Titanium-gadolinium alloy composition with excellent neutron absorption capacity and tensile properties, and neutron absorbing structural materials manufactured using the alloy

Abstract

The present invention improves upon the low strength of conventional Al-B4C composite-based neutron absorbers and exhibits superior ductility, allowing it to function not only as a neutron absorber but also as a structural neutron absorber. It also overcomes the problems inherent in boron-containing corrosion-resistant steels, such as poor formability during manufacturing and high brittleness in the finished product. It also eliminates the need for separate supports or structural materials, significantly improving the efficiency and manufacturing economy of spent nuclear fuel storage facility design and construction. It also enables designs that can store more spent nuclear fuel within a given volume of spent nuclear fuel storage vessel, maximizing utilization of limited space.

Description

[Technical Field]

[0001] The present invention relates to a titanium-gadolinium-based neutron-absorbing structural alloy composition having excellent neutron absorption capacity and tensile properties, and a neutron-absorbing structural material manufactured using the same. More specifically, the present invention relates to a neutron-absorbing structural alloy composition having excellent mechanical strength, ductility, and neutron absorption capacity compared to conventional commercial Al-B4C composites or boron-containing corrosion-resistant steels, and a neutron-absorbing structural material manufactured using the same. [Background technology]

[0002] Spent nuclear fuel, which is discarded after being used to generate electricity at nuclear power plants, is stored in wet or dry storage facilities before reaching the final disposal stage. During this process, the spent nuclear fuel undergoes cooling and radioactive decay. During the storage and transportation process, neutron absorbers are installed between spent nuclear fuel bundles to prevent neutron multiplication due to nuclear fission reactions of radioactive nuclides remaining in the spent nuclear fuel and the resulting criticality. Neutron poisons contained in the neutron absorbers absorb neutrons of various energies, maintaining the subcriticality of the spent nuclear fuel storage system. Commonly known neutron poisons include boron (B), gadolinium (Gd), cadmium (Cd), indium (In), hafnium (Hf), and samarium (Sm).

[0003] In addition, the spent nuclear fuel storage industry is focusing on the development of so-called neutron-absorbing structural materials that not only have neutron absorption capabilities but also have structural capabilities themselves, in order to efficiently store spent nuclear fuel and efficiently design and manufacture spent nuclear fuel storage systems. In other words, if a neutron absorber can also function as a structural material in addition to neutron absorption, it will not require a separate support or structural material, which will increase the efficiency and manufacturing economy of the design and manufacturing of spent nuclear fuel storage facilities. In some cases, it will be possible to design a spent nuclear fuel storage container with a certain volume so that more spent nuclear fuel can be stored, maximizing the usability of limited space.

[0004] However, the neutron poisons and composite materials using them used in neutron absorbing structural materials that have been researched to date have limitations in terms of their neutron absorbing performance and their application to neutron absorbing structural materials that themselves have the performance of structural materials due to the following problems:

[0005] First, Al-B4C composites (or Al-B4C cermets) used as neutron absorbers are typically manufactured in the form of plates via powder metallurgy processes. However, these composites suffer from low strength and high brittleness, making them difficult to use as neutron absorber structures. Specifically, Al-B4C composites contain a high percentage of brittle B4C particles dispersed within the matrix metal, which can reduce the structural stability of the composite or make it highly susceptible to impact or breakage. Therefore, reducing the volume fraction of the B4C particles to reduce brittleness reduces neutron absorption, making it difficult to achieve the intended purpose of the neutron absorber. Finally, the commonly used Al-B4C composites require a separate structural material, limiting their ability to maximize space utilization.

[0006] Second, while research is underway into new additives or materials to improve the low strength of Al-B4C composites, no research has been reported on structural materials that have excellent neutron absorption capacity, high strength, and high ductility, and that satisfy all the efficiency and cost-effectiveness of the design and construction of spent nuclear fuel storage facilities. More specifically, there have been attempts to use high-strength Al alloy powders (Al3000, 5000, and 6000 series alloys) in addition to pure aluminum powder in Al-B4C composites, but in these cases, the strength of the Al alloy matrix is very high, making it difficult to manufacture into plate material when a large amount of B4C is added. In addition, boron-containing corrosion-resistant steel (BORS, ASTM A887-20, Grade A) is commercially available and manufactured by powder metallurgy using austenitic corrosion-resistant steel 304 alloy with up to 2.25% by weight (wt.%) of boron (B). However, because it is manufactured using powder metallurgy, the manufacturing costs are relatively high, which is disadvantageous in terms of the efficiency and manufacturing economy of the design and manufacturing of spent nuclear fuel storage facilities.

