Large-grain uranium dioxide fuel pellet and preparation method thereof
By adjusting the oxygen-uranium ratio of uranium dioxide powder and field-assisted sintering technology, the problem of poor fission gas release after doping the grain growth aid was solved, and uranium dioxide fuel pellets with large grains with low defect concentration were prepared, which improved safety performance.
Patent Information
- Application Number
- CN202510589725.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-08
- Publication Date
- 2025-08-01
AI Technical Summary
In the prior art, after doping with grain growth aids, the fission gas release effect is poor, and sometimes even increases, which cannot effectively improve safety performance.
By adjusting the oxygen-uranium ratio of uranium dioxide powder under an oxidative atmosphere, increasing the point defect concentration, and using field-assisted sintering technologies such as discharge plasma sintering or rapid hot press sintering, atom diffusion is accelerated under the action of electric fields, promoting grain growth, and finally obtaining large grains and low defect concentration fuel core pellets without adding grain growth aids.
It is achieved without doping grain growth aids to obtain large-grain uranium dioxide fuel pellets with an average particle size of 35 to 45 μm, which has high density and low defect concentration, which significantly reduces fission gas release and improves safety performance.
Smart Images
Figure CN120413103A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the field of uranium fuel preparation, and particularly to a large-grain uranium dioxide fuel pellet and a preparation method thereof. Background Art
[0002] Uranium dioxide has advantages such as a high melting point, good anti-irradiation performance, resistance to steam corrosion, and a stable crystal structure, and is the preferred material for commercial nuclear fuel. However, the Fukushima nuclear accident has exposed that there are still potential safety hazards in the existing uranium dioxide fuel pellet - Zr alloy cladding system.
[0003] Increasing the grain size of uranium dioxide is an effective method to improve its safety performance. After the grain size of uranium dioxide increases, the average distance for fission gas to diffuse from the grain interior to the grain boundary increases, and more fission gas can be accommodated inside the grains, thereby reducing the release amount. There are two existing methods for increasing the grain size of uranium dioxide. The first is to increase the atomic diffusion rate of uranium dioxide and enlarge the grain size by extending the sintering time and increasing the sintering temperature. This method has an insignificant effect on increasing the size of uranium dioxide, and at the same time faces the problem of high-temperature volatilization of uranium dioxide.
[0004] Therefore, currently the mainstream adopts the second method, which is to dope a small amount of grain growth additives into uranium dioxide to increase the grain size. The influence mechanism of grain growth additives on the grain size is mainly reflected in two aspects: one is to increase the point defect concentration in uranium dioxide and enhance the diffusion rate of uranium atoms; the other is to form a liquid eutectic phase with uranium dioxide and accelerate the grain boundary movement. The types of grain growth additives include Cr2O3, TiO2, Al2O3, SiO2, Nb2O5, MnO, etc., and the addition amount is generally less than 1 wt.%. Nuclear energy agencies in countries such as the United States, France, the United Kingdom, Sweden, and Japan have prepared large-grain uranium dioxide fuel pellets with a grain size reaching 30 - 120 μm based on this technical route and obtained valuable in-pile irradiation data. The in-pile irradiation results of Westinghouse and Studsvik companies show that the fission gas release of the large-grain uranium dioxide fuel pellets obtained after doping with grain growth additives does indeed decrease, but the degree of decrease is much lower than the theoretical value. In-pile irradiation tests in the United Kingdom and Japan even obtained the result that the fission gas release of the large-grain uranium dioxide fuel pellets obtained after doping with grain growth additives increases instead. This is because after doping with grain growth additives, a large number of point defects are formed inside the uranium dioxide fuel pellets. While enhancing the diffusion rate of uranium atoms and promoting grain growth, it also accelerates the diffusion rate of fission gas inside the grains. This effect greatly reduces the inhibitory effect of large-grain uranium dioxide on fission gas and even promotes fission gas release. Summary of the Invention
[0005] In view of the problem that in the prior art, the effect of inhibiting fission gas by large-grained uranium dioxide is greatly reduced, and even the release of fission gas will be promoted, the present invention provides a large-grained uranium dioxide fuel pellet and a preparation method thereof.
[0006] The technical method of the present invention is as follows:
[0007] A preparation method of a large-grained uranium dioxide fuel pellet, comprising: S1. heating uranium dioxide powder in an oxidizing atmosphere to obtain uranium dioxide powder with an increased point defect concentration; S2. performing field-assisted (also known as field-assisted) sintering on the uranium dioxide powder with an increased point defect concentration to obtain a large-grained uranium dioxide fuel pellet.
