Method for producing uranium-nitride-beryllium composite fuel pellets

Abstract

Embodiments of the present application relate to the manufacture of fuel loaded in a non-radioactive shell, and particularly to a method for preparing a uranium-nitrogen-rhenium composite fuel pellet, comprising the following steps: S1: preparing a mixed powder of metallic uranium powder and metallic rhenium powder; S2: preparing a UNRe mixed powder using the mixed powder obtained in step S1; S3: pressing the UNRe mixed powder obtained in step S2 to obtain a composite fuel compact; and S4: reacting the composite fuel compact obtained in step S3 with a mixed gas of nitrogen and ammonia at a predetermined temperature and a predetermined vacuum degree for a predetermined time to obtain a high-density composite fuel pellet. The method for preparing a uranium-nitrogen-rhenium composite fuel pellet according to the embodiments of the present application can introduce rhenium elements into the fuel pellet, and the rhenium elements can block the reaction between the fuel pellet and the cladding material of the reactor, thereby improving the stability of the fuel pellet.

Description

Technical Field

[0001] The embodiments of this application relate to the manufacture of fuel contained in a non-radioactive casing, and more specifically to a method for preparing a uranium-nitrogen-rhenium composite fuel pellet. Background Technology

[0002] The statements herein are provided merely as background information in connection with this application and do not necessarily constitute prior art.

[0003] Space nuclear propulsion refers to the use of nuclear reactors mounted on spacecraft to power their movement. It is a key technology for deep space exploration and cargo transportation. Space nuclear propulsion is characterized by its long operational lifespan, low susceptibility to the space environment, and high stability, making it an important research direction in the field of space exploration. Summary of the Invention

[0004] A brief overview of this application is provided below to offer a basic understanding of certain aspects thereof. It should be understood that this overview is not an exhaustive summary of the application. It is not intended to identify key or essential parts of the application, nor is it intended to limit its scope. Its purpose is merely to present certain concepts in a simplified form as a prelude to the more detailed description that follows.

[0005] The embodiments of this application provide a method for preparing uranium-nitrogen-rhenium composite fuel pellets, the method comprising the following steps: S1: preparing a mixed powder of metallic uranium powder and metallic rhenium powder; S2: preparing a mixed powder of uranium and rhenium using the mixed powder obtained in step S1; S3: pressing the mixed powder of uranium and rhenium obtained in step S2 to obtain a composite fuel compact; S4: reacting the composite fuel compact obtained in step S3 with a mixed gas of nitrogen and ammonia under predetermined temperature and predetermined vacuum conditions for a predetermined time to obtain a high-density composite fuel pellet.

[0006] The preparation method provided in the embodiments of this application can prepare uranium-nitrogen-rhenium composite fuel pellets. Rhenium can be introduced into the fuel pellets, and rhenium can prevent the fuel pellets from reacting with the reactor cladding material, thereby improving the stability of the fuel pellets. Attached Figure Description

[0007] Other objects and advantages of this application will become apparent from the following description of embodiments of this application with reference to the accompanying drawings, and will help to provide a comprehensive understanding of this application.

[0008] Figure 1 This is a flowchart illustrating the preparation of uranium-nitrogen-rhenium composite fuel pellets provided in an embodiment of this application.

[0009] Figure 2This is a flowchart of surface treatment of high-density composite fuel pellets provided in an embodiment of this application.

[0010] Figure 3 This is a schematic diagram of the ball mill used in preparing mixed powders.

[0011] Figure 4 This is a flowchart of the preparation of UNRe mixed powder provided in the embodiments of this application.

[0012] Figure 5 This is a schematic diagram of the thermopressor used in the preparation of UNRe mixed powder.

[0013] Figure 6 This is a schematic diagram of the structure of the oxygen-free glove box used in the preparation of UNRe mixed powder.

[0014] Figure 7 This is a schematic diagram of the high-temperature sintering furnace used to obtain sintered mixed metal powders.

[0015] Figure 8 This is a flowchart of the preparation of composite fuel compacts provided in an embodiment of this application.

[0016] Figure 9 This is a schematic diagram of the cold isostatic press used in the preparation of composite fuel compacts.

[0017] Explanation of reference numerals in the attached figures:

[0018] 30. Ball mill; 50. Warm press; 60. Oxygen-free glove box; 70. High-temperature sintering furnace; 90. Cold isostatic press.

