A kind of matrix graphite powder for high temperature / ultra-high temperature gas cooled reactor fuel element and its preparation method and matrix graphite, fuel element

Abstract

The application provides a kind of matrix graphite powder for high temperature / ultra-high temperature gas cooled reactor fuel element and its preparation method and matrix graphite, fuel element, belong to high temperature / ultra-high temperature gas cooled reactor technical field.The raw material of the matrix graphite powder of the application adds the nuclear grade graphene powder, so that the subsequent graphene modified matrix graphite can be prepared, the matrix graphite has high thermal conductivity, low thermal expansion coefficient and thermal expansion anisotropy, low wear rate, and can ensure the safe operation of fuel element under high temperature / ultra-high temperature condition of reactor.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature / ultra-high-temperature gas-cooled reactor technology, specifically relating to a matrix graphite powder for fuel elements of high-temperature / ultra-high-temperature gas-cooled reactors, its preparation method, matrix graphite, and fuel elements. Background Technology

[0002] High-temperature gas-cooled reactors (HTRs) are one of the key reactor types developed for fourth-generation nuclear energy systems, characterized by high coolant outlet temperatures and excellent inherent safety. Fuel elements are the energy source of HTRs, powered by the enormous energy released from nuclear fission of the coated fuel particles. Fuel elements can be classified into cylindrical fuel elements and spherical fuel elements according to their shape, but both are composed of a matrix graphite and coated fuel particles. The main functions of the matrix graphite include: (1) Structural support. The matrix graphite acts as a carrier for the coated fuel particles, ensuring their uniform dispersion within it. The matrix graphite also provides the fuel elements with sufficient mechanical strength, enabling them to effectively withstand the applied load and ensuring safe circulation within the reactor core. (2) Neutron moderation. After fast neutrons generated by nuclear fission collide with carbon atoms, they are slowed down into thermal neutrons, which continue to trigger chain reactions. (3) Thermal conductivity. The heat energy generated by nuclear fission needs to be transferred to the helium coolant through the matrix graphite in a timely manner, and its good thermal conductivity ensures the full utilization of thermal energy.

[0003] Currently, my country's 10MW high-temperature gas-cooled reactor experimental reactor (HTR-10) and high-temperature gas-cooled reactor demonstration project (HTR-PM) both utilize spherical fuel elements. Spherical fuel elements, structurally, consist of a 50mm diameter fuel zone (core) and a 5mm thick unfueled zone (shell). Compositionally, they consist of a graphite matrix and coated fuel particles (uniformly dispersed within the 50mm fuel zone). The graphite matrix used in fuel elements comprises approximately 71wt% nuclear-grade natural graphite powder, 18wt% nuclear-grade artificial graphite powder, and 11wt% phenolic resin carbon. Graphite matrix accounts for over 95% of both volume and mass in spherical fuel elements, profoundly determining their thermodynamic properties, oxidation resistance, and wear resistance, among other in-reactor performance characteristics.

[0004] Following criticality, grid connection, and full-power operation of both reactors, the HTR-PM successfully completed its 168-hour continuous operation test on December 6, 2023, commencing commercial operation. This milestone marks the official start of the commercialization of high-temperature gas-cooled reactors (HTGRs), indicating that fourth-generation nuclear power will enter a rapid development phase, and the demand for HTR commercial reactors will continue to increase. The future development direction of HTGRs is the ultra-high-temperature gas-cooled reactor (UHTGR). UHTGRs not only possess inherently better safety and higher power generation efficiency, but their core outlet temperature will also be further increased, expected to reach 750-950℃. Higher core outlet and operating temperatures place higher demands on the comprehensive performance of the matrix graphite, especially its thermal conductivity, thermal expansion, and wear resistance.

[0005] Matrix graphite should possess good thermal conductivity to ensure timely transfer of heat generated by the fission of coated fuel particles, thereby reducing the core temperature of the coated fuel particles and improving reactor safety. Studies have shown that the thermal conductivity of matrix graphite decreases with increasing temperature, and its thermal conductivity further decreases after irradiation. Therefore, in order to ensure the safe operation of fuel elements at higher core temperatures and maintain good heat transfer efficiency during in-reactor service, it is essential to prepare matrix graphite with better thermal conductivity.

[0006] The coefficient of thermal expansion of the matrix graphite increases with increasing temperature and exhibits anisotropy. Since future ultra-high temperature gas-cooled reactor core temperatures will be even higher, and significant temperature gradients exist in certain regions, thermal stress may occur in the matrix graphite. Excessive thermal stress can threaten the safety of both the matrix graphite and fuel elements, and in severe cases, may even lead to fuel element rupture. Therefore, it is essential to ensure that the matrix graphite maintains a low coefficient of thermal expansion and low anisotropy at high temperatures.

[0007] The wear resistance of the matrix graphite at high temperatures is also a key concern, especially since spherical fuel elements need to circulate within the reactor core. Friction between the spheres and between the spheres and the circulation path can easily generate dust, affecting the normal operation of the reactor. Improving the wear resistance of the matrix graphite is an effective way to reduce dust levels.

[0008] Given the complex and harsh application environment of ultra-high temperature gas-cooled reactors, it is extremely urgent and necessary to develop a matrix graphite with high thermal conductivity, low coefficient of thermal expansion, anisotropic thermal expansion, and low wear rate to ensure the safe operation of fuel elements under ultra-high temperature conditions in the reactor. Summary of the Invention

[0009] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, embodiments of this invention propose a matrix graphite powder for high-temperature / ultra-high-temperature gas-cooled reactor fuel elements, its preparation method, matrix graphite, and fuel elements. The matrix graphite prepared from this matrix graphite powder possesses high thermal conductivity, low coefficient of thermal expansion, anisotropic thermal expansion, and low wear rate, ensuring the safe operation of fuel elements under high-temperature / ultra-high-temperature reactor conditions.