[0007] Third, in an effort to replace the conventional Al-B4C composite and improve the economic viability of austenitic corrosion-resistant alloy 304, a material manufactured through a wrought process (casting / rolling) rather than powder metallurgy (ASTM A887-20, Grade B) has been commercialized. While this is economically advantageous, it produces a large amount of brittle (Fe,Cr)2B compounds, making hot working in sheet form difficult and resulting in very low ductility and impact toughness even after fabrication. Furthermore, the presence of a high volume fraction of (Fe,Cr)2B compounds limits weldability for fabricating spent nuclear fuel storage baskets (square tubes). Furthermore, the limited amount of boron that can be added reduces criticality control performance.

[0008] Therefore, there is an urgent need to research new materials that can address the problems of boron-containing corrosion-resistant steel, such as poor formability during the manufacturing process and high brittleness of the finished product, while also resolving the strength and brittleness issues of Al-B4C composites, and that can simultaneously function as a neutron absorber and structural material by having a certain level of neutron absorption capacity or higher. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been devised to overcome the above-mentioned problems. The object of the present invention is to provide an alloy composition for a neutron-absorbing structural material, which can improve the low strength of conventional Al-B4C composite-based neutron absorbers and exhibit superior ductility, thereby simultaneously performing not only the neutron absorbing function but also the function of a neutron-absorbing structural material that has the performance of a structural material itself, and a neutron-absorbing structural material made thereof.

[0010] In addition, the present invention aims to provide a neutron-absorbing structural alloy composition that overcomes the problems of boron-containing corrosion-resistant steel, such as low formability during the manufacturing process and high brittleness of the finished product, and does not require a separate support or structural material, thereby significantly improving the efficiency and manufacturing economy of the design and manufacturing of spent nuclear fuel storage facilities, enabling designs that allow more spent nuclear fuel to be stored in a spent nuclear fuel storage container with a certain volume, and maximizing usability in limited spaces. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the present invention provides a neutron-absorbing structural alloy composition including a base metal and 2 to 49 wt % of gadolinium (Gd) based on the total weight of the base metal.

[0012] The base metal is preferably titanium.

[0013] The neutron absorbing structural alloy composition is characterized in that it does not contain hafnium (Hf).

[0014] The present invention also includes the above-mentioned neutron absorbing structural alloy composition and a residual amount of oxygen, and a part of the gadolinium in the neutron absorbing structural alloy composition may be dissolved in the base metal, and the remaining part may be dispersed in the form of an α-gadolinium phase (α-Gd phase).

[0015] The residual oxygen content is also characterized by being less than 0.3% by weight relative to the total weight of the neutron absorbing structural material.

[0016] The neutron absorbing structural material is characterized in that it satisfies all of the following relational expressions (1) to (3). (1) Yield strength less than 550 MPa (2) Ultimate tensile strength less than 650 MPa (3) Total elongation of 22% or more

[0017] The present invention also provides a method for manufacturing a neutron-absorbing structural material, including a first step of preparing the neutron-absorbing structural alloy composition described in claim 1, a second step of melting the neutron-absorbing structural alloy composition to produce a molten ingot, a third step of hot-forging the molten ingot and rolling it to produce a rolled material, and a fourth step of heat-treating the rolled material.

[0018] The fourth step is characterized by performing a β-phase heat treatment in which the alloy is heat-treated at a temperature of 900°C or higher and then cooled in air or water, or by performing a recrystallization heat treatment at a temperature of 900°C or lower. [Effects of the Invention]

