CA3162473A1 - Method for producing uranium carbide/mwcnt disc which is isol target material, and uranium carbide/mwcnt disc produced by same - Google Patents

Abstract

The present invention relates to an ISOL system target material, and more specifically relates to a method in which a large-area MWCNT/uranium oxide disc is produced, and the disc is heat treated, thereby producing a uranium carbide/MWCNT disc, and also to a disc produced via the method.

Description

DESCRIPTION
TITLE OF INVENTION: METHOD FOR PRODUCING URANIUM CARBIDE/MWCNT
DISC WHICH IS ISOL TARGET MATERIAL, AND URANIUM CARBIDE/MWCNT
DISC PRODUCED BY SAME
Technical Field [1] The present invention relates to an IOS system target material and, more particularly, to a method for manufacturing uranium carbide/MWCNT disk by preparing a large-area MWCNT/uranium oxide disk and heat-treating the disk and to a disk manufactured by the method.
Background Art

[2] An ISOL system is a system that produces various kinds of neutron-excess rare isotopes (RI) through proton-induced fission by injecting a proton beam (70 MeV, 10 kW) onto a target material existing in the system. During the RI
production process, the inside of the ISOL system has a high temperature of about 2,000 C due to the large amount of heat generated by fission. In dissipating the heat generated from a target material to the outside of the product and discharging the generated rare isotopes, the thermal conductivity and porosity of the target material and the particle size of a uranium material are important.

Date Recue/Date Received 2022-05-19

[3] The target material that has been considered to be feasible so far is a disk in which a uranium material is uniformly distributed in a matrix. A
multi-wall carbon nanotube (MWCNT), which is a carbon allotrope, is cohmonly used as the matrix. This matrix is a net-like structure and is advantageous in that it is porous and has high thermal conductivity.

[4] Examples of the uranium material distributed in the matrix include metal uranium, uranium oxide, and uranium carbide. In the case of metal uranium, although the density of metal uranium (i.e., nuclear fission density) is high, it cannot be used because it has a melting point of 1132 C, which is lower than the internal temperature (2,000 C) of the ISOL internal system. That is, there is likelihood that the metal uranium melts in the system. Uranium carbide has advantages of being able to produce rare isotopes in a large amount due to a higher fission density than uranium oxide and of efficiently discharging heat generated by fission to the outside due to the high thermal conductivity thereof. For these reasons, it is preferable to use uranium carbide as the target material.

[5] The target material has been conventionally prepared by powder metallurgy. That is, in the conventional target material production method, uranium oxide powder is charged into dies of a press molding apparatus to produce a molded disk, and then the molded disk is subjected to a carbothermic Date Recue/Date Received 2022-05-19 reduction process so that uranium oxide is converted into uranium carbide. However, this conventional production method can produce relatively small-area disks with satisfactory quality but cannot produce large-area thin disks with good quality because the large-area thin disks may be bent in the press molding process in which uranium oxide powder is press-molded into a molded disk or in the high-temperature heat treatment process in which the molded disk is converted into uranium carbide.
Disclosure Technical Problem

[6] The present invention has been made to solve the problems of the conventional art and the objective of the present invention is to provide a method for manufacturing a large-area thin uranium carbide disk without breakage and bending and to provide a uranium carbide disk manufactured by the method.
Technical Solution

[7] In order to achieve the above objective, there is provided a method for manufacturing a uranium carbide/MWCNT disk, the method including: drying and wet grinding uranium oxide to prepare a uranium oxide powder; mixing and dispersing the uranium oxide powder and MWCNTs in a solvent to prepare a homogeneous uranium oxide/MWCNT mixed solution; drying the Date Recue/Date Received 2022-05-19 homogeneous mixed solution to prepare a uranium oxide/MWCNT
mixed powder; inserting the mixed powder into a mold of a press molding apparatus and then press-molding the mixed powder into a molded uranium oxide/MWCNT disk; and heat-treating the molded disk.

[8] The griding may be performed for 200 to 240 minutes at 500 rpm using a planetary mill.

