A method for preparing UO2 fuel microspheres

Abstract

The application relates to a method for preparing UO2 fuel microspheres, comprising the following steps: preparing a uranium-containing eutectic, preparing microspheres by using a vibration dispersion method or a microfluidic method, denitrogenation, reduction, sintering; the uranium-containing eutectic prepared by using UNH and urea as raw materials is used for preparing UO2 fuel microspheres, replaces the glue preparation process of a conventional sol-gel method, and improves the density and uranium content of a ball preparation mother liquor. The method provided by the application can greatly simplify the ball preparation process flow of UO2 fuel microspheres, reduce the use of process reagents and the generation amount of process waste liquid, so as to provide a UO2 fuel microsphere preparation method which is high in production efficiency, more economical and safe.

Description

Technical Field

[0001] This invention belongs to the field of nuclear fuel preparation technology, specifically relating to a method for preparing UO2 fuel microspheres. Background Technology

[0002] In the field of nuclear fuel preparation, UO2 powder is typically prepared using the following processes: Process 1 involves dissolving nuclear-pure uranium powder (such as U3O8 powder) in nitric acid and then adding ammonia to obtain ammonium diuranate (ADU) precipitate. ADU is then pyrolyzed at high temperature and reduced with hydrogen to obtain nuclear-pure UO2 powder. Process 2 involves obtaining UO2 powder from nuclear-pure UF6, which is further divided into two preparation routes: one is the ADU wet method, which involves hydrolyzing UF6, adding ammonia to obtain ADU precipitate, and then pyrolyzing and reducing it to obtain nuclear-pure UO2 powder; the other is the integrated dry method (IDR method), which uses a converter to react UF6 with water at high temperature to generate UO2F2, which is then further reduced with hydrogen to UO2 powder. The shape and size of UO2 particles in the UO2 powder prepared by the ADU method and the IDR method cannot be controlled or standardized.

[0003] In the preparation of fuel pellets for advanced fourth-generation nuclear power reactors—high-temperature gas-cooled reactors—which offer better power generation efficiency and safety, UO2 microspheres with consistent shape and size are needed as the core of TRISO-coated fuel particles to obtain tristructural isotropic (TRISO) coated fuel particles. Currently, the main process for preparing UO2 microspheres with consistent shape and size is the sol-gel method, including external gelation, internal gelation, and full gelation. The sol-gel method essentially involves solidifying compounds containing highly chemically active components through solution, sol, and gel processes, followed by heat treatment to produce solid oxides or other compounds. The Institute of Nuclear and New Energy Technology at Tsinghua University has conducted systematic and in-depth research on the sol-gel method for preparing UO2 microspheres and developed a complete full gelation process. The full gelation process requires mixing uranyl nitrate aqueous solution with complexing agents / buffers, binders, crosslinking agents, and curing agents according to a specific formula and multiple steps to form a colloid. This involves the combined use of various reagents and is a relatively complex process. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing UO2 fuel microspheres, which can replace the sol-gel process with a eutectic mixture. By combining vibration dispersion or microfluidic methods, UO2 microspheres with a controllable particle size in the micrometer range can be prepared. Furthermore, this method can greatly simplify the process of preparing UO2 fuel microspheres, reduce the use of process reagents, and reduce the amount of process waste liquid generated.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is: a method for preparing UO2 fuel microspheres, the method comprising the following steps:

[0006] S1. Preparation of uranium-containing eutectic: Dry uranyl nitrate hexahydrate solid and urea solid are mixed in a certain proportion and continuously ground under stirring until the mixture of uranyl nitrate hexahydrate and urea becomes a completely clear liquid, thus obtaining uranium-containing eutectic.

[0007] S2. Preparation of microspheres: Uranium-containing droplets are prepared, solidified into microspheres, washed and dried to obtain dried microspheres.

[0008] S3, Denitrification: The dried microspheres are heated to 450-500℃ under an argon atmosphere according to a specified program and held at that temperature for more than 30 minutes to obtain UO3 microspheres;

[0009] S4. Reduction: The UO3 microspheres are heated to 680-900°C in an argon atmosphere according to a set program, and then reduced in a hydrogen atmosphere to obtain UO2 microspheres.

[0010] S5. Sintering: The UO2 microspheres are sintered in a hydrogen atmosphere, and the temperature is raised to 1500℃ according to a certain program and held for 3-4 hours to obtain UO2 microspheres with micron particle size.

[0011] Furthermore, the molar ratio of the solid uranyl nitrate hexahydrate to the solid urea is 1:4 or 1:9.

