Composite fuel and preparation method thereof

Abstract

The invention discloses a composite fuel and a preparation method thereof, the composite fuel obtained by using the preparation method has a specific organization structure microcosmically, i.e., a U3Si2 phase forms a continuous network structure, and the continuous network structure can coat a UN phase with an island-shaped structure characteristic. Therefore, the composite fuel has excellent comprehensive performance such as good water corrosion resistance and high heat conductivity.

Description

Technical Field

[0001] The present invention relates to the technical field of nuclear fuel, and particularly to a composite fuel and a preparation method thereof. Background Art

[0002] At present, the research and development of accident-tolerant fuels mainly include three aspects: improvement of cladding materials, research and development of advanced fuel pellets, and research and development of advanced claddings. Among them, the focus of research and development of advanced fuel pellets is to develop new fuels with better thermal conductivity and fission product containment capabilities than UO2 fuels.

[0003] UO2 fuels are used in commercial nuclear reactor fuels due to their performance advantages such as high melting point, corrosion resistance, low high-temperature creep, and good irradiation stability. However, UO2 fuel pellets still have deficiencies under accident conditions. Its main disadvantage is that the UO2 fuel has poor thermal conductivity, which is not conducive to the effective removal of the residual heat of the reactor under accident conditions. In view of the above disadvantages of the currently used UO2 fuels, the next-generation advanced fuel pellets need to focus on improving the thermal conductivity of the fuel and reducing the temperature of the fuel during operation, so as to improve the inherent safety of the fuel.

[0004] UN fuels and U3Si2 fuels have characteristics such as good thermal conductivity and high uranium density, and are one of the important candidate fuels for the next-generation accident-tolerant fuels. However, research shows that UN fuels are prone to react with water, which limits their application in pressurized water reactors. In order to improve the water corrosion resistance of UN fuels, some research scholars utilized the good water corrosion resistance of U3Si2 fuel pellets and developed UN / U3Si2 composite fuel pellets by compounding UN fuels with U3Si2 fuels. However, simply compounding UN fuels with U3Si2 fuels can improve the water corrosion resistance of UN / U3Si2 composite fuels, but the effect is not significant. Summary of the Invention

[0005] The embodiments of the present application provide a composite fuel and a preparation method thereof. The composite fuel obtained by using this preparation method has a specific organizational structure microscopically, that is, the U3Si2 phase forms a continuous network structure, and the continuous network structure can coat the UN phase with the characteristics of an island structure, so that the composite fuel has excellent comprehensive properties such as good water corrosion resistance and high thermal conductivity.

[0006] In a first aspect, the present application provides a preparation method of a composite fuel, including:

[0007] Perform two-step sintering on the green body of the UN / U3Si2 composite fuel pellets to obtain the composite fuel; wherein, the two-step sintering includes a first sintering and a second sintering, the sintering time of the first sintering is greater than that of the second sintering, and the sintering temperature of the first sintering is less than that of the second sintering; the composite fuel includes a U3Si2 phase and a UN phase, wherein the U3Si2 phase forms a continuous network structure, and the continuous network structure can coat the UN phase with an island-like structure feature.

[0008] In some embodiments, the method for preparing the green body of the UN / U3Si2 composite fuel pellets includes:

[0009] Dissolve the binder in absolute ethanol to obtain a binder solution;

[0010] Mix the U3Si2 powder and the binder solution evenly to obtain a slurry of the U3Si2 powder containing the binder;

[0011] Add the slurry of the U3Si2 powder containing the binder to the UN powder to obtain a material and mix evenly; wherein, the particle size of the U3Si2 powder is smaller than that of the UN powder;

[0012] Dry the absolute ethanol in the material to obtain a UN, U3Si2 mixed fuel powder;

[0013] Press and form the UN, U3Si2 mixed fuel powder to obtain the green body of the UN / U3Si2 composite fuel pellets.

[0014] In some embodiments, the binder is polyethylene glycol, liquid paraffin or polyvinyl butyral.

[0015] In some embodiments, the step of adding the slurry of the U3Si2 powder containing the binder to the UN powder to obtain a material and mix evenly includes:

[0016] Add the slurry of the U3Si2 powder containing the binder to the UN powder in multiple times to obtain a material, and stir evenly each time the slurry is added.

[0017] In some embodiments, the sintering temperature of the first sintering is 1400°C - 1650°C; the sintering temperature of the second sintering is not lower than 1665°C.

[0018] In some embodiments, the sintering time of the first sintering is 1h - 8h.

[0019] In some embodiments, the sintering time of the second sintering is less than or equal to 1h.

[0020] In some embodiments, the particle size of the U3Si2 powder is less than 5 μm.

[0021] In some embodiments, the particle size of the UN powder is 1 μm - 30 μm.