[9] The uranium oxide may be either 1J308 or 1J02.

[10] The uranium oxide powder may have a particle size in a range of 0.10 pm to 0.25 pm.

[11] The press-molding apparatus includes a pair of anvils, and each of the anvils may have a concave or convex shape.

[12] The anvil may have a depth in a range of 50 to 100 pm.

[13] The carbon-thermal reduction step may be performed at a temperature in a range of 1250 C to 2000 C and a pressure in a range of 10-4 torr to 10-6 torr.

[14] The carbothermic reduction reaction may be performed after inserting a spacer between each of the disks.

[15] In addition, the present invention provides a uranium carbide/MWCNT disk prepared by the method described above.

[16] The uranium carbide/MWCNT disk may have a thickness in a range of 1.0 um to 1.5 um. The uranium carbide/MWCNT disk may have a diameter in a range of 45 um to 55 um.
Advantageous Effects Date Recue/Date Received 2022-05-19

[17] With the use of the present invention, it is possible to manufacture a large-area thin uranium carbide/MWCNT disk without breakage or bending.
Description of Drawings

[18] FIG. 1 is a process flowchart illustrating a method of manufacturing a uranium carbide disk, according to the present invention.

[19] FIG. 2 is a view illustrating a uranium carbide disk manufactured by the manufacturing method of the present invention.

[20] FIG. 3 is a photograph of the microstructure of MWCNTs.

[21] FIG. 4 is a photograph of the microstructure of a U308/MWCNT mixed powder.

[22] FIG. 5 is a particle size analysis graph of 1J308 powder.

[23] FIG. 6 is a conceptual diagram of a hydraulic molding apparatus having anvils.

[24] FIG. 7 is a conceptual diagram of the convex and concave anvils used to manufacture a molded disc of the present invention.

[25] FIG. 8 is a view illustrating a large-diameter uranium oxide/MWCNT disk manufactured with or without anvils, in which

[26] (a) shows a case where only upper and lower punches are used without using anvils, and

[27] (b) shows a case where a concave anvil and a convex anvil Date Recue/Date Received 2022-05-19 are used in combination.

[28] FIG. 9 is a view illustrating the shapes of uranium carbide/MWCNT disks depending on the presence and absence of a spacer in a carbothermal reduction step, in which the left figure corresponds to the case where the spacer is present and the right figure corresponds to the case where the spacer is absent.
Best Mode

[29] Hereinafter, the present invention will be described in detail.

[30] In the step of preparing a uranium oxide powder, the uranium oxide may be U308 or UO2õ, (0x0.15). UO2 (theoretical density: 10.96 g/cm3) sintered pellets are converted into U308 (theoretical density: 8.38 g/cm3) pellets when the UO2 sintered pellets are oxidized at an atmospheric atmosphere and at about 400 C to 500 C. At this time, powdering occurs due to volume expansion attributable the density difference between materials. There are many microcracks on the surface of the particles of the resulting powder. Since 1J308 has a constant 0/U ratio of 2.67, it maintains a stable state regardless of the atmosphere, thereby being powdered under atmospheric conditions. The uranium oxide can be easily powdered due to microcracks on the surface of the 1J308 particles, and an accurate amount of MWCNT measured according to the Date Recue/Date Received 2022-05-19 stoichiometric ratio can be added. On the other hand, when using a powder of UO2..õ (generally 0)c0.15), the 0/U ratio must be measured to weight and add the accurate amount of MWCNT
according to the 0/U ratio, or a reduction reaction in a hydrogen atmosphere at 600 C to 700 C must be performed to obtain 1J02. In addition, since the UO2 obtained through the reduction reaction is highly hygroscopic and easily oxidized, the 1J02 powder must be stored in an atmosphere of an inert gas such as argon.

