A method of manufacturing a spherical fuel element

Abstract

The application relates to a preparation method of a spherical fuel element, which comprises the following steps: (1) pressing base graphite powder and clad fuel particles into a spherical fuel element ball blank; (2) turning the spherical fuel element ball blank into an ellipsoid; and (3) performing low-temperature carbonization and high-temperature purification on the ellipsoid-type fuel element ball blank obtained after turning; wherein the base graphite powder has specific parameters, and preferably, the low-temperature carbonization and high-temperature purification are continuously heat-treated in the same heating equipment. According to the application, the recycling of the base graphite can be realized by changing the turning and low-temperature carbonization sequence; in addition, the preparation method of the spherical fuel element according to the application can realize the continuous low-temperature carbonization and high-temperature purification in the same equipment, reduces material transportation, greatly shortens the heat treatment time, and significantly improves the production efficiency.

Description

Technical Field

[0001] This invention relates to the field of nuclear fuel preparation, and more specifically to a method for preparing spherical fuel elements. Background Technology

[0002] The spherical fuel elements used in my country's pebble bed high-temperature gas-cooled reactors have a diameter of approximately 60 mm. The interior of each element is a fuel zone, consisting of approximately 12,000 coated fuel particles (TRISO particles) with a diameter of approximately 1 mm dispersed within a graphite matrix, forming spheres approximately 50 mm in diameter. The fuel zone is surrounded by a non-fuel zone with a thickness of approximately 5 mm. In existing methods for preparing spherical fuel elements, the preforms, after being cold-pressed isostatically, require processing including low-temperature carbonization, machining, and high-temperature purification.

[0003] In this process, the graphite matrix fragments generated during the turning process are carbonized, altering their chemical composition and making them unrecyclable, thus generating a large amount of radioactive solid waste. If a process of turning first, followed by carbonization and purification is adopted, the size of the blank after turning needs to be strictly controlled so that it can meet the size requirements of spherical fuel elements after heat treatment. However, the size change of the blank during heat treatment is not only related to the known loose density and particle size distribution, but also to many other influencing factors.

[0004] Furthermore, the spherical fuel elements, after being pressed and molded in this process, require carbonization and purification. During carbonization and purification, issues arise such as separate low-temperature carbonization and high-temperature purification processes, numerous material loading and unloading operations, and low energy utilization. The heat treatment stage, a crucial production process in the preparation of spherical fuel elements, requires significant improvement in its processing methods and production efficiency. Adopting a continuous heat treatment method that directly purifies the material after carbonization without cooling could effectively solve the above problems. However, realizing this concept requires overcoming challenges related to material arrangement, temperature uniformity, the impact of positive to negative pressure conversion on the equipment, and the selection of materials such as heating elements and furnace linings.

[0005] With the development of various advanced reactor types such as high-temperature gas-cooled reactors and molten salt reactors, higher requirements have been placed on reducing solid waste emissions, lowering production costs, and improving production efficiency in the process of preparing spherical fuel elements. Therefore, it is necessary to improve and optimize the existing production processes.

[0006] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0007] Purpose of the invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing spherical fuel elements.

[0009] Solution

[0010] To achieve the objective of this invention, a method for preparing a spherical fuel element is provided, comprising the following steps:

[0011] The matrix graphite powder and coated fuel particles are pressed into spherical fuel element blanks;

[0012] The spherical fuel element blank is machined into an ellipsoid;

[0013] The ellipsoidal fuel element blanks obtained after turning are subjected to low-temperature carbonization and high-temperature purification.

[0014] The matrix graphite powder has the following parameters: resistivity, particle size distribution, loose density, tap density, and Hausner coefficient, wherein the resistivity is 40-60 mΩ·cm, the mass percentage of particles with a particle size of <32µm after air sieve sieving is 35-45%, <63µm is 50-60%, <160µm is 65-75%, <400µm is 80-85%, and <800µm is not less than 95%, and the loose density is 0.50-0.55 g / cm³. 3 The tap density is 0.75-0.85 g / cm³. 3 The Hausner coefficient is no greater than 1.55; and the ellipsoidal fuel element blank is controlled to be parallel to the pressing direction (e.g., Figure 1 The dimension in the axial direction (as shown) is 2.2%-2.5% larger than the dimension of the target fuel element in that direction, controlling the ellipsoidal fuel element blank in the direction perpendicular to the pressing direction (e.g., Figure 1 The dimension in the lateral direction (as shown) is 1.3-1.6% larger than the dimension of the target fuel element in that direction.

[0015] In one possible implementation, the difference between the size of the resulting spherical fuel element parallel to the pressing direction and the size perpendicular to the pressing direction is no more than 0.5%.

[0016] In one possible implementation, when obtaining a high-temperature gas-cooled reactor spherical fuel element with a diameter of 59.6-60.2 mm, the size of the ellipsoidal fuel element blank in the direction parallel to the pressing direction is controlled to be 61.4 mm, and the size of the ellipsoidal fuel element blank in the direction perpendicular to the pressing direction is controlled to be 60.8 mm.

[0017] In one possible implementation, the matrix graphite cut from the pressed spherical fuel element blank is collected and placed in a kneader with a small amount of alcohol added for 4 hours. The resulting extruded, dried, and pulverized matrix graphite powder has a resistivity of approximately 50-60 mΩ·cm and a tap density of approximately 0.80-0.85 g / cm³. 3 The loose bulk density is approximately 0.51-0.54 g / cm³. 3 Subsequently, the recovered matrix graphite powder was added to new matrix graphite powder and mixed thoroughly for pressing spherical fuel elements. The mass ratio of the recovered matrix graphite powder added did not exceed 20%. The main technical indicators of the resulting spherical fuel elements, such as density and crushing strength, all met the requirements.

[0018] In one possible implementation, low-temperature carbonization and high-temperature purification are carried out in the same heating equipment for continuous heat treatment. The high-low temperature continuous heat treatment furnace includes a furnace body, a heating and insulation system, a temperature control system, a loading and unloading system, a vacuum system, a waste discharge system, a process gas path system, a pneumatic system, a water path system, a tail gas combustion system, and an electrical control system.

