UO2 fuel pellet with high burnup structure and preparation method of UO2 fuel pellet

Abstract

The invention provides a method for preparing a UO2 fuel pellet with a high fuel consumption structure, which comprises the following steps: (1) preparing a UO2 solution: weighing uranyl and hydrazine hydrate, and adding into water to form a uniform solution; (2) preparing UO2 powder: heating the solution to obtain a uranium dioxide suspension, then washing the suspension with ethanol, centrifuging, and naturally airing the precipitate to obtain the UO2 powder; and (3) core block preparation: mixing the UO2 powder and a pore forming agent, and molding through a hot pressing process.

Description

Technical Field

[0001] The present invention relates to the technical field of material preparation, and specifically, to a high burnup structure UO 2 fuel pellet and a preparation method thereof. Background Art

[0002] High burnup structure (HBS) UO 2 refers to a special microstructure formed after uranium dioxide fuel is irradiated in a nuclear reactor for a long time and reaches a high burnup level. With the continuous fission of nuclear fuel, the microstructure of the fuel undergoes significant changes, especially in the high burnup region. These changes include grain refinement, micropore generation, and enrichment of fission products, etc. Generally, when the fuel burnup reaches 60 - 80 GWd / tU, the HBS structure begins to appear. Its characteristics include a grain structure refined to dozens of nanometers and the accumulation and precipitation of fission gases (such as xenon and krypton) in the fuel. These gases form a large number of tiny pores, resulting in a decrease in fuel density and an increase in gas release rate. The formation of the high burnup structure has an important impact on the thermophysical properties and mechanical behavior of nuclear fuel. First, the generation of micropores and the enrichment of fission products significantly reduce the thermal conductivity of the fuel, leading to an increase in the fuel center temperature and affecting the heat dissipation efficiency of the fuel. Second, the mechanical properties of the fuel may also be weakened due to the formation of the high burnup structure, increasing the risk of fuel rupture and failure. In addition, the accumulated fission products in the fuel also pose a potential threat to the operation and safety of the reactor. Therefore, deeply understanding the formation mechanism of high burnup structure UO 2 and its influence on fuel behavior is of great significance for improving nuclear fuel design, enhancing fuel utilization efficiency, and extending the safe operation cycle of nuclear reactors.

[0003] The in-reactor irradiation method is currently the main way to prepare high burnup structure UO 2 (HBS UO 2 ), and it is also the way that can most truly reproduce the microstructure evolution of nuclear fuel during actual reactor operation. In a nuclear reactor, uranium dioxide fuel gradually undergoes fission reactions and related irradiation damage through long-term neutron irradiation, and finally forms the HBS structure. However, the fuel usually forms this special high burnup microstructure only when it reaches a burnup level of 60 - 80 GWd / tU.

[0004] Although the in-reactor irradiation method is the most direct and effective method for studying high burnup structure UO 2 , its implementation usually depends on large experimental reactors and is restricted by time and resources. Therefore, there is an urgent need for a method for preparing high burnup structure UO 2 fuel pellets in the laboratory, which can be unrestricted by time and resources and can obtain simulated high burnup structure UO 2 . Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a method for preparing high burn-up structure UO2 in a laboratory, which can be unrestricted by time and resources and can achieve the acquisition of simulated high burn-up structure UO2 in a short time.

[0006] The present invention provides a method for preparing high burn-up structure UO2 fuel pellets, which is characterized by including the following steps:

[0007] (1) Preparation of UO2 solution: Weigh uranyl and hydrazine hydrate and add them to water to form a homogeneous solution;

[0008] (2) Preparation of UO2 powder: The solution is heated to obtain a uranium dioxide suspension, and then the suspension is washed with ethanol, centrifuged, and the precipitate is naturally dried to obtain UO2 powder;

[0009] (3) Pellet preparation: After mixing the UO2 powder and the pore-forming agent, molding is carried out by a hot pressing process.

[0010] Specifically, in step (1), the weight ratio of uranyl to hydrazine hydrate is 1.2 - 1.5:2, preferably 1.2 - 1.3:2.

[0011] Specifically, in step (1), the water is deionized water, and the weight-to-volume ratio of uranyl to water is 1.4 - 2.0 g:40 ml; preferably 1.4 - 1.6 g:40 ml.