[0008] In step S1, the oxygen-to-uranium ratio of the uranium dioxide powder is 2.01 - 2.09. The average particle size of the uranium dioxide powder is 1 - 5 μm. The heating temperature is 100 - 250 °C, the heat preservation time is 0.5 - 1.5 h, and the oxidizing atmosphere is an air atmosphere.
[0009] In step S2, the field-assisted sintering includes spark plasma sintering or rapid hot pressing sintering.
[0010] Specifically, the steps of spark plasma sintering include: when loading the uranium dioxide powder with an increased point defect concentration into a graphite mold, transferring it to a spark plasma sintering furnace; applying pressure to the graphite mold, and applying a pulsed current for sintering in a sintering protective atmosphere, cooling with the furnace and demolding to obtain a large-grained uranium dioxide fuel pellet. Here, when applying pressure to the graphite mold, the applied pressure is 30 - 60 MPa. When applying a pulsed current for sintering, the pulse duty cycle is set to 7 / 12 - 9 / 12, the heating rate is 100 - 200 °C / min, the heating temperature is 1500 - 1600 °C, and the heat preservation time is 20 - 30 min.
[0011] Specifically, the steps of rapid hot pressing sintering include: loading the uranium dioxide powder with an increased point defect concentration into a graphite mold, transferring it to a rapid hot pressing sintering furnace; applying pressure to the graphite mold, and applying a direct current for sintering in a sintering protective atmosphere, cooling with the furnace and demolding to obtain a large-grained uranium dioxide fuel pellet. Here, when applying pressure to the graphite mold, the applied pressure is 30 - 60 MPa. When applying a direct current for sintering, the heating rate is 100 - 200 °C / min, the heating temperature is 1500 - 1600 °C, and the heat preservation time is 20 - 30 min.
[0012] The present invention also provides a large-grained uranium dioxide fuel pellet, which is obtained by the above preparation method. The average particle size of the large-grained uranium dioxide fuel pellet is 35 - 45 μm, and the density of the large-grained uranium dioxide fuel pellet is more than 95%.
[0013] The beneficial effects of the present invention are:
[0014] The present invention first heats uranium dioxide powder in an oxidizing atmosphere. During this process, the oxygen-uranium ratio of the uranium dioxide powder is increased, thereby increasing the point defect concentration. The uranium dioxide powder with the increased point defect concentration is then subjected to external field-assisted sintering. Under the action of the electric field, the charged defects in the powder are accelerated to migrate, promote atomic diffusion, and accelerate grain growth. After sintering, the point defect concentration is significantly reduced, thereby obtaining a fuel pellet with large grains and low defect concentration. BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Figure 1 Schematic diagram of point defect concentration adjustment;
[0016] Figure 2 This is the first metallographic image of the large-grain uranium dioxide fuel pellets prepared in Example 1;
[0017] Figure 3 This is the second metallographic image of the large-grain uranium dioxide fuel pellets prepared in Example 1;
[0018] Figure 4 This is the first metallographic image of the large-grain uranium dioxide fuel pellets prepared in Example 2;
[0019] Figure 5 This is the second metallographic image of the large-grain uranium dioxide fuel pellets prepared in Example 2. DETAILED DESCRIPTION
[0020] The following is a clear and complete description of the technical solution of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.
[0021] The concept of the present invention is that, without doping with grain growth aids, the most ideal solution to increase grain size is to try to increase the point defect concentration in the original uranium dioxide powder, accelerate the atomic diffusion rate and grain growth rate during the sintering process, and restore the point defect concentration to a lower level after sintering is completed, thereby inhibiting the diffusion of fission gases inside the grains.
[0022] Based on this, the present invention proposes a method for preparing large-grained uranium dioxide fuel pellets. Without adding grain growth aids, by regulating the oxygen-uranium ratio of uranium dioxide powder, the point defect concentration is increased, and with the assistance of an external electric field, the atomic rate diffusion is accelerated. At the same time, during the sintering process, the defect concentration decreases due to processes such as defect recombination and the reaction of defects with the environment. Through the above process, large-grained uranium dioxide fuel pellets with a low point defect concentration can be obtained without adding grain growth aids.
[0023] Specifically, the present invention provides a method for preparing large-grained uranium dioxide fuel pellets, including:
[0024] S1. Heat the uranium dioxide powder in an oxidizing atmosphere to obtain uranium dioxide powder with an increased point defect concentration.
[0025] In step S1, the oxygen-uranium ratio of the uranium dioxide powder is 2.01 - 2.09. For example, the oxygen-uranium ratio of the uranium dioxide powder is 2.04, 2.05, 2.07, 2.08.
[0026] In step S1, the average particle size of the uranium dioxide powder is 1 - 5 μm. For example, the average particle size of the uranium dioxide powder is 3 μm, 4 μm, 5 μm.