[0019] It should be noted that the accompanying drawings are not necessarily drawn to scale, but are shown only in a schematic manner without affecting the reader's understanding. Detailed Implementation

[0020] Exemplary embodiments of this application will be described below with reference to the accompanying drawings. For clarity and brevity, not all features of actual implementations are described in the specification. However, it should be understood that many implementation-specific decisions must be made in the development of any such actual embodiment to achieve the developer's specific goals, such as complying with constraints related to the system and business, and these constraints may vary depending on the implementation. Furthermore, it should be understood that while development work can be very complex and time-consuming, such development work is merely a routine task for those skilled in the art who benefit from the content of this application.

[0021] It should also be noted that, in order to avoid obscuring this application with unnecessary details, only the equipment structure and / or processing steps closely related to the solution according to this application are shown in the accompanying drawings, while other details that are not closely related to this application are omitted.

[0022] The key component of space nuclear power is the space nuclear reactor. Traditional nuclear reactors mostly use uranium dioxide fuel, but space nuclear reactors require fuel to operate stably for a long time in a high-temperature, high-radiation environment. Uranium dioxide fuel cannot achieve long-term stable operation, so it cannot be used as fuel for space nuclear reactors. Other types of fuel need to be selected. Nitride fuel has the advantages of high thermal conductivity, high uranium density, and low evaporation rate, and can operate stably for a long time, so it can be used as fuel for space nuclear reactors.

[0023] However, single-phase uranium nitride fuel undergoes nitrogen decomposition at high temperatures and reacts with niobium-zirconium alloy, resulting in poor compatibility between it and the niobium-zirconium alloy reactor cladding material. Therefore, when selecting single-phase uranium nitride fuel as the fuel for a nuclear reactor, a barrier needs to be added between the fuel and the cladding material to prevent them from coming into contact with each other.

[0024] The commonly used method is to add an ultrathin rhenium tube between the single-phase uranium nitride fuel and the cladding material to prevent them from contacting each other. However, the processing technology of the ultrathin rhenium tube is complex and the processing cost is also very high.

[0025] In response to at least one of the above-mentioned technical problems, embodiments of this application provide a method for preparing uranium-nitrogen-rhenium composite fuel pellets.

[0026] like Figure 1 The diagram illustrates a flowchart of the preparation of uranium-nitrogen-rhenium composite fuel pellets according to an embodiment of this application. The process includes the following steps: S1: preparing a mixed powder of metallic uranium powder and metallic rhenium powder; S2: preparing a mixed powder of uranium and rhenium using the mixed powder obtained in step S1; S3: pressing the mixed powder of uranium and rhenium obtained in step S2 to obtain a composite fuel compact; S4: reacting the composite fuel compact obtained in step S3 with a mixed gas of nitrogen and ammonia under predetermined temperature and vacuum conditions for a predetermined time to obtain a high-density composite fuel pellet. The composite fuel pellets prepared according to the method provided in the embodiments of this application can achieve a relative density of over 90%, where relative density refers to the ratio of the actual density to the theoretical density of the composite fuel pellet, and the same applies below.

[0027] The preparation method provided in the embodiments of this application can prepare uranium-nitrogen-rhenium composite fuel pellets. Rhenium can be introduced into the fuel pellets, and rhenium can prevent the fuel pellets from reacting with the reactor cladding material, thereby improving the stability of the fuel pellets.

[0028] In some embodiments, after the high-density composite fuel pellets are prepared, their surface can be treated, such as... Figure 2 The diagram illustrates a flowchart of surface treatment for high-density composite fuel pellets according to an embodiment of this application. The process may include the following steps: S5: Mixing rhenium metal powder and the UNRe mixed powder obtained in step S2 to obtain a mixed powder; S6: Burying the composite fuel pellet obtained in step S4 into the mixed powder obtained in step S5 and compacting it; S7: Passing nitrogen gas into the compacted product obtained in step S6 at a predetermined temperature for a predetermined time to obtain uranium-nitrogen-rhenium composite fuel pellets.

[0029] By treating the surface of high-density composite fuel pellets, the rhenium content on the surface can be increased, which can better prevent contact between the fuel pellets and the cladding material. In particular, after surface treatment, a coating with a high rhenium content can be formed on the surface of the uranium-nitrogen-rhenium composite fuel pellets, and the thickness of the coating can reach 20μm to 60μm.

