METHOD FOR PREPARING UAL 2 POWDER AND UA1 2 POWDER PREPARED ACCORDING TO THE SAME CROSS-REFERENCES TO RELATED APPLICATION 5 This patent application claims the benefit of priority from Korean Patent Application No. 10-2013-0165175, filed on 12, 27, 2013, the contents of which are incorporated herein by reference. 10 BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preparing a 15 UA-l 2 powder and a UA-l 2 powder prepared according to the same, and more particularly, to a method for preparing a UAl 2 powder in which an uranium ingot and an aluminum ingot are mixed at a specific ratio to prepare a molten alloy, and only UAl 2 powder is selectively prepared by a simple process through a 20 centrifugal atomization method, and to a UAl 2 powder prepared according to the method. 2. Description of the Related Art 99mTc is an element that occupies about 80% of the demand 25 for medical diagnostic radioisotopes and is a medical 1 radioisotope which is importantly utilized in a nuclear medical diagnosis of diseases. 99mTc is an artificial element that does not exist in nature, and is a daughter nuclide generated by the radioactive decay of 99Mo. 5 Meanwhile, there are two major methods for preparing 99Mo which is the only mother nuclide of the medical radioisotope 99mTc. A first method is a method for extracting 99Mo from products generated by the nuclear fission of uranium, and a second method is a method for obtaining 99Mo by irradiating 10 neutrons to 98Mo. However, with respect to the second method, a raw material 98Mo is difficult to be obtained and accordingly, is expensive, and radiation intensity is weak. Therefore, the first method is mainly used. 99Mo prepared by the foregoing methods generates 99mTc by 15 means of radioactive decay, and the generated 99mTc is used for a disease diagnosis. However, since 99Mo has a very short half-life of 66 hours, a high-enriched uranium target having an enrichment degree of 90% or more has been used until recently in order to 20 increase productivity. Herein, the phrase "an enrichment degree of 90% or more" denotes that among uranium isotopes, highly fissionable U-235 is included in an amount of 90% or more and U-238 is included in an amount of 10% or less. The policy to reduce an enrichment degree of uranium in a 25 irradiation target material for medical radioisotope 99Mo from 2 a high enrichment of about 90% to an enrichment of 20% or less, has been globally carried out since 1996 led mainly by the United States and the international atomic energy agency (IAEA) as a nuclear non-proliferation policy. 5 However, there is a limitation in that a yield of 99Mo decreases when the enrichment degree of the uranium in the irradiation target material is reduced from a high enrichment of about 90% to an enrichment of 20% or less. Accordingly, efforts are underway toward a direction of increasing a total 10 content of uranium instead of reducing an enrichment degree of 235-U from an enrichment of 90% to a low enrichment of 20%, and if possible, studies have been carried out on principle by following a method for not altering a target process and shape that have been typically used. 15 For example, Argonne National Laboratory in the United States has developed a target having a shape in which a thin uranium metal foil having a thickness of 120 pm to 150 pm is inserted between two aluminum cylinders. When neutrons are irradiated to the thin uranium metal 20 foil in a nuclear reactor, deformation occurs due to an anisotropic microstructure, and volume expansion occurs because air bubbles or the number of atoms are increased by a fission gas. The foregoing volume expansion rapidly increases due to the fact that the higher the temperature is, the 25 greater the atomic diffusion motion is. Therefore, a target 3 is developed with the foregoing design in order to effectively dissipate a lot of heat generated from nuclear fission. However, there are limitations in that a target of a large-scale 99Mo producer used in a process for preparing 99Mo 5 has generally a plate shape, and a process becomes complicated because aluminum cylindrical housings are removed after nuclear fission and then a post treatment has to be performed. Meanwhile, the target of the large-scale 99Mo producer uses currently aluminide which is an alloy of high-enriched 10 uranium and aluminum, and is a material melt-alloyed by adding about 18 wt% of an aluminum metal to a uranium metal. The aluminide has a microstructure in which UA1 3 and UA1 4 phases are precipitated and are dispersed in an Al matrix during cooling. 