[0027] In step S1, heat the uranium dioxide powder in an oxidizing atmosphere to increase the oxygen-uranium ratio of the uranium dioxide powder. During this process, the point defect concentration of the uranium dioxide powder increases. Here, the heating temperature is 100 - 250 °C, the holding time is 0.5 - 1.5 h, and the oxidizing atmosphere is an air atmosphere.
[0028] S2. Perform external field-assisted sintering on the uranium dioxide powder with an increased point defect concentration to obtain large-grained uranium dioxide fuel pellets.
[0029] In step S2, mechanical pressure and an electric field are applied simultaneously during the sintering process. The electric field can be applied through a spark plasma sintering furnace or a rapid hot pressing sintering furnace. Under the action of the electric field, charged defects in the powder will accelerate migration, promote atomic diffusion, and accelerate grain growth. During the migration of defects, there are various processes that reduce the defect concentration, such as defect recombination and the reaction of defects with the sintering environment, as Figure 1 shown. After sintering, the defect concentration will decrease significantly. Therefore, fuel pellets with both large grains and a low defect concentration can be obtained without adding grain growth aids.
[0030] In step S2, the external field-assisted sintering includes spark plasma sintering or rapid hot pressing sintering.
[0031] Specifically, the steps of spark plasma sintering include: when loading the uranium dioxide powder with increased point defect concentration into a graphite mold, transferring it to a spark plasma sintering furnace; applying pressure to the graphite mold, and applying a pulsed current for sintering under a sintering protective atmosphere, followed by furnace cooling and demolding to obtain large-grained uranium dioxide fuel pellets. Here, when applying pressure to the graphite mold, the applied pressure is 30 - 60 MPa. When applying a pulsed current for sintering, the duty cycle of the current pulse is set to 7 / 12 - 9 / 12, the heating rate is 100 - 200 °C / min, the heating temperature is 1500 - 1600 °C, and the holding time is 20 - 30 min. The sintering protective atmosphere is high-purity argon at 0.04 MPa. For example, the applied pressure is 40 MPa, 50 MPa. The duty cycle of the current pulse is set to 8 / 12. The heating rates are 150 °C / min, 170 °C / min, 190 °C / min. The heating temperatures are 1550 °C, 1580 °C. The holding times are 23 min, 25 min, 28 min.
[0032] Specifically, the steps of rapid hot pressing sintering include: when loading the uranium dioxide powder with increased point defect concentration into a graphite mold, transferring it to a rapid hot pressing sintering furnace; applying pressure to the graphite mold, and applying a direct current for sintering under a sintering protective atmosphere, followed by furnace cooling and demolding to obtain large-grained uranium dioxide fuel pellets. Here, when applying pressure to the graphite mold, the applied pressure is 30 - 60 MPa. When applying a direct current for sintering, the heating rate is 100 - 200 °C / min, the heating temperature is 1500 - 1600 °C, and the holding time is 20 - 30 min. The sintering protective atmosphere is high-purity argon at 0.04 MPa. For example, the applied pressure is 40 MPa, 50 MPa. The heating rates are 150 °C / min, 170 °C / min, 190 °C / min. The heating temperatures are 1550 °C, 1580 °C. The holding times are 23 min, 25 min, 28 min.
[0033] In the present invention, the uranium dioxide powder is first heated in an oxidizing atmosphere. During this process, the oxygen-uranium ratio of the uranium dioxide powder is increased to further increase the point defect concentration. Then, the uranium dioxide powder with increased point defect concentration is subjected to external field-assisted sintering. Under the action of an electric field, the charged defects in the powder will accelerate migration, promote atomic diffusion, and accelerate grain growth. After sintering, the point defect concentration will drop significantly, and fuel pellets with both large grains and low defect concentration characteristics are obtained.
[0034] The present invention also provides a large-grained uranium dioxide fuel pellet. The large-grained uranium dioxide fuel pellet is obtained by the above preparation method. The average particle size of the large-grained uranium dioxide fuel pellet is 35 - 45 μm, and the density of the large-grained uranium dioxide fuel pellet is more than 95%.
[0035] In the present invention, unless otherwise specified, all the raw materials for preparation are commercially available products well-known to those skilled in the art.
[0036] The present invention will be described in detail below through examples and experimental examples. However, these are only examples and do not limit the present invention in any form.
[0037] Example 1
[0038] Step 1: Select uranium dioxide powder with an average particle size of 3 μm and an oxygen-to-uranium ratio of 2.04. Keep the uranium dioxide powder at 200 °C in an air atmosphere for 0.5 h to obtain uranium dioxide powder with an increased point defect concentration.