[31] As a method of grinding 1J308 powder, dry grinding or wet grinding is generally used. However, to remove heat generated during high-speed rotation, it is preferable to use wet grinding. The grinding may be performed using a planetary mill. Preferably, the griding may be performed for 200 to 240 minutes at 500 rpm using a planetary mill. The uranium oxide powder obtained through grinding under the conditions may have a particle size of 0.10 pm to 0.25 pm.

[32] MWCNTs in the step of preparing the uranium oxide/MWCNT
mixture have a porous structure. As shown in FIG. 3, MWCNTs have a porous structure like an unraveled skein with tangled fine threads. The particles of the uranium oxide powder homogeneously distributed in the porous net structure are converted into uranium carbide through heat treatment and fixed in place. The net structure of MWCNTs locally generates heat during the collision of proton beams, thereby preventing the Date Recue/Date Received 2022-05-19 generation of a large amount of heat and facilitating the release of heat. However, since MWCNTs have a tangled skein structure, the MWCNTs are difficult to disperse. Therefore, it is preferable to use short MWCNTs and add a dispersant.
Ethanol is used as a solvent, and a dispersant may be added to obtain a homogeneous mixture. The dispersant serves as a binder and a dispersant at the same time.

[33] The step of preparing the uranium oxide/MWCNT mixed powder includes drying the mixed solution and performing grinding.
The powder agglomerates immediately after the dispersion is dried and thus the powder is unsuitable for molding.
Therefore, it is preferable that the mixed powder preparation includes the grinding step.

[34] The step of manufacturing the uranium oxide/MWCNT disk may be performed using a concave or convex anvil. FIG. 6 illustrates a molding apparatus for press-molding using the prepared 1J308/MWCNT/dispersant mixed powder. There are hydraulic or mechanical types of molding apparatuses, but a hydraulic molding apparatus is used in the embodiment. There are also double-acting or single-acting hydraulic molding apparatuses, but the single-axis hydraulic molding apparatus is used in the embodiment. The hydraulic molding apparatus generally includes dies and two punches (i.e., upper and lower punches). The shape of the face of the punch coming into contact with the powder may vary. As the diameter of the punch Date Recue/Date Received 2022-05-19 increases, it is more difficult to make a punch with a complicated shape, and the loss is increased in case of failure.
Therefore, the surface of the punch is generally flat. When the powder loaded into the die is pressed into a large-area thin disk with a large diameter by the flat surface of the punch, most of the molding disks are bent or damaged after being drawn out from the molding apparatus. This is because, as the diameter of the disk increases, the flow of powder does not smoothly occur on the flat surface of the punch during the pressing, so that it is difficult to uniformly transfer the pressure. There is a problem in that the disk is bent during molding when using a general flat anvil. However, with the use of a concave or convex anvil, it is possible to manufacture a defect-free disk by offsetting the bending force during molding.

[35] Next, in the step of manufacturing the uranium carbide/MWCNT disk, the carbothermal reduction reaction is preferably performed at a high temperature of about 2,000 (= or more and a vacuum pressure of 10- 6 torr or less. The purpose of performing the reaction at or above 2,000 C is to prevent the shrinkage of the uranium oxide/MWCNT disk and to enable the uranium oxide/MWCNT disk to be maintained in a stable state in the groove of a graphite container because the internal temperature of the ISOL system exceeds 2000 C due to the high-Date Recue/Date Received 2022-05-19 temperature heat generated by induced nuclear fission. In addition, the purpose of performing the reaction in a high vacuum condition of 10-6 torr or less is to completely convert 1J308 to UC2 and to remove volatile substances or impurities generated from the dispersant or the like.

[36] 1J308 is converted to uranium carbide by the following two-step chemical reaction.

