Composite uranium nitride pellet manufacturing method and composite uranium nitride pellet

Abstract

The invention discloses a composite uranium nitride pellet manufacturing method and a composite uranium nitride pellet, and belongs to the field of nuclear power. The manufacturing method of the composite uranium nitride pellet comprises the following steps: sintering uranium nitride powder serving as a raw material to prepare the uranium nitride pellet; preparing graphite slurry by taking graphite powder, a solvent, an adhesive and a dispersing agent as raw materials; and coating the graphite slurry on the surface of the uranium nitride pellet, drying and roasting to obtain a finished product of the composite uranium nitride pellet with the graphite coating. According to the method, a firm graphite coating can be formed on the surface of the uranium nitride pellet, mechanical friction between the uranium nitride pellet and a fuel cladding is reduced, fission products are contained and sealed, the service life of a fuel assembly is prolonged, and the reliability of the fuel assembly is improved.

Description

Technical Field

[0001] The invention belongs to the field of nuclear power, and in particular relates to a composite uranium nitride pellet manufacturing method and a composite uranium nitride pellet. Background Art

[0002] Compared with the uranium oxide pellets used in existing commercial reactors, uranium nitride pellets have the advantages of high melting point, high thermal conductivity, and high uranium density, and are considered to have good application prospects in new reactors with higher safety and economy. However, as the degree of burnup of uranium nitride pellets increases, the accumulation of fission products in the pellets easily leads to swelling; at the same time, the release of fission products such as iodine and cesium at high temperatures intensifies, which easily induces corrosion and cracking at the contact position between the cladding and the pellets, and even leads to penetration of the cladding and accidents. Therefore, providing a method for manufacturing uranium nitride pellets that can improve the safety of uranium nitride pellets is of positive significance for improving the inherent safety of fuel and improving the reliability of reactors. Summary of the invention

[0003] The purpose of the present invention is to provide a method for manufacturing a composite uranium nitride pellet to improve the safety of the uranium nitride pellet. The present invention also provides a composite uranium nitride pellet.

[0004] According to an embodiment of one aspect of the present invention, a method for manufacturing a composite uranium nitride pellet is provided, the method comprising the following steps:

[0005] Step a): providing uranium nitride powder, and sintering the uranium nitride powder into uranium nitride pellets;

[0006] Step b): providing graphite powder, a solvent, a binder and a dispersant, and mixing them into a graphite slurry;

[0007] Step c): coating the graphite slurry on the surface of the uranium nitride pellet and drying;

[0008] Step d): vacuum calcining the uranium nitride pellets coated with the graphite slurry in step c) to obtain finished composite uranium nitride pellets.

[0009] The method can form a graphite coating on the surface of the uranium nitride pellet. The graphite coating can lubricate the fuel pellet, reduce the mechanical damage of the pellet to the cladding, and at the same time reduce the material stress of the cladding at the contact position between the cladding and the pellet, thereby reducing the risk of stress corrosion. Furthermore, the graphite coating can block and adsorb fission products generated during the service of the pellet, thereby reducing the risk of iodine-induced stress corrosion of the cladding material.

[0010] Furthermore, in some embodiments, step a) further includes

[0011] The uranium nitride powder is mixed with 0.5wt%-18wt% of a sintering aid, and the mixture is pressed into a uranium nitride green body pellet.

[0012] Furthermore, in some embodiments, in the step a), the sintering temperature of the uranium nitride pellet is 1750° C.-1900° C., and the sintering time is 28 min-100 min.

[0013] Furthermore, in some embodiments, in step b), the solvent includes one or a combination of isopropyl alcohol, n-butanol, and ethylene glycol; the binder includes one or a combination of resin and ethyl cellulose; and the dispersant is configured as cellulose acetate.

[0014] Furthermore, in some embodiments, in the step b), the purity of the graphite powder is not less than 99%.

[0015] Furthermore, in some embodiments, in the step b), the graphite slurry is obtained by mixing and ball-milling the graphite powder, the solvent, the binder and the dispersant.

[0016] Furthermore, in some embodiments, in the step c), the coating thickness of the graphite slurry is 6 μm-50 μm.

[0017] Furthermore, in some embodiments, in the step c), the drying temperature does not exceed 200°C.

[0018] Furthermore, in some embodiments, in the step d), the calcination temperature is 300° C.-400° C., and the calcination time is 2 h-4 h.

[0019] According to an embodiment of another aspect of the present invention, there is provided a composite uranium nitride pellet, which comprises a uranium nitride substrate and a graphite coating, wherein the graphite coating is coated on the surface of the uranium nitride substrate; the composite uranium nitride pellet is manufactured by the composite uranium nitride pellet manufacturing method provided in any of the aforementioned embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The figure is a schematic diagram of the process of manufacturing a composite uranium nitride pellet in one embodiment.