[0019] The present invention improves upon the low strength of conventional Al-B4C composite-based neutron absorbers and exhibits superior ductility, allowing them to function not only as neutron absorbers but also as structural neutron absorbers. Furthermore, it overcomes the problems inherent in boron-containing corrosion-resistant steels, such as poor formability during manufacturing and high brittleness in the finished product. Since it does not require a separate support or structural material, it significantly improves the efficiency and manufacturing economy of spent nuclear fuel storage facility design and construction. It also enables designs that allow for the storage of more spent nuclear fuel within a given volume of spent nuclear fuel storage vessel, maximizing the use of limited space. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a graph showing the neutron absorption capacity of a neutron absorbing structural material according to one embodiment of the present invention. [Figure 2] FIG. 2 is a graph showing a room-temperature tensile test curve of a neutron absorbing structural material according to one embodiment of the present invention. [Figure 3] FIG. 3 is a low-magnification microstructure image of a neutron absorbing structural material according to one embodiment of the present invention. [Figure 4] FIG. 4 is a high-magnification microstructure image of a neutron absorbing structural material according to one embodiment of the present invention. [Figure 5] FIG. 5 is a microstructure image showing the spatial distribution of crystal orientation in a neutron absorbing structural material according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention may be embodied in various different forms and is not limited to the embodiments set forth herein.

[0022] Previously, the boron-containing corrosion-resistant steel or Al-B4C composite materials used in spent nuclear fuel storage facilities had problems such as poor formability during the manufacturing process and high brittleness and low strength in the finished product. However, there has been insufficient research into neutron poisons and composite materials using these that can simultaneously function as neutron absorbers and structural materials, which has limited the ability to improve the efficiency and manufacturing economy of the design and manufacturing of spent nuclear fuel storage facilities and maximize the utilization of the limited space in spent nuclear fuel storage containers.

[0023] Therefore, the present invention seeks to solve the above-mentioned problems by providing a neutron-absorbing structural alloy composition comprising a base metal and 2-49 wt % gadolinium (Gd) based on the total weight of the base metal.

[0024] Therefore, the present invention can improve the low strength of conventional Al-B4C composite-based neutron absorbers and exhibit superior ductility, allowing it to function not only as a neutron absorber but also as a structural neutron absorber. Furthermore, it overcomes the problems of boron-containing corrosion-resistant steel, such as poor formability during manufacturing and high brittleness of the finished product. Since it does not require a separate support or structural material, it significantly improves the efficiency and manufacturing economy of spent nuclear fuel storage facility design and construction, enabling designs that can store more spent nuclear fuel within a given volume of spent nuclear fuel storage vessel, maximizing the use of limited space.

[0025] The neutron absorbing structural material according to the present invention will be specifically described below.

[0026] The present invention, which is applied to a spent nuclear fuel storage system, is manufactured in the form of a plate and then fabricated and installed in the form of a square tube that wraps around the periphery of the spent nuclear fuel bundle. It must absorb neutrons emitted from the spent nuclear fuel and reduce the criticality of the spent nuclear fuel to 0.95 or less. In addition, the spent nuclear fuel storage industry, to which the present invention pertains, is focusing on the development of so-called neutron-absorbing structural materials that have not only neutron-absorbing capabilities but also structural capabilities themselves in order to efficiently store spent nuclear fuel and efficiently design and manufacture spent nuclear fuel storage systems. If a neutron absorber can also function as a structural material in addition to neutron absorption, it would eliminate the need for a separate support or structure, thereby improving the efficiency and manufacturing economy of the design and manufacture of spent nuclear fuel storage facilities. In some cases, it would be possible to design a spent nuclear fuel storage container with a certain volume so that more spent nuclear fuel can be stored, maximizing the usability of limited space. Therefore, the material to be developed in this invention is a neutron-absorbing structural material that not only has an excellent level of neutron absorption ability that can reduce the criticality in spent nuclear fuel storage systems to 0.95 or less, but also can function as a structural material at the same time.

[0027] Therefore, the neutron-absorbing structural alloy composition according to the present invention comprises a base metal and a large amount of gadolinium (Gd) added to the base metal.

[0028] The neutron absorbing structural material of the present invention simultaneously serves as a structure for supporting spent nuclear fuel and as a criticality control element for suppressing nuclear reactions, and effectively releases decay heat emitted from spent nuclear fuel to the outside, thereby improving thermal stability by suppressing the temperature rise of spent nuclear fuel.

[0029] For this purpose, the base metal may be a metal having excellent corrosion resistance and specific strength so as to be able to function as a structural material, and most preferably titanium (Ti) may be used.