[37] 1. 17308 + 19C -> 31JC2 + 18C + CO2 (600 C to 700 C)

[38] 2. 17308 + 18C - 31JC2 + 6C + CO2 (1250 C to 2000 C)

[39] In general, heat treatment may be performed in a state in which several cylindrical disks are stacked. However, in the case where the disks are thin, since the disks may stack to each other, it is difficult to separate the disks from each other at the time of separating the disks after the heat treatment. Therefore, it is difficult to obtain quality disks with a good shape. Therefore, as shown in FIG. 9, multiple cylindrical disks are vertically placed in the respective grooves formed at regular intervals in the graphite container, and then loaded into a furnace and heat-treated. However, even in this case, when a thin large-area molded disk is heat-treated, the disk contracts due to sintering and bends due to the weight thereof (see the right side of FIG. 9). Therefore, to solve this problem, as shown in FIG. 9, graphite spacers having different thicknesses were installed. That is, by inserting the cylindrical disk between the spacers, it is Date Recue/Date Received 2022-05-19 possible to manufacture a flat disk without bending during the heat treatment.

[40] The uranium carbide/MWCNT disk manufactured by the method according to the present invention has a diameter of 45 to 55 him and a thickness of 1.0 to 1.5 hmt.

[41] The details of the present invention described with reference to the accompanying drawings are presented only for illustrative purposes, and it will be apparent to those skilled in the art that the scope of the present invention is not limited thereto.

[42] 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 skill in the art that the scope of the present disclosure is not to be construed as being limited by these examples.

[43] Example - Method of Manufacturing Uranium Carbide/MWCNT
Disk

[44] As illustrated in FIG. 1, a uranium carbide disk was manufactured through grinding, mixing and dispersing, molding, and heat treatment of a raw material powder.

[45] The grinding of the raw material powder is a process of pulverizing U308 powder, which is a uranium raw material, using a planetary ball mill. The volume of a container used for the grinding is 100 mL, and the grinding surface is made of Date Recue/Date Received 2022-05-19 tungsten carbide. The U308 powder pulverized by one grinding process was 18 g, and as the media used in the mill, 130 g of a mixture of tungsten carbide balls with diameters of 3 hmt, 1.6 hmt, and 1 mm. After quantification of 1J308 powder and tungsten carbide balls, wet grinding was performed with about 20 mL of IPA. The planetary ball mill was operated at a rotation speed of 500 rpm for a total of 6 hours with cycles of 3-minute grinding and 1-minute stop.
Tungsten carbide balls were filtered out to be separated from the pulverized U308 powder, and the pulverized 1J308 powder was dried with a rotary evaporator so that the IPA was volatilized and fine 1J308 dry powder was prepared. The U308 powder was in an agglomerate state immediately after the drying, and thus the U308 powder was difficult to mix with other raw materials. Thus, the U308 dry powder was pulverized with a mortar. FIG. 5 is a particle size analysis result of the U308 dry powder according to the net grinding time (not including the stop time). The grinding was performed for at least 200 minutes so that the powder had a particle size of 0.2 pm or less.

[46] The pulverized 1J308 powder was uniformly mixed with multi-walled carbon nanotubes (MWCNTs) to prepare a 1J308/MWCNT mixed powder. Since MWCNTs have a tangled skein-like microstructure as illustrated in FIG. 3, there is a problem in that the MWCNTs are difficult to disperse. To solve this problem, MWCNTs with a relatively short length of 2 pm or less were used, and a Date Recue/Date Received 2022-05-19 dispersant (WinSperse4090, Youngjin Corporation) that improves the dispersibility of carbon-based materials was used.

[47] After weighting 6.2 g of the 1J308 powder, 1.75 g of the MWCNTs, 0.52 g of the dispersant, and about 400 mL of ethanol, the raw materials were wet-mixed and dispersed using an ultrasonic cleaner and a stirrer at the same time. In the case, the output power of the ultrasonic cleaner was maintained at the maximum value, and the stirrer was maintained at 150 rpm for 3 hours. The mixed U308/MWCNT mixed powder was dried for about 30 minutes at a temperature in the range of 50 C to 70 C
using a rotary evaporator. The powder immediately after the drying was in an agglomerated state and was thus unsuitable for molding. Therefore, the powder was pulverized with a mortar and sieve.