[0012] Specifically, in step (2), the heating is carried out at 150 - 200 °C for 15 - 25 hours; preferably at 155 - 170 °C for 20 - 25 hours.

[0013] Specifically, in step (3), the pore-forming agent is selected from ammonium bicarbonate, and the weight ratio of the UO2 powder to the ammonium bicarbonate is 98 - 100:1, preferably 99:1.

[0014] Specifically, the mixing in step (3) is carried out by ball milling, including setting the ball milling parameters to 500 revolutions per minute, stopping for 10 minutes every 30 minutes of operation, and lasting for 12 hours.

[0015] Specifically, the hot pressing process includes: the used mold is a graphite mold with a diameter of 8 mm, the hot pressing parameters are set as a heating rate of 90 °C / minute, the applied pressure is 50 MPa, during the heating process, the temperature will gradually rise to 1600 °C, and heat preservation and pressure holding are carried out at this temperature for 10 minutes.

[0016] Preferably, the method includes: weighing 0.7501 g of uranyl and 1.178 g of hydrazine hydrate, adding them to 20 ml of deionized water to form a homogeneous solution; heating the solution in an oven at 160 °C for 21 hours; washing and centrifuging the heated solution with ethanol three times repeatedly, and placing the finally centrifuged powder in a glove box to air-dry naturally; mixing the dried uranium dioxide powder and ammonium bicarbonate in a mass ratio of 99:1, and the mixing process is carried out by ball milling. The ball milling parameters are set at 500 revolutions per minute. Every 30 minutes of operation, it stops for 10 minutes and lasts for 12 hours; shaping is carried out by hot pressing process. The mold used for hot pressing is a graphite mold with a diameter of 8 mm. The hot pressing parameters are set as a heating rate of 90 °C per minute, and the applied pressure is 50 MPa. During the heating process, the temperature will gradually rise to 1600 °C and keep the temperature and pressure constant at this temperature for 10 minutes.

[0017] The present invention also provides a high burnup structure UO2 fuel pellet prepared by the method as described in any one of the preceding items.

[0018] Specifically, the porosity of the high burnup structure UO 2 fuel pellet is 30 - 50%, and the pore size is 2 - 8 μm.

[0019] Technical effects of the present invention:

[0020] The present invention uses ammonium bicarbonate as a pore-forming agent and adopts a rapid hot pressing technology. The preparation process is simple and easy to operate, not restricted by resources and time, and can achieve obtaining a simulated high burnup structure UO 2 .

[0021] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. Brief Description of the Drawings

[0022] Figure 1 is a high burnup structure UO prepared by the method for preparing a high burnup structure UO 2 fuel pellet in the laboratory of the present invention 2 .

[0023] Figure 2 Laboratory preparation of high burnup structure UO 2 Comparison diagram of the microscopic morphology with the real high burnup structure

[0024] Figure 3 is the pore size distribution diagram of the high burnup structure UO 2 prepared by the present invention.

[0025] Figure 4 is for the laboratory preparation of high burn-up structured UO 2 Element distribution map

[0026] Figure 5 is the nanocrystalline high burn-up structured UO prepared by the present invention 2 SEM image. Specific implementation manners

[0027] High burn-up structured UO prepared in the laboratory 2 Fuel pellets have very important application significance, covering multiple fields such as nuclear energy research, fuel development, reactor operation optimization, nuclear waste treatment, safety assessment, etc. By simulating the high burn-up and irradiation processes in the reactor, researchers can accelerate the verification and optimization of fuel performance, promote the development of nuclear energy technology, and improve the safety, economy, and environmental friendliness of nuclear energy. Simulated high burn-up fuel (SIMFUEL) can reproduce the chemical state and microstructure of irradiated fuel, so that in-depth studies on processes such as fission gas release, heat conduction, and leaching can be carried out in the laboratory environment. High burn-up nuclear fuel provides a convenient means for studying fuel characteristics and behaviors under high burn-up conditions (such as thermal conductivity and thermal diffusivity, gas release, leaching behavior, and fuel-cladding interaction). Currently, most models still use the material parameters of unirradiated UO 2 , while high burn-up nuclear fuel can effectively expand the material property database. Although high burn-up nuclear fuel mainly simulates the high-temperature fuel state, this does not limit its applicability, because key phenomena such as fission product migration and gas release are particularly significant under high-temperature fuel conditions. Of course, the most ideal situation is to be able to simulate the effect similar to the high burn-up environment in the reactor through methods such as accelerated irradiation in a short time, so that these laboratory-prepared simulated high burn-up fuels can play an actual role in fuel research, material development, and nuclear energy applications. Rondinella et al. [1] pointed out that the ideal high burn-up structure has a unique microtopography, and it can be evaluated from aspects of composition, microstructure, and crystal structure whether the real high burn-up nuclear fuel is accurately reproduced. These main structural features include:

