FR2870628A1 - METHOD FOR MANUFACTURING SINTERED DOUBLE NUCLEAR FUEL PASTILLE - Google Patents

Abstract

The present invention provides a method for making a sintered double nuclear fuel pellet (10). The method comprises the steps of: preparing a first powder composed of a material selected from the group consisting of UO2 and UO2-Er2O3, and a second powder consisting of UO2-Gd2O3 and a sintering additive; producing a double pellet comprising an annular outer portion (2) composed of the first powder and a cylindrical inner portion (1) composed of the second powder; and sintering the double pellet under a reducing gas atmosphere, wherein the sintering additive contains manganese from 0.001% to 2% by weight based on the total weight of the cylindrical inner portion (1).

Description

PROCESS FOR MANUFACTURING A FUEL PASTIL

NUCLEAR DOUBLE SINTERED

  This application is based on, and claims the priority of, Korean Application No. 2004-35571, filed May 19, 2004.

  The present invention relates to a process for producing a sintered double-fueled nuclear fuel pellet, and more particularly to a process for manufacturing a sintered double-fuel pellet consisting of a cylindrical inner part composed of UO2-Gd2O3 and an annular outer part. composed of UO2-Er2O3 or UO2, which is capable of preventing the occurrence of cracks due to the densification rate difference between the two parts during sintering.

  In general, the sintered pellet most widely used as nuclear fuel is a pellet of sintered uranium dioxide (UO2). The sintered uranium dioxide pellet has a height of about 10 mm and a diameter of about 8 mm in a cylindrical form for practical application. The sintered UO2 pellet contains 1 to 5% by weight of U235 and produces nuclear fission energy while U235 decreases by the action of neutrons when used in a nuclear reactor. In addition to the sintered UO2 pellet, a sintered fuel absorbent pellet containing neutron absorbing materials, such as gadolinium (Gd) or erbium (Er), is used in a nuclear reactor to control the number of neutrons.

  The sintered fuel absorbent chip is generally the same size as the sintered UO2 chip.

  Gd or Er is uniformly distributed throughout the sintered UO2 pellet. The sintered fuel absorbent chip is commonly represented by (U, Gd) 02 or (U, Er) 02r and is manufactured according to the following procedure.

  First, a UO 2 powder is mixed with a powder of Gd 2 O 3 or Er 2 O 3 R and sprayed to prepare a UO 2 -Gd 2 O 3 powder or a UO 2 -Er 2 O 3 powder. At this time, the content of Gd2O3 and the content of Er2O3 are limited to about 15% or less and 4% by weight or less, respectively.

  After that, the powdered powder is subjected to compression molding to produce a pellet, which is sintered by heating under a reducing gas atmosphere of 1600 to 1800 C for 2 to 4 hours. 70% of the theoretical value, and that of the sintered pellet represents about 95 of the theoretical value. During sintering, the Gd or Er is dissolved in the UO2 network structure. The reducing gas may be hydrogen gas, or a mixed gas of hydrogen gas with at least one gas selected from water vapor, inert gases and carbon dioxide.

  Korean Patent No. 0281169 (published November 2000), issued to Korea Atomic Energy Research Institute, discloses that a double structure composed of a cylindrical inner portion and an annular outer portion made of different materials is advantageous over a structure using only a pellet of (U, Gd) O2 or sintered (U, Er) O2 as sintered fuel absorbent pellet, since the double structure shows improved nuclear performance in a nuclear reactor. FIG. 1 is a schematic view showing the structure of a conventional double sintered nuclear fuel pellet having a cylindrical inner portion 1 and an annular outer portion 2.

  The dual sintered fuel absorbent nuclear fuel pellet is manufactured by charging different nuclear fuel powders into the inner and outer portions, molding the portions to produce a dual pellet, and sintering the dual pellet.

  However, the sintering process may have a number of problems based on the fact that the inner part and the outer part are composed of different materials. That is, the annular outer portion is composed of a mixed powder of UO2-Er2O3 and the cylindrical inner portion is composed of a mixed powder of UO2-Gd2O3. In the sintering process (also referred to as densification) for increasing the density of the wafer while contracting it, a large difference in densification rate of the cylindrical inner portion and the annular outer portion made of different materials is induced, which generates an undesirable internal voltage at the interface between the two parts. As a result, large gaps or cracks occur at the interface of the final sintered double nuclear fuel pellet.

