Large grain uo2 pellets for high burnup fuel rods and method of making same

Abstract

The application discloses a large-grain UO2 pellet for high-burnup fuel rods and a preparation method thereof. The large-grain UO2 pellet is prepared by doping Cr2O3 and Al2O3, the doping amount of Cr2O3 is 500ppm-1000ppm, and the doping amount of Al2O3 is 50ppm-200ppm. The large grain size is realized under the condition of low Cr2O3 doping amount by the synergistic effect of Cr2O3 and Al2O3, and the second phase is avoided to be precipitated. The addition of Al2O3 reduces the diffusion efficiency of fission gas under the condition of high burnup, guarantees the pinning performance of fission products, avoids the aggregation, connection and release of fission gas at the interface, improves the oxide distribution uniformity, improves the performance stability of the pellet, and meets the performance requirements of the high-burnup fuel rod in a reactor.

Description

Technical Field

[0001] The present invention relates to the technical field of UO2 ceramic fuel materials, and in particular to a large-grain UO2 pellet for high-burnup fuel rods and a preparation method thereof. Background Art

[0002] Improving the average fuel rod burnup and extending the fuel refueling cycle are important means to improve the economic efficiency of reactors. The grain size of currently operating UO2 fuel pellets is generally less than 10μm. The distance for fission gas atoms in the pellets to diffuse to the grain boundaries is small, and the fission gas atoms gather at the grain boundaries, forming grain boundary bubbles. As the burnup increases, the number of grain boundary bubbles increases, and the bubble size increases, causing the grain boundary bubbles to connect and the fission gas to be released. At the same time, the increase in the number and size of grain boundary bubbles will cause the pellets to swell faster. In addition, for currently operating fuel rods, when the fuel element reaches the first refueling cycle, the UO2 pellets come into contact with the Zr alloy cladding, and the pellets and the Zr alloy cladding undergo physical and chemical interaction (PCI). Under high burnup transient conditions, when the transient power exceeds the threshold, the PCI stress will be too high, the fuel rod Zr alloy cladding will be damaged, and thus the fuel rod failure will occur.

[0003] To increase fuel rod burnup and extend reactor refueling time, international research has been conducted on optimizing fuel pellet designs. These include the use of high-uranium-density fuel and fully ceramic-coated dispersed microencapsulated fuel. However, the low melting point of U-Si in high-uranium-density fuel reduces the core operating temperature margin. The large neutron absorption cross section of nitrogen in uranium-free (UN) fuel compromises reactor economics, and UN fuel exhibits poor resistance to aqueous corrosion, compromising reactor safety. Fully ceramic-coated dispersed microencapsulated fuel pellets, however, suffer from a low uranium loading, failing to meet the uranium loading requirements of large pressurized water reactors. International research has also been conducted on modifying UO2 pellets, including increasing their thermal conductivity and increasing their size. While increasing pellet thermal conductivity reduces core operating temperature, it also increases pellet hardness, reducing the fuel rod's resistance to PCI and posing safety risks due to high fuel rod burnup. Increasing UO2 pellet grain size primarily involves increasing sintering temperatures and extending holding times, but this results in insignificant grain growth and is difficult to implement for large-scale production. Foreign countries also prepare large-grain UO2 pellets by adding sintering aids including Cr2O3, Nb2O5, TiO2, Al2O3, etc. However, the fission gas release and swelling of pellets doped with TiO2 and Nb2O5 are even higher than those of ordinary grain pellets, and the performance cannot meet the design requirements of high-burnup fuel elements; for pellets doped with Cr2O3, a high doping amount is required to obtain large-grain size pellets, which is difficult to mix uniformly during the mixing process and difficult to carry out large-scale production; it is difficult to obtain large-grain size UO2 pellets by doping with Al2O3.