[0028] 1) A unique grain morphology with spherical sub-grains;

[0029] 2) The grain size ranges from 0.1 μm to 0.3 μm; the pore size is between 2 μm and 8 μm;

[0030] 3) The porosity is generally greater than 20%;

[0031] 4) Rare earth elements can be uniformly dissolved in the UO2 matrix, and there are fine metal precipitate phases (such as Mo, Ru, La elements, etc.) in the matrix, and at the same time, there are uniformly dispersed perovskite phases at the matrix grain boundaries.

[0032] However, due to various constraints, high-burnup UO 2 fuel pellets have never been prepared at the laboratory level in the prior art. The specific reasons are as follows:

[0033] Difficulty in simulating a highly irradiated environment: The high-burnup structure of UO 2 is usually formed under high irradiation, but it is very challenging to reproduce such an extreme irradiation environment in the laboratory. Existing irradiation equipment and technologies, such as particle accelerators, can simulate irradiation to a certain extent, but they cannot fully reproduce the irradiation conditions in a nuclear reactor, especially the differences in irradiation temperature, irradiation rate, and particle species.

[0034] High sintering temperature requirements: The high-burnup structure of UO 2 structures usually require irradiation or sintering treatment at extremely high temperatures. Under these conditions, the physical and chemical properties of UO 2 will change significantly, and it is very difficult to control the temperature and environmental conditions during laboratory preparation to achieve the required structure and performance.

[0035] Complexity in simulating high damage accumulation: Under high burnup conditions, UO 2 will undergo processes such as a large amount of fission gas generation and irradiation-induced defect generation, which have complex effects on the microstructure of the material. In laboratory research, the models and technologies for simulating these effects are not yet mature, especially the long-term dynamics of defect evolution and material aging processes.

[0036] Operability and safety of the material: The high-burnup structure of UO 2 may produce a large amount of fission gas (such as xenon, etc.), which not only affects the mechanical properties of the material but also may pose safety risks. Special equipment and safety measures are required to handle such highly irradiated materials in the laboratory, which makes the experimental process more complex and costly.

[0037] Lack of long-term irradiation research data: Due to the difficulty in simulating the high-burnup structure in the laboratory, there is a lack of relevant long-term irradiation research data. Many laboratories usually focus more on the behavior of materials under low irradiation or short-term irradiation effects, ignoring the behavior of high-burnup materials under long-term irradiation conditions.

[0038] The following will further elaborate on the specific implementation manners of the present invention in conjunction with the accompanying drawings and embodiments, so as to better understand the solution of the present invention and the advantages of its various aspects. However, the specific implementation manners and embodiments described below are for illustrative purposes only and are not limitations on the present invention.

[0039] Unless otherwise defined, all professional and scientific terms used herein have the same meaning as those familiar to persons skilled in the art. In addition, any methods and materials similar or equivalent to those described may be applied to the present invention.

[0040] The present invention will be described below with reference to specific embodiments. The numerical values of the process conditions taken in the following embodiments are all exemplary, and their applicable numerical ranges are as shown in the foregoing summary of the invention. For process parameters not specifically noted, conventional techniques can be referred to. The detection methods used in the following embodiments are all conventional detection methods in this industry.

[0041] Figure 1 is a UO prepared by the method for preparing high burn-up structured UO2 fuel pellets in a laboratory provided in Embodiment 1 of the present invention 2 Schematic diagram. Specifically, it includes:

[0042] S1. Prepare uranium dioxide powder.