15 When the aluminide has the foregoing microstructure, the aluminide may prevent deterioration of behavior due to a temperature rise because thermal conductivity is very good to maintain the core temperature low, but has a low uranium content of 1.5 g-U/cc. 20 Since among intermetallic compounds of the uranium aluminide, UA-l 2 has a uranium content higher than those of UAl 3 and UAl 4 , UAI 2 powder is firstly prepared, and thus a dispersed material, which is mixed with aluminum powder to be rolled, has a uranium content increased by about 3.0 g-U/cc. 25 Meanwhile, Japan Patent Publication No. 1995-218697 has 4 disclosed, as a typical technology relating a method for preparing a uranium target, a method for preparing a 99Mo generating target using low-enrichment uranium and a 99Mo generating target prepared of low-enrichment uranium. In 5 detail, Japan Patent Publication No. 1995-218697 has disclosed a method for preparing a first target for generating a fission product, the method includes: (a) selecting a first base material that has a first surface, a second surface, an edge portion and a predetermined thickness; (b) preparing the first 10 surface of the first base material for accommodating foil that has a first surface, a second surface and a predetermined thickness, and is comprised of a fissile material; (c) contacting the first surface of the foil with the first surface of the base material such that the foil is removed 15 from the first base material later; (d) selecting a second base material that has a first surface, a second surface, an edge portion and a predetermined thickness; (e) preparing the first surface of the second base material for accommodating the foil such that the foil is removed from the second base 20 material later; (f) installing the edge portion of the first base material on the edge portion of the second base material such that the second surface of the first base material and the second surface of the second base material are exposed to the ambient atmosphere, and the foil is inserted between the 25 first base material and the second base material not to be 5 exposed to the ambient atmosphere; and (g) compressing the exposed second surface of the first base material and the exposed second surface of the second base material, to ensure a mechanically slidable insertion-contact between the foil and 5 the first surface of the first base material, and between the foil and the first surface of the second base material. Accordingly, the inventors of the present invention found a method for selectively preparing UA-l 2 powder from a specific ratio of a molten aluminum-uranium alloy by using a 10 centrifugal atomization method during conducting studies on a method for preparing UAl 2 powder that is stable and has a high uranium density, and thus completed the present invention. A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission 15 that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. Throughout the description and claims of the specification, the word "comprise" and variations of the word, 20 such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps. SUMMARY OF THE INVENTION One aspect of the present invention is to provide a 25 method for preparing a UAl 2 powder. 6 Another aspect of the present invention is to provide a UA1 2 powder prepared according to the preparing method. Still another aspect of the present invention is to provide a method for preparing a low-enriched uranium target. 5 Even another aspect of the present invention is to provide a low-enriched uranium target prepared according to the preparing method. A further aspect of the present invention is to provide a low-enriched uranium target being used as an irradiation 10 target for medical radioisotope Mo-99. In order to achieve the aspects, the present invention provides a method for preparing a UAl 2 powder, the method comprising: mixing an aluminum ingot and a uranium ingot such that aluminum is mixed in a range of 10 weight% to 25 weight% 15 with respect to the total weight of the mixture, and then melting the mixture under a vacuum atmosphere (step 1) ; and preparing the molten alloy melted in the step 1 into powder by using a centrifugal atomization method (step 2). The present invention also provides UAl 2 powder prepared 20 according to the preparing method. Furthermore, the present invention provides a method for preparing a low-enriched uranium target, the method comprising: mixing an aluminum ingot and a uranium ingot such that 25 aluminum is mixed in a range of 10 weight% to 25 weight% with respect to the total weight of the mixture, and then melting the mixture under a vacuum atmosphere (step 1); preparing the molten alloy melted in the step 1 into powder to produce a UAl 2 powder by using a centrifugal 30 atomization method (step 2); mixing a UAl 2 powder and an aluminum powder to prepare a powder compact (step 3); and rolling the powder compact of the step 3 (step 4). 