[0010] This invention provides a matrix graphite powder for fuel elements in high-temperature / ultra-high-temperature gas-cooled reactors. The raw materials include nuclear-grade graphene powder, nuclear-grade natural graphite powder, nuclear-grade artificial graphite powder, nuclear-pure phenolic resin ethanol solution, and ethanol. The mass percentage of the nuclear-grade graphene powder to the sum of the masses of the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder is 0.2-5%.

[0011] The advantages and technical effects of the matrix graphite powder in this embodiment of the invention are as follows:

[0012] (1) In this embodiment of the invention, the nuclear-grade graphene powder is added to the raw materials of the matrix graphite powder so that the prepared matrix graphite powder contains graphene, thereby achieving the purpose of modifying the matrix graphite by modifying the matrix graphite powder with graphene.

[0013] (2) Since graphene has better thermal conductivity than natural graphite and artificial graphite, a more developed thermal conductivity network is constructed by modifying the matrix graphite with graphene. Compared with the matrix graphite in the prior art that is not modified with graphene, the matrix graphite in the present invention has better thermal conductivity.

[0014] (3) In the embodiments of the present invention, the thermal expansion coefficient of the matrix graphite is reduced and the wear resistance of the matrix graphite is improved by modifying the matrix graphite with graphene.

[0015] (4) In this embodiment of the invention, ethanol is used as the dispersant for the nuclear-grade graphene powder in the raw materials of the matrix graphene powder. This not only effectively disperses the graphene, but also allows the ethanol to evaporate directly during the subsequent heating process, thus avoiding the introduction of impurities.

[0016] In some embodiments, the mass percentage of the nuclear-grade graphene powder to the sum of the masses of the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder is 0.4-3%.

[0017] In some embodiments, with the total mass of the nuclear-grade graphene powder, the nuclear-grade natural graphite powder, the nuclear-grade artificial graphite powder, and the phenolic resin in the nuclear-pure phenolic resin ethanol solution being 100%, the sum of the mass fractions of the nuclear-grade graphene powder and the nuclear-grade natural graphite powder is 64%, the mass fraction of the nuclear-grade artificial graphite powder is 16%, and the mass fraction of the phenolic resin in the nuclear-pure phenolic resin ethanol solution is 20%.

[0018] In some embodiments, the specific surface area of ​​the nuclear-grade graphene powder is 10-30 m². 2 / g, with an average particle size of 20-50μm, total ash content ≤50μg / g, and total equivalent boron content ≤1μg / g.

[0019] In some embodiments, the loose packing density of the matrix graphite is 0.50-0.55 g / cm³. 3 The average particle size is 40-80 μm, and the maximum particle size is ≤350 μm.

[0020] In addition, this invention also provides a method for preparing matrix graphite powder for high-temperature / ultra-high-temperature gas-cooled reactor fuel elements, comprising the following steps:

[0021] S1. The nuclear-grade graphene powder and ethanol are mixed and then subjected to ultrasonic treatment to obtain a graphene-ethanol suspension;

[0022] S2. The graphene ethanol suspension and the nuclear-grade phenolic resin ethanol solution are mixed and then subjected to ultrasonic stirring to obtain a graphene phenolic resin ethanol suspension.

[0023] S3. The nuclear-grade natural graphite powder, the nuclear-grade artificial graphite powder, and the graphene phenolic resin ethanol suspension are mixed and kneaded to obtain a paste.

[0024] S4. The paste is subjected to granulation, drying and pulverization to obtain the matrix graphite powder.

[0025] The advantages and technical effects of the method for preparing the matrix graphite powder in this embodiment of the invention are as follows:

[0026] (1) The preparation method of this invention achieves good dispersion of graphene in ethanol through step S1, then achieves good dispersion of graphene in phenolic resin ethanol solution through step S2, and then achieves good dispersion of graphene in paste through step S3. Through the stepwise implementation of the above three steps, the graphene is finally uniformly dispersed in the matrix graphene. The uniformly dispersed graphene helps to further construct a more perfect thermally conductive network, helps to reduce the thermal expansion of the matrix graphene, and at the same time improves the mechanical strength and wear resistance of the matrix graphene.

[0027] (2) In step S1 of the preparation method of this embodiment, ethanol is selected as the dispersant to ensure that the phenolic resin added later can be dissolved, and the ethanol can evaporate without residue in the subsequent drying stage, thus avoiding the introduction of impurities into the graphite matrix system.

[0028] (3) In the preparation method of the present invention, step S1 adopts ultrasonic treatment and step S2 adopts ultrasonic stirring treatment, both of which are to improve the dispersion uniformity of graphene and thus improve the comprehensive performance of the matrix graphite.

[0029] In some embodiments, the ultrasonic power of the ultrasonic treatment is 500-1500w, and the ultrasonic time is 2-5h.

[0030] In some embodiments, the ultrasonic stirring time is 0.5-2 hours and the stirring speed is 100-800 rpm.

[0031] In some embodiments, the temperature of the ultrasonic stirring treatment is 40-60°C.

[0032] In addition, this invention also provides a matrix graphite for high-temperature / ultra-high-temperature gas-cooled reactor fuel elements, which is obtained by pressing, heat treatment and turning of the matrix graphite powder of this invention.

[0033] The advantages and technical effects of the graphite matrix in this embodiment of the invention are as follows:

[0034] Compared to existing technologies that do not use graphene for modification, the graphene in this invention is modified with graphene, which improves the thermal conductivity, reduces the coefficient of thermal expansion, and enhances the wear resistance of the graphene.

[0035] In addition, this embodiment of the invention also provides a high-temperature / ultra-high-temperature gas-cooled reactor fuel element, wherein the fuel region and the fuel-free region of the fuel element are based on graphite according to this embodiment of the invention.

