Patent Application
20220223303
The present disclosure relates to nuclear fuel pellets capable of suppressing an increase in weight due to oxidation in a steam atmosphere and a method for manufacturing the same. More particularly, the present disclosure relates to a method of preparing sintered pellets capable of suppressing an increase in the weight by inhibiting surface oxidation using a principle that the uranium oxide powder contains chromium, manganese, and silicon oxide as additives, and creates a liquid phase upon sintering to produce a nuclear fuel sintered pellet, which promotes the movement of uranium atoms, making the grain thereof larger and finally forming a film on the grain boundaries.
Nuclear power generation companies emphasized the need to improve nuclear fuel performance for economical operation because there it is required to lower the unit cost of electricity production.
Nuclear fuel development companies had developed long-term/high-combustibility sintered pellets from the 1980s to the early 2000s. However, as interest in the safety of nuclear power has recently increased, the newly developed nuclear fuel is also required to have improved safety performance.
In order to develop a nuclear fuel with an increased operating margin to improve the safety of a nuclear reactor core, nuclear fuel manufacturing companies have improved the performance of uranium dioxide (UO2) sintered pellets by adding oxides in a concentration of several hundred to several thousand ppm per weight. According to licensing reports (for example, Licensing Topical Report, GNF NEDC-33106P, Rev.2/AREVAANP-10340NP) prepared by existing nuclear fuel manufacturing companies (GNF, AREVA) for commercial production and for supply of sintered pellets containing development additives, it can be seen that not only experiments on economical combustion but also experiments related to safety evaluation were conducted. In particular, the weight increase due to the oxidation reaction of the UO2 sintered pellets caused by the inflow of cooling water or steam into the damaged fuel rod due to fuel rod damage was evaluated.
In general, damage to a fuel rod during in-furnace combustion causes corrosion of the UO2 sintered pellets in a water or steam atmosphere at 360° C. to 1200° C. As shown in Reaction Scheme 1 below, the sintered pellets are oxidized as the ratio of O/U=2.0 of UO2 gradually increases for each step.
[Reaction Formula 1]
UO2→U3O7/U4O9→U3O8
U3O8 generated after a total of second times phase transformations is fragmented and separated from UO2 because of the change in the crystal structure due to the phase transformation. The crystal structure maintains a cubic structure from UO2 to U4O9 but changes to an orthorhombic structure from U3O8, and the density decreases by about 20% to 8.35 g/cm3 (volume increases), so the internal stress is generated. This is because the corresponding stress eventually exceeds the fracture stress, and fragmentation occurs. Fragmentation caused by phase transformation to U3O8 due to UO2 oxidation is directly related to the leakage of radioactive fissile material out of the fuel rod when the fuel rod is damaged. Therefore, the oxidation resistance of UO2 has a great influence on the safety margin of the nuclear reactor.
The process of fragmentation is as follows. An initial oxidation reaction occurs from the surface. Oxygen atoms fill the lattice voids of UO2. At this time, the valence of the existing U is changed from +4 to +6 to satisfy electron neutrality in the lattice. Accordingly, the bonding force between atoms becomes stronger, and the spacing between atoms becomes narrower. As a result, the density increases by about 10%, and the surface of UO2, where the initial oxidation occurred, shrinks as a whole. As a result, microcracks occur at grain boundaries where interatomic bonding is weak, and oxygen moves rapidly along the grain boundary cracks created in this way. The grain boundary is a fast diffusion path for oxygen atoms and a high-energy state in which the bonds between atoms are broken, so oxidation proceeds quickly.
In K. Une, Journal of Nuclear Materials, 232 (1996) 240-247, as a result of an oxidation test of UO2 in water at 340° C. for 50 hours, it was reported that the penetration depth of the corrosion layer decreased as the grain size increased. In the method proposed by Korean Patent No. 10-0446587, the grain size obtained by adding oxidizing additives to have a weight ratio of (Mn+Cr+Al)/U of 0.005% to 0.15% by weight and sintering in a weakly oxidizing (gas ratio: CO2/H2=0.3% to 1.6% by weight) atmosphere was 4 to 6 times larger than that of 8 μm, which is the crystal size of a general UO2 sintered pellets. In general, the creep rate increases as the grain size increases.