[0019] In one possible implementation, the furnace body includes a furnace shell, a furnace door, and a furnace shell base. The furnace shell has a horizontal double-layer structure, and the furnace door is located at the front and rear of the furnace shell, with a heat-shielding door inside. The furnace body is the main supporting structure of the equipment and is equipped with a thermocouple insertion / removal mechanism, a temperature measuring device, an infrared thermometer measuring base, a temperature uniformity testing base, a pressure measuring device, an observation hole, an automatic exhaust valve, and a mechanical balance valve. The furnace door is hinged, and both the front and rear furnace doors are pneumatically driven for easy operation. The furnace shell is equipped with an automatic venting valve, a manual venting valve, and a mechanical balance valve to ensure safe and reliable operation of the equipment. The furnace shell is sealed using a special water-cooled rubber sealing ring for vacuum furnaces.

[0020] In one possible implementation, the heating and insulation system is housed within the furnace shell. The system includes a furnace liner, an insulation layer, and a heating element. A leak-proof heat-sealing layer provides a stepped connection between the furnace door and the furnace liner. The insulation layer is located on the inner wall of the furnace liner, and the heating element is disposed within the insulation layer. The stepped connection provides excellent heat insulation and allows for rapid cooling. The equipment uses water-cooled electrodes (T2 copper) to introduce electrical energy into the heating element within the furnace shell, employing a low-voltage, high-current power supply method to ensure equipment and personnel safety and high reliability.

[0021] In one possible implementation, the temperature control system includes a thermocouple for low-temperature temperature measurement and control, and an infrared thermometer for high-temperature temperature measurement and control. The infrared thermometer and thermocouple have the function of automatic switching and mutual calibration between high-temperature and low-temperature temperature measurement and control. When the infrared thermometer is contaminated while measuring high temperatures, it can be cleaned online to ensure the accuracy of temperature measurements at high temperatures. The system adopts segmented measurement: tungsten-rhenium thermocouples are used for temperature control at low temperatures (<1200℃); the infrared thermometer is used for temperature control at high temperatures (≥1200℃). When switching between infrared thermometer and thermocouple temperature measurement and control, calibration is performed between the infrared thermometer and thermocouple temperature measurement. During calibration, the furnace temperature points corresponding to the infrared temperature measurement and thermocouple temperature measurement are kept consistent. A ventilation device is provided at the infrared thermometer measurement point, and its opening and closing are controlled by a valve.

[0022] In one possible implementation, the loading and unloading system is housed within a heating and insulation system. The system includes a muffle box and supporting plates. The muffle box is located within the heating element. The supporting plates are equipped with positioning holes for spherical fuel elements, graphite column support holes, and exhaust gas discharge holes. Multiple supporting plates are placed within the muffle box, separated by graphite columns. Each supporting plate can hold multiple spherical fuel elements. The positioning holes prevent the spherical fuel elements from sticking together during low-temperature carbonization. The exhaust gas discharge holes ensure that decomposition products and volatiles from the purification process enter the tail gas combustion system through the single outlet at the bottom of the muffle box, thus achieving directional coke removal. By placing graphite columns between the supporting plates, the strength of the supporting plates is ensured, preventing excessive deformation or even collapse due to the weight of the spherical fuel elements.

[0023] In one possible implementation, the vacuum system includes a vacuum pump and a vacuum exhaust pipe, with the vacuum pump connected to the furnace shell via the vacuum exhaust pipe. The vacuum system also includes a pneumatic vacuum valve, a dust filter, a bellows, and a vacuum gauge. The vacuum exhaust pipe is equipped with a pneumatic vacuum valve to control the opening and closing of the vacuum system. The system is interlocked with the heating and insulation system to control the vacuum level inside the furnace; if the vacuum level inside the equipment does not reach the set requirement under low-temperature conditions, the heating and insulation system will not start. A quick-release vacuum leak detection interface is provided on the vacuum exhaust pipe for system leak detection.

[0024] In one possible implementation, the waste discharge system includes a low-temperature carbonization waste discharge pipeline and a high-temperature purification waste discharge pipeline. The low-temperature carbonization waste discharge pipeline includes a micro-positive pressure exhaust pipe and a micro-positive pressure exhaust valve. The micro-positive pressure exhaust pipe is connected to the bottom of the muffle furnace through a bottom opening in the furnace shell. The micro-positive pressure exhaust valve is installed on the micro-positive pressure exhaust pipe. The high-temperature purification waste discharge pipeline includes a vacuum pump and a vacuum exhaust pipe. The vacuum pump is connected to the furnace shell through the vacuum exhaust pipe. All organic products generated during the decomposition of phenolic resin during low-temperature carbonization must be discharged directionally, ensuring no residual condensate remains in the furnace. Installing a micro-positive pressure exhaust valve at the end of the micro-positive pressure exhaust pipe allows for precise adjustment and maintenance of the micro-positive pressure within the furnace.

[0025] In one possible implementation, the process gas system includes a protective gas inlet, an inlet valve, a volumetric flow meter, a micro-positive pressure exhaust pipe, and a micro-positive pressure exhaust valve. The protective gas inlet is connected to the top of the furnace shell, and the inlet valve and volumetric flow meter are installed on the protective gas inlet. The micro-positive pressure exhaust pipe is connected to the bottom of the muffle box through a bottom opening in the furnace shell, and the micro-positive pressure exhaust valve is installed on the micro-positive pressure exhaust pipe. The process gas system can achieve constant flow and constant pressure control inside the furnace. That is, the flow rate of argon gas entering the furnace is first set to be constant, and the opening degree of the micro-positive pressure exhaust valve is controlled to ensure precise control of the micro-positive pressure inside the furnace. The design of argon gas entering the muffle box needs to achieve directional coke discharge to ensure that there is no liquid accumulation in the muffle box and furnace lining. A furnace pressure detection system is provided to monitor the furnace pressure in real time. The furnace pressure is controlled under constant flow conditions. When the furnace pressure is too high or too low, the furnace pressure is kept constant by adjusting the size of the exhaust port of the micro-positive pressure exhaust pipe.

