Carbon Composite Heat Transfer Tube for Molten Salt Reactor and Manufacturing Method Thereof
Patent Information
- Application Number
- KR1020260133774
- Authority / Receiving Office
- KR · KR
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2026-07-21
- Publication Date
- 2026-09-01
Smart Images
Figure PAT00004_ABST
Abstract
Description
Technology Field
[0001] The present invention relates to a carbon composite heat transfer tube applied to a heat exchanger of a molten salt reactor (MSR) in which high-temperature corrosive molten salt circulates, and a method for manufacturing the same. More specifically, the present invention relates to a heat transfer tube for a reactor and a method for manufacturing the same, having a structure in which a carbon / carbon composite (C / C Composite)-based base tube (110) is woven and molded into a three-dimensional structure including a circumferential direction (113) of carbon fibers (111), and carbon nanotubes (120) oriented along the direction orthogonal to the fiber axis of the carbon fibers (111) are provided on the surface of the carbon fibers (111) of the base tube (110) or within the carbon / carbon composite matrix (112) to selectively form a heat transfer path in the radial direction of the tube, and a surface sealing layer (130) made of pyrolytic graphite (PyG) material blocks the penetration of molten salt and fission gas. Background Technology
[0003] The Molten Salt Reactor (MSR), one of the Generation 4 (Gen-IV) nuclear power systems, uses molten fluoride salts such as LiF-BeF₂ and LiF-NaF-KF (FLiNaK) or molten chloride salts based on NaCl-MgCl₂ as a fuel dissolution medium and primary coolant, and typically operates under high temperature conditions of 600°C to 750°C. MSRs possess excellent advantages such as high thermal efficiency (44–47%) due to high-temperature operation, inherent safety resulting from operating pressures near atmospheric pressure, and the possibility of online nuclear fuel processing. Consequently, commercialization is being actively pursued in the form of Small Modular Reactors (SMRs), such as Kairos Power’s Hermes reactor and Terrestrial Energy’s IMSR.
[0004] The heat exchanger (160) of the MSR is a key device that transfers thermal energy between a primary system in which radioactive molten salt circulates and a non-radioactive secondary system, and the heat transfer tube (100) forms the physical boundary between the two systems and the primary radiation shielding boundary. The inner wall of the heat transfer tube (100) must satisfy a design life of several decades under extreme conditions in which it is exposed to the molten salt flow path (161) in which radioactive high-temperature molten salt flows, and the outer wall to the secondary coolant flow path (162) in which the secondary coolant flows. Therefore, the selection of the material for the heat transfer tube (100) is recognized as one of the key technical challenges for the commercialization of the MSR.
[0005] The current standard material for MSR heat transfer tubes is Hastelloy N (a Ni-Mo-Cr superalloy). Hastelloy N possesses excellent corrosion resistance to molten fluoride salts and has a track record of approximately 14,000 hours of operation in the Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory. However, under long-term (>100,000 hours) operating conditions, complex degradation mechanisms manifest, including intergranular corrosion caused by the tellurium (Te) component of fluoride salts, surface degradation due to the selective leaching of Mo, and a decrease in creep resistance due to radiation. In particular, under conditions where the operating temperature exceeds 700°C, the corrosion rate of current nickel-based superalloys, including Hastelloy N, increases rapidly, and it has been pointed out that there is a limitation in that it is difficult to meet the design life of over 40 years targeted by commercial MSRs.
[0006] Carbon / carbon composites (C / C Composite) are all-carbon materials in which carbon fibers (111) are bonded to a carbon / carbon composite matrix (112). They maintain structural strength even at high temperatures of 2000°C or higher, have excellent chemical compatibility with molten fluoride salts, high radiation irradiation stability, and possess characteristics such as a very small neutron absorption cross-section. Due to these characteristics, carbon / carbon composites are being studied as a promising candidate for MSR heat exchanger heat transfer tube materials. However, carbon / carbon composites have an inherently high porosity (typically 10-20%) due to the characteristics of the manufacturing process, and there is a fundamental problem in that macro-cracks and micro-cracks induced during the high-temperature heat treatment process act as penetration pathways for molten salt and fission gases (Xe-135, Kr-89, etc.) into the substrate, causing a risk of radioactive material leaking into the secondary system.
[0007] As a preliminary study to solve the pore problem of carbon / carbon composites, research has been reported on blocking the penetration of molten FLiNaK salts and fission gases by applying a pyrrolitic carbon (PyC) or pyrrolitic graphite (PyG) coating via chemical vapor deposition (CVD) to a core graphite (isotropic graphite such as IG-110) substrate. While these studies are valid in the context of sealing pores in core graphite substrates, they have limitations in that they do not present a technique for forming a uniform PyG sealing layer on both the inner and outer walls in a special geometric structure of a heat transfer tube made of a 3D woven carbon fiber-based carbon / carbon composite, nor do they present a method for improving through-wall thermal conductivity, which is a key indicator of heat exchanger performance.
[0008] Another technical challenge for carbon / carbon composite heat transfer tubes is the low radial thermal conductivity (λ_r). While carbon fiber composites have high thermal conductivity of 50 to 400 W / mK in the fiber axis direction, they have relatively low thermal conductivity of 5 to 20 W / mK in the through-wall direction (radial direction) perpendicular to the fiber axis. This is primarily due to phonon scattering caused by the interface of the carbon / carbon composite matrix (112) between adjacent fibers. Given that the thermal conductivity of current Hastelloy N at 700°C is approximately 11 to 12 W / mK, the through-wall thermal conductivity of carbon / carbon composites can be disadvantageous in heat exchanger design, and it is difficult to achieve the high heat flux required for the compact heat exchanger design demanded by SMRs.