[0004] To obtain high-performance, large-grain UO2 pellets, researchers studied the effect of adding TiO2 powder on the grain size of UO2 pellets. The results showed that the grain size of the UO2 pellets was 9 μm when no TiO2 was added, while the grain size reached 94 μm when the TiO2 addition reached 0.15 wt%. While the grain size met the requirements for large-grain pellets, the swelling and fission gas release of TiO2-doped large-grain UO2 pellets were higher than those of conventional UO2 pellets. Therefore, while TiO2 doping can produce large-grain UO2 pellets, it cannot meet the requirements of high-burnup, long-refueling cycle fuel rods. Other researchers have shown that the grain size of pellets doped with Cr2O3 can reach approximately 40 μm, but the Cr2O3 second phase appears in the pellets. Furthermore, the prepared pellets exhibit uneven grain size, with different grain sizes coexisting. This leads to uneven overall performance of the pellets. Other studies have shown that while doping with Cr2O3 can produce large-grain UO2 pellets, the required doping level (1600 ppm) is far greater than the solid solubility of Cr2O3 in UO2, leading to the precipitation of a second phase of Cr2O3. This creates an interface between the second phase and the matrix in the UO2 pellets, where fission gases accumulate, connect, and release. Furthermore, high Cr2O3 doping levels can lead to uneven oxide distribution in industrial production, resulting in unstable pellet performance. This is inconvenient for large-scale industrial applications and makes it difficult to meet industrial production requirements.

[0005] In view of this, this patent application is filed. Summary of the Invention

[0006] The technical problem to be solved by the present invention is that the existing UO2 pellets have poor fission gas containment capacity, large swelling volume, high interaction force with the cladding, unstable pellet performance under high burnup (burnup > 48000GWd / tU is called high burnup), and cannot take into account the large grain size and second phase precipitation as well as meet the performance requirements of long-life high burnup fuel rods.

[0007] The first object of the present invention is to provide a method for preparing large-grain UO2 pellets for high burnup fuel rods, wherein the large-grain UO2 pellets are prepared by doping Cr2O3 and Al2O3, wherein the doping amount of Cr2O3 is 500ppm~1000ppm and the doping amount of Al2O3 is 50ppm~200ppm.

[0008] In the embodiment of the present invention, large-grain UO2 pellets are prepared by doping with Cr2O3 and Al2O3, and the doping amounts of Cr2O3 and Al2O3 are rationally designed. At this doping amount, the synergistic effect of Cr2O3 and Al2O3 is utilized, and the grain growth of the UO2 pellets is achieved by adding a relatively low content of Cr2O3, thereby obtaining large-grain pellets and avoiding the precipitation of the second phase. The addition of Al2O3 reduces the diffusion efficiency of fission gases under high burnup conditions, ensures the pinning performance of fission products, avoids the aggregation, connection, and release of fission gases at the interface, improves the uniformity of oxide distribution, improves the stability of pellet performance, and meets high burnup requirements.

[0009] In the embodiment of the present invention, when the doping amount of Cr2O3 is less than 500ppm and greater than 1000ppm, large grain size and second phase precipitation cannot be obtained respectively. When the doping amount of Al2O3 is less than 50ppm, the effect of being unable to pin down fission gas atoms will appear. When the doping amount of Al2O3 is greater than 200ppm, the high amount of liquid phase will lead to the unfavorable factor of sintering deformation of UO2 pellets.

[0010] In an optional embodiment, the Cr2O3 content in the finished pellets after doping and sintering is less than or equal to 300 ppm, and the Al2O3 content is less than or equal to 10 ppm. When the Cr2O3 content in the finished pellets exceeds 300 ppm, the pellets are prone to forming a second phase, and the neutron absorption cross section of the Cr in the pellets is large, affecting the economic efficiency of the pellets. When the Al2O3 content in the finished pellets exceeds 10 ppm, the melting point of the pellets is lowered, affecting the safety characteristics of the pellets.