[0012] Furthermore, in step S2, the dried microspheres are prepared using a vibration dispersion method. The method for preparing microspheres using the vibration dispersion method includes the following specific steps:

[0013] S211. Preparation of uranium-containing droplets: The uranium-containing eutectic is dispersed into uranium-containing droplets through a vibrating nozzle of a specific size at a set flow rate;

[0014] S212, Solidification into microspheres: The uranium-containing droplets are solidified in an ammonia column to obtain solidified microspheres;

[0015] S213. Washing and drying: Take out the solidified microspheres, wash the solidified microspheres with dilute ammonia water, and dry them in an oven at 110°C for more than 1 hour to obtain the dried microspheres.

[0016] Furthermore, the volume ratio of water to concentrated ammonia in the ammonia column is 1:5 to 1:6;

[0017] The concentrated ammonia solution has a mass percentage concentration of 25-28%.

[0018] Furthermore, in step S213, the dilute ammonia solution is 1 mol / L dilute ammonia solution;

[0019] Wash the cured microspheres with dilute ammonia water 3 to 5 times;

[0020] Each time the solidified microspheres are washed with 1 mol / L dilute ammonia, the volume of the 1 mol / L dilute ammonia is 1 to 1.5 times that of the solidified microspheres.

[0021] Furthermore, in step S2, the dried microspheres are prepared using a microfluidic method, which is based on a eutectic-microfluidic molding process equipment.

[0022] The method for preparing the dried microspheres using microfluidics includes the following specific steps:

[0023] S221. Preparation of uranium-containing droplets: The uranium-containing eutectic is placed in a eutectic test tube and the n-octanol solution is placed in a n-octanol solution test tube. The uranium-containing eutectic is used as the dispersed phase and the n-octanol solution is used as the continuous phase. The liquid in the eutectic test tube and the n-octanol solution test tube is driven into the connecting tube by the pressure controller of the microfluidic instrument through the pressure difference. The uranium-containing eutectic enters the continuous phase channel of the microfluidic chip from the eutectic inlet and the n-octanol solution enters the dispersed phase channel of the microfluidic chip from the n-octanol solution inlet. In the microfluidic chip, the uranium-containing eutectic is sheared into continuous uranium-containing droplets by the n-octanol solution, and the uranium-containing eutectic and the n-octanol solution merge at the outlet of the microfluidic chip to obtain a n-octanol solution carrying uranium-containing droplets.

[0024] S222, Curing into microspheres: A curing liquid is added to a curing tube, and the n-octanol solution containing uranium droplets enters the curing liquid in the curing tube and is cured and aged into cured microspheres;

[0025] S223. Washing and drying: The cured microspheres are taken out from the curing tube, washed with anhydrous ethanol, and then dried in an oven to obtain dried microspheres.

[0026] Furthermore, by selecting microfluidic chips of different specifications, the size of the uranium-containing droplets can be adjusted within the range of 10 μm to 700 μm.

[0027] Furthermore, the curing solution is prepared as follows: First, ammonia water and n-octanol solution are thoroughly mixed, and then the aqueous phase in the lower layer is removed by phase separation to obtain an ammonia-saturated n-octanol solution, which is the curing solution.

[0028] Furthermore, step S222 includes the following specific steps:

[0029] After adding the curing liquid into the curing tube, petroleum ether is added to cover the curing liquid, serving as the upper liquid phase in the curing tube.

[0030] The n-octanol solution containing uranium droplets first enters the petroleum ether in the curing tube, and then enters the curing liquid.

[0031] Furthermore, the curing tube is a glass tube with a piston at the bottom;

[0032] In step S223, the solidified microspheres are removed from the solidification tube by: drawing the liquid phase from the upper end of the solidification tube, and then removing the solidified microspheres from the lower end of the solidification tube.

[0033] The beneficial effects of this invention are as follows: The method for preparing UO2 fuel microspheres provided by this invention involves the following steps: preparing a uranium-containing eutectic, preparing microspheres using a vibration dispersion method or a microfluidic method, denitrification, reduction, and sintering; using UNH and urea as raw materials to prepare the uranium-containing eutectic, UO2 fuel microspheres are prepared, replacing the conventional sol-gel method's gel preparation process, thus improving the density and uranium content of the microsphere-forming mother liquor. The method provided by this invention can greatly simplify the UO2 fuel microsphere-forming process, reduce the use of process reagents and the amount of process waste liquid generated, thereby providing a more efficient, economical, and safer method for preparing UO2 fuel microspheres. The specific beneficial effects of this invention are mainly reflected in:

[0034] (1) The uranium-containing eutectic, which is liquid at room temperature, is obtained by directly mixing uranyl nitrate hexahydrate and urea in a certain proportion as the mother liquor for making balls, which replaces the preparation of the adhesive in the sol-gel method (which requires mixing uranyl nitrate aqueous solution with complexing agent / buffer, binder, crosslinking agent and curing agent according to a certain formula and process to prepare a colloid). The process is simple, has fewer steps and is more efficient.