[0009] Meanwhile, a concept for improving through-thickness thermal conductivity in carbon fiber reinforced composites by growing carbon nanotubes (CNTs) radially on the fiber surface to form a so-called "Fuzzy Fiber" structure has been reported for polymer matrix composites. In this concept, carbon nanotubes grown perpendicular to the fiber axis form thermal bridges between adjacent fibers to overcome thermal resistance at the fiber-matrix interface. However, this technology has been limited to polymer matrix composites, and no technology has been disclosed that simultaneously solves the issues of carbon nanotube viability in carbon / carbon composites involving high-temperature carbonization and graphitization processes (1000–3000°C), design of in-situ CVD growth conditions for carbon nanotubes on carbon substrates, and prevention of metal catalyst contamination in the core material environment. The problem to be solved
[0011] The first problem that the present invention aims to solve is to effectively block the penetration of high-temperature molten salt and fission gas through the inherent pores and cracks of carbon / carbon composite heat transfer tubes, thereby preventing the leakage of radioactive materials into the secondary system and ensuring the long-term safe operation of the MSR.
[0012] The second problem that the present invention aims to solve is to increase the heat transfer efficiency of an MSR heat exchanger by overcoming the thermal bottleneck phenomenon at the fiber-matrix interface that structurally occurs in carbon fiber reinforced composite structures and effectively improving the through-wall thermal conductivity of the heat transfer tubes.
[0013] The third problem that the present invention aims to solve is to implement a process method that makes it realistically possible to manufacture a helical coil-shaped carbon / carbon composite heat transfer tube that meets the compact design requirements of Small Modular Reactors (SMRs). Conventionally, it has been widely believed that manufacturing helical carbon / carbon composite heat transfer tubes is technically very difficult due to reasons such as the inability to extract mandrels and the occurrence of cracks caused by anisotropic shrinkage in the curvature. The present invention aims to present multiple realistic manufacturing routes to overcome this technical prejudice. means of solving the problem
[0015] The carbon composite heat transfer tube for a nuclear reactor according to the present invention for solving the above problem comprises a carbon / carbon composite (C / C Composite)-based base tube (110) in which carbon fibers (111) are woven and molded into a three-dimensional structure including a circumferential direction (113), and carbon nanotubes (120) that exist on the surface of the carbon fibers (111) of the base tube (110) or within the carbon / carbon composite matrix (112) and are oriented along the direction orthogonal to the fiber axis of the carbon fibers (111) to form a radial direction heat transfer path of the base tube (110).
[0016] The carbon nanotube (120) may be grown in-situ by chemical vapor deposition (CVD) via a catalyst layer (121) on the surface of the carbon fiber (111) of the substrate tube (110). In this case, the catalyst layer (121) may be composed of a carbon-based non-metallic catalyst that does not leave any radioactive metal elements, or it may be composed of a metal catalyst containing at least one of Fe, Ni, and Co, wherein the metal catalyst is substantially removed by acid washing or heat treatment after the growth of the carbon nanotube (120) is completed.
[0017] The carbon nanotube (120) may be aligned such that the orientation angle (θ) (122) with respect to the tube radius direction is within the range of 0° or more and 30° or less, and the volume fraction of the carbon nanotube (120) may be 1% or more and 10% or less of the total volume of the substrate tube (110).
[0018] The above-mentioned tube (110) can be formed into a spiral coil shape (150), and the spiral coil shape (150) can be formed by directly printing it using an additive manufacturing method, by creep molding by applying a load to a spiral jig (152) while heating a straight carbon / carbon composite tube to 1800°C or higher in an inert atmosphere, or by winding carbon fibers onto a spiral sacrificial mandrel (151) made of carbon foam material and then destroying the sacrificial mandrel (151) during the carbonization process.
[0019] The heat transfer tube (100) of the present invention may further include a surface sealing layer (130) made of pyrrolite graphite material that is deposited by CVD on the inner wall surface and the outer wall surface of the substrate tube (110) to block the penetration of molten salt or fission gas. The surface sealing layer (130) includes an inner wall sealing layer (131) and an outer wall sealing layer (132), and may include a high-density impregnated area (133) that is physically penetrated and filled into the micropores on the surface of the substrate tube (110). The pyrrolite graphite of the surface sealing layer (130) may have an anisotropic structure in which the crystal plane (ab-plane) (134) is oriented parallel to the wall surface of the substrate tube (110), so that the thermal conductivity in the radial direction of the tube is higher than the thermal conductivity in the axial direction of the tube.
[0020] The heat transfer tube (100) of the present invention may further include an outer protective layer (140) made of silicon carbide (SiC) material that is additionally deposited by CVD on the outer surface of the surface sealing layer (130) to prevent high-temperature oxidation of pyrolitic graphite.