[0011] In an optional embodiment, the following steps are included:

[0012] (1) Weighing a certain amount of UO2 powder and mixing it with Cr2O3 and Al2O3 powder to obtain a first mixed powder;

[0013] (2) mixing the first mixed powder with U3O8, ammonium oxalate and zinc stearate to obtain a second mixed powder;

[0014] (3) pressing the second mixed powder into a molded green body;

[0015] (4) sintering the formed green body at high temperature, and then performing surface and size processing, surface decontamination, and drying to obtain a finished UO2 pellet;

[0016] The weight ratio of the Cr2O3 and Al2O3 powders is 10:1 to 5:1, and the amount of the mixed Cr2O3 and Al2O3 powder added is 500ppm to 1200ppm. This addition and ratio ensures that the Cr2O3 and Al2O3 form a liquid phase compound during the sintering process, increasing the grain growth rate of the UO2 pellets and producing large-grain pellets. Furthermore, the Cr2O3 and Al2O3 can be completely dissolved in the UO2 pellets without the formation of a second phase. Furthermore, the formation of solid-solution Al atoms can effectively pin fission gas atoms, reducing the fission gas atomic diffusion coefficient, thereby improving the fission gas containment capacity of the large-grain pellets.

[0017] In an optional embodiment, a ball mill is used to mix the UO2 powder, Cr2O3, and Al2O3 powders in step (1). The ball mill speed is 300-500 r / min, and the ball milling time is greater than or equal to 16 hours. If the ball mill speed is too high, the centrifugal force of the powder will be large and the mixing will be uneven. If the speed is too low, the powders will not be fully mixed, affecting the mixing efficiency. If the mixing time is less than 16 hours, the powders will not be fully mixed, which will lead to problems such as pellet deformation, cracking, and small grain size during the subsequent sintering process.

[0018] In an optional embodiment, in step (2), based on the mass of UO2 powder, Cr2O3 and Al2O3 mixed powder, the mass of U3O8 powder is added in an amount of 8% to 15%, the mass of ammonium oxalate is added in an amount of 1% to 5%, and the mass of zinc stearate is added in an amount of 0.1% to 0.5%;

[0019] In step (2), a double cone mixer is used to obtain the second mixed powder, the mixing speed is 10 r / min to 20 r / min, and the mixing time is 10 hours to 24 hours.

[0020] In an optional embodiment, during the press molding process in step (3), the molding pressure is 200 MPa to 400 MPa, and the holding time is 30 seconds to 60 seconds. When the molding pressure is less than 200 MPa, the green density of the core block is low, making sintering and densification difficult. When the molding pressure is greater than 300 MPa or the holding time is greater than 60 seconds, the friction between the core block and the mold is large, and the core block is prone to defects such as corner chipping during the demolding process. When the holding time is less than 30 seconds, the pressure is not sufficiently transmitted along the axial direction of the core block, and the core block is prone to transverse cracks.

[0021] In an optional embodiment, in step (4), the high-temperature sintering temperature is 1750°C to 1800°C, and the holding time in the high-temperature sintering zone is greater than or equal to 8 hours. Compared with conventional sintering, the temperature is increased by about 50°C. This is mainly to ensure that Cr2O3 and Al2O3 are fully volatilized on the basis of reducing the grain size of the pellets, so that the Cr2O3 content in the pellets is less than or equal to 300ppm and the Al2O3 content is less than or equal to 10ppm. However, sintering temperatures greater than 1800°C will damage the sintering furnace and cause significant deformation of the pellets.

[0022] In an optional embodiment, the density of the UO2 pellets obtained after high-temperature sintering in step (4) is 93.5% to 96.5%.

[0023] In an optional embodiment, the feed speed when processing the surface of the core block in step (4) is 0.01 mm to 0.02 mm;

[0024] The drying temperature is 100℃~150℃, and the drying time is 1h~3h.