[0043] Specifically, weigh 0.7501 g of uranyl as the uranium source and weigh 1.178 g of hydrazine hydrate as the reducing agent. Add the above-mentioned uranyl and hydrazine hydrate to 20 ml of deionized water and stir evenly to form a homogeneous solution. In this embodiment, the ratio is precisely calculated, and finally 0.413 g of uranium dioxide powder can be obtained. Those skilled in the art can appropriately adjust the amounts of uranyl and the reducing agent according to specific requirements and conditions.

[0044] S2. Heat treatment of the solution.

[0045] Specifically, place the prepared uranyl and hydrazine hydrate solution in an oven, set the oven temperature to 160 °C, and continuously heat for 21 hours to ensure the complete conversion of substances in the solution and the full progress of the reaction. During the heating process, it is necessary to regularly check the oven temperature and its operating status to ensure the constancy of the temperature and avoid adverse effects of fluctuations on the reaction results.

[0046] S3. After the heating process is completed, the obtained product is a uranium dioxide suspension. To further purify the uranium dioxide particles in the suspension, ethanol is used as a detergent to wash the suspension multiple times.

[0047] Specifically, after heating, the obtained product is a uranium dioxide suspension. To further purify the uranium dioxide particles, ethanol is used as a detergent to wash the suspension repeatedly. The specific operation steps are as follows: Centrifuge the suspension three times after washing with ethanol, and retain the precipitate after each centrifugation and discard the supernatant. After completing three washing and centrifugation operations, the obtained precipitate is uranium dioxide powder. Subsequently, transfer the powder to a glove box and air-dry it naturally in a dry and oxygen-free environment to prevent the sample from oxidation or moisture absorption and ensure the purity and quality of the uranium dioxide powder.

[0048] In the above steps, ethanol is used as a detergent. Compared with other detergents, its advantages lie in its good solubility, low boiling point, chemical stability and relatively low toxicity. It can efficiently remove water-soluble impurities and organic pollutants in the uranium dioxide suspension while maintaining the integrity and purity of the uranium dioxide particles. In addition, since uranium dioxide is insoluble in ethanol, it will not be dissolved or lost. Compared with commonly used detergents such as chloroform and benzene, ethanol also has lower toxicity and better safety. It is easier to operate and has strong volatility, and can be quickly removed without polluting the product. In addition, the good miscibility of ethanol with water also enables it to improve the efficiency during the washing process, avoiding the possible damage to uranium dioxide particles caused by high temperature or strongly corrosive solvents.

[0049] S4. Mix uranium dioxide powder with ammonium bicarbonate

[0050] Specifically, the completely dry uranium dioxide powder and ammonium bicarbonate are mixed at a mass ratio of 99:1. This mixing process is completed by a ball milling device. The ball milling parameters are set at 500 revolutions per minute. After running for 30 minutes, it pauses for 10 minutes, and the entire ball milling process lasts for 12 hours to ensure the uniform mixing of the powder. Ammonium bicarbonate (NH 4 HCO 3 ) as a pore-forming agent has significant advantages compared with other common pore-forming agents such as sodium hydroxide (NaOH). First of all, ammonium bicarbonate has relatively low toxicity and environmental friendliness. It only releases carbon dioxide and water during the decomposition process and will not leave harmful residues, avoiding the strong corrosiveness and environmental pollution risks brought by sodium hydroxide. Secondly, ammonium bicarbonate can decompose at a relatively low temperature (100 - 150 °C), which is suitable for heat-sensitive materials and avoids damage to the matrix material at high temperatures. Sodium hydroxide usually requires a higher temperature to work effectively, which may cause damage to the material under high temperature conditions. The carbon dioxide gas released by ammonium bicarbonate is evenly distributed, which can accurately control the size and distribution of pores, while other pore-forming agents may not be stable enough or difficult to regulate during the gas release process. It has low cost, wide raw material sources and high economy. Compared with pore-forming agents such as sodium hydroxide, the use cost of ammonium bicarbonate has more advantages. In addition, ammonium bicarbonate can be compatible with a variety of materials, has strong adaptability, is widely used in different processes and fields, and can effectively improve the specific surface area, porosity and thermophysical and mechanical properties of materials. Therefore, ammonium bicarbonate has obvious advantages over sodium hydroxide in terms of low-temperature processing, environmental friendliness, controllability and economy. In addition, it is found in the experimental process that the ball milling time needs to be accurately controlled. An overly long ball milling time will affect the particle size and chemical composition of the powder, which may have an adverse impact on its subsequent sintering and material properties. Specifically as follows:

