Uranium dioxide pellets for nuclear fuel with improved nuclear fission gas trapping capability and method for producing same

Abstract

The present application relates to a kind of uranium dioxide pellets and its preparation method, the uranium dioxide pellets contain oxide additive, so as to improve the adsorption performance of nuclear fission gas of uranium dioxide pellets for nuclear fuel, while promoting the growth of grain size, by adding sintering additive La2O3-Al2O3-SiO2 in uranium dioxide, so as to generate liquid phase in the sintering process of uranium dioxide pellets to accelerate the movement of material to promote grain growth, since the vapor pressure of liquid phase is lower, less volatilization in the sintering process, so the performance of additive can be effectively exerted, and the liquid phase of coating grain boundary can effectively adsorb nuclear fission gas cesium.

Description

Technical Field

[0001] This invention relates to a pellet and its preparation method, the pellet containing oxide additives to promote the growth of grain size in uranium dioxide pellets for nuclear fuel and improve the nuclear fission gas capture capability. Background Technology

[0002] Since 1970, research has focused on improving the performance of uranium dioxide pellets for nuclear power plants. Initially, the development of uranium dioxide pellets has emphasized promoting grain size growth through the addition of oxides, thereby preventing fuel rod breakage that could occur during normal and transition operations due to pellet-cladding interaction (PCI). Based on numerous studies worldwide on additives, nuclear fuel manufacturers AREVA (now Framatome) and Westinghouse Atom have completed the development of Cr2O3 and Cr2O3-Al2O3, respectively, and have obtained or are in the process of obtaining regulatory approval for commercial development.

[0003] When the cladding and pellet come into contact, fuel rod breakage caused by PCI begins to occur above 30 GWD / MTU. At this point, the pellet applies external force in the radial direction of the cladding, causing mechanical deformation and leading to fracture. However, pellets with a large-grained microstructure, resulting from the addition of oxides, undergo plastic deformation before causing cladding deformation, thereby eliminating the mutual stress between the pellet and cladding caused by heating and volume expansion. Furthermore, the grain boundary area, which serves as a channel for the escape of various fission gases generated through nuclear reactions, is reduced, thus slowing the rate at which fission gases escape from the pellet. From the perspective of accelerating brittle fracture by reacting the escaping fission gases to the inner surface of the fuel rod with ceramic compounds, this is a characteristic of pellets with large grains that are important for improving the safety of nuclear fuel rods. As mentioned above, the role of sintering additives that reduce PCI damage is essentially to increase the grain size of the uranium dioxide pellet. This is because during the sintering process of uranium dioxide, oxide additives promote the movement of uranium cations at the sintering temperature. As a result, the developed microstructure improves the safety of the combustion process in the nuclear power plant furnace and the operating margin of the power plant.

[0004] Following the 2011 Fukushima Daiichi nuclear disaster, which resulted in a radioactive leak, research resumed worldwide with the goal of improving the performance of nuclear fuels to enhance safety. Ultimately, the development of Accident Tolerant Fuel (ATF) began with pellets and cladding. Global ATF nuclear fuel development began in 2012 with research and development investment from the U.S. Department of Energy. Research, currently used to obtain suitable final candidates, has progressed to the level of unit testing, testing lead test rods (LTRs) after irradiation testing. Among the ATF pellets developed to date, uranium dioxide-based candidates include: composite morphologies with silicon carbide whiskers (SiC whiskers) or diamond particles dispersed within uranium dioxide pellets, and metallic micro-unit morphologies forming a metallic mesh within a uranium dioxide matrix—all aimed at improving thermal conductivity. However, in terms of dimensional stability during preparation and in-furnace combustion, uranium dioxide pellets with added oxides and large grains (containing additives to reduce PCI breakage) are considered the most viable final candidates for ATF pellets, but are still under development.

[0005] Generally, performance improvements are observed in pellets with an average grain size of 40 μm or larger (two-dimensional measurement) through grain growth. According to the research results of C. Delafoy et al. (Proceedings of the 2007 International Conference on Light Water Reactor Fuel Performance), as a result of oxidation tests, pellets with an average grain size of around 20 μm showed a weight reduction of up to 5 times compared to pellets with an average grain size of 40 μm or larger. This demonstrates that large-grain pellets exhibit higher oxidation resistance.