[0022] In a second aspect, the present application provides a composite fuel, comprising a U3Si2 phase and a UN phase, wherein the U3Si2 phase forms a continuous network structure, and the continuous network structure can coat the UN phase having an island-like structural feature.

[0023] A composite fuel and a preparation method thereof provided by the present application. The composite fuel obtained by using this preparation method has a specific microstructure microscopically, that is, the U3Si2 phase forms a continuous network structure, and the continuous network structure can coat the UN phase having an island-like structural feature, so that the composite fuel has excellent comprehensive properties such as good water corrosion resistance and high thermal conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Exemplarily shows a morphology diagram of a composite fuel provided according to some embodiments. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] In order to better understand the above technical solutions, the technical solutions of the present application will be described in detail below through specific embodiments.

[0026] Since the UN fuel pellets have poor water corrosion resistance and are prone to react with water, while the U3Si2 fuel pellets have better water corrosion resistance. Therefore, in order to improve the water corrosion resistance of the UN fuel pellets, a composite fuel is reasonably designed in the embodiments of the present application. The composite fuel has a specific microstructure, that is: the U3Si2 phase forms a continuous network structure, and the U3Si2 phase can coat the UN phase having an island-like structural feature. Through this microstructure design, the excellent water corrosion resistance characteristic of U3Si2 can be fully utilized to provide a protective effect for the UN phase in the UN / U3Si2 composite fuel pellets, thereby effectively improving the water corrosion resistance of the UN fuel pellets. In order to make the composite fuel have the above specific microstructure, the embodiments of the present application propose a preparation method of the composite fuel. Through this preparation method, the composite fuel can have the above specific microstructure, and thus has excellent comprehensive properties. Figure 1 Exemplarily shows a morphology diagram of a composite fuel provided according to some embodiments.

[0027] An embodiment of the present application provides a method for preparing a composite fuel, which includes performing two-step sintering on a green body of a UN / U3Si2 composite fuel pellet to obtain the composite fuel; wherein, the two-step sintering includes a first sintering and a second sintering, the sintering time of the first sintering is greater than that of the second sintering, and the sintering temperature of the first sintering is less than that of the second sintering; the composite fuel includes a U3Si2 phase and a UN phase, wherein the U3Si2 phase forms a continuous network structure, and the continuous network structure can coat the UN phase with an island-like structure feature.

[0028] Specifically, the step of performing two-step sintering on the green body of the UN / U3Si2 composite fuel pellet to obtain the composite fuel includes performing the first sintering and the second sintering on the green body of the UN / U3Si2 composite fuel pellet successively. In the embodiment of the present application, first, the green body of the UN / U3Si2 composite fuel pellet is subjected to the first sintering to increase its density to 80% T.D. - 95% T.D., and then the second sintering is performed, that is, sintering is performed above the melting point of U3Si2 to melt the U3Si2 phase in the pellet and coat the UN phase, thereby obtaining a network structure. The composite fuel finally obtained in the embodiment of the present application includes a U3Si2 phase and a UN phase, wherein the U3Si2 phase forms a continuous network structure, and the continuous network structure can coat the UN phase with an island-like structure feature. The U3Si2 phase used to form the continuous network structure can be referred to as the network phase.

[0029] In some embodiments, the method for preparing the green body of the UN / U3Si2 composite fuel pellet includes: dissolving a binder in absolute ethanol to obtain a binder solution; mixing the U3Si2 powder and the binder solution evenly to obtain a slurry of the U3Si2 powder containing the binder; adding the slurry to the UN powder to obtain a mixed fuel powder and mixing evenly; wherein the particle size of the U3Si2 powder is smaller than that of the UN powder; drying the absolute ethanol in the material to obtain a UN and U3Si2 mixed fuel powder; pressing and forming the UN and U3Si2 mixed fuel powder to obtain a green body of the UN / U3Si2 composite fuel pellet.

[0030] In this embodiment, since UN reacts easily with water, when selecting a binder, a binder that can be dissolved in an organic solvent can be selected. The binder can be polyethylene glycol, and of course, liquid paraffin, polyvinyl butyral (PVB), etc. can also be selected.

[0031] In some embodiments, the step of adding the slurry of the U3Si2 powder containing the binder to the UN powder to obtain a material and mixing evenly includes: adding the slurry of the U3Si2 powder containing the binder to the UN powder in multiple times to obtain a material, and stirring evenly each time the slurry is added.

[0032] In this embodiment, the slurry is stirred evenly each time it is added, that is, during the mixing process of UN and U3Si2 fuels, the U3Si2 powder can partially or completely coat the UN powder.

[0033] In some embodiments, the particle size of the U3Si2 powder is less than 5 μm. Specifically, it can be ball-milled by a planetary ball mill, and the U3Si2 powder is sieved using a sieve to obtain U3Si2 powder with a particle size less than 5 μm.