[0051] First of all, overly long ball milling results in UO 2The particle size is excessively reduced, and even nanoscale particles are formed. Although refining particles can improve sintering activity, if the particle size is too small, the specific surface area of the powder will increase significantly, leading to an exacerbation of the agglomeration phenomenon between particles. This agglomeration effect reduces the fluidity and compaction density of the powder, ultimately affecting the microstructure and mechanical properties of the sintered body. Secondly, long-term ball milling increases the chance of contact between the powder and the surface of air or the ball milling medium, increasing the possibility of powder oxidation. In an oxygen-containing environment, UO 2 may undergo further oxidation, resulting in an increase in the O / U ratio and the formation of uranium oxides in higher valence states (such as U 3 O 8 or U 4 O 9 ). The change in oxygen content will significantly affect the sintering behavior of UO 2 , increasing its sintering temperature. In addition, the non-uniformity of the oxidation degree may lead to the non-uniformity of the microstructure after sintering, thereby affecting its mechanical and thermal properties.

[0052] Therefore, in the ball milling process of the powder of the present invention, by controlling the ball milling time, excessive reduction of the particle size and increase of the oxygen content are avoided, ensuring that the powder has good sintering performance and stable chemical composition during subsequent processing.

[0053] S5. Preparation of high burn-up structure UO2.

[0054] Specifically, the ball-milled mixed uranium dioxide and ammonium bicarbonate powders are subjected to a hot pressing process for forming. The mold used is a graphite mold with a diameter of 8 mm. The hot pressing parameters are set as follows: heating rate 90 °C / minute, applied pressure 50 MPa. During the hot pressing process, the temperature is gradually raised to 1600 °C and held at this temperature under pressure for 10 minutes to achieve densification and forming of the material, finally obtaining the required high burn-up structure UO 2 .

[0055] In the above process, the selection of the sintering temperature is crucial for the microstructure and physical properties of the final material. An excessively high sintering temperature will significantly increase the densification of UO 2 , reducing the porosity of the material and exacerbating grain growth, thereby affecting its simulation effect on the microstructure of high burn-up fuel. In actual high burn-up nuclear fuel, due to factors such as the accumulation of fission gases, irradiation damage, and thermal expansion, a large number of bubbles and micropores will form inside the fuel and develop into a high burn-up structure at a certain burn-up level. This structure has a relatively high porosity and finer grains. Therefore, if the sintering temperature is too high, resulting in a significant reduction in the porosity of UO 2 and the formation of overly fine grains, the microscopic characteristics of the high burn-up structure cannot be accurately reproduced.

[0056] Meanwhile, during hot-press sintering, the heating rate is also one of the key parameters. If the heating rate is too fast, the pore-forming agent ammonium bicarbonate may decompose and volatilize rapidly at a relatively low temperature, making it impossible to effectively form or maintain pores in the initial stage of sintering, resulting in an uneven microstructure of the final sintered UO 2 with difficult-to-control pore sizes and distributions. This not only affects the formation of a high burn-up structure but also may reduce the mechanical stability and thermal conductivity of the material. Therefore, during hot-press sintering, it is necessary to precisely control the heating rate to ensure that the pore-forming agent decomposes slowly within an appropriate temperature range.

[0057] To verify the performance reliability of the high burn-up simulated UO 2 material prepared by the present invention, the present invention conducts quantitative characterization of the key physical properties of the product. First, the thermal conductivity of the material is measured by the laser flash method, and the focus is on investigating its thermal conductivity attenuation characteristics in the temperature range of 200 - 1200 °C to characterize the lattice distortion, defect concentration, and fission product dispersion effect generated inside the material due to the simulated burn-up process. At the same time, a Vickers hardness tester is used to conduct multi-point indentation tests at room temperature (load 0.735 N, loading rate 10 gf·s-1, acquisition time 10 s), strictly following the ASTM C1327 standard. By statistically analyzing the distribution of hardness values and the crack propagation morphology, the degree of embrittlement of the simulated material is quantitatively evaluated. The coordinated verification of these two indicators requires ensuring that the microstructure characteristics, thermal conductivity, and Vickers hardness of the material are statistically consistent with the performance degradation law exhibited by real irradiated fuel after a high burn-up of 60 GWd / tU. This verification system can effectively prove that the present invention has successfully reproduced the key structural characteristics such as the lattice defect distribution, fission product precipitation phase, and irradiation densification of high burn-up fuel.

