A multi-stage reinforced connection method of an anti-accident SiC / SiC fuel cladding tube and a SiC end plug

Abstract

The application relates to a multistage reinforced connection method of an anti-accident SiC / SiC fuel cladding tube and a SiC end plug. The connection method comprises the following steps: (1) polishing, polishing, ultrasonic cleaning and drying the connection part of the SiC / SiC fuel cladding tube and the SiC end plug; (2) mixing SiC whiskers, SiC powder, a ceramic precursor, a catalyst, a binder and a solvent to obtain a paste-like sealing agent; (3) uniformly applying the paste-like sealing agent to the connection surface of the SiC / SiC fuel cladding tube and the SiC end plug, and placing the connection surface in a chemical vapor deposition system to sequentially perform ceramicization and densification of the connection layer, so that a multistage reinforced connection layer of the anti-accident SiC / SiC fuel cladding tube and the SiC end plug is finally obtained.

Description

Technical Field

[0001] This invention belongs to the field of ceramic material joining technology, specifically relating to a multi-stage reinforced joining method for an accident-resistant SiC / SiC fuel cladding tube and a SiC end plug. Background Technology

[0002] Currently, both power plant and marine nuclear reactors are primarily pressurized water reactors (PWRs). The fuel cladding in a PWR encloses the nuclear fuel core, transfers fission heat from the fuel to the coolant, and shields against radiation products from nuclear fission. Its integrity directly affects the normal operation and lifespan of the PWR, making it one of the most crucial reactor safety barriers. Currently, commercial PWRs predominantly use zirconium alloys for fuel cladding. However, zirconium alloy claddings suffer from serious safety defects under Loss-of-Water Accidents (LOCA): metallic zirconium reacts with high-temperature steam to generate hydrogen, potentially leading to the meltdown of the fuel cladding tubes or even a hydrogen explosion. Therefore, developing new accident-tolerant fuel cladding materials is imperative.

[0003] SiC / SiC composite materials possess excellent properties such as high strength and toughness, high thermal conductivity and low coefficient of thermal expansion, high neutron economy, and resistance to high-pressure hydrothermal and high-temperature steam corrosion. They are widely recognized worldwide as a candidate material for accident-tolerant fuel cladding. In actual service environments, to ensure the sealing of fuel elements, the SiC / SiC fuel cladding tube must be connected to the SiC end plug. The connection layer between these two components becomes a crucial factor affecting the overall performance of the component, often becoming a weak point in the SiC / SiC fuel element under both normal and severe accident conditions.

