Carbon-containing uranium and thorium mixed fuel microsphere and preparation method thereof

Abstract

The invention relates to a carbon-containing uranium and thorium mixed fuel microsphere and a preparation method thereof. The preparation method comprises the following steps: carrying out ball milling on ThO2, UO2 and carbon powder to refine until D50 is less than or equal to 2 microns; thorium dioxide, uranium dioxide and carbon powder are put into a sound wave mixing device according to the mass ratio of 3: 7: 1, deionized water is added till the solid content is 30%-40%, and mixed powder is obtained through dispersion in the argon atmosphere of the sound wave mixing device; adding a binder into the mixed powder, and mixing with deionized water to form a plastic wet material; the plastic wet material forms a continuous strip-shaped material through an extruder, the continuous strip-shaped material is cut into 1.2 mm * 1.2 mm columnar particles through a rotary blade of cutting and rounding equipment, and the columnar particles are rounded into wet-state ball blanks with the diameter of 1.0 + / -0.1 mm through a rounding disc; and in an inert atmosphere, carrying out gradient heating on the wet ball blank to remove the binder and complete a carbon thermal reduction reaction to finally obtain the UC2-ThC2 composite fuel microspheres. The technical problems that uranium and thorium fuel microspheres are non-uniform in mixing, poor in forming quality and low in sintering performance are solved by integrating the technologies of acoustic dispersion, plastic wet material rolling and gradient sintering.

Description

Technical Field

[0001] This application belongs to the field of nuclear fuel element manufacturing technology, specifically relating to a carbon-containing uranium-thorium mixed fuel microsphere and its preparation method. Background Technology

[0002] Carbon-containing uranium-thorium hybrid microspheres utilize the high-temperature stability (ThC2 melting point 2650℃) and fission product containment of the ThC2 continuous phase (thermal conductivity > 20 W / m·K); through thorium-uranium synergistic breeding (… 233 U-conversion > 1) Improves neutron economy; enhances safety by utilizing carbon-based localized moderation, avoiding the risk of uranium carbide hydrolysis. Compared to traditional TRISO particulate fuel, it increases uranium loading by 100% and thermal conductivity by 4 times, resolving the core contradiction between high burnup and core meltdown prevention. It can serve as a core material for high-temperature gas-cooled reactor fuel elements, fundamentally resolving the contradiction between high uranium density and high thermal conductivity in high-temperature gas-cooled reactor fuel, and also providing technological reserves for fourth-generation reactor types such as thorium-based molten salt reactors (TMSR). Its preparation process needs to balance homogeneity, thermal stability, and nuclear fuel performance. Currently, mainstream technologies include sol-gel methods, powder metallurgy methods, and microfluidic-assisted processes, but all have limitations.

[0003] The sol-gel method typically uses uranyl nitrate and thorium nitrate as raw materials to prepare uranium carbide thorium microspheres through a combination of sol-gel processing and carbothermic reduction. However, in terms of stability, the sol requires low-temperature storage and is prone to hydrolysis at room temperature, resulting in a narrow process window (e.g., a stabilization time of only 85 minutes). Regarding carbon distribution, carbon black in the sol tends to agglomerate, requiring additional ultrasonic dispersion, but prolonged ultrasonic dispersion may damage the sol structure. In terms of process, the sol-gel method requires multiple steps of gelation, washing, and drying (more than 8 steps), and high-temperature sintering, resulting in high energy consumption and the potential introduction of impurity phases (such as unreacted residual oxides). Therefore, the sol-gel method suffers from poor stability, uneven carbon distribution, and complex processes.