[0026] In one possible implementation, the pneumatic system includes an air source triplet, a solenoid directional valve, and air tubing. The pneumatic system provides power to the pneumatic actuators of the equipment, and this equipment employs a distributed arrangement of pneumatic control valves.

[0027] In one possible implementation, the water system is housed within a horizontal double-layer structure of the furnace shell, and is a closed-loop water inlet and return system. The water system includes main inlet and outlet pipes, temperature and pressure resistant hoses, manual valves, a pressure gauge, a flow indicator, and temperature and flow sensors. A temperature and flow sensor is used at the return end of the water-cooled electrode to monitor water flow in real time, ensuring equipment safety. Pressure and temperature sensors are installed on the main inlet pipe to detect inlet water pressure and temperature, and alarms are provided for water shortage and over-temperature. A warning time is given when low cooling water flow or pressure alarms are triggered, or when the return water temperature exceeds the warning limit. If the fault cannot be resolved within the warning time, the equipment implements protective measures, and the inlet water automatically switches to emergency water inlet. Both the cooling circulating water system and the emergency water system are equipped with electric or pneumatic three-way valves for inlet and return. When low cooling water flow or pressure alarms are triggered, or when the return water temperature exceeds the warning limit, the electric or pneumatic three-way valve on the inlet branch automatically switches to emergency water inlet, and the electric or pneumatic three-way valve on the return branch automatically switches to emergency drainage. The water system ensures that the surface temperature of the furnace shell does not exceed 60°C when the equipment is kept at 1900°C.

[0028] In one possible implementation, the exhaust gas combustion system is connected to the end of a slightly positive pressure exhaust pipe, and the exhaust gas combustion system includes a heating element and a compressed air supply path. The exhaust gas combustion system is also equipped with a temperature control system to ensure that the small organic molecules generated during the low-temperature carbonization process are fully oxidized in compressed air under different temperature conditions, transforming them into CO2 and H2O.

[0029] In one possible implementation, the electrical control system includes a programmable logic controller (PLC) and an operation panel. The PLC controls the electrical components of the entire equipment. The operation panel is a touchscreen operating system, which enables animated simulations of the entire equipment's operating status and online control of each control system on the touchscreen. Related program controls and heating regimes can be directly set on the operation panel, and experimental parameters and related data can be automatically stored and retrieved. The control unit of the electrical control system includes the controlled system, a touchscreen, and related buttons and indicator lights. An audible and visual alarm is installed above the control cabinet; the equipment emits audible and visual alarm signals when faults such as over-temperature, over-pressure, or insufficient vacuum occur.

[0030] In one possible implementation, preferably, the heating element is high-purity isostatic graphite. High-quality high-purity isostatic graphite ensures uniform heating radiation.

[0031] In one possible implementation, preferably, the muffle box is sealed with graphite cover plates on both sides. During loading, the support plate is pulled out; after loading, the support plate can be pushed back into the muffle box, with multiple support plates separated by graphite pillars. After loading is complete, the muffle box is sealed with graphite cover plates on both sides.

[0032] In one possible implementation, preferably, the ash content of the heating element, muffle box, and support plate is no higher than 100 ppm, and the ash content of the insulation layer is no higher than 1000 ppm. The heating element, muffle box, and support plate are all purified by passing halogen elements at temperatures above 2200℃ to avoid contamination of the nuclear-grade spherical fuel elements within the furnace during heat treatment.

[0033] In one possible implementation, preferably, the infrared thermometer has a fixed centering structure, which ensures accurate measurement and is not easily eccentric. The optical temperature measurement window has an anti-contamination function, and the inner surface of the lens has a cleaning device that can clean the lens surface online at high temperatures.

[0034] In one possible implementation, preferably, a cooling water jacket is provided outside the micro-positive pressure exhaust pipe, so that the waste liquid generated during the carbonization process is condensed in the micro-positive pressure exhaust pipe and collected at the bottom, which can improve the effect of directional coke discharge in the furnace.

[0035] In one possible implementation, preferably, the vacuum pump includes a mechanical pump and a Roots pump, and a differential pressure valve is provided in front of the vacuum pump to prevent vacuum pump oil from returning to the vacuum exhaust pipe when the vacuum pump suddenly stops.

[0036] The heat treatment method using the aforementioned high-low temperature continuous heat treatment furnace includes low-temperature carbonization and high-temperature purification of spherical fuel elements, with the low-temperature carbonization and high-temperature purification processes being carried out continuously in the same equipment. The heat treatment method includes the following steps:

[0037] Step 1: Place the spherical fuel element into the positioning hole on the support plate, and place multiple layers of the support plate into the muffle box. Close the loading and unloading system, close the furnace door and the heat shield door, and open the water system for cooling the furnace shell surface.

[0038] Step 2: Start the vacuum pump to evacuate the furnace shell, then fill the protective gas inlet with argon gas to a slightly positive pressure, start the heating element to heat up, and carbonize the spherical fuel element.

[0039] Step 3: The temperature is raised from room temperature to 320℃ at a rate not exceeding 1℃ / min; the temperature is raised from 320℃ to 700℃ at a rate not exceeding 0.6℃ / min; the temperature is raised from 700℃ to 800℃ at a rate not exceeding 1.5℃ / min; and the temperature is held at 800℃ for 1 hour. During the carbonization process, the exhaust gas is discharged to the tail gas combustion system through a micro-positive pressure exhaust pipe, and the waste liquid is collected after condensation in the micro-positive pressure exhaust pipe.

[0040] Step 4: Close the micro-positive pressure exhaust valve, and turn on the mechanical pump and Roots pump of the vacuum pump in sequence to continuously evacuate the furnace and continue to heat up to purify the spherical fuel element.