[0021] The purpose of the above drawings is to explain the present invention in detail so that those skilled in the art can understand the technical concept of the present invention, but it is not intended to limit the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below through specific embodiments in conjunction with the accompanying drawings.

[0023] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present invention. The phrase appearing in various locations in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive independent or alternative embodiments. Those skilled in the art should be able to understand that the embodiments herein may be combined with other embodiments without causing structural conflicts.

[0024] In the description herein, "plurality" means at least two.

[0025] Nuclear power is a clean, efficient and sustainable energy source, and is an important choice for addressing climate change and ensuring energy security. Nuclear fuel pellets are the core components of nuclear power plants, and their materials and preparation processes directly affect the performance and safety of nuclear power plants.

[0026] At present, commercial light water reactor nuclear power plants mainly use uranium oxide as the nuclear fuel pellet material. As a mature nuclear fuel material, uranium oxide has excellent properties such as good chemical / irradiation stability, strong water corrosion resistance, and high melting point. However, uranium oxide fuel pellets also have some disadvantages, mainly low thermal conductivity and uranium density. These disadvantages limit the power density and burnup rate of the reactor, and increase the operating cost and risk of the reactor.

[0027] In order to overcome the shortcomings of uranium oxide fuel pellets and improve the economy and safety of nuclear power plants, people have begun to explore new nuclear fuel pellet materials. Among them, uranium nitride (UN) is a candidate material with high potential. Uranium nitride has the advantages of high melting point (3123K), high thermal conductivity (27W / m·K), and high uranium density (14.32g / cm3). These advantages are conducive to the design of advanced reactors with higher safety and better economy. It also has the potential to be applied to existing commercial light water reactors to improve the accident resistance and economy of existing reactors.

[0028] However, when the fuel pellet reaches a certain burnup, the pellet swells due to the gaseous and solid fission products, and the cladding creeps inward under the pressure difference between the inside and outside, and the two come into contact. Under conditions with more drastic power changes, the high temperature of the pellet will lead to an increase in the release of fission products (such as iodine or cesium), which will diffuse to the inner surface of the cladding. The stress of the cladding is higher at the radial crack of the pellet, and stress corrosion cracking (SCC) may occur at the contact surface between the pellet and the cladding, resulting in smaller intergranular cracks. Under some extreme conditions, these cracks will continue to develop and eventually penetrate the cladding (typical PCI failure). The PCI effect (Pellet-to-Cladding Interaction) between the pellet and the cladding is one of the main modes of failure of the rod cladding material.

[0029] Graphite has the following performance characteristics: 1) Graphite has an extremely low neutron absorption cross section and a large scattering cross section. 2) Graphite has good thermal conductivity, and its thermal conductivity is about 0.3K / (cm·S·℃), which is second only to aluminum and equivalent to brass. 3) Graphite has good lubrication properties and is often used as a lubricant. 4) The chemical properties of graphite are very stable. 5) Graphite has good radiation resistance and can serve in the reactor for a long time, up to 30 to 40 years. By coating the uranium nitride pellets with graphite slurry, the fission products produced by the pellets can be wrapped, stress corrosion cracking can be reduced, and the risk of PCI effect can be greatly reduced. Therefore, an efficient and low-cost graphite coating process is needed to manufacture uranium nitride fuel pellets that can greatly mitigate the PCI effect.

[0030] To this end, an embodiment of one aspect of the present invention provides a method for manufacturing a composite uranium nitride pellet coated with a graphite protective layer. Figure 1 As shown, the method comprises the following steps:

[0031] Step a): providing a uranium nitride powder raw material, mixing the uranium nitride powder raw material with a sintering aid, and pressing the mixture into a uranium nitride green body pellet by a press.

[0032] In a preferred embodiment, the sintering aid accounts for 0.5wt%-18wt%.

[0033] Subsequently, the uranium nitride green body pellets are sintered in vacuum or inert atmosphere to obtain uranium nitride pellets.

[0034] In a preferred embodiment, the sintering temperature is 1750° C.-1900° C., and the holding time during the sintering process is 28 min-100 min.

[0035] Step b): providing graphite powder, a solvent, a binder and a dispersant, and mixing them into a graphite slurry.

[0036] In a preferred embodiment, the purity of the graphite powder is not less than 99%.

[0037] The solvent, binder and dispersant are non-ionic organic materials that are non-corrosive to metals and can be easily gasified and removed under heating and roasting conditions. Specifically, in a preferred embodiment, the solvent can be one or a combination of organic solvents such as isopropyl alcohol, n-butyl alcohol, cellulose acetate, ethylene glycol, etc. that do not affect the performance of the uranium nitride pellet material; the binder can be resin or ethyl cellulose or a mixture of the two; and the dispersant can be cellulose acetate. The graphite powder is mixed with the solvent, binder and dispersant and then ball-milled to obtain a uniform graphite slurry.

