needle. After expelling the air bubbles in the syringe, insert the needle into the rubber stopper and inject the precursor solution into the round-bottom flask through the syringe. Take the round-bottom flask according to: "Rotate and shake for 10 minutes + Ultrasonic treatment for 20 minutes" as an operation unit and repeat it 3 times.

[0075] (2) Connect the round-bottom flask to a rotary evaporator and perform rotary evaporation under reduced pressure. After the xylene solvent evaporates, the polycarbosilane precipitates from the xylene and coats the surface of the UN powder. Transfer the UN powder coated with the precursor to an atmosphere furnace. After evacuating to below 1 Pa, heat it to 850 °C at a rate of 0.5 °C / min under a flowing high-purity argon atmosphere and hold for 1 hour, then cool it with the furnace. Grind the UN powder coated with the precursor in a glove box for 10 minutes.

[0076] (3) The same as (3) in Example 1.

[0077] (4) Sintering of the composite pellets. Transfer the assembled mold and the sample therein from the previous step into an SPS sintering furnace, evacuate to 5 Pa, and then introduce a flowing high-purity argon atmosphere for protection. Heat the mold and the sample to 1700 °C at a heating rate of 200 °C / min and simultaneously apply a slow pressure of 55 MPa. Hold at 1700 °C and 55 MPa for 15 minutes. Cool with the furnace to room temperature and then break the vacuum. After sintering is completed, cool with the furnace to room temperature. After breaking the vacuum, take out the mold and the sample therein.

[0078] (5) The same as (5) in Example 1.

[0079] Example 3

[0080] (1) In a glove box under a high-purity argon atmosphere, weigh 3 g of high-purity UN powder with an average particle size of 3 μm into a 250 mL round-bottom flask and seal it with a rubber stopper. Completely dissolve 0.6 g of nuclear pure grade polycarbosilane precursor in 5 mL of tetrahydrofuran. After complete dissolution, draw it into a 10 mL syringe with a needle. After expelling the air bubbles in the syringe, insert the needle into the rubber seal and inject the tetrahydrofuran solution of polycarbosilane into the round-bottom flask through the syringe. Take the round-bottom flask and repeat the operation unit of "rotary oscillation for 10 minutes + ultrasonic treatment for 20 minutes" 3 times.

[0081] (2) Connect the round-bottom flask to a rotary evaporator and perform rotary evaporation. After the tetrahydrofuran solvent evaporates, the polycarbosilane precipitates from the tetrahydrofuran and coats the surface of the UN powder. Transfer the UN powder coated with the precursor to an atmosphere furnace. After evacuating to below 1 Pa, heat it to 250 °C at a rate of 0.5 °C / min under a flowing high-purity argon atmosphere and hold for 1 hour, then heat it to 850 °C at a rate of 2 °C / min and hold for 1 hour, and then cool it with the furnace. Grind the UN powder coated with the precursor in a glove box for 10 minutes.

[0082] (3) The same as (3) in Example 1.

[0083] (4) The same as (4) in Example 1.

[0084] (5) is the same as (5) in Example 1.

[0085] (6) The uranium nitride / silicon carbide composite pellets obtained in (5) were isothermally heat-treated in high-purity argon at 1900 °C for 10 hours.

[0086] Test Example

[0087] The composite pellet structures obtained in Examples 1 to 3 are similar. Taking the composite pellet obtained in Example 1 as an example for testing.

[0088] The XRD of the composite pellet obtained in Example 1 was investigated. The results are as Figure 2 shown. There is no obvious difference between the XRD pattern of the composite pellet after coating and heat treatment and the XRD pattern of the uranium nitride powder before coating, which proves that the coating and heat treatment did not change the basic phase of the UN powder.

[0089] The low-magnification SEM images of the composite pellets obtained in the examples are as Figure 3 shown, showing that the composite pellets are dense and have no obvious pore defects. The two phases are clearly distinguishable and the interface is obvious. As Figure 4 shown is the high-magnification SEM image of the composite pellet prepared in Example 1 and its corresponding X-ray energy-dispersive spectroscopy (EDX) surface scan image, which proves that the dark area in the SEM image is SiC and the light area is UN, and the two are closely combined and the interface is obvious.

[0090] The apparent densities of the composite pellets obtained in Examples 1 to 3 were measured to be 97.50% TD, 97.23% TD, and 96.20% TD, respectively.

[0091] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without conflict, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0092] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present application. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present application.

Claims

1. A composite pellet, characterized in that, Comprising: A dispersed phase, the dispersed phase comprising uranium nitride; A continuous phase, the continuous phase comprising silicon carbide and elemental carbon and being continuously distributed, and the dispersed phase being dispersed in the continuous phase.

2. The composite pellet according to claim 1, wherein The content of uranium nitride in the composite pellet is generally not less than 50 wt%; Optionally, the content of silicon carbide in the composite pellet is generally not greater than 50 wt%.

3. The composite pellet according to claim 1, wherein The uranium nitride is in particulate form and is discontinuously distributed within the composite pellet; Optionally, the silicon carbide is continuously distributed in a three-dimensional network within the composite pellet.

4. A method for preparing the composite pellet according to any one of claims 1 to 3, characterized in that, Comprising: Adding uranium nitride powder to an organic solvent containing a silicon carbide precursor for mixing treatment to obtain a precursor solution; Performing a drying treatment on the precursor solution, maintaining the temperature, and grinding to obtain a mixed powder; Performing a sintering treatment on the mixed powder to obtain the composite pellet.

5. The method according to claim 4, characterized in that, The average particle size of the uranium nitride powder is 1 - 30 μm; Optionally, the silicon carbide precursor includes at least one of polycarbosilane, polymethylsilane, polydimethylsilane, polysilane, polysilyne, polydimethylsiloxane; Optionally, the organic solvent includes at least one of benzene, xylene, tetrahydrofuran, n-hexane, cyclohexane, N,N-dimethylformamide, dimethyl sulfoxide; Optionally, the mixing treatment is carried out in an inert atmosphere; Optionally, the temperature maintenance is carried out in an inert atmosphere or in a vacuum.

6. The method according to claim 4, characterized in that The drying treatment includes at least one of natural evaporation, heating evaporation, reduced-pressure evaporation, reduced-pressure rotary evaporation, freeze drying, spray drying; Optionally, the temperature maintenance is carried out at 500 - 1000 °C for 0.5 - 5 h; Optionally, the sintering treatment includes spark plasma sintering, hot pressing sintering, hot isostatic pressing sintering; preferably, spark plasma sintering; Optionally, the heating rate of the spark plasma sintering is 5 - 250 °C / min, the target temperature is 1300 - 2200 °C, the holding time is 1 - 120 min, and the pressure is 20 - 200 MPa.

7. The method according to claim 4, wherein The sintering treatment is carried out in an inert atmosphere or in a vacuum.

8. The method according to claim 4, characterized in that, The sintering mold used in the sintering treatment contains a graphite paper or graphite foil lining; Optionally, after the sintering treatment is completed, the obtained sintered product is polished.

9. The method according to claim 8, wherein Further comprising: Thermally treating the polished product at a constant temperature in an inert atmosphere at 1100 - 1900 °C for 1 - 100 h.

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