However, from the viewpoint of changes in the sintered state due to furnace combustion, the oxidation rate cannot be reduced simply by the grain size. The reason is that as the degree of combustion increases, fission products accumulate inside the fuel, swelling occurs, and at the same time, internal stress is applied due to heat gradient, causing cracks throughout the sintered pellets to progress through grain boundaries. In addition, on the outside of the UO2 sintered pellets having an average degree of combustion of 40 GWd/tM or more, a porous rim structure in which bubbles are scattered in the UO2 matrix is formed at the grain boundary. After all, since such cracks and rim structures are formed on grain boundary surfaces with broken bonds vulnerable to oxidation, the inflow of an oxidizing agent from the outside increases the oxidation reactivity explosively. Therefore, even if the oxidation reaction rate is reduced by simply making the grain boundaries larger, grain size growth cannot be the perfect solution from the viewpoint of material deterioration resulting from in-furnace combustion.
In addition, the liquid phase that may exist at the grain boundary mentioned in the present disclosure is, as shown in the phase diagram of K. T. Jacob, Can. Metall. Q., 20 89-92 (1981), MnO—Al2O3 appears to form a liquid phase at a temperature of 1540° C., and thus, in a general UO2 sintering atmosphere, Cr2O3 is reduced to CrO and then volatilized. In addition, through several experiments, it can be seen that MnO—Al2O3 also undergoes rapid volatilization in an oxidizing atmosphere. As a result, the effect of the additive of the patent on suppressing steam oxidation that proceeds along the grain boundary is considered to be insignificant due to the volatilization of Cr2O3 and MnO—Al2O3.
In the method suggested by Korean Patent No. 10-0521638, UO2 containing SiO2, CaO, and Cr2O3 (weight ratio, 35 to 55:45 to 65:1 to 7) additives are sintered at 1700° C. in H2+5% CO2 atmosphere for 4 hours to form a liquid phase in grain boundaries and apply external stress to show the rapid creep deformation. Through this, a result of offsetting the stress transferred to the clad surrounding the UO2 sintered pellets can be obtained. However, such an increase in creep deformation rate was obtained only when an excess of 3000 ppm (0.3% by weight) or more was added. In addition, since the grain size is also small (about 6 μm to 8 μm), the embodiment cannot be a good solution in terms of resistance to oxidation at high temperature because the grain boundary area where the rapid oxidation progress by high-temperature steam is triggered is large. In addition, CaO and CaCO3, which are alkali oxides, are very active in reactivity with steam or water as the main components of lime and are not suitable as grain coating materials for oxidation inhibition.
Accordingly, in order to improve the oxidation resistance of a nuclear fuel pellets, the present inventors have devised a method for lowering the oxidation reaction rate not only to reduce the area of a region vulnerable to oxidation reaction by accelerating the grain growth rate but also suppress contact with the oxidizing agent by coating the grain boundary with an oxide with excellent oxidation resistance and low volatility.
An objective of the present disclosure is to improve the safety of nuclear power plants by suppressing the release of nuclear fission materials flowing out to coolant together with corrosion products of UO2 by lowering the rate of nuclear fuel pellets oxidation due to the steam atmosphere when nuclear fuel rods used in nuclear power plants are damaged.
In order to achieve the above objective, the present disclosure provides uranium dioxide nuclear fuel pellets. According to an aspect of the present disclosure, uranium dioxide nuclear fuel pellets include: uranium dioxide (UO2); and a sintering additive made of Cr2O3, MnO, and SiO2.
The sintering additive is 0.05% to 0.16% by weight per 100% by weight of UO2, and the sintering additive may be mixed with 20% to 40% by weight of Cr2O3, 30% to 50% by weight of MnO, and 20% to 40% by weight of SiO2.