[0004] Currently, the main bonding methods for nuclear-grade SiC and SiC / SiC composites include: diffusion bonding using metals such as Mo / Ti; bonding through in-situ reaction of the T-Si-C ternary system to generate the MaX phase; bonding using the NITE process with SiC nanoparticles; in-situ glass-ceramic bonding; and precursor-converted ceramic bonding. The literature “Koyanagi T, Katoh Y, Terrani KA, et al. Hydrothermal corrosion of silicon carbide joints without radiation[J]. Journal of Nuclear Materials, 2016, 481: 226-233.” discloses specific examples of metal diffusion bonding, in-situ MAX phase bonding, and NITE bonding. However, the metal and MAX phase bonding layer will generate a series of microcracks after high dose irradiation, which will have an adverse effect on the overall sealing performance of the fuel element. The NITE process usually uses Al2O3, Y2O3 and SiO2 as sintering aids, and its bonding temperature is often as high as 1800℃, which is far beyond the service temperature of domestic third-generation SiC continuous fibers. This will cause damage to the fiber structure and have a negative impact on the mechanical and thermal performance of SiC / SiC fuel cladding tubes. Chinese patents (publication numbers CN110903102A, CN109336634B, and CN112851389A) disclose methods for connecting SiC / SiC composite materials or SiC ceramics to nuclear materials using CaO-Y2O3-Al2O3-SiO2, CaO-MgO-Al2O3-SiO2, and CaO-Al2O3-SiO2-Li2O glass-ceramic bonding, respectively. However, oxide glass-ceramic bonding has an inherent defect of low thermal conductivity, which not only reduces the heat transfer efficiency between nuclear fuel and coolant but also generates significant thermal stress between the SiC / SiC cladding tube and the SiC end plug under the high heat flux density gradient in the LOCA state. Chinese patent (publication number CN108191432B) discloses a process for connecting SiC / SiC composite screws and precursor conversion ceramics. In this process, the precursor releases a large amount of gas during pyrolysis, leaving many pores inside the product, and also experiences significant volume shrinkage, resulting in a low matrix density. Taking all the above factors into consideration, in the existing precursor conversion ceramic bonding process, adding or generating in-situ homogeneous filler SiC is an effective means to ensure high airtightness, excellent mechanical and thermal properties, and low thermal stress of the bonding layer. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of existing accident-resistant SiC / SiC fuel cladding tubes and SiC end plugs in terms of insufficient bonding performance. This invention provides a method that combines ceramic precursor conversion with chemical vapor deposition to introduce multi-level SiC whiskers and in-situ SiC nanowire networks homogeneous with SiC / SiC cladding tubes and SiC end plugs into a porous SiC framework converted from ceramic precursors. This achieves SiC / SiC fuel cladding tube and SiC end plug bonding at low temperature (without damaging the SiC / SiC composite material), with low thermal stress (the bonding layer can maintain high structural stability in the LOCA state), and with excellent mechanical and thermal properties.

[0006] This invention provides a multi-stage reinforced connection method for an accident-resistant SiC / SiC fuel cladding tube and a SiC end plug, comprising: (1) Grind, polish, ultrasonically clean and dry the connection between the SiC / SiC fuel cladding tube and the SiC end plug; (2) SiC whiskers, SiC powder, ceramic precursor, catalyst, binder and solvent are thoroughly mixed to obtain a paste-like encapsulant. The ceramic precursor is at least one of polycarbosilane, polymethylsilane and polyallylcarbosilane. The catalyst is an organic compound of a transition metal. The binder is a compound of phenolic resin and organosilicon resin. The volume ratio of SiC whiskers, SiC powder, ceramic precursor, catalyst, binder and solvent is (5~25):(10~20):(10~15):(1~2):(3~6):(40~50). (3) Apply the paste-like encapsulant evenly to the connection surface between the SiC / SiC fuel cladding tube and the SiC end plug, and place it in a chemical vapor deposition system to ceramicize and densify the connection layer. Finally, a multi-level reinforced connection layer between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug can be obtained.

[0007] Preferably, in step (1), the SiC / SiC fuel cladding tube is a SiC / SiC composite material made of third-generation SiC continuous fibers wound in one dimension or woven in two dimensions and deposited with PyC interface, SiC matrix and SiC coating, with a relative density of 85~95%, an inner diameter of 7.5~10mm and a wall thickness of 0.5~1.5mm.

[0008] Preferably, in step (1), the relative density of the SiC end plug is 95~99%, the diameter of the part that is in close contact with the casing tube is 7.3~9.7mm and the length is 0.5~2cm, and the diameter of the part that is in contact with the external environment is 8.5~11.6mm and the thickness is 1~5mm.

[0009] Preferably, in step (2), the SiC whiskers have an aspect ratio of (20~50):1, a diameter of 0.2~0.5μm, and a purity of >99%.

[0010] Preferably, in step (2), the particle size distribution of the SiC powder is 20μm:5μm:0.5μm=5:3:2, and the purity is >99%.

[0011] Preferably, in step (2), the ceramic precursor has an average relative molecular mass > 1000 and a purity > 95%.