[0035] (2) The uranium-containing eutectic obtained is very stable, the additive urea is low in cost and can be completely removed when heated to above 150°C;

[0036] (3) Uranium-containing eutectic melt, as the mother liquor for pelletizing, has a higher density and a higher uranium content compared to the gel solution of the sol-gel method; as a precursor mother liquor for pelletizing, the density of the gel solution from the full gelation method is approximately 1.5 g / cm³. 3 It contains 1.25 mol / L of uranium, while the density of a 1:4 type eutectic can reach 1.95 g / cm³. 3 The uranium content reaches 2.63 mol / L; the density of the 1:9 type eutectic can reach 1.73 g / cm³. 3 The uranium content reaches 1.66 mol / L;

[0037] (4) Use ammonia water or an ammonia-saturated octanol solution as the curing solution. No heating is required during the curing process.

[0038] (5) The reagents used in the method provided by the present invention, such as urea, octanol and ammonia, are all inexpensive. They eliminate the need for conventional sol-gel process reagents such as polyvinyl alcohol (PVA) and toxic reagents such as tetrahydrofurfuryl alcohol and hexamethylenetetramine. This effectively reduces production costs and provides better economy and safety.

[0039] (6) The technical solution provided by the present invention can prepare UO2 microspheres with a controllable particle size range of 10μm to 700μm and a narrow particle size distribution range, which can be used as a new and economical UO2 microsphere preparation solution. Attached Figure Description

[0040] Figure 1 This is a schematic flowchart of a method for preparing UO2 fuel microspheres according to an embodiment of the present invention.

[0041] Figure 2 This is a process flow diagram of the preparation of UO2 fuel microspheres by the eutectic method according to an embodiment of the present invention.

[0042] Figure 3 This is a process flow diagram of the preparation of UO2 fuel microspheres by the all-gel method according to an embodiment of the present invention.

[0043] Figure 4 This is a schematic diagram of the molding process equipment for eutectic-microfluidic materials as described in an embodiment of the present invention. Detailed Implementation

[0044] The technical solutions in the embodiments of the present invention will be further described clearly and completely below with reference to the accompanying drawings and examples. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0045] Eutectic mixtures (DES) are generally composed of two or three components, often obtained by mixing metal salts with hydrogen bond donors. The components are interconnected by forming hydrogen bonds and have low lattice energy, which results in the melting point of the eutectic mixture being lower than the melting point of any one component alone. Macroscopically, most eutectic mixtures are liquid at room temperature.

[0046] In their 2019 article "Structure and properties of 'Type IV' lanthanide nitrate hydrate: urea deepeutectic solvents" published in ACS Sustainable Chemistry & Engineering, Hammond et al. studied the properties and structure of eutectic mixtures formed by mixing trivalent lanthanide metals such as cerium, protactinium, and neodymium with urea, confirming the hydrogen bonding between water and urea. In their 2021 article "Electrochemical and thermodynamic insights on actinide type (IV) deep eutectic solvent" published in the Journal of Molecular Liquids, Ruma Gupta et al. discovered that eutectic mixtures also form when uranyl nitrate hexahydrate (UNH) is mixed with urea in a certain proportion. By adjusting the ratio of uranyl nitrate hexahydrate to urea (molar ratio of 1:4), its melting point can be reduced to below 0°C. Furthermore, UO2 nanocrystals were prepared by electrochemical reduction of this eutectic mixture.

[0047] During the implementation of this invention, the inventors discovered through research that a eutectic can also be formed when the molar ratio of uranyl nitrate hexahydrate to urea is 1:9. Moreover, the 1:4 type uranium-containing eutectic exhibits crystal precipitation after being left at room temperature for several weeks, while the 1:9 type uranium-containing eutectic remains very stable after being left at room temperature for several months.

[0048] In the process of implementing the inventive concept, the inventors discovered that using a eutectic mixture of uranyl nitrate and urea instead of the sol-gel method for the preparation of uranium-containing microspheres has at least the following advantages: (1) Compared with the process flow of the full gel method for preparing UO2 fuel microspheres (e.g., Figure 3 As shown in the diagram, the process flow diagram for preparing UO2 fuel microspheres using the eutectic method is as follows: Figure 2As shown), the process of pelletizing mother liquor is greatly simplified, improving production efficiency. The full gel method requires mixing uranyl nitrate aqueous solution with complexing agent / buffer urea, binder polyvinyl alcohol (PVA), crosslinking agent tetrahydrofurfuryl alcohol (4-HF), and curing agent hexamethylenetetramine (HMTA) according to a certain formula and process to prepare a colloid; the eutectic method only requires directly mixing UNH and solid urea in a certain proportion. (2) The use of process reagents is reduced. In the full gel method, five process reagents are required: PVA, 4-HF, urea, HMTA, and ammonia. In the eutectic method, only two or three process reagents are required: urea, ammonia, and (octanol). (3) The generation of waste liquid is reduced, improving the efficiency, economy, and safety of the production process. This not only increases the density of the pelletizing precursor mother liquor (the density of the full gel method is about 1.4 to 1.5 g / cm³), but also improves the production efficiency, economy, and safety of the production process. 3 The density of the 1:4 type eutectic reaches 1.95 g / cm³. 3 It can improve the uranium content (the uranium content of the gel in the full gelation method is about 1.25 mol / L, while the uranium content of the eutectic can reach 2.63 mol / L), and can greatly simplify the preparation process of fuel microspheres, reduce the use of process reagents and the generation of waste liquid, improve production efficiency and safety, and reduce production costs.