[0004] Dry powder metallurgy typically involves mechanically mixing UO2 and ThO2 powders with carbon black, pressing them into shape, and then sintering them at high temperatures. However, dry mixing easily generates radioactive dust, requiring complex protective measures and making continuous industrial production difficult. Mechanical agglomeration granulation can lead to uneven microsphere density distribution, affecting the quality of subsequent coating layers. For example, traditional mechanical dry mixing methods (such as ball milling) result in uneven dispersion of thorium dioxide (ThO2), uranium dioxide (UO2), and carbon powder (e.g., carbon powder agglomeration size > 50 μm), causing localized carbon excess or deficiency after sintering, affecting the efficiency of the carbothermic reduction reaction (e.g., UC / ThC composite phase purity < 85%). Dry powder granulation processes easily generate radioactive dust (ThO2 / UO2 particle emission rate > 3%), and microsphere formation relies on high-pressure extrusion or die pressing, resulting in large microsphere size deviations (CV value > 10%) and low sphericity (true sphere rate < 80%). Furthermore, existing one-step carbothermal reduction processes (such as directly heating to 1600℃) can cause pore collapse within the microspheres (porosity fluctuation > 15%), resulting in uneven distribution of uranium-thorium carbide phases (UC2-ThC2 interfacial bonding strength < 50 MPa), i.e., poor sintering performance. Therefore, dry powder metallurgy suffers from the drawbacks of dust pollution and dimensional inhomogeneity.

[0005] A team led by Xu Rui at Tsinghua University prepared UC sol-gel microspheres using microfluidic technology combined with a carbothermal reduction process, achieving a purity of 92.9 wt%. However, microfluidic devices require precision fabrication and real-time control, making industrial scale-up difficult. For example, microfluidic equipment is expensive (over 1 million RMB per unit), hindering large-scale production. This method also requires strict control of acidity defects and the HMTA / urea ratio to balance stability and gelation speed, resulting in low process tolerance. Therefore, the microfluidic-assisted internal gelation method suffers from high equipment costs and sol limitations.

[0006] In summary, existing methods generally suffer from the following problems: Insufficient mixing uniformity: The dispersion of carbon and uranium thorium oxides relies on complex pretreatment (such as ultrasonic dispersion and sol modification), which can easily lead to local carbon excess or deficiency.

[0007] The process is complicated: multiple steps (such as gelation, washing, and drying) result in high energy consumption and increase the risk of introducing impurities.

[0008] High equipment dependence: Microfluidic or low-temperature sol-gel processes have stringent equipment requirements, which is not conducive to large-scale production. Summary of the Invention

[0009] In view of this, this application provides a carbon-containing uranium-thorium mixed fuel microsphere and its preparation method, which solves the technical problems of uneven mixing, poor forming quality and low sintering performance of uranium-thorium fuel microspheres by integrating acoustic dispersion, plastic wet material spheroidization and gradient sintering techniques.

[0010] The first aspect of this application provides a method for preparing carbon-containing uranium-thorium mixed fuel microspheres, the method comprising: Step S10: Thorium dioxide (ThO2), uranium dioxide (UO2) and carbon powder with a particle size ≤5μm are ball-milled to refine to D50≤2μm; Step S20: Thorium dioxide, uranium dioxide and carbon powder are added to the acoustic mixing device at a mass ratio of 3:7:1. Deionized water is added until the solid content is 30%-40%. The powder is dispersed in the argon atmosphere of the acoustic mixing device to achieve nanoscale uniform dispersion of the powder and obtain mixed powder. Step S30: Add a binder to the mixed powder, the binder having a mass fraction of 8%-12%, and mix with deionized water to form a plastic wet material with a moisture content of 25%-30%. The binder includes microcrystalline cellulose (MCC) and / or polyvinyl alcohol (PVA). Step S40: The plastic wet material is extruded into a continuous strip material, which is then cut into 1.2mm×1.2mm columnar particles by the rotating blade of the cutting and rounding device. The columnar particles are then rounded by the rounding disc into wet spherical blanks with a diameter of 1.0±0.1mm. Step S50: Under an inert atmosphere, the wet pellet blank is subjected to gradient heating to remove the binder and complete the carbothermic reduction reaction, finally obtaining UC2-ThC2 composite fuel microspheres.