[0012] Preferably, in step (2), the transition metal in the organic compound of the transition metal is iron (thermal neutron absorption cross section 2.56 barns), chromium (thermal neutron absorption cross section 3.1 barns), or nickel (thermal neutron absorption cross section 4.49 barns), with a purity >99%, such as bis(cyclopentadiene)chromium, ferrocene, or bis(cyclopentadiene)nickel.

[0013] Preferably, in step (2), the mass ratio of phenolic resin to organosilicon resin in the binder is (1~3):1.

[0014] Preferably, in step (2), the solvent is at least one of xylene, acetone, and cyclohexane, with a purity > 99%.

[0015] Preferably, in step (3), the ceramicization process of the connecting layer includes: evacuating to 100~1000Pa, introducing Ar at a flow rate of 50~200mL / min, heating to 1350~1450℃ at a rate of 5~10℃ / min, and holding at the temperature for 1~5h to achieve ceramicization of the precursor in the connecting layer.

[0016] Preferably, in step (3), the densification process of the connecting layer includes: maintaining a vacuum of 100~1000Pa and an Ar flow rate of 50~200mL / min, cooling to 800~1000℃ at a rate of 5~10℃ / min, using methyltrichlorosilane as the gas source for the SiC matrix, Ar as the dilution gas, and H2 as the carrier gas for chemical vapor deposition; wherein, the deposition time is 60~180h, the flow ratio of the carrier gas H2 to methyltrichlorosilane is (5~15):1, and the flow rate of the carrier gas H2 is 60~300mL / min.

[0017] In this invention, by adjusting the formulation ratio of the encapsulant and combining it with the densification of the original porous SiC matrix (conversion of the precursor in the encapsulant) and SiC whisker framework (integrated in the encapsulant) in the chemical vapor deposition process, a multi-level microstructure including SiC whiskers, SiC matrix and SiC nanowires is formed by precisely controlling various process parameters. Beneficial effects

[0018] The multi-stage reinforced connection process between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug of this invention has the following advantages: (1) Near homogeneous bonding between SiC / SiC composite material and CVD-SiC was achieved, effectively relieving interlayer thermal stress.

[0019] (2) The connection layer from micron-sized SiC whiskers and SiC matrix to in-situ SiC nanowires has a multi-level strengthening effect, which gives it excellent mechanical properties. Moreover, the morphology and volume fraction of each component are controllable, the process is simple and versatile.

[0020] (3) Unlike traditional glass sealants, it improves the resistance of the bonding layer to radiation damage under normal operation in the reactor and the structural stability under accident conditions, effectively preventing the leakage of radioactive materials generated by the fission of the core fuel in the cladding tube and improving the overall safety margin of the reactor. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a low-magnification scanning electron microscope image of the connecting layer; Figure 3 The image shows a high-magnification scanning electron microscope image of the bonding layer (needle-shaped particles are added SiC whiskers, bamboo-shaped particles are in-situ generated SiC whiskers, and other granular particles are SiC powder). Detailed Implementation

[0022] To further illustrate the invention's content, features, and practical effects, the invention will be described in detail below with reference to embodiments. It should be noted that the modification methods of the invention are not limited to these specific implementation methods. Equivalent substitutions and modifications made by those skilled in the art based on their reading of the invention's content, without departing from the spirit and essence of the invention, are also within the scope of protection claimed by this invention.

[0023] The following exemplarily illustrates a multi-stage reinforced connection process between an accident-resistant SiC / SiC fuel cladding tube and a SiC end plug provided by the present invention (e.g. Figure 1 (As shown).

[0024] The connection between the SiC / SiC fuel cladding tube and the SiC end plug is ground, polished, ultrasonically cleaned, and dried.

[0025] In an optional embodiment, the SiC / SiC fuel cladding tube is a SiC / SiC composite material made by one-dimensional winding (or two-dimensional weaving) of third-generation SiC continuous fibers and deposition of PyC interface, SiC matrix and SiC coating, with a relative density of 85~95%, an inner diameter of 7.5~10mm and a wall thickness of 0.5~1.5mm.

