Preparation method and application of uranium nitride spherical particles

Abstract

The invention discloses a uranium nitride spherical particle preparation method, which comprises: mixing a carbon source and a solution to prepare gel particles; the carbon source comprises diamond powder; the gel particles are subjected to pretreatment and carbon thermal reduction nitridation treatment, and the uranium nitride spherical particles are obtained. According to the preparation method, the C / U molar ratio can be effectively controlled, and the prepared uranium nitride spherical particles have the advantages of being compact in structure, smooth in surface and high in purity.

Description

Technical Field

[0001] The present invention relates to the technical field of nitride nuclear fuels, and particularly relates to a preparation method and application of uranium nitride spherical particles. Background Art

[0002] Uranium nitride (UN) has characteristics such as high melting point, high uranium density, high thermal conductivity, good compatibility, and easy recovery. In future advanced micro nuclear reactors, it is expected to replace the use of uranium dioxide (UO 2 ). Uranium nitride coated fuel (UN-TRISO) particles not only have high thermal conductivity and good irradiation stability, but also can effectively prevent the release of fission products, and can be applied to advanced microreactors such as gas-cooled microreactors.

[0003] At present, the gel process combined with carbothermal reduction nitridation technology route is usually adopted to prepare uranium nitride spherical particles. Among them, the gel process is a forming process, including the internal gel process and the external gel process. Carbothermal reduction nitridation is a synthesis route. However, in the prior art, the gel solution of the internal gel process is sensitive to temperature, and it is required that the gel solution be prepared and stored in an environment of 0-4°C, which has high requirements for equipment and processes and is not conducive to large-scale production.

[0004] In the carbothermal reduction nitridation route, the stoichiometric ratio of carbon required to prepare UN to UO 3 is mostly 2.5. When using the internal gel process for forming, in order to ensure sufficient carbon source, the actual ratio is usually 2.65. In an inert gas environment, the organic matter (raw material) in the formula will also crack to generate carbon during the heat treatment process, further increasing the uncertainty of the C / U molar ratio in the particles. Existing research has shown that precisely controlling the C / U molar ratio is an important factor for preparing high-purity UN by the carbothermal reduction nitridation method. In addition, in the existing research on preparing uranium nitride spherical particles by the external gel combined with carbothermal reduction nitridation technology, due to the relatively high content of organic matter, the carbon-containing uranium gel particles will crack during heat treatment in an inert gas.

[0005] In order to remove the organic matter as completely as possible, currently it is necessary to increase the heat treatment temperature in air or extend the pretreatment time. However, when the heat treatment temperature in air is too high, it will cause the oxidation of the carbon source in the particles, resulting in incomplete carbothermal reduction reaction and impure product phase. If the amount of carbon source is increased to ensure sufficient carbon source required in the subsequent reaction, it will cause the problem of more carbon impurities in the product. In addition, the density of the uranium nitride spherical particles prepared by the above method also needs to be further improved.

[0006] Therefore, there is an urgent need for a preparation method of uranium nitride spherical particles that can effectively control the C / U molar ratio, so as to further improve the performance such as purity and compactness of uranium nitride spherical particles. Summary of the Invention

[0007] The present invention aims to solve at least one of the technical problems existing in the prior art to a certain extent. To this end, the present invention provides a method for preparing uranium nitride spherical particles. The preparation method of the present invention can effectively control the C / U molar ratio and obtain uranium nitride spherical particles with a dense structure, a smooth surface, and excellent performance.

[0008] Therefore, in the first aspect of the present invention, the present invention proposes a method for preparing uranium nitride spherical particles, including the following steps:

[0009] Mix a carbon source and a solution to prepare gel particles; the carbon source includes diamond powder;

[0010] Pre-treat the gel particles and perform carbothermal reduction nitridation treatment to obtain uranium nitride spherical particles.

[0011] Thus, the preparation method of the present invention can effectively control the C / U molar ratio, and the prepared uranium nitride spherical particles have a dense structure, a relatively high purity, and excellent overall performance.

[0012] In some embodiments, the diamond powder satisfies at least one of the following:

[0013] (1) The average particle size of the diamond powder is 0.1 - 10 μm;

[0014] (2) The specific surface area of the diamond powder is 15 - 30 m 2 / g.

[0015] In some embodiments, the C / U molar ratio of the gel particles is 2.4 - 2.6.

[0016] In some embodiments, the pre-treatment is carried out in an air atmosphere; the pre-treatment satisfies at least one of the following:

[0017] (A) The heating rate of the pre-treatment is 0.5 - 2 °C / min;

[0018] (B) The temperature of the pre-treatment is 360 - 560 °C;

[0019] (C) The time of the pre-treatment is 3 - 7 h.

