Sintering Additive for Forming a Coating Film That Can Improve the Oxidation Resistance of Nuclear Fuel Pellets and Preparation Method Thereof

Abstract

The present invention relates to a uranium dioxide nuclear fuel pellet having high oxidation resistance in a steam atmosphere and a method for preparing the same. The method includes: 1) a step of adding and mixing a sintering additive powder containing Cr2O3, MnO, and SiO2 to uranium dioxide powder to prepare a mixed powder; 2) a step of compression molding the mixed powder to prepare a green body; and 3) a step of sintering the green body by heating in a weakly oxidizing atmosphere with an oxygen potential of -581.9 to -218.2 kJ / mol. In the uranium dioxide nuclear fuel pellet, 0.05 to 0.16 wt% of the sintering additive powder is contained relative to 100 wt% of uranium dioxide, and the sintering additive powder contains Cr2O3, MnO, and SiO2. According to the present invention, chromium and manganese oxides are contained in the uranium oxide powder, and the liquid phase generated during sintering for preparing the nuclear fuel pellet promotes grain growth and forms a coating film at the grain interface to inhibit the reaction with steam, thereby restricting the oxidation reaction. It has the effect of being able to improve the oxidation resistance of high-temperature steam near the peak cladding temperature (PCT) of 1204 °C (≈2200 °F) in a loss-of-coolant accident to reduce the leakage amount of nuclear fission substances caused by fuel rod damage.

Description

Technical Field

[0001] The present invention relates to a nuclear fuel pellet capable of suppressing the phenomenon of weight increase due to oxidation in a water vapor atmosphere, and more particularly, to a uranium oxide powder containing chromium, manganese, and silicon oxide as additives, and forming a liquid phase during sintering for preparing a nuclear fuel pellet, and the liquid phase promotes the movement of uranium atoms to increase the grain size, and finally forms a coating film at the grain boundary to suppress oxidation, thereby reducing the corresponding weight increase rate. Background Art

[0002] Nuclear power generation companies emphasize the necessity of improving the performance of nuclear fuels required for economic operation in order to reduce the production cost of electric energy. From 1980 to the early 2000s, the development of long-cycle / high-burnup pellets was carried out mainly by nuclear fuel development companies. However, in recent years, the attention to the safety of nuclear power generation has increased, and higher safety performance is also required for newly developed nuclear fuels.

[0003] In order to develop nuclear fuels with an increased operating margin that can improve the safety of the reactor core, nuclear fuel manufacturing companies improve the performance of uranium dioxide pellets by adding oxides in units of several hundred to several thousand weight ppm. According to the licensing topical reports (Licensing Topical Report, GNF NEDC-33106P, Rev.2 / AREVA ANP-10340NP) prepared by existing nuclear fuel manufacturing companies (GNF, AREVA) for commercial production and supply of pellets containing additives, not only simple experiments on economical combustion are carried out, but also experiments related to safety evaluation are emphasized. In particular, the amount of weight increase caused by the oxidation reaction of uranium dioxide pellets due to the inflow of cooling water or water vapor when the fuel rod is damaged is evaluated.

[0004] During the in-reactor combustion process, fuel rod damage usually causes corrosion of uranium dioxide pellets in an atmosphere of water or steam at 360-1200°C. As shown in the following reaction formula 1, as the oxygen-uranium ratio O / U = 2.0 ratio of uranium dioxide gradually increases in each stage, the pellets are oxidized.

[0005] Reaction formula 1:

[0006] Uranium dioxide → U3O7 / U4O9 → U3O8

[0007] After a total of two phase transitions occur, the generated U3O8 fragments and falls off from uranium dioxide due to the change in crystal structure caused by the phase transition. That is, the reason is that the cubic crystal structure is maintained from uranium dioxide to U4O9, but from U3O8, it becomes an orthorhombic crystal structure, and the density decreases by about 20% to 8.35 g / cm 3(Volume increases), resulting in internal stress. This stress eventually exceeds the fracture stress and fragmentation occurs. The fragmentation caused by the oxidation of uranium dioxide triggering the phase change of U3O8 is directly related to the leakage of radioactive fission substances outside the fuel rod when the fuel rod is damaged. Therefore, the oxidation resistance of uranium dioxide has a great impact on the safety margin of the nuclear reactor.