[0025] A second object of the present invention is to provide a large-grain UO2 pellet for high burnup fuel rods, produced by the production method described in any one of the above items, wherein the large-grain UO2 pellet has an average grain size of greater than or equal to 40 μm, a pellet surface porosity of less than or equal to 0.5%, a pellet pore size of less than or equal to 45 μm, and no doping or precipitation of a second phase.

[0026] The density of the large-grain UO2 pellets is 93.5% to 96.5% of the theoretical density; the mass content of uranium in the pellets is greater than or equal to 87.7%;

[0027] When conducting thermal stability tests on the core blocks, the density increment of the core blocks in the thermal stability test was less than 1.29% under the conditions of being 30°C higher than the sintering temperature, the test holding time being greater than or equal to 24 hours, and the test atmosphere being H2. There was no decrease in the core block density; the stoichiometric ratio of the core blocks was 2.000±0.010; and the total H content of the core blocks was less than or equal to 0.8μg / gUO2.

[0028] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0029] An embodiment of the present invention provides a method for preparing large-grain UO2 pellets for high-burnup fuel rods. The large-grain UO2 pellets are prepared by doping with Cr2O3 and Al2O3, and the doping amounts of Cr2O3 and Al2O3 are rationally designed. At these doping amounts, the synergistic effect of Cr2O3 and Al2O3 is utilized to achieve large grain size at a low Cr2O3 doping amount, thereby avoiding second phase precipitation. The addition of Al2O3 reduces the diffusion efficiency of fission gases under high burnup conditions, ensures the pinning performance of fission products, prevents the aggregation, connection, and release of fission gases at the interface, improves the uniformity of oxide distribution, and enhances the stability of pellet performance, thereby meeting the performance requirements of high-burnup fuel rods in the stack.

[0030] The large-grain UO2 pellets for high-burnup fuel rods obtained by the preparation method of the embodiment of the present invention have an average grain size greater than or equal to 40 μm, a porosity less than or equal to 0.5% on the pellet surface, a pore size less than or equal to 45 μm in the pellet, and no doped or precipitated second phase; the density of the large-grain UO2 pellets is 93.5% to 96.5% of the theoretical density; the mass content of uranium in the pellets is greater than or equal to 87.7%; when conducting a thermal stability test on the pellets, under the conditions of a sintering temperature 30°C higher than the sintering temperature, a test holding time greater than or equal to 24 hours, and a test atmosphere of H2, the density increment of the pellets in the thermal stability test is less than 1.29%, and there is no decrease in the pellet density; the stoichiometric ratio of the pellets is 2.000±0.010; and the total hydrogen content of the pellets is less than or equal to 0.8 μg / gUO2, meeting the requirements for high burnup and long refueling cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the examples. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be considered as limiting the scope. A person of ordinary skill in the art can also derive other relevant drawings based on these drawings without inventive effort. In the drawings:

[0032] Figure 1 The microstructure diagram of the finished UO2 pellets obtained in Example 1 and the comparative example;

[0033] Among them, a) corresponds to Example 2, doped with 2000 ppm of Cr2O3; b) corresponds to Example 3, doped with 10 ppm of Al2O3; c) corresponds to Example 5, doped with 1000 ppm Cr2O3 and 50 ppm TiO2; d) corresponds to Example 4, doped with 1000 ppm Cr2O3 and 50 ppm Nb2O5; e) corresponds to Example 1, doped with 1000 ppm Cr2O3 and 50 ppm Al2O3.

[0034] Figure 2 The curves of the fission product diffusion rate of UO2 pellets with different doping elements vary with temperature.

[0035] Figure 3 The graph is a graph showing the change in grain size of the finished UO2 pellets at different sintering temperatures according to an embodiment of the present invention.

[0036] Figure 4 This is the XRD pattern of the large-grain UO2 pellets obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with examples. The illustrative embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Generally, the components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations.