[0020] In some embodiments, the method for preparing the gel particles includes using an internal gel process or an external gel process, preferably the external gel process; in the preparation method of the external gel process, the solution includes at least one of: uranyl nitrate solution with insufficient acid, urea, ammonium nitrate, tetrahydrofurfuryl alcohol, and polyvinyl alcohol solution.

[0021] In some embodiments, the solution further includes a dispersant; the dispersant satisfies at least one of the following:

[0022] (a) The dispersant includes at least one of ammonium citrate, polyvinylpyrrolidone, polyethyleneimine, ammonium polyacrylate, and tetramethylammonium hydroxide, preferably polyethyleneimine or ammonium polyacrylate;

[0023] (b) The addition amount of the dispersant is 0.07 - 0.09% of the mass of the carbon source.

[0024] In some embodiments, in the step of carbothermal reduction nitridation treatment, it includes:

[0025] Roasting treatment, carbonization treatment, and nitridation treatment to obtain the spherical uranium nitride particles; the carbothermal reduction nitridation treatment satisfies at least one of the following:

[0026] (i) The roasting treatment is carried out in a protective atmosphere; the gas of the protective atmosphere includes argon and / or helium; the roasting treatment includes: with a heating rate of 0.5 °C / min, keeping warm at 200 °C, 400 °C, and 500 °C for 1 h respectively;

[0027] (ii) The carbonization treatment is carried out in a protective atmosphere; the gas of the protective atmosphere includes argon and / or helium; the carbonization treatment includes: heating to 600 °C at a rate of 0.5 °C / min, then heating to 1400 °C at a rate of 5 °C / min, and then heating to 1500 °C at a rate of 0.5 °C / min;

[0028] (iii) The nitridation treatment is carried out in nitrogen; the nitridation treatment includes: keeping warm at 1500 - 2000 °C for 3 - 7 h.

[0029] In some embodiments, after the carbothermal reduction nitridation treatment, it further includes a heat preservation treatment; the heating rate of the heat preservation treatment is 5 °C / min, the temperature is 1750 - 2000 °C, and the heat preservation time is 4 - 6 h.

[0030] In the second aspect of the present invention, the present invention provides a spherical uranium nitride particle, including the spherical uranium nitride particle prepared by the method of the first aspect. Thus, the spherical uranium nitride particle of the present invention has high purity, dense structure, and excellent performance.

[0031] In the third aspect of the present invention, the present invention provides the application of the spherical uranium nitride particle obtained by the preparation method of the first aspect, or the spherical uranium nitride particle of the second aspect, in nuclear fuel or nuclear materials.

[0032] Compared with the prior art, the beneficial technical effects achieved by the present invention are:

[0033] The carbon source of the present invention uses diamond powder to prepare spherical uranium nitride particles, which can obtain spherical uranium nitride particles with a dense structure and good performance at a relatively high pretreatment temperature and a short treatment time, and accurately control the C / U molar ratio during the carbothermal reduction nitridation process to obtain spherical uranium nitride particles with a high purity.

[0034] Additional aspects and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0036] Figure 1 is the dispersion effect diagram of gel particles using different dispersants in the present invention;

[0037] Figure 2 is the microscopic morphology diagram of uranium nitride spherical particles pretreated at different temperatures for 5 h in Comparative Example 1 of the present invention; among them, (a) and (a') are treated at 300 °C, (b) and (b') are treated at 350 °C, (c) and (c') are treated at 400 °C, (d) and (d') are treated at 450 °C, and (e) and (e') are treated at 500 °C;

[0038] Figure 3 is the optical photograph of uranium nitride spherical particles pretreated with different carbon sources in Comparative Example 1 and Example 5 of the present invention; among them, (a) is the uranium nitride spherical particle of Comparative Example 1, and (b) is the uranium nitride spherical particle of Example 5;

[0039] Figure 4 is the optical photograph of uranium nitride spherical particles prepared in Examples 2 to 4 of the present invention; among them, (a) is the uranium nitride spherical particle of Example 2, (b) is the uranium nitride spherical particle of Example 3, and (c) is the uranium nitride spherical particle of Example 4;

[0040] Figure 5 is the XRD pattern of uranium nitride spherical particles prepared in Examples 2 to 4 of the present invention; among them, 1500 °C corresponds to Example 2, 1750 °C corresponds to Example 3, and 2000 °C corresponds to Example 4;