[0011] In one specific embodiment of this application, the parameters of the acoustic mixing device are set as follows: frequency 20kHz-40kHz, power density 0.5-1.5W / cm². 3 Amplitude: 5μm-50μm; Time: 5 minutes-60 minutes; Temperature: <60℃.

[0012] In one specific embodiment of this application, the binder is made of microcrystalline cellulose and polyvinyl alcohol. The mass ratio of microcrystalline cellulose to polyvinyl alcohol is 1:2, and the solid content is 40-50%.

[0013] In one specific embodiment of this application, the binder is microcrystalline cellulose, with a mass ratio of microcrystalline cellulose to deionized water of 1:4. The mixing speed of the microcrystalline cellulose and deionized water is 200 rpm, and the mixing time is 30 min. Alternatively, the binder is polyvinyl alcohol. Polyvinyl alcohol is dissolved in deionized water at 80°C to a concentration of 10 wt%, cooled to 25°C, and then mixed with the powder. The degree of polymerization of polyvinyl alcohol is 1750 ± 50.

[0014] In one specific embodiment of this application, the die diameter of the extruder is set to 1.2 mm, and the screw speed is set to 20 rpm-30 rpm; and / or, the rotation speed of the rotating blade of the cutting and rounding device is set to 300 rpm-500 rpm, and the blade spacing is 0.8 mm-1.2 mm; and / or, the tilt angle of the rounding disc is 45°, and the rotation speed is 300 rpm-500 rpm.

[0015] In one specific embodiment of this application, step S20 includes: Step S21: Thorium dioxide, uranium dioxide, and carbon powder are added to the mixing tank of the acoustic mixing device at a mass ratio of 3:7:1, and deionized water is added until the solid content is 30%-40%. Step S22: Start the acoustic mixing device and run it in pulse mode for 30 minutes, controlling the temperature to ≤40℃. The pulse mode is 2 minutes of operation and 1 minute of intermittent operation. Step S23: After mixing, the slurry is centrifuged and dehydrated to obtain a uniform powder with a moisture content of 15%-20%.

[0016] In one specific embodiment of this application, the centrifugal dehydration process is carried out at a speed of 3000 rpm for 10 minutes.

[0017] In one specific embodiment of this application, step S40 includes: Step S41: The plastic wet material is extruded into a continuous strip material, and then cut into 1.2mm×1.2mm cylindrical particles by the rotating blade of the cutting and rounding equipment; Step S42: The columnar particles are dropped into a rolling disc and rolled into wet spherical blanks with a diameter of 1.0±0.1mm under the action of centrifugal force. The single forming time is 3-5 minutes. Step S43: The wet spherical blank is dried with hot air at 40℃ for 30 minutes to achieve preliminary shaping.

[0018] In one specific embodiment of this application, step S50 includes: Step S51, Pre-sintering stage: Under Ar atmosphere, pre-sintering is carried out at 600℃-800℃ with a temperature increase of 2℃ / min to remove the binder and form a porous skeleton with a porosity of 40%-50%; Step S52, Carbothermic Reduction Stage: Carbothermic reduction is carried out in a mixed atmosphere of Ar and H2, with the temperature increased to 1200℃-1400℃ at a rate of 5℃ / min. The volume fraction of H2 in the mixed atmosphere is 10%. Step S53, Densification Stage: Increase the temperature to 1500℃-1600℃ at a rate of 3℃ / min, perform densification sintering, hold for 2 hours, and maintain a vacuum degree ≤10. -2 Pa, finally yielded UC2-ThC2 composite fuel microspheres.

[0019] The second aspect of this application provides a carbon-containing uranium-thorium mixed fuel microsphere, which is prepared using the method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to the first aspect of this application.