[0030] In some embodiments, the uranium powder and rhenium powder have different particle sizes in step S1. Specifically, the uranium powder can have a particle size of 30 μm to 50 μm, and the rhenium powder can have a particle size of 10 μm to 20 μm. This difference in particle size between the uranium and rhenium powder facilitates thorough mixing during the preparation process.

[0031] In some embodiments, step S1 may further include the following steps: S11: performing low-temperature ball milling on the mixed powder of metallic uranium powder and metallic rhenium powder; S12: drying the product obtained in S11 for a predetermined time to obtain the mixed powder of metallic uranium powder and metallic rhenium powder.

[0032] In some embodiments, in step S11, the mixed powder is subjected to low-temperature ball milling, which can make the mixed powder brittle to facilitate subsequent processing. The ball milling temperature can be 0°C to 20°C.

[0033] like Figure 3 The diagram shows a schematic of a ball mill used in preparing mixed powders. In preparing the mixed powders, uranium powder and rhenium powder are placed in a ball mill 30 for low-temperature ball milling. The ball mill 30 is equipped with a control panel, through which the ball milling parameters can be adjusted. Specifically, the relevant ball milling parameters for preparing the mixed powders may include the following: placing uranium powder, rhenium powder, and cemented carbide grinding balls into a cemented carbide container for processing; selecting ethanol as the grinding medium; a ball-to-powder ratio of (3–9):1; and a ball milling time of 10–30 hours.

[0034] In some embodiments, in step S12, the product obtained after ball milling can be placed in a drying oven and vacuum dried for 3 to 5 hours before being taken out to obtain a mixed powder of metallic uranium powder and metallic rhenium powder.

[0035] In some embodiments, in step S2, such as Figure 4 The diagram shows a flowchart of the preparation of UNRe mixed powder according to an embodiment of this application. The process may include the following steps: S21: The mixed powder obtained in step S1 is formed into a compact under a pressure of 100-200 MPa; S22: The compact obtained in step S21 is subjected to a predetermined vacuum degree and a predetermined temperature with flowing ammonia gas for a predetermined time, followed by hydrogen gas for a predetermined time, to obtain a sintered body of mixed metal powder; S23: The sintered body of mixed metal powder obtained in step S22 is crushed and screened to obtain UNRe mixed powder.

[0036] In some embodiments, in step S21, the compact is pressed under a pressure of 100 to 200 MPa. Since the pressing pressure is relatively low, the resulting compact has a low density and pores are formed inside, which facilitates the entry of gas into the compact and provides convenient conditions for processing processes that require gas.

[0037] In some embodiments, in step S22, the predetermined temperature can be 1200°C to 1400°C, the predetermined time for introducing ammonia can be 2h to 4h, and the predetermined time for introducing hydrogen can be 1h to 3h. The hydrogen introduced after the ammonia can remove the residual ammonia in the compact and can also remove impurities in the compact.

[0038] like Figure 5 , Figure 6 and Figure 7 As shown, Figure 5 The diagram shows the structure of the thermopressor used in preparing UNRe mixed powder. Figure 6 The diagram shows the structure of the oxygen-free glove box used in the preparation of UNRe mixed powder. Figure 7 This diagram shows the structure of a high-temperature sintering furnace used to obtain sintered mixed metal powders.

[0039] In step S21, the mixed powder obtained in step S1 can be placed into a stainless steel mold with a diameter of 10mm to 30mm and then sent to a thermopressor 50 for pressing.

[0040] In step S22, the pressed blank obtained in step S21 can be placed in a high-temperature sintering furnace 70 for high-temperature sintering to obtain a mixed powder metal sintered body. The specific process includes the following: after the mixed metal powder is placed in the high-temperature sintering furnace 70, the high-temperature sintering furnace 70 is evacuated; when the vacuum degree of the high-temperature sintering furnace 70 reaches the predetermined vacuum degree, flowing ammonia gas is introduced and the temperature is raised to the predetermined temperature, and the predetermined temperature is maintained while ammonia gas is continuously introduced; after 2h to 4h, flowing hydrogen gas is introduced, and the predetermined temperature is maintained while hydrogen gas is continuously introduced for 1h to 3h to obtain a mixed metal powder sintered body.