[0039] It should be noted that similar reference numerals and letters represent similar items in the following figures. Therefore, once an item is defined in one figure, it does not need to be further defined or explained in subsequent figures. In the absence of conflict, the embodiments and features in the embodiments of this application can be combined with each other.

[0040] In the description of the embodiments of the present application, the terms "center", "up", "down", "left", "right", "vertical", "longitudinal", "lateral", "horizontal", "inside", "outside", "front", "back", "top", "bottom", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, or are the orientations or positional relationships in which the product of the application is conventionally placed when in use, or are the orientations or positional relationships conventionally understood by those skilled in the art. They are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0041] In the description of the present invention, unless otherwise expressly specified or limited, the terms "disposed," "having," "installed," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention in specific contexts.

[0042] Example 1:

[0043] A large-grain UO2 pellet for high burnup fuel rods is prepared by the following method:

[0044] (1) Weigh a certain amount of UO2 powder and mix it with Cr2O3 and Al2O3 powders. The Cr2O3 doping level is 1000 ppm, and the Al2O3 doping level is 50 ppm. The three powders are mixed in a ball mill at a speed of 300 r / min for 16 hours to obtain a first mixed powder.

[0045] (2) The first mixed powder obtained in step (1) is mixed with U3O8, ammonium oxalate, and zinc stearate, wherein the UO2 powder, Cr2O3, and Al2O3 mixed powder are used as the reference mass, the U3O8 powder is added in an amount of 8% by mass, the ammonium oxalate is added in an amount of 1% by mass, and the zinc stearate is added in an amount of 0.1% by mass. A double cone mixing is used, the mixing speed is 10 r / min, and the mixing time is 10 hours to obtain a second mixed powder.

[0046] (3) The second mixed powder obtained in step (2) is pressed on a rotary press at a molding pressure of 200 MPa and a holding time of 30 s to obtain a molded green body. The surface state of the molded green body is inspected, and obvious defects such as chipping and cracks are not allowed on the surface. If the above defects exist, the green body is crushed and recycled.

[0047] (4) The green body obtained in step (3) is subjected to high-temperature sintering. The sintering atmosphere is high-purity H2. A pusher-boat sintering furnace is used for sintering. The holding time in the high-temperature zone of the sintering is 9 hours. The sintering temperature is 1750°C, and a UO2 pellet with a density of 93.5% is obtained.

[0048] (5) The high-density UO2 pellets obtained in step (4) are subjected to surface and dimensional processing. The surface of the UO2 pellets is processed by centerless grinding at a feed rate of 0.01 mm. The processed UO2 pellets are rinsed in a water tank to remove surface contamination. The cleaned pellets are then dried at a temperature of 100° C. for 1 hour to obtain finished UO2 pellets.

[0049] Example 2:

[0050] A large-grain UO2 pellet for high burnup fuel rods is prepared by the following method:

[0051] (1) Weigh a certain amount of UO2 powder and mix it with Cr2O3 and Al2O3 powders. The Cr2O3 doping level is 500 ppm, and the Al2O3 doping level is 200 ppm. The three powders are mixed in a ball mill at a speed of 300 r / min for 16 hours to obtain a first mixed powder.

[0052] (2) The first mixed powder obtained in step (1) is mixed with U3O8, ammonium oxalate, and zinc stearate, wherein the UO2 powder, Cr2O3, and Al2O3 mixed powder are used as the reference mass, the U3O8 powder is added in an amount of 15% by mass, the ammonium oxalate is added in an amount of 5% by mass, and the zinc stearate is added in an amount of 0.5% by mass. A double cone mixing is used at a mixing speed of 10 r / min and a mixing time of 10 hours to obtain a second mixed powder.

[0053] (3) The second mixed powder obtained in step (2) is pressed on a rotary press at a molding pressure of 200 MPa and a holding time of 30 s to obtain a molded green body. The surface state of the molded green body is inspected, and obvious defects such as chipping and cracks are not allowed on the surface. If the above defects exist, the green body is crushed and recycled.