[0041] Step 5: Raise the temperature from 800℃ to 1500℃ at a rate not exceeding 5℃ / min; raise the temperature from 1500℃ to 1700℃ at a rate not exceeding 3℃ / min; raise the temperature from 1700℃ to 1900℃ at a rate not exceeding 1℃ / min; hold the temperature at 1900℃ for 1 hour, and then begin cooling.

[0042] Step 6: When the temperature drops below 1000℃, argon gas is introduced into the protective gas inlet. When the temperature drops below 600℃, the heat shield door is opened. When the temperature drops below 100℃, the furnace door is opened. After the temperature drops to room temperature, the graphite covers at both ends of the muffle box are opened to remove the spherical fuel element.

[0043] Preferably, the carbonization atmosphere in step three is a slightly positive pressure argon atmosphere, with constant flow and constant pressure control, argon flow rate controlled at 300-800 L / min, furnace pressure at 4-8 kPa, and carbonization time at approximately 28.5-30 hours, preferably 28.5 hours.

[0044] Preferably, the purification process in step five is carried out under vacuum conditions.

[0045] Beneficial effects

[0046] This invention enables the recycling of matrix graphite by changing the sequence of turning and low-temperature carbonization.

[0047] Because of the above technical solutions, this invention can be used not only for the continuous heat treatment of spherical fuel element blanks, but also for the preparation of matrix graphite sphere products. It realizes the continuous operation of low-temperature carbonization and high-temperature purification in the same equipment, reduces material handling, and shortens the heat treatment time of spherical fuel elements from about 90 hours of separate heat treatment to about 50 hours of continuous heat treatment, which can improve production efficiency by 44.4%. Attached Figure Description

[0048] One or more embodiments are illustrated by way of example with reference to the accompanying drawings, and these illustrative examples are not intended to limit the embodiments. The term "illustrative" as used herein means "serving as an example, embodiment, or illustration." Any embodiment illustrated herein as "illustrative" is not necessarily to be construed as superior to or better than other embodiments.

[0049] Figure 1 This is a schematic diagram illustrating the control of the dimensions of the ellipsoidal fuel element blank in parallel with the pressing direction (i.e., axial direction) and perpendicular with the pressing direction (i.e., lateral direction) when preparing spherical fuel elements according to the method of the present invention.

[0050] Figure 2 This is a flowchart of the preparation of spherical fuel elements in Embodiments 1 and 2 of the present invention;

[0051] Figure 3 This is a schematic diagram of the high and low temperature continuous heat treatment furnace described in Embodiment 3 of the present invention.

[0052] Explanation of key figure labels:

[0053] 1-Furnace shell, 2-Furnace liner, 3-Insulation layer, 4-Heating element, 5-Muffle box, 6-Material support plate, 7-Protective gas inlet, 8-Micro-positive pressure exhaust pipe, 9-Micro-positive pressure exhaust valve, 10-Tail gas combustion system, 11-Vacuum exhaust pipe, 12-Vacuum pump, 13-Furnace shell base. Detailed Implementation

[0054] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprising of," etc., will be understood to include the stated elements or components, without excluding other elements or other components.

[0055] Furthermore, to better illustrate the present invention, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that the present invention can be practiced without certain specific details. In some embodiments, materials, elements, methods, and means well known to those skilled in the art are not described in detail in order to highlight the spirit of the invention.

[0056] Example 1

[0057] In this embodiment, a spherical fuel element was prepared according to the method of the present invention (the process is as follows). Figure 2 As shown in the figure, the preparation method specifically includes the following steps:

[0058] The matrix graphite powder and coated fuel particles are pressed into spherical fuel element blanks;

[0059] The spherical fuel element blank is machined into an ellipsoid;

[0060] The ellipsoidal fuel element blanks obtained after turning are subjected to continuous heat treatment in a high-low temperature continuous heat treatment furnace for low-temperature carbonization and high-temperature purification.

[0061] The matrix graphite powder has the following parameters: resistivity 58.76 mΩ·cm, air-flow sieve particle size distribution of <32µm: 40.8% by mass, <63µm: 56.2% by mass, <160µm: 69.8% by mass, <400µm: 84.3% by mass, <800µm: 96.8% by mass, loose density 0.53 g / cm³, tapped density 0.81 g / cm³, and Hausner coefficient 1.53. Furthermore, the size of the ellipsoidal fuel element blank parallel to the pressing direction is 2.33% larger than the target fuel element's size in that direction, and the size of the ellipsoidal fuel element blank perpendicular to the pressing direction is 1.35% larger than the target fuel element's size in that direction. The resulting spherical fuel element has a size difference of 0.41% between the parallel and perpendicular pressing directions.

[0062] The matrix graphite cut from the pressed spherical fuel element blanks is collected and placed in a kneader with a small amount of alcohol. The mixture is kneaded for 4 hours, then extruded, dried, and pulverized. The pulverized matrix graphite powder is recovered (the resistivity of the recovered matrix graphite powder is approximately 50-60 mΩ·cm, and the tap density is approximately 0.80-0.85 g / cm³). 3 The loose bulk density is approximately 0.51-0.54 g / cm³. 3 The recovered matrix graphite powder was then pressed into spherical fuel elements. The main technical indicators of the resulting spherical fuel elements, such as density and crushing strength, all met the requirements.

[0063] After the furnace body cooled down naturally, the heat-treated spherical fuel elements were removed and their various performance indicators were tested, as shown in Table 1 below. All performance indicators met the technical requirements.

[0064] Table 1. Performance indicators of spherical fuel elements prepared using a method for preparing spherical fuel elements.

[0065] Performance indicators Technical Requirements Measured average Dimensions parallel to the pressing direction (mm) 59.6-60.2 60.02±0.03 Dimensions perpendicular to the pressing direction (mm) 59.6-60.2 59.96±0.04 <![CDATA[Density (g / cm 3 )]]> 1.70-1.77 1.74 Thermal conductivity (1000℃, W / mK) ≥25.0 33.4 Anisotropy (RT-500℃) ≤1.3 1.16 <![CDATA[Crushing strength (kN) a > ≥18.0 24.6 / 21.8 Number of ball drops ≥50 ≥50 Wear rate (mg / h) ≤6.0 1.48 <![CDATA[Corrosion rate (mg / cm 2 h) b > ≤1.3 0.75

[0066] a Parallel and perpendicular to the pressing direction;

[0067] b The corrosion temperature and time were 1000℃ for 10h, and the atmosphere was He + 1 vol% H2O.