[0030] In addition, boron, which has been dispersed in base metals for neutron absorption, is a typical neutron poison used in the nuclear industry for neutron absorption. Naturally occurring boron is 10 B and 11 Two isotopes of B make up approximately 19.9% and 80.1% of the total. 10 B has been used in neutron poisoning because it exhibits a high neutron absorption cross section.

[0031] However, the boron particles ( 10 Conventional Al-B4C composites (or Al-B4C cermets) containing B generally contain a high proportion of B4C particles, resulting in low strength and high brittleness, making them unsuitable for use as structural materials. To improve the strength of Al-B4C composites, high-strength Al alloy powders (Al3000, 5000, and 6000 series alloys) are sometimes used in addition to pure Al powder. However, in these cases, the strength of the Al alloy matrix is so high that adding a large amount of B4C makes it difficult to manufacture them into sheet materials.

[0032] Therefore, unlike conventional Al-B4C composites that contain boron for neutron absorption, the neutron absorbing structural material of the present invention uses gadolinium to solve the problems caused by the brittleness of B4C and the problems with plate forming that arise from its high strength.

[0033] Gadolinium, like boron (B), cadmium (Cd), indium (In), hafnium (Hf), and samarium (Sm), is an element used as a neutron poison in the nuclear industry due to its high neutron absorption capacity. Boron, which is used as a neutron poison in many commercial neutron absorbers, such as boron-containing corrosion-resistant steel and Al-B4C composites, has a thermal neutron absorption cross section of 767 barn. Gadolinium, however, has a thermal neutron absorption cross section of 49,700 barn, approximately 64 times higher. Therefore, the present invention achieves significantly better neutron absorption capacity than boron, even with the addition of a small amount of gadolinium.

[0034] More specifically, referring to FIG. 1 showing the neutron absorption capacity of the neutron absorbing structural material manufactured by the titanium-gadolinium neutron absorbing structural alloy composition in which titanium is used as the base metal and different gadolinium contents are added to titanium according to a preferred embodiment of the present invention, it can be seen that the embodiment of the neutron absorbing structural material according to the present invention exhibits a neutron absorption capacity superior to that of Al-B4C composite and boron-containing corrosion-resistant steel, which are commercial neutron absorbing materials. That is, the neutron absorption capacity of the titanium-gadolinium alloy depending on the gadolinium content is calculated based on the boron equivalent (B eq ) is calculated based on the following mathematical formula 1 and compared with an Al-B4C composite and boron-containing corrosion-resistant steel, which are commercial neutron absorbers. When 6.5 wt.% or more of gadolinium is added to titanium, it is found that the neutron absorption capacity is superior to that of the Al-40 wt.% B4C composite, which has the highest neutron absorption capacity among commercial neutron absorbers. In other words, Example 4 is found to have a superior neutron absorption capacity to that of the Al-40 wt.% B4C composite.

[0035] [Formula 1] B eq =2.532×Gd weight% For this reason, the gadolinium content of the neutron-absorbing structural alloy composition according to the present invention is 2 to 49 wt %, preferably 4 to 20 wt %, and most preferably 6 to 20 wt %. If the gadolinium content of the neutron-absorbing structural alloy composition according to the present invention is less than 2 wt %, the neutron absorption capacity may be reduced due to a lack of gadolinium. On the other hand, if the gadolinium content of the neutron-absorbing structural alloy composition according to the present invention is more than 20 wt %, the neutron absorption capacity may be excellent, but the corrosion resistance and oxidation resistance may be reduced, and the raw material cost may increase. That is, the present invention allows the production of a neutron-absorbing structural material with a gadolinium content of more than 20 wt %, and since the addition of 20 wt % already exhibits sufficient neutron absorption capacity, there is no need to add more gadolinium in a typical spent nuclear fuel storage facility. The amount of gadolinium added may be appropriately selected depending on the intended use and environment.

[0036] Although hafnium (Hf) may be further added, its neutron absorption cross section is 104 barn, which is 1 / 477 of that of gadolinium (49,700 barn), so the addition of hafnium does not sufficiently improve neutron absorption, and the addition of hafnium increases the strength of the base metal but reduces its ductility, which not only contradicts the purpose of the present invention, which is to develop a neutron-absorbing structural material with excellent ductility and formability, but also has the problem of drastically increasing manufacturing costs as an expensive alloying element. For this reason, the alloy composition for the neutron-absorbing structural material according to the present invention does not need to further contain hafnium.