In addition, another aspect of the present disclosure is to provide a method of manufacturing uranium dioxide nuclear fuel pellets. The method for manufacturing nuclear fuel pellets, the method includes steps of: 1) mixing sintering additive powders consisting of Cr2O3, MnO, and SiO2 to uranium dioxide (UO2) powder to prepare a mixed powder; 2) preparing a molded body by compression molding; and 3) heating and sintering the molded body under a reducing atmosphere. The sintering additive powder of step 1) may be added in an amount of 0.05% to 0.16% by weight per 100% by weight of UO2, and the sintering additive powder of step 1) may be mixed with 20% to 40% by weight of Cr2O3, 30% to 50% by weight of MnO, and 20% to 40% by weight of SiO2.
The compression molding pressure of step 2) may be 3 tons/cm2.
The heating and sintering temperature of step 3) may be 1730° C. to 1760° C., and in the reducing atmosphere, an oxygen potential may be −581.9 kJ/mol to −218.2 kJ/mol.
According to the present disclosure as described above, the UO2 sintered pellets to which Cr2O3, MnO, and SiO2 are added have large crystal grains, and at the same time, show high oxidation resistance in a high-temperature steam atmosphere due to an additive film formed at the grain boundaries. Therefore, due to the oxidation of UO2, UO2 becomes U3O8 and reduces the amount of UO2 oxide that is finely fragmented and falls apart, thereby preventing the loss of fission materials to the cooling water when the fuel rod is damaged.
Hereinafter, embodiments of the present disclosure will be described in detail.
The present disclosure provides nuclear fuel pellets having excellent oxidation resistance capable of lowering an oxidation rate of a UO2 sintered pellets at a high temperature, and a preparation method using the same. The nuclear fuel pellets of the present disclosure include a sintering additive made of Cr2O3, MnO, and SiO2, which is sintered in a reducing atmosphere to form a liquid phase to promote grain growth, and as a result, to form an additive film at the grain boundary, thereby lowering the oxidation rate of the UO2 sintered pellets at high temperature.
According to the present disclosure,
The total amount of the sintering additive added in step (S11) may be 0.05% to 0.16% by weight per 100% by weight of UO2. When the amount of the sintering additive is less than 0.05% by weight, sufficient grain growth cannot be promoted, and a liquid fraction capable of coating grain boundaries is not generated. When the amount of the sintering additive is 0.16% by weight or more, since thermal neutrons required for the nuclear fission chain reaction are shielded by additional elements with a large thermal neutron absorption cross-sectional area, the concentration of fissionable U-235 is also less economical. Therefore, the range in which resistance to oxidation due to high-temperature steam may be effectively exhibited and thermal neutron economic feasibility may be maintained is preferably 0.05% to 0.16% by weight.
In step (S11), the sintering additive may be mixed in a ratio of 20% to 40% by weight of Cr2O3, 30% to 50% by weight of MnO, and 20% to 40% by weight of SiO2 per 100% by weight of the sintering additive.
Cr2O3
When Cr2O3 is added to the UO2 matrix, vacant point defects of U4+ ions in the lattice are generated to satisfy charge neutrality in the matrix, and thus, the grain growth of the UO2 sintered pellets is promoted by increasing the diffusion rate of the U4+ ions. In the case of a sintered pellets doped with 0.16% by weight of Cr2O3 per 100% by weight of UO2 manufactured by AREVA Co., the range of 0.05% by weight of Cr2O3 that can be dissolved in the UO2 matrix was excessively exceeded. Excessively exceeded Cr2O3 is to further promote grain growth by reducing Cr2O3 that is not dissolved in the UO2 sintering temperature range to a liquid CrO form.