[0054] (4) The green body obtained in step (3) is subjected to high-temperature sintering in a high-purity H2 atmosphere in a pusher-boat sintering furnace. The holding time in the high-temperature sintering zone is 9 hours and the sintering temperature is 1800°C to obtain a UO2 pellet with a density of 95.5%.

[0055] (5) The high-density UO2 pellets obtained in step (4) are subjected to surface and dimensional processing. The surface of the UO2 pellets is processed by centerless grinding at a feed rate of 0.01 mm. The processed UO2 pellets are rinsed in a water tank to remove surface contamination. The cleaned pellets are then dried at a temperature of 100° C. for 1 hour to obtain finished UO2 pellets.

[0056] Example 3:

[0057] A large-grain UO2 pellet for high burnup fuel rods is prepared by the following method:

[0058] (1) Weigh a certain amount of UO2 powder and mix it with Cr2O3 and Al2O3 powders. The Cr2O3 doping level is 600 ppm, and the Al2O3 doping level is 100 ppm. The three powders are mixed in a ball mill at a speed of 300 r / min for 16 hours to obtain a first mixed powder.

[0059] (2) The first mixed powder obtained in step (1) was mixed with U3O8, ammonium oxalate, and zinc stearate, wherein the UO2 powder, Cr2O3, and Al2O3 mixed powder were used as the reference mass, the U3O8 powder was added in an amount of 11% by mass, the ammonium oxalate was added in an amount of 3% by mass, and the zinc stearate was added in an amount of 0.3% by mass. A double cone mixing was performed at a mixing speed of 10 r / min for 10 hours to obtain a second mixed powder.

[0060] (3) The second mixed powder obtained in step (2) is pressed on a rotary press at a molding pressure of 200 MPa and a holding time of 30 s to obtain a molded green body. The surface state of the molded green body is inspected, and obvious defects such as chipping and cracks are not allowed on the surface. If the above defects exist, the green body is crushed and recycled.

[0061] (4) The green body obtained in step (3) is subjected to high-temperature sintering in a high-purity H2 atmosphere in a pusher-boat sintering furnace. The holding time in the high-temperature sintering zone is 9 hours and the sintering temperature is 1800°C to obtain a UO2 pellet with a density of 96.5%.

[0062] (5) The high-density UO2 pellets obtained in step (4) are subjected to surface and dimensional processing. The surface of the UO2 pellets is processed by centerless grinding at a feed rate of 0.01 mm. The processed UO2 pellets are rinsed in a water tank to remove surface contamination. The cleaned pellets are then dried at a temperature of 100° C. for 1 hour to obtain finished UO2 pellets.

[0063] Comparative Example 1:

[0064] The difference between Comparative Example 1 and Example 1 is that only Cr2O3 is doped, and the doping amount of Cr2O3 is the same as that in Example 1, and the rest is also the same as that in Example 1.

[0065] Comparative Example 2:

[0066] The difference between Comparative Example 2 and Example 1 is that the doping amount of Cr2O3 is 2000ppm, and the rest is the same as Example 1.

[0067] Comparative Example 3:

[0068] The difference between Comparative Example 3 and Example 1 is that the doping amount of Al2O3 is 10ppm, and the rest is the same as Example 1.

[0069] Comparative Example 4:

[0070] The difference between Comparative Example 4 and Example 1 is that Cr2O3 and Nb2O5 are doped, and the doping amount of Nb2O5 is the same as the doping amount of Al2O3. The rest is the same as Example 1.

[0071] Comparative Example 5:

[0072] The difference between Comparative Example 5 and Example 1 is that Cr2O3 and TiO2 are doped, and the doping amount of TiO2 is the same as the doping amount of Al2O3. The rest is the same as Example 1.