[0008] The fragmentation process is as follows. The initial oxidation reaction occurs from the surface, and oxygen atoms fill the lattice voids of uranium dioxide. At this time, in order to satisfy the electron neutrality within the lattice, the existing U atoms change from +4 to +6. As a result, the binding force between atoms becomes stronger and the distance between atoms becomes narrower. Consequently, the density increases by about 10%, and thus the entire surface of the uranium dioxide where the initial oxidation starts shrinks. As a result, microcracks appear at the grain boundaries where the atomic binding is weak, and oxygen rapidly moves along the grain boundary gaps generated in this way. The grain boundary not only becomes a fast diffusion channel for oxygen atoms but also a position with a high energy state where the atomic bonds break, so the oxidation proceeds rapidly.

[0009] In K. Une Journal of Nuclear Materials, 232(1996)240 - 247, the results of a 50 - hour oxidation test of uranium dioxide in water at 340 °C were published, showing that the penetration depth of the corrosion layer decreases as the grain size increases. In the method disclosed in Korean Patent No. 10 - 0446587, it is added according to the weight ratio of (Mn + Cr + Al) / U of 0.005 wt% to 0.15 wt%, and the grain size obtained by sintering in a weakly oxidizing (gas ratio: CO2 / H2 = 0.3 - 1.6%) atmosphere is 4 - 6 times larger than the grain size of 8 μm of general uranium dioxide pellets. Generally, the creep rate increases as the grain size grows.

[0010] However, from the perspective of the changes in the pellet state caused by in - furnace combustion, the oxidation rate cannot be reduced solely by the grain boundary size. The reason is that as the burnup increases, fission products accumulate inside the fuel and swelling occurs, and at the same time, due to the thermal gradient, internal stress is exerted, so the pellets crack through the grain boundaries everywhere. In addition, at the outer edge of uranium dioxide pellets with an average burnup of more than 40 GWd / tM, a porous rim structure in which bubbles are dispersed in the uranium dioxide matrix is formed at the crystal interface. As a result, since these cracks and rim structures are grain boundary surfaces where the bonds that are easily oxidized are broken, the oxidant flowing in from the outside explosively increases the oxidation reactivity. Therefore, even if the oxidation reaction rate is simply reduced by increasing the grain boundary, from the perspective of material degradation caused by in - furnace combustion, the growth of the grain size does not become a perfect solution.

[0011] In addition, the liquid phase that can also exist at grain boundaries mentioned in this invention, as shown in the phase diagram of K.T. Jacob, Can. Metall. Q., 2089 - 92 (1981), seems to be the liquid phase formed by MnO - Al2O3 at 1540 °C, which results in the volatilization of Cr2O3 after being reduced to CrO in the general sintering atmosphere of uranium dioxide. Moreover, through various experiments, it is known that MnO - Al2O3 volatilizes rapidly in an oxidizing atmosphere. Therefore, it is judged that due to the volatilization of Cr2O3 and MnO - Al2O3, the additive effect of this patent is very little in suppressing the steam oxidation phenomenon occurring along grain boundaries.

[0012] In the method disclosed in Korean Patent No. 10 - 0521638, it shows that uranium dioxide containing additives SiO2, CaO, Cr2O3 (weight ratio, 35 - 55:45 - 65:1 - 7) is sintered in an H2 + 5% CO2 atmosphere at 1700 °C to form a liquid phase at grain boundaries, and shows the result of rapid creep deformation when an external stress is applied, and points out that the result of being able to offset the stress transmitted to the cladding surrounding the pellet can be obtained. However, it also gives the result that this creep strain rate will increase only when an excess addition of 3000 ppm (0.3 wt%) or more is made. In addition, since the grain size is very small, about 6 - 8 μm, the crystal interface area that is rapidly oxidized by high - temperature steam is large, so it cannot be a good solution in terms of high - temperature oxidation resistance. And, as basic oxides, CaO and CaCO3, which are the main components of lime, are equally very active in reacting with steam or water, so they are not suitable as grain - boundary coating substances for suppressing oxidation.

[0013] Therefore, the present inventors propose a method of reducing the area of the part where oxidation reaction is likely to occur by promoting the grain growth rate in order to improve the oxidation resistance of nuclear fuel pellets, and coating the grain interfaces with oxides having excellent oxidation resistance and low volatility to inhibit contact with oxidants, thereby reducing the oxidation reaction rate.