[0026] The SiC end plug has a relative density of 95-99%, a diameter of 7.3-9.7 mm and a length of 0.5-2 cm at the part that is in close contact with the casing tube, and a diameter of 8.5-11.6 mm and a thickness of 1-5 mm at the part that is in contact with the external environment. Its preparation processes include chemical vapor deposition (Braun J, Cédric Sauder, Lamon J, et al. Influence of an original manufacturing process on the properties and microstructure of SiC / SiC tubular composites[J]. Composites Part A: Applied Science and Manufacturing, 2019, 123: 170-179. DOI: 10.1016 / j.compositesa. 2019.04.031.) or nano-transient eutectic (Fitriani P, Sharma AS, Septiadi A, et al. Fabrication of tubular SiCf / SiC using different preform architectures by electrophoretic deposition and hotpressing[J]. Ceramics International, 2017, 43(10): 7618-7626. DOI: 10.1016 / j.ceramint. 2017.03.056.).

[0027] SiC whiskers, SiC powder, ceramic precursor, catalyst, binder and solvent are thoroughly mixed to obtain a paste-like encapsulant.

[0028] In an optional embodiment, the SiC whiskers have an aspect ratio of (20~50):1, a diameter of 0.2~0.5μm, and a purity >99%. The purpose of adding SiC whiskers in this invention is to serve as an inert reinforcing phase to improve the room temperature and high temperature mechanical properties of the bonding layer.

[0029] The particle size distribution of the SiC powder is 20μm:5μm:0.5μm=5:3:2, and the purity is >99%.

[0030] The ceramic precursor is at least one of polycarbosilane, polymethylsilane, and polyallylcarbosilane, with an average relative molecular mass >1000 and a purity >95%.

[0031] The catalyst is at least one organotransition metal compound selected from iron (thermal neutron absorption cross-section 2.56 barns), chromium (thermal neutron absorption cross-section 3.1 barns), and nickel (thermal neutron absorption cross-section 4.49 barns), with a purity >99%; for example, dicyclopentadiene chromium, ferrocene, or bis(cyclopentadiene)nickel are selected. The catalyst functions as follows: the nano-metal particles formed by pyrolysis at high temperature serve as nucleation sites for the gas-phase precursor, promoting the in-situ growth of enhanced whiskers.

[0032] The binder is a mixture of phenolic resin and organosilicon resin in a mass ratio of (1~3):1, for example 2:1; the function of the binder is to impart bonding strength to the bonding layer at room temperature, and the carbon and silicon produced by pyrolysis at high temperature can regulate the stoichiometry of the bonding layer.

[0033] The solvent is at least one of organic solvents such as xylene, acetone, and cyclohexane, with a purity > 99%.

[0034] The volume ratio of the SiC whiskers, SiC powder, ceramic precursor, catalyst, binder and solvent is (5~25):(10~20):(10~15):(1~2):(3~6):(40~50). The advantage of the encapsulant of the present invention is that the added SiC whiskers can work synergistically with the in-situ generated SiC whiskers to enhance the structure and maintain the structural stability of the encapsulation bonding layer under high temperature thermal shock environment.

[0035] A paste-like encapsulant is evenly applied to the interface between the SiC / SiC fuel cladding tube and the SiC end plug. The interface is then placed in a chemical vapor deposition system for ceramicization and densification of the interface layer, ultimately resulting in a multi-level reinforced interface layer between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug.