[0036] The advantages and technical effects of the high-temperature / ultra-high-temperature gas-cooled reactor fuel elements of this invention are as follows:

[0037] Because the high-performance matrix graphite of the present invention is used, the high-temperature / ultra-high-temperature gas-cooled reactor fuel element of the present invention can operate safely at higher core temperatures and maintain good heat transfer efficiency during in-core service, while effectively reducing dust. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of an ultrasonic stirrer;

[0039] Figure 2 This is a flowchart of the preparation process of the matrix graphite spheres. Detailed Implementation

[0040] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0041] The embodiments of the present invention provide raw materials including nuclear-grade graphene powder, nuclear-grade natural graphite powder, nuclear-grade artificial graphite powder, nuclear-pure phenolic resin ethanol solution, and ethanol, wherein the mass of the nuclear-grade graphene powder accounts for 0.2-5% of the sum of the masses of the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder.

[0042] Graphene is a two-dimensional nanomaterial composed of carbon atoms through sp2 hybridization. It possesses significant advantages in mechanical and thermal properties, with a theoretical tensile strength reaching 125 GPa and an elastic modulus as high as 1.1 TPa. At room temperature, the thermal conductivity of a single layer of graphene can reach 5000 W / m·K. The embodiments of this invention utilize graphene to modify the matrix graphite, which helps to further improve the comprehensive properties of the matrix graphite (high thermal conductivity, low coefficient of thermal expansion and anisotropic thermal expansion, low wear rate), enabling its performance to meet the application conditions of high-temperature / ultra-high-temperature gas-cooled reactors. Currently, there is no relevant research on the use of graphene to modify the matrix graphite; this invention fills a gap in this research field.

[0043] In the raw materials of the matrix graphite powder of this invention, the addition amount of nuclear-grade graphene powder is 0.2-5%, for example, 0.2%, 0.4%, 1%, 2%, 3%, 4%, 5%, etc. This addition amount is calculated as a percentage of the mass of the nuclear-grade graphene powder to the sum of the masses of the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder. When the addition amount of nuclear-grade graphene powder is less than 0.2%, it is difficult to improve the overall performance of the matrix graphite. When the addition amount of nuclear-grade graphene powder is greater than 5%, the dispersion effect of the nuclear-grade graphene powder is poor, and it is prone to agglomeration, resulting in a more deteriorated overall performance of the matrix graphite compared to the matrix graphite without graphene modification in the prior art.

[0044] Preferably, in order to further improve the overall performance of the matrix graphite, the mass percentage of the nuclear-grade graphene powder to the sum of the masses of the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder is 0.4-3%.

[0045] In this embodiment of the invention, the content of components other than nuclear-grade graphene powder in the matrix graphite powder is not particularly limited and can be set with reference to existing technology. Currently, mature commercial matrix graphite powder is made from 64 wt% nuclear-grade natural graphite powder, 16 wt% nuclear-grade artificial graphite powder, and 20 wt% nuclear-pure phenolic resin as raw materials, through processes such as mixing, granulation, drying, and pulverization. Therefore, in order to further improve the comprehensive performance of the matrix graphite, with the total mass of the nuclear-grade graphene powder, the nuclear-grade natural graphite powder, the nuclear-grade artificial graphite powder, and the phenolic resin in the nuclear-pure phenolic resin ethanol solution being 100%, the sum of the mass fractions of the nuclear-grade graphene powder and the nuclear-grade natural graphite powder is 64%, the mass fraction of the nuclear-grade artificial graphite powder is 16%, and the mass fraction of the phenolic resin in the nuclear-pure phenolic resin ethanol solution is 20%.

[0046] In some embodiments, the specific surface area of ​​the nuclear-grade graphene powder is 10-30 m². 2 / g, for example, 10m 2 / g, 15m 2 / g、20m 2 / g、25m 2 / g、30m 2The average particle size is 20-50 μm, such as 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, etc. The loose density, surface energy, and specific surface area of ​​the nuclear-grade graphene powder differ significantly from those of the nuclear-grade natural graphene powder and the nuclear-grade artificial graphene powder. Even a small amount added can have a significant impact on the matrix graphite system. The above parameter ranges were determined after comprehensively considering the average particle size of the nuclear-grade natural graphene powder and the nuclear-grade artificial graphene powder and after experimental verification. Therefore, the nuclear-grade graphene powder selected in this embodiment of the invention is an ultrafine powder with a high specific surface area. If the average particle size of the nuclear-grade graphene powder is too small or too large, it will affect the particle size and particle size distribution of the final matrix graphene powder. If the specific surface area of ​​the nuclear-grade graphene powder is too low or too high, it will affect the mixing process of the matrix graphene powder, including ethanol content, paste dryness, and mixing uniformity. Therefore, the specific surface area and average particle size of the nuclear-grade graphene powder are preferably within the above ranges.

[0047] In some embodiments, the total ash content of the nuclear-grade graphene powder is ≤50 μg / g, for example, 0 μg / g, 10 μg / g, 20 μg / g, 30 μg / g, 40 μg / g, 50 μg / g, etc., and the total equivalent boron content is ≤1 μg / g, for example, 0 μg / g, 0.2 μg / g, 0.4 μg / g, 0.6 μg / g, 0.8 μg / g, 1 μg / g, etc. Since the nuclear-grade graphene powder used in the embodiments of the present invention is for preparing nuclear reactor materials, its total ash content and total equivalent boron content must be strictly limited, requiring a total ash content ≤50 μg / g and a total equivalent boron content ≤1 μg / g.

[0048] In some embodiments, the loose packing density of the matrix graphite powder is 0.50-0.55 g / cm³. 3 For example, 0.50 g / cm 3 0.51g / cm 3 0.52g / cm 3 0.53g / cm 3 0.54g / cm 3 0.55g / cm 3 The average particle size is 40-80 μm, such as 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, etc., and the maximum particle size is ≤350 μm, such as 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, etc. When the loose packing density, average particle size, and maximum particle size of the matrix graphite powder are within the above ranges, it is beneficial to further improve the comprehensive performance of the matrix graphite.