[0041] Figure 6 is the optical photograph of uranium nitride spherical particles prepared with different C / U molar ratios in Comparative Example 2 of the present invention; among them, (a) is C / U molar ratio 2.1, (b) is C / U molar ratio 2.3, (c) is C / U molar ratio 2.5, (d) is C / U molar ratio 2.7, and (e) is C / U molar ratio 2.9;

[0042] Figure 7 is the microstructural photograph of uranium nitride spherical particles prepared in Comparative Example 3 of the present invention;

[0043] Figure 8Microstructure photographs of uranium nitride spherical particles prepared in Examples 2 to 4 of the present invention; among them, (a) and (d) are uranium nitride spherical particles of Example 2, (b) and (e) are uranium nitride spherical particles of Example 3, and (c) and (f) are uranium nitride spherical particles of Example 4. Detailed implementation manners

[0044] The embodiments of the present invention will be described in detail below. The following described embodiments are exemplary and are only used to explain the present invention, and should not be construed as a limitation of the present invention.

[0045] It should be noted that the terms "first" and "second" are only used for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features. Further, in the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more.

[0046] In the ranges disclosed herein, the endpoints and any value are not limited to the exact range or value, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and a single point value, and between single point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0047] In this document, the term "comprising" or "including" is an open expression, that is, it includes the content specified by the present invention, but does not exclude other aspects of the content.

[0048] In the first aspect of the embodiments of the present invention, a method for preparing uranium nitride spherical particles is proposed, including the following steps:

[0049] Mix a carbon source and a solution to prepare gel particles; the carbon source includes diamond powder;

[0050] Pre-treat the gel particles and perform carbothermal reduction nitridation treatment to obtain uranium nitride spherical particles.

[0051] In the present invention, since the thermal stability of diamond powder is usually higher than that of amorphous carbon, the use of diamond powder as a carbon source can effectively increase the pretreatment temperature of the gel particles without causing oxidation reactions. In the pretreatment stage, the carbon-containing, oxygen-containing and other organic matter in the gel particles will undergo oxidative decomposition and release, and be eliminated as thoroughly as possible, effectively preventing the organic matter from generating residual carbon during the subsequent carbon thermal reduction nitridation process, so as to ensure that almost all of the carbon in the carbon thermal reduction nitridation treatment comes from the carbon in the carbon source, thereby better controlling the C / U molar ratio in the uranium nitride spherical particles, preventing the particles from cracking in subsequent treatments and ensuring the purity of the final product. In addition, the inventors have also found that the use of diamond powder as a carbon source to prepare uranium nitride spherical particles can also significantly increase their density, with an apparent density of up to 11.45 g / cm 3 The apparent density of uranium nitride spherical particles prepared with amorphous carbon powder as the carbon source is 9.15 g / cm 3 Therefore, the preparation method of the present invention can effectively control the C / U molar ratio and obtain uranium nitride spherical particles with high purity, dense structure and excellent overall performance.

[0052] In some embodiments of the present invention, the diamond powder satisfies at least one of the following:

[0053] (1) The average particle size of diamond powder is 0.1 to 10 μm;

[0054] (2) The specific surface area of ​​diamond powder is 15 to 30 m 2 / g.

[0055] As an example, the average particle size of the diamond powder is 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0056] As an example, the specific surface area of ​​diamond powder is 15m 2 / g, 17m 2 / g, 19m 2 / g, 21m 2 / g, 23m 2 / g, 25m 2 / g, 27m 2 / g, 29m 2 / g, 30m 2 / g, etc.

[0057] In some embodiments of the present invention, the C / U molar ratio of the gel particles is 2.4 to 2.6, preferably 2.6.

[0058] In the process of preparing uranium nitride spherical particles, accurately controlling the C / U molar ratio is the key to preparing UN with a relatively high purity. Therefore, when the C / U molar ratio of the gel particles of the present invention is 2.4 to 2.6, preferably 2.6, it can not only ensure the full progress of the subsequent carbothermal reduction reaction, but also reduce the carbon impurity content in the uranium nitride spherical particles, thereby obtaining uranium nitride spherical particles with a relatively high purity.

[0059] As an example, the C / U molar ratio is 2.4, 2.42, 2.44, 2.46, 2.48, 2.50, 2.52, 2.54, 2.56, 2.58, 2.6, etc.

[0060] In some embodiments of the present invention, the preparation method of the gel particles includes using an internal gel process or an external gel process, preferably the external gel process; in the preparation method of the external gel process, the solution includes at least one of uranyl nitrate solution with insufficient acid, urea, ammonium nitrate, tetrahydrofurfuryl alcohol, and polyvinyl alcohol solution.