  In an effort to overcome these problems, US Patent No. 4,678,629 proposes a method comprising sintering a cylindrical inner portion (UO2-Gd2O3) and an annular outer portion (UO2) separated from each other and adjusting the sintered cylindrical inner portion in the annular sintered outer portion. However, since this process requires precise fabrication and treatment of the sintered parts, it is not an effective approach for practical application.

  Alternatively, M. Fisher (J. Nucl Mater., 138, 242-247 (1986)) reported a process for making a sintered double pellet comprising an outer portion of ThO 2 and an inner portion of UO 2 via two-stage sintering. According to this method, the UO2 inner part is first sintered, fitted into a ThO2 pellet, and sintered again to produce a sintered pellet having no cracks. However, this process also involves the adaptation of the sintered UO2 inner part into the ThO2 pellet, which makes the whole process more complicated. Furthermore, since this method does not provide a basic solution to a large difference in densification rate between the cylindrical inner part and the annular outer part, the occurrence of cracks still can not be avoided, resulting in a 2870628 5 deterioration of sintered pellet resistance and damage during handling. In particular, the thermal conductivity is reduced, resulting in a reduction of the operational performance in a nuclear reactor.

  Thus, there is a need in the art for a process for making a cracked, split-free, non-cracked, fuel-absorbing nuclear fuel pellet from the difference in densification rate between a cylindrical inner portion and an annular outer portion composed of different materials. during sintering.

  The present invention has been realized in view of the above problems of the prior arts, and it is an object of the present invention to provide a method for manufacturing a sintered double-fueled nuclear fuel pellet comprising a cylindrical inner portion composed of UO2Gd2O3 and a annular outer part composed of UO2-Er2O3 or UO2r which is able to prevent the occurrence of cracks at the interface between the two parts by decreasing the difference in densification rate of the two parts during sintering.

  In order to achieve the above object of the present invention, there is provided a method for making a sintered dual fuel pellet, comprising the steps of: preparing a first powder composed of a material selected from the group consisting of UO2 and UO2-Er2O3r and a second powder composed of UO2-Gd2O3 and a sinter additive; producing a double pellet comprising an annular outer portion composed of the first powder and a cylindrical inner portion composed of the second powder; and sintering the double pellet under a reducing gas atmosphere, wherein the sintering additive contains manganese of 0.001% to 2% by weight based on the total weight of the cylindrical inner portion (or second powder).

  Preferably, the content of Er2O3 in the UO2-Er2O3 mixture comprising the first powder may be about 4% by weight or less, and the content of Gd2O3 in the UO2-Gd2O3 mixture comprising the second powder may be in the range from about 4% to about 15% by weight.

  Preferably, the density of the sintered double nuclear fuel pellet may be greater than 94% of the theoretical value.

  Preferably, the sintering additive may be at least one selected from the group consisting of pure manganese, manganese oxide (MnO), manganese sulfide (MnS), manganese fluoride, and manganese chloride. . It is preferred to use manganese compounds, such as manganese oxide (MnO) and manganese sulfide (MnS), having a relatively high melting point (e.g. 1785 C and 1610 C, respectively). .

  The reducing gas may be hydrogen gas, or a mixed gas of hydrogen gas with at least one gas selected from water vapor, inert gases and carbon dioxide. The double pellet can be sintered in the temperature range of about 1600 ° C to about 1800 ° C.

  In one embodiment of the present invention, at least a portion of the UO2 may be replaced with PuO2.

  The present invention is characterized by adding the sintering additive containing about 0.001 to about 2 weight percent manganese, such as pure manganese or the manganese compound, based on the total weight of the cylindrical inner portion, by relative to the cylindrical inner part. Manganese dissolves in the UO2 and Gd2O network during sintering and then generates lattice defects, such as uranium gaps, due to the valence difference. Thus, the pure manganese or the added manganese compound facilitates the densification of the UO2-Gd2O3 composing the cylindrical inner portion to reduce the generation of internal tension between the cylindrical inner portion and the annular outer portion.

  In general, the UO2-Er2O3 component annular outer portion shows densification behavior similar to that of pure UO2, but the UO2-Gd2O3 component cylindrical inner portion has a very low densification rate in a temperature range predetermined sintering.