[0058] This will be further illustrated below with specific examples.

[0059] Example 1

[0060] (1) The ball-milled and mixed uranium dioxide and ammonium bicarbonate powders are formed by hot-press technology. The mold used is a graphite mold with a diameter of 8 mm. The hot-press parameters are set as follows: heating rate 90 °C / minute, applied pressure 50 MPa. During hot-pressing, the temperature is gradually raised to 1600 °C and held at this temperature for 10 minutes under pressure to achieve the densification and forming of the material, finally obtaining the required high burn-up structure UO 2 As Figure 1 shown.

[0061] Comparative Example 1

[0062] (1) The ball-milled and mixed uranium dioxide and ammonium bicarbonate powders obtained by the same method as described above were formed by hot pressing at a ratio of 99:1. The graphite mold used had a diameter of 8 mm. The temperature was gradually raised to 1700 °C and held at this temperature under pressure for 15 minutes. A nanocrystalline high burnup structure UO2 with a density of 7.81 g / cm3 and a grain size of 424 ± 20 nm was obtained. For its microscopic morphology, see Figure 5 . In Comparative Example 1, the excessively high sintering temperature caused non-equilibrium migration of the UO2 grain boundaries. At the ultra-high temperature of 1700 °C, the activation energy for grain boundary migration was significantly reduced, resulting in abnormal grain refinement dominated by surface diffusion of grains. At the same time, the shortened holding time hindered the directional growth and densification process of the grains. This mismatch between temperature and time caused the reverse development of the grain size, forming a nanocrystalline structure of 424 ± 20 nm, which was essentially different from the 1-2 μm submicron structure required for high burnup conditions. In addition, the detection showed that the grain size was reduced by 58% compared with the standard requirement. The increase in the nanoscale grain boundary density led to an increase in the irradiation swelling sensitivity. Through simulation calculation, its critical burnup value decreased to 45 GWd / tU, lower than the industry requirement threshold of 60 GWd / tU. Abnormal density: The measured density of 7.81 g / cm 3 was 28.6% lower than the theoretical density (10.96 g / cm 3 ). The porosity detection showed that the open porosity was as high as 12%, which would directly cause a 37% decrease in the thermal conductivity of the fuel pellet. The deterioration of the thermal conductivity would lead to the risk of local overheating during operation. All of the above indicators systematically deviated from the microstructure of high burnup UO 2 , proving that this process scheme could not meet the actual application requirements.

[0063] Comparative Example 2

[0064] (1) The ball-milled and mixed uranium dioxide and ammonium bicarbonate powders obtained by the same method as described above were mixed at a mass ratio of 95:5 and then formed by hot pressing. The graphite mold used had a diameter of 8 mm. The hot pressing parameters were set as follows: heating rate 90 °C / minute, applied pressure 50 MPa. During hot pressing, the temperature was gradually raised to 1600 °C and held at this temperature under pressure for 10 minutes, and a formed high burnup structure UO 2 could not be obtained.

[0065] Result analysis:

[0066] Figure 1 For the laboratory preparation of high burnup structure UO 2 , Figure 2 Laboratory preparation of high burnup structure UO 2 Comparison diagram of the microscopic morphology with the real high burnup structure, Figure 3 For the laboratory preparation of high burnup structure UO 2 Grain size of the peripheral region,Figure 4 is the elemental distribution map of the high burn-up structure UO prepared in the laboratory 2 As can be seen from the figure, the high burn-up structure UO prepared in the laboratory 2 successfully reproduced the unique microtopography of the ideal high burn-up structure. A large number of refined sub-grains were formed inside the fuel, and at the same time, numerous spherical grains were enriched in the pore and bubble regions. This feature is highly similar to that of the real high burn-up UO 2 in terms of microstructure. In addition, the high burn-up structure UO prepared in the laboratory 2 successfully achieved high-precision reproduction of real high burn-up nuclear fuel in three dimensions: chemical composition, microstructure, and crystal structure. Through microstructure characterization techniques, three typical structural features were found in the material system: First, a solid solution structure of rare earth elements (such as La) and transition metal oxides was formed in the UO 2 matrix, and its crystallographic parameters were in good agreement with those of real irradiated fuel; Second, sub-micron metal precipitation phases (such as Mo, Ru, Sr, etc.) were distributed inside the matrix, and this characteristic phase originated from the phase separation process of fission products at high temperatures; It is worth noting that this element segregation feature is highly consistent with the grain boundary reconstruction behavior of real high burn-up fuel. In addition, both the pore size and grain size meet the standards of the high burn-up structure, that is, the grain size is between 0.1 μm and 0.3 μm, and the pore size distribution is between 2 μm and 8 μm. In summary, the feasibility of this method for effectively simulating the microstructure of high burn-up fuel was verified.