[0061] In some embodiments of the present invention, the pretreatment is carried out in an air atmosphere; the pretreatment satisfies at least one of the following:

[0062] (A) The heating rate of the pretreatment is 0.5 to 2 °C / min, preferably 1 °C / min;

[0063] (B) The temperature of the pretreatment is 360 to 560 °C, preferably 400 to 450 °C;

[0064] (C) The time of the pretreatment is 3 to 7 h, preferably 5 h.

[0065] In the present invention, heat-treating the gel particles in an air atmosphere at 360 to 560 °C for 3 to 7 h can oxidize and decompose the organic substances in the solution and release them, preventing the formation of residual carbon during the subsequent carbothermal reduction nitridation process, which may cause the C / U molar ratio to be difficult to control, and at the same time avoiding the doping of carbon impurities in the uranium nitride spherical particles.

[0066] When the internal gel process is used for the preparation method of the gel particles, a part of the urea in the solution complexes with uranyl ions (UO2 2+ ) and a part also reacts with hexamethylenetetramine (HMTA) to form urea-formaldehyde resin. After the sol gels, there are organic substances such as urea and urea-formaldehyde resin inside the gel particles, and the washing process cannot remove them all. To ensure the integrity of the particles during the heat treatment process, the heating rate must be slow enough under inert gas protection. During the preparation process of the external gel process, the presence of polyvinyl alcohol (PVA) makes the spherical particles prone to cracking during the heat treatment process in an inert gas environment. The decomposition behavior of PVA in different atmospheres affects UO 3·The main factors for the integrity and cracking of C particles. Under an inert atmosphere, a large amount of gas is released during the pyrolysis of PVA, and the residual carbon generated causes channel blockage, and the gas cannot be discharged in time. At this time, UO 3 ·C particles are prone to cracking. Under an air atmosphere, PVA undergoes oxidative decomposition, with less carbon residue, and in-situ decomposition can also generate gas channels. At this time, UO 3 ·C particles are not prone to cracking. Therefore, the technical means of pretreating carbon-containing uranium gel particles under an air atmosphere at a certain temperature can effectively prevent cracking and ensure the integrity of the particles.

[0067] As an example, the heating rate for pretreatment is 0.5 °C / min, 0.8 °C / min, 1.0 °C / min, 1.2 °C / min, 1.5 °C / min, 1.7 °C / min, 2 °C / min, etc.

[0068] As an example, the pretreatment temperature is 360 °C, 380 °C, 400 °C, 420 °C, 440 °C, 460 °C, 480 °C, 500 °C, 520 °C, 540 °C, 560 °C, etc.

[0069] As an example, the pretreatment time is 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, 6 h, 6.5 h, 7 h, etc.

[0070] Thus, the preparation method of the present invention can effectively control the C / U molar ratio and obtain spherical uranium nitride particles with higher purity and better performance.

[0071] In some embodiments of the present invention, the solution further includes a dispersant; the dispersant satisfies at least one of the following:

[0072] (a) The dispersant includes at least one of ammonium citrate, polyvinylpyrrolidone, polyethyleneimine, ammonium polyacrylate, and tetramethylammonium hydroxide, preferably polyethyleneimine or ammonium polyacrylate;

[0073] (b) The addition amount of the dispersant is 0.07-0.09% of the mass of the carbon source.

[0074] Uranyl nitrate used in the sol-gel process for preparing uranium nitride is acidic, and the pH value is usually around 2. After the addition of the carbon source, a stable suspension cannot be formed due to severe agglomeration. The stable mechanisms of powders usually include electrostatic stability mechanism, steric hindrance stability mechanism, electrostatic-steric hindrance stability mechanism, depletion stability mechanism, and half-vacancy stability mechanism. The inventor conducted dispersion experiments with five dispersants, namely ammonium citrate, polyvinylpyrrolidone, polyethyleneimine, ammonium polyacrylate, and tetramethylammonium hydroxide (observing after standing for 0.5 h after making the suspension), in order to improve the dispersibility of the carbon source powder in the acidic solution. Among them, in the acidic solution with pH = 2, the dispersion effects of polyethyleneimine and ammonium polyacrylate are better, as Figure 1 shown.

[0075] In some embodiments of the present invention, after preparing the gel particles, washing, drying, and heat treatment are further included; the heat treatment includes: drying the gel particles at 60 - 80 °C for 12 h.