[0014] Prior art documents

[0015] Patent documents

[0016] Patent Document 1: Korean Patent No. 10 - 0446587 Authorization Date: August 23, 2004

[0017] Patent Document 2: Korean Patent No. 10 - 0521638 Authorization Date: October 6, 2005

[0018] Non - patent documents

[0019] Non-Patent Document 1: GNF, Additive Fuel Pellets for GNF Designs, NEDO-33406 (2009)

[0020] Non-Patent Document 2: AREVA, Incorporation of Chromia-Doped Fuel Properties in AREVA Apporved Methdos, ANP-10340NP (2016)

[0021] Non-Patent Document 3: K. Une, Journal of Nuclear Materials, 232 (1996) p. 240 - 247.

[0022] Non-Patent Document 4: K. T. Jacob, Cam. Metall. Q., (1981) p. 89 - 92 Summary of the Invention

[0023] Problems to be Solved by the Invention

[0024] An object of the present invention is to improve the safety of a nuclear power plant by reducing the oxidation rate of nuclear fuel pellets due to the formation of a steam atmosphere caused by the damage of nuclear fuel rods used in the nuclear power plant, and suppressing the release of nuclear fission substances flowing out into the cooling water together with corrosion products of uranium dioxide.

[0025] Means for Solving the Problems

[0026] To achieve the above object, according to one aspect of the present invention, there is provided a uranium dioxide nuclear fuel pellet comprising: uranium dioxide (UO2); and a sintering additive containing Cr2O3, MnO and SiO2.

[0027] Relative to 100 wt% of uranium dioxide, the sintering additive is 0.05 wt% to 0.16 wt%, and the sintering additive can be mixed in a ratio of 20 wt% to 40 wt% of Cr2O3, 0 wt% to 50 wt% of MnO3, and 20 wt% to 40 wt% of SiO2.

[0028] And, according to another aspect of the present invention, there is provided a method for preparing uranium dioxide nuclear fuel pellets, comprising: 1) a step of adding and mixing a sintering additive powder containing Cr2O3, MnO and SiO2 to uranium dioxide (UO2) powder to prepare a mixed powder; 2) a step of compression molding the mixed powder to prepare a molded body; and 3) a step of heating and sintering the molded body in a reducing atmosphere. Relative to 100 wt% of uranium dioxide, 0.05 to 0.16 wt% of the sintering additive powder in the step 1) may be added, and the sintering additive powder in the step 1) may be mixed in a ratio of 20 wt% to 40 wt% of Cr2O3, 30 wt% to 50 wt% of MnO, and 20 wt% to 40 wt% of SiO2.

[0029] The pressure of the compression molding in the step 2) may be 3 tons / cm 2 (ton / cm 2 ).

[0030] The sintering temperature for heating in the step 3) may be 1730 °C to 1760 °C, and based on the oxygen potential, the reducing atmosphere may be -581.9 kJ / mol to -218.2 kJ / mol.

[0031] Effects of the Invention

[0032] According to the present invention, there are the following technical effects: Due to the addition of Cr2O3, MnO and SiO2, the uranium dioxide pellets according to the present invention have large grains and a coating film formed at the grain boundaries, so they exhibit high oxidation resistance in a high-temperature steam atmosphere. By reducing the amount of oxidized uranium dioxide that exfoliates due to the oxidation of uranium dioxide to form uranium trioxide and fine fragmentation, it is possible to prevent the loss of nuclear fission substances to the cooling water when the fuel rod is damaged. Brief Description of the Drawings

[0033] Figure 1 It is a simplified process flow chart of a method for preparing uranium dioxide pellets according to an embodiment of the present invention.

[0034] Figure 2 It is thermodynamic data showing the calculated value of the oxygen potential at which the oxygen-uranium ratio (O / U ratio) of uranium dioxide pellets remains at 2.0 at the sintering temperature according to an embodiment of the present invention.

[0035] Figure 3 It is a chart showing the state and composition ratio of Cr2O3, MnO and SiO2 according to an embodiment of the present invention.

[0036] Figure 4SEM photographs and X-ray spectroscopy (energy dispersive spectrometer, EDS) results showing the microstructure of uranium dioxide pellets doped with 10 wt% of Cr2O3, MnO, and SiO2 according to an embodiment of the present invention.

[0037] Figure 5 Optical microscope photographs showing the microstructure of uranium dioxide pellets doped with 0.1 wt% of Cr2O3, MnO, and SiO2 according to an embodiment of the present invention and the pellets of Comparative Examples 4 to 6.