  For these reasons, prior art has the problem that the occurrence of cracks can be induced due to undesired voltage generated at the interface of a sintered chip. To solve this problem, the present invention provides a method for reducing a large difference in densification rate by adding pure manganese or the manganese compound as a sintering additive to the powder component of the cylindrical inner portion. The manganese used here is dissolved in the UO2 lattice structure to facilitate sintering of the double pellet, and to increase the rate of densification of the UO2-Gd2O3 composing the cylindrical inner portion, thereby decreasing the difference in size. the densification rate between the UO2-Gd2O3 composing the cylindrical inner part and the UO2-Er2O3 or the pure UO2 composing the annular outer part.

  Regardless of the types of manganese compound, namely, manganese oxide (MnO), manganese sulfide (MnS), manganese fluoride and manganese chloride, the desired sintering promotion effects may be obtained.

  In addition, the process of the present invention can be applied to the manufacture of another sintered double nuclear fuel pellet in which part of the UO2 is replaced by plutonium dioxide (PuO2), which is a nuclear material having the same network structure as UO2.

  The above and other objects, features, and advantages of the present invention will be more readily understood from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a schematic view showing the structure a conventional sintered double nuclear fuel pellet; Fig. 2 is a graph showing changes in densification rate with increasing temperatures; Figs. 3a and 3b show cross-sectional views showing the structure of a sintered dual-fuel pellet made by a method of the present invention; and Figs. 4a and 4b show cross-sectional views showing the structure of a sintered dual-fuel pellet made by a conventional method.

  Hereinafter, the principle and functions of the present invention will be explained in more detail with reference to the accompanying drawings.

  FIG. 2 is a graph illustrating the effect of manganese oxide (MnO) as a sintering additive, and shows changes in the rate of densification of respective compounds that can be used to make a sintered nuclear fuel pellet, with increasing temperatures.

  Referring to FIG. 2, the 2% by weight UO 2 -Er 2 O 3 (a) composing an annular outer portion has the highest densification rate at 1220 ° C, and the 10% by weight UO 2 -Gd 2 O 3. (b) composing a cylindrical inner portion has the highest densification rate at 1510 C. As a result, both sides experience a large difference in densification rate during sintering. That is, the cylindrical inner portion is densified at a higher temperature than the annular outer portion.

  However, when the manganese oxide (MnO) is added in amounts of 0.01%, 0.02%, 0.05% and 0.1% by weight to 10% by weight UO 2 -Gd 2 O 3 ( designated c, d, e and f in Fig. 2, respectively), based on the total weight of the inner portion, the densification behavior of the 10 wt% UO2-Gd2O3 changes to be similar to that of UO2-Er2O3 at 2% by weight (denoted by a). More particularly, while the amount of manganese oxide (MnO) added increases from 0.01% to 0.1% by weight, temperatures showing the highest densification rate are close to 1220 C.

  As experimentally demonstrated above, the densification of the pellet is greatly increased by the addition of the manganese compound to the UO2-Gd2O3 mixed powder. These results show that manganese is dissolved in the UO2 and Gd2O3 network at an early stage of sintering and facilitates densification by generating lattice defects, such as uranium deficiencies. In addition, it can be confirmed that the difference in densification speed between the inner part and the outer part decreases with decreasing sintering temperatures.

  In conclusion, a sintered double pellet having no cracks at the interface between the inner and outer portions can be manufactured by adding a sintering additive containing manganese to the UO2-Gd2O3 composite powder component of the inner portion, producing a lozenge and sintering the lozenge. The sinter additive contains manganese at 0.001% to 2% by weight based on the total weight of the interior portion. When the amount of manganese added is less than 0.001% by weight, sufficient sinter promotion effects can not be obtained. On the other hand, when the amount exceeds 2% by weight, characteristics inherent to the mixed powder of UO2-Gd2O3 composing the inner part can be degraded. It is preferred that the sinter additive can be added as pure manganese or manganese compound.

  The present invention will now be described in more detail with reference to the following specific examples. These examples illustrate a process for producing a sintered double nuclear fuel pellet comprising an inner portion composed of (U, Gd) O 2 and an outer portion composed of (U, Er) O 2.

  First, a mixture of 2 wt. Of Er203 powder and UO2 powder was charged to a tubular mixer and then blended for 1 hour to prepare a mixed 2 wt.% UO 2 -Er 2 O 3 powder for an annular outer portion of a dual nuclear fuel. Separately, a mixture of manganese oxide (MnO) and Gd2O3 powder was ball milled using zirconia balls for 12 hours to prepare an MnO-containing Gd2O3 powder. Then, the thus prepared powder was mixed with UO 2 powder in a tubular mixer for 1 hour, and then pulverized with a pestle and mortar for 10 minutes to prepare a mixed 10% UO 2 -Gd 2 O 3 powder. % by weight - MnO at 0.1% by weight for a cylindrical inner part.