[0076] In some embodiments of the present invention, in the step of carbothermal reduction nitridation treatment, it includes:

[0077] Roasting treatment, carbonization treatment, and nitridation treatment to obtain the spherical uranium nitride particles; the carbothermal reduction nitridation treatment satisfies at least one of the following:

[0078] (i) The roasting treatment is carried out in a protective atmosphere; the gas of the protective atmosphere includes argon and / or helium; the roasting treatment includes: with a heating rate of 0.5 °C / min, keeping warm at 200 °C, 400 °C, and 500 °C for 1 h respectively;

[0079] (ii) The carbonization treatment is carried out in a protective atmosphere; the gas of the protective atmosphere includes argon and / or helium; the carbonization treatment includes: heating to 600 °C at a rate of 0.5 °C / min, then heating to 1400 °C at a rate of 5 °C / min, and then heating to 1500 °C at a rate of 0.5 °C / min;

[0080] (iii) The nitridation treatment is carried out in nitrogen; the nitridation treatment includes: keeping warm at 1500 - 2000 °C for 3 - 7 h, preferably keeping warm at 1500 °C for 5 h.

[0081] In the present invention, after the carbonization treatment, the nitridation treatment can be carried out at the same temperature and by switching to a nitrogen atmosphere, avoiding the reaction being too fast due to increasing the temperature, thereby preventing the particle composition and structure from being uneven and causing cracking.

[0082] In some embodiments of the present invention, after the nitridation treatment is completed, it is switched to a protective atmosphere and cooled to room temperature at a cooling rate of 10 °C / min to obtain the spherical uranium nitride particles.

[0083] In some embodiments of the present invention, after the carbothermal reduction nitridation treatment, heat preservation treatment is further included; the heating rate of the heat preservation treatment is 5 °C / min, the temperature is 1750 - 2000 °C, and the heat preservation time is 4 - 6 h. The heat preservation treatment is mainly used for densifying the particles. At this time, the role of the nitrogen atmosphere is to prevent the solid solution of carbon elements to ensure the purity of uranium nitride, that is, to reduce the carbon impurity content.

[0084] In the second aspect of the embodiments of the present invention, the present invention provides a spherical uranium nitride particle, including the spherical uranium nitride particle prepared by the method of the first aspect. Thus, the spherical uranium nitride particle of the present invention has high purity, a dense structure, and excellent performance.

[0085] In a third aspect of the embodiments of the present invention, the present invention proposes the use of the spherical uranium nitride particles obtained by the preparation method of the first aspect, or the spherical uranium nitride particles of the second aspect in nuclear fuel or nuclear material.

[0086] It will be understood by those skilled in the art that the features and advantages described above for the spherical uranium nitride particles are also applicable to this application and will not be described in detail here.

[0087] The scheme of the present invention will be explained below in conjunction with the embodiments. It will be appreciated by those skilled in the art that the following embodiments are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Where specific techniques or conditions are not indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used are not indicated by the manufacturer and are all conventional products that can be obtained commercially.

[0088] Example 1: Preparation of carbon-containing uranium gel particles by exogel process

[0089] Urea was added to 2.38 mol / L underacid uranyl nitrate solution (ADUN), the molar ratio of urea to U element was 2.35, and the mixture was fully stirred and put into a water bath to boil the glue, the temperature was set to 85°C, and after heating for 2 hours, ammonium nitrate was added, the molar ratio of ammonium nitrate to U element was 0.432, polyethyleneimine (PEI, the amount of which was 0.09% of the diamond powder) was added as a dispersant, and stirred evenly to form a mixed solution. Then, a 17.8 m 2 / g, diamond powder (C / U molar ratio of 2.6) with an average particle size of 0.22μm is added to the mixed solution in small amounts and continuously, and ultrasonic stirring is performed at the same time. This step takes no less than 20 minutes to ensure that the carbon powder is fully dispersed. The prepared carbon-uranium colloid is added to the dispersion container, and the flow rate is controlled to 8mL / min by a high-precision valve. The colloid is extruded and ejected from the nozzle (inner diameter of 0.60mm), and is broken into equal volume liquid segments under the action of the exciter (excitation frequency of 100Hz), and then dispersed spherical droplets are formed under the action of surface tension. During the dispersion process, a laser holographic monitoring system is used to assist in measuring the size and out-of-roundness of the droplets. The droplets pass through the air zone (height of 10cm) and enter the ammonia zone (height of 10cm). At this time, the substances on the surface of the droplets react with ammonia to form a gel layer. The spherical particles of the surface gel continue to fall, collide with ammonia water, until they are immersed, and continue to react with ammonia water. After fully aging for 12 hours, gel particles are obtained. The fully aged gel particles were washed three times with deionized water and dried in an oven at 60°C for 12 h to obtain carbon-containing uranium gel particles.

