Microstructure-controllable un fuel pellets and preparation method thereof

Abstract

This invention belongs to the field of nuclear fuel technology, specifically relating to a microstructure-controllable UN fuel pellet and its preparation method. The method involves using UO2 powder and high-purity carbon powder as raw materials, mixing and ball-milling them to prepare a mixed powder. The mixed powder is then pressed into a blank, which undergoes a carbothermic reaction under a nitrogen atmosphere. The carbothermic powder is then pressed into a blank and decarburized in a nitrogen-hydrogen mixed gas to obtain UN powder. The UN powder is then densified and sintered to obtain high-purity UN fuel pellets. During the densification and sintering process, the grain size of the UN fuel pellets is controlled by adjusting sintering parameters under vacuum conditions. This invention offers a simple preparation method that introduces no additives or additional elements, does not affect the service performance of large-grain fuel pellets, and reduces the pressure of spent fuel reprocessing.

Description

Technical Field

[0001] This invention belongs to the field of nuclear fuel technology, specifically relating to a UN fuel pellet with controllable microstructure and its preparation method. Background Technology

[0002] Uranium nitride (UN) fuel is considered a potential candidate for accident-tolerant fuel in light water reactors and for fourth-generation reactors due to its advantages such as high uranium atomic density, high melting point, high thermal conductivity, good high-temperature stability, and good compatibility with liquid metals. One of the key technologies for UN fuel preparation is the synthesis process of high-purity, high-sintering-activity UN powder. However, UN powder preparation faces several key challenges: the UN preparation process is complex, and powder particle size and impurities are difficult to control; uranium nitride is chemically reactive and readily reacts with oxygen and water in the air; and U and N elements do not readily combine, resulting in high reaction temperatures and long durations in the UN preparation process, requiring high equipment safety and consuming significant energy. Internationally, there are currently several technical routes for UN preparation, mainly including the carbothermal reduction route, the metal nitridation route, the sol-gel route, and the uranium fluoride nitridation route. Among these, the carbothermal reduction nitridation route was proposed by the Battelle Memorial Institute (BMI) in 1968. This route has been extensively studied and the process is relatively mature, but the prepared UN powder contains excessively high levels of carbon and oxygen impurities. Because the lattice constant of uranium nitride (UN) is highly sensitive to carbon content, carbon impurities easily fill lattice vacancies, forming carbonitrides during heat treatment. This alters the lattice constant of UN and thus affects its powder properties. The presence of oxygen impurities leads to the precipitation of UO2 in UN, thereby affecting the sintering activity and service performance of the UN powder. Controlling oxygen content in uranium nitride fuel preparation is an international challenge; therefore, when using the carbothermic reduction nitride route to prepare UN powder, controlling the carbon and oxygen impurity content in the final product is crucial.

