Patent Publication Number: US-2021166825-A1

Title: Joining and sealing pressurized ceramic structures

Description:
PRIORITY CLAIM AND RELATED PATENT APPLICATIONS 
     This patent document claims priority to and benefits of U.S. Provisional Patent Application No. 62/574,721 filed on Oct. 19, 2017 with the same title. 
    
    
     TECHNICAL FIELD 
     This patent document relates to systems, structures, devices, and fabrication processes in connection with casing, housing or cladding structures for holding nuclear fuel materials for use in nuclear reactors, or in connection with heat exchangers, or nosecones or nozzles, or with flow channel inserts. 
     BACKGROUND 
     Many nuclear reactors use a fissile material as the fuel for nuclear reactions to generate power. The fuel is usually held in robust containers, such as fuel rods, that can endure high operating temperatures of nuclear reactions and maintain the structural integrity in an intense neutron radiation environment. It is desirable that fuel structures maintain their shape and structural integrity over a sufficient period (e.g., several years) within the reactor core, thereby preventing the leakage of fission products into the reactor coolant. Other structures, such as heat exchangers, nozzles, nosecones, flow channel inserts, or related components, also require high temperature performance, corrosion resistance, and specific, non-planar geometries where high dimensional accuracy is important. 
     SUMMARY 
     This patent document relates to systems, structures, devices, and fabrication processes for ceramic matrix composites suitable for use in a nuclear reactor environment and other applications requiring materials that can withstand high temperatures and/or highly corrosive environments. 
     In one exemplary aspect, a method of joining and sealing ceramic structures is disclosed. The method includes forming a joint of a ceramic structure including a tubular structure and an end plug located inside the tubular structure using a sealing material, wherein the end plug is structured to include a hole that goes through a top surface and a bottom surface of the end plug; filling the ceramic structure with a desired gas composition through the hole; heating a material into a molten form using a heat source; and directing the material into the hole, wherein the material solidifies to seal the end plug. 
     In some embodiments, the forming of the joint includes: applying the sealing material between the ceramic structure and the end plug, wherein the sealing material includes a preceramic polymer and a plurality of inclusions; forming a solid ceramic from the sealing material; and crystallizing the solid ceramic to form a crystalline matrix comprising a same ceramic polymorph as the ceramic structure and the end plug, the plurality of inclusions being disposed within the crystalline matrix. 
     In some embodiments, the inclusions can include spheres, flakes, whiskers, fibers, or irregular shapes comprising the ceramic polymorph. In some implementations, the sealing material can be cured at a first temperature and pyrolized at a second temperature higher than the first temperature to form the solid ceramic, and the solid ceramic is crystallized at a third temperature higher than the second temperature. 
     In some embodiments, the method can be implemented to include creating a low pressure in the ceramic structure; and strengthening the joint under the low pressure, before the filling of the desired gas composition, by applying to the crystalline matrix a substantially gas impermeable sealing layer, the substantially gas impermeable sealing layer comprising the same ceramic polymorph as the ceramic structure and the end plug. The strengthening the joint can be performed using chemical vapor infiltration (CVI). In some implementations, the low pressure can be created by removing a substantial amount of gas from the ceramic structure. The crystalline matrix may further include cracks, pores, or voids, and the substantially gas impermeable sealing layer may penetrate partially or fully into the crystalline matrix via the cracks, pores, or voids. 
     In some embodiments, the desired gas composition includes helium gas. In some implementations, the melted material may have a high melt-temperature. For example, the melted material includes oxide, silicon, or a transition metal. 
     In some embodiments, the method includes creating a low pressure in the tubal structure; and strengthening the joint under the low pressure before the filling of the desired gas composition. In some embodiments, the method also includes applying the sealing material to an outer surface of the end plug; and strengthening the end plug under a second low pressure. The strengthening can be performed using chemical vapor deposition (CVD). In some implementations, the material in the molten form solidifies while flowing in the hole. In some implementations, the material in the molten form solidifies after the heat source is removed. In some embodiments, the method further includes heat treating the joined ceramic structure and the end plug at a temperature of at least 1350° C. 