[0036] In an optional embodiment, the ceramization and densification process of the connecting layer is as follows: vacuuming to 100-1000 Pa, introducing Ar at 50-200 mL / min, heating to 1350-1450 °C at 5-10 °C / min and holding for 1-5 h to achieve ceramization of the precursor in the connecting layer; subsequently, maintaining the same vacuum and Ar flow rate, cooling to 800-1000 °C at 5-10 °C / min, using MTS as the gas source for the SiC matrix, Ar as a dilution gas to control the reaction rate of chemical vapor infiltration, and H2 as the carrier gas for MTS via bubbling, introducing it into the chemical vapor reactor. The deposition time is 60-180 h, the flow ratio of the carrier gas H2 to MTS is 15:1-5:1, and the carrier gas flow rate is 60-300 mL / min.

[0037] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below. Example 1

[0038] (1) Select a SiC / SiC fuel cladding tube with a length of ~10cm, a relative density of ~90%, an inner diameter of ~10mm, and a wall thickness of ~0.8mm, and a SiC end plug with a relative density of ~98% (specific structural parameters are as follows). Figure 1 As shown in the figure, the connection between the SiC / SiC fuel cladding tube and the SiC end plug is ground, polished, ultrasonically cleaned and dried.

[0039] (2) SiC whiskers (length-to-diameter ratio of 20:1, diameter of 0.3μm), SiC powder, polycarbosilane, dicyclopentadiene chromium, binder (a compound of phenolic resin and organosilicon resin with an effective mass ratio of 2:1) and acetone are thoroughly mixed in a volume ratio of 25:15:10:2:3:50 to obtain a paste-like encapsulant.

[0040] (3) A paste-like encapsulant is uniformly applied to the connection surface between the fuel cladding tube and the end plug. The connection layer is then placed in a chemical vapor deposition system for ceramicization and densification. Specific process parameters are as follows: vacuum to 1000 Pa, Ar is introduced at 120 mL / min, and the temperature is increased to 1350℃ at 5℃ / min and held for 1 hour to achieve ceramicization of the precursor in the connection layer. Subsequently, maintaining the same vacuum and Ar flow rate, the temperature is decreased to 1000℃ at 7℃ / min. MTS is used as the gas source for the SiC matrix, Ar is used as a dilution gas to control the reaction rate of chemical vapor infiltration, and H2 is introduced into the chemical vapor reactor as the carrier gas for MTS via bubbling. The deposition time is 120 hours, the flow ratio of the carrier gas H2 to MTS is 15:1, and the carrier gas flow rate is 180 mL / min. Finally, a multi-stage reinforced connection layer between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug is obtained.

[0041] Testing revealed that the multi-stage reinforced bonding layer between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug has a thickness of 137 μm, a thermal conductivity of 56.6 W / (m•K), and a coefficient of thermal expansion of 4.3 × 10⁻⁶. -6 / ℃. The end plug ejection test load was 2485N, and the load retention rate of the sample was 79.3% after one hot (1200℃)-cold (25℃) cycle.

[0042] Figure 2 The image shows a low-magnification scanning electron microscope image of the connecting layer. As can be seen from the image, the formed needle-like interlocking network structure is extremely uniform and free from defects such as component segregation and macroscopic pores.

[0043] Figure 3 The image shows a high-magnification scanning electron microscope (SEM) image of the connecting layer. As can be seen from the image, the high aspect ratio needle-like and columnar grains are the initially introduced SiC whiskers, the bamboo-like high aspect ratio grains are the subsequently in-situ generated SiC whiskers, and the irregularly structured grains such as plates and granules are SiC powder used as fillers. Example 2

[0044] (1) Select a SiC / SiC fuel cladding tube with a length of ~10cm, a relative density of ~85%, an inner diameter of ~8.5mm, and a wall thickness of ~1.5mm, and a SiC end plug with a relative density of ~95% (specific structural parameters are as follows). Figure 1 As shown in the figure, the connection between the SiC / SiC fuel cladding tube and the SiC end plug is ground, polished, ultrasonically cleaned and dried.

[0045] (2) SiC whiskers (length-to-diameter ratio of 40:1, diameter of 0.2μm), SiC powder, polyallyl carbosilane, ferrocene, binder (a mixture of phenolic resin and silicone resin with an effective mass ratio of 2:1) and cyclohexane are thoroughly mixed in a volume ratio of 15:10:15:1:4.5:40 to obtain a paste-like encapsulant.