[0003] Since the Fukushima nuclear accident, in order to reduce the probability of core meltdown and severe accidents, the nuclear industry has proposed the concept of Accident Tolerant Fuel (ATF). UN fuel pellets, as a potential ATF, have advantages over conventional UO2 fuel, including increased thermal conductivity, improved mechanical properties, enhanced radiation resistance, and improved lattice redox properties. Therefore, UN fuel can extend the refueling cycle of nuclear reactor fuel assemblies (deepening reactor fuel burnup) and improve fuel safety, making it a promising candidate for accident-tolerant fuel. For fuel pellets, increasing grain size extends the average path of fission products diffusion to grain boundaries, effectively reducing fission gas release and alleviating end-of-life fuel rod internal pressure, thus enabling safe operation to higher burnup levels. Simultaneously, larger grain pellets have smaller grain boundary areas, less restriction on dislocation movement, and are "softer," which helps mitigate pellet-cladding interaction (PCI). For example, Westinghouse research shows that defects such as bubbles generated after irradiation of large-grain fuel pellets are distributed within the grains, while bubbles in conventionally sized fuel pellets tend to accumulate at grain boundaries. Research from MIT indicates that large-grain fuel pellets release less fission gas and have lower internal pressure, making them more suitable for meeting fuel rod pressure design criteria. Secondly, because fission gas can be stored within the grains of large-grain fuel pellets rather than accumulating as bubbles at grain boundaries, these pellets exhibit less volume swelling, reducing pressure at the pellet-cladding interface and effectively mitigating pellet-cladding interactions. Similarly, grain refinement is a common method for improving material properties; it can significantly enhance strength, plasticity, corrosion resistance, and thermal stability. Therefore, exploring the impact of different microstructures on the performance of UN fuel pellets by artificially controlling their grain size is crucial for improving nuclear fuel preparation processes. However, large-grain fuel pellets are currently mainly prepared by doping with growth promoters. However, the introduction of non-U elements will increase the difficulty of spent fuel reprocessing, and the presence of growth promoters may reduce the radiation resistance of fuel pellets. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a microstructure-controllable UN fuel pellet and its preparation method. This invention employs a carbothermal reduction method to prepare UN powder. Taking into account factors such as the activity of the raw material powder, the carbon / uranium molar ratio, the reaction atmosphere, the reaction temperature, and the time during UN powder preparation, and considering the grain size, phase stability, and mechanical properties of the UN fuel pellet, a DC rapid hot-pressing sintering method is used to design and prepare UN fuel pellets with different microstructures without growth promoters. The UN fuel pellets have extremely low carbon and oxygen content, and the specific component of the fuel pellet is UN.

[0005] This invention is specifically achieved through the following technical solutions: The first objective of this invention is to provide a method for preparing UN fuel pellets with controllable microstructure, comprising the following steps: UO2 powder and high-purity carbon powder were mixed and ball-milled to prepare a mixed powder.

[0006] After the mixed powder is pressed into a blank, it undergoes a carbothermic reaction under a nitrogen atmosphere. The reaction that occurs during the carbothermic reaction is: UO2 + C + N2 → UN x C y + CO.

[0007] The carbothermic powder is pressed into a blank and then decarburized in a nitrogen-hydrogen mixture to obtain UN powder. The reaction that occurs during this process is: UN x C y +H2+N2→ UN+CH4. During the reaction, the technical problems of controlling and removing C and O impurities are overcome by adjusting the atmosphere composition, heating time and temperature parameters.

[0008] UN powder is densified and sintered to obtain UN fuel pellets.

[0009] During the densification sintering process, under vacuum conditions, the grain size of the UN fuel pellets is controlled by adjusting sintering parameters, including: Sintering temperature: 600℃~1900℃, sintering pressure: 50MPa~500MPa, holding time: 10min~30min. During the sintering process, the sintering temperature has a dominant effect on the density and hardness of the sample. At the same temperature, the higher the sintering pressure, the higher the density and hardness.

[0010] The above method allows the grain size of UN fuel pellets to vary from nanometers to hundreds of micrometers. Preferably, the grain size of the UN fuel pellets is 600 nm to 126 μm.

[0011] By using the above-described method of the present invention, and by controlling the atmosphere composition, heating time and temperature parameters during the decarbonization process, the technical problem of the difficulty in controlling and removing C and O impurities is overcome. The prepared UN fuel pellets have an oxygen content of 158 ppm and a carbon content of 2540 ppm.

[0012] Preferably, the molar ratio of UO2 powder to high-purity carbon powder is 1:2.2~4.0, more preferably 1:2.4; the purity of UO2 powder is ≥99.99%, and the purity of UO2 powder is ≥99.99%. 235 U content is 0.2 at%, the rest is 238 U; High-purity carbon powder with a purity of ≥99.9%.

[0013] Preferably, when mixing and ball milling, wet ball milling is used, the medium is ethanol, and the ball milling parameters are: rotation speed 250 rpm / min~500 rpm / min, forward and reverse rotation 10 min~15 min, pause 5 min~10 min, total time 20 h~40 h.

[0014] Preferably, the temperature of the carbothermic reaction is 1600℃~1800℃, more preferably 1700℃, and the time is ≥5h (i.e. not less than 5h).