[0068] Example 2

[0069] In this embodiment, a spherical fuel element was prepared according to the method of the present invention (the process is as follows). Figure 2 As shown in the figure, the preparation method specifically includes the following steps:

[0070] The matrix graphite powder and coated fuel particles are pressed into spherical fuel element blanks;

[0071] The spherical fuel element blank is machined into an ellipsoid;

[0072] The ellipsoidal fuel element blanks obtained after turning are subjected to continuous heat treatment in a high-low temperature continuous heat treatment furnace for low-temperature carbonization and high-temperature purification.

[0073] The matrix graphite powder has the following parameters: resistivity 55.68 mΩ·cm, air-flow sieve particle size distribution of <32µm (39.6% by mass), <63µm (55.3% by mass), <160µm (68.9% by mass), <400µm (83.8% by mass), <800µm (96.2% by mass), loose density 0.54 g / cm³, tapped density 0.83 g / cm³, and Hausner coefficient 1.537. Furthermore, the size of the ellipsoidal fuel element blank parallel to the pressing direction is 2.35% larger than the target fuel element's size in that direction, and the size of the ellipsoidal fuel element blank perpendicular to the pressing direction is 1.33% larger than the target fuel element's size in that direction. The resulting spherical fuel element has a size difference of 0.39% between the parallel and perpendicular pressing directions.

[0074] The matrix graphite cut from the pressed spherical fuel element blanks is collected and placed in a kneader with a small amount of alcohol. The mixture is kneaded for 4 hours, then extruded, dried, and pulverized. The pulverized matrix graphite powder is recovered (the resistivity of the recovered matrix graphite powder is approximately 50-60 mΩ·cm, and the tap density is approximately 0.80-0.85 g / cm³). 3 The loose bulk density is approximately 0.51-0.54 g / cm³. 3 The recovered matrix graphite powder was then pressed into spherical fuel elements. The main technical indicators of the resulting spherical fuel elements, such as density and crushing strength, all met the requirements.

[0075] After the furnace body cooled down naturally, the heat-treated spherical fuel elements were removed and their various performance indicators were tested, as shown in Table 2 below. All performance indicators met the technical requirements.

[0076] Table 2 shows the performance indicators of spherical fuel elements prepared using a method for preparing spherical fuel elements.

[0077] Performance indicators Technical Requirements Measured average Dimensions parallel to the pressing direction (mm) 59.6-60.2 60.01±0.03 Dimensions perpendicular to the pressing direction (mm) 59.6-60.2 59.97±0.05 <![CDATA[Density (g / cm 3 )]]> 1.70-1.77 1.75 Thermal conductivity (1000℃, W / mK) ≥25.0 34.5 Anisotropy (RT-500℃) ≤1.3 1.18 <![CDATA[Crushing strength (kN) a > ≥18.0 25.6 / 21.6 Number of ball drops ≥50 ≥50 Wear rate (mg / h) ≤6.0 1.55 <![CDATA[Corrosion rate (mg / cm 2 h) b > ≤1.3 0.72

[0078] a Parallel and perpendicular to the pressing direction;

[0079] b The corrosion temperature and time were 1000℃ for 10h, and the atmosphere was He + 1 vol% H2O.

[0080] Example 3

[0081] like Figure 3 As shown, the high and low temperature continuous heat treatment furnace used in Examples 1 and 2 includes a furnace body, a heating and heat preservation system, a temperature control system, a loading and unloading system, a vacuum system, a waste discharge system, a process gas system, a pneumatic system, a water system, a tail gas combustion system 10, and an electrical control system.

[0082] The furnace body includes a furnace shell 1, a furnace door, and a furnace shell base 13. The furnace shell 1 has a horizontal double-layer structure. The furnace door is located at the front and rear of the furnace shell 1, and a heat-shielding door is installed inside the furnace door. The furnace body is the main supporting structure of the equipment and is equipped with a thermocouple insertion / removal mechanism, a temperature measuring device, an infrared thermometer measuring base, a temperature uniformity testing base, a pressure measuring device, an observation hole, an automatic exhaust valve, and a mechanical balance valve. The furnace door is hinged and the locking of both the front and rear furnace doors is pneumatically driven for convenient operation. The furnace shell 1 is equipped with an automatic venting valve, a manual venting valve, and a mechanical balance valve to ensure safe and reliable operation of the equipment. The furnace shell 1 is sealed using a special rubber sealing ring with water cooling for vacuum furnaces.

[0083] The heating and insulation system is installed inside the furnace shell 1. This system is used to start the heating and maintain the temperature of the equipment. The system includes a furnace chamber 2, an insulation layer 3, and a heating element 4. A stepped connection with the furnace door and the furnace chamber 2 is provided by a leak-proof heat-sealing layer. The insulation layer 3 is located on the inner wall of the furnace chamber 2, and the heating element 4 is located within the insulation layer 3. The stepped connection provides good heat insulation and allows for rapid cooling. The equipment introduces electrical energy into the heating element 4 inside the furnace shell 1 via water-cooled electrodes (T2 copper). The low-voltage, high-current power supply ensures equipment and personnel safety and high reliability. The heating element 4 is made of high-purity isostatic graphite. High-quality high-purity isostatic graphite ensures uniform heating radiation.