[0006] In the research results of K. Fuglesang HWR-1161 (2016), it was published that after combustion in an experimental reactor, the amount of nuclear fission gas measured from pellets with grains larger than 40 μm was significantly lower than that of typical pellets with grains of around 10 μm. In the event of an accident in a nuclear reactor, the permissible standard for the amount of radioactive source term that can be released to the outside, based on the "Accident source term" of US NRC 10 CFR 50.67, is no more than 25 rem from the exclusion area boundary. Large-grain pellets have the effect of reducing the amount of radioactive source that can leak to the outside under accident conditions.

[0007] Generally, gaseous nuclear fission materials can significantly influence atmospheric coverage. Therefore, Oak Ridge National Laboratory reported a positive result at CNF11-19: the reaction of nuclear fission gases with materials within a fuel cell can form compounds that prevent atmospheric diffusion. Since cesium (Cs) is a typical nuclear fission gas, stabilized by combining with silicon dioxide to form the Cs₂Si₄O₃ compound, Japan has developed additives to capture fission-produced gases in fuel cells containing large-grained silicon dioxide.

[0008] Japanese Patent Nos. 2603382 and 3999843 disclose methods for obtaining large grains and thus achieving higher creep rates by adding 0.01 to 0.25 wt% Al₂O₃ and SiO₂ in various ratios. Dr. Matsunaga's 2014 doctoral dissertation at Osaka University, entitled "Study on the Interaction with Cesium / Iodine and Helium Oxide Fuels," reported results on the capture of cesium in the aluminosilicate liquid phase formed by the additives Al₂O₃ and SiO₂. The dissertation ultimately demonstrated that the additives not only promote grain growth through addition, but also capture radioactive nuclear fission gases present within a uranium dioxide matrix. According to the report by T. Matsuda et al., IAEA-TECDOC-1036 (1996), the liquid phase formed by Al2O3 and SiO2 occurs much earlier than the sintering temperature of uranium dioxide. Due to high vapor pressure, it volatilizes during sintering, resulting in weight loss. Dr. Matsunaga's doctoral dissertation also confirmed that with the addition of 0.025 wt% and 0.25 wt% Al2O3-SiO2, the average grain size remained around 20 μm (two-dimensional measurement). Considering this, while sintering temperatures and specific sintering atmospheres can promote material movement to facilitate grain growth, this performance decreases with the volatilization of SiO2-containing compounds that can capture cesium, a nuclear fission product generated during combustion in a nuclear reactor. That is, even with increased additive content to promote more active grain growth and adsorption of fission material, the effect of the Al2O3-SiO2 additive is limited due to the reduced liquid phase fraction caused by volatilization.

[0009] It is known that adding rare earth metal oxides to oxides commonly used as refractory materials can increase the glass transition temperature (Tg) of the refractory material. g ) or melting point (T mThe function of this process is to increase the bonding strength of non-bridging oxygen atoms (NBO) by creating a high field strength in the bonds of amorphous metal-oxygen compounds existing in the glassy or liquid phase at high temperatures, thereby raising the phase transition temperature. However, the effect on the boiling point (T) is currently unclear. b What kind of impact will it have?

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent No. 02603382 (Grant Date: January 29, 1997)

[0013] Patent Document 2: Japanese Patent No. 03999843 (Grant Date: August 17, 2007)

[0014] Non-patent literature

[0015] Non-patent literature 1: C. Felafoy et al., WRFPM 2007, San Francisco, USA (2007), p1071.

[0016] Non-patent literature 2: K. Fuglesang, OECD HALDEN REACTOR PROJECT, HWR-1161 (2016) p14.

[0017] Non-patent literature 3: T. Matsuda et al., IAEA-TECDOC-1036 (1996), pp. 9-17.