[0049] In addition, this invention also provides a method for preparing matrix graphite powder for high-temperature / ultra-high-temperature gas-cooled reactor fuel elements, characterized by comprising the following steps:

[0050] S1. The nuclear-grade graphene powder and ethanol are mixed and then subjected to ultrasonic treatment to obtain a graphene-ethanol suspension;

[0051] S2. The graphene ethanol suspension and the nuclear-grade phenolic resin ethanol solution are mixed and then subjected to ultrasonic stirring to obtain a graphene phenolic resin ethanol suspension.

[0052] S3. The nuclear-grade natural graphite powder, the nuclear-grade artificial graphite powder, and the graphene phenolic resin ethanol suspension are mixed and kneaded to obtain a paste.

[0053] S4. The paste is subjected to granulation, drying and pulverization to obtain the matrix graphite powder.

[0054] The key to the preparation method of this invention lies in ensuring that graphene is uniformly dispersed in the graphite matrix. Uniformly dispersed graphene helps to further construct a more complete thermally conductive network, helps to reduce the thermal expansion of the graphite matrix, and at the same time improves the mechanical strength and wear resistance of the graphite matrix. Therefore, the preparation method of this invention achieves the aforementioned objectives by performing steps S1, S2, and S3 in stages, rather than mixing all raw materials together and then performing kneading, granulation, drying, and pulverization.

[0055] In step S1, ethanol is used as the dispersant for dispersing the nuclear-grade graphene powder. The advantages of ethanol include its ability to dissolve phenolic resin and its ability to evaporate without residue during subsequent drying, making it a suitable solvent. To avoid introducing impurities into the graphite matrix system, other dispersants should not be used in this step. To ensure uniform dispersion of the nuclear-grade graphene powder in ethanol, the concentration of the dispersion should not be too high, generally below 5% by mass.

[0056] In step S1, the nuclear-grade graphene powder is dispersed using ultrasound. This step is a prerequisite for ensuring good dispersion of graphene in the phenolic resin ethanol solution in the subsequent step S2. Ultrasound is an effective method for dispersing graphene. Ultrasound waves cause high-frequency oscillations in the graphite slurry, dispersing the graphene through ultrasonic cavitation. Repeated experiments have proven that under ultrasonic power conditions of 500-1500W (e.g., 500W, 600W, 800W, 1000W, 1200W, 1500W), and for 2-5 hours (e.g., 2h, 3h, 4h, 5h), good dispersion of the nuclear-grade graphene powder in ethanol can be achieved. If the ultrasonic power is too low or the ultrasonic time is too short, the nuclear-grade graphene powder will remain in an agglomerated state and will not be completely dispersed. Conversely, if the ultrasonic power is too high or the ultrasonic time is too long, the particle size of the nuclear-grade graphene powder will decrease, and in severe cases, it may even lead to re-agglomeration and reduce the preparation efficiency.

[0057] The ultrasonic treatment in step S1 can be performed by using an ultrasonic oscillator, a probe-type ultrasonic instrument, or a two-dimensional material stripper to disperse the nuclear-grade graphene powder.

[0058] Step S2 is a prerequisite for ensuring the uniform dispersion of graphene in the graphite matrix. To uniformly disperse graphene in the phenolic resin ethanol solution, this step employs an ultrasonic stirrer. Ultrasound and stirring help to uniformly disperse graphene in the phenolic resin. Among these, the ultrasonic time and stirring speed are key parameters. Too short an ultrasonic time or too slow a stirring speed will not guarantee uniform dispersion of graphene in the phenolic resin ethanol solution, while too long an ultrasonic time or too fast a stirring speed may have the opposite effect, potentially leading to graphene agglomeration due to increased particle collisions. Experimental verification has shown that ultrasonic times of 0.5-2 hours (e.g., 0.5h, 1h, 1.5h, 2h) and stirring speeds of 100-800 rpm (e.g., 100rpm, 200rpm, 400rpm, 600rpm, 800rpm) are suitable preparation parameters. As for the ultrasonic power, it can be set to 500-1500W, such as 500W, 600W, 800W, 1000W, 1200W, 1500W, etc.

[0059] In some embodiments, the ultrasonic stirring treatment in step S2 is performed under heating conditions. Specifically, it can be performed using methods such as... Figure 1 The ultrasonic stirrer equipped with an ultrasonic heating device is used. The heating temperature is preferably maintained within the range of 40-60℃, such as 40℃, 45℃, 50℃, 55℃, and 60℃. Maintaining a suitable viscosity of the phenolic resin during the mixing process is crucial. The ultrasonic stirring treatment under heating conditions in step S2 helps reduce the viscosity of the phenolic resin, thereby promoting more uniform dispersion of graphene. If the heating temperature in step S2 is too low, the viscosity of the phenolic resin will be high, affecting the uniformity of subsequent mixing. Conversely, if the heating temperature in step S2 is too high, it will cause ethanol to evaporate too quickly, also affecting the uniformity of subsequent mixing.

[0060] In step S3, a certain mass of the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder can be weighed and poured together into the mixing tank of a kneader for dry mixing. The kneader is equipped with a Z-shaped stirring paddle, and the stirring speed is 20-100 rpm, such as 20 rpm, 40 rpm, 60 rpm, 80 rpm, 100 rpm, etc. The kneading time is 10-30 min, such as 10 min, 15 min, 20 min, 25 min, 30 min, etc. After the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder are evenly mixed, the dispersed graphene phenolic resin ethanol suspension is poured into the mixing tank of the kneader for kneading. The stirring speed is 20-100 rpm, such as 20 rpm, 40 rpm, 60 rpm, 80 rpm, 100 rpm, etc., and the kneading time is 2-10 h, such as 2 h, 4 h, 6 h, 8 h, 10 h, etc.