[0038] Figure 6 Optical microscope photographs showing the microstructure of uranium dioxide pellets doped with 10 wt% of Cr2O3, MnO, and Al2O3 prepared according to Comparative Example 5 of the present invention.

[0039] Figure 7 A graph showing the weight gain per unit surface area over time in the case of performing a high-temperature steam oxidation experiment on the pellets prepared according to the embodiments of the present invention and the pellets prepared according to Comparative Examples 1 to 6.

[0040] Figure 8 A graph showing together the grain sizes of the pellets prepared according to the embodiments of the present invention and the pellets prepared according to Comparative Examples 1 to 6 and the measurement values of the high-temperature steam oxidation experiment. Detailed Description of the Invention

[0041] Hereinafter, embodiments of the present invention will be described in detail.

[0042] The present invention provides a nuclear fuel pellet and a method for preparing the same. The nuclear fuel pellet includes sintering additives made of Cr2O3, MnO, and SiO2, and is sintered in a reducing atmosphere to form a liquid phase to promote grain growth, and finally a coating film is formed at the grain boundaries, thereby being able to reduce the oxidation rate of the uranium dioxide pellet at high temperatures and having excellent oxidation resistance.

[0043] Figure 1 A process flow chart showing the method for preparing the nuclear fuel pellet according to the present invention. Refer to Figure 1 , the method for preparing the nuclear fuel pellet of the present invention includes: 1) a step (S11) of adding and mixing an additive powder containing Cr2O3, MnO, and SiO2 to uranium dioxide (UO2) powder to prepare a mixed powder; 2) a step (S12) of compression molding the mixed powder to prepare a molded body; and 3) a step (S13) of heating and sintering the molded body in a reducing atmosphere.

[0044] 1) In step (S11), the total amount of the sintering additive added may be 0.05 wt% to 0.16 wt% of the sintering additive relative to 100 wt% of uranium dioxide. When the content of the sintering additive is less than 0.05 wt%, not only the grain growth cannot be sufficiently promoted, but also the liquid phase fraction capable of coating the grain boundaries will not be formed. When it is above 0.16 wt%, since the thermal neutrons required for the nuclear fission chain reaction are shielded by the additive elements with a large thermal neutron absorption cross section, the economy of enriching fissile U-235 also decreases. Therefore, the range that can have effective oxidation resistance to high-temperature steam and maintain thermal neutron economy is preferably 0.05 wt% to 0.16 wt%.

[0045] 1) The sintering additive in step (S11) can be obtained by mixing in the following proportions: 20 wt% to 40 wt% of Cr2O3, 30 wt% to 50 wt% of MnO, and 20 wt% to 40 wt% of SiO2 are added relative to 100 wt% of the sintering additive.

[0046] (1) Cr2O3

[0047] When Cr2O3 is added in the uranium dioxide matrix, in order to satisfy the charge neutrality in the matrix, vacancy defects of U ions in the lattice will appear. Therefore, as the diffusion rate of U ions increases, the grain growth of the uranium dioxide pellets can be promoted. In the case of pellets doped with 0.16 wt% of Cr2O3 relative to 100 wt% of uranium dioxide of AREVA, it seriously exceeds the range of 0.05 wt% of Cr2O3 that can be dissolved in the uranium dioxide matrix. This is to reduce the undissolved Cr2O3 in the uranium dioxide sintering temperature range to the liquid phase form of CrO to further promote grain growth. 4+ ions, so as the diffusion rate of U 4+ ions increases, the grain growth of the uranium dioxide pellets can be promoted. In the case of pellets doped with 0.16 wt% of Cr2O3 relative to 100 wt% of uranium dioxide of AREVA, it seriously exceeds the range of 0.05 wt% of Cr2O3 that can be dissolved in the uranium dioxide matrix. This is to reduce the undissolved Cr2O3 in the uranium dioxide sintering temperature range to the liquid phase form of CrO to further promote grain growth.