[0037] Furthermore, as described above, the present invention adds gadolinium to ensure excellent neutron absorption, and at the same time, the added gadolinium reacts with oxygen remaining in the base metal to reduce the oxygen content in the base metal, thereby improving the ductility of the base metal.

[0038] More specifically, referring to Table 2 and the room-temperature tensile test curves in Figure 2, tensile test results for neutron-absorbing structural materials manufactured using titanium-gadolinium neutron-absorbing structural alloy compositions containing titanium as the base metal and varying amounts of gadolinium added to titanium according to preferred embodiments of the present invention, generally show an increase in strength and a decrease in ductility with the addition of alloying elements. However, the alloys manufactured using the alloy compositions and heat treatment processes proposed in the present invention show the opposite behavior when gadolinium is added up to 10 wt.%. That is, the comparative example without gadolinium exhibits a yield strength of 550 MPa and a total elongation of 23.8%, while the examples with gadolinium added up to 10 wt.% show a decrease in strength and an increase in elongation as the amount of gadolinium increases. For example, in Example 4, the yield strength decreased to 395 MPa and the total elongation increased to 42.6% compared to the comparative example.

[0039] There are two reasons why the strength of neutron-absorbing structural materials decreases and ductility increases with increasing gadolinium content: First, increasing gadolinium content suppresses the lath martensitic transformation when cooled from the beta phase region at high temperatures, and second, the gadolinium α phase, which exists as a second phase, absorbs oxygen that existed as an interstitial element in the titanium α phase, which serves as the matrix, forming a thin gadolinium oxide (Gd2O3) on the surface of the gadolinium α phase, thereby reducing the oxygen concentration in the titanium α phase.

[0040] Furthermore, even though the yield strength decreased as the amount of gadolinium added increased, all of the examples showed higher strength and elongation than the previously mentioned conventional boron-containing corrosion-resistant steel (UNS S30467, Type 304B7, Grade B: 1.75 to 2.25 wt.%), which is a commercial neutron-absorbing structural material and exhibits a yield strength of approximately 205 MPa and a total elongation of 6%.

[0041] As described above, the present invention improves upon the low strength of conventional Al-B4C composite-based neutron absorbers and exhibits excellent ductility, allowing it to function not only as a neutron absorber but also as a structural neutron absorber. Furthermore, it overcomes the problems inherent in boron-containing corrosion-resistant steels, such as poor formability during manufacturing and high brittleness in the finished product. Since it does not require a separate support or structural material, it significantly improves the efficiency and manufacturing economy of spent nuclear fuel storage facility design and construction, enabling designs that can store more spent nuclear fuel within a given volume of spent nuclear fuel storage vessel, maximizing the use of limited space.

[0042] Next, the neutron absorbing structural material according to the present invention will be described, but to avoid duplication, explanations of parts that have the same technical concept as the alloy composition of the neutron absorbing structural material described above will be omitted.

[0043] The neutron absorbing structural material according to the present invention contains the above-mentioned neutron absorbing structural material alloy composition, a residual amount of oxygen, and contains 2 to 49 wt % of gadolinium based on the total weight of the base metal.

[0044] More specifically, referring to Figure 3, which is a low-magnification microstructure image of a neutron-absorbing structural material according to a preferred embodiment of the present invention, the white spherical or elongated particles indicated by the arrows represent gadolinium particles, and it can be seen that gadolinium oxide (Gd2O3) is formed on the surface and in some parts of these particles. In other words, it can be seen that the fraction of these (Gd + Gd2O3) composite phase particles increases as the amount of gadolinium added increases. From the results of Figures 1 and 2 and Tables 1 and 2, it can be seen that the increase in the fraction of these composite phase particles and the resulting decrease in the oxygen content in the titanium-based metal allow the present invention to exhibit excellent elongation.

[0045] In order to exhibit such ductility, the residual oxygen content may be less than 0.3 wt % based on the total weight of the neutron absorbing structural material, and more preferably less than 0.2 wt %. In this case, if the residual oxygen content exceeds 0.3 wt % based on the total weight of the neutron absorbing structural material, the strength increases, but the ductility decreases, which may cause a problem of reduced formability in forging, hot rolling, and cold rolling processes.