[0061] In step S4, the paste obtained by kneading is granulated, then dried in an oven at 80-90℃ for 8-15 hours, and finally pulverized to obtain the matrix graphite powder of this embodiment of the invention, with a loose packing density of 0.50-0.55 g / cm³. 3 The average particle size is 40-80 μm, and the maximum particle size does not exceed 350 μm.

[0062] In addition, this invention also provides a matrix graphite for high-temperature / ultra-high-temperature gas-cooled reactor fuel elements, which is obtained by pressing, heat treatment and turning of the matrix graphite powder of this invention.

[0063] Since the matrix graphite powder in this embodiment of the invention is a graphene-modified matrix graphite powder, the matrix graphite in this embodiment of the invention has high thermal conductivity, low coefficient of thermal expansion and anisotropy of thermal expansion, and low wear rate. Specifically, the thermal conductivity -AX at 1000℃ is 40-45 W·m. -1 ·K -1 Thermal conductivity (TR) is 43-46 W·m. -1 ·K -1 The coefficient of thermal expansion at 500℃ is 3.3-3.5 × 10⁻⁶. -6 K -1 The coefficient of thermal expansion (TR) at 500℃ is 3.1-3.3×10⁻⁶. -6 K -1 The anisotropy of thermal expansion is 1.05-1.10, and the wear rate is 2.20-2.5 mg / ball. -1 ·h -1 .

[0064] In addition, this embodiment of the invention also provides a high-temperature / ultra-high-temperature gas-cooled reactor fuel element, wherein the fuel region and the fuel-free region of the fuel element are based on graphite according to this embodiment of the invention.

[0065] Because the high-performance matrix graphite of the present invention is used, the high-temperature / ultra-high-temperature gas-cooled reactor fuel element of the present invention can operate safely at higher core temperatures and maintain good heat transfer efficiency during in-core service, while effectively reducing dust.

[0066] The present invention will now be described in detail with reference to the embodiments and accompanying drawings.

[0067] Example 1

[0068] A method for preparing graphene-modified matrix graphite powder is shown below:

[0069] S1. Using a probe-type ultrasonic transducer (SCIENTZ-IID, Xinzhi Biotechnology), 10g of nuclear-grade graphene powder (specific surface area 15.8m²) was subjected to ultrasonic treatment. 2 The graphene ethanol suspension was obtained by dispersing graphene in 1000 mL of ethanol (analytical grade) with an average particle size of 28.5 μm, a total ash content of 15 μg / g, and a total equivalent boron content of 0.2 μg / g. The suspension was then subjected to ultrasonication at room temperature with a power of 900 W for 4 h.

[0070] S2. The ultrasonically purified graphene-ethanol suspension was poured into 1200g of qualified nuclear-grade phenolic resin ethanol solution. The specific parameters of the phenolic resin were: solid content of 52wt%, relative molecular weight of 1050, softening point of 110℃, ash content of 32μg / g, and char content of 55wt%. An ultrasonic stirrer (FS-2000, Changsha Wanrong Powder Equipment Technology Co., Ltd.) was used to simultaneously ultrasonicate, stir, and heat the graphene-phenolic resin ethanol suspension. The ultrasonic power was 600W, the stirring speed was 500rpm, the stirring time was 30min, and the heating temperature was 50℃, resulting in a graphene-phenolic resin ethanol suspension.

[0071] S3. Weigh 1990g of nuclear-grade natural graphite powder (average particle size 27.5μm, specific surface area 4.2m²). 2 The total ash content is 15.0 μg / g, the total equivalent boron content is 0.3 μg / g, and the loose bulk density is 0.35 g / cm³. 3 500g of nuclear-grade artificial graphite powder (average particle size 23.5μm, specific surface area 1.6m²). 2 / g, loose bulk density is 0.80g / cm³ 3 A total of 2490g of nuclear-grade natural graphite powder and nuclear-grade artificial graphite powder were poured into the mixing tank of a kneader for dry mixing at 60 rpm for 20 minutes. Then, a graphene phenolic resin ethanol suspension was poured into the mixing tank of the kneader for kneading at 60 rpm for 6 hours to obtain a paste.

[0072] S4. The mixed paste is granulated, dried in a 90℃ oven for 10 hours, and then pulverized to obtain graphene-modified matrix graphene powder with a graphene content of 0.4% (i.e., the mass of nuclear-grade graphene powder accounts for 0.4% of the total mass of nuclear-grade natural graphene powder and nuclear-grade artificial graphene powder), labeled as MG-GNP0.4%. The loose bulk density of the graphene-modified matrix graphene powder in this embodiment is 0.52 g / cm³. 3 The average particle size is 49.8 μm, and the maximum particle size is 332 μm.

[0073] Example 2

[0074] A method for preparing graphene-modified matrix graphite powder is shown below:

[0075] S1. Using a probe-type ultrasonic transducer (SCIENTZ-IID, Xinzhi Biotechnology), 75g of nuclear-grade graphene powder (specific surface area 15.8m²) was subjected to ultrasonic treatment. 2 The sample (with an average particle size of 28.5 μm, a total ash content of 15 μg / g, and a total equivalent boron content of 0.2 μg / g) was dispersed in 1000 mL of ethanol (analytical grade) and subjected to ultrasonication at room temperature with a power of 900 W for 5 h.

[0076] S2. The ultrasonically dispersed graphene-ethanol suspension was poured into 1200g of qualified nuclear-grade phenolic resin ethanol solution. The specific parameters of the phenolic resin were: solid content of 52wt%, relative molecular weight of 1050, softening point of 110℃, ash content of 32μg / g, and char content of 55wt%. An ultrasonic stirrer (FS-2000, Changsha Wanrong Powder Equipment Technology Co., Ltd.) was used to simultaneously perform ultrasonication, stirring, and heating on the graphene-phenolic resin ethanol suspension. The ultrasonic power was 600W, the stirring speed was 500rpm, the stirring time was 30min, and the heating temperature was 50℃, resulting in a graphene-phenolic resin ethanol suspension.