[0018] Non-patent literature 4: T. Matsuda, Doctoral Dissertation, Osaka University (2014), p30. Summary of the Invention

[0019] The problem that the invention aims to solve

[0020] The purpose of this invention is to provide a uranium dioxide pellet containing additives and a method for preparing the same, which can increase the Si content in uranium dioxide used as nuclear fuel in nuclear power plants while stably forming a liquid phase to promote grain growth, thereby effectively capturing the fissile material cesium. By utilizing pellets with improved microstructure, it is also possible to ensure combustion margin in the nuclear power plant furnace to cope with PCI damage and nuclear power plant accidents.

[0021] means for solving problems

[0022] To achieve the above objectives, the present invention provides a uranium dioxide pellet for nuclear fuel, comprising uranium dioxide and nuclear fuel additives, said additives comprising lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃), and silicon oxide (SiO₂). By adding the liquid phase formed by the sintering additives La₂O₃-Al₂O₃-SiO₂, uranium ions move rapidly to promote grain growth, and the additives can be coated onto the grain interface, thereby improving the capture capability of cesium fission gas generated during combustion in the nuclear power plant furnace.

[0023] The total content of the additive may be 0.05 to 0.15 parts by weight relative to 100 parts by weight of uranium dioxide.

[0024] The content of lanthanum oxide may be from 0.005 to 0.015 parts by weight relative to 100 parts by weight of uranium dioxide.

[0025] The alumina in the additive can be 0.015 to 0.045 parts by weight.

[0026] The amount of silicon dioxide in the additive can be from 0.03 to 0.09 parts by weight.

[0027] The grain size of the uranium dioxide core is greater than 40 μm, and the Si volatilization rate obtained by formula 1 can be less than 20%.

[0028] Formula 1:

[0029] Si volatilization rate (%) = (Weight of Si volatilized / Weight of Si added) × 100

[0030] Furthermore, this invention provides a method for preparing uranium dioxide pellets, specifically, a method for preparing uranium dioxide pellets for nuclear fuel, comprising: (a) a step of preparing uranium dioxide powder; (b) a step of mixing lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃), and silicon oxide (SiO₂) to prepare an additive; (c) a step of adding the additive to the uranium dioxide powder and mixing it to prepare a mixed powder; (d) a step of compressing the mixed powder to prepare a shaped body; and (e) a step of heating and sintering the shaped body inside a sintering furnace in a reducing atmosphere. Therefore, this invention can effectively form large grains by adding lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃), and silicon oxide (SiO₂) to uranium dioxide powder used as nuclear fuel pellets and sintering it in a weakly reducing atmosphere to form a high-temperature stable liquid phase.

[0031] The total content of the additive may be 0.05 to 0.15 parts by weight relative to 100 parts by weight of uranium dioxide.

[0032] The content of lanthanum oxide may be from 0.005 to 0.015 parts by weight relative to 100 parts by weight of uranium dioxide.

[0033] The alumina in the additive can be 0.015 to 0.045 parts by weight.

[0034] The amount of silicon dioxide in the additive can be from 0.03 to 0.09 parts by weight.

[0035] The grain size of the uranium dioxide core is above 40 μm, and the Si volatilization rate obtained by Formula 1 can be less than 20%.

[0036] Formula 1:

[0037] Si volatilization rate (%) = (Weight of Si volatilized / Weight of Si added) × 100

[0038] Step (e) is performed at 1730°C to 1780°C, and the hydrogen injection rate can be maintained at 200 ml to 2,000 ml / min.

[0039] The effects of the invention

[0040] According to the present invention described above, by adding sintering additives La2O3-Al2O3-SiO2 to uranium dioxide to increase the Si content, the capture performance of nuclear fission gases such as cesium in uranium dioxide pellets is improved.

[0041] At the same time, according to the present invention, since the liquid phase formed during the sintering of the core accelerates the movement of the material, it can promote the grain growth of the uranium dioxide core. Since the liquid phase covers the grain boundaries, it can effectively adsorb the nuclear fission gas cesium.

[0042] Furthermore, due to the aforementioned effects, not only is PCI damage during the sintering process of uranium dioxide pellets reduced, but the safety margin for responding to accident scenarios is also improved. Attached Figure Description

[0043] Figure 1 The following is a process flow diagram illustrating one aspect of the preparation method of uranium dioxide pellets according to the present invention.