Therefore, in the nuclear fuel sintered additive according to this disclosure, Cr2O3 should be added in an amount of less than 0.05% by weight per 100% by weight of UO2, which is a range that may be dissolved in UO2, to prevent the formation of a liquid phase Cr2O3 alone, because a dense oxide film cannot be formed in the case of a liquid phase formed only of Cr2O3. Therefore, Cr2O3 should react with MnO and SiO2 to form a dense compound. At this time, 0.015% by weight or more of Cr2O3 should be added per 100% by weight of UO2 in order to make the minimum compound fraction capable of exhibiting oxidation resistance performance. Therefore, it is preferable to add 0.015% to 0.05% by weight of Cr2O3 per 100% by weight of UO2.
MnO
MnO exists in a solid form because its solubility is low in the UO2 matrix, and its phase transformation does not occur in a liquid phase even at a sintering temperature when added in a single composition, which eventually hinders crystal grain growth. However, when MnO reacts with Cr2O3 and SiO2, a liquid compound is formed from a temperature lower than the sintering temperature (1730° C. to 1780° C.). As shown in the Cr2O3—MnO—SiO2 three-component phase diagram at 1500° C. in
SiO2
SiO2 has excellent fission gas capture performance capable of reacting with fission products generated by nuclear fission to form a compound. In addition, as shown in the state diagram of
The compound of this composition is to exhibit an oxidation resistance that is about 5 times higher than that of pure UO2 in a steam atmosphere of 1200° C.
Step (S12) is mixing and molding the additive together with the UO2 powder. After mixing using a Nauta mixer, the mixed powder is put into the molding mold, and the molded body is prepared at a pressure of 3 tons/cm2.
Step (S13) is sintering the molded body, and sintering may be performed at a temperature range of 1730° C. to 1760° C. for 4 to 6 hours. Sintering may be performed in an atmosphere in which an oxygen potential is −581.9 kJ/mol to −218.2 kJ/mol (reducing atmosphere). In this case, referring to
Referring to
The sintered uranium dioxide nuclear fuel pellets of the present disclosure include: uranium dioxide (UO2); and a sintering additive consisting of Cr2O3, MnO, and SiO2.
The sintering additive may be 0.05% to 0.16% by weight per 100% by weight of UO2.
The sintering additive may be mixed in a ratio of 20% to 40% by weight of Cr2O3, 30% to 50% by weight of MnO, and 20% to 40% by weight of SiO2 per 100% by weight of the sintering additive.
Hereinafter, the present disclosure will be described in more detail through examples. These examples are only for illustrating the present disclosure, and it will be apparent to those of ordinary skilled in the art that the scope of the present disclosure is not to be construed as being limited by these examples.
An additive consisting of Cr2O3, MnO, and SiO2 in a total amount of 0.1% by weight was added to the UO2 powder. At this time, the ratio of Cr2O3, MnO, and SiO2 constituting 0.1% by weight was 3:4:3, respectively (see Table 1). After mixing for 4 hours in a 3-axis rotary mixer, the molded body was prepared by compressing at 3 ton/cm2 pressure. The molded body was heated to 1750° C. at a rate of 5° C./min and then sintered for 4 hours. The atmosphere kept the oxygen potential at −380 kJ/mol during sintering.
In order to confirm the minimum required liquid fraction for improving oxidation resistance and growing grain size, UO2 sintered pellets were prepared using the methods in Comparative Examples 1 to 2 (see Table 1). In addition, in order to confirm the deterioration of the oxidation resistance performance due to a ratio exceeding an appropriate Cr2O3, UO2 sintered pellets were prepared in Comparative Example 3 (see Table 1) using the same method as the preparing method of the Example.
For comparison with Example, pure UO2 sintered pellets without additives were prepared by the same preparing process as in Example.
Although crystal grain growth is promoted by the additive, in order to confirm the effect of liquid phase volatilization under oxidation conditions on the deterioration in oxidation resistance, an additive consisted of Cr2O3, MnO, and Al2O3 was added in an amount of 0.1% by weight. At this time, the ratio of Cr2O3, MnO, and Al2O3 constituting 0.1% by weight was 7:2:1, respectively. UO2 sintered pellets were prepared in the same method as the preparing method of the Example.