[0049] Microfluidics is a technique that enables the formation of liquid-encapsulated droplets or bubbles from two immiscible liquids or liquids and gases. Compared to traditional macroscopic techniques, microdroplet technology offers advantages such as high efficiency, high throughput, and low consumption, and is therefore widely used in microchemical, biochemical, and pharmaceutical fields. Microfluidic methods can rapidly and massively prepare droplets of specific sizes, and allow for precise control of the required liquid volume. Researchers at the University of Science and Technology of China, including Yang Yating and Li Xiang, have successfully prepared cerium-containing and uranium-containing microspheres using microfluidics, and conducted a series of studies on the factors affecting microsphere size and formation rate, confirming the feasibility of using microfluidics in the sol-gel method to prepare UO2 fuel microspheres. By employing eutectic mixtures combined with different droplet dispersion techniques, such as vibrational dispersion and microfluidics, it is possible to prepare microspheres with controllable sizes. In the implementation of this invention, by using uranium-containing eutectic materials in conjunction with microfluidic technology, we can prepare UO2 fuel microspheres with a particle size ranging from 10 micrometers to 700 micrometers.

[0050] like Figures 1-4 As shown in the figure, an embodiment of the present invention provides a method for preparing UO2 fuel microspheres, the method comprising the following steps:

[0051] S1. Preparation of uranium-containing eutectic: First, dry uranyl nitrate hexahydrate solid and urea solid are mixed in a mortar in a certain proportion and continuously ground under stirring until the mixture of uranyl nitrate hexahydrate and urea becomes a completely clear liquid, thus obtaining uranium-containing eutectic.

[0052] The molar ratio of the solid uranyl nitrate hexahydrate to the solid urea is 1:4 or 1:9.

[0053] S2. Preparation of microspheres: Uranium-containing droplets are prepared, solidified into microspheres, washed and dried to obtain dried microspheres.

[0054] The dried microspheres are prepared by vibration dispersion or microfluidic method, wherein the microfluidic method is based on a eutectic-microfluidic molding process equipment;

[0055] The method for preparing the dried microspheres using the vibration dispersion method includes the following specific steps:

[0056] S211. Preparation of uranium-containing droplets: The uranium-containing eutectic is dispersed into uranium-containing droplets through a vibrating nozzle of a specific size at a set flow rate;

[0057] S212, Solidification into microspheres: The uranium-containing droplets are solidified in an ammonia column to obtain solidified microspheres;

[0058] The volume ratio of water to concentrated ammonia in the ammonia column is 1:5 to 1:6, and the mass percentage concentration of the concentrated ammonia is 25% to 28%.

[0059] S213. Washing and drying: Take out the solidified microspheres, wash the solidified microspheres 3 to 5 times with 1 mol / L dilute ammonia water to wash away ammonium nitrate and some urea, and dry them in an oven at 110°C for more than 1 hour to obtain dried microspheres.

[0060] Each time the solidified microspheres are washed with 1 mol / L dilute ammonia, the volume of the 1 mol / L dilute ammonia is approximately 1 to 1.5 times that of the solidified microspheres.

[0061] The method for preparing the dried microspheres using microfluidics includes the following specific steps:

[0062] S221. Preparation of Uranium-Containing Droplets: The prepared uranium-containing eutectic is placed in eutectic test tube I, and the n-octanol solution is placed in n-octanol solution test tube II. Using the uranium-containing eutectic as the dispersed phase and the n-octanol solution as the continuous phase, the pressure at the inlet of each test tube (eutectic test tube I and n-octanol solution test tube II) is set by the pressure controller of the microfluidic instrument X, thereby causing the liquids in the two test tubes to enter the connecting tube through the pressure difference. The two liquids enter the microfluidic chip in the connecting tube through different inlets. Specifically, the uranium-containing eutectic enters the microfluidic chip from eutectic inlet ①, and the n-octanol solution enters the microfluidic chip from n-octanol solution inlet ②, and the two converge at the microfluidic chip outlet ③.

[0063] By adjusting the pressure of the two phases (uranium-containing eutectic phase and n-octanol solution phase), the uranium-containing eutectic phase is sheared into continuous uranium-containing droplets by the n-octanol solution within the microfluidic chip. These continuous uranium-containing droplets are then encapsulated in the octanol solution, resulting in a n-octanol solution containing uranium-containing droplets, which flows out from outlet ③. Furthermore, by adjusting the pressure of the two phases, the size of the uranium-containing droplets can be adjusted within a certain range.