[0015] During decarbonization, the hydrogen volume content in the mixed gas is 3%, the temperature is not lower than 1500℃ (≥1500℃), and the time is ≥6h (i.e., not less than 6h).

[0016] A second objective of this invention is to provide a UN fuel pellet with controllable microstructure, prepared using the method described above. The grain size of the UN fuel pellet varies from the nanometer to the hundreds of micrometer scale. Preferably, the grain size ranges from 600 nm to 126 μm. The prepared UN fuel pellet exhibits a stable single-phase structure, uniform composition distribution, low carbon and oxygen content, and good microstructure and mechanical properties.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention uses UO2 powder and high-purity carbon powder as raw materials, mixes and ball-mills them to prepare a mixed powder; after pressing the mixed powder into a blank, the blank undergoes a carbothermic reaction under a nitrogen atmosphere; the carbothermic powder is then pressed into a blank and decarburized in a nitrogen-hydrogen mixed gas to obtain UN powder; the UN powder is then densified and sintered to obtain UN fuel pellets; this invention controls the grain size of the UN fuel pellets by changing the sintering parameters, including: sintering temperature 600℃~1900℃, sintering pressure 50MPa~ This invention enables the preparation of UN fuel pellets with grain sizes ranging from nanometers to hundreds of micrometers at 500 MPa and a holding time of 10 to 30 minutes. The preparation process involves no additives, introduces no additional elements, and does not affect the service performance of large-grain fuel pellets, thus reducing the pressure on spent fuel reprocessing. The invention's simple preparation method provides a simple and feasible approach for artificially controlling the grain size of UN fuel pellets and exploring the impact of different microstructures on their performance, which is beneficial for advancing the performance improvement and application research of UN fuel pellets.

[0018] This invention employs a direct current rapid hot pressing sintering method to prepare UN fuel pellets, performing high-temperature sintering at 600℃~1900℃. This directly yields UN fuel pellets with different designed microstructures, and the preparation method is simple. The UN fuel pellets prepared by this invention have low carbon and oxygen impurity content, with an oxygen content of 158ppm and a carbon content of 2540ppm (third-party verification). The oxygen content within the fuel pellets is the lowest reported in currently available domestic and international literature. Attached Figure Description

[0019] Figure 1 In the image, (a) is a physical image of UO2 powder raw material, and (b) is a physical image of high-purity carbon powder raw material.

[0020] Figure 2 This is a photograph of the mixed powder obtained after high-energy ball milling.

[0021] Figure 3 A photograph of a blank made from mixed powder.

[0022] Figure 4 This is a photograph of the high-purity UN powder obtained after decarbonization.

[0023] Figure 5 This is a physical image of UN fuel pellets obtained by DC rapid sintering at 1600℃.

[0024] Figure 6 The results show the carbon and oxygen content of UN fuel pellets sintered at 1600℃, where (a) is the carbon content test result and (b) is the oxygen content test result.

[0025] Figure 7 The XRD patterns of UN fuel pellets obtained under different sintering conditions in Examples 1 to 6 are shown.

[0026] Figure 8 The images show the SEM morphology of the fracture surfaces of UN fuel pellets prepared in Examples 1 to 6: (a) 600℃-400MPa-30min, (b) 600℃-500MPa-10min, (c) 700℃-500MPa-10min, (d) 1600℃-50MPa-10min, (e) 1850℃-50MPa-10min, and (f) 1900℃-50MPa-30min. Detailed Implementation

[0027] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below with reference to specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention. Unless otherwise specified, the experimental methods and detection methods described in the following embodiments are conventional methods; unless otherwise specified, the reagents and materials described are commercially available.

[0028] Example 1 (1) Preparation of raw materials: UO2 powder was produced by International Bio-analytical Industries Inc. of the United States, with a purity of ≥99.9%. 235 U content is 0.2 at%, the rest is238 U. High-purity carbon powder (derived from acetylene, 50% compressed) was produced by Sinopharm Chemical Reagent Shaanxi Co., Ltd., with a purity of 99.9%. Figure 1 In the image, (a) shows a physical image of UO2 powder, and (b) shows a physical image of high-purity carbon powder raw material powder. To prevent the sample powder from becoming damp or deteriorating during powder processing, the sample powder was pre-placed in a glove box with a controlled H2O content of less than 0.01 ppm and an oxygen concentration of less than 4 ppm.