[0046] Therefore, the neutron absorbing structural material according to the present invention can satisfy all of the following relational expressions (1) to (3).

[0047] (1) Yield strength less than 550 MPa (2) Ultimate tensile strength less than 650 MPa (3) Total elongation of 22% or more The method for manufacturing the neutron absorbing structural material according to the present invention will be described below, but to avoid duplication, explanations of parts that share the same technical concept as the neutron absorbing structural material described above will be omitted.

[0048] The method for manufacturing a neutron-absorbing structural material according to the present invention includes a first step of preparing the above-mentioned neutron-absorbing structural material alloy composition, a second step of melting the neutron-absorbing structural material alloy composition to produce a molten ingot, a third step of hot-forging the molten ingot and then rolling it to produce a rolled material, and a fourth step of heat-treating the rolled material.

[0049] The first step is to prepare a neutron-absorbing structural alloy composition containing a base metal and 2 to 49 wt % of gadolinium (Gd) based on the total weight of the base metal.

[0050] The base metal may be a metal having excellent corrosion resistance and specific strength so as to function as a structural material, and most preferably titanium (Ti) may be used.

[0051] Next, the second step is a step of primarily melting the neutron absorbing structural alloy composition prepared in the first step to prepare a melted ingot.

[0052] The method for melting the neutron-absorbing structural alloy composition may be any of various methods widely used for melting titanium alloys among known common base metals, as long as it meets the objectives of the present invention, but preferably, vacuum plasma melting, vacuum arc melting, vacuum electron beam melting, etc. According to a preferred embodiment of the present invention, the melting may be performed 3 to 10 times using vacuum plasma melting to produce a molten ingot.

[0053] Next, the third step is a step of hot forging the ingot and then rolling it to produce a rolled material, that is, the third step includes a step of hot forging the molten ingot and a step of hot rolling the hot-forged material, or cold rolling the hot-forged material, or hot rolling and then cold rolling the hot-forged material.

[0054] The hot forging, hot rolling, or cold rolling can be any conventional hot forging, hot rolling, or cold rolling process known in the art, as long as it meets the objectives of the present invention. For example, the material can be heat-treated at 900-1200°C for 0.5-4 hours, followed by hot forging with a thickness reduction of 40-80%. The hot-forged material can then be heat-treated at 1000-1200°C for 0.5-2 hours, followed by hot rolling with a final thickness reduction of 30-70% to produce a rolled material. During hot rolling, it is advantageous to maintain the temperature of the material so that it does not drop below 900°C from the perspective of hot rolling performance. If the temperature of the material drops below this level during hot rolling, the material can be further maintained at 1100°C for 20 minutes before hot rolling, and then water-cooled after hot rolling.

[0055] In addition, the hot forged material can be manufactured into a plate material through cold rolling instead of hot rolling, and in this case, the hot forged material can be cold rolled at room temperature with a final thickness reduction rate of 20 to 50% to manufacture a rolled material. In addition, the hot rolled plate material can be further cold rolled, and when cold rolling the hot rolled plate material, cold rolling can be performed at room temperature with a thickness reduction rate of 10 to 30%.

[0056] Next, the fourth step is a β-phase heat treatment process in which the alloy is heat-treated at a temperature of 900°C or higher and then cooled in air or water, or a recrystallization heat treatment process at a temperature of 900°C or lower.

[0057] The fourth heat treatment may be a B-phase heat treatment process performed at 900 to 1100°C for 0.5 to 2 hours. However, if sufficient strain energy is accumulated in the rolled material to cause recrystallization through the hot rolling or cold rolling, a recrystallization heat treatment in the α-phase region below 900°C may be used as the final heat treatment.

[0058] That is, after the first step, the subsequent forming process for the molten ingot can be any of hot forging, hot rolling, cold rolling, etc., and intermediate heat treatment can be performed between these processes as needed.

[0059] The present invention will be described in more detail below with reference to examples. However, it should be understood that the following examples are not intended to limit the scope of the present invention, but are intended to aid in understanding the present invention. [Example]

[0060] Example 1: Manufacturing of neutron absorbing structural material After preparing the neutron-absorbing structural alloy composition shown in Table 1 below, it was remelted six times using vacuum plasma melting to prepare ingots. The melted ingots were then heat-treated at 1150°C for 2 hours, hot-forged with a thickness reduction rate of 70%, and water-cooled after forging.