[0084] The temperature control system includes a thermocouple for low-temperature temperature measurement and control, and an infrared thermometer for high-temperature temperature measurement and control. The infrared thermometer and thermocouple have automatic switching and mutual calibration functions for high-temperature and low-temperature temperature measurement and control. The temperature control system is used to precisely control the equipment's heating rate and target temperature. When the infrared thermometer is contaminated at high temperatures, it can be cleaned online to ensure the accuracy of temperature measurements at high temperatures. The system adopts segmented measurement: tungsten-rhenium thermocouples are used for temperature control at low temperatures (<1200℃); the infrared thermometer is used for temperature control at high temperatures (≥1200℃). When switching between infrared thermometer and thermocouple temperature measurement and control, calibration is performed, ensuring that the furnace temperature points corresponding to the infrared and thermocouple measurements remain consistent during calibration. A ventilation device is provided at the infrared thermometer measurement point, controlled by a valve. The infrared thermometer has a fixed, centered structure, ensuring accurate measurement and preventing eccentricity. The optical temperature measurement window has anti-contamination functionality, and the inner surface of the lens has a cleaning device that allows for online cleaning of the lens surface under high-temperature conditions.

[0085] The loading and unloading system is located within the heating and insulation system. The system includes a muffle box 5 and supporting plates 6. The muffle box 5 is housed within the heating element 4. The supporting plates 6 have positioning holes for spherical fuel elements, graphite column support holes, and exhaust gas discharge holes. Multiple supporting plates 6 are placed within the muffle box 5, separated by graphite columns. Each supporting plate 6 can hold multiple spherical fuel elements. The positioning holes prevent the spherical fuel elements from sticking together during low-temperature carbonization. The exhaust gas discharge holes ensure that decomposition products and volatiles from the purification process enter the tail gas combustion system 10 through the sole outlet at the bottom of the muffle box 5, thus achieving directional coke removal. Graphite columns are placed between the supporting plates 6 to ensure their strength and prevent excessive deformation or even collapse due to the weight of the spherical fuel elements. The muffle box 5 is sealed with graphite cover plates on both sides. During loading, pull out the support plate 6. After loading is complete, push the support plate 6 into the muffle box 5. After loading is complete, seal the sides of the muffle box 5 with graphite cover plates.

[0086] The ash content of the heating element 4, muffle box 5, and support plate 6 is no higher than 100 ppm, and the ash content of the insulation layer 3 is no higher than 1000 ppm. The heating element 4, muffle box 5, and support plate 6 are all purified by passing halogen elements at a temperature above 2200℃ to avoid contamination of the nuclear-grade spherical fuel elements in the furnace during the heat treatment process.

[0087] The vacuum system includes a vacuum pump 12 and a vacuum exhaust pipe 11. The vacuum pump 12 is connected to the furnace shell 1 via the vacuum exhaust pipe 11. The vacuum pump 12 includes both a mechanical pump and a Roots pump. A differential pressure valve is installed before the vacuum pump 12 to prevent vacuum pump oil from returning to the vacuum exhaust pipe 11 when the vacuum pump 12 suddenly stops. The vacuum system also includes a pneumatic vacuum valve, a dust filter, a bellows, and a vacuum gauge. The vacuum exhaust pipe 11 is equipped with a pneumatic vacuum valve to control the opening and closing of the vacuum system. The system is interlocked with the heating and insulation system to control the vacuum level inside the furnace. When the vacuum level inside the equipment does not reach the set requirement under low-temperature conditions, the heating and insulation system will not be able to start. A quick vacuum leak detection interface is provided on the vacuum exhaust pipe 11 for system leak detection.

[0088] The waste discharge system includes a low-temperature carbonization waste discharge pipeline and a high-temperature purification waste discharge pipeline. The low-temperature carbonization waste discharge pipeline includes a micro-positive pressure exhaust pipe 8 and a micro-positive pressure exhaust valve 9. The micro-positive pressure exhaust pipe 8 is connected to the bottom of the muffle box 5 through the bottom opening of the furnace shell 1. The micro-positive pressure exhaust valve 9 is installed on the micro-positive pressure exhaust pipe 8. A cooling water jacket is provided outside the micro-positive pressure exhaust pipe 8 so that the waste liquid generated during the carbonization process is condensed in the micro-positive pressure exhaust pipe and collected at the bottom, which can improve the effect of directional coke discharge in the furnace.

[0089] The high-temperature purification waste discharge pipeline includes a vacuum pump 12 and a vacuum exhaust pipe 11. The vacuum pump 12 is connected to the furnace shell 1 via the vacuum exhaust pipe 11. This prevents the tail gas from condensing and blocking the low-temperature carbonization waste discharge pipeline, thus allowing it to connect with the subsequent tail gas combustion system 10. All organic products generated during the decomposition of phenolic resin during low-temperature carbonization must be discharged in a directed manner, ensuring no residual condensate remains in the furnace. A micro-positive pressure exhaust valve 9 is installed at the end of the micro-positive pressure exhaust pipe 8, enabling precise adjustment of the micro-positive pressure inside the furnace.

[0090] The process gas system includes a protective gas inlet 7, an inlet valve, a volumetric flow meter, a micro-positive pressure exhaust pipe 8, and a micro-positive pressure exhaust valve 9. The protective gas inlet 7 is connected to the top of the furnace shell 1. An inlet valve and a volumetric flow meter are installed on the protective gas inlet 7. The micro-positive pressure exhaust pipe 8 is connected to the bottom of the muffle box 5 through a bottom opening in the furnace shell 1. The micro-positive pressure exhaust valve 9 is installed on the micro-positive pressure exhaust pipe 8. The process gas system can achieve constant flow and constant pressure control within the furnace. Specifically, the argon gas flow rate into the furnace is first set to be constant, and the opening degree of the micro-positive pressure exhaust valve 9 is controlled to ensure precise control of the micro-positive pressure within the furnace. The design for argon gas entering the muffle box 5 must achieve directional coke discharge to ensure no liquid accumulation in the muffle box 5 and the furnace liner 2. A furnace pressure detection system is provided to monitor the furnace pressure in real time. The furnace pressure is controlled under constant flow conditions. When the furnace pressure is too high or too low, the furnace pressure is kept constant by adjusting the size of the exhaust port of the micro-positive pressure exhaust pipe 8.

[0091] The process gas path system and vacuum system are designed to create an atmospheric environment that meets the process requirements.