In order to investigate the low oxidation resistance when the liquid phase is formed by the additive but the grain growth is insufficient, an additive composed of Cr2O3, CaO, and SiO2 was added so as to be 0.1% by weight. At this time, the ratio of Cr2O3, CaO, and SiO2 constituting 0.1% by weight was 4:5:1, respectively. UO2 sintered pellets were prepared in the same manner as the preparing method of the Example.
The grain sizes of the UO2 sintered pellets prepared in Examples and Comparative Examples 1 to 6 were measured using a straight-line crossing method, and the results are shown in Table 2 and
After mechanically cutting the cross section of the sintered pellets prepared by the methods of the Example and Comparative Examples,
the surface microstructure of the sintered pellets was observed with an optical microscope through polishing and heat etching. The results are shown in
A high-temperature steam oxidation experiment was performed with the sintered pellets prepared by the methods of the Example and Comparative Examples 1 to 6 above. The sintered pellets prepared by the methods of the Example and Comparative Examples 1 to 6 were oxidized by exposing the sintered pellets to steam at 1200° C., and a thermogravimetric analyzer was used to measure the weight increase in real-time. At this time, the resulting weight increase was calculated and expressed per unit surface area because the oxidation reaction area increased as the surface area increased. Each of the sintered pellets was loaded into a thermogravimetric analyzer, and argon gas flowed thereto, 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 40 ml/min and oxidation was performed for 20 hours, and the weight was observed to increase over time.
As shown in
When Cr2O3—MnO—SiO2 of Comparative Example 1 was added in an amount of 0.05% by weight per 100% by weight of UO2, the crystal grain size, as well as the high-temperature oxidation resistance, seem similar to those of the Example. However, when 0.04% by weight was added as in Comparative Example 2, crystal grain growth and resistance to high-temperature oxidation were reduced due to a decrease in the liquid fraction formed by the additive.
As in Comparative Example 3, when the Cr2O3 additive was added in an amount of 0.07% by weight per 100% by weight of UO2, the Cr2O3 additive was added in an excess ratio of MnO (0.02% by weight) and SiO2 (0.01% by weight), so that a liquid phase consisting of Cr2O3—MnO—SiO2 component was not sufficiently produced. However, although the grain size is increased due to the liquid phase generated by Cr2O3 alone due to the reduction of Cr2O3 that did not form a liquid phase without MnO and SiO2, the oxidation resistance performance according to the additive self-oxidation and insufficient Cr2O3—MnO—SiO2 liquid fraction in an oxidizing atmosphere seemed to be degraded.
This is because the area of the grain boundary is large since the general UO2 grain size of Comparative Example 4 was less than 10 μm, and thus an oxidation reaction due to penetration of high-temperature steam has actively occurred.
As in Comparative Example 5, Cr2O3—MnO—Al2O3 added UO2 was composed of large grains of 40 μm or more, but as shown in
As in Comparative Example 6, Cr2O3, CaO, and SiO2 added UO2 has a liquid phase formed at a grain boundary but has an average grain size of fewer than 10 μm, the grain boundary area in which the oxidation reaction rate occurs rapidly is large, and thus, the oxidation seems to have occurred four times faster compared to the embodiment of the Example.
As described above, it will be apparent to those skilled in the art that such a specific technique is merely a preferred embodiment, and thus the scope of the present disclosure is not limited thereto. Accordingly, it is intended that the substantial scope of the present disclosure be defined by the appended claims and their equivalents.
All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2019-0118276 | Sep 2019 | KR | national |
This patent application is a continuation of International Application No. PCT/KR2019/014543, filed on Oct. 31, 2019, which claims the benefit of priority to Korean Patent Application No. 10-2019-0118276, filed on Sep. 25, 2019, the entireties of which are incorporated herein by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/KR2019/014543 | Oct 2019 | US |
| Child | 17701240 | US |