  The mixed 2% by weight UO 2 -Er 2 O 3 powder was loaded into the annular outer portion and the mixture of UO 2 -Gd 2 O 3 at 10% by weight-MnO at 0.1% by weight was loaded into the cylindrical interior, that it was then subjected to compression molding to produce a double pellet. The double pellet is shown in Figure 1. A number of methods for dual compaction is already known. In the present example, compaction was carried out according to the process described in Korean Patent No. 0354544 (published September 16, 2002), issued to Korea Atomic Energy Research Institute, et al. After that, the double pellet was sintered as follows. The pellet was heated to 1700 C at a rate of 5 K / min and then maintained under a reducing gas atmosphere at 1700 C for 4 hours to make a sintered nuclear fuel pellet. At this time, hydrogen, or a mixed gas of hydrogen gas with at least one gas chosen from water vapor, inert gases and carbon dioxide may be used as the reducing gas. In the present example, a mixed gas of 3% H 2 -CO 2 was used. The density of the sintered pellet was about 97% of the theoretical density.

  The sintered nuclear fuel pellet thus manufactured was polished along a face parallel to an axial direction. The occurrence of cracks and defects at the interface has been observed. Figure 3a shows a photograph of the overall cross-section of the sintered double-fuel pellet made in Example 1. Figure 3b shows an optical microscope image showing the vicinity of the sintered double-sintered nuclear fuel pellet interface. . As shown in FIGS. 3a and 3b, the addition of manganese oxide (MnO) to the inner part makes it possible to manufacture the double sintered nuclear fuel pellet whose interface is clean without the occurrence of cracks.

  As can be seen from the graph shown in FIG. 2, the inner part composed of the mixed powder of UO 2 -Gd 2 O 3 at 10% by weight containing 0.1% by weight of manganese oxide (MnO), corresponding to 0.077% by weight of manganese, at the maximum densification rate at 1280 ° C., which is similar to the temperature (about 1220 ° C.) at which the outer portion composed of the 2% by weight UO 2 -Er 2 O 3 mixed powder at the maximum densification rate. Accordingly, the final sintered double nuclear fuel pellet can be manufactured without the occurrence of interfacial cracks.

  By a procedure similar to that of Example 1, a mixture of 2% by weight of powder of Er.2O3 and powder of UO2 was charged to a tubular mixer and then mixed for 1 hour to prepare a mixture. 2% UO2-Er2O3 mixed powder for an annular outer portion of a dual nuclear fuel.

  Separately, 10% by weight of Gd2O3r powder which had previously been ball milled using zirconia balls for 12 hours was added to a powder of UO2r and mixed in a tubular mixer. for 1 hour, and sprayed with a pestle and mortar for 10 minutes to prepare a mixed powder of UO2-Gd2O3 at 10% by weight, for an inner portion, the composition of which was different from that prepared in Example 1. That is, the procedure was carried out in the same manner as in Example 1, except that no manganese oxide was added to the powder forming the inner part.

  The 2% by weight UO2-Er2O3 mixed powder was charged to the outer portion and the 10% by weight UO2-Gd2O3 mixed powder was charged to the inner portion. After that, the two parts were compacted and then sintered to make a sintered nuclear fuel pellet.

  The sintered nuclear fuel pellet thus manufactured was polished along a face parallel to an axial direction. The occurrence of cracks and defects at the interface has been observed. Figure 4a shows a photograph of the overall cross-section of the sintered double-fuel pellet made in Comparative Example 1. Figure 4b shows an optical microscope image showing the vicinity of the dual nuclear fuel pellet interface. sintered. As shown in FIGS. 4a and 4b, numerous cracks have been observed in the inner and outer portions of the sintered double nuclear fuel pellet.

  As explained earlier, the occurrence of cracks in the double sintered nuclear fuel pellet made in Comparative Example 1 is due to non-uniform densification resulting from a large difference in sintering velocity of the outer and inner portions.

  More particularly, as shown in FIG. 2, the inner portion of the sintered double nuclear fuel pellet composed of the 10% by weight UO 2 -Gd 2 O 3 mixed powder has a maximum densification rate at 1510.degree. of the dual sintered nuclear fuel pellet composed of the 2% by weight UO2Er2O3 mixed powder at a maximum densification rate at 1220 C. As a result, there is a large difference in contraction at the interface with increasing sintering temperatures. This difference generates a voltage at the interface, resulting in the occurrence and growth of cracks.