     In another exemplary aspect, a method of joining and sealing ceramic structures is disclosed. The method includes forming a joint of a ceramic structure and an end plug using a sealing material, wherein the end plug has a body including a hole that goes through a top surface and a bottom surface of the end plug, and a pin positioned in the hole; placing a material in the hole of the end plug; applying heat to a section of the ceramic structure near the end plug to heat the material; and applying pressure to the body or the pin so that the pin presses the heated material in the hole to seal the end plug. 
     In some embodiments, the method also includes applying the sealing material between the ceramic structure and the end plug, wherein the sealing material includes a preceramic polymer and a plurality of inclusions; forming a solid ceramic from the sealing material; and crystallizing the solid ceramic to form a crystalline matrix comprising a same ceramic polymorph as the ceramic structure and the end plug, the plurality of inclusions being disposed within the crystalline matrix. 
     In some embodiments, the inclusions include spheres, flakes, whiskers, fibers, or irregular shapes comprising the ceramic polymorph. In some implementations, the sealing material is cured at a first temperature and pyrolized at a second temperature higher than the first temperature to form the solid ceramic, and the solid ceramic is crystallized at a third temperature higher than the second temperature. 
     In some embodiments, the method also includes strengthening the joint under the low pressure by forming a substantially gas impermeable sealing layer on the crystalline matrix, the substantially gas impermeable sealing layer comprising the same ceramic polymorph as the ceramic structure and the end plug. The strengthening the joint may be performed using chemical vapor infiltration (CVI). The crystalline matrix may further include cracks, pores, or voids, and the substantially gas impermeable sealing layer may penetrate partially or fully into the crystalline matrix via the cracks, pores, or voids. 
     In some embodiments, the method also includes filling the ceramic structure with a desired gas composition through the hole. In some implementations, the pressure can be applied to the pin by increasing a pressure of the desired gas composition. 
     In another exemplary aspect, a device for sealing an end of a ceramic structure is disclosed. The device includes a device body that includes a ceramic material and is shaped to include a first surface having a first opening and a second surface having a second opening, wherein the first opening and the second opening form a hollow space in the device body, and wherein the first opening and the second opening form a hollow space in the device body that provides a passage into an inner area of the ceramic structure and can be filled with a sealing material to seal the passage. 
     In some embodiments, a diameter of the first opening and a diameter of the second opening are substantially the same. In some embodiments, the ceramic material includes silicon carbide (SiC). In some embodiments, the device body includes a subsection that is tapered along a center axis of the device. 
     In some embodiments, the device also includes a pin shaped to fit into the hollow space as part of the passage in the device body and to press the sealing material to seal the passage. In some implementations, the pin includes a ceramic material. 
     In another exemplary aspect, a nuclear fuel rod is disclosed. The nuclear fuel rod includes a tubular structure that includes a first ceramic material, a first plug joined with a first end of the tubular structure using a first sealing material; a second plug joined with a second end of the tubular structure using said first sealing material, wherein the second plug comprising a body that is shaped to include a first surface having a first opening and a second surface having a second opening, the first opening and the second opening forming a hollow space in the body of the second plug; a second sealing material disposed within the hollow space of the second plug such that the second material seals the second plug; and one or more nuclear fuel pellets positioned within the tubular structure. 
     In some embodiments, the ceramic material includes silicon carbide (SiC). The first sealing material may be a preceramic polymer. In some implementations, the preceramic polymer includes a plurality of inclusions. For example, the preceramic polymer is polycarbosilane and the inclusions are in a form of silicon carbide powder. In some implementations, the first plug has a tapered body. The first plug may include a silicon carbide material. 
     In some embodiments, the body of the second plug includes a section that is tapered along a center axis of the second plug. The second plug may include a silicon carbide material. In some implementations, the second plug includes a pin or insert shaped to fit into the hollow space as part of the passage in the device body and to press the second sealing material to seal the second plug. The pin or the insert may include a silicon carbide material. In some embodiments, the second sealing material includes oxide, silicon, or a transition metal. 
     In another exemplary aspect, an apparatus for sealing a ceramic structure is disclosed. The apparatus includes a chamber for holding the ceramic structure; a gas inlet coupled to the chamber for directing a gas composition to or from the chamber; and a plurality of coils arranged outside of the chamber, wherein the plurality of coils are capable of induction heating to raise a temperature of a section of the ceramic structure held within the chamber. 