[0077] S3. Weigh 1925g of nuclear-grade natural graphite powder (average particle size 27.5μm, specific surface area 4.2m²). 2 The total ash content is 15.0 μg / g, the total equivalent boron content is 0.3 μg / g, and the loose bulk density is 0.35 g / cm³. 3 500g of nuclear-grade artificial graphite powder (average particle size 23.5μm, specific surface area 1.6m²). 2 / g, loose bulk density is 0.80g / cm³ 3A total of 2425g of nuclear-grade natural graphite powder and nuclear-grade artificial graphite powder were poured into the mixing tank of a kneader for dry mixing at 60 rpm for 20 minutes. Then, a graphene phenolic resin ethanol suspension was poured into the mixing tank of the kneader for kneading at 60 rpm for 6 hours to obtain a paste.

[0078] S4. The mixed paste is granulated, dried in a 90℃ oven for 10 hours, and then pulverized to obtain graphene-modified matrix graphene powder with an addition amount of 3% (i.e., the mass of nuclear-grade graphene powder accounts for 3% of the total mass of nuclear-grade natural graphene powder and nuclear-grade artificial graphene powder), labeled as MG-GNP3%. The loose packing density of the graphene-modified matrix graphene powder in this embodiment is 0.51 g / cm³. 3 The average particle size is 48.4 μm, and the maximum particle size is 325 μm.

[0079] Example 3

[0080] A method for preparing graphene-modified matrix graphite powder is shown below:

[0081] S1. Using a probe-type ultrasonic transducer (SCIENTZ-IID, Xinzhi Biotechnology), 125g of nuclear-grade graphene powder (specific surface area 15.8m²) was subjected to ultrasonic treatment. 2 The graphene ethanol suspension was obtained by dispersing graphene in 1000 mL of ethanol (analytical grade) with an average particle size of 28.5 μm, a total ash content of 15 μg / g, and a total equivalent boron content of 0.2 μg / g. The suspension was then subjected to ultrasonication at room temperature with a power of 900 W for 5 h.

[0082] S2. Pour the ultrasonically dispersed graphene-ethanol suspension into 1200g of qualified nuclear-grade phenolic resin ethanol solution. The specific parameters of the phenolic resin are: solid content of 52wt%, relative molecular weight of 1050, softening point of 110℃, ash content of 32μg / g, and char residue of 55wt%. An ultrasonic stirrer (FS-2000, Changsha Wanrong Powder Equipment Technology Co., Ltd.) is used to simultaneously perform ultrasonication, stirring, and heating on the graphene-phenolic resin ethanol suspension. The ultrasonic power is 600W, the stirring speed is 500rpm, the stirring time is 30min, and the heating temperature is 50℃.

[0083] S3. Weigh 1875g of nuclear-grade natural graphite powder (average particle size 27.5μm, specific surface area 4.2m²). 2 The total ash content is 15.0 μg / g, the total equivalent boron content is 0.3 μg / g, and the loose bulk density is 0.35 g / cm³. 3 500g of nuclear-grade artificial graphite powder (average particle size 23.5μm, specific surface area 1.6m²). 2 / g, loose bulk density is 0.80g / cm³ 3 A total of 2375g of nuclear-grade natural graphite powder and nuclear-grade artificial graphite powder were poured into the mixing tank of a kneader for dry mixing at 60 rpm for 20 minutes. Then, a graphene-phenolic resin-ethanol suspension was poured into the mixing tank of the kneader for kneading at 60 rpm for 6 hours to obtain a paste.

[0084] S4. The mixed paste is granulated, dried in a 90℃ oven for 10 hours, and then pulverized to obtain graphene-modified matrix graphene powder with a graphene content of 5% (i.e., the mass of nuclear-grade graphene powder accounts for 5% of the total mass of nuclear-grade natural graphene powder and nuclear-grade artificial graphene powder), labeled as MG-GNP5%. The loose packing density of the graphene-modified matrix graphene powder in this embodiment is 0.50 g / cm³. 3 The average particle size is 47.8 μm, and the maximum particle size is 318 μm.

[0085] Comparative Example 1

[0086] A method for preparing graphene-modified matrix graphite powder is shown below:

[0087] S1. Using a probe-type ultrasonic transducer (SCIENTZ-IID, Xinzhi Biotechnology), 150g of nuclear-grade graphene powder (specific surface area 15.8m²) was subjected to ultrasonic treatment. 2 The graphene ethanol suspension was obtained by dispersing graphene in 1000 mL of ethanol (analytical grade) with an average particle size of 28.5 μm, a total ash content of 15 μg / g, and a total equivalent boron content of 0.2 μg / g. The suspension was then subjected to ultrasonication at room temperature with a power of 900 W for 5 h.

[0088] S2. Pour the ultrasonically dispersed graphene-ethanol suspension into 1200g of qualified nuclear-grade phenolic resin ethanol solution. The specific parameters of the phenolic resin are: solid content of 52wt%, relative molecular weight of 1050, softening point of 110℃, ash content of 32μg / g, and char content of 55wt%. An ultrasonic stirrer (FS-2000, Changsha Wanrong Powder Equipment Technology Co., Ltd.) is used to simultaneously perform ultrasonication, stirring, and heating on the graphene-phenolic resin ethanol suspension. The ultrasonic power is 600W, the stirring speed is 500rpm, the stirring time is 30min, and the heating temperature is 50℃.