7 Furthermore, the present invention provides a low enriched uranium target prepared according to the preparing method. In a method for preparing UA1 2 powder of the present 5 invention, UA1 2 powder may be selectively prepared by centrifugal atomizing a molten alloy of uranium and aluminum having a specific composition. Since the UAl2 powder shows good irradiation performance and has a high uranium density, the volume of a fuel particle may be reduced. 10 Also, since a process is simple, a yield rate may be improved to obtain a lot of powder. Furthermore, since the UAl 2 is prepared in a small size, the swelling of UAl2 powder according to neutron irradiation and a temperature change is low to be suitable for preparing a 15 target. Since the UAl 2 is small and has a spherical shape, thereby reducing a defective rate such as a thickness reduction of a cladding material due to an excessive dog-bone when a rolling process is performed after preparing a powder compact, and thereby raising a uranium content in a target. 20 BRIEF DESCRIPTION OF THE DRAWINGS The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in 25 conjunction with the accompanying drawings, in which: 8 FIG. 1 is a diagram showing a typical method for preparing uranium aluminide powder and a method for preparing UAl 2 powder and a method for a low-enriched uranium target by using the UAl 2 powder according to Example of the present 5 invention; FIG. 2 is a graph showing a binary phase diagram of uranium-aluminum. FIG. 3 is a photograph showing UAl 2 powder prepared in Example 1; and 10 FIG. 4 is a photograph showing an X-ray and a cross section of a core region of a low-enriched uranium target prepared in Example 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS 15 The present invention provides a method for preparing a UAl 2 powder, which includes mixing an aluminum ingot and a uranium ingot such that aluminum is included in a range of 10 weight% to 25 weight% with respect to the total weight of the mixture, and melting the mixture under a vacuum atmosphere 20 (step 1); and preparing the molten alloy melted in the step 1 into powder by using a centrifugal atomization method (step 2). Hereinafter, a method for preparing a UAl 2 powder according to the present invention will be described in detail 25 for each step. 9 In a method for preparing a UA1 2 powder according to the present invention, the step 1 is a step of mixing an aluminum ingot and a uranium ingot such that aluminum is included in a range of 10 weight% to 25 weight% with respect to the total 5 weight of the mixture, and melting the mixture under a vacuum atmosphere. A target using a uranium aluminide powder is that a UAlX particle that is a mixed phase of UAl 2 , UAl 3 and UAl 4 , is dispersed in an aluminum matrix. 10 UAl 4 is an intermetallic compound having an orthorhombic crystal structure that has a wide range stoichiometrically and is crystallographically anisotropic, and is unsuitable as a nuclear fuel because UA-l 4 has a weak characteristic with respect to irradiation. On the other hand, since UAl 3 has a 15 simple cubic structure that is crystallographically isotropic, UAl 3 is stable with respect to irradiation, and has excellent ductility compared to UAl 4 , so UAl 3 may be easily prepared. Meanwhile, since UAl 2 is a material having a face centered cubic structure that is crystallographically 20 isotropic, shows a stable irradiation behavior like UAl 3 , and has a high uranium density (6.64 g-U/cm), UAl 2 may reduce the volume of a fuel particle, but since UAl 2 is spontaneously ignitable due to high ignition, it is difficult to prepare UAl 2 in a single phase. 