[0020] The beneficial effects of the technical solution in this application are as follows: by integrating the technologies of acoustic dispersion, plastic wet material spheroidization, and gradient sintering, the core problems of uneven mixing, poor forming quality, and low sintering performance of uranium-thorium fuel microspheres are solved, providing an innovative solution for low-cost and high-reliability manufacturing of high-temperature gas-cooled reactor fuel elements, thorium-based molten salt reactor elements, etc. Attached Figure Description

[0021] Figure 1 The diagram shown is a flowchart illustrating a method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to an embodiment of this application. Detailed Implementation

[0022] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] At least one embodiment of this application provides a method for preparing carbon-containing uranium-thorium mixed fuel microspheres, referencing... Figure 1 The preparation method of the carbon-containing uranium-thorium mixed fuel microspheres includes the following steps.

[0024] Step S10: Thorium dioxide (ThO2), uranium dioxide (UO2), and carbon powder with a particle size ≤5μm are ball-milled to refine to D50≤2μm.

[0025] It should be noted that step S10 is a raw material pretreatment step.

[0026] Step S20: Thorium dioxide, uranium dioxide and carbon powder are added to the acoustic mixing device at a mass ratio of 3:7:1. Deionized water is added until the solid content is 30%-40%. The powder is dispersed in the argon atmosphere of the acoustic mixing device to achieve nanoscale uniform dispersion and obtain mixed powder.

[0027] Specifically, the components are dispersed in an argon atmosphere using an acoustic mixing device. The high-frequency cavitation effect of the acoustic mixing device (also known as acoustic cavitation effect) is utilized to achieve uniform mixing of the three components at the nanoscale. For example, the carbon powder agglomeration size is <5μm, and the mixing uniformity deviation is <3%. Carbon powder dispersion (SEM-EDS analysis): carbon element distribution deviation ≤3%; mixing uniformity (laser particle size analyzer): D90 ≤5μm.

[0028] It should be noted that step S20 is an acoustic-assisted mixing step, which employs acoustic-assisted nanoscale mixing technology. The acoustic mixing device can be equipped with a gas-liquid two-phase circulation system.

[0029] Step S30: Add a binder to the mixed powder, the binder having a mass fraction of 8%-12%, and mix with deionized water to form a plastic wet material with a moisture content of 25%-30%. The binder includes microcrystalline cellulose (MCC) and / or polyvinyl alcohol (PVA).

[0030] In some embodiments, the rheological properties of the wet plastic stock can be controlled, for example, the tensile strength of the wet plastic stock can be controlled between 0.8 and 1.2 MPa. The tensile strength of the wet plastic stock can be measured using a universal testing machine based on the STM D638 standard. The elongation at break of the wet plastic stock can also be further controlled between 150% and 200%.

[0031] Step S40: The plastic wet material is extruded into a continuous strip material, which is then cut into 1.2mm×1.2mm columnar particles by the rotating blade of the cutting and rounding equipment. The columnar particles are then rounded by the rounding disc into wet spherical blanks with a diameter of 1.0±0.1mm.

[0032] Step S50: Under an inert atmosphere, the wet pellet blank is subjected to gradient heating to remove the binder and complete the carbothermic reduction reaction, finally obtaining UC2-ThC2 composite fuel microspheres.

[0033] According to the technical solution provided in the embodiments of this application, step S20 adopts acoustic wave-assisted nanoscale mixing technology, utilizing the acoustic wave cavitation effect to achieve nanoscale uniform dispersion of uranium thorium oxide and carbon powder (e.g., carbon powder distribution standard deviation <3%), increasing the contact area of ​​the carbothermic reduction reaction by 5 times, and improving the purity of the UC2-ThC2 composite phase from 85% in the prior art to over 92%, thereby improving mixing uniformity and reaction efficiency and avoiding the stability problem of sol-gel process; steps S30 and S40 adopt plastic wet material spheroidizing technology, by adding binders such as microcrystalline cellulose / polyvinyl alcohol to the mixed powder, and combining with cutting and spheroidizing equipment, using wet granulation process to replace dry mixing and pressing, the radioactive dust emission rate is reduced to <0.5%, and the cutting and spheroidizing equipment achieves high-precision microsphere forming, simplifying the microsphere forming process and improving dimensional consistency; step S50 adopts gradient carbothermic reduction sintering process, reducing phase contamination and improving UC 2 / The purity of the ThC2 composite phase (compared to the upper limit of 92.9 wt% in the literature). In addition, the process steps for preparing carbon-containing uranium-thorium mixed fuel microspheres have been simplified from the traditional 8 steps to 4 steps (mixing → granulation → pre-firing → sintering), reducing equipment costs by 70% and enabling continuous production of millions of microspheres per year.

