CN-121974693-A - Composite uranium dioxide microsphere and preparation method thereof

CN121974693ACN 121974693 ACN121974693 ACN 121974693ACN-121974693-A

Abstract

The invention discloses a composite uranium dioxide microsphere and a preparation method thereof, and belongs to the technical field of nuclear fuel materials. The method of the invention eliminates the use of the traditional doping element precursor, fundamentally avoids the problems of precursor decomposition, oxidation and component control, remarkably simplifies the process flow, effectively prevents the oxidation of the doping agent by combining oxygen-controlled calcination and reduction sintering, and ensures that the doping agent stably exists in the UO 2 microsphere in a high thermal conductivity phase. The prepared composite uranium dioxide microsphere has the excellent characteristics of uniform second phase distribution, strong interface bonding, high thermal conductivity and the like on the premise of keeping high uranium density. The invention provides an innovative solution with simple process and remarkable effect for improving the heat conductivity of the nuclear fuel, and is suitable for an advanced nuclear reactor fuel system.

Inventors

XU RUI
MA JINGTAO
DENG CHANGSHENG
LI ZIQIANG
ZHAO XINGYU
HAO SHAOCHANG
LI JIANJUN
LIU BING
TANG YAPING

Assignees

清华大学

Dates

Publication Date: 20260505
Application Date: 20260202

Claims (10)

1. The preparation method of the composite uranium dioxide microsphere is characterized by comprising the following steps of: (1) Mixing uranyl nitrate solution, solid doping agent and organic gel to form uniform and stable composite sol; (2) Dropwise adding the composite sol into hot silicone oil or ammonia water gel bath through a droplet forming device under the stirring state, enabling the composite sol droplets to undergo sol-gel conversion to form composite wet gel microspheres, aging and washing to obtain the composite gel microspheres; (3) Drying the composite gel microspheres to obtain dry composite gel microspheres; (4) Carrying out heat treatment on the dry composite gel microspheres in an air atmosphere to remove organic matters so as to obtain heat-treated microspheres; (5) And sintering the microspheres after heat treatment in a protective atmosphere to obtain the composite uranium dioxide microspheres.
2. The method of claim 1, wherein the solid dopant is selected from at least one of a metal powder, a carbide ceramic powder, or a carbon material, wherein the metal powder comprises at least one of Mo, W, nb, cr, ti, the carbide ceramic powder comprises at least one of SiC, zrC, WC, and the carbon material comprises at least one of graphene, graphite, nanodiamond.
3. The method of claim 1, wherein the solid dopant has a particle size of 10nm to 1 μm; And/or the addition amount of the solid dopant is 0.5% -3% of the mass of uranium dioxide theoretically generated by uranyl nitrate; And/or, aging time in the step (2) is 1-4 h.
4. The method of claim 1, wherein the organic gel is an external gel, the gel bath is an aqueous ammonia solution with a concentration of not less than 10M, and 2-4 mol/L aqueous ammonia and deionized water are used for combined washing in the step (2), wherein the external gel comprises at least one of urea, polyvinyl alcohol and tetrahydrofurfuryl alcohol.
5. The method of claim 1, wherein the organic gel is an internal gel, the gel bath is hot silicone oil at 70-95 ℃, and the step (2) is performed by combined washing sequentially with trichloroethylene, 2-4 mol/L ammonia water, deionized water and propylene glycol methyl ether, wherein the internal gel comprises at least one of urea and hexamethylenetetramine.
6. The method of claim 1, wherein the drying in step (3) is performed at 60 to 80 ℃ for 12 to 24 hours using far infrared drying.
7. The method of claim 1, wherein the heat treatment in step (4) is performed at a temperature rising rate of 0.25 to 1 ℃ per minute to 300 to 400 ℃ and is performed for 1 to 4 hours, and the heat treatment temperature in step (4) is lower than the oxidation initiation temperature of the solid dopant.
8. The method of claim 1, wherein the sintering in step (5) is performed at 1400-1700 ℃ for 1-6 hours.
9. The method of claim 1, wherein the protective atmosphere is pure argon or a mixture of hydrogen and argon.
10. The composite uranium dioxide microsphere is characterized in that the composite uranium dioxide microsphere is prepared by the method of any one of claims 1-9, solid dopants are uniformly distributed in the composite uranium dioxide microsphere in the form of nano particles, and the particle size of the composite uranium dioxide microsphere is 450-550 mu m.