[0021] The manufacturing method of the present invention comprises the steps of: (a) producing a filament composite of a carbon precursor matrix and a carbon fiber (111) into a tubular three-dimensional precursor structure by a multi-axis additive manufacturing method; (b) forming a carbon / carbon composite substrate tube (110) by pyrolysis of the three-dimensional precursor structure in an inert atmosphere and densification by chemical vapor penetration (CVI); (c) applying a catalyst layer (121) to the surface of the carbon fiber (111) of the substrate tube (110) and then supplying a hydrocarbon gas to grow carbon nanotubes (120) in situ in a direction orthogonal to the fiber axis of the carbon fiber (111); and (d) forming a pyrrolitic graphite sealing layer by CVD on the inner and outer wall surfaces of the substrate tube (110). After step (b) and before step (c) above, the method may further include a step of forming a spiral tube shape by applying a load to a spiral jig (152) while heating the substrate tube (110) to 1800°C or higher in an inert atmosphere to create a spiral tube shape. Effects of the invention
[0023] The carbon composite heat transfer tube for a nuclear reactor according to the present invention has carbon nanotubes (120) oriented in a direction orthogonal to the fiber axis, which act as a thermal bridge at the interface between the carbon fiber (111) and the carbon / carbon composite matrix (112), thereby physically overcoming the thermal bottleneck phenomenon inherent to carbon fiber reinforced composites in the through-wall direction at the nanostructure level. Carbon fiber composites have anisotropy in which the thermal conductivity in the fiber axis direction is very high, but the thermal conductivity in the direction perpendicular to the fiber axis (through-wall direction) is significantly low. This is because the thermal resistance at the interface between the fiber and the matrix acts as a dominant bottleneck for heat transfer in the through-wall direction. According to the approach in the present invention, in which carbon nanotubes (120) are oriented in a direction orthogonal to the fiber axis to overcome this interfacial thermal resistance with a physical nanostructure, an improvement of 300 to 650% in transverse thermal conductivity can be achieved experimentally, thereby producing the effect of improving the heat transfer coefficient of the through-wall of the carbon / carbon composite heat transfer tube to a level equivalent to or greater than that of the conventional Hastelloy N heat transfer tube.
[0024] The pyrrolite graphite surface sealing layer (130) of the present invention is simultaneously applied to both the inner and outer walls of the substrate tube (110), thereby providing the effect of simultaneously blocking two independent risks—penetration of high-temperature, high-pressure molten salt toward the inner wall and diffusion of fission gases (Xe-135, Kr-89, etc.) into the substrate—with a single coating structure. High-density deposition of pyrrolite graphite by the CVD method forms a high-density impregnation region (133) that fills even the surface pores of the carbon substrate, thereby providing a level of airtightness that is difficult to achieve with a simple surface coating, and a helium gas permeability of 10 -12 It is possible to reduce it to the level of m² / s.
[0025] The anisotropic structure of the pyrrolite graphite surface sealing layer (130) has an ab-plane (134) oriented parallel to the wall of the substrate tube (110), thereby providing an additional heat transfer enhancement effect that increases the thermal conductivity of the pyrrolite graphite itself in the radial direction of the tube more than in the axial direction of the tube. This effect works synergistically with the heat transfer enhancement effect inside the substrate tube (110) by the carbon nanotube (120) to maximize the through-wall heat transfer coefficient of the entire heat transfer tube (100).
[0026] The heat transfer tube (100) with a spiral coil shape (150) increases the heat exchange area per unit installation volume by 1.5 to 3 times compared to a straight tube, enabling the design of a compact heat exchanger (160) required for a small modular reactor (SMR). In addition, the secondary flow (Dean Vortex) (180) generated by the spiral flow disturbs the fluid thermal boundary layer, improving the convective heat transfer coefficient by 10 to 30% compared to a straight tube, and exerts an additional effect of suppressing local freezing in low-velocity zones of high-viscosity molten salt.
[0027] The combination of the carbon / carbon composite substrate tube (110), which is the core component of the present invention, and the carbon nanotube (120) oriented orthogonally to the fiber axis, is technically non-obvious in that the prior art has not specifically taught or suggested this combination. The Fuzzy Fiber concept, which improves transverse thermal conductivity by orienting carbon nanotubes orthogonally to the fiber axis in carbon fiber composites, has been disclosed as being limited to polymer matrix composites. In order to apply this to carbon / carbon composites, it is only possible to realize it by simultaneously solving complex technical challenges such as the survival of carbon nanotubes in a high-temperature carbonization process, the design of in-situ CVD growth conditions for the carbon substrate, and the prevention of radioactive contamination by using non-metallic catalysts or post-removal of metallic catalysts. The existence of these challenges and the combination of solutions are recognized as having a technical inventive step, as they go beyond the level that a person skilled in the art can obviously reach from known prior art. Furthermore, there was a technical prejudice that manufacturing carbon / carbon composite heat transfer tubes in a spiral shape was difficult due to reasons such as the inability to extract a conventional mandrel and the occurrence of cracks caused by anisotropic shrinkage of the curvature. However, the present invention overcomes this prejudice by presenting multiple manufacturing paths, such as 3D printing-based direct molding, high-temperature creep molding, and sacrificial mandrel winding. Brief explanation of the drawing