[0034] In some embodiments, the particle size of the UN powder is 1 μm - 30 μm. Specifically, it can be ball-milled by a planetary ball mill, and the UN powder is sieved using a sieve to obtain UN powder with a particle size of 1 μm - 30 μm. In this embodiment, the particle sizes of the U3Si2 fuel powder and the UN fuel powder need to be graded. Generally, the particle size of the U3Si2 fuel powder needs to be smaller, and the particle size of the UN fuel powder needs to be larger. Then, during the preparation process, the U3Si2 fuel powder needs to be fully mixed with the binder first to modify the surface of the U3Si2 fuel (to make it have a certain viscosity), and then the U3Si2 fuel powder containing the binder is mixed with the UN fuel, so that the U3Si2 fuel powder can wrap the UN fuel powder to achieve an effect similar to coating.

[0035] In some embodiments, the sintering temperature of the first sintering is 1400°C - 1650°C; the sintering temperature of the second sintering is not lower than 1665°C. In this embodiment, the green body of the UN / U3Si2 composite fuel pellet is first sintered at a low temperature (1400°C - 1650°C) to increase the pellet density to 80% T.D. - 95% T.D. At this stage, the sintering temperature is lower than the melting point of U3Si2 (1665°C). Therefore, microscopically, the U3Si2 phase and the UN phase are only simply combined. Then, sintering is carried out above the melting point of U3Si2. Among them, the sintering temperature is above 1665°C and the sintering time is 0 h - 1 h. The U3Si2 in the pellet is melted, and the melted U3Si2 phase can coat the UN phase under the action of surface tension, thereby obtaining a network structure.

[0036] In some embodiments, the sintering time of the first sintering is 1 h - 8 h, and the sintering time of the second sintering is less than or equal to 1 h. In this embodiment, during the second sintering, the sintering temperature is higher than the melting point of the U3Si2 fuel phase. At this time, if the sintering time is too long, it is not easy to control the external dimensions of the pellet. Therefore, a shorter time, that is, less than or equal to 1 h, needs to be selected.

[0037] In some embodiments, during the preparation of the composite fuel, the particle size of the U3Si2 powder is less than 5 μm, the particle size of the UN powder is 1 μm to 30 μm, and during the mixing process of the UN and U3Si2 fuels, the U3Si2 fuel can partially or completely coat the UN powder. During the sintering process of the composite fuel, it is sintered in two steps. First, the green body of the UN / U3Si2 composite fuel pellet is sintered at 1400 °C - 1650 °C for a long time (1 h - 8 h) to obtain a fuel pellet with a density of 80% T.D. - 95% T.D., and then it is sintered at a temperature above 1665 °C for a short time (less than or equal to 1 h) to finally obtain the desired composite fuel.

[0038] Example 1

[0039] Using a planetary ball mill, the UN powder and the U3Si2 powder were respectively ball-milled. A sieve was used to screen the U3Si2 powder to obtain U3Si2 powder with a particle size less than 2 μm. Polyethylene glycol was dissolved in absolute ethanol at a ratio of 2 wt.%, and the U3Si2 powder and the polyethylene glycol solution were weighed at a mass ratio of 1 wt.%, and the U3Si2 powder and the polyethylene glycol solution were fully mixed evenly. The UN powder was weighed at a mass ratio of m(UN):m(U3Si2 powder) of 7:3, where the average particle size of the UN powder was 15 μm. The slurry containing the U3Si2 powder was added to the UN powder in 3 portions. Each time it was added, it was fully stirred evenly, and the absolute ethanol in the material was dried at 50 °C. Finally, the UN and U3Si2 mixed fuel powder was obtained.

[0040] Using a molding press, the above-mentioned mixed fuel powder was pressed into shape to obtain a green body of the UN / U3Si2 composite fuel pellet. Using a vacuum sintering furnace, first, the green body of the UN / U3Si2 composite fuel pellet was sintered at 1600 °C for 4 h to obtain an initial composite fuel pellet with a density of about 90% T.D., and then the initial composite fuel pellet was sintered at 1675 °C for 0.5 h to finally obtain a composite fuel with a density of about 95% T.D., where the microstructure morphology of the composite fuel is as Figure 1 shown. It can be seen from Figure 1 that the composite fuel has a specific microstructure, that is, the U3Si2 phase inside the composite fuel forms a continuous network structure, and the U3Si2 phase can coat the UN phase with an island-like structure characteristic.