[0064] By selecting microfluidic chips of different specifications, the size of the uranium-containing droplets at the microfluidic chip outlet ③ can be adjusted within a range of 10μm to 700μm.

[0065] S222, Curing into microspheres: Curing liquid IV is added to curing tube A. The n-octanol solution containing uranium droplets enters the curing liquid IV in curing tube A and is completely cured and aged into cured microspheres.

[0066] The preparation method of the curing solution IV is as follows: First, ammonia water and n-octanol solution are thoroughly mixed, and then the aqueous phase in the lower layer is removed by phase separation to obtain an ammonia-saturated n-octanol solution, which is the curing solution IV.

[0067] Specifically, the mass percentage concentration of the ammonia water is 25-28%.

[0068] Optionally, after adding curing liquid IV to curing tube A, petroleum ether III is added to cover the curing liquid IV, serving as the upper liquid phase in curing tube A to prevent uranium-containing droplets from adsorbing at the opening of curing tube A.

[0069] Optionally, a small amount of ethanol is added to the ammonia-saturated n-octanol solution obtained in the curing liquid IV and then thoroughly mixed to facilitate the entry of the n-octanol solution carrying uranium-containing droplets from the upper liquid phase (petroleum ether III) into the curing liquid IV.

[0070] In one specific embodiment, the n-octanol solution containing uranium droplets first enters petroleum ether IV in curing tube A, and then enters curing liquid IV (n-octanol solution saturated with ammonia containing a small amount of ethanol) under the action of gravity, and reacts with ammonia to generate ammonium diuranate and solidify. After the formed curing system is left to stand at room temperature for several hours, it is completely cured and aged into solidified microspheres.

[0071] S223. Washing and drying: Take the cured microspheres out of the curing tube A, wash the cured microspheres with anhydrous ethanol, and then dry the washed microspheres in an oven to obtain dried microspheres.

[0072] Specifically, the curing tube A is a glass tube with a piston at the bottom.

[0073] Specifically, the liquid phase in curing tube A is drawn out from the top, and then the cured microspheres are taken out from the bottom of curing tube A. The cured microspheres are washed with anhydrous ethanol 3 to 5 times to remove n-octanol and some of the urea in the cured microspheres. The washed microspheres are then kept in an oven at 60°C for more than 30 minutes to obtain dried microspheres.

[0074] S3. Denitrification: The dried microspheres are heated to 160-200℃ and held for 15 minutes under an argon atmosphere according to a specified program. Then, the temperature is further increased to 400℃ and held for more than 30 minutes to remove urea, ammonium nitrate and other organic residues. The temperature is then further increased to 450-500℃ and held for 30 minutes to obtain UO3 microspheres.

[0075] S4. Reduction: The UO3 microspheres are heated to 500-600°C in an argon atmosphere according to a set program, then switched to a 4% hydrogen-argon (V / V) mixture and held for 30 minutes; then the temperature is further increased to 680-900°C and reduced to a hydrogen atmosphere to obtain UO2 microspheres.

[0076] S5. Sintering: The UO2 microspheres are sintered in a hydrogen atmosphere, and the temperature is raised to 1500℃ according to a certain program and held for 3-4 hours to obtain UO2 microspheres with micron particle size.

[0077] The following examples further illustrate specific embodiments of the present invention.

[0078] Example 1: Preparation of UO2 microspheres using a eutectic process combined with vibrational dispersion (Part 1)

[0079] (1) Preparation of uranium-containing eutectic: First, 10g (0.02mol) of dried uranyl nitrate hexahydrate crystals were ground into powder in a mortar, and 4.78g (0.08mol) of dried urea crystals were ground into powder in a mortar. Then, the two powders were mixed in the mortar, and the mixture was continuously ground while stirring. After about ten minutes, the mixture became a completely clear liquid, thus obtaining the uranium-containing eutectic, and its density was measured to be 1.95g / cm³. 3 The uranium content is 32.1% (g / g) or 2.63 mol / L, and the water content is 14.67% (g / g).

[0080] (2) Preparation of microspheres: In this embodiment, the vibration dispersion method is used to prepare microspheres, including the following specific steps:

[0081] (2-1) Preparation of uranium-containing droplets: The uranium-containing eutectic is dispersed into uranium-containing droplets through a vibrating nozzle with an inner diameter of 0.7 to 1.5 mm at a flow rate of about 10 ml / min;

[0082] (2-2) Solidification into microspheres: The uranium-containing droplets are solidified in an ammonia column to obtain solidified microspheres;

[0083] The ammonia column is prepared by mixing water and concentrated ammonia, wherein the volume ratio of water to concentrated ammonia is 1:5 to 1:6, the mass percentage concentration of the concentrated ammonia is 25 to 28%, and the mass percentage concentration of the prepared ammonia column is 4.1 to 5.6%.