[0034] In summary, the embodiments of this application solve the core problems of uneven mixing, poor forming quality, and low sintering performance of uranium-thorium fuel microspheres by integrating acoustic dispersion, plastic wet material spheroidization, and gradient sintering technologies. This provides an innovative solution for the low-cost and high-reliability manufacturing of fuel elements for high-temperature gas-cooled reactors and thorium-based molten salt reactors.

[0035] In at least one embodiment of this application, the parameters of the acoustic mixing device are set as follows: frequency 20kHz-40kHz, power density 0.5-1.5W / cm². 3 Amplitude: 5μm-50μm; Time: 5 minutes-60 minutes; Temperature: <60℃.

[0036] In at least one embodiment of this application, the binder is microcrystalline cellulose and polyvinyl alcohol. The mass ratio of microcrystalline cellulose to polyvinyl alcohol is 1:2, and the solid content is 40-50%. Thus, by innovatively proposing a microcrystalline cellulose (MCC) / polyvinyl alcohol (PVA)-deionized water binder system (MCC / PVA mass ratio 1:2, solid content 40-50%), the tensile strength of the prepared plastic wet material is >0.8MPa, and the elongation at break is >150%.

[0037] In at least one embodiment of this application, the binder is microcrystalline cellulose, and the mass ratio of microcrystalline cellulose to deionized water is 1:4. The mixing rate of the microcrystalline cellulose and deionized water is 200 rpm, and the mixing time is 30 min.

[0038] In at least one embodiment of this application, the binder is polyvinyl alcohol (PVA). PVA is dissolved in deionized water at 80°C to a concentration of 10 wt%, cooled to 25°C, and then mixed with the mixed powder. The degree of polymerization of PVA is 1750 ± 50.

[0039] In at least one embodiment of this application, the die diameter of the extruder is set to 1.2 mm, and the screw speed is set to 20 rpm-30 rpm; and / or, the rotation speed of the rotating blades of the cutting and rounding device is set to 300 rpm-500 rpm, and the blade spacing is 0.8 mm-1.2 mm; and / or, the tilt angle of the rounding disc is 45°, and the rotation speed is 300 rpm-500 rpm. Thus, a high-sphericity wet spherical blank can be directly formed using the cutting and rounding device, resulting in a wet spherical blank diameter of 1.0 ± 0.1 mm, a CV value < 5%, and a true sphericity > 95%, achieving a CV value reduction from 10% to 5% and a true sphericity increase from 80% to 95%.

[0040] In at least one embodiment of this application, steps S21 to S23 are a specific implementation of step S20.

[0041] Step S21: Thorium dioxide, uranium dioxide and carbon powder are added to the mixing tank of the acoustic mixing device at a mass ratio of 3:7:1, and deionized water is added until the solid content is 30%-40%.

[0042] Step S22: Start the acoustic mixing device and run it in pulse mode for 30 minutes, controlling the temperature to ≤40℃. The pulse mode consists of 2 minutes of operation followed by a 1-minute interval.

[0043] Step S23: After mixing, the slurry is centrifuged and dehydrated to obtain a uniform powder with a moisture content of 15%-20%.

[0044] In at least one embodiment of this application, the centrifugal dehydration process is carried out at a speed of 3000 rpm for 10 minutes.

[0045] In at least one embodiment of this application, steps S41 to S43 are a specific implementation of step S40.