[0073] Example and comparative example test results:

[0074] 1. Conduct microstructural observations, such as Figure 1 As shown in Figure 1 a is the microstructure diagram of comparative example 2, Figure 1 b is the microstructure diagram of comparative example 3, Figure 1 c is the microstructure diagram of comparative example 5, Figure 1 d is the microstructure diagram of comparative example 4, Figure 1 e is the microstructure diagram of Example 1.

[0075] Depend on Figure 1 The results of the examples and comparative examples show that when the doping amount of Cr2O3 is 2000ppm, a large amount of second phase precipitation occurs in the pellets; when the doping amount of Al2O3 is 10ppm, the pellet grain size is small and large grain sintering cannot be achieved; when doping with Cr2O3 and TiO2, a large amount of second phase appears at the grain boundary; when doping with Cr2O3 and Nb2O5, large grain pellets are obtained, but the fission gas diffusion rate is greatly increased (such as Figure 2 shown).

[0076] 2. The fission product diffusion rate test results using different doping elements were studied, such as Figure 2 As shown by Figure 2 It can be seen that doping TiO2, Nb2O5, MgO, etc. alone will increase the fission product diffusion rate, resulting in a decrease in the pellet's fission gas containment capacity. Doping Cr2O3 also increases the fission product diffusion rate at high temperatures, but the increase is smaller, while doping Al2O3 can significantly reduce the fission product diffusion rate.

[0077] 3. The changes in the grain size of the finished UO2 pellets at different sintering temperatures were studied. The results are as follows Figure 3As shown in , it shows that the average grain size of the UO2 pellets is 40 μm when the temperature ranges from 1750°C to 1800°C, proving that the present invention obtains large-sized grain pellets.

[0078] 4. Determine the physical structure. The results are as follows: Figure 4 As shown, the XRD pattern results of Example 1 show that the large-grain UO2 pellets obtained by doping Cr2O3 and Al2O3 are composed of a single UO2 phase without the presence of other impurity phases.

[0079] Therefore, in the embodiment of the present invention, by doping with Cr2O3 and Al2O3, large-grain UO2 pellets can be prepared, and under the action of Al2O3, the diffusion of fission gas is reduced to ensure the pinning performance of fission products, avoid the accumulation, connection and release of fission gas at the interface, improve the uniformity of oxide distribution, improve the stability of pellet performance, and meet high fuel consumption requirements.

[0080] This invention relates to the design and preparation of large-grain UO2 fuel. By optimizing the grain size, doping oxide type, and content of UO2 pellets, the invention achieves large-grain UO2 pellets that meet the requirements of high burnup and long refueling cycles. For high-burnup and long-refueling cycle UO2 pellets, the invention proposes the following specifications: grain size ≤ 40 μm, open porosity ≤ 0.5%, pellet uranium mass content ≤ 87.7%, the absence of precipitated second phases in the UO2 matrix, and pellet hydrogen content ≤ 0.8 μg / g UO2.

[0081] The present invention prepares large-grain UO2 pellets by doping with Cr2O3 and Al2O3. The Cr2O3 doping level is 500ppm to 1000ppm, and the Al2O3 doping level is 50ppm to 200ppm. A two-step powder mixing process ensures uniform mixing of the doping oxides with the UO2 powder. The pellets are then mixed with a density-reducing material, U3O8, a pore-forming agent, ammonium oxalate, and a binder, zinc stearate. The pellets are then pressed into a green compact and sintered in a H2 atmosphere at a temperature of 1750°C to 1800°C, yielding UO2 pellets with a density of 93.5% to 96.5%. This invention addresses the issue of excessive Cr2O3 doping, which can lead to oxide precipitation. It also addresses the problems of existing UO2 pellets at high burnup, such as poor fission gas containment, large swelling, and high interaction with the cladding, which cannot meet the design requirements of high-burnup fuel rods. The present invention also simplifies the preparation process and is amenable to industrial production, resolving the challenges of industrial production of large-grain UO2 pellets.