[0029] FIG. 1 is a schematic diagram of the overall configuration of the heat exchanger (160) of the molten salt reactor (170). FIG. 2 is an overall perspective view of a heat transfer tube (100) according to one embodiment of the present invention. FIG. 3 is an enlarged view showing the three-dimensional woven structure of carbon fibers (111) of a substrate tube (110), showing circumferentially oriented fibers (113), axially oriented fibers (114), and radially oriented fibers (115). FIG. 4 is a vertical cross-sectional view of the wall of the heat transfer tube (100), showing the entire layer structure of the base tube (110), carbon nanotube (120), surface sealing layer (130) and outer protective layer (140). FIG. 5 is an enlarged schematic diagram showing the fiber axis orthogonal orientation mechanism of a carbon nanotube (120), showing the orientation angle (θ) (122) of a carbon nanotube (120) grown from a catalyst layer (121) on the surface of a carbon fiber (111). FIG. 6 is a diagram showing the principle of forming a radial heat transfer path by carbon nanotubes (120), comparing the thermal bottleneck at the fiber-matrix interface in the absence of CNTs with the formation of a radial heat bridge after the introduction of CNTs. FIG. 7 is an anisotropic crystal orientation cross-sectional view of a surface sealing layer (130), showing the crystal structure of pyrrolite graphite in which a crystal plane (ab-plane) (134) is oriented parallel to the wall surface of the substrate tube (110). FIG. 8 is an enlarged cross-sectional view showing a high-density impregnation area (133) of a surface sealing layer (130). FIG. 9 is a schematic diagram of a CVD process for simultaneously forming an inner wall sealing layer (131) and an outer wall sealing layer (132). FIG. 10 is a process flowchart for manufacturing a 3D printing-based spiral heat transfer tube (100). FIG. 11 is a schematic diagram of a high-temperature creep forming process, showing the spiral deformation process of a straight base tube (110) using a spiral jig (152). FIG. 12 is a schematic diagram of the manufacturing process of a spiral heat transfer tube (100) using a sacrificial mandrel (151). FIG. 13 is a schematic diagram of the process for applying a catalyst layer (121) and growing carbon nanotubes (120) in situ. FIG. 14 is a flow analysis diagram of a secondary flow (Dean Vortex) (180) occurring in a heat transfer tube (100) of a spiral coil shape (150). FIG. 15 is a comparison of the performance of the heat transfer tube (100) of the present invention and a conventional Hastelloy N metal heat transfer tube in terms of thermal conductivity, corrosion resistance, and radiation safety. FIG. 16 is a graph showing the change in thermal conductivity in the radial direction of the tube according to the volume fraction of carbon nanotubes (120). Specific details for implementing the invention
[0030] Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. As the present invention is susceptible to various modifications and may have various embodiments, specific embodiments are illustrated in the drawings and described in detail in the detailed description. In the following description, the same reference numerals refer to the same components.
[0031] [Example 1: Three-dimensional woven structure of a substrate tube]
[0032] The heat transfer tube (100) of the present invention includes a base tube (110) based on a carbon / carbon composite as a core structural element. The base tube (110) is composed of carbon fibers (111) and a carbon / carbon composite matrix (112), and the carbon fibers (111) are woven and molded into a three-dimensional structure including a circumferential direction (113). Specifically, the carbon fibers (111) of the base tube (110) are composed of a three-dimensional woven architecture including circumferentially oriented fibers (113) oriented in the circumferential direction of the tube, axially oriented fibers (114) oriented in the longitudinal direction of the tube, and radially oriented fibers (115) oriented in the thickness direction of the tube wall. Among these, the circumferentially oriented fibers (113) play a key role in the operating principle of the present invention in that they provide geometric conditions that allow carbon nanotubes (120) to be oriented in a direction orthogonal to the fiber axis, that is, in the tube radius direction.
[0033] The carbon fiber (111) may be a PAN (polyacrylonitrile) or pitch-based carbon fiber. Pitch-based carbon fiber has a significantly higher thermal conductivity (200–900 W / mK) in the fiber axial direction compared to PAN-based carbon fiber, which is advantageous for improving the overall thermal performance of the heat transfer tube (100). It is preferable that the volume fraction of the fiber used in the three-dimensional weaving be 40–60 vol% of the total substrate tube (110) in terms of balancing structural strength and matrix impregnation.
[0034] The carbon / carbon composite matrix (112) is formed by a liquid phase impregnation-carbonization (LPI) process using pitch or phenolic resin as a precursor, or a chemical vapor infiltration (CVI) process using a hydrocarbon gas such as methane (CH₄) or propane (C₃H₅). A number of impregnation-carbonization or CVI cycles are repeated until a target density (1.8 g / cm³ or more) is reached. After densification is complete, the substrate tube (110) undergoes graphitization treatment at 2000 to 3000°C in an Ar atmosphere to improve crystallinity, increase thermal conductivity, and enhance radiation resistance. However, when introducing carbon nanotubes (120) into the substrate tube (110), it is preferable to limit the final graphitization temperature to 1500°C or lower to preserve the structure of the carbon nanotubes (120), or to adopt a sequence in which carbon nanotubes (120) are introduced after the substrate tube (110) is completely graphitized.
[0035] [Example 2: Fiber axis orthogonal orientation structure of carbon nanotubes]
[0036] In the heat transfer tube (100) of the present invention, carbon nanotubes (120) are present on the surface of carbon fibers (111) of the substrate tube (110) or within the carbon / carbon composite matrix (112), and are oriented along a direction orthogonal to the fiber axis of the carbon fibers (111) to form a tube-radial heat transfer path of the substrate tube (110). When the carbon nanotubes (120) are oriented in a direction perpendicular to the fiber axis of the carbon fibers (111), a nanoscale thermal bridge is formed across the interface of the carbon / carbon composite matrix (112) between the carbon fibers (111). This thermal bridge suppresses fiber-matrix interface phonon scattering, which is the main cause of hindering tube-radial heat transfer in conventional carbon / carbon composites, and directly establishes a tube-radial heat transfer path by utilizing the excellent axial thermal conductivity (individual CNT: 1000~6600 W / mK) of the individual carbon nanotubes (120).