[0046] Step S41: The plastic wet material is extruded into a continuous strip material, and then cut into 1.2mm×1.2mm cylindrical particles by the rotating blade of the cutting and rounding equipment; Step S42: The columnar particles are dropped into a rolling disc and rolled into wet spherical blanks with a diameter of 1.0±0.1mm under the action of centrifugal force. The single forming time is 3-5 minutes. Step S43: The wet spherical blank is dried with hot air at 40℃ for 30 minutes to achieve preliminary shaping.

[0047] The diameter of the wet spherical blank was measured using a laser particle size analyzer: 1.0±0.1mm; the sphericity was determined using image analysis software: true sphericity ≥95% (roundness ≥0.95).

[0048] In at least one embodiment of this application, steps S51 to S53 are a specific manifestation of step S50.

[0049] Step S51, Pre-sintering stage: Under Ar atmosphere, pre-sintering is carried out at 600℃-800℃ with a temperature increase of 2℃ / min to remove the binder and form a porous skeleton with a porosity of 40%-50%; It should be noted that the MCC / PVA decomposition rate in step S51 is ≥99%.

[0050] Step S52, Carbothermic Reduction Stage: Carbothermic reduction is carried out in a mixed gas of Ar and H2 atmospheres, with the temperature increased to 1200℃-1400℃ at 5℃ / min. The volume fraction of H2 in the mixed gas is 10%.

[0051] It should be noted that step S52 ensures a complete reaction between ThO2 / UO2 and carbon (UC2-ThC2 composite phase purity > 92%). The reaction equation for step S52 is as follows: ThO2 + UO2 + 6C = UC2 + ThC2 + 2CO2↑ Step S53, Densification Stage: Increase the temperature to 1500℃-1600℃ at a rate of 3℃ / min, perform densification sintering, hold for 2 hours, and maintain a vacuum degree ≤10. -2 Pa, finally yielded UC2-ThC2 composite fuel microspheres.

[0052] It should be noted that step S53 eliminates residual porosity (final porosity < 5%) and improves interfacial bonding strength (UC2-ThC2 interfacial strength > 80 MPa). Tests show that the composite phase of the UC2-ThC2 composite fuel microspheres is ≥ 92% (XRD quantitative analysis of phase purity), the density of the UC2-ThC2 composite fuel microspheres is ≥ 95% of the theoretical density (Archimedes method), and the compressive strength of the UC2-ThC2 composite fuel microspheres is ≥ 200 MPa (universal testing machine, ISO 185 standard).

[0053] In the above embodiments, by designing a three-stage gradient sintering process to control the pore evolution (porosity fluctuates from 15% to a stable <5%), the interfacial bonding strength of UC2-ThC2 is increased by 60% (from 50MPa to 80MPa), and the compressive strength of the microspheres is >200MPa, thereby enhancing the sintering performance and structural stability of the final UC2-ThC2 composite fuel microspheres.

[0054] At least one embodiment of this application also provides a carbon-containing uranium-thorium mixed fuel microsphere, which is prepared by a method for preparing carbon-containing uranium-thorium mixed fuel microspheres in any of the above embodiments of this application.

[0055] The carbon-containing uranium-thorium mixed fuel microspheres have good sphericity, uniform size, and a particle size of 100~1000μm.

[0056] The carbon-containing uranium-thorium mixed fuel microspheres prepared using the methods described above in this application will be illustrated below with specific examples.

[0057] Example 1: Microcrystalline cellulose (MCC) binder system Raw material ratio: ThO2:UO2:charcoal powder = 3:7:1 (mass ratio), MCC addition 10wt%; Sound wave mixing: frequency 30kHz, power 1.0W / cm² 3 Mixing time: 30 min; Rolling parameters: blade speed 600 rpm, rolling disc speed 400 rpm; Sintering process: Carbothermic reduction at 1400℃ for 4 hours, densification at 1600℃ for 2 hours; result: Microsphere diameter: 1.02±0.08mm (CV=4.2%); Carbon distribution uniformity: EDS surface scan deviation 2.8%; Density after sintering: 10.2 g / cm³ 3 (Theoretical value 10.5g / cm³) 3 ); Compressive strength: 215MPa.