[0061] Next, the hot-forged material was heat-treated at 1100°C for 1 hour and then immediately hot-rolled with a final thickness reduction rate of 60%. During hot rolling, the temperature of the target material was maintained so as not to drop below 900°C. If the temperature of the material dropped below this during the hot rolling process, it was maintained at 1100°C for an additional 20 minutes before hot-rolling and then water-cooled.

[0062] Subsequently, the hot-rolled material was heat-treated at 1000°C for 1 hour and then air-cooled.

[0063] Examples 2 to 6: Manufacturing of neutron absorbing structural materials The neutron absorbing structural material was manufactured in the same manner as in Example 1, except that the alloy composition was changed as shown in Table 1 below.

[0064] Comparative Example A neutron absorbing structural material having the composition shown in Table 1 below was prepared, and then the same procedure as in Example 1 was carried out except that gadolinium was not added.

[0065] [Table 1]

[0066] Experimental example 1: Evaluation of neutron absorption capacity The neutron absorption capacity was evaluated for Examples 1 to 6 and the Comparative Example, and the results are shown in Figure 1. In this case, the black circles in Figure 1 indicate the Gd addition amounts of the Examples and the corresponding boron equivalents, and the gray squares indicate the boron equivalents of an Al-40 wt.% BC composite as a commercial neutron absorber (Ti-6.5 wt.% alloy has the same boron equivalent).

[0067] 1, it can be seen that the embodiment of the neutron absorbing structural material according to the present invention exhibits a neutron absorption capacity superior to that of Al-40 wt.% B4C composite as a commercial neutron absorbing material. That is, the neutron absorption capacity of titanium-gadolinium alloys depending on the gadolinium content is calculated based on the boron equivalent (B eq) is calculated based on the following mathematical formula 1 and compared with an Al-40 wt.% B4C composite as a commercial neutron absorber. When 6.5 wt.% or more of gadolinium is added to titanium, it is found that the neutron absorption capacity is superior to that of the Al-40 wt.% B4C composite, which has the highest neutron absorption capacity among commercial neutron absorbers. In other words, Examples 4, 5, and 6 have superior neutron absorption capacity to the Al-40 wt.% B4C composite.

[0068] [Number 1] B eq =2.532×Gd weight%

[0069] Experimental Example 2: Evaluation of tensile properties The tensile properties of Examples 1 to 6 and the Comparative Example were evaluated, and the results are shown in FIG.

[0070] [Table 2]

[0071] Referring to Table 2 and the room-temperature tensile test curve in Figure 2, it can be seen that while the addition of alloying elements generally increases strength and decreases ductility, the alloys manufactured using the alloy composition and heat treatment process proposed in the present invention exhibit the opposite behavior. That is, the comparative example without gadolinium exhibited a yield strength of 550 MPa and a total elongation of 23.8%, while the examples with gadolinium added exhibited a decrease in strength and an increase in elongation as the gadolinium content increased to 10 wt.%. For example, in Example 4, the yield strength decreased to 395 MPa and the total elongation increased to 42.6% compared to the comparative example. However, when the gadolinium content was further increased, as in Examples 5 and 6, the neutron absorption capacity increased proportionally, but the strength and elongation further decreased.

[0072] Experimental Example 3: Low and high magnification images Low- and high-magnification microstructure images were measured for Examples 1 to 6 and the Comparative Example, and the results are shown in FIGS. 3 and 4, respectively.

[0073] Referring to Figure 3, the white spherical or elongated particles indicated by arrows are gadolinium particles, and gadolinium oxide (Gd2O3) has formed on the surface and in some parts of these particles. The formation of oxide on the surface of the gadolinium particles means that oxygen in the titanium matrix has been absorbed by the gadolinium surface, and the fraction of these (Gd + Gd2O3) composite phase particles increased as the amount of gadolinium added increased.

[0074] Figure 4 is a high-magnification microstructure image observed with a scanning electron microscope, in which small-angle grain boundaries and irregularities due to defects such as potentials are observed on the surface after chemical etching. The directionality of the irregularities indicates that defects are generated and aligned in a specific direction within a given grain.