[0037] The orientation angle (θ) (122) formed by the orientation direction of the carbon nanotube (120) and the tube radial direction is preferably within the range of 0° to 30°. As the orientation angle (θ) (122) approaches 0°, the efficiency of the tube radial heat transfer path is maximized, and if the orientation angle (θ) (122) exceeds 30°, the contribution to tube radial heat transfer decreases and the possibility of bundling of the carbon nanotube (120) increases. The orientation angle (θ) (122) can be measured by X-ray diffraction (XRD) analysis, Raman spectroscopy, or transmission electron microscopy (TEM) cross-sectional analysis.
[0038] It is preferable to set the volume fraction of carbon nanotubes (120) to 1 vol% or more and 10 vol% or less of the total volume of the substrate tube (110). If the volume fraction is less than 1 vol%, the thermal bridge density by carbon nanotubes (120) is insufficient, so the effect of improving thermal conductivity in the radial direction of the tube is negligible. On the other hand, if the volume fraction exceeds 10 vol%, bundling between carbon nanotubes (120) intensifies, increasing interfacial thermal resistance and potentially lowering the mechanical strength of the substrate tube (110). An optimal balance between improving thermal conductivity in the radial direction of the tube and maintaining mechanical properties is achieved at a volume fraction of 3 to 7 vol%.
[0039] [Example 3: Introduction of Carbon Nanotubes by CVD In-situ Growth]
[0040] An example of a method for growing carbon nanotubes (120) in situ on the surface of carbon fibers (111) of a base tube (110) is described. First, surface activation is performed by treating the surface of carbon fibers (111) of the base tube (110) with an 80% HNO₃ aqueous solution to introduce carboxyl groups (-COOH). Next, the base tube (110) is immersed in a catalyst precursor solution (a 0.05 to 0.5 mol / L aqueous solution of Fe(NO₃)₃·9H₂O or Ni(NO₃)₂·6H₂O) to adsorb the catalyst precursor onto the surface of the carbon fibers (111), and calcined at 300 to 500°C in an Ar atmosphere to form a catalyst layer (121) composed of Fe₂O₃ or NiO nanoparticles on the surface of the carbon fibers (111).
[0041] A substrate tube (110) having a catalyst layer (121) formed thereon is mounted in a CVD reactor, and the catalyst layer (121) is activated into Fe or Ni metal nanoparticles by reducing it with H₂ gas at 800 to 900°C. Then, a CVD is performed at 900 to 1100°C for 20 minutes to 2 hours while supplying CH₄ (or C₂H₂) gas diluted to a concentration of 3 to 30 vol% to grow carbon nanotubes (120) in situ from the catalyst layer (121) on the surface of the carbon fiber (111) in a direction orthogonal to the fiber axis. At this time, the length of the carbon nanotube (120) can be controlled in the range of 0.1 to 10 μm depending on the CVD time.
[0042] [Example 4: Growth of Carbon Nanotubes Using Non-Metallic Carbon-Based Catalysts]
[0043] An example of using a carbon-based non-metallic catalyst to prevent radioactive contamination in a nuclear material environment is described. Nanodiamonds (average particle size 5 nm), silicon carbide (SiC) nanoparticles, or amorphous carbon nanoparticles rich in sp³ bonds can be used as non-metallic carbon-based catalysts. These catalysts are dispersed and coated on the surface of carbon fibers (111), and then induce the growth of carbon nanotubes (120) under CVD conditions of 1100 to 1300°C. Since the use of a non-metallic carbon-based catalyst prevents radioactive metal elements such as Fe, Ni, and Co from remaining in the substrate tube (110), the risk of generating radioactive isotopes such as Fe-59, Co-60, and Ni-63 in a neutron irradiation environment is fundamentally blocked, which provides significant advantages in terms of nuclear safety and radiation protection.
[0044] [Example 5: Post-removal of metal catalyst after use]
[0045] When prioritizing the growth efficiency of carbon nanotubes (120), an example is described in which carbon nanotubes (120) are grown using a metal catalyst comprising at least one of Fe, Ni, and Co, and then the metal catalyst is removed. After the growth of carbon nanotubes (120) is completed, an acid washing treatment is performed by immersing the substrate tube (110) in a 5-30 wt% aqueous HCl solution or an aqueous HNO₃ solution for 30 minutes to 2 hours to dissolve and remove metal nanoparticles remaining on the surface of the carbon fiber (111). Alternatively, the substrate tube (110) is vacuumed (10 -3 When heated to 700–900°C in an atmosphere of Torr or less, the metal catalyst can be vaporized and removed. After acid washing or heat treatment is completed, energy dispersive X-ray analysis (EDX) or inductively coupled plasma mass spectrometry (ICP-MS) is used to confirm that the residual metal content is below the acceptance limit for nuclear materials (Fe < 1 wt ppm, etc.).
[0046] [Example 6: Helical coil-shaped heat transfer tube]
[0047] The heat transfer tube (100) of the present invention may be formed in a spiral coil shape (150). The heat transfer tube (100) in a spiral coil shape (150) increases the heat transfer area within the volume of a unit heat exchanger (160) by 1.5 to 3 times or more compared to a straight tube, thereby enabling the design of a compact heat exchanger (160) required in a small modular reactor (SMR). As geometric parameters of the spiral coil shape (150), it is preferable that the coil diameter (D) be 100 to 500 mm, the tube outer diameter (d) be 10 to 30 mm, and the coil pitch (P) be 1.2 to 2.0 times the tube outer diameter (d) in terms of balancing compactness and flow resistance.