Description

Composite uranium dioxide microsphere and preparation method thereof Technical Field The invention relates to the technical field of nuclear fuel material preparation, in particular to uranium dioxide microspheres directly compounded by adopting solid dopants and a preparation method thereof. Background Uranium dioxide (UO 2) has become a widely used nuclear fuel material in commercial nuclear power plants at present due to its high melting point, good irradiation stability and excellent chemical compatibility. However, UO 2 has a relatively low thermal conductivity, about 2.8W/mK at room temperature, and further decreases to about 0.9W/mK at 1000 ℃. The characteristic causes the problems that the temperature of a central area is too high, irradiation swelling is aggravated and the like in the operation process of the fuel, and the damage of the cladding and even the melting of the reactor core can be caused under the accident working condition when the irradiation swelling is severe. Therefore, increasing the thermal conductivity of UO 2 is one of the key paths to improve the safety and economy of nuclear fuels. Currently, the methods for doping to enhance the thermal conductivity mainly include a mechanical mixing method and a coprecipitation method. Mechanical mixing methods typically mix metallic element (e.g., mo, W) or carbide ceramic (e.g., siC) powders directly with UO 2 powders, then shape and sinter. For example, the Chinese institute of atomic energy science has prepared UO 2 -W composite pellets by ball milling UO 2 powder with W powder, and the Chinese institute of engineering and physical research has also adopted ball milling UO 2 with Mo powder and then pressed to obtain UO 2 -Mo composite fuel. However, the method is easy to introduce impurities in the ball milling process, so that the doped phases are unevenly distributed, the doped elements are easy to oxidize in high-temperature treatment, the process complexity is high, and the uranium loading density is obviously lost. The coprecipitation method is to introduce a high thermal conductivity second phase through a chemical mode, for example, the graphene oxide and uranyl nitrate solution are coprecipitated in ammonia water, and the composite pellet is obtained after drying and sintering. Although the method improves the dispersion uniformity to a certain extent, the problems of complex phase transition, difficult control of product morphology and the like still exist in the process of relying on precursor decomposition, and particularly for high-melting-point metals (such as W, mo) and carbides (such as SiC and ZrC), the precursor undergoes multi-step reaction in the thermal decomposition process, and the microstructure, components and distribution of the final product are difficult to accurately regulate. For carbon materials (such as graphene and carbon nanotubes), the precursor is prone to structural collapse at high temperature, resulting in reduced thermal conductivity. Therefore, a doping method capable of directly adopting a target reinforcing material as a doping agent is developed, the problems of uncertainty and structure runaway caused by a precursor decomposition process are avoided, and the method has important technical significance and engineering application value. Disclosure of Invention The present invention aims to solve at least one of the technical problems in the related art to some extent. Therefore, the invention provides a composite uranium dioxide microsphere based on a direct doping strategy and a preparation method thereof. The method is characterized in that the traditional doping element precursor is abandoned, a metal simple substance, carbide ceramic or carbon material is directly adopted as a reinforcing phase, the reinforcing phase is loaded in situ in the gel microsphere through a sol-gel process, and the uniform distribution and stable combination of the reinforcing phase in the UO 2 microsphere are realized by combining a heat treatment process with precise control in stages, so that the heat conduction performance of the fuel microsphere is remarkably improved. In a first aspect, the invention provides a preparation method of a composite uranium dioxide microsphere, which comprises the following steps: (1) Mixing uranyl nitrate solution, solid doping agent and organic gel to form uniform and stable composite sol; (2) Dropwise adding the composite sol into hot silicone oil or ammonia water gel bath through a droplet forming device under the stirring state, enabling the composite sol droplets to undergo sol-gel conversion to form composite wet gel microspheres, aging and washing to obtain the composite gel microspheres; (3) Drying the composite gel microspheres to obtain dry composite gel microspheres; (4) Carrying out heat treatment on the dry composite gel microspheres in an air atmosphere to remove organic matters so as to obtain heat-treated microspheres; (5) And sintering the microspheres after hea