[0075] Experimental Example 4: Low-magnification microstructure image of crystal orientation spatial distribution Crystal orientation spatial distribution microstructure images were measured using electron backscatter diffraction for Examples 1 to 6 and the Comparative Example, and the results are shown in Figure 5. In the case of pure titanium (Comparative Example) without added gadolinium, a titanium α phase was generated that underwent a phase transition to an acicular shape during cooling in the titanium B phase region. In Example 1, in which 1 wt.% gadolinium was added, an acicular α phase was also generated, but the size of the titanium B phase, which is a stable phase at high temperatures, became smaller, resulting in the refinement of the acicular α phase. When the amount of gadolinium added was increased from 3 wt.% to 20 wt.%, in all cases, an equiaxed α phase was generated instead of an acicular shape, and its size also tended to gradually refine as the amount of gadolinium added increased. In the crystal orientation spatial distribution microstructure image of FIG. 5, it can be seen that in the gadolinium-added example, the particles are expressed in various hues. This means that after the transformation from the titanium B phase to the titanium α phase, a preferred orientation or crystallographic texture, which would result in an abundance of particles with a specific crystal orientation, is not developed.

[0076] From the results shown in Figures 1, 2, 3, 5, and Tables 1 and 2, the fraction of the Gd phase and the (Gd + Gd2O3) composite phase increases with increasing gadolinium content. This increases the strength of titanium and titanium alloys and reduces ductility by absorbing oxygen on the surface of the gadolinium particles, thereby reducing the oxygen concentration in the titanium matrix. Therefore, as the gadolinium content increases up to 10 wt.% (Examples 1, 2, 3, and 4), elongation also increases. When the gadolinium content increases above 10 wt.% (Examples 5 and 6), the amount of Gd2O3 oxide formed on the gadolinium surface increases, weakening the interfacial bonding strength between the Gd2O3 oxide and the titanium matrix, resulting in fracture at relatively low strains during tensile testing. However, even with this lower elongation, the composite exhibits significantly better ductility than the commercially available Al-40%B4C composite (elongation of 1-2%).

[0077] Based on Experimental Examples 1 to 4, it can be seen that the neutron-absorbing structural material of the present invention can improve upon the low strength of conventional Al-B4C composite-based neutron absorbers and exhibit superior ductility, thereby simultaneously performing the function of a neutron-absorbing structural material with the performance of a structural material itself, while also overcoming the problems of Al-B4C composites and boron-containing corrosion-resistant steels, such as poor formability during the manufacturing process and high brittleness of the finished product. Furthermore, since the present invention does not require a separate support or structural material, it significantly improves the efficiency and manufacturing economy of spent nuclear fuel storage facility design and manufacturing, and enables the storage of more spent nuclear fuel within a given volume of spent nuclear fuel storage vessel, maximizing the usability of limited space.

Claims

1. Base metals and and 2 to 49 weight percent gadolinium (Gd) based on the total weight of the base metal.

2. 2. The neutron absorbing structural alloy composition of claim 1, wherein said base metal is titanium.

3. 2. The neutron absorbing structural alloy composition of claim 1, which is free of hafnium (Hf).

4. The neutron absorbing structural alloy composition of claim 1; and the remaining oxygen amount, A neutron absorbing structural material in which a portion of the gadolinium in the neutron absorbing structural alloy composition is dissolved in the base metal as a solid solution, and the remaining portion is dispersed in the form of an α-gadolinium phase (α-Gd phase).

5. 5. The neutron absorbing structural material according to claim 4, wherein the residual oxygen content is less than 0.3% by weight based on the total weight of the neutron absorbing structural material.

6. 5. The neutron absorbing structural material according to claim 4, wherein the following relational expressions (1) to (3) are all satisfied: (1) Yield strength of less than 550 MPa (2) Ultimate tensile strength of less than 650 MPa (3) Total elongation of 22% or more

7. A first step of providing a neutron absorbing structural alloy composition according to claim 1; a second step of melting the neutron-absorbing structural alloy composition to produce a melted ingot; a third step of hot forging the molten ingot and then rolling it to produce a rolled material; A fourth step of heat treating the rolled material.

8. 8. The method for manufacturing a neutron absorbing structural material according to claim 7, wherein the fourth step is a beta phase heat treatment step in which heat treatment is performed at a temperature of 900°C or higher followed by air or water cooling, or a recrystallization heat treatment step at a temperature of 900°C or lower.

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