[0044] Figure 2 This is the equilibrium state diagram of the Al2O3-SiO2 binary system.

[0045] Figure 3 This is the equilibrium state diagram of the three-component system La2O3-Al2O3-SiO2.

[0046] Figure 4The results show the distribution of metal elements within the microstructure of a uranium dioxide pellet containing 5 parts by weight of La2O3-Al2O3-SiO2 additives according to one embodiment of the present invention, using energy dispersive X-ray spectrometry (EDS).

[0047] Figure 5 Photographs (×1,000 magnification) of the microstructures of La2O3-Al2O3-SiO2 and Al2O3-SiO2 pellets prepared according to one method of the present invention, with 0.1 parts by weight and 0.09 parts by weight of uranium dioxide respectively added to 100 parts by weight of uranium dioxide.

[0048] Figure 6 The results of grain size measurement and quantitative analysis of Si element of uranium dioxide pellets prepared according to embodiments of the present invention are shown.

[0049] Figure 7 The results of grain size measurement and quantitative analysis of Si element of uranium dioxide pellets prepared according to a comparative example of the present invention are shown. Detailed Implementation

[0050] The present invention will now be described in detail.

[0051] This invention provides a uranium dioxide pellet for nuclear fuel, comprising: uranium dioxide; and an additive containing lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃), and silicon oxide (SiO₂). The total content of the additive is 0.05 to 0.15 parts by weight relative to 100 parts by weight of uranium dioxide. The content of lanthanum oxide is 0.005 to 0.015 parts by weight relative to 100 parts by weight of uranium dioxide. The aluminum oxide in the additive is 0.015 to 0.045 parts by weight, and the silicon oxide in the additive is 0.03 to 0.09 parts by weight. The additive, containing lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃), and silicon oxide (SiO₂), forms a liquid phase at the sintering temperature to accelerate the movement of uranium atoms, thereby promoting grain growth. Furthermore, a coating is formed at the grain interface, thereby adsorbing nuclear fission gases generated during combustion in the nuclear power plant furnace.

[0052] In addition, the present invention provides a method for preparing uranium dioxide pellets. Specifically, a method for preparing uranium dioxide pellets for nuclear fuel is provided, comprising: (a) mixing La2O3, Al2O3, and SiO2 to prepare an additive (S11); (b) adding 0.05 to 0.1 parts by weight of the additive to 100 parts by weight of uranium dioxide powder and mixing them to prepare a mixed powder (S12); (c) compressing the mixed powder to prepare a shaped body (S13); and (d) heating and sintering the shaped body in a sintering furnace under a reducing atmosphere at a temperature of 1730°C to 1780°C (S14).

[0053] Regarding the additive, the content of lanthanum oxide is 0.005 to 0.015 parts by weight relative to 100 parts by weight of uranium dioxide, the content of aluminum oxide in the additive is 0.015 to 0.045 parts by weight, and the content of silicon oxide in the additive may be 0.03 to 0.09 parts by weight.

[0054] Figure 1 A process diagram illustrating the preparation method of the additive-containing uranium dioxide pellet of the present invention is provided. See also... Figure 1 Step S11 involves mixing additives used as uranium dioxide oxides. These additives include La₂O₃, Al₂O₃, and SiO₃. The ratio of oxides to additives in step S11 is as follows: Figure 3 As shown in the equilibrium ternary system state diagram of La2O3-Al2O3-SiO2, La2O3, Al2O3, and SiO3 are added according to the composition range with the highest liquid phase fraction, in the ratio (by weight).

[0055] -Lanthium oxide (La2O3)-

[0056] Lanthanum oxide (La₂O₃) is not typically used as an additive in uranium dioxide pellets due to its high neutron absorption rate. It primarily dissolves in the aluminosilicate liquid phase formed by Al₂O₃ and SiO₂ above 1500℃ to increase viscosity. Based on this effect, it is mainly used to increase the transition temperature and melting point of refractory materials used at high temperatures. Although no research results have been published to date on the increase in melting point of aluminosilicate due to increased viscosity, the addition of La₂O₃ to suppress the volatilization of aluminosilicate during uranium dioxide sintering can not only effectively promote grain growth but also help improve the capture performance of nuclear fission gases during the combustion process in nuclear power plant furnaces.