[0090] Example 2: Preparation of spherical uranium nitride particles

[0091] The carbon-containing uranium gel particles are obtained according to the preparation method of Example 1, and are subjected to pretreatment and carbothermal reduction nitridation treatment. The steps are as follows:

[0092] Pretreatment: In an air atmosphere, the temperature is raised to 400 °C at a heating rate of 1 °C / min and held for 5 h, and then cooled to room temperature in the furnace and taken out;

[0093] Roasting treatment: In an argon atmosphere, the temperature is raised to 200 °C, 400 °C and 500 °C at a heating rate of 0.5 °C / min and held for 1 h respectively;

[0094] Carbonization treatment: In an argon atmosphere, the temperature is raised to 600 °C at a heating rate of 0.5 °C / min, then raised to 1400 °C at a heating rate of 5 °C / min, and then raised to 1500 °C at a heating rate of 0.5 °C / min;

[0095] Nitridation treatment: In a nitrogen atmosphere, hold at 1500 °C for 5 h; finally, in an argon atmosphere, cool to room temperature at a cooling rate of 10 °C / min to obtain uranium nitride spherical particles, as shown in Figure 4 (a), and the XRD pattern of the uranium nitride spherical particles is as shown in Figure 5 shown.

[0096] Example 3: Preparation of uranium nitride spherical particles

[0097] The carbon-containing uranium gel particles are obtained according to the preparation method of Example 1, and then subjected to pretreatment and carbothermal reduction nitridation treatment. The steps are as follows:

[0098] Pretreatment: In an air atmosphere, the temperature is raised to 400 °C at a heating rate of 1 °C / min and held for 5 h, and then cooled to room temperature in the furnace and taken out;

[0099] Roasting treatment: In an argon atmosphere, the temperature is raised to 200 °C, 400 °C and 500 °C at a heating rate of 0.5 °C / min and held for 1 h respectively;

[0100] Carbonization treatment: In an argon atmosphere, the temperature is raised to 600 °C at a heating rate of 0.5 °C / min, then raised to 1400 °C at a heating rate of 5 °C / min, and then raised to 1500 °C at a heating rate of 0.5 °C / min;

[0101] Nitridation treatment: In a nitrogen atmosphere, hold at 1500 °C for 5 h; then raise the temperature to 1750 °C at a heating rate of 5 °C / min and hold for 5 h; finally, in an argon atmosphere, cool to room temperature at a cooling rate of 10 °C / min to obtain uranium nitride spherical particles, as shown in Figure 4 (b), and the XRD pattern of the uranium nitride spherical particles is as shown in Figure 5 shown.

[0102] Example 4: Preparation of uranium nitride spherical particles

[0103] The carbon-containing uranium gel particles were obtained according to the preparation method of Example 1, and then pretreated and subjected to carbothermal reduction nitridation treatment. The steps are as follows:

[0104] Pretreatment: In an air atmosphere, the temperature was raised to 400 °C at a heating rate of 1 °C / min and held for 5 h, and then cooled to room temperature in the furnace and taken out;

[0105] Calcination treatment: In an argon atmosphere, the temperature was raised to 200 °C, 400 °C, and 500 °C at a heating rate of 0.5 °C / min and held for 1 h respectively;

[0106] Carbonization treatment: In an argon atmosphere, the temperature was raised to 600 °C at a heating rate of 0.5 °C / min, then raised to 1400 °C at a heating rate of 5 °C / min, and then raised to 1500 °C at a heating rate of 0.5 °C / min;

[0107] Nitridation treatment: In a nitrogen atmosphere, it was held at 1500 °C for 5 h; then the temperature was raised to 2000 °C at a heating rate of 5 °C / min and held for 5 h; finally, in an argon atmosphere, it was cooled to room temperature at a cooling rate of 10 °C / min to obtain spherical uranium nitride particles, as shown in Figure 4 (c), and the XRD pattern of the spherical uranium nitride particles is as shown in Figure 5 shown.