[0029] (2) High-energy ball milling: UO2 powder and high-purity carbon powder were mixed in a molar ratio of 1:2.4. The mixed powder was then wet-milled to fully mix the UO2 powder and high-purity carbon powder and break down the particles, thereby improving the effect of the carbothermic reduction reaction. The high-energy ball milling process was carried out in a zirconium oxide ball milling jar under an argon atmosphere. The ball milling parameters were: rotation speed 500 rpm / min, forward and reverse rotation 15 min, pause 10 min, total time 40 h, and 20 ml of ethanol was added for wet ball milling. Figure 2 This is a photograph of the mixed powder obtained after high-energy ball milling.

[0030] (3) Carbothermic reaction: The ball-milled mixed powder is pressed into a preform by a tablet press and placed in a high-temperature tube furnace. The carbothermic reaction is carried out in a nitrogen atmosphere at 1700℃ for 5 hours. The reaction process is UO2 + C + N2 → UN x C y + CO. Figure 3 A photograph of a blank made from mixed powder.

[0031] (4) Hydrogen reduction: The powder that has completed the carbothermic reaction is pressed into a preform using a tablet press, and then subjected to decarburization treatment in a nitrogen-hydrogen mixture (hydrogen content of 3%) at 1500℃ for 6 hours. During this process, UN x C y +H2+N2→ UN+CH4. By adjusting the atmosphere composition, heating time and temperature parameters during the reaction, the technical problems of controlling and removing C and O impurities are overcome, and high-purity UN powder is obtained. Figure 4 This is a photograph of the high-purity UN powder obtained after decarbonization treatment.

[0032] (5) Densification sintering: High-purity UN powder is densified in an FHP-808 DC rapid sintering equipment. During the sintering process, the chamber is kept under vacuum, and the temperature is monitored by an infrared thermometer aligned with the temperature measuring hole of the mold. During the heating process, the target temperature of 600℃ is reached at a heating rate of 100℃ / min, followed by holding for 30min at a sintering pressure of 400MPa. After the holding period, the powder is cooled to room temperature in the vacuum chamber at a rate of 50℃ / min. High-density UN fuel pellets can be obtained in a shorter sintering time through field-assisted sintering technology.

[0033] Example 2 Steps (1) to (4) are the same as in Example 1, except that in step (5) the sintering parameters are changed, specifically: (5) Densification sintering: High-purity UN powder is densified and sintered in an FHP-808 DC rapid sintering equipment. During the sintering process, the chamber is kept under vacuum, and the temperature is monitored by an infrared thermometer aligned with the temperature measuring hole of the mold. During the heating process, the target temperature of 600℃ is reached at a heating rate of 100℃ / min, and then held for 10 min. The sintering pressure is 500MPa. After the holding period, the powder is cooled to room temperature in the vacuum chamber at a rate of 50℃ / min to obtain UN fuel pellets.

[0034] Example 3 Steps (1) to (4) are the same as in Example 1, except that in step (5) the sintering parameters are changed, specifically: (5) Densification sintering: High-purity UN powder is densified and sintered in an FHP-808 DC rapid sintering equipment. During the sintering process, the chamber is kept under vacuum, and the temperature is monitored by an infrared thermometer aligned with the temperature measuring hole of the mold. During the heating process, the target temperature of 700℃ is reached at a heating rate of 100℃ / min, and then held for 10min. The sintering pressure is 500MPa. After the holding period, the powder is cooled to room temperature in the vacuum chamber at a rate of 50℃ / min to obtain UN fuel pellets.