[0041] In step S23, the sintered mixed metal powder obtained in step S22 can be placed in an oxygen-free glove box 60 for crushing and screening. The sintered mixed metal powder can be mechanically crushed to obtain crushed products. During screening, the crushed products can be passed through a 60-80 mesh sieve to obtain UNRe mixed powder.

[0042] In some embodiments, in step S3, such as Figure 8 The diagram shows a flowchart of the preparation of composite fuel compacts provided by an embodiment of this application. The process may include the following steps: S31: adding stearic acid of a predetermined concentration to the mixed powder obtained in step S2 and mixing; S32: pressing the compact at 100-200°C and 300-500 MPa to form a pre-compressed compact; S33: re-compressing the pre-compressed compact from step S32 at 500-700 MPa to obtain the composite fuel compact.

[0043] In step S31, the mixed powder obtained in step S2 and stearic acid can be placed in a three-dimensional motion mixer for mixing. The predetermined concentration of stearic acid can be 0.5% to 1.5%, and the mixing time can be 3 to 5 hours. In step S32, the mixed powder obtained in step S31 can be placed into a stainless steel mold with a diameter of 10 mm to 15 mm, and then fed into a warm press 50 for pressing to obtain a pre-pressed blank. Figure 9 The diagram shows a schematic of the cold isostatic press used to prepare the composite fuel compact. In step S33, the pre-pressed compact obtained in step S32 can be placed into the cold isostatic press 90 for re-pressing to obtain the composite fuel compact.

[0044] In step S33, the pre-pressed billet obtained in step S32 is re-pressed, which can increase the density of the billet and effectively eliminate defects formed during the pressing process. For example, re-pressing can effectively eliminate porosity.

[0045] In some embodiments, in step S1, the mass ratio of metallic uranium powder to metallic rhenium powder is (30-90):1, to avoid affecting the concentration of uranium in the fuel pellets due to the high concentration of rhenium.

[0046] In some embodiments, in step S5, the molar ratio of rhenium powder to UNRe mixed powder is (2-4):1, which can increase the rhenium content in the coating formed after surface treatment.

[0047] In some embodiments, the predetermined vacuum level in step S22 is (5-7)×10⁻⁶. -3 Pa.

[0048] In some embodiments, the predetermined vacuum degree in step S4 is (2 to 4) × 10⁻⁶. -3 Pa.

[0049] In some embodiments, the predetermined temperature in step S4 is 1700℃~1900℃, and the predetermined time is 3h~5h. In step S4, the composite fuel compact obtained in step S3 can be placed in a high-temperature sintering furnace 70 for processing to obtain composite fuel pellets. The specific process may include the following: placing the composite fuel compact into the high-temperature sintering furnace 70 and evacuating the furnace; after the vacuum reaches a predetermined level, introducing a mixture of nitrogen and ammonia into the furnace, wherein the ammonia content is 5%; heating the furnace to a predetermined temperature and maintaining it for a predetermined time to obtain high-density composite fuel pellets. Introducing a mixture of nitrogen and ammonia in step S4 helps remove impurities from the composite fuel compact, achieving one-time forming of the composite fuel pellets.

[0050] In some embodiments, in step S5, rhenium metal powder and the UNRe mixed powder obtained in step S2 can be mixed in a ball mill 30 at a molar ratio of (2-4):1, wherein the particle size of the rhenium metal powder is 10μm-20μm, and the relevant ball milling parameters may include the following: rhenium metal powder, the UNRe mixed powder obtained in step S2 and cemented carbide grinding balls are placed in a cemented carbide container for processing, argon is selected as the protective gas for ball milling, the ball-to-material ratio is (5-10):1, and the ball milling time is 20h-30h.

[0051] In some embodiments, in step S7, the predetermined temperature can be 1400℃~1600℃, and the predetermined time can be 20h~40h. The compacted product obtained in step S6 can be placed in a high-temperature sintering furnace 70, nitrogen gas is introduced and the temperature is raised to a predetermined temperature, and nitrogen gas is continuously introduced for a predetermined time while maintaining the predetermined temperature to obtain uranium-nitrogen-rhenium composite fuel pellets.