25 In a typical method for preparing a uranium aluminide 10 nuclear fuel, UAl, is alloyed, melted and casted to be prepared in an ingot, the ingot is pulverized to prepare UAlx powder, the UAl powder is mixed with aluminum powder, and the resultant mixture is compacted and rolled to prepare a 5 dispersed nuclear fuel. However, the foregoing alloy-melt casting method is difficult to obtain a pure UAl 2 single phase because U-Al intermetallic compounds coexist with each other, accordingly, uranium density in the nuclear fuel is limited within 2.6 g 10 U/cc at most, and when a uranium content is raised, a lot of defectives are generated in a rolling for preparing a target. In the present invention, a uranium ingot and an aluminum ingot are charged into an induction heating furnace at a specific composition ratio, and then are melted in a high 15 temperature to prepare a powder through a centrifugal atomization method. Accordingly, the uranium aluminide powder of single phase UAl 2 having a stable irradiation behavior and a simple preparing process, and in which fission products may be efficiently solid-solutioned, may be prepared. 20 At this time, the aluminum ingot and the uranium ingot are mixed with each other such that aluminum is included in a range of 10 weight% to 25 weight%, preferably, in a range of 10 weight% to 20 weight% or 20 weight% to 25 weight% with respect to the total weight of the mixture. When aluminum is 25 included in an amount of less than 10 weight% with respect to 11 the total weight of the mixture, a great amount of aluminum is evaporated due to a low melting point of aluminum, so that a fraction of a phase existing in a U state may be greater than that of a UAl 2 phase, and when aluminum is included in an 5 amount of more than 25 weight% with respect to the total weight of the mixture, a lot of a UAl 3 phase may exist to reduce uranium density in the powder. The melting of the step 1 may be performed in a temperature range of 1,800'C to 2,000C. 10 When the melting of the step 1 is performed at a temperature of less than 1,800'C, a molten alloy of uranium and aluminum is not properly formed and thus may not be discharged on a disk through a nozzle, and when the melting of the step 1 is performed at a temperature of more than 2,000'C, costs may 15 be increased due to an unnecessary temperature rise. The vacuum atmosphere may have a pressure of 10-3 torr or 20 torr. When the vacuum atmosphere has a pressure of less than 10-3 torr, costs may be increased due to generation of 20 unnecessary vacuum, and when the vacuum atmosphere has a pressure of more than 30 torr, aluminum is evaporated to raise a pressure in a melting chamber, so that the possibility of explosion is increased. In a method for preparing a UAl 2 powder according to the 25 present invention, the step 2 is a step of preparing the 12 molten alloy melted in the step 1 into a powder by using a centrifugal atomization method. The centrifugal atomization method of the present invention may be performed by using an apparatus for preparing 5 a nuclear fuel powder disclosed in Korea Patent No. 10-279880. In detail, the centrifugal atomization method is a method in which when a molten alloy is discharged on a disk through a nozzle, a fine particle is formed by a centrifugal force to fly to a chamber wall, is cooled by a cooling gas during the 10 flying, and is formed into a spherical fine powder when reaching the chamber wall, to be collected in a collection container. At this time, a size of the alloy particle powder may be differently distributed from about 50 pm to about 500 pm 15 according to a rotation speed, a molten metal supply amount, a disk size, and molten metal temperature. Since a particle of the molten metal has a very large specific surface area, the particle of the molten metal has a fast solidification rate to have a fine crystal grain structure. 20 When a particle is prepared through the foregoing method, a preparing process is simple, a loss rate of the particle is lowered in preparing of the particle to improve productivity, and also impurities are prevented from being mixed into the particle, thereby considerably raising purity. 25 Also, another reason why it is preferable to prepare a 13 uranium particle powder by using the centrifugal atomization method rather than a typical method in which a uranium alloy ingot is prepared and then pulverized to prepare a uranium particle powder, is because a generated particle has a 5 spherical shape and is a fine particle. When powder is prepared through the typical method, a crystal grain size is about several hundreds pm, but when powder is prepared through the centrifugal atomization method, a fine crystal grain structure having a size of about several pm is formed. 