[0084] (2-3) Washing and drying: Take out the solidified microspheres, wash the solidified microspheres 3 to 5 times with 1 mol / L dilute ammonia water to remove ammonium nitrate and some urea, and dry them in an oven at 110°C for more than 1 hour to obtain dried microspheres.

[0085] Each time the solidified microspheres are washed with 1 mol / L dilute ammonia, the volume of the 1 mol / L dilute ammonia is approximately 1 to 1.5 times that of the solidified microspheres.

[0086] (3) Denitrification: The dried microspheres are heated to 160-200℃ at a heating rate of 10℃ / min under an argon atmosphere and held for 15 min. Then the temperature is increased to 400℃ and held for 30 min to remove urea, ammonium nitrate and other organic residues. The temperature is then increased to 450-500℃ and held for 30 min to obtain UO3 microspheres.

[0087] (4) Reduction: The UO3 microspheres are heated to 500-600°C at a heating rate of 10°C / min under an argon atmosphere, then switched to a 4% hydrogen-argon (V / V) mixture, held for 30 min, and then heated to 680-900°C. After that, the UO2 microspheres are reduced to a hydrogen atmosphere for 10-15 min.

[0088] (5) Sintering: The UO2 microspheres are sintered in a hydrogen atmosphere, heated to 1500℃ at a heating rate of 10℃ / min, and held for 3-4 hours to obtain UO2 microspheres with a particle size of 0.5-1mm.

[0089] Example 2: Preparation of UO2 microspheres using a eutectic process combined with vibrational dispersion (Part Two)

[0090] (1) Preparation of uranium-containing eutectic: First, 10g (0.02mol) of dried uranyl nitrate hexahydrate crystals were ground into powder in a mortar, and 10.76g (0.18mol) of dried urea crystals were ground into powder in a mortar. Then, the two powders were mixed in the mortar, and the mixture was continuously ground while stirring. After about ten minutes, the mixture became a completely clear liquid, thus obtaining the uranium-containing eutectic, and its density was measured to be 1.73g / cm³. 3 The uranium content is 22.8% (g / g) or 1.66 mol / L, and the water content is 10.45% (g / g).

[0091] Steps (2) to (5) in Example 2 are the same as in Example 1, and will not be repeated here.

[0092] Example 3: Preparation of UO2 microspheres using a eutectic process combined with microfluidic technology (Part 1)

[0093] (1) Preparation of uranium-containing eutectic: First, 10g (0.02mol) of dried uranyl nitrate hexahydrate crystals were ground into powder in a mortar, and 4.78g (0.08mol) of dried urea crystals were ground into powder in a mortar. Then, the two powders were mixed in the mortar, and the mixture was continuously ground while stirring. After about ten minutes, the mixture became a completely clear liquid, thus obtaining the uranium-containing eutectic, and its density was measured to be 1.95g / cm³. 3 The uranium content is 32.1% (g / g) or 2.63 mol / L, and the water content is 14.67% (g / g).

[0094] (2) Microsphere preparation: In this embodiment, microspheres are prepared using a microfluidic method, which is based on a eutectic-microfluidic molding process and equipment; including the following specific steps:

[0095] (2-1) Preparation of uranium-containing droplets: such as Figure 3As shown, the prepared uranium-containing eutectic was placed in eutectic test tube I, and the n-octanol solution was placed in n-octanol solution test tube II. Using the uranium-containing eutectic as the dispersed phase and the n-octanol solution as the continuous phase, the pressure at the inlet of eutectic test tube I was set to 405 mPa and the pressure at the inlet of n-octanol solution test tube II was set to 605 mPa by the pressure controller of the microfluidic instrument X. The pressure difference drove the two liquids into the microfluidic chip through two different inlets (eutectic inlet ① and n-octanol solution inlet ②). By selecting a 100 μm microfluidic chip (the microfluidic chip size can be selected from 10 μm to 1 mm, with different inlet and outlet pressures corresponding to different sizes), the diameter of the uranium-containing droplet at the microfluidic chip outlet ③ was obtained to be 100 μm. In this process, uranium-containing eutectic enters the continuous phase channel of the microfluidic chip from the eutectic inlet ①, and n-octanol solution enters the dispersed phase channel of the microfluidic chip from the n-octanol solution inlet ②. The two converge at the microfluidic chip outlet ③. The uranium-containing eutectic is sheared into continuous uranium-containing droplets by the n-octanol solution. The continuous uranium-containing droplets are encapsulated in the octanol solution to obtain a n-octanol solution carrying uranium-containing droplets, which flows out from the outlet ③.