[0052] The following three examples illustrate in detail the process of preparing uranium-nitrogen-rhenium composite fuel pellets according to the methods provided in the embodiments of this application.

[0053] Example 1

[0054] First, uranium powder with a particle size of 30 μm and rhenium powder with a particle size of 10 μm were taken at a mass ratio of 30:1. The uranium and rhenium powders were then placed in a ball mill 30. A cemented carbide container and grinding balls were selected for processing. Ethanol was chosen as the grinding medium, the ball-to-powder ratio was 3:1, the grinding time was 10 hours, and the grinding temperature was 0°C. After mixing, the mixture was removed from the ball mill 30 and placed in a drying oven for vacuum drying for 3 hours to obtain a mixed powder of uranium and rhenium powders. Through the above steps, an ultrafine mixed powder can be obtained, with a fineness range reaching below 1 micrometer (μm).

[0055] Next, the mixed powder of uranium and rhenium metals is loaded into a 10mm diameter stainless steel mold and then fed into a warm press 50, where it is formed into a compact under a pressure of 100MPa. The compact is then placed in a high-temperature sintering furnace 70 and evacuated to a vacuum level of 5×10⁻⁶. -3 After Pa, ammonia gas is introduced, and the temperature is raised to 1200℃~1400℃. This temperature is maintained for 2 hours, then hydrogen gas is introduced, and the temperature is maintained for 1 hour to obtain a sintered body of mixed metal powder. Next, the sintered body is placed in an oxygen-free glove box 60 and mechanically crushed to obtain the crushed product. This crushed product is then sieved through a 60-mesh sieve to obtain UNRe mixed powder. The UNRe mixed powder obtained through this method has high rhenium purity, reaching up to 99.9% in rhenium powder.

[0056] Next, the UNRe mixed powder and 0.5% stearic acid were placed in a three-dimensional motion mixer and mixed thoroughly for 3 hours to obtain a mixed product. Then, the mixed product was taken out and placed into a stainless steel mold with a diameter of 10 mm. It was then sent to a warm press 50 and pressed at 100℃ and 300MPa to obtain a pre-pressed blank. The pre-pressed blank was then placed in a cold isostatic press 90 and re-pressed at 500MPa to obtain a composite fuel compact with a relative density of 60.1%.

[0057] Next, the composite fuel compact is placed in a high-temperature sintering furnace 70 and evacuated to a vacuum level of 2×10. -3 After Pa, a mixture of nitrogen and ammonia gas is introduced, with the ammonia content in the mixture being 5%. The temperature is raised to 1700℃ and maintained at the current temperature for 3 hours to obtain composite fuel pellets with a relative density of 90.2%.

[0058] Next, rhenium metal powder and UNRe mixed powder with a particle size of 10 μm were taken at a molar ratio of 2:1. The extracted rhenium metal powder and UNRe mixed powder were placed in a ball mill 30. A cemented carbide container and grinding balls were selected for processing. Argon gas was selected as the ball mill protective gas. The ball-to-material ratio was 5:1, and the ball milling time was 20 h. After the mixing was completed, the mixed product was taken out from the ball mill 30. Composite fuel pellets with a relative density of 90.2% were embedded in the mixed product and compacted. The compacted product was placed in a high-temperature sintering furnace 70. Nitrogen gas was introduced and the temperature was raised to 1700℃. After maintaining the current temperature for 20 h, uranium-nitrogen-rhenium composite fuel pellets with a coating thickness of 20.4 μm were obtained.

[0059] Example 2

[0060] First, uranium powder with a particle size of 40 μm and rhenium powder with a particle size of 15 μm were taken at a mass ratio of 60:1. The uranium powder and rhenium powder were placed in a ball mill 30. A cemented carbide container and grinding balls were selected for processing. Ethanol was selected as the grinding medium. The ball-to-powder ratio was 6:1. The ball milling time was 20 h and the ball milling temperature was 10 °C. After mixing, the mixed product was taken out from the ball mill 30 and placed in a drying oven for vacuum drying for 4 h to obtain a mixed powder of uranium powder and rhenium powder.