  As shown in Figure 4b, the cracks begin to grow at the interface between the inner and outer portions, and propagate through the inner portion in a direction perpendicular to the interface and through the outer portion in a direction virtually parallel to the interface. This crack propagation behavior assumes that a tensile stress is generated parallel to a lengthwise direction in the inner portion, and a compressive force is generated parallel to a lengthwise direction. in the outer part. As a result, it appears that cracks occur due to a generated voltage while an inner portion having a relatively low densification rate contracts after the outer portion having a relatively high sintering rate is sintered.

  In contrast, according to Example 1 of the present invention, the sintered double nuclear fuel pellet can be manufactured without the occurrence of cracks at the interface between the inner and outer portions by adding manganese oxide ( MnO) as a sintering additive to the cylindrical inner portion to decrease the densification rate difference of the two parts during sintering. In addition to manganese oxide (MnO), the additive used herein may be pure manganese, manganese sulfide (MnS), manganese fluoride, manganese chloride or the combination thereof. Regardless of the type of these manganese compounds, sintering promoting effects similar to those of Example 1 can be obtained. It is preferable to use manganese compounds having a relatively high melting point.

  Although the present invention has been described herein with reference to the preceding examples and accompanying drawings, it is intended that the scope of the present invention be defined by the following claims. Accordingly, those skilled in the art will appreciate that various substitutions, modifications, and various changes are possible without departing from the technical spirit of the present invention as described in the appended claims. It should be understood that such substitutions, modifications and changes are within the scope of the present invention.

  That is, although the UO2-containing sintered double fuel pellet has been described in the previous examples and accompanying drawings, the method of the present invention can be applied to the manufacture of Sintered double nuclear fuel containing different materials. For example, although the process of the present invention is applied to the manufacture of another sintered double nuclear fuel pellet in which part of the UO 2 is replaced by plutonium dioxide (PuO 2), which is a nuclear material having the same network structure as the UO2r the same crack prevention effect can be hoped for. It should of course be understood that this modified embodiment is within the scope and spirit of the invention as described in the appended claims.

  As appears from the description below

  in accordance with the present invention, the sintered double nuclear fuel pellet may be manufactured, which has little or no cracks at the interface between the annular cylindrical and outer inner portions by adding a small amount of pure manganese or the compound of manganese as a sintering additive to the mixed powder of UO2-Gd2O3 composing the inner part. Because the sintered double sintered nuclear fuel pellet has high mechanical strength and exhibits superior heat transfer efficiency, it is expected to greatly improve performance in a nuclear reactor.

  Of course, the invention is not limited to the embodiments described above and shown, from which we can provide other modes and other embodiments, without departing from the scope of the invention. .

Claims (8)

  A process for producing a sintered double-sintered fuel pellet (10), comprising the steps of: preparing a first powder composed of a material selected from the group consisting of UO2 and UO2-Er2O3r and a second powder consisting of UO2- Gd2O3 and a sintering additive; producing a double pellet comprising an annular outer portion (2) composed of the first powder and a cylindrical inner portion (1) composed of the second powder; and sintering the reducing gas pellet, wherein the sintering additive contains manganese from 0.001% to 2% by weight based on the total weight of the cylindrical inner portion (1).   The process according to claim 1, wherein the content of Er2O3 in the UO2-Er2O3 mixture comprising the first powder is 4% by weight or less.   The process according to claim 1, wherein the content of Gd2O3 in the UO2-Gd2O3 blend comprising the second powder is in the range of 4% to 15% by weight.   The method of claim 1, wherein the density of the sintered double nuclear fuel pellet (10) is greater than 94% of the theoretical value.   double under an atmosphere of The process according to claim 1, wherein the sintering additive is at least one compound selected from the group consisting of pure manganese, manganese oxide (MnO), manganese sulfide (MnS), fluoride of manganese and manganese chloride.   6. The method of claim 1, wherein the reducing gas is hydrogen gas, or a mixed gas of hydrogen gas with at least one gas selected from water vapor, inert gases and carbon dioxide.   7. The method of claim 1, wherein the double pellet is sintered in the temperature range between 1600C and 1800C.   8. The method of claim 1, wherein at least a portion of the UO2 component annular outer portion (2) and the cylindrical inner portion (1) is replaced by PuO2.