[0089] S3. Weigh 1850g of nuclear-grade natural graphite powder (average particle size 27.5μm, specific surface area 4.2m²). 2 The total ash content is 15.0 μg / g, the total equivalent boron content is 0.3 μg / g, and the loose bulk density is 0.35 g / cm³. 3500g of nuclear-grade artificial graphite powder (average particle size 23.5μm, specific surface area 1.6m²). 2 / g, loose bulk density is 0.80g / cm³ 3 A total of 2375g of nuclear-grade natural graphite powder and nuclear-grade artificial graphite powder were poured into the mixing tank of a kneader for dry mixing at 60 rpm for 20 minutes. Then, a graphene phenolic resin ethanol suspension was poured into the mixing tank of the kneader for kneading at 60 rpm for 6 hours to obtain a paste.

[0090] S4. The mixed paste was granulated, dried in a 90℃ oven for 10 hours, and then pulverized to obtain graphene-modified matrix graphene powder with a graphene content of 6% (i.e., the mass of nuclear-grade graphene powder accounts for 6% of the total mass of nuclear-grade natural graphene powder and nuclear-grade artificial graphene powder), labeled as MG-GNP6%. The loose bulk density of the graphene-modified matrix graphene powder in this comparative example is 0.49 g / cm³. 3 The average particle size is 46.9 μm, and the maximum particle size is 309 μm.

[0091] Comparative Example 2

[0092] For comparison, a matrix graphite powder without added graphene was prepared, namely the matrix graphite powder used in HTR-PM fuel elements in the prior art.

[0093] The preparation method of the matrix graphite powder is as follows:

[0094] S1. Take 2000g of nuclear-grade natural graphite powder (average particle size 27.5μm, specific surface area 4.2m²). 2 The total ash content is 15.0 μg / g, the total equivalent boron content is 0.3 μg / g, and the loose bulk density is 0.35 g / cm³. 3 500g of nuclear-grade artificial graphite powder (average particle size 23.5μm, specific surface area 1.6m²). 2 / g, loose bulk density is 0.80g / cm³ 3 1200g of qualified nuclear-grade phenolic resin ethanol solution was poured into the mixing tank of a kneader and kneaded at a speed of 60 rpm for 6 hours to obtain a paste.

[0095] S2. The mixed paste was granulated, dried in an oven at 90℃ for 10 hours, and then pulverized using a pulverizer to obtain the matrix graphite powder of this comparative example, labeled as MGC. The loose packing density of the matrix graphite powder of this comparative example was 0.52 g / cm³. 3 The average particle size is 50.2 μm, and the maximum particle size is 345 μm.

[0096] Performance testing

[0097] 1. The performance of the matrix graphite powders MG-GNP0.4%, MG-GNP3%, MG-GNP5%, MG-GNP6%, and MGC was tested. The test results are shown in Table 1.

[0098] Table 1. Performance test results of the matrix graphite powder in each embodiment and comparative example

[0099]

[0100] As shown in Table 1, with the increase of graphene content from 0.4% to 5%, the loose density, average particle size, and maximum particle size of the matrix graphite powder decreased. When the graphene content reached 6%, the loose density of the matrix graphite powder was only 0.49 g / cm³. 3 The loose packing density of the matrix graphite powder used in HTR-PM fuel elements does not meet the requirement of ≥0.5 g / cm³. 3 Technical requirements.

[0101] 2. Since the mass of the matrix graphite accounts for more than 95% of the total mass of the spherical fuel element, matrix graphite spheres without coated fuel particles are usually used instead of spherical fuel elements for performance testing. Matrix graphite powders obtained in the various embodiments and comparative examples were used to prepare matrix graphite spheres for performance verification. The preparation process of the matrix graphite spheres mainly includes core sphere pre-compression, fuel-free zone preparation, final compression, heat treatment, and turning. A schematic diagram of the preparation process is shown below. Figure 2 The specific conditions for each process are as follows:

[0102] (1) Core ball pre-compression: The matrix graphite powder is filled into the special rubber mold for core ball compression and pre-compressed according to the compression conditions of 2.5MPa pre-compression pressure and 20s holding time.

[0103] (2) Preparation of fuel-free zone: In another larger-sized special rubber mold, a certain amount of matrix graphite powder is first added to the lower half of the mold, then the core ball is placed into the mold, and after the upper half of the mold is closed, matrix graphite powder is continued to be filled into the mold until the powder fills the mold.

[0104] (3) Final pressing: The cold isostatic pressing or isostatic pressing process is adopted, the pressing pressure is 300MPa, the holding time is 5min, and the matrix graphite sphere green blank is obtained by pressing.

[0105] (4) Heat treatment: The heat treatment includes two stages: carbonization and purification. Carbonization is carried out under an argon atmosphere, with a heating rate of 2℃ / min, a carbonization temperature of 800℃, and a holding time of 1h. Carbonization pyrolyzes the phenolic resin into glassy carbon, providing good adhesion. Purification is carried out under an argon atmosphere, with a heating rate of 5℃ / min, a purification temperature of 1900℃, and a holding time of 1h, followed by cooling to room temperature.

[0106] (5) Turning: The heat-treated matrix graphite spheres are turned on a lathe. The diameter of the matrix graphite spheres after turning is in the range of 59.5-60.5mm.

[0107] Graphite spheres prepared using graphene-modified matrix graphite powders MG-GNP0.4%, MG-GNP3%, MG-GNP5%, and MG-GNP6% were labeled MGC-GNP0.4%, MGC-GNP3%, MGC-GNP5%, and MGC-GNP6%, respectively. For comparison, matrix graphite spheres were prepared using MG matrix graphite powder with the same preparation process and labeled MGC.

[0108] The average density, thermal conductivity, crushing strength, wear rate, linear thermal expansion coefficient, and thermal expansion anisotropy of the matrix graphite spheres were tested using the ET / J 20236 test method for spherical fuel elements in high-temperature gas-cooled reactors. The test results are shown in Tables 2 and 3. The thermal conductivity and crushing strength of the matrix graphite spheres are both anisotropic, where the AX direction represents the axial direction and the TR direction represents the equatorial direction. The total ash content and total equivalent boron content of the matrix graphite spheres were tested using the test procedures for major projects of high-temperature gas-cooled reactor nuclear power plants. The test results are shown in Table 2.