[0061] Next, the mixed powder of metallic uranium powder and metallic rhenium powder is loaded into a stainless steel mold with a diameter of 20 mm, and then fed into a warm press 50 to be formed into a compact under a pressure of 150 MPa; then the compact is placed in a high-temperature sintering furnace 70 and vacuumed to 6×10 -3 After Pa, ammonia gas is introduced, the temperature is raised to 1300℃, and the current temperature is maintained for 3 hours. Then, the gas introduced is changed to hydrogen gas, and the current temperature is maintained for 2 hours to obtain a sintered body of mixed metal powder. Next, the sintered body of mixed metal powder is placed in an oxygen-free glove box 60 and mechanically crushed to obtain crushed products. The crushed products are then screened through a 70-mesh sieve to obtain UNRe mixed powder.

[0062] Next, the UNRe mixed powder and 1% stearic acid were thoroughly mixed in a three-dimensional motion mixer for 4 hours to obtain a mixed product. Then, the mixed product was taken out and placed into a stainless steel mold with a diameter of 12.5 mm. It was then sent to a warm press 50 and pressed at 150°C and 400 MPa to obtain a pre-pressed blank. The pre-pressed blank was then placed in a cold isostatic press 90 and re-pressed at 600 MPa to obtain a composite fuel compact with a relative density of 62.3%.

[0063] Next, the composite fuel compact is placed in a high-temperature sintering furnace 70 and evacuated to a vacuum level of 3×10. -3After Pa, a mixture of nitrogen and ammonia gas is introduced, with the ammonia content in the mixture being 5%. The temperature is raised to 1800℃ and maintained at the current temperature for 4 hours to obtain composite fuel pellets with a relative density of 90.6%.

[0064] Next, rhenium metal powder and UNRe mixed powder with a particle size of 15μm were taken at a molar ratio of 3:1. The extracted rhenium metal powder and UNRe mixed powder were placed in a ball mill 30. A cemented carbide container and grinding balls were selected for processing. Argon gas was selected as the ball mill protective gas. The ball-to-material ratio was 7.5:1, and the ball milling time was 25h. After the mixing was completed, the mixed product was taken out from the ball mill 30. Composite fuel pellets with a relative density of 90.6% were embedded in the mixed product and compacted. The compacted product was placed in a high-temperature sintering furnace 70. Nitrogen gas was introduced and the temperature was raised to 1500℃. After maintaining the current temperature for 30h, uranium-nitrogen-rhenium composite fuel pellets with a coating thickness of 41.5μm were obtained.

[0065] Example 3

[0066] First, uranium powder with a particle size of 50 μm and rhenium powder with a particle size of 20 μm were taken at a mass ratio of 90:1. The uranium powder and rhenium powder were placed in a ball mill 30. A cemented carbide container and grinding balls were selected for processing. Ethanol was selected as the grinding medium. The ball-to-powder ratio was 9:1. The ball milling time was 30 h and the ball milling temperature was 20 °C. After mixing, the mixed product was taken out from the ball mill 30 and placed in a drying oven for vacuum drying for 5 h to obtain a mixed powder of uranium powder and rhenium powder.

[0067] Next, the mixed powder of uranium and rhenium metals is loaded into a 30mm diameter stainless steel mold and then fed into a warm press 50, where it is formed into a compact under a pressure of 200MPa. The compact is then placed in a high-temperature sintering furnace 70 and evacuated to a vacuum level of 7×10⁻⁶. -3 After Pa, ammonia gas is introduced, the temperature is raised to 1400℃, and the current temperature is maintained for 4 hours. Then, the gas introduced is changed to hydrogen gas, and the current temperature is maintained for 3 hours to obtain a sintered body of mixed metal powder. Next, the sintered body of mixed metal powder is placed in an oxygen-free glove box 60 and mechanically crushed to obtain crushed products. The crushed products are then screened through an 80-mesh sieve to obtain UNRe mixed powder.

[0068] Next, the UNRe mixed powder and 1.5% stearic acid were thoroughly mixed in a three-dimensional motion mixer for 5 hours to obtain a mixed product. The mixed product was then removed and placed into a stainless steel mold with a diameter of 15 mm. It was then fed into a warm press 50 and pressed at 200°C and 500 MPa to obtain a pre-pressed blank. The pre-pressed blank was then placed into a cold isostatic press 90 and re-pressed at 700 MPa to obtain a composite fuel compact with a relative density of 64.7%.