[0048] Therefore, in the nuclear fuel sintering additive of the present invention, it is necessary to prevent the addition amount of Cr2O3 from being less than the solubility range of uranium dioxide, that is, the addition amount of Cr2O3 is less than 0.05 wt% relative to 100 wt% of uranium dioxide and forms a liquid phase alone. The reason is that when a liquid phase is formed only by Cr2O3, a dense oxide film cannot be formed. Therefore, it is necessary to react with MnO and SiO2 to form a dense compound. Here, in order to prepare the minimum compound fraction that can exhibit antioxidant performance, it is necessary to add more than 0.015 wt% of Cr2O3 relative to 100 wt% of uranium dioxide. Therefore, preferably, 0.015 wt% to 0.05 wt% of Cr2O3 is added relative to 100 wt% of uranium dioxide.

[0049] (2)MnO

[0050] When MnO has low solubility in the uranium dioxide matrix and is added as a single component, even at the sintering temperature, it does not undergo a phase change to become a liquid phase. Instead, it exists in a solid phase form, ultimately hindering grain growth. However, if it starts to react with Cr2O3 and SiO2 before the sintering temperature (1730 °C to 1780 °C), a liquid phase compound is formed starting from a low temperature. As Figure 3 shown in the ternary system state diagram of Cr2O3 - MnO - SiO2 at 1500 °C, it can be seen that when the MnO content is increased, the liquid phase fraction increases. As a result, the increase in the liquid phase fraction promotes the grain growth of uranium dioxide. Therefore, the higher the MnO ratio, the better. However, in the impurity concentration standard for nuclear fuel pellets, it is stipulated that the total amount of SiO2 and MnO added shall not exceed 0.12 wt% relative to 100 wt% of uranium dioxide. Therefore, preferably, MnO is added in an amount of 0.06 wt% or less. And, in order to coat the grain boundaries where the oxidation reaction initially starts with the Cr2O3 - MnO - SiO2 compound capable of maintaining at least the antioxidant performance to inhibit the reaction between uranium dioxide and the oxidant, preferably, at least 0.02 wt% or more of MnO is added.

[0051] (3)SiO2

[0052] SiO2 has excellent nuclear fission gas capture performance capable of reacting with nuclear fission products generated by nuclear fission to form compounds. And, as Figure 3 shown in the state diagram, SiO2 forms a liquid phase compound near the sintering temperature together with Cr2O3 and MnO to promote grain growth. However, in order to meet the impurity concentration standard for nuclear fuel pellets, preferably, 0.05 wt% or less of SiO2 is added relative to 100 wt% of uranium dioxide. In order to meet the minimum liquid phase volume fraction required for the liquid phase coating of the grain boundaries of Cr2O3 - MnO - SiO2 to exhibit antioxidant performance, preferably, 0.015 wt% or more of SiO2 is added relative to 100 wt% of uranium dioxide.

[0053] This is to be able to exhibit antioxidant properties about 5 times higher than that of pure uranium dioxide in a steam atmosphere at 1200 °C.

[0054] 2) Step (S12) is a step of mixing and molding additives together with uranium dioxide powder, showing a method of using a Nauta mixer to mix and then putting the mixed powder into a molding die and using a pressure of 3 tons / cm 2 to prepare a molded body.

[0055] 3) Step (S13) is the step of sintering the formed body, which can be sintered at a temperature range of 1730 °C to 1760 °C for 4 to 6 hours. The sintering atmosphere is an oxygen potential of -581.9 kJ / mol to -218.2 kJ / mol (reducing atmosphere). At this time, referring to Figure 2 it can be known that the oxygen-uranium ratio (O / U ratio) is 2.0 in this oxygen potential atmosphere, which is more stable. As a reference, when the sintering atmosphere is below -581.9 kJ / mol or above -218.2 kJ / mol, the oxygen-uranium ratio (O / U ratio) of uranium dioxide increases to more than 2.0, resulting in crystal structure deformation and cracks generated outside or inside the pellet.

[0056] Referring to Figure 3 , it can be confirmed that a liquid phase of the added oxide can be formed starting from a temperature of 1500 °C, which is below the target sintering temperature of 1730 °C to 1760 °C, as Figure 4 it can be confirmed that there are oxides containing Cr, Mn, and Si. The oxide containing Cr, Mn, and Si surrounds the grain boundaries of uranium dioxide, and thus it can be judged that it exists in a liquid phase at the sintering temperature. Since a coating film with excellent oxidation resistance is formed at the grain boundaries through the above process, the result that the weight increase due to oxidation is about 1 / 5 lower than that of pure uranium dioxide can be obtained.

[0057] The uranium dioxide nuclear fuel pellet of the present invention comprises: uranium dioxide (UO2); and a sintering additive containing Cr2O3, MnO, and SiO2.