10 In the present invention, since a uranium particle has a spherical shape, a rolling is well performed, and a swelling of the small grain size according to neutron irradiation and a temperature change is low to improve target performance. The present invention provides a UAl 2 powder prepared 15 according to the preparing method. Since UAl 2 is a material having a face centered cubic structure that is crystallographically isotropic, shows a stable irradiation performance like UAl 3 , and has a high uranium density (6.64g-U/cm 3 ), UAl 2 can reduce the volume of a 20 fuel particle. A uranium ingot and an aluminum ingot are charged into an induction heating furnace at a specific composition ratio, and then are melted in high temperature to prepare a UAl 2 powder according to the present invention through a centrifugal 25 atomization method. Accordingly, the UA-l 2 may be prepared 14 through a simple preparing process compared to a typical pulverizing method and thus be mass-producted, thereby providing the UAl 2 at a low price, and the UAl 2 may be provided as a single phase UAl 2 particle powder having a stable 5 irradiation performance and in which fission products may be efficiently solid-solutioned. At this time, the UAl 2 powder may use a particle having a diameter of 10 pm to 150 pm, and only the particle having the diameter may be selected through sieving. 10 The swelling of a small-sized powder is low according to neutron irradiation and a temperature change to be advantageous for preparing a target, thereby considerably reducing a defective rate such as a thickness reduction of a cladding material due to an excessive dog-bone when a rolling 15 process is performed after preparing a powder compact, and thereby raising a uranium content in the target. The present invention provides a method for preparing a low-enriched uranium target, which includes: mixing a UAl 2 powder and an aluminum powder to prepare a powder compact 20 (step 1); and rolling the powder compact of the step 1 (step 2). Hereinafter, a method for preparing a low-enriched uranium target is described in detail for each step. In a method for preparing a low-enriched uranium target, 25 the step 1 is a step of mixing the UA-l 2 powder and the 15 aluminum powder to prepare a powder compact. The UA1 2 powder may be mass-produced, so that a low enriched uranium target may be prepared at a lower cost, and also, since the swelling of a UAl 2 powder is low according to 5 neutron irradiation and a temperature change, the UAl 2 powder is advantageous for preparing a target, thereby reducing a defective rate. Generally, a powder compact used in preparing a Mo99 irradiation target may be prepared by mixing a uranium powder 10 and an aluminum powder available commercially. At this time, when an aluminum powder in a lump state is used, since the aluminum powder is nonuniformly mixed to reduce homogeneity of a nuclear fuel, a commercial aluminum powder having a size of 100 meshes or less may be used. An aluminum powder sieved may 15 be heated in a high vacuum furnace to remove impurities and moisture. When UAl 2 powder and aluminum powder are uniformly mixed, and then the mixed powder is compacted, a powder compact of UA-l 2 and Al may be prepared. At this time, it is preferable that the UAl 2 powder be 20 mixed in a range of 40 volume% to 50 volume% with respect to total weight of the mixed powder of the UAl 2 powder and the aluminum powder. The reason is for preparing a high density target instead of using low-enriched uranium. When the UAl 2 powder is mixed in an amount of less than 25 40 volume%, an amount of 99Mo generated by nuclear fission is 16 decreased to reduce a production amount, and on the contrary, when the UA-l 2 powder is mixed in an amount of more than 50 volume%, a rolling is difficult. In a method for preparing a low-enriched uranium target, 5 the step 2 is a step of rolling the powder compact of the step 1. The rolling of the step 2 may be performed through a known method, and for example, a typical method for preparing a uranium target for producing 99Mo in which a powder compact 10 is charged into a sandwich type frame, and then the sandwich type frame is assembled and welded to roll the powder compact, may be used as it is, so that additional process costs are unnecessary. The powder compact may be rolled to be prepared in a 15 plate shape, but a shape of the rolled powder compact is not limited thereto. At this time, after performing the step 2, a heat treatment of the rolled powder compact may be further included. 