[0046] (3) A paste-like encapsulant is uniformly applied to the connection surface between the fuel cladding tube and the end plug. The connection layer is then placed in a chemical vapor deposition system for ceramicization and densification. Specific process parameters are as follows: vacuum to 100 Pa, Ar is introduced at 50 mL / min, and the temperature is raised to 1450℃ at 7℃ / min and held for 1 hour to achieve ceramicization of the precursor in the connection layer. Subsequently, maintaining the same vacuum level and Ar flow rate, the temperature is lowered to 900℃ at 5℃ / min. MTS is used as the gas source for the SiC matrix, Ar is used as a dilution gas to control the reaction rate of chemical vapor infiltration, and H2 is introduced into the chemical vapor reactor as the carrier gas for MTS via bubbling. The deposition time is 60 hours, the flow ratio of the carrier gas H2 to MTS is 5:1, and the carrier gas flow rate is 60 mL / min. Finally, a multi-level reinforced connection layer between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug is obtained.

[0047] Testing revealed that the multi-stage reinforced bonding layer between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug has a thickness of 164 μm, a thermal conductivity of 49.7 W / (m•K), and a coefficient of thermal expansion of 4.9 × 10⁻⁶. -6 / ℃. The end plug ejection test load was 2162N, and the load retention rate of the sample was 86.4% after one hot (1200℃)-cold (25℃) cycle. Example 3

[0048] (1) Select a SiC / SiC fuel cladding tube with a length of ~10cm, a relative density of ~95%, an inner diameter of ~7.5mm, and a wall thickness of ~0.5mm, and a SiC end plug with a relative density of ~99% (specific structural parameters are as follows). Figure 1 As shown in the figure, the connection between the SiC / SiC fuel cladding tube and the SiC end plug is ground, polished, ultrasonically cleaned and dried.

[0049] (2) SiC whiskers (length-to-diameter ratio of 50:1, diameter of 0.5μm), SiC powder, polymethylsilane, bis(cyclopentadiene)nickel, binder (a mixture of phenolic resin and organosilicon resin with an effective mass ratio of 2:1) and xylene are thoroughly mixed in a volume ratio of 5:20:12.5:1.5:6:45 to obtain a paste-like encapsulant.

[0050] (3) A paste-like encapsulant is uniformly applied to the connection surface between the fuel cladding tube and the end plug. The connection layer is then placed in a chemical vapor deposition system for ceramicization and densification. Specific process parameters are as follows: vacuum to 500 Pa, Ar is introduced at 200 mL / min, and the temperature is increased to 1400℃ at 10℃ / min and held for 5 hours to achieve ceramicization of the precursor in the connection layer. Subsequently, maintaining the same vacuum and Ar flow rate, the temperature is decreased to 800℃ at 10℃ / min. MTS is used as the gas source for the SiC matrix, Ar is used as a dilution gas to control the reaction rate of chemical vapor infiltration, and H2 is introduced into the chemical vapor reactor as the carrier gas for MTS via bubbling. The deposition time is 180 hours, the flow ratio of the carrier gas H2 to MTS is 10:1, and the carrier gas flow rate is 300 mL / min. Finally, a multi-stage reinforced connection layer between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug is obtained.

[0051] Testing revealed that the multi-stage reinforced bonding layer between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug has a thickness of 111 μm, a thermal conductivity of 44.3 W / (m•K), and a coefficient of thermal expansion of 3.7 × 10⁻⁶. -6 / ℃. The end plug ejection test load was 2793N, and the load retention rate of the sample was 82.7% after one hot (1200℃)-cold (25℃) cycle.