[0057] In order to suppress the volatilization of Si used to capture fission-generating gases by preventing the volatilization of aluminosilicates, in this invention, preferably, 0.005 parts by weight or more of La2O3 are added relative to 100 parts by weight of uranium dioxide powder. However, near the sintering temperature, its effect can be limited by increasing the viscosity of the aluminosilicate liquid phase that promotes grain size growth; therefore, less than 0.015 parts by weight of La2O3 should be added relative to 100 parts by weight of uranium dioxide.

[0058] -Aluminum oxide (Al2O3) and silicon oxide (SiO2)-

[0059] pass Figure 2 The Al2O3-SiO2 equilibrium diagram confirms that Al2O3 and SiO2 together form a liquid phase starting at 1546℃. This aluminosilicate liquid phase promotes the formation of U+ within the sintering temperature range (1700–1780℃). 4+ Ion diffusion promotes grain growth. Regarding the liquid phase fraction of aluminosilicates forming from 1546℃, it increases sharply when the ratio of SiO2 to Al2O3 is more than twice that of SiO2. Therefore, selecting an Al2O3:SiO2 ratio of at least approximately 1:2 can result in a high liquid phase fraction, thereby promoting grain growth.

[0060] Therefore, in this invention, when 0.005 to 0.015 parts by weight of La2O3 are added relative to 100 parts by weight of uranium dioxide, 0.015 to 0.045 parts by weight of Al2O3 are preferably added relative to 100 parts by weight of uranium dioxide powder. Furthermore, 0.03 to 0.09 parts by weight of SiO2 are preferably added relative to 100 parts by weight of uranium dioxide powder.

[0061] In step S11 of this invention, uniform mixing is performed using zirconia spheres with a diameter (Φ) of 5 mm and a Turbula mixer capable of triaxial rotational mixing. In step S11, only the additives are mixed initially because the additive system is a multi-component system, thus maintaining the compositional ratio between the additive powders within the uranium dioxide master powder when mixed with it. The liquid phase obtained by adding the sintering additive La2O3-Al2O3-SiO2 allows uranium ions to move rapidly to promote grain growth and coat the grain interfaces, thereby improving the capture capability of cesium fission gas generated during combustion in a nuclear power plant furnace.

[0062] When adding the additive powder La2O3-Al2O3-SiO3 added in step S12, the additive content is limited to 0.05 to 0.15 parts by weight relative to 100 parts by weight of uranium dioxide powder. Adding more than 0.05 parts by weight in step S12 is to form a sufficient liquid phase. On the other hand, the reason for limiting the additive content to no more than 0.1 parts by weight is to minimize the decrease in neutron economy caused by adding elements with high thermal neutron absorption cross-sections, and because adding more than 0.1 parts by weight would inhibit grain growth. Therefore, in order to grow grains to an average size of 40 μm or more and to coat the grains with a SiO2-containing liquid phase with high adsorption capacity for nuclear fission materials, the addition amount is limited to 0.05 to 0.1 parts by weight. The mixture is uniformly mixed using zirconia balls with a diameter (Φ) of 5 mm and a Turbula mixer in a triaxial rotational motion for 4 hours.

[0063] Step S13 involves compacting the mixed additive powder and uranium dioxide powder by feeding the mixed powder into a molding die at a rate of 2.5 tons / cm³. 2 Pressure preparation of molded parts.

[0064] Step S14 involves sintering the prepared molded body. 100% hydrogen gas is injected at a rate of 200 ml / min to 2,000 ml / min at a temperature ranging from 1700°C to 1780°C, and sintering is carried out for 3 to 5 hours. The injection rate of 100% hydrogen gas at a rate of 200 ml / min or higher in step S14 is to suppress the rise in oxygen potential caused by trace amounts of oxygen released from uranium dioxide or the refractory material, thereby maintaining a reducing sintering atmosphere. On the other hand, the rate is kept below 2,000 ml / min to reduce the pressure exerted internally due to the formation of condensate at the exhaust port by trace amounts of water vapor emitted during sintering, which could cause pipe blockage. This process allows for the preparation of grain cores with an average grain size of 40 μm or higher.