[0092] The pneumatic system includes an air source triplet, a solenoid directional valve, and air pipes. The pneumatic system provides power to the pneumatic actuators of the equipment; this equipment employs a distributed arrangement of pneumatic control valves.

[0093] The water system is used for cooling the equipment surface. It is housed within a horizontal double-layer structure of the furnace shell 1 and features a closed-loop inlet and outlet system. The system includes main inlet and outlet pipes, temperature and pressure resistant hoses, manual valves, a pressure gauge, a flow indicator, and temperature and flow sensors. A temperature and flow sensor is used at the return end of the water-cooled electrode to monitor water flow in real time, ensuring equipment safety. Pressure and temperature sensors are installed on the main inlet pipe to detect inlet pressure and temperature, and alarms are provided for water shortage and over-temperature. A warning time is given when low cooling water flow or pressure alarms are triggered, or when the return water temperature exceeds the warning limit. If the fault cannot be resolved within the warning time, the equipment implements protective measures, and the inlet water automatically switches to emergency water supply. Both the cooling circulating water system and the emergency water system are equipped with electric or pneumatic three-way valves for inlet and return. When low cooling water flow or pressure alarms are triggered, or when the return water temperature exceeds the warning limit, the electric or pneumatic three-way valve on the inlet branch automatically switches to emergency water supply, and the electric or pneumatic three-way valve on the return branch automatically switches to emergency drainage. The water system ensures that the surface temperature of the furnace shell 1 does not exceed 60°C when the equipment is kept at 1900°C.

[0094] The exhaust gas combustion system 10 is connected to the end of the micro-positive pressure exhaust pipe 8. The exhaust gas combustion system 10 includes a heating component and a compressed air supply path. The exhaust gas combustion system 10 is also equipped with a temperature control system, which enables the small organic molecules generated during the low-temperature carbonization process to be fully oxidized in compressed air under different temperature conditions, transforming them into CO2 and H2O.

[0095] The electrical control system includes a programmable logic controller (PLC) and an operation panel. This system controls the electrical components of the entire equipment, including the touchscreen, thermocouples, gas and water valves, and pumps. The operation panel is a touchscreen operating system, enabling animated simulations of the entire equipment's operating status and online control of each control system. Related program controls and heating regimes can be directly set on the operation panel, and experimental parameters and related data can be automatically stored and retrieved. The electrical control system includes the controlled system, a touchscreen, and relevant buttons and indicator lights. An audible and visual alarm is installed above the control cabinet; it emits alarm signals when the equipment experiences faults such as over-temperature, over-pressure, or insufficient vacuum.

[0096] A heat treatment method using the aforementioned continuous heat treatment equipment for spherical fuel elements includes low-temperature carbonization and high-temperature purification of the spherical fuel elements, wherein the low-temperature carbonization and high-temperature purification processes are carried out continuously in the same equipment. The heat treatment method includes the following steps:

[0097] Step 1: Place 200 spherical fuel elements into the positioning holes on the material support plate 6, and place multiple layers of the material support plate 6 into the muffle box 5. Close the loading and unloading system, close the furnace door and the heat shield door, and open the water system for cooling the surface of the furnace shell 1.

[0098] Step 2: Start vacuum pump 12 to evacuate the furnace shell 1, then fill the protective gas inlet 7 with argon gas to a slightly positive pressure, start heating element 4 to heat up and carbonize the spherical fuel element.

[0099] Step 3: The temperature is raised from room temperature to 320℃ at a rate not exceeding 1℃ / min; from 320℃ to 700℃ at a rate not exceeding 0.6℃ / min; and from 700℃ to 800℃ at a rate not exceeding 1.5℃ / min. The temperature is then held at 800℃ for 1 hour. During the carbonization process, the waste gas is discharged to the tail gas combustion system 10 through the micro-positive pressure exhaust pipe 8, and the waste liquid is collected after condensation in the micro-positive pressure exhaust pipe 8. The carbonization atmosphere is a micro-positive pressure argon atmosphere, with constant flow and constant pressure control. The argon flow rate is controlled at 300-800 L / min, the furnace pressure is 4-8 kPa, and the carbonization time is approximately 28 hours.

[0100] Step 4: Close the micro-positive pressure exhaust valve 9, and turn on the mechanical pump and Roots pump of vacuum pump 12 in sequence to continuously evacuate the furnace and continue to heat up to purify the spherical fuel element.

[0101] Step 5: Raise the temperature from 800℃ to 1500℃ at a rate not exceeding 5℃ / min; raise the temperature from 1500℃ to 1700℃ at a rate not exceeding 3℃ / min; raise the temperature from 1700℃ to 1900℃ at a rate not exceeding 1℃ / min; hold the temperature at 1900℃ for 1 hour, then begin cooling. The purification process is carried out under vacuum conditions.

[0102] Step 6: When the temperature drops below 1000℃, argon gas is introduced into the protective gas inlet 7. When the temperature drops below 600℃, the heat shield door is opened. When the temperature drops below 100℃, the furnace door is opened. After the temperature drops to room temperature, the graphite cover plates at both ends of the muffle box 5 are opened, and the spherical fuel element is removed.

[0103] The entire continuous heat treatment process for carbonization and purification takes approximately 40 hours, plus the time required for cooling, totaling about 50 hours. No visible condensate is present inside furnace shell 1. Organic matter generated during the decomposition of phenolic resin is discharged through a directional coke discharge device, condensed, collected, and then oxidized in the tail gas combustion system 10.