[0108] Example 5: Preparation of spherical uranium nitride particles

[0109] The carbon-containing uranium gel particles were obtained according to the preparation method of Example 1, except that the C / U molar ratio in Example 5 was 2.4. Then, pretreatment and carbothermal reduction nitridation treatment were carried out. The steps are as follows:

[0110] Pretreatment: In an air atmosphere, the temperature was raised to 450 °C at a heating rate of 1 °C / min and held for 5 h, and then cooled to room temperature in the furnace and taken out;

[0111] Calcination treatment: In an argon atmosphere, the temperature was raised to 200 °C, 400 °C, and 500 °C at a heating rate of 0.5 °C / min and held for 1 h respectively;

[0112] Carbonization treatment: In an argon atmosphere, the temperature was raised to 600 °C at a heating rate of 0.5 °C / min, then raised to 1400 °C at a heating rate of 5 °C / min, and then raised to 1500 °C at a heating rate of 0.5 °C / min;

[0113] Nitridation treatment: In a nitrogen atmosphere, it was held at 1500 °C for 5 h; then the temperature was raised to 2000 °C at a heating rate of 5 °C / min and held for 5 h; finally, in an argon atmosphere, it was cooled to room temperature to obtain spherical uranium nitride particles.

[0114] Comparative Example 1: Preparation of spherical uranium nitride particles using amorphous carbon powder as the carbon source at different pretreatment temperatures

[0115] Prepare uranium nitride spherical particles according to the method of Example 2, except that the carbon source in Comparative Example 1 is TPX-1408 amorphous carbon powder, the C / U molar ratio is 2.5, and the pretreatment temperatures in air atmosphere are 300 °C, 350 °C, 400 °C, 450 °C, and 500 °C respectively. The internal pictures of the uranium nitride spherical particles obtained by pretreatment at different temperatures for 5 h are as Figure 2 shown.

[0116] As Figure 2 can be seen, from the overall morphology of the particles, the carbon source is evenly distributed inside the particles, but obvious agglomeration occurs (size about 10 - 20 μm). At 300 °C ( Figure 2 a, Figure 2 a’), 350 °C ( Figure 2 b, Figure 2 b’), the internal carbon particles are darker in color after treatment. At 400 °C ( Figure 2 c, Figure 2 c’), the internal carbon particles are in an intermediate state, showing grayish-white, as Figure 2 indicated by the arrow in c’. There is a gap between the carbon particles and the matrix. At 450 °C ( Figure 2 d, Figure 2 d’), there is an obvious separation between the internal carbon particles and the matrix (as Figure 2 indicated by the arrow in d’), the color is the same as that of the matrix, and holes (about 20 μm) appear. At 500 °C ( Figure 2 e, Figure 2 e’), the internal carbon particles are severely consumed (as Figure 2 indicated by the arrow in e’), and the hole size is larger than that at 400 °C. This shows that when using amorphous carbon powder as the carbon source, the air pretreatment temperature cannot exceed 350 °C.

[0117] Figure 3 Optical photo comparison of uranium nitride spherical particles pretreated at 350 °C for 5 h in air atmosphere with TPX-1408 amorphous carbon powder as the carbon source in Comparative Example 1 ( Figure 3 a) and uranium nitride spherical particles pretreated at 450 °C for 5 h with diamond powder as the carbon source in Example 5 ( Figure 3 b). As Figure 3 can be seen, the surface of the particles in (a) is rough, usually due to the non-dense structure; while the surface of the particles in (b) is smooth, complete, and without cracking.

[0118] Thus, it shows that the uranium nitride spherical particles prepared by the present invention with diamond powder as the carbon source have a dense, complete, high-purity, and excellent performance structure.

[0119] Comparative Example 2: Prepare uranium nitride spherical particles with amorphous carbon powder as the carbon source at different C / U molar ratios

[0120] Prepare uranium nitride spherical particles according to the method of Example 2, except that the carbon source in Comparative Example 2 is TPX-1408 amorphous carbon powder, and the C / U molar ratios are 2.1, 2.3, 2.5, 2.7, and 2.9 respectively. The optical photos of the uranium nitride spherical particles prepared under different C / U molar ratios are as Figure 6 shown, where Figure 6 a to Figure 6 e have C / U molar ratios of 2.1, 2.3, 2.5, 2.7, and 2.9 respectively.

[0121] It can be seen from Figure 6 that the surfaces of the uranium nitride spherical particles obtained with amorphous carbon powder as the carbon source at different C / U molar ratios are relatively rough.

[0122] It can be seen from the comparison between Figure 4 and Figure 6 that the uranium nitride spherical particles prepared in Examples 2 to 4 using diamond powder as the carbon source by the external gelation combined carbothermal reduction nitridation method have uniform particle sizes and smooth surfaces.

[0123] Comparative Example 3: Prepare uranium nitride spherical particles using amorphous carbon powder as the carbon source

[0124] Prepare uranium nitride spherical particles according to the methods of Examples 2 to 4 respectively, except that the diamond powder in Examples 2 to 4 is replaced with TPX-1408 amorphous carbon powder. The microstructural photos of the uranium nitride spherical particles prepared in Comparative Example 3 are as Figure 7 shown.