[0069] Next, the composite fuel compact is placed in a high-temperature sintering furnace 70 and evacuated to a vacuum level of 4×10. -3 After Pa, a mixture of nitrogen and ammonia gas is introduced, with the ammonia content in the mixture being 5%. The temperature is raised to 1900℃ and maintained at the current temperature for 5 hours to obtain composite fuel pellets with a relative density of 90.9%.

[0070] Next, rhenium metal powder and UNRe mixed powder with a particle size of 20 μm were taken at a molar ratio of 3:1. The extracted rhenium metal powder and UNRe mixed powder were put into ball mill 30. A container and grinding balls made of hard alloy were selected for processing. Argon gas was selected as the ball mill protective gas. The ball-to-material ratio was 10:1 and the ball milling time was 30 h. After the mixing was completed, the mixed product was taken out from ball mill 30. Composite fuel pellets with a relative density of 90.9% were buried in the mixed product and compacted. The compacted product was put into high-temperature sintering furnace 70. Nitrogen gas was introduced and the temperature was raised to 1600℃. After maintaining the current temperature for 40 h, uranium-nitrogen-rhenium composite fuel pellets with a coating thickness of 60.7 μm were obtained.

[0071] Regarding the embodiments of this application, it should also be noted that, without conflict, the embodiments of this application and the features in the embodiments can be combined with each other to obtain new embodiments.

[0072] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. The scope of protection of this application shall be determined by the scope of the claims.

Claims

1. A method for preparing uranium-nitrogen-rhenium composite fuel pellets, characterized in that, Includes the following steps: S1: Preparation of a mixed powder of metallic uranium powder and metallic rhenium powder; S2: Prepare UNRe mixed powder using the mixed powder obtained in step S1; S3: Press the UNRe mixed powder obtained in step S2 to obtain a composite fuel compact; S4: The composite fuel compact obtained in step S3 is reacted with a mixture of nitrogen and ammonia under predetermined temperature and vacuum conditions for a predetermined time to obtain high-density composite fuel pellets. Step S2 also includes the following steps: S21: The mixed powder obtained in step S1 is molded into a compact under a pressure of 100~200MPa; S22: The compact obtained in step S21 is subjected to a predetermined vacuum degree and a predetermined temperature, through which ammonia gas is introduced for a predetermined time, and then hydrogen gas is introduced for a predetermined time to obtain a sintered body of mixed metal powder. S23: The sintered mixed metal powder obtained in step S22 is crushed and screened to obtain UNRe mixed powder.

2. The preparation method according to claim 1, characterized in that, Also includes: S5: Mix the rhenium metal powder and the UNRe mixed powder obtained in step S2 to obtain a mixed powder; S6: The composite fuel pellets obtained in step S4 are embedded in the mixed powder obtained in step S5 and compacted. S7: The compacted product obtained in step S6 is passed through nitrogen gas at a predetermined temperature for a predetermined time to obtain uranium-nitrogen-rhenium composite fuel pellets.

3. The method according to claim 1, characterized in that, In step S1, the particle sizes of the metallic uranium powder and the metallic rhenium powder are different.

4. The method according to claim 1, characterized in that, Step S1 also includes the following steps: S11: Low-temperature ball milling of a mixture of metallic uranium powder and metallic rhenium powder; S12: Dry the product obtained in S11 for a predetermined time to obtain a mixed powder of uranium metal powder and rhenium metal powder.

5. The method according to claim 1, characterized in that, Step S3 also includes the following steps: S31: Add the mixed powder obtained in step S2 to a predetermined concentration of stearic acid and mix. S32: Press the mixed powder obtained in step S31 into a pre-pressed blank at 100-200℃ and 300-500MPa pressure; S33: The pre-pressed blank obtained in step S32 is re-pressed at 500-700MPa to obtain a composite fuel blank.

6. The method according to claim 1, characterized in that, In step S1, the mass ratio of the metallic uranium powder to the metallic rhenium powder is (30~90):

1.

7. The method according to claim 2, characterized in that, The molar ratio of the rhenium metal powder to the UNRe mixed powder is (2~4):

1.

8. The method according to claim 1, characterized in that, The predetermined vacuum degree mentioned in step S22 is (5~7)×10 -3 Pa.

9. The method according to claim 1, characterized in that, The predetermined vacuum degree mentioned in step S4 is (2~4)×10 -3 Pa.

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