[0109] Table 2. Performance of matrix graphite spheres prepared from matrix graphite powder in each embodiment and comparative example

[0110]

[0111] It should be noted that the high-temperature gas-cooled reactor (HTGR) has just entered the formal commercial operation stage, while the ultra-high-temperature gas-cooled reactor (UHTGR) represents the future development direction of HTGRs, and a relevant technical indicator system has not yet been established. Therefore, this invention still uses the technical indicators of the HTGR demonstration project HTR-PM.

[0112] Table 2 shows that the properties of the MGC-GNP 0.4%, MGC-GNP 3%, and MGC-GNP 5% matrix graphite spheres all meet the technical requirements of HTR-PM. However, when the graphene content reaches 6%, the average density of the matrix graphite spheres is lower than the technical requirement of 1.70 g / cm³. 3 Furthermore, performance deterioration occurs, with reduced thermal conductivity and crushing strength. This is mainly due to the excessive amount of graphene added, which leads to a decrease in dispersion. The agglomeration of graphene easily causes stress concentration and reduces the continuity of the thermal conductive network.

[0113] Adding graphene helps reduce the wear rate of the matrix graphite spheres; when the graphene addition amount is 3%, the wear rate of the matrix graphite spheres is reduced by 25%. Furthermore, the total ash content and total equivalent boron content of the matrix graphite spheres in all embodiments and comparative examples meet the technical requirements for HTR-PM matrix graphite spheres.

[0114] Table 3. Coefficient of thermal expansion and degree of thermal expansion anisotropy of the matrix graphite spheres

[0115]

[0116] Table 3 shows the linear coefficient of thermal expansion and the degree of thermal expansion anisotropy of the matrix graphite spheres in each embodiment and comparative example. The technical requirements for HTR-PM matrix graphite spheres stipulate that the degree of thermal expansion anisotropy should not exceed 1.3. It is evident that the matrix graphite spheres prepared in each embodiment and comparative example meet these requirements. When the graphene content reaches 6%, the degree of thermal expansion anisotropy reaches 1.19, which is higher than that of other embodiments and comparative examples.

[0117] In summary, the performance of the graphene-modified matrix graphite spheres prepared from the graphene-modified matrix graphite powder is superior to that of the unmodified matrix graphite spheres. The amount of graphene added in the graphene-modified matrix graphite powder should not exceed 5%.

[0118] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0119] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A matrix graphite powder for fuel elements in high-temperature / ultra-high-temperature gas-cooled reactors, characterized in that, The raw materials include nuclear-grade graphene powder, nuclear-grade natural graphite powder, nuclear-grade artificial graphite powder, nuclear-pure phenolic resin ethanol solution, and ethanol, wherein the mass percentage of the nuclear-grade graphene powder to the sum of the masses of the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder is 0.2-1%; The method for preparing the matrix graphite powder for the fuel elements of the high-temperature / ultra-high-temperature gas-cooled reactor includes the following steps: S1. The nuclear-grade graphene powder and ethanol are mixed and then subjected to ultrasonic treatment to obtain a graphene-ethanol suspension; S2. The graphene ethanol suspension and the nuclear-grade phenolic resin ethanol solution are mixed and then subjected to ultrasonic stirring to obtain a graphene phenolic resin ethanol suspension. S3. The nuclear-grade natural graphite powder, the nuclear-grade artificial graphite powder, and the graphene phenolic resin ethanol suspension are mixed and kneaded to obtain a paste. S4. The paste is subjected to granulation, drying and pulverization to obtain the matrix graphite powder.

2. The matrix graphite powder according to claim 1, characterized in that, The mass percentage of the nuclear-grade graphene powder to the sum of the masses of the nuclear-grade natural graphite powder and the nuclear-grade artificial graphite powder is 0.4-1%.

3. The matrix graphite powder according to claim 1 or 2, characterized in that, With the total mass of the nuclear-grade graphene powder, the nuclear-grade natural graphite powder, the nuclear-grade artificial graphite powder, and the phenolic resin in the nuclear-pure phenolic resin ethanol solution being 100%, the sum of the mass fractions of the nuclear-grade graphene powder and the nuclear-grade natural graphite powder is 64%, the mass fraction of the nuclear-grade artificial graphite powder is 16%, and the mass fraction of the phenolic resin in the nuclear-pure phenolic resin ethanol solution is 20%.

4. The matrix graphite powder according to claim 1 or 2, characterized in that, The specific surface area of ​​the nuclear-grade graphene powder is 10-30 m². 2 / g, with an average particle size of 20-50μm, total ash content ≤50μg / g, and total equivalent boron content ≤1μg / g.

5. The matrix graphite powder according to claim 1 or 2, characterized in that, The loose packing density of the matrix graphite powder is 0.50-0.55 g / cm³. 3 The average particle size is 40-80 μm, and the maximum particle size is ≤350 μm.

6. The matrix graphite powder according to claim 1, characterized in that, The ultrasonic power of the ultrasonic treatment is 500-1500w, and the ultrasonic time is 2-5h.

7. The matrix graphite powder according to claim 6, characterized in that, The ultrasonic stirring treatment lasts for 0.5-2 hours, and the stirring speed is 100-800 rpm.

8. The matrix graphite powder according to claim 7, characterized in that, The temperature of the ultrasonic stirring treatment is 40-60℃.

9. A matrix graphite for fuel elements in high-temperature / ultra-high-temperature gas-cooled reactors, characterized in that, It is obtained by pressing, heat treatment and turning of the matrix graphite powder as described in any one of claims 1-8.

10. A high-temperature / ultra-high-temperature gas-cooled reactor fuel element, characterized in that, The fuel region and the fuel-free region of the fuel element comprise the matrix graphite as described in claim 9.

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