[0125] Figure 8 are the microstructural photos of the uranium nitride spherical particles prepared in Examples 2 to 4, where (a) and (d) are the microstructures of the uranium nitride spherical particles in Example 2, (b) and (e) are the microstructures of the uranium nitride spherical particles in Example 3, and (c) and (f) are the microstructures of the uranium nitride spherical particles in Example 4.

[0126] It can be seen from the comparison between Figure 7 and Figure 8 that in Comparative Example 3, since amorphous carbon powder is used as the carbon source, there are uneven voids in the internal structure of the uranium nitride spherical particles in Figure 7 ; while the uranium nitride spherical particles in Figure 8 have a more dense and uniform microstructure. Thus, it shows that using diamond powder as the carbon source can obtain uranium nitride spherical particles with better quality and higher density at a higher pretreatment temperature.

[0127] In the description of this specification, the description referring to terms such as "one embodiment", "some embodiments", "examples", "specific examples", or "some examples", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0128] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing spherical uranium nitride particles, characterized in that: The following steps are involved: Mixing a carbon source and a solution to prepare gel particles; the carbon source includes diamond powder; The gel particles are pretreated and carbon thermal reduction nitridation treated to obtain uranium nitride spherical particles.

2. The method according to claim 1, characterized in that The diamond powder satisfies at least one of the following: (1) The average particle size of the diamond powder is 0.1 to 10 μm; (2) The specific surface area of ​​the diamond powder is 15 to 30 m 2 / g.

3. The method according to claim 1 or 2, characterized in that: The C / U molar ratio of the gel particles is 2.4 to 2.

6.

4. The method according to any one of claims 1 to 3, characterized in that: The pretreatment is carried out in an air atmosphere; the pretreatment satisfies at least one of the following: (A) The heating rate of the pretreatment is 0.5 to 2°C / min; (B) the pretreatment temperature is 360 to 560° C.; (C) The pretreatment time is 3 to 7 hours.

5. The method according to any one of claims 1 to 4, characterized in that: The preparation method of the gel particles includes adopting an inner gel process or an outer gel process, preferably an outer gel process; in the preparation method of the outer gel process, the solution includes: at least one of an under-acidified uranyl nitrate solution, urea, ammonium nitrate, tetrahydrofurfuryl alcohol, and a polyvinyl alcohol solution.

6. The method according to any one of claims 1 to 5, characterized in that: The solution further comprises a dispersant; the dispersant satisfies at least one of the following: (a) the dispersant comprises at least one of ammonium citrate, polypropylene pyrrolidone, polyethylene imine, ammonium polyacrylate and tetramethylammonium hydroxide, preferably polyethylene imine or ammonium polyacrylate; (b) The amount of the dispersant added is 0.07 to 0.09% of the mass of the carbon source.

7. The method according to any one of claims 1 to 6, characterized in that: The steps of the carbon thermal reduction nitriding treatment include: The uranium nitride spherical particles are obtained by calcination, carbonization and nitridation. The carbon thermal reduction nitridation treatment satisfies at least one of the following conditions: (i) the calcination treatment is carried out in a protective atmosphere; the gas of the protective atmosphere includes argon and / or helium; the calcination treatment includes: heating at a rate of 0.5°C / min, and keeping at 200°C, 400°C and 500°C for 1 hour respectively; (ii) the carbonization treatment is carried out in a protective atmosphere; the gas of the protective atmosphere comprises argon and / or helium; the carbonization treatment comprises: heating to 600°C at 0.5°C / min, then heating to 1400°C at 5°C / min, and then heating to 1500°C at 0.5°C / min; (iii) The nitriding treatment is carried out in nitrogen; the nitriding treatment comprises: keeping the temperature at 1500-2000° C. for 3-7 hours.

8. The method according to any one of claims 1 to 7, characterized in that: After the carbon thermal reduction nitriding treatment, a heat preservation treatment is also included; the heating rate of the heat preservation treatment is 5°C / min, the temperature is 1750-2000°C, and the heat preservation time is 4-6h.

9. A spherical uranium nitride particle, characterized in that: Spherical uranium nitride particles prepared by the method according to any one of claims 1 to 8.

10. Use of the spherical uranium nitride particles obtained by the preparation method according to any one of claims 1 to 8, or the spherical uranium nitride particles according to claim 9 in nuclear fuel or nuclear material.

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