     In some embodiments, the chamber is made of quartz. The chamber can have a uniform cross-section in some implementations. In some embodiments, the chamber has a small cross-section at a first end and a large cross-section at a second end. For example, a diameter of the first end is around 70 mm. In another example, a diameter of the second end is around 110 mm. 
     In some embodiments, the gas composition includes He or Ar. In some embodiments, the coils are radiofrequency (RF) coils. In some implementations, the apparatus also includes an outer chamber and a flange that are robust against a high operating temperature. The outer chamber can have a height of around 540 mm. 
     In another exemplary aspect, a method of sealing a ceramic structure is disclosed to include positioning the ceramic structure into a chamber of a sealing device; disposing a plug on an end of the ceramic structure, wherein a sealing material is positioned between the ceramic structure and the plug; placing a susceptor block adjacent to the plug; and driving a plurality of induction coils arranged outside of the chamber with a varying electric current to heat the end of the ceramic structure and the susceptor block to a high temperature to join the plug and parts of the ceramic structure in contact with the plug at the end of the ceramic structure, thus sealing the end of the ceramic structure. 
     In some embodiments, the seal is obtained by a chemical vaper infiltration (CVI) process. In some embodiments, the method also includes removing a section of the susceptor block after the seal is obtained. In some implementations, the method further includes placing a layer of silicon between the plug and the susceptor block. The seal can be obtained by a liquid silicon flow process. The layer of silicon can be configured to melt at the high temperature and to react with the susceptor block. In some embodiments, the method also includes directing a gas composition into the ceramic structure before the heating of the end of the ceramic structure. 
     In yet another exemplary aspect, a system of sealing a ceramic structure is disclosed to include a gas storage to supply a gas composition, an apparatus for sealing a ceramic structure, comprising: a chamber for holding the ceramic structure; a gas inlet coupled to the chamber for directing the gas composition to the chamber; and a plurality of coils arranged outside of the chamber, wherein the coils are capable of induction heating to raise a temperature of a section of the ceramic structure held within the chamber; one or more temperature monitors for monitoring one or more temperatures of the apparatus, and one or more pressure regulators for controlling pressure of the apparatus. 
     In some embodiments, the one or more temperature monitors include a thermocouple for monitoring a temperature of the ceramic structure. In some implementations, the one or more temperature monitors include a pyrometer to monitor a temperature of the section of the ceramic structure. The one or more temperature monitors may further include a temperature monitor to report a temperature of the chamber. 
     In some embodiments, the one or more pressure regulators include a back pressure regulator. In some implementations, the system also includes a filter for filtering an exhaust gas emitted from the apparatus. In some embodiments, the system further includes a pre-filter gas pressure monitor for monitoring a pressure of the exhaust gas before passing the filter. The system may also include a post-filter gas pressure monitor for monitoring a pressure of the exhaust gas after passing the filter. 
     In one exemplary aspect, a device for sealing a ceramic structure with an end plug is disclosed to include a body that is shaped to fit between one opening end of the ceramic structure and the end plug to seal the opening end with the end plug, wherein the body includes a first surface having a first opening, a second surface having a second opening, and a side wall connecting the first surface and the second surface, and wherein the first opening and the second opening form a hollow space in the body to enable the end plug to be coupled to the device. 
     In some embodiments, the insert also includes a raised part extending from the first surface and protruding from the side wall. In some embodiments, the insert is made of a transition metal. The transition metal may be molybdenum. 
     In another exemplary aspect, a method of manufacturing an insert for sealing or joining a ceramic structure with an end plug is disclosed. The method includes fabricating a part that includes a body that is shaped to fit between one opening end of the ceramic structure and the end plug to seal the opening end with the end plug. The body includes a first surface having a first opening, a second surface having a second opening, and a side wall connecting the first surface and the second surface. The first opening and the second opening form a hollow space in the body to enable the end plug to be coupled to the part. The method also includes cleaning the fabricated part, and polishing the cleaned part to reduce surface blemish. 
     In some embodiments, the cleaning is performed using an ultrasonic bath. In some implementations, the polishing of the cleaned part includes electro-polishing the cleaned part in an acid bath. 