[0058] Example 2: Polyvinyl alcohol (PVA) adhesive system Raw material ratio: ThO2:UO2:charcoal powder = 3:7:1, PVA addition 8wt%; Sound wave mixing: frequency 35kHz, power 1.2W / cm² 3 Mixing time: 40 min; Rolling parameters: blade speed 700 rpm, rolling disc speed 450 rpm; Sintering process: Optimized to a three-stage process (pre-firing at 600℃ → reduction at 1400℃ → densification at 1550℃); result: Microsphere diameter: 0.98±0.07mm (CV=3.9%); Carbon distribution uniformity: EDS deviation 2.5%; Density after sintering: 10.4 g / cm³ 3 ; Compressive strength: 230MPa.

[0059] Example 3: Optimization of Process Parameters Areas for improvement: The sound wave mixing has been upgraded to a dual-frequency mode (20kHz + 40kHz) with a power density of 1.5W / cm². 3 ; The moisture content of wet materials is reduced to 28%, improving rounding efficiency; A gradient H2 concentration (10%→5%→0%) is introduced during the sintering stage.

[0060] result: Microsphere diameter uniformity: CV = 3.1%; Phase purity: UC2+ThC2=93.5% (XRD); Porosity: 4.2% (SEM image analysis); Compressive strength: 245MPa.

[0061] In Examples 1 to 3 above, the mixing uniformity was analyzed using SEM-EDS surface scanning analysis of the carbon element distribution standard deviation. Microsphere size was determined using a laser diffraction particle size analyzer (Malvern Mastersizer 3000). Sintered density was determined based on the Archimedes method (ASTM B962 standard). Mechanical properties were tested using a universal testing machine (ISO 185 standard, loading rate 0.5 mm / min).

[0062] It should be noted that the combination of the technical features in the embodiments of this application is not limited to the combination methods described in the embodiments of this application or the combination methods described in specific embodiments. All technical features described in this application can be freely combined or combined in any way, unless they contradict each other.

[0063] As indicated in this application and claims, unless the context clearly indicates otherwise, the words "a," "an," and / or "the" do not specifically refer to the singular and may also include the plural. Generally speaking, the term "comprising" only indicates that it includes the explicitly identified steps and elements, which do not constitute an exclusive list, and the method or apparatus may also include other steps or elements.

[0064] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications or equivalent substitutions made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing carbon-containing uranium-thorium mixed fuel microspheres, characterized in that, include: Step S10: Thorium dioxide (ThO2), uranium dioxide (UO2) and carbon powder with a particle size ≤5μm are ball-milled to refine to D50≤2μm; Step S20: Thorium dioxide, uranium dioxide and carbon powder are added to the acoustic mixing device at a mass ratio of 3:7:

1. Deionized water is added until the solid content is 30%-40%. The powder is dispersed in the argon atmosphere of the acoustic mixing device to achieve nanoscale uniform dispersion of the powder and obtain mixed powder. Step S30: Add a binder to the mixed powder, the binder having a mass fraction of 8%-12%, and mix with deionized water to form a plastic wet material, the moisture content of the plastic wet material being 25%-30%, the binder including microcrystalline cellulose and / or polyvinyl alcohol; Step S40: The plastic wet material is extruded into a continuous strip material, which is then cut into 1.2mm×1.2mm columnar particles by the rotating blade of the cutting and rounding device. The columnar particles are then rounded by the rounding disc into wet spherical blanks with a diameter of 1.0±0.1mm. Step S50: Under an inert atmosphere, the wet pellet blank is subjected to gradient heating to remove the binder and complete the carbothermic reduction reaction, finally obtaining UC2-ThC2 composite fuel microspheres.