[0065] See Figure 3 As shown in the equilibrium state diagram of the La2O3-Al2O3-SiO2 system, a liquid phase can be formed in this sintering process. For example, ... Figure 4 As shown, in order to observe the liquid phase, by confirming the signal emitted from the region composed of lanthanum (La), aluminum (Al), and silicon (Si) elements that coat the uranium dioxide grains in a core containing an excess of 5 parts by weight of La2O3-Al2O3-SiO2 relative to 100 parts by weight of uranium dioxide, it can be clearly confirmed that an amorphous liquid phase coated with grain boundaries has actually formed.

[0066] By using the additives and processes of this invention, a chip with reduced PCI damage and excellent accident safety can be prepared by efficiently forming a liquid phase to improve the capture performance of nuclear fission gas, promoting particle growth, and coating the grain boundaries with adsorbent components of nuclear fission material.

[0067] The present invention will be described in more detail below through embodiments. Those skilled in the art will understand that these embodiments are merely illustrative and the scope of the invention is not limited to these embodiments.

[0068] Example 1: Preparation of uranium dioxide (UO2) pellets

[0069] The La2O3, Al2O3, and SiO2 additives were three-dimensionally homogenized for 4 hours using zirconia balls with a diameter (Φ) of 5 mm and a Turbula mixer, according to the composition (addition ratio and parts by weight) listed in Table 1 below. Regarding the additives, 0.1 parts by weight of the additives were added relative to 100 parts by weight of uranium dioxide powder, and the mixture was stirred for 4 hours using a Turbula mixer, followed by mixing with 3.5 tons / cm³ of water. 2 The molded body was prepared by pressure compaction. Then, the temperature of the molded body was increased to 1730°C at a rate of 5°C / min, and sintered at 1730°C for 4 hours. The sintering atmosphere was regulated by injecting 100% hydrogen at a rate of 250 ml / min.

[0070] Examples 2 to 12:

[0071] The same preparation method as in Example 1 was used, but uranium dioxide (UO2) pellets were prepared using additives with the composition described in Table 1 below.

[0072] Comparative Example 1:

[0073] The same preparation method as in Example 1 was used, but uranium dioxide (UO2) pellets were prepared using the composition of additives without La2O3 as described in Table 2 below.

[0074] Comparative Examples 2 to 16:

[0075] The same preparation method as in Example 1 was used, but uranium dioxide (UO2) pellets were prepared using the composition of additives described in Table 2 below.

[0076] Measurement example: Confirming the distribution location of metallic elements

[0077] The preparation method was the same as in Example 1, but to more clearly observe the liquid phase, energy dispersive X-ray spectroscopy (EDS) was used to observe the distribution of metal elements in the microstructure of the uranium dioxide pellet with 5 parts by weight of the additive La2O3-Al2O3-SiO2. The observation results are as follows. Figure 4 As shown.

[0078] See Figure 4 It can be seen that, with Figure 3 The La2O3-Al2O3-SiO23 components are the same, and a liquid phase is also formed in this invention.

[0079] Measurement Example 1: Quantitative Analysis of Metallic Elements (Measurement of Si's Resistance to Volatilization)

[0080] After sintering the cores prepared in Examples 1 to 12 and Comparative Examples 1 to 16, an inductively coupled plasma spectrometer (ICP) was used to quantify the Si metal element.

[0081] This analysis allows for the measurement of the amount of silicon dioxide, which possesses excellent nuclear fission gas trapping capabilities, remaining in the core after volatilization during the sintering process, from the additives added before sintering. The quantitative analysis of metallic silicon (Si) was also performed to quantitatively analyze silicon oxide (SiO2) added in oxide form.

[0082] By measuring the silicon metal element content in the uranium dioxide pellets prepared in Examples 1 to 12 and Comparative Examples 1 to 16, the volatile content and the volatile ratio (%) shown in Formula 1 can be calculated, and the results are shown in Table 1 (Examples), Table 2 (Comparative Examples), and... Figure 6 (Example) Figure 7 (Comparative example) is shown.