     In yet another exemplary aspect, a method of joining a ceramic structure with an end plug and an insert is disclosed to include placing the insert between the ceramic structure and the end plug to form an assembly, positioning the assembly in an inert gas composition, heating the assembly under a first temperature and a first pressure, and annealing the assembly for a duration of time to relieve interfacial residual stress. 
     In some embodiments, the inert gas composition is helium. In some embodiments, the first temperature is beyond 1500° C. The duration of time can be between 2 to 4 hours. 
     In some embodiments, the heating of the assembly includes applying a force to create the first pressure to the assembly, and hot-pressing the assembly under the first temperature and the first pressure. The force may be between 0.5 to 5 kN. 
     The above and other aspects and their implementations are described in greater detail in the drawings, the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an exemplary fuel assembly for a nuclear reactor. 
         FIG. 1B  shows an exemplary heat exchanger for capturing heat. 
         FIG. 2A  shows an exemplary schematic diagram of sealing the first end of a SiC structure for a nuclear reactor. 
         FIG. 2B  is a flowchart representation of a method for sealing two articles. 
         FIG. 2C  shows a schematic diagram of intermediate structures during joining. 
         FIG. 2D  shows an exemplary schematic diagram of reinforcing a joint. 
         FIG. 3A  shows an exemplary schematic diagram of an end plug with a fill-hole. 
         FIG. 3B  shows another exemplary schematic diagram of an alternative end plug with a fill-hole. 
         FIG. 3C  shows another exemplary schematic diagram of an alternative end plug with a fill-hole. 
         FIG. 4A  shows an exemplary fill-hole. 
         FIG. 4B  illustrates an exemplary cross section image showing a partial fill-hole in an end plug obtained by X-ray computed tomography (XCT). 
         FIG. 5  shows an exemplary schematic diagram for the multi-step approach of sealing the second end of the SiC cladding using the alternative end plug. 
         FIG. 6  shows an exemplary configuration to seal the fill-hole. 
         FIG. 7A  shows an exemplary configuration to seal the fill-hole with a pin. 
         FIG. 7B  shows another exemplary configuration to seal the fill-hole with a pin. 
         FIG. 7C  shows another exemplary configuration to seal the fill-hole with a pin. 
         FIG. 7D  shows yet another exemplary configuration to seal the fill-hole with a pin. 
         FIG. 8  is a flow chart illustrating an exemplary method of joining and sealing ceramic structures. 
         FIG. 9  is a flow chart illustrating another exemplary method of joining and sealing ceramic structures. 
         FIG. 10A  shows a side view of an exemplary insert. 
         FIG. 10B  shows a cross-section of an exemplary insert. 
         FIG. 11A  shows a side view of another exemplary insert. 
         FIG. 11B  shows a cross-section view of another exemplary insert. 
         FIG. 12A  shows a side view of an exemplary insert placed in the cladding structure with an end plug. 
         FIG. 12B  shows a perspective view of an exemplary insert placed in the cladding structure with an end plug. 
         FIG. 13A  shows a side view of another exemplary insert placed in the cladding structure with an end plug. 
         FIG. 13B  shows a perspective view of another exemplary insert placed in the cladding structure with an end plug. 
         FIG. 14  shows exemplary macro- and microstructures of an insert and a SiC plug after an annealing process. 
         FIG. 15  shows an exemplary flow chart for a method of manufacturing an insert for sealing a ceramic structure with an end plug. 
         FIG. 16  shows an exemplary flow chart for a method of sealing or joining a ceramic structure with an end plug and an insert. 
         FIG. 17  shows an exemplary fuel rod after joining and sealing both ends of a ceramic structure using processes in accordance with one or more embodiments of the present technology. 
         FIG. 18  shows an exemplary schematic diagram of a furnace that can be used for several joining processes to join and seal ceramic parts. 
         FIG. 19  shows an exemplary schematic diagram of the inner sleeve, the narrow tubal section, and coils. 
         FIG. 20  shows another exemplary schematic diagram of the inner sleeve, the narrow tubal section, and coils. 
         FIG. 21  shows an exemplary configuration of the furnace system for a chemical vapor infiltration (CVI) process. 