[0058] Relative to 100 wt% of the uranium dioxide, the sintering additive can be 0.05 to 0.16 wt%, and the sintering additive can be obtained by mixing in the following proportions: relative to 100 wt% of the sintering additive, 20 wt% to 40 wt% of Cr2O3, 30 wt% to 50 wt% of MnO, and 20 wt% to 40 wt% of SiO2 are added.

[0059] The present invention will be described in more detail below through examples. These examples are only used to illustrate the present invention, and it is obvious to those skilled in the art that the scope of the present invention is not limited to the interpretation through these examples.

[0060] Examples

[0061] An additive containing Cr2O3, MnO, and SiO2 was added to the uranium dioxide powder to a total amount of 0.1 wt%. Here, the proportions of Cr2O3, MnO, and SiO2 constituting the 0.1 wt% additive were 3:4:3 (see Table 1), and it was mixed in a three-axis rotary mixer for 4 hours, and then at 3 tons / cm 2It is compressed under pressure to prepare a formed body. The formed body is heated to 1750 °C at a rate of 5 °C / min and then sintered for 4 hours. Among them, the sintering atmosphere is maintained at an oxygen potential of -380 kJ / mol.

[0062] Comparative Examples 1 to 3

[0063] In order to confirm the minimum liquid phase fraction required for improving oxidation resistance and grain size growth, uranium dioxide pellets were prepared in Comparative Examples 1 and 2 (see Table 1), and in order to confirm the decrease in oxidation resistance performance caused by exceeding the appropriate ratio of Cr2O3, uranium dioxide pellets were prepared using the same preparation method as in the examples.

[0064] Comparative Example 4

[0065] For comparison with the examples, pure uranium dioxide pellets without additives were prepared by the same preparation process as in the examples.

[0066] Comparative Example 5

[0067] In order to confirm the influence of the additive promoting grain growth but the liquid phase volatilization under oxidation conditions on the decrease in oxidation resistance, an additive containing Cr2O3, MnO, and Al2O3 was added to 0.1 wt%. Here, the proportions of Cr2O3, MnO, and Al2O3 constituting 0.1 wt% were 7:2:1 respectively, and uranium dioxide pellets were prepared by the same preparation method as in the examples.

[0068] Comparative Example 6

[0069] In order to understand the low oxidation resistance when the additive promotes liquid phase formation but the grain growth is weak, an additive containing Cr2O3, CaO, and SiO2 was added to 0.1 wt%. Here, the proportions of Cr2O3, CaO, and SiO2 constituting 0.1 wt% were 4:5:1 respectively, and uranium dioxide pellets were prepared by the same preparation method as in the examples.

[0070] Measurement Example 1: Measuring grain size

[0071] The grain sizes of the uranium dioxide pellets prepared through the examples and Comparative Examples 1 to 6 were measured using the linear intercept method, and the results are shown in Tables 2 and 8.

[0072] Table 1:

[0073]

[0074] Measurement Example 2: Observing the microstructure

[0075] After mechanically cutting the cross-section of the pellets prepared through the examples and comparative examples, they were polished and thermally etched, and the surface of the microstructure of the pellets was observed with an optical microscope. The results are as followsFigure 5 as shown

[0076] Measurement Example 3: High-temperature steam oxidation experiment

[0077] A high-temperature steam oxidation experiment was carried out using the pellets prepared through the above-described Examples and Comparative Examples 1 to 6. The pellets prepared through the above-described Examples and Comparative Examples 1 to 6 were exposed to steam at 1200 °C for oxidation. A thermo gravimetric analyzer was used to measure the increase in weight in real time. Here, since the larger the surface area, the larger the oxidation reaction area, the increase in weight as a result was marked by calculation per unit surface area. Each pellet was separately loaded into the thermo gravimetric analyzer and argon gas was introduced, and the temperature was raised to 1200 °C at a rate of 30 °C / min. After reaching the target temperature of 1200 °C, steam was injected at a rate of 40 ml / min while oxidation was carried out for 20 hours, and the weight increase over time was observed. Figure 7 The time-weight increase amount / surface area graph shows the high-temperature steam oxidation results of the uranium dioxide pellets prepared through the Examples and Comparative Examples 1 to 6.