Claims

1. A method for multistage strengthening of an accident resistant SiC / SiC fuel cladding tube to a SiC end plug, characterized in that, include: (1) Grind, polish, ultrasonically clean and dry the connection between the SiC / SiC fuel cladding tube and the SiC end plug; (2) SiC whiskers, SiC powder, ceramic precursor, catalyst, binder and solvent are thoroughly mixed to obtain a paste-like encapsulant. The ceramic precursor is at least one of polycarbosilane, polymethylsilane and polyallylcarbosilane. The catalyst is an organic compound of a transition metal. The binder is a compound of phenolic resin and organosilicon resin. The volume ratio of SiC whiskers, SiC powder, ceramic precursor, catalyst, binder and solvent is (5~25):(10~20):(10~15):(1~2):(3~6):(40~50). (3) Apply the paste-like encapsulant evenly to the connection surface between the SiC / SiC fuel cladding tube and the SiC end plug, and place it in a chemical vapor deposition system to sequentially ceramicize and densify the connection layer. Finally, a multi-level reinforced connection layer including SiC whiskers, SiC matrix and SiC nanowires can be obtained between the accident-resistant SiC / SiC fuel cladding tube and the SiC end plug.

2. The connection method according to claim 1, characterized in that, In step (1), the SiC / SiC fuel cladding tube is a SiC / SiC composite material made of third-generation SiC continuous fibers wound in one dimension or woven in two dimensions and deposited with PyC interface, SiC matrix and SiC coating. The relative density is 85~95%, the inner diameter is 7.5~10mm and the wall thickness is 0.5~1.5mm.

3. The connection method according to claim 1 or 2, characterized in that, In step (1), the relative density of the SiC end plug is 95~99%, the diameter of the part that is in close contact with the SiC / SiC fuel cladding tube is 7.3~9.7mm and the length is 0.5~2cm, and the diameter of the part that is in contact with the external environment is 8.5~11.6mm and the thickness is 1~5mm.

4. The connection method according to claim 1, characterized in that, In step (2), the aspect ratio of the SiC whiskers is (20~50):1, the diameter is 0.2~0.5μm, and the purity is >99%; the particle size distribution of the SiC powder is 20μm:5μm:0.5μm=5:3:2, and the purity is >99%.

5. The connection method according to claim 1, characterized in that, In step (2), the ceramic precursor has an average relative molecular mass > 1000 and a purity > 95%.

6. The connection method according to claim 1, characterized in that, In step (2), the transition metal in the organic compound of the transition metal is iron, chromium, or nickel.

7. The connection method according to claim 6, characterized in that, The organic compound of the transition metal is dicyclopentadiene chromium, ferrocene, or bis(cyclopentadiene)nickel.

8. The connection method according to claim 1, characterized in that, In step (2), the mass ratio of phenolic resin and organosilicon resin in the binder is (1~3):

1.

9. The connection method according to claim 1, characterized in that, In step (2), the solvent is at least one of xylene, acetone, and cyclohexane, with a purity > 99%.

10. The connection method according to claim 1, characterized in that, In step (3), the ceramicization process of the connecting layer includes: evacuating to 100~1000Pa, introducing Ar at a flow rate of 50~200mL / min, heating to 1350~1450℃ at a rate of 5~10℃ / min, and holding at the temperature for 1~5h to achieve ceramicization of the precursor in the connecting layer.

11. The connection method according to claim 1, characterized in that, In step (3), the densification process of the connecting layer includes: maintaining a vacuum of 100~1000Pa and an Ar flow rate of 50~200mL / min, cooling to 800~1000℃ at a rate of 5~10℃ / min, using methyltrichlorosilane as the gas source for the SiC matrix, Ar as the dilution gas, and H2 as the carrier gas for chemical vapor deposition; wherein, the deposition time is 60~180h, the flow ratio of the carrier gas H2 to methyltrichlorosilane is (5~15):1, and the flow rate of the carrier gas H2 is 60~300mL / min.

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