[0096] (2-2) Solidification into microspheres: Add curing liquid IV to curing tube A in advance, and then add 20 ml of petroleum ether III to cover the curing liquid IV as the upper liquid phase in curing tube A to prevent uranium-containing droplets from adsorbing at the opening of curing tube A.

[0097] The curing solution is prepared as follows: First, mix 30 ml of ammonia solution with a mass percentage concentration of 25-28% with 600 ml of n-octanol solution by shaking thoroughly, then let it stand to separate the phases and remove the aqueous phase in the lower layer; then take 550 ml of the upper layer of ammonia-saturated n-octanol solution, add 10 ml of anhydrous ethanol to it and mix thoroughly to obtain the curing solution IV.

[0098] Finally, the n-octanol solution containing uranium droplets was added to the curing tube A. The n-octanol solution containing uranium droplets first entered petroleum ether III in the curing tube A, and then entered the n-octanol solution saturated with ammonia under the action of gravity. It reacted with ammonia to generate ammonium diuranate and was cured. After the curing system was left to stand at room temperature for several hours, it was completely cured and aged into cured microspheres.

[0099] Specifically, the curing tube A is a glass tube with a piston at the bottom, and the dimensions of the curing tube A are an outer diameter of 32mm, an inner diameter of 29mm, and a length of 1000mm.

[0100] (2-3) Washing and drying: The liquid phase in the curing tube A is drawn out from the upper part ④ of the curing tube, and then the cured microspheres are taken out from the lower part of the curing tube A. The cured microspheres are washed with anhydrous ethanol 3 to 5 times, and then the washed microspheres are kept in an oven at 60℃ for more than 30 minutes to obtain dried microspheres.

[0101] Each time the cured microspheres are washed with anhydrous ethanol, the volume of the anhydrous ethanol is approximately 1 to 2 times that of the cured microspheres.

[0102] (3) Denitrification: The dried microspheres are heated to 160-200℃ at a heating rate of 10℃ / min under an argon atmosphere and held for 15 min. Then the temperature is increased to 400℃ and held for 30 min to remove urea, ammonium nitrate and other organic residues. Then the temperature is increased to 450-500℃ and held for 30 min to obtain UO3 microspheres.

[0103] (4) Reduction: The UO3 microspheres are heated to 500-600°C at a heating rate of 10°C / min under an argon atmosphere, then switched to a 4% hydrogen-argon (V / V) mixture and held for 30 min; then the temperature is further increased to 680-900°C and reduced to a hydrogen atmosphere for 10-15 min to obtain UO2 microspheres.

[0104] (5) Sintering: The UO2 microspheres are sintered in a hydrogen atmosphere, heated to 1500℃ at a heating rate of 10℃ / min, and held for 3-4 hours to obtain UO2 microspheres with micron particle size.

[0105] Example 4: Preparation of UO2 microspheres using a eutectic process combined with microfluidic technology (Part 2)

[0106] (1) Preparation of uranium-containing eutectic: First, 10g (0.02mol) of dried uranyl nitrate hexahydrate crystals were ground into powder in a mortar, and 10.76g (0.18mol) of dried urea crystals were ground into powder in a mortar. Then, the two powders were mixed in the mortar, and the mixture was continuously ground while stirring. After about ten minutes, the mixture became a completely clear liquid, thus obtaining the uranium-containing eutectic, and its density was measured to be 1.73g / cm³. 3 The uranium content is 22.8% (g / g) or 1.66 mol / L, and the water content is 10.45% (g / g).

[0107] Steps (2) to (5) in Example 4 are the same as in Example 3, and will not be repeated here.

[0108] The methods described in this invention are not limited to the specific embodiments described above. The embodiments are merely illustrative examples of this invention, and this invention can also be implemented in other specific ways or forms without departing from the spirit or essential characteristics of this invention. Therefore, the described embodiments should be considered illustrative rather than limiting in any respect. The scope of this invention should be defined by the appended claims, and any variations equivalent to the intent and scope of the claims should also be included within the scope of this invention.

Claims

1. A method for preparing UO2 fuel microspheres, characterized in that, The method includes the following steps: S1. Preparation of uranium-containing eutectic: Dry solid uranyl nitrate hexahydrate and solid urea are mixed at a molar ratio of 1:9 and continuously ground under stirring until the mixture of uranyl nitrate hexahydrate and urea becomes a completely clear liquid, thus obtaining a uranium-containing eutectic; the 1:9 type uranium-containing eutectic remains stable after being left at room temperature for several months. S2. Preparation of microspheres: Uranium-containing droplets are prepared, solidified into microspheres, washed and dried to obtain dried microspheres. S3, Denitrification: The dried microspheres are heated to 450~500℃ under an argon atmosphere according to a specified program and held at that temperature for more than 30 minutes to obtain UO3 microspheres; In step S3, the dried microspheres are heated to 160-200°C and held for 15 minutes under an argon atmosphere according to a specified program, and then heated to 400°C and held for more than 30 minutes to remove urea, ammonium nitrate and other organic residues; then heated to 450-500°C and held for 30 minutes to obtain UO3 microspheres. S4. Reduction: The UO3 microspheres are heated to 500~600℃ in an argon atmosphere according to a set program, then switched to a 4% hydrogen-argon (V / V) mixed gas and held at that temperature for 30 minutes; then the temperature is further increased to 680~900℃, and then the hydrogen atmosphere is switched to reduce the UO2 microspheres. S5. Sintering: The UO2 microspheres are sintered in a hydrogen atmosphere, and the temperature is raised to 1500℃ according to a certain program and held for 3~4 hours to obtain UO2 microspheres with micron particle size.