[0048] In a heat transfer tube (100) with a spiral coil shape (150), a secondary flow (Dean Vortex) (180) is induced by centrifugal force when molten salt flows along the spiral path. The secondary flow (180) continuously disturbs the thermal boundary layer, improving the convective heat transfer coefficient by 10 to 30% compared to a straight tube. In addition, continuous fluid mixing by the secondary flow (180) improves temperature uniformity within the molten salt path (161) and has the effect of suppressing the risk of local freezing of high-viscosity molten salt. The secondary flow (180) is effectively generated under conditions where the Dean number (De = Re×√(d / 2R_c), where Re is the Reynolds number and R_c is the coil radius) is 11.6 or higher, and in the spiral coil shape (150) parameter of the present invention, the Dean number sufficiently satisfies this under the normal operating flow rate conditions of the MSR.
[0049] [Example 7: Manufacture of Spiral Heat Transfer Tube Based on Multi-Axis Additive Manufacturing (3D Printing)]
[0050] As an embodiment of the manufacturing method of the present invention, a method for manufacturing a helical carbon / carbon composite heat transfer tube using a multi-axis additive manufacturing method is described. This method has the advantage of fundamentally resolving the problem of being unable to extract the helical mandrel.
[0051] In step (a), a composite filament is prepared in which continuous carbon fibers (111) based on pitch or polyacrylonitrile (PAN) are impregnated into a pitch precursor matrix, and the composite filament is mounted on a 6-axis robotic 3D printer. A 3D model of a target helical coil shape (150) is designed in CAD software, and the CAD model is created with dimensions that compensate in advance for the shrinkage rate (about 3-5%) in the carbonization process. The robotic 3D printer forms a 3D precursor structure by printing layers by layer, with the orientation direction of the carbon fibers (111) in each layer being predominantly in the circumferential direction (113).
[0052] In step (b), the output three-dimensional precursor structure is mounted in an Ar atmosphere pyrolysis furnace and heated to 1200°C at a heating rate of 5°C / min and maintained for 2 hours to perform pyrolysis treatment. Since the carbon / carbon composite precursor obtained after pyrolysis has low density and high porosity, an isothermal chemical vapor infiltration (CVI) process using CH₄ (10–20 vol%) gas is performed in multiple cycles at 1100°C to densify it to a density greater than the target density (≥1.7 g / cm³). Between each cycle, the closed layer formed on the surface is removed by machining to secure a gas diffusion path into the internal pores.
[0053] In step (c), a catalyst layer (121) is applied to the surface of the carbon fibers (111) of the densified substrate tube (110) by the method of Example 3 or Example 4, and CVD is performed to grow carbon nanotubes (120) in situ. By passing a CVD reaction gas through the internal flow path of the substrate tube (110) in the shape of a spiral coil (150), the growth of carbon nanotubes (120) on the surface of the inner wall carbon fibers (111) is promoted, and by supplying a reaction gas from the outside, the growth of carbon nanotubes (120) on the outer wall is simultaneously achieved.
[0054] In step (d), CH₄ (5-15 vol%) hydrocarbon gas is injected at 1000-1200°C through the inner wall channel (molten salt channel (161)) of the substrate tube (110), and at the same time, a hydrocarbon gas atmosphere of the same composition is created around the outer wall to simultaneously CVD deposit an inner wall sealing layer (131) and an outer wall sealing layer (132). The deposition pressure is set to 10-50 Torr and the deposition time to 10-20 hours to achieve a final pyrrolite graphite sealing layer thickness of 50-200 μm.
[0055] [Example 8: Manufacture of spiral heat transfer tube by high-temperature creep forming]
[0056] As another embodiment of the manufacturing method of the present invention, a method of deforming a carbon / carbon composite substrate tube (110) manufactured in a straight tube shape into a spiral coil shape (150) by high-temperature creep molding is described.
[0057] First, a straight-shaped carbon / carbon composite substrate tube (110) is manufactured in accordance with steps (a) and (b) of Example 7. Next, the substrate tube (110) is placed on a spiral jig (152) made of a carbon material (graphite or carbon / carbon composite), and a load is applied to both ends of the substrate tube (110) using carbon clamps or weights. In this state, it is heated to 2000–2500°C in a high-temperature furnace in an Ar atmosphere to form micro-cracks within the pitch matrix of the carbon / carbon composite substrate tube (110), and a spiral coil shape (150) is imparted along the shape of the spiral jig (152) by high-temperature creep plastic deformation. After molding, the spiral coil shape (150) is permanently fixed by slow cooling in an Ar atmosphere. The micro-cracks formed by creep molding after molding are filled in the subsequent CVD PyG sealing process. After high-temperature creep molding, steps (c) and (d) are performed sequentially to form carbon nanotubes (120) and a surface sealing layer (130).
[0058] [Example 9: Manufacture of spiral heat transfer tube by sacrificial mandrel winding]
[0059] As another embodiment of the manufacturing method of the present invention, a filament winding method using a helical sacrificial mandrel (151) made of carbon foam is described. An open-pore carbon foam block with a density of 0.2 to 0.5 g / cm³ is machined by CNC lathe to produce a helical sacrificial mandrel (151) corresponding to a target helical coil shape (150). A pitch-impregnated carbon fiber (111) filament is multilayer-winded onto the helical sacrificial mandrel (151) such that the circumferential direction (113) is dominant, and then cured. After curing, a carbonization treatment is performed at 1000 to 1200°C in an Ar atmosphere. During carbonization, the helical sacrificial mandrel (151) made of carbon foam material gradually disappears, and only the wound carbon fiber (111) composite remains in the helical coil shape (150). Subsequently, steps of CVI densification, carbon nanotube (120) in situ growth, and PyG CVD coating are performed sequentially.