[0035] Example 4 Steps (1) to (4) are the same as in Example 1, except that in step (5) the sintering parameters are changed, specifically: (5) Densification sintering: High-purity UN powder is densified and sintered in an FHP-808 DC rapid sintering equipment. During the sintering process, the chamber is kept under vacuum, and the temperature is monitored by an infrared thermometer aligned with the temperature measuring hole of the mold. During the heating process, the target temperature of 1600℃ is reached at a heating rate of 100℃ / min, and then held for 10min. The sintering pressure is 50MPa. After the holding period, the powder is cooled to room temperature in the vacuum chamber at a rate of 50℃ / min to obtain UN fuel pellets.

[0036] Example 5 Steps (1) to (4) are the same as in Example 1, except that in step (5) the sintering parameters are changed, specifically: (5) Densification sintering: High-purity UN powder is densified and sintered in an FHP-808 DC rapid sintering equipment. During the sintering process, the chamber is kept under vacuum, and the temperature is monitored by an infrared thermometer aligned with the temperature measuring hole of the mold. During the heating process, the target temperature of 1850℃ is reached at a heating rate of 100℃ / min, and then held for 10min. The sintering pressure is 50MPa. After the holding period, the powder is cooled to room temperature in the vacuum chamber at a rate of 50℃ / min to obtain UN fuel pellets.

[0037] Example 6 Steps (1) to (4) are the same as in Example 1, except that in step (5) the sintering parameters are changed, specifically: (5) Densification sintering: High-purity UN powder is densified and sintered in an FHP-808 DC rapid sintering equipment. During the sintering process, the chamber is kept under vacuum, and the temperature is monitored by an infrared thermometer aligned with the temperature measuring hole of the mold. During the heating process, the target temperature of 1900℃ is reached at a heating rate of 100℃ / min, and then held for 30min. The sintering pressure is 50MPa. After the holding period, the powder is cooled to room temperature in the vacuum chamber at a rate of 50℃ / min to obtain UN fuel pellets.

[0038] In Examples 1 to 6, the grain size of UN fuel pellets was controlled by changing the sintering parameters, achieving the preparation of UN fuel pellets with grain sizes ranging from nanometers to hundreds of micrometers, and determining the sintering conditions for UN fuel pellets with different microstructures. Table 1 shows the sintering parameters for UN fuel pellets. Figure 5 This is a physical image of UN fuel pellets obtained by sintering at 1600℃. Figure 6 The figure shows the results of a third-party carbon and oxygen content test of UN fuel pellets sintered at 1600℃, which shows that the oxygen content in the fuel pellets is the lowest in currently available domestic and international literature.

[0039] Table 1 Sintering conditions for UN fuel pellets The microstructure and properties of the UN fuel pellets prepared in the above embodiments were analyzed as follows: (1) Phase structure analysis The sintered sample was ground and polished, and then subjected to XRD testing. The test results are as follows: Figure 7As shown, at a DC hot-pressing rapid sintering temperature of 600℃ to 1900℃, the characteristic diffraction peak positions of the UN fuel pellets match those of PDF#04-003-3033 (face-centered cubic structure UN) in the Jade standard database, indicating that the densified fuel pellets at a DC hot-pressing rapid sintering temperature of 600℃ to 1900℃ are UN.

[0040] (2) Microstructural analysis Figure 8 The images show the SEM morphology of the fracture surfaces of UN fuel pellets prepared in Examples 1 to 6. (a) Example 1: 600℃-400MPa-30min; (b) Example 2: 600℃-500MPa-10min; (c) Example 3: 700℃-500MPa-10min; (d) Example 4: 1600℃-50MPa-10min; (e) Example 5: 1850℃-50MPa-10min; and (f) Example 6: 1900℃-50MPa-30min. The average densities of the UN fuel pellets under different sintering parameters in Examples 1 to 6 were 65.6%, 72.6%, 74.7%, 95.3%, 95.8%, and 98.6%, respectively, with average grain sizes of approximately 760nm, 700nm, 600nm, 10μm, 52μm, and 126μm, respectively. The density and grain size measurements of UN fuel pellets prepared by DC rapid sintering are shown in Table 2. The density of the UN fuel pellets increases synchronously with increasing sintering temperature.