[0038] Step c): Dilute the graphite slurry with isopropanol according to temperature, ambient humidity and other conditions to adjust the graphite slurry to a viscosity suitable for coating. Use a spraying device to coat the graphite slurry on the surface of the uranium nitride pellets prepared in step a), and dry it in an electric oven at a temperature not exceeding 200° C. to gasify the organic solvent and part of the binder and dispersant therein.

[0039] In different embodiments, the coating of graphite slurry can be performed once or in multiple times; according to the size of the fuel pellets and the matching relationship between the fuel pellets and the cladding tube, the thickness of the graphite slurry coating can be 6 μm-50 μm.

[0040] Step d): The dried uranium nitride pellets in step c) are placed in a heat treatment furnace and calcined in a vacuum environment to fully solidify the graphite slurry layer and completely gasify the organic components therein to obtain a finished composite uranium nitride pellet.

[0041] In a preferred embodiment, the calcination temperature is 300° C.-400° C., and the calcination time is 2 h-4 h. After calcination, a certain strength bond is formed between the graphite coating and the uranium nitride substrate, thereby coating and protecting the uranium nitride pellets.

[0042] The surface of the composite uranium nitride pellet prepared by the above method is formed with a firmly attached graphite coating, which can lubricate the pellet, reduce the friction between the pellet and the fuel cladding when the pellet is loaded into the cladding and during long-term service, and reduce the mechanical damage of the pellet to the cladding; at the same time, it can reduce the hoop stress of the cladding material at the contact position between the cladding and the pellet, thereby reducing the risk of stress corrosion. Furthermore, the graphite coating can block and adsorb the fission products in the pellet, weaken or avoid iodine stress corrosion (ISCC) at the contact position between the cladding and the pellet, and improve the service life and reliability of the fuel assembly.

[0043] The purpose of the above embodiments is to further explain the present invention in detail in conjunction with the accompanying drawings so that those skilled in the art can understand the technical concept of the present invention. Within the scope of the present invention, the optimization or equivalent replacement of the method steps involved, and the combination of the implementation methods in different embodiments without conflict of structure and principle, all fall within the protection scope of the present invention.

Claims

1. A method for manufacturing a composite uranium nitride pellet, characterized in that: The following steps are involved: Step a): providing uranium nitride powder, and sintering the uranium nitride powder into uranium nitride pellets; Step b): providing graphite powder, a solvent, a binder and a dispersant, and mixing them into a graphite slurry; Step c): coating the graphite slurry on the surface of the uranium nitride pellet and drying; Step d): vacuum calcining the uranium nitride pellets coated with the graphite slurry in step c) to obtain finished composite uranium nitride pellets.

2. The method for manufacturing composite uranium nitride pellets according to claim 1, characterized in that: The step a) further comprises The uranium nitride powder is mixed with 0.5wt%-18wt% of a sintering aid, and the mixture is pressed into a uranium nitride green body pellet.

3. The method for manufacturing composite uranium nitride pellets according to claim 1 or 2, characterized in that: In the step a), the sintering temperature of the uranium nitride pellet is 1750° C.-1900° C., and the sintering time is 28 min-100 min.

4. The method for manufacturing composite uranium nitride pellets according to claim 1, characterized in that: In the step b), the solvent includes one or a combination of isopropanol, n-butanol, and ethylene glycol; the binder includes one or a combination of resin and ethyl cellulose; and the dispersant is cellulose acetate.

5. The method for manufacturing composite uranium nitride pellets according to claim 1 or 4, characterized in that: In the step b), the purity of the graphite powder is not less than 99%.

6. The method for manufacturing composite uranium nitride pellets according to claim 1 or 4, characterized in that: In the step b), the graphite slurry is obtained by mixing and ball-milling the graphite powder, the solvent, the binder and the dispersant.

7. The method for manufacturing composite uranium nitride pellets according to claim 1, characterized in that: In the step c), the coating thickness of the graphite slurry is 6 μm-50 μm.

8. The method for manufacturing composite uranium nitride pellets according to claim 1 or 7, characterized in that: In the step c), the drying temperature does not exceed 200°C.

9. The method for manufacturing composite uranium nitride pellets according to claim 1, characterized in that: In the step d), the calcination temperature is 300° C.-400° C., and the calcination time is 2 h-4 h.

10. A composite uranium nitride pellet, characterized in that: It comprises a uranium nitride substrate and a graphite coating, wherein the graphite coating is coated on the surface of the uranium nitride substrate; the composite uranium nitride pellet is manufactured by the composite uranium nitride pellet manufacturing method according to any one of claims 1 to 9.

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