[0060] [Example 10: Formation of Pyrrolite Surface Sealing Layer]
[0061] A method for forming a surface sealing layer (130) made of pyrrolitic graphite material on the inner wall surface and the outer wall surface of a substrate tube (110) is described. The surface sealing layer (130) includes an inner wall sealing layer (131) formed on the inner wall and an outer wall sealing layer (132) formed on the outer wall. The substrate tube (110) is mounted in a CVD reactor, and an inner wall sealing layer (131) is formed by performing CVD at 1000 to 1200°C while supplying a CH₄ (or C₃H₅) gas diluted with Ar at a concentration of 5 to 20 vol% through the inner wall flow path of the substrate tube (110). At the same time, an outer wall sealing layer (132) is formed by creating a hydrocarbon gas atmosphere of the same composition in the space outside the substrate tube (110) inside the reactor. During the CVD process, hydrocarbon gas diffuses into the open pores on the surface of the substrate tube (110) and deposits carbon on the pore walls, thereby gradually reducing the pore diameter and finally forming a high-density impregnated region (133) with sealed pore outlets. After the completion of CVD, the porosity is reduced to less than 2%, and the helium gas permeability is 10 -12 By reducing it to the m² / s level, it is possible to effectively block fission gases such as Xe-135.
[0062] In an example for forming an anisotropic pyrrolite graphite surface sealing layer (130), the CVD substrate temperature is raised to 1200°C or higher and the hydrocarbon gas partial pressure is kept low (1 to 5 Torr) to grow anisotropic pyrrolite graphite in which the crystal plane (ab-plane) (134) is oriented parallel to the wall of the substrate tube (110). The anisotropic surface sealing layer (130) thus formed has an anisotropic structure in which the tube radial thermal conductivity (λ_r) is higher than the tube axial thermal conductivity (λ_a), and this works synergistically with the improvement of radial heat transfer inside the substrate tube (110) by carbon nanotubes (120) to maximize the through-wall heat transfer coefficient of the entire heat transfer tube (100).
[0063] [Example 11: Formation of Silicon Carbide External Protective Layer]
[0064] An example is described of forming an outer protective layer (140) made of silicon carbide (SiC) on the outer surface of the outer wall sealing layer (132) of a heat transfer tube (100). The outer protective layer (140) serves to prevent oxidative degradation that may occur when pyrolite graphite is exposed to a high-temperature oxidizing environment. A substrate tube (110) having the outer wall sealing layer (132) formed thereon is mounted in a CVD reactor, and a mixed gas of SiCl₄ (or CH₃SiCl₃) + H₂ + C₃H₅ (or CH₄) is supplied at 1000 to 1150°C to deposit a SiC outer protective layer (140) with a thickness of 10 to 100 μm on the surface of the outer wall sealing layer (132). SiC has good chemical stability against molten fluoride salts and provides a protective function that prevents oxidation of pyrolite graphite. However, the outer protective layer (140) is not applied to the inner wall that is in direct contact with the molten salt channel (161), but is applied only to the outer wall that is exposed to the secondary coolant channel (162), thereby providing a protective function optimized for the special operating conditions of the MSR.
[0065] [Example 12: Performance Comparison with Conventional Hastelloy N Heat Transfer Tubes]
[0066] The performance of the heat transfer tube (100) according to the present invention is compared with that of a conventional Hastelloy N heat transfer tube. The heat transfer tube (100) manufactured by the method of Example 7 (base tube (110) + 4 vol% carbon nanotube (120) + 100 μm anisotropic pyrrolitic graphite surface sealing layer (130)) has a weight increase rate of less than 0.02 wt% in the FLiNaK salt (650°C, 1 atm) penetration test, showing significantly superior salt penetration blocking performance compared to a C / C composite without carbon nanotubes (weight increase rate 13.5 wt%). The tube radial thermal conductivity is improved by about 300% compared to the conventional C / C composite by introducing 4 vol% carbon nanotubes (120), achieving about 18 to 25 W / mK, which can be confirmed to be a level exceeding Hastelloy N (about 12 W / mK at 700°C). In addition, in a 1000-hour immersion test in 700°C FLiNaK salt, the surface sealing layer (130) of the heat transfer tube (100) remains stable without peeling or cracking, whereas the Hastelloy N specimen shows a surface corrosion depth of 15 μm and traces of intergranular corrosion, confirming that the heat transfer tube (100) of the present invention exhibits significantly superior performance in terms of long-term corrosion resistance. Explanation of the symbols
[0068] 100: Heat transfer tube (carbon composite heat transfer tube for nuclear reactors) 110: Material tube 111: Carbon fiber 112: Carbon / Carbon Composite Matrix 113: Circumferentially oriented fibers 114: Axially oriented fibers 115: Radially oriented fibers 120: Carbon nanotubes (CNT) 121: Catalyst layer 122: CNT orientation angle (θ) 130: Surface sealing layer (pyrolitic graphite, PyG) 131: Inner wall sealing layer 132: Exterior wall sealing layer 133: High-density impregnation area 134: Crystal plane (ab-plane) 140: Outer protective layer (silicon carbide, SiC) 150: Spiral coil shape 151: Sacrifice Mandrill 152: Spiral Jig 160: Heat exchanger 161: Molten salt flow path 162: Secondary coolant flow path 170: Reactor body 180: Secondary flow (Dean vortex)
Claims