[0041] (3) Mechanical property testing Hardness was measured using a Vickers hardness tester. A diamond indenter was pressed into the sample surface under a load of 9.8 N and held for 10 seconds to obtain the Vickers hardness of the prepared UN fuel pellets. The test was repeated ten times for each sample. The average hardness values ​​of the UN fuel pellets under different sintering parameters in Examples 1-6 were 1.7 GPa, 2.8 GPa, 3.1 GPa, 5.8 GPa, 6.1 GPa, and 7.5 GPa, respectively. The Vickers hardness measurement results of the UN fuel pellets prepared by DC rapid sintering are shown in Table 2. The hardness of the UN pellets increased synchronously with increasing sintering temperature. The increase in hardness was consistent with the increase in density and grain size.

[0042] Table 2. Characterization results of mechanical properties of UN fuel pellets As can be seen from the above results, this invention controls the grain size of UN fuel pellets by changing sintering parameters, achieving the preparation of UN fuel pellets with grain sizes ranging from nanometers to hundreds of micrometers. During the preparation process, no additives are used and no additional elements are introduced, which does not affect the service performance of large-grain fuel pellets and reduces the pressure of spent fuel reprocessing. The preparation method of this invention is simple and provides a simple and feasible method to support the artificial control of the grain size of UN fuel pellets and to explore the influence of different microstructures on the performance of UN fuel pellets. This is conducive to promoting the performance improvement and application research of UN fuel pellets.

[0043] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, it is intended to include any modifications and variations that fall within the scope of the claims and their equivalents.

Claims

1. A method for preparing UN fuel pellets with controllable microstructure, characterized in that, Includes the following steps: UO2 powder and high-purity carbon powder were mixed and ball-milled to prepare a mixed powder. After the mixed powder is pressed into a blank, the blank undergoes a carbothermic reaction under a nitrogen atmosphere. The powder after carbothermic reaction is pressed into a blank, and then decarburized in a nitrogen-hydrogen mixture to obtain UN powder. UN powder is densified and sintered to obtain UN fuel pellets; During the densification sintering process, under vacuum conditions, the grain size of the UN fuel pellets is controlled by adjusting sintering parameters, including: Sintering temperature: 600℃~1900℃, sintering pressure: 50MPa~500MPa, holding time: 10 min ~ 30 min.

2. The method for preparing a microstructure-controllable UN fuel pellet according to claim 1, characterized in that, The grain size of UN fuel pellets varies from nanometers to hundreds of micrometers.

3. The method for preparing a microstructure-controllable UN fuel pellet according to claim 2, characterized in that, The grain size of UN fuel pellets ranges from 600 nm to 126 μm.

4. The method for preparing a microstructure-controllable UN fuel pellet according to claim 1, characterized in that, The UN fuel pellets contain 158 ppm of oxygen and 2540 ppm of carbon.

5. The method for preparing a microstructure-controllable UN fuel pellet according to claim 1, characterized in that, The molar ratio of UO2 powder to high-purity carbon powder is 1:2.2~4.0; the purity of UO2 powder is ≥99.99%, and the purity of UO2 powder is ≥99.99%. 235 U content is 0.2 at%, the rest is 238 U; High-purity carbon powder with a purity of ≥99.9%.

6. The method for preparing a microstructure-controllable UN fuel pellet according to claim 1, characterized in that, When mixing and ball milling, wet ball milling is used, with ethanol as the medium. The ball milling parameters are: rotation speed 250 rpm / min~500 rpm / min, forward and reverse rotation 10 min~15 min, pause 5 min~10 min, total time 20 h~40 h.

7. The method for preparing a microstructure-controllable UN fuel pellet according to claim 1, characterized in that, The temperature for the carbothermic reaction is 1600℃~1800℃; the temperature for decarburization is not lower than 1500℃.

8. A UN fuel pellet with controllable microstructure, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 7.

9. The microstructure-controllable UN fuel pellet according to claim 8, characterized in that, UN fuel pellets have grain sizes ranging from nanometers to hundreds of micrometers, with an oxygen content of 158 ppm and a carbon content of 2540 ppm.

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