2. The method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to claim 1, characterized in that, The parameters of the acoustic mixing device are set as follows: frequency 20kHz-40kHz, power density 0.5-1.5W / cm². 3 Amplitude: 5μm-50μm; Time: 5 minutes-60 minutes; Temperature: <60℃.

3. The method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to claim 1, characterized in that, The binder uses microcrystalline cellulose and polyvinyl alcohol, with a mass ratio of 1:2 and a solid content of 40-50%.

4. The method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to claim 1, characterized in that, The binder is microcrystalline cellulose, with a mass ratio of microcrystalline cellulose to deionized water of 1:

4. The stirring rate during the mixing of microcrystalline cellulose and deionized water is 200 rpm, and the mixing time is 30 min. Alternatively, the binder is polyvinyl alcohol, which is dissolved in deionized water at 80℃ to a concentration of 10 wt%, cooled to 25℃, and then mixed with the powder. The degree of polymerization of polyvinyl alcohol is 1750±50.

5. The method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to claim 1, characterized in that, The die diameter of the extruder is set to 1.2 mm, and the screw speed is set to 20 rpm-30 rpm; and / or, the rotation speed of the rotating blades of the cutting and rounding device is set to 300 rpm-500 rpm, and the blade spacing is 0.8 mm-1.2 mm; and / or, the tilt angle of the rounding disc is 45°, and the rotation speed is 300 rpm-500 rpm.

6. The method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to claim 1, characterized in that, Step S20 includes: Step S21: Thorium dioxide, uranium dioxide, and carbon powder are added to the mixing tank of the acoustic mixing device at a mass ratio of 3:7:1, and deionized water is added until the solid content is 30%-40%. Step S22: Start the acoustic mixing device and run it in pulse mode for 30 minutes, controlling the temperature to ≤40℃; the pulse mode is 2 minutes of operation followed by 1 minute of intermittent operation. Step S23: After mixing, the slurry is centrifuged and dehydrated to obtain a uniform powder with a moisture content of 15%-20%.

7. The method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to claim 6, characterized in that, The centrifugation process was carried out at 3000 rpm for 10 minutes.

8. The method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to claim 1, characterized in that, Step S40 includes: Step S41: The plastic wet material is extruded into a continuous strip material, and then cut into 1.2mm×1.2mm cylindrical particles by the rotating blade of the cutting and rounding equipment; Step S42: The columnar particles are dropped into a rolling disc and rolled into wet spherical blanks with a diameter of 1.0±0.1mm under the action of centrifugal force. The single forming time is 3-5 minutes. Step S43: The wet spherical blank is dried with hot air at 40℃ for 30 minutes to achieve preliminary shaping.

9. A method for preparing carbon-containing uranium-thorium mixed fuel microspheres according to any one of claims 1 to 8, characterized in that, Step S50 includes: Step S51, Pre-sintering stage: Under Ar atmosphere, pre-sintering is carried out at 600℃-800℃ with a temperature increase of 2℃ / min to remove the binder and form a porous skeleton with a porosity of 40%-50%; Step S52, Carbothermic Reduction Stage: Carbothermic reduction is carried out in a mixed atmosphere of Ar and H2, with the temperature increased to 1200℃-1400℃ at a rate of 5℃ / min. The volume fraction of H2 in the mixed atmosphere is 10%. Step S53, Densification Stage: Increase the temperature to 1500℃-1600℃ at a rate of 3℃ / min, perform densification sintering, hold for 2 hours, and maintain a vacuum degree ≤10. -2 Pa, finally yielded UC2-ThC2 composite fuel microspheres.

10. A carbon-containing uranium-thorium mixed fuel microsphere, characterized in that, The carbon-containing uranium-thorium mixed fuel microspheres are prepared using the preparation method of carbon-containing uranium-thorium mixed fuel microspheres as described in any one of claims 1 to 9.

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