[0082] The large-grain UO2 pellets of the present invention have low irradiation swelling and fission gas release rate, which solves the problems of existing UO2 pellets such as poor fission gas containment capacity under high burnup, large swelling, high interaction force with the cladding, and inability to meet the design requirements of high burnup fuel rods.

[0083] Anything not mentioned or elaborated in detail in the embodiments of the present invention can be obtained by using existing technologies.

[0084] The above specific implementation methods further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific implementation methods of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing large-grain UO2 pellets for high burnup fuel rods, characterized in that: Large-grain UO2 pellets were prepared by doping Cr2O3 and Al2O3, with the Cr2O3 doping amount being 600ppm~1000ppm and the Al2O3 doping amount being 100ppm~200ppm; The preparation method comprises the following steps: (1) weighing a certain amount of UO2 powder and mixing it with Cr2O3 and Al2O3 powder to obtain a first mixed powder; (2) mixing the first mixed powder with U3O8, ammonium oxalate and zinc stearate to obtain a second mixed powder; (3) pressing the second mixed powder into a molded product to obtain a molded green body; (4) The formed green body is sintered at high temperature, and then surface and size processing, surface decontamination, and drying are performed to obtain the finished UO2 pellets.

2. The method for preparing large-grain UO2 pellets for high burnup fuel rods according to claim 1, characterized in that: After doping and sintering, the content of Cr2O3 in the finished pellets is less than or equal to 300ppm, and the content of Al2O3 is less than or equal to 10ppm.

3. The method for preparing large-grain UO2 pellets for high burnup fuel rods according to claim 1, characterized in that: The weight ratio of the Cr2O3 and Al2O3 powders is 10:1-5:1, and the addition amount of the Cr2O3 and Al2O3 mixed powders is 500ppm-1200ppm.

4. The method for preparing large-grain UO2 pellets for high burnup fuel rods according to claim 1, characterized in that: In step (1), a ball mill is used to mix UO2 powder, Cr2O3 and Al2O3 powder. The rotation speed of the ball mill is 300 r / min~500 r / min, and the ball milling time is greater than or equal to 16 hours.

5. The method for preparing large-grain UO2 pellets for high burnup fuel rods according to claim 1, characterized in that: In step (2), based on the mixed powder of UO2 powder, Cr2O3 and Al2O3 as the base mass, the added U3O8 powder accounts for 8% to 15% by mass, the ammonium oxalate accounts for 1% to 5% by mass, and the zinc stearate accounts for 0.1% to 0.5% by mass; In step (2), a double cone mixer is used to obtain a second mixed powder, the mixing speed is 10 r / min to 20 r / min, and the mixing time is 10 hours to 24 hours.

6. The method for preparing large-grain UO2 pellets for high burnup fuel rods according to claim 5, characterized in that: In step (3), during the pressing process, the molding pressure is 200 MPa to 400 MPa, and the holding time is 30 s to 60 s.

7. The method for preparing large-grain UO2 pellets for high burnup fuel rods according to claim 5, characterized in that: In step (4), the high-temperature sintering temperature is 1750°C to 1800°C, and the holding time in the high-temperature sintering zone is greater than or equal to 8 hours.

8. The method for preparing large-grain UO2 pellets for high burnup fuel rods according to claim 5, characterized in that: The density of the UO2 pellets obtained after high-temperature sintering in step (4) is 93.5% to 96.5%.

9. The method for preparing large-grain UO2 pellets for high burnup fuel rods according to claim 1, characterized in that: In step (4), the feed speed when processing the core block surface is 0.01mm~0.02mm; The drying temperature is 100℃~150℃, and the drying time is 1h~3h.

10. A large-grain UO2 pellet for high burnup fuel rods, prepared according to the method for preparing large-grain UO2 pellets for high burnup fuel rods according to any one of claims 1 to 9.

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