[0083] Formula 1:

[0084] Si volatilization rate (%) = (Weight of Si volatilized / Weight of Si added) × 100

[0085] Measurement Example 2: Grain Size Measurement

[0086] The photographs (×1,000 magnification) taken using a microtome optical microscope for the core blocks of Example 1 and Comparative Example 1 are as follows: Figure 5 As shown, the grain size of the chips prepared in Examples 1 to 12 and Comparative Examples 1 to 16 was measured, and the results are summarized in Table 1 (Examples) and Table 2 (Comparative Examples). The figures are as follows. Figure 6 (Example) and Figure 7 (Comparative example) is shown.

[0087] Table 1:

[0088]

[0089] Table 2:

[0090]

[0091] Measurement results: Si volatility resistance and grain size

[0092] -Lanthium oxide (La2O3)-

[0093] See Table 1 ( Figure 6 ) and Table 2 ( Figure 7 By comparing Example 1 (grain size: 45.5 μm, Si volatility: 13.4%) with Comparative Example 1 (grain size: 21.5 μm, Si volatility: 64.9%), it can be seen that the presence or absence of lanthanum oxide (La2O3) increases the volatility resistance of Si and the size of the grains.

[0094] Comparing Examples 1 and 3, it can be seen that maintaining the same content of alumina (Al2O3) and silicon oxide (SiO2) and adjusting the content of lanthanum oxide (La2O3) also has the effect of improving the volatility resistance of Si and increasing grain size. Referring to Examples 5 and 6 respectively, it can be seen that the presence of lanthanum oxide (La2O3) and an appropriate content range are important factors for improving the volatility resistance of Si and increasing grain size.

[0095] Comparative Examples 2, 5, 2-1, and 8 show that the minimum content of lanthanum oxide (La2O3) can improve the volatility resistance of Si while promoting grain growth. When the minimum content of lanthanum oxide (La2O3) reaches 0.005 parts by weight or more relative to 100 parts by weight of uranium dioxide (UO2), the volatility resistance of Si is improved and grain growth is promoted. In contrast, when the content is less than 0.005 parts by weight, the volatility resistance of Si is slightly improved, but grain growth is slow.

[0096] Comparative examples 4, 6, 9-1, and 9-2 show that while improving the anti-volatility of Si, the maximum content of lanthanum oxide (La2O3) that allows for grain growth is increased. The grain size and Si volatilization rate of the chips prepared in Comparative Examples 9-1 and 9-2 indicate that adding excess lanthanum oxide (La2O3) reduces the Si volatilization rate, but decreases the grain size.

[0097] -Aluminum oxide (Al2O3)-

[0098] Comparing Example 7 with Comparative Examples 10-1 and 10-2, it can be seen that while improving the volatility resistance of Si, the minimum content of alumina (Al2O3) required for grain growth is increased. It is evident that when the minimum content of alumina (Al2O3) reaches 0.015 parts by weight or more relative to 100 parts by weight of uranium dioxide (UO2), grain growth is promoted while improving the volatility resistance of Si. In contrast, when the content is less than 0.015 parts by weight, although the volatility resistance of Si is slightly improved, grain growth is slow.

[0099] Comparing Example 8 with Comparative Examples 11-1 and 11-2, it can be seen that while improving the volatility resistance of Si, the maximum content of alumina (Al2O3) at which the grain size can grow is increased. It is evident that when the maximum content of alumina (Al2O3) exceeds 0.045 parts by weight relative to 100 parts by weight of uranium dioxide (UO2), the volatility resistance of Si is slightly improved, but grain growth is slow.

[0100] -Silicon oxide (SiO2)-

[0101] Comparing Example 9 and Comparative Example 12, it can be seen that while improving the volatility resistance of Si, the minimum content of silicon oxide (SiO2) required for grain growth was increased. It is evident that when the minimum content of silicon oxide (SiO2) reaches 0.03 parts by weight or more relative to 100 parts by weight of uranium dioxide (UO2), grain growth is promoted while improving the volatility resistance of Si. In contrast, when the content is less than 0.03 parts by weight, although the volatility resistance of Si is slightly improved, grain growth is slow.