[0078] Table 2:

[0079]

[0080] As Figure 7 and Figure 8 shown, the pellet prepared through the Example was 0.184 mg / mm 2 , and Comparative Example 4 was 0.980 mg / mm 2 (5.3 times that of the Example), Comparative Example 5 was 0.592 mg / mm 2 (3.2 times that of the Example), Comparative Example 6 was 0.941 mg / mm 2 (5.1 times that of the Example), with respect to the weight increase per surface area.

[0081] In the case of adding 0.05 wt% of Cr2O3-MnO-SiO2 to 100 wt% of uranium dioxide in Comparative Example 1, it had a grain size and high-temperature oxidation resistance similar to those of the Example. However, in the case of adding 0.04 wt% in Comparative Example 2, the liquid phase fraction formed by the additive decreased, resulting in a decrease in grain growth and high-temperature oxidation resistance.

[0082] As shown in Comparative Example 3, when 0.07 wt% of the additive Cr2O3 was added relative to 100 wt% of uranium dioxide, Cr2O3 was added in excess relative to MnO (0.02 wt%) and SiO2 (0.01 wt%). Therefore, the liquid phase composed of the Cr2O3-MnO-SiO2 components was not sufficiently formed. However, the Cr2O3 that did not form a liquid phase with MnO and SiO2 was reduced and the separately formed liquid phase caused an increase in grain size. However, the oxidation of the additive itself in an oxidizing atmosphere and the insufficient Cr2O3-MnO-SiO2 liquid phase fraction caused a decrease in oxidation resistance.

[0083] Since the average grain size of the general uranium dioxide in Comparative Example 4 was less than 10 μm, the area of the grain boundaries was large. At the same time, due to the penetration of high-temperature steam, the oxidation reaction occurred actively.

[0084] As shown in Comparative Example 5, the uranium dioxide added with Cr2O3-MnO-Al2O3 was composed of large grains of 40 μm or more. However, as Figure 6 shown, due to the volatilization of Cr2O3 or MnO-Al2O3, the high-temperature steam reacted quickly with the grains through the pores formed at the grain boundaries. Therefore, the oxidation rate was about three times higher than that of the Examples.

[0085] As shown in Comparative Example 6, the uranium dioxide added with Cr2O3, CaO and SiO2 had a liquid phase formed at the grain boundaries. However, since the average grain size was less than 10 μm and the area of the grain boundaries where the oxidation reaction occurred quickly was large, the oxidation was about four times faster than that of the Examples.

[0086] As described above, specific parts of the content of the present invention have been described in detail. It is obvious to those of ordinary skill in the art that these specific descriptions are only preferred embodiments, and the scope of the present invention is not limited thereto. Therefore, it should be understood that the substantial scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A uranium dioxide nuclear fuel pellet, characterized in that, Comprising uranium dioxide (UO2) and a sintering additive, the sintering additive comprising Cr2O3, MnO, and SiO2; The sintering additive is 0.05 to 0.16 parts by weight relative to 100 parts by weight of the uranium dioxide; The sintering additive comprises 20 wt% to 40 wt% of Cr2O3, 30 wt% to 50 wt% of MnO, and 20 wt% to 40 wt% of SiO2; The sintering additive forms a liquid phase existing at the grain boundaries of the uranium dioxide.

2. A method for preparing uranium dioxide nuclear fuel pellets, characterized in that, Including: 1) A step of adding and mixing a sintering additive powder comprising Cr2O3, MnO, and SiO2 to uranium dioxide (UO2) powder to prepare a mixed powder; 2) A step of compression molding the mixed powder to prepare a molded body; And 3) A step of sintering the molded body in an atmosphere with an oxygen potential of -581.9 kJ / mol to -218.2 kJ / mol; The sintering additive powder in the step 1) is mixed in the following ratio: the sintering additive powder is 0.05 to 0.16 parts by weight relative to 100 parts by weight of the uranium dioxide; The sintering additive powder in the step 1) comprises 20 wt% to 40 wt% of Cr2O3, 30 wt% to 50 wt% of MnO, and 20 wt% to 40 wt% of SiO2; The sintering additive forms a liquid phase existing at the grain boundaries of the uranium dioxide.

3. The method for preparing uranium dioxide nuclear fuel pellets according to claim 2, characterized in that The pressure for compression molding in step (2) is 3 tons / cm 2 .

4. The method for preparing uranium dioxide nuclear fuel pellets according to claim 2, characterized in that The step 3) is carried out under the conditions of 1730 °C to 1760 °C.

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