2. The method for preparing UO2 fuel microspheres according to claim 1, characterized in that, In step S2, the dried microspheres are prepared using a vibration dispersion method. The method for preparing the dried microspheres using the vibration dispersion method includes the following specific steps: S211. Preparation of uranium-containing droplets: The uranium-containing eutectic is dispersed into uranium-containing droplets through a vibrating nozzle of a specific size at a set flow rate; S212, Solidification into microspheres: The uranium-containing droplets are solidified in an ammonia column to obtain solidified microspheres; S213. Washing and drying: Take out the solidified microspheres, wash the solidified microspheres with dilute ammonia water, and dry them in an oven at 110°C for more than 1 hour to obtain the dried microspheres.

3. The method for preparing UO2 fuel microspheres according to claim 2, characterized in that, The volume ratio of water to concentrated ammonia in the ammonia column is 1:5 to 1:

6. The mass percentage concentration of the concentrated ammonia solution is 25-28%.

4. The method for preparing UO2 fuel microspheres according to claim 2, characterized in that, In step S213, the dilute ammonia solution is 1 mol / L dilute ammonia solution; Wash the solidified microspheres 3-5 times with 1 mol / L dilute ammonia water; Each time the solidified microspheres are washed with 1 mol / L dilute ammonia, the volume of the 1 mol / L dilute ammonia is 1 to 1.5 times that of the solidified microspheres.

5. The method for preparing UO2 fuel microspheres according to claim 1, characterized in that, In step S2, the dried microspheres are prepared using a microfluidic method, which is based on a eutectic-microfluidic molding process equipment. The method for preparing the dried microspheres using microfluidics includes the following specific steps: S221. Preparation of uranium-containing droplets: The uranium-containing eutectic is placed in a eutectic test tube and the n-octanol solution is placed in a n-octanol solution test tube. The uranium-containing eutectic is used as the dispersed phase and the n-octanol solution is used as the continuous phase. The liquid in the eutectic test tube and the n-octanol solution test tube is driven into the connecting tube by the pressure difference through the pressure controller of the microfluidic instrument. The uranium-containing eutectic enters the continuous phase channel of the microfluidic chip from the eutectic inlet, and the n-octanol solution enters the dispersed phase channel of the microfluidic chip from the n-octanol solution inlet. Inside the microfluidic chip, the uranium-containing eutectic is sheared into continuous uranium-containing droplets by the n-octanol solution, and the uranium-containing eutectic and the n-octanol solution merge at the outlet of the microfluidic chip to obtain an n-octanol solution carrying uranium-containing droplets. S222, Curing into microspheres: A curing liquid is added to a curing tube, and the n-octanol solution containing uranium droplets enters the curing liquid in the curing tube and is cured and aged into cured microspheres; S223. Washing and drying: The cured microspheres are taken out from the curing tube, washed with anhydrous ethanol, and then dried in an oven to obtain dried microspheres.

6. The method for preparing UO2 fuel microspheres according to claim 5, characterized in that, By selecting microfluidic chips of different specifications, the size of the uranium-containing droplets can be adjusted from 10 μm to 700 μm.

7. The method for preparing UO2 fuel microspheres according to claim 5, characterized in that, The curing solution is prepared as follows: First, ammonia water and n-octanol solution are thoroughly mixed, and then the aqueous phase in the lower layer is removed by phase separation to obtain an ammonia-saturated n-octanol solution, which is the curing solution.

8. The method for preparing UO2 fuel microspheres according to claim 5, characterized in that, Step S222 includes the following specific steps: After adding the curing liquid into the curing tube, petroleum ether is added to cover the curing liquid, serving as the upper liquid phase in the curing tube. The n-octanol solution containing uranium droplets first enters the petroleum ether in the curing tube, and then enters the curing liquid.

9. The method for preparing UO2 fuel microspheres according to claim 8, characterized in that: The curing tube is a glass tube with a piston at the bottom; In step S223, the solidified microspheres are removed from the solidification tube by: drawing the liquid phase from the upper end of the solidification tube, and then removing the solidified microspheres from the lower end of the solidification tube.

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