[0104] Comparative Example 1:

[0105] As can be seen from Examples 1 and 2, the spherical fuel elements prepared using the method for preparing spherical fuel elements disclosed in this invention meet all relevant technical requirements. Compared with the original process of first low-temperature carbonization, then turning, and then high-temperature purification, the continuous heat treatment time for low-temperature carbonization and high-temperature purification in this method is only 50 hours, which is about 44.4% less than the approximately 90 hours of separate heat treatment in the original process. The power consumption of the entire heat treatment process is reduced from about 3000 kW*h to 1800 kW*h, saving 40% of energy. The matrix graphite collected during the turning process of a single spherical fuel element is about 25-30g, which can be directly recycled and reused for the preparation of spherical fuel elements after short-term kneading, extrusion, drying, and pulverization. Without considering the cost of recycling solid waste from the original process, if we only consider the cost of recycling the matrix graphite (approximately 200 yuan / kg), the matrix graphite collected during the machining of a single spherical fuel element costs 5-6 yuan. Based on the current high-temperature gas-cooled reactor demonstration project requiring 300,000 spherical fuel elements annually, this could save 1.5 million to 1.8 million yuan, while also reducing 7.5-9 tons of radioactive solid waste, resulting in significant economic benefits.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a spherical fuel element, comprising the following steps: (1) Press the matrix graphite powder and coated fuel particles into spherical fuel element blanks; (2) Turning the spherical fuel element blank into an ellipsoid; and (3) The ellipsoidal fuel element blanks obtained after turning are subjected to low-temperature carbonization and high-temperature purification; The matrix graphite powder has the following parameters: resistivity 40-60 mΩ·cm, air-flow sieve particle size distribution of <32µm with a mass percentage of 35-45%, <63µm with a mass percentage of 50-60%, <160µm with a mass percentage of 65-75%, <400µm with a mass percentage of 80-85%, <800µm with a mass percentage of not less than 95%, loose density 0.50-0.55 g / cm3, tapped density 0.75-0.85 g / cm3, and Hausner coefficient not greater than 1.

55. The low-temperature carbonization and the high-temperature purification are carried out in the same heating equipment for continuous heat treatment.

2. The preparation method according to claim 1, wherein, The size of the ellipsoidal fuel element blank in the direction parallel to the pressing direction is 2.2-2.5% larger than the size of the target fuel element in that direction, and the size of the ellipsoidal fuel element blank in the direction perpendicular to the pressing direction is 1.3-1.6% larger than the size of the target fuel element in that direction.

3. The preparation method according to claim 2, wherein, The difference between the dimension of the resulting spherical fuel element parallel to the pressing direction and the dimension perpendicular to the pressing direction does not exceed 0.5%.

4. The preparation method according to any one of claims 1-3, wherein, To obtain spherical fuel elements for high-temperature gas-cooled reactors with diameters of 59.6-60.2 mm, the size of the ellipsoidal fuel element blank parallel to the pressing direction is controlled to be 61.4 mm, and the size of the ellipsoidal fuel element blank perpendicular to the pressing direction is controlled to be 60.8 mm.

5. The preparation method according to any one of claims 1-3, further comprising: The matrix graphite cut off from the pressed spherical fuel element blanks is collected, placed in a kneader, alcohol is added, kneaded, extruded, dried and crushed, and the crushed matrix graphite powder is recovered. Subsequently, the recovered matrix graphite powder is added to new matrix graphite powder and mixed evenly for pressing spherical fuel elements.

6. The preparation method according to claim 5, wherein, The recovered matrix graphite powder has a resistivity of 50-60 mΩ·cm and a tap density of 0.80-0.85 g / cm³. 3 The loose bulk density is 0.51-0.54 g / cm³. 3 .

7. The preparation method according to claim 5, wherein, The mass ratio of the recycled matrix graphite powder added to the new matrix graphite powder shall not exceed 20%.

8. The preparation method according to any one of claims 1-3, wherein, The low-temperature carbonization and the high-temperature purification are carried out using a high-low temperature continuous heat treatment furnace.

9. The preparation method according to claim 8, wherein, The method for continuous heat treatment using a high-low temperature continuous heat treatment furnace includes the following steps: Step 1: Place the spherical fuel element into the positioning hole on the support plate, and place multiple layers of the support plate into the muffle box. Close the loading and unloading system, close the furnace door and the heat shield door, and open the water system for cooling the furnace shell surface. Step 2: Start the vacuum pump to evacuate the furnace shell, then fill the protective gas inlet with argon gas to a slightly positive pressure, start the heating element to heat up, and carbonize the spherical fuel element. Step 3: The temperature is raised from room temperature to 320℃ at a rate not exceeding 1℃ / min; the temperature is raised from 320℃ to 700℃ at a rate not exceeding 0.6℃ / min; the temperature is raised from 700℃ to 800℃ at a rate not exceeding 1.5℃ / min; and the temperature is held at 800℃ for 1 hour. During the carbonization process, the exhaust gas is discharged to the tail gas combustion system through a micro-positive pressure exhaust pipe, and the waste liquid is collected after condensation in the micro-positive pressure exhaust pipe. Step 4: Close the micro-positive pressure exhaust valve, and turn on the mechanical pump and Roots pump of the vacuum pump in sequence to continuously evacuate the furnace and continue to heat up to purify the spherical fuel element. Step 5: Raise the temperature from 800℃ to 1500℃ at a rate not exceeding 5℃ / min; raise the temperature from 1500℃ to 1700℃ at a rate not exceeding 3℃ / min; raise the temperature from 1700℃ to 1900℃ at a rate not exceeding 1℃ / min; hold the temperature at 1900℃ for 1 hour, and then begin cooling. Step 6: When the temperature drops below 1000℃, argon gas is introduced into the protective gas inlet. When the temperature drops below 600℃, the heat shield door is opened. When the temperature drops below 100℃, the furnace door is opened. After the temperature drops to room temperature, the graphite covers at both ends of the muffle box are opened to remove the spherical fuel element.

10. The preparation method according to claim 9, wherein, The carbonization atmosphere in step three is a slightly positive pressure argon atmosphere, with constant flow and constant pressure control. The argon flow rate is controlled at 300-800 L / min, the furnace pressure is 4-8 kPa, and the carbonization time is 28.5-30 hours.

11. The preparation method according to claim 10, wherein, The carbonization process takes 28.5 hours.

12. The preparation method according to any one of claims 9-11, wherein, The purification process in step five is carried out under vacuum conditions.

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