[0039] Step 2: Load the uranium dioxide powder obtained in Step 1 into a graphite mold with an inner diameter of 8.5 mm and transfer it to a spark plasma sintering furnace. Apply a mechanical pressure of 50 MPa to the graphite mold, set the current pulse duty ratio to 8 / 12, and the sintering protective atmosphere is 0.04 MPa high-purity argon. Heat it to 1500 °C at a heating rate of 200 °C / min, keep it warm for 20 min, and then cool it with the furnace. Take out the mold from the sintering furnace and demold it to obtain a large-grained uranium dioxide fuel pellet with a diameter of about 8.5 mm. The density of the large-grained uranium dioxide fuel pellet is 98%, and the average grain size is about 35 μm, as Figure 2 and Figure 3 shown.
[0040] Example 2
[0041] Step 1: Select uranium dioxide powder with an average particle size of 3 μm and an oxygen-to-uranium ratio of 2.08. Keep it at 100 °C in an air atmosphere for 1.5 h to obtain uranium dioxide powder with an increased point defect concentration.
[0042] Step 2: Externally field-assisted sintering. Load the uranium dioxide powder obtained in Step 1 into a graphite mold with an inner diameter of 12.5 mm and transfer it to a rapid thermal pressing sintering furnace. Apply a mechanical pressure of 50 MPa to the graphite mold, and the sintering protective atmosphere is 0.04 MPa high-purity argon. Heat it to 1600 °C at a heating rate of 100 °C / min, keep it warm for 30 min, and then cool it with the furnace. Take out the mold from the sintering furnace and demold it to obtain a large-grained uranium dioxide fuel pellet with a diameter of about 12.5 mm. The density of the large-grained uranium dioxide fuel pellet is 95%, and the average grain size is about 45 μm, as Figure 4 and Figure 5 shown.
[0043] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.
Claims
1. A method for preparing large-grained uranium dioxide fuel pellets, characterized in that, Including: S1. Heating uranium dioxide powder in an oxidizing atmosphere to obtain uranium dioxide powder with an increased point defect concentration; S2. Subjecting the uranium dioxide powder with an increased point defect concentration to field-assisted sintering to obtain large-grained uranium dioxide fuel pellets.
2. The preparation method according to claim 1, characterized in that, When heating in the oxidizing atmosphere, the heating temperature is 100 - 250 °C, the heat preservation time is 0.5 - 1.5 h, and the oxidizing atmosphere is an air atmosphere.
3. The preparation method according to claim 1, characterized in that, The oxygen-to-uranium ratio of the uranium dioxide powder is 2.01 - 2.
09.
4. The preparation method according to claim 1, characterized in that, The average particle size of the uranium dioxide powder is 1 - 5 μm.
5. The preparation method according to claim 1, characterized in that, The field-assisted sintering includes spark plasma sintering or rapid hot pressing sintering.
6. The preparation method according to claim 5, characterized in that, The steps of the spark plasma sintering include: When loading the uranium dioxide powder with an increased point defect concentration into a graphite mold, transferring it to a spark plasma sintering furnace; Applying pressure to the graphite mold, applying a pulsed current for sintering in a sintering protective atmosphere, cooling with the furnace and demolding to obtain large-grained uranium dioxide fuel pellets.
7. The preparation method according to claim 6, characterized in that When applying pressure to the graphite mold, the applied pressure is 30 - 60 MPa; When applying the pulsed current for sintering, the duty cycle of the current pulse is set to 7 / 12 - 9 / 12, the heating rate is 100 - 200 °C / min, the heating temperature is 1500 - 1600 °C, and the heat preservation time is 20 - 30 min.
8. The preparation method according to claim 5, characterized in that, The steps of the rapid hot pressing sintering include: Loading the uranium dioxide powder with an increased point defect concentration into a graphite mold and transferring it to a rapid hot pressing sintering furnace; Applying pressure to the graphite mold, applying a direct current for sintering in a sintering protective atmosphere, cooling with the furnace and demolding to obtain large-grained uranium dioxide fuel pellets.
9. The preparation method according to claim 8, characterized in that, When applying pressure to the graphite mold, the applied pressure is 30 - 60 MPa; When applying the direct current for sintering, the heating rate is 100 - 200 °C / min, the heating temperature is 1500 - 1600 °C, and the heat preservation time is 20 - 30 min.
10. A large-grain uranium dioxide fuel pellet, characterized in that, The large-grained uranium dioxide fuel pellets are obtained by the preparation method according to any one of claims 1 - 9. The average particle size of the large-grained uranium dioxide fuel pellets is 35 - 45 μm, and the density of the large-grained uranium dioxide fuel pellets is more than 95%.