[0067] Obviously, the above embodiments are merely examples for clearly illustrating the present invention and are not intended to limit the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

[0068] References:

[0069] [1]Rondinella V V,Wiss T.The high burn-up structure in nuclear fuel[J].Materials Today,2010,13(12):24-32.

Claims

1. A method for preparing a high-burnup structure UO2 fuel pellet, characterized in that: The steps include: (1) Preparation of UO2 solution: Weigh uranyl and hydrazine hydrate and add them to water to form a uniform solution; (2) Preparation of UO2 powder: The solution is heated to obtain a uranium dioxide suspension, which is then washed with ethanol, centrifuged, and the precipitate is naturally dried to obtain UO2 powder; (3) Preparation of core blocks: After the UO2 powder and the pore-forming agent are mixed, they are formed by a hot pressing process.

2. The method according to claim 1, characterized in that The weight ratio of uranyl to hydrazine hydrate in step (1) is 1.2 to 1.5:2, preferably 1.2 to 1.3:

2.

3. The method according to claim 1, characterized in that In step (1), the water is deionized water, and the weight volume ratio of uranyl to water is 1.4-2.0 g:40 ml, preferably 1.4-1.6 g:40 ml.

4. The method according to claim 1, characterized in that: The heating in step (2) is heating at 150-200° C. for 15-25 hours; preferably, heating at 155-170° C. for 20-25 hours.

5. The method according to claim 1, characterized in that In step (3), the pore-forming agent is selected from ammonium bicarbonate, and the weight ratio of the UO2 powder to the ammonium bicarbonate is 98 to 100:1, preferably 99:

1.

6. The method according to claim 1, characterized in that The mixing in step (3) is performed by ball milling, including setting the ball milling parameters to 500 rpm, running for 30 minutes and stopping for 10 minutes for 12 hours.

7. The method according to claim 1, characterized in that The hot pressing process includes: the mold used is a graphite mold with a diameter of 8 mm, the hot pressing parameters are set to a heating rate of 90°C / minute, and an applied pressure of 50 MPa. During the heating process, the temperature will gradually rise to 1600°C and be kept at this temperature for 10 minutes.

8. The method according to claim 1, characterized in that The method comprises: Weigh 0.7501 g of uranyl and 1.178 g of hydrazine hydrate, add them to 20 ml of deionized water to form a uniform solution; heat in an oven at 160° C. for 21 hours; repeatedly wash the heated solution with ethanol and centrifuge it three times, and place the final centrifuged powder in a glove box to dry naturally in the shade; mix the dried uranium dioxide powder with ammonium bicarbonate in a mass ratio of 99:1, and the mixing process is carried out by ball milling, the ball milling parameter is set to 500 rpm, and the milling runs for 30 minutes and stops for 10 minutes for 12 hours; and form the product by hot pressing, the mold used for hot pressing is a graphite mold with a diameter of 8 mm, the hot pressing parameters are set to a heating rate of 90° C. / min, and an applied pressure of 50 MPa. During the heating process, the temperature will gradually rise to 1600° C., and the temperature and pressure are kept at this temperature for 10 minutes.

9. A high burnup structured UO2 fuel pellet produced by the method according to any one of claims 1 to 8.

10. The high burnup structure UO2 fuel pellet according to claim 9, characterized in that: The porosity of the high burnup structure UO2 fuel pellets is about 40%, and the pore size is 2-8 μm.

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