20 Generally, in order to extract 99Mo from the prepared target, Al in the prepared target should be dissolved in a NaOH solution. However, since a UAl 2 phase is insoluble in the NaOH solution, the UAl 2 phase needs to be converted into a UAlx phase. The rolled powder compact prepared in the step 2 25 may be heat-treated in a heating furnace having a temperature 17 of 450'C to 550'C to prepare a target that contains the UAlx phase. The present invention provides a low-enriched uranium target prepared according to the preparing method. 5 A typical U-Al alloy powder fuel is prepared in a state in which a uranium aluminide (UAlz) particle is dispersed in an aluminum matrix, and when an aluminum content is raised to the maximum by using low-enriched uranium, the aluminum content may be raised up to 3 g-U/cc in theory, but when the 10 uranium content is raised to 3g-U/cc, defectives are generated. Also, French CERCA company preparing a dispersed target using low-enriched uranium has announced that when a typical pulverized powder is used, 2.6g-U/cc is the upper limit of 15 uranium density in a dispersed target prepared by using UAl 2 , the reason is why the more a particle charging amount is, the higher a defective rate is, and UAl 2 all reacts and is converted into UAlx. However, a low-enriched uranium target prepared by using 20 a spherical uranium aluminide particle powder through a centrifugal atomization method of the present invention has advantages in that the uranium content thereof is far higher than those of typical products, irradiation performance is excellent and productivity is the same as that of a highly 25 enriched uranium target. 18 At this time, uranium density of the low-enriched uranium target may be in a range of 2.6 g-U/cc to 3.0 g-U/cc. Hereinafter, the present invention will be described in more detail though Examples. However, the following Examples 5 are strictly for the purpose of explaining the present invention, and do not limit the scope of the present invention. <Example 1> Step 1: 117.7 g of an aluminum ingot and 470.5 g of a 10 uranium ingot were mixed with each other such that aluminum is included in an amount of 20 weight% with respect to the total weight of the mixture. This is a ratio in consideration of evaporability of aluminum in melting such that weight% of aluminum is 18.5 weight%. 15 The temperature of the mixture was raised up to 1,860'C in an induction heating furnace under a vacuum atmosphere of 10-3 torr, and then a molten uranium alloy was prepared. Step 2: The molten uranium alloy was poured onto the rotating disk to manufacture a UAl 2 powder by using a 20 centrifugal atomization method. <Example 2> Step 1: 21.18 g of a UAl 2 powder having a size of 150 im or less, which was manufactured in Example 1, and 7.86 g of commercial an aluminum powder having a size of 100 mesh or 25 less, which was heated in a high vacuum furnace and from which 19 impurities and moisture were removed, were mixed with each other. At this time, the mixture has been measured such that uranium density is 3.0 g-U/cc. The mixture was uniformly mixed for 3 hours by using a 5 spex mill, and was green-compacted by using a hydraulic press to manufacture a powder compact. Step 2: The powder compact in the Step 1 was charged into a groove of a center of a picture frame, and the picture frame was covered with an upper and lower aluminum cover plate, 10 welded and then bonded. After that, a rolling was performed to prepare a plate-shaped uranium alloy target. <Example 3> The plate rolled in the Step 2 of Example 2 was heat treated in a heating furnace having a temperature of 500'C to 15 prepare a plate-shaped uranium alloy target containing a UAlX phase. <Experimental Example> The UAl 2 powder prepared in Example 1 and the uranium alloy target prepared in Example 2 were observed by using a 20 scanning electron microscope (SEM) and an X-ray, and then the observed result was shown in FIGS. 3 and 4 As shown in FIG. 3, it may be seen that a fine particle having a diameter of 50 im to 150 im are manufactured. Also, as shown in FIG. 4, it may be seen that a uranium 25 aluminide particle having a diameter of 50 im are contained in 20 the uranium alloy target while maintaining a spherical shape. Like this, it may be seen that since the UAl 2 powder prepared according to the present invention has a fine-sized particle and thus is low in swelling according to neutron 5 irradiation and a temperature change, a good target may be prepared. 21