Claim 1 A carbon composite heat transfer tube for a nuclear reactor applied to a heat exchanger of a nuclear reactor in which a high-temperature corrosive molten salt circulates, comprising: a base tube based on a carbon / carbon composite (C / C Composite) in which carbon fibers are woven and molded into a three-dimensional structure including a circumferential direction; and carbon nanotubes (CNTs) present on the surface of the carbon fibers of the base tube or within the carbon composite matrix, oriented along a direction orthogonal to the fiber axis of the carbon fibers to form a heat transfer path in the radial direction of the base tube. Claim 2 A carbon composite heat transfer tube for a nuclear reactor according to claim 1, characterized in that the carbon nanotubes are grown in-situ by chemical vapor deposition (CVD) via a catalyst layer on the surface of carbon fibers of the substrate tube. Claim 3 A carbon composite heat transfer tube for a nuclear reactor, characterized in that, in paragraph 2, the catalyst layer is composed of a carbon-based non-metallic catalyst, and does not leave any neutron-activating metal elements in the substrate tube after the growth of the carbon nanotube. Claim 4 A carbon composite heat transfer tube for a nuclear reactor according to claim 2, wherein the catalyst layer comprises a metal catalyst including at least one of Fe, Ni, and Co, and the metal catalyst is substantially removed from the substrate tube by acid washing or heat treatment after the growth of the carbon nanotube is completed. Claim 5 A carbon composite heat transfer tube for a nuclear reactor according to claim 1, characterized in that the carbon nanotubes are aligned such that the orientation angle (θ) with respect to the radial direction of the tube is within the range of 0° or more and 30° or less. Claim 6 A carbon composite heat transfer tube for a nuclear reactor according to claim 1, characterized in that the volume fraction of the carbon nanotube is 1% or more and 10% or less of the total volume of the substrate tube. Claim 7 A carbon composite heat transfer tube for a nuclear reactor according to claim 1, characterized in that the above-described tube is formed in a helical coil shape, thereby increasing the heat transfer area per unit volume and inducing a Dean Vortex by helical flow, which improves the heat transfer efficiency of the heat exchanger. Claim 8 A carbon composite heat transfer tube for a nuclear reactor according to claim 7, characterized in that the above-described tube is formed by extruding a filament, which is a composite of a carbon precursor matrix and a carbon fiber, into a spiral tube-shaped three-dimensional precursor structure using a multi-axis additive manufacturing method, and densifying the three-dimensional precursor structure by pyrolysis and chemical vapor infiltration (CVI) in an inert atmosphere. Claim 9 In claim 7, the above-described tube is characterized by having a spiral tube shape formed by applying a load to a spiral jig while heating a carbon / carbon composite tube densified in the form of a straight tube to a temperature of 1800°C or higher in an inert atmosphere to form a spiral tube shape. Claim 10 A carbon composite heat transfer tube for a nuclear reactor according to claim 7, characterized in that the above-described tube has a spiral tube shape formed by filament winding carbon fibers onto a spiral sacrificial mandrel made of carbon foam material and then eliminating the sacrificial mandrel through resin impregnation and carbonization processes. Claim 11 A carbon composite heat transfer tube for a nuclear reactor according to claim 1, further comprising a surface sealing layer made of pyrolytic graphite (PyG) that is deposited by chemical vapor deposition (CVD) on the inner wall surface and the outer wall surface of the substrate tube to block the penetration of molten salt or fission gas into the interior of the substrate tube. Claim 12 A carbon composite heat transfer tube for a nuclear reactor, characterized in that, in claim 11, the surface sealing layer comprises a high-density impregnated region that is physically penetrated and filled into the micropores (porosity) present on the surface of the substrate tube. Claim 13 A carbon composite heat transfer tube for a nuclear reactor, characterized in that, in claim 11, the pyrrolitic graphite of the surface sealing layer has an anisotropic structure in which the crystal plane (ab-plane) is oriented parallel to the wall surface of the substrate tube, and the thermal conductivity in the radial direction is higher than the thermal conductivity in the axial direction. Claim 14 A carbon composite heat transfer tube for a nuclear reactor, further comprising an outer protective layer made of silicon carbide (SiC) that is additionally deposited by chemical vapor deposition (CVD) on the outer surface of the surface sealing layer to prevent high-temperature oxidation of the pyrrolitic graphite in claim 11. Claim 15 A method for manufacturing a carbon composite heat transfer tube for a nuclear reactor according to claim 1, comprising: (a) a step of outputting a filament, which is a composite of a carbon precursor matrix and a carbon fiber, into a tubular three-dimensional precursor structure using a multi-axis additive manufacturing method; (b) a step of pyrolyzing the three-dimensional precursor structure in an inert atmosphere and densifying it by chemical vapor deposition (CVI) to form a carbon / carbon composite substrate tube; (c) a step of applying a catalyst layer to the surface of the carbon fiber of the substrate tube and then supplying a hydrocarbon gas to grow carbon nanotubes in situ in a direction orthogonal to the fiber axis of the carbon fiber; and (d) a step of forming a pyrrolitic graphite sealing layer on the inner and outer wall surfaces of the substrate tube by chemical vapor deposition (CVD); Claim 16 A method for manufacturing a carbon composite heat transfer tube for a nuclear reactor according to claim 15, further comprising the step of imparting a spiral tube shape by creep forming the substrate tube by applying a load to a spiral jig while heating it to 1800°C or higher in an inert atmosphere after step (b) and before step (c).