[48] The 1J308/MWCNT powder was molded using the mold of FIG. 6.
In this case, a molded disk having a diameter of 50 01111 can be formed using the anvil of FIG. 7. FIG. 8 is a view showing the degree of warpage of the molded disk according to the presence or absence of the anvil of the present invention. The diameter and thickness of the molded disk for use as the ISOL target of RISP is 50 0mt and about 1.3 mm. That is, since the molded disk is long and thin, when a conventional anvil is used, bending occurs. The molding pressure was about 200 MPa which was maintained for 1 minute.

[49] The 1J308/MWCNT disk becomes a uranium carbide disks through Date Recue/Date Received 2022-05-19 heat treatment in a vacuum condition. The operating temperature of the RISP 10-kW ISOL system is about 2000 C.
Therefore, the heat treatment temperature must be 2000 C or higher. However, since the limit temperature of the currently available heat treatment system is 1700 C, the heat treatment was carried out at a maximum of 1670 C with a vacuum degree of 10-3 torr maintained.

[50] The molded disks of the 1J308/MWCNT/ dispersant were placed at regular intervals in a graphite container as shown in FIG.
9, and graphite spacers were inserted between each of the molded disks to prevent the molded particles from being warped during the sintering.

[51] A heat treatment apparatus was heated up to 1670 C in a way that the temperature was constantly raised by IA per minute by the Joule heating method and then the temperature raising was paused when the vacuum level exceeded 10-3 torr. The temperature raising was paused in a temperature range of around 100 C at which moisture volatilized, in a temperature of 200 C
to 500 C in which the dispersant volatilized, and in a temperature range of 1250 C or higher in which uranium carbide was synthesized. When the vacuum degree was lowered to 10-5 torr or less when the temperature was maintained at 1670 C, the heat treatment was finished. As a result, a thin large-area uranium carbide/MWCNT disk as shown in FIG. 9 was produced.

[52] As described above, a particular part of the present Date Recue/Date Received 2022-05-19 disclosure has been described in detail, and the specific description is only a preferred embodiment, and the fact that the scope of the present disclosure is not limited thereby will be clear that to those of ordinary skill in the art. Thus, the substantial scope of the present invention will be defined by the appended claims and their equivalents.
Date Recue/Date Received 2022-05-19

Claims (11)

1. A method of manufacturing a uranium carbide/MWCNT disk, the method comprising:
drying and wet grinding uranium oxide to prepare a uranium oxide powder;
mixing and dispersing the uranium oxide powder and MWCNTs in a solvent to prepare a homogeneous uranium oxide/MWCNT mixed solution;
drying the mixed solution to prepare a uranium/MWCNT mixed powder;
inserting the mixed powder into a mold of a press molding apparatus and performing press-molding to obtain a uranium oxide/MWCNT disk; and causing the molded disk to undergo a carbothermic reduction reaction.

2. The method of claim 1, wherein the grinding is performed at a speed of 500 rpm for 200 to 240 minutes using a planetary mill.

3. The method of claim 1, wherein the uranium oxide is U308 or UO2.

4. The method of claim 1, wherein the uranium oxide powder has a particle size in a range of 0.10 to 0.25 pm.

5. The method of claim 1, wherein the press molding apparatus is equipped with a pair of anvils, and each of the anvils is a concave or convex anvil.

6. The method of claim 5, wherein each of the anvils has a depth in a range of 50 to 100 pm.

7. The method of claim 1, wherein the carbothermic reduction reaction may be performed at a temperature in a range of 1250 C to 2000 C and a pressure in a range of 10-4torr to 10-torr.

8. The method of claim 1, wherein the carbothermic reduction reaction is performed after inserting spacers between each of the molded disks.

9. A uranium carbide/MWCNT disk manufactured by the method of any one of claim 1 or 8,

10. The uranium carbide/MWCNT disk of claim 9, wherein the uranium carbide/MWCNT disk has a thickness in a range of 1.0 to 1. 5 mut .

11. The uranium carbide/MWCNT disk of claim 9, wherein the uranium carbide/MWCNT disk has a diameter in a range of 45 to 55 hmt.