[0041] Example 2

[0042] Using a planetary ball mill, the U3Si2 powder was first ball-milled. A sieve was used to screen the U3Si2 powder to obtain U3Si2 powder with a particle size less than 2 μm. Polyethylene glycol was dissolved in absolute ethanol at a ratio of 2 wt.%. The U3Si2 powder and the polyethylene glycol solution were weighed according to a mass ratio of 1 wt.%, and the U3Si2 powder and the polyethylene glycol solution were thoroughly mixed evenly. UN powder was weighed according to a mass ratio of m(UN):m(U3Si2 powder) of 8:2. Among them, the average particle size of the UN powder was 20 μm. The slurry containing the U3Si2 powder was added to the UN powder in two portions. Each time it was added, it was stirred evenly, and the absolute ethanol in the material was dried at 70 °C. Finally, a UN and U3Si2 mixed fuel powder was obtained.

[0043] Using a molding press, the above UN and U3Si2 mixed fuel powder was pressed into a shape to obtain a green body of a UN / U3Si2 composite fuel pellet. Using a vacuum sintering furnace, the green body of the UN / U3Si2 composite fuel pellet was first sintered at 1550 °C for 4 h to obtain an initial composite fuel pellet with a density of about 87% T.D., and then the initial composite fuel pellet was sintered at 1700 °C for 0.3 h to finally obtain a composite fuel with a density of about 96% T.D. The prepared composite fuel also has a specific microstructure, that is, the U3Si2 phase inside the pellet forms a continuous network structure, and the U3Si2 phase can coat the UN phase with an island-like structure feature.

[0044] In the embodiment of the present application, a composite fuel is also provided, including a U3Si2 phase and a UN phase, wherein the U3Si2 phase forms a continuous network structure, and the continuous network structure can coat the UN phase with an island-like structure feature.

[0045] In summary, a composite fuel and a preparation method thereof provided by the present invention. The composite fuel obtained by using this preparation method has a specific organizational structure microscopically, that is, the U3Si2 phase forms a continuous network structure, and the continuous network structure can coat the UN phase with an island-like structure feature, so that the composite fuel has excellent comprehensive properties such as good water corrosion resistance and high thermal conductivity.

[0046] It is readily understood by those skilled in the art that, on the premise of no conflict, the above-mentioned advantageous means can be freely combined and superimposed. The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application. The above is only the preferred implementation manner of the present application. It should be noted that for those of ordinary skill in the art in this technical field, several improvements and variations can be made without departing from the technical principle of the present application, and these improvements and variations should also be regarded as within the protection scope of the present application.

Claims

1. A method for preparing a composite fuel, characterized in that: include: A two-step sintering is performed on a UN / U3Si2 composite fuel pellet green body to obtain a composite fuel; wherein the two-step sintering includes a first sintering and a second sintering, the sintering time of the first sintering is greater than the sintering time of the second sintering, and the sintering temperature of the first sintering is less than the sintering temperature of the second sintering; the composite fuel includes a U3Si2 phase and a UN phase, wherein the U3Si2 phase forms a continuous network structure, and the continuous network structure can cover the UN phase with an island-like structural feature.

2. The method according to claim 1, characterized in that The method for preparing the UN / U3Si2 composite fuel pellet green body comprises: dissolving the adhesive in anhydrous ethanol to obtain an adhesive solution; The U3Si2 powder and the adhesive solution are mixed evenly to obtain a slurry of the U3Si2 powder containing the adhesive; Add the slurry of the U3Si2 powder containing the adhesive to the UN powder to obtain a material, and mix them evenly; wherein the particle size of the U3Si2 powder is smaller than that of the UN powder; Drying the anhydrous ethanol in the material to obtain UN and U3Si2 mixed fuel powder; The UN and U3Si2 mixed fuel powders are pressed and molded to obtain UN / U3Si2 composite fuel pellet green bodies.

3. The method according to claim 2, characterized in that The adhesive is polyethylene glycol, liquid paraffin or polyvinyl butyral.

4. The method according to claim 2, characterized in that: The step of adding the slurry of the U3Si2 powder containing the adhesive into the UN powder to obtain the material and mixing them uniformly comprises: The slurry of U3Si2 powder containing the binder is added to UN powder in multiple times to obtain the material, wherein the slurry is stirred evenly each time it is added.

5. The method according to claim 1, characterized in that The sintering temperature of the first sintering is 1400°C-1650°C; the sintering temperature of the second sintering is not less than 1665°C.

6. The method according to claim 1, characterized in that The sintering time of the first sintering is 1h-8h.

7. The method according to claim 6, characterized in that The sintering time of the second sintering is less than or equal to 1 hour.

8. The method according to claim 2, characterized in that: The particle size of the U3Si2 powder is less than 5 μm.

9. The method according to claim 2, characterized in that: The particle size of the UN powder is 1 μm-30 μm.

10. A composite fuel, characterized in that: It includes U3Si2 phase and UN phase, wherein the U3Si2 phase forms a continuous network structure, and the continuous network structure can cover the UN phase with island-like structural characteristics.

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