[0102] Comparing Example 10 and Comparative Example 13, it can be seen that while the volatility resistance of Si is improved, the maximum content of silicon oxide (SiO2) at which the grain size can grow is increased. It is also known that when the maximum content of silicon oxide (SiO2) exceeds 0.09 parts by weight relative to 100 parts by weight of uranium dioxide (UO2), although the volatility resistance of Si is slightly improved, grain growth is slow.

[0103] -Total content of additives-

[0104] By comparing Example 2 with Comparative Examples 2-1 and 2-2, it can be confirmed that while the volatility resistance of Si is improved, the minimum content of additives that allows for grain size growth is increased. Comparative Examples 2-1 and 2-2 show that when the total content of additives is less than 0.05 parts by weight relative to 100 parts by weight of uranium dioxide, the volatility resistance of Si is improved due to the presence of lanthanum oxide (La2O3), but grain size does not grow.

[0105] Comparative Examples 4 and 14 to 16 show that while improving the volatility resistance of Si, the maximum content of additives that allow for grain size growth is also increased. Comparative Examples 14 to 16 show that when the total content of additives exceeds 0.15 parts by weight relative to 100 parts by weight of uranium dioxide, the presence of lanthanum oxide (La₂O₃) improves the volatility resistance of Si, but grain size does not grow. The foregoing describes specific parts of the invention; those skilled in the art should understand that these specific techniques are merely preferred embodiments, and the scope of the invention is not limited thereto. Therefore, the essential scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A uranium dioxide pellet for use as nuclear fuel, characterized in that, Contains uranium dioxide and nuclear fuel additives. The nuclear fuel additives include lanthanum oxide (La2O3), aluminum oxide (Al2O3), and silicon oxide (SiO2). The nuclear fuel additive therein is an amorphous liquid phase that coats the grain boundaries of the uranium dioxide; The content of the nuclear fuel additive is 0.05 to 0.15 parts by weight relative to 100 parts by weight of the uranium dioxide.

2. The uranium dioxide pellet according to claim 1, characterized in that, The content of lanthanum oxide is 0.005 to 0.015 parts by weight relative to 100 parts by weight of uranium dioxide, the content of aluminum oxide is 0.015 to 0.045 parts by weight, and the content of silicon oxide is 0.03 to 0.09 parts by weight.

3. The uranium dioxide pellet according to claim 1, characterized in that, The uranium dioxide core has a grain size of 40 μm to 60 μm, and the Si volatilization rate, based on Formula 1, is less than 20%. Formula 1: Si volatilization rate (%) = (weight of Si volatilized / weight of Si added) × 100.

4. A method for preparing uranium dioxide pellets, characterized in that, include: (a) Steps for preparing uranium dioxide powder; (b) The step of mixing lanthanum oxide (La2O3), aluminum oxide (Al2O3), and silicon oxide (SiO2) to prepare an additive; (c) The step of adding the additive to the uranium dioxide powder and mixing it to prepare a mixed powder; (d) The step of compressing the mixed powder to prepare a molded article; as well as (e) The step of heating and sintering the molded body inside a sintering furnace in a reducing atmosphere. Wherein, after sintering, the additive is an amorphous liquid phase that coats the grain boundaries of the uranium dioxide; The amount of the additive is 0.05 to 0.15 parts by weight relative to 100 parts by weight of the uranium dioxide.

5. The method for preparing uranium dioxide pellets according to claim 4, characterized in that, The content of lanthanum oxide is 0.005 to 0.015 parts by weight relative to 100 parts by weight of uranium dioxide, the content of aluminum oxide is 0.015 to 0.045 parts by weight, and the content of silicon oxide is 0.03 to 0.09 parts by weight.

6. The method for preparing uranium dioxide pellets according to claim 4, characterized in that, Step (e) is performed at a temperature of 1730°C to 1780°C.

7. The method for preparing uranium dioxide pellets according to claim 4, characterized in that, In step (e), the hydrogen injection rate is maintained at 200 ml to 2,000 ml / min.

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