         FIG. 22  shows an exemplary configuration of the furnace system for a liquid silicon flow process. 
         FIG. 23  shows an exemplary schematic diagram of the monitoring mechanism for the furnace system. 
         FIG. 24  shows another exemplary schematic diagram of the exhaust monitoring mechanism for the furnace system. 
         FIG. 25  shows an exemplary flow chart for a method of sealing an end of the SiC cladding. 
     
    
    
     DETAILED DESCRIPTION 
     Nuclear fuel used in a nuclear reactor is usually held in fuel rods capable of enduring high operating temperatures and an intense neutron radiation environment. Fuel structures need to maintain their shape and structural integrity over a long period of time within the reactor core, thereby preventing the leakage of fission products into the reactor coolant of a reactor.  FIG. 1A  shows an example of a nuclear fuel rod assembly  100  formed of a bundle of fuel rods  101  used in a nuclear reactor. Each rod has a hollow interior to contain nuclear fuel pellets  103  such as Uranium-containing pellets and spacer grids  105  are used to hold the rods in the assembly. A reactor is designed to hold nuclear fuel rod assemblies that provide sufficient nuclear fuels for power generation when the reactor is in operation. Various fuel rods may be implemented. Some nuclear reactors use zirconium cladding, for example. The fuel rods in this document use Silicon carbide ceramic matrix composites (CMCs) for improved performance. 
     Silicon carbide (SiC) can be used for nuclear applications due to its high temperature strength and chemical inertness. SiC fibers can be used to construct ceramic matrix composites (CMCs) in a high purity SiC matrix (SiC/SiC) to provide increased fracture toughness and can be used as cladding materials for advanced high temperature fission reactors and first wall materials in fusion reactors. SiC/SiC composites can also be designed to enhance reactor safety as cladding for various reactors such as light water reactors (LWRs), where their oxidation kinetics in high temperature steam during accident conditions are superior to zirconium alloys by several orders of magnitude. 
     SiC composites can also be used in variety of high temperature applications such as heat exchangers to recuperate high temperature waste heat from aluminum recycling, syngas production, or gasification-combined-cycle plants.  FIG. 1B  shows an example of a heat exchanger in a counter-flow heat exchanger configuration used in various applications. In this example, one or more hot fluid channels are provided to direct hot fluid from the left of the heat exchanger into thermal conductive hot fluid tubes to pass through the heat exchange to exit on the right side to release the heat of the hot fluid inside the heat exchanger while a cold fluid is directed into the heat exchanger in a generally opposite direction of the hot fluid in the thermal conductive hot fluid tubes to absorb part of the heat released by the hot fluid and then exit the heat exchange at an elevated temperature. The cold and hot fluids (e.g., gas flows or liquid flows) in this example are in thermal contact for the heat exchange but are separately recycled so the heat energy in the hot fluid is transferred to the cold fluid for a desired use. SiC composites are high temperature compatible and exhibit good corrosion resistance, and can be used in counter-flow and various other heat exchangers to effectively address corrosion problems that are escalating as crude oils are often contaminated with naphthenic acid, sulfur, carbon dioxide and hydrogen sulfide. 
     The aerospace field also has a wide variety of applications that are an ideal match for the high temperature strength of SiC composites: nosecones, shrouds, airfoils, turbine blades and other jet engine components. In all cases, the geometry of a fiber preform must be maintained during the fabrication process to produce a ceramic matrix composite, near net-shape component. 
     The manufacture of the SiC composites for various applications usually includes several steps. First, a SiC composite structure is manufactured to include an inner hollow passage through the SiC composite structure which is to be sealed. The Sic composite structure can have a tubular, tubal or non-tubular shape with sidewalls and an inner hollow passage surrounded by the sidewalls. Second, a first end of the SiC structure is joined and sealed with a first SiC end plug. A joint between a first article (e.g., the SiC composite structure) and a second article (e.g., the SiC end plug) may include a matrix comprising a ceramic polymorph that extends between the first and second articles. A plurality of inclusions that includes a ceramic polymorph may be distributed throughout the matrix. In some embodiments, a sealing layer that includes a ceramic polymorph can be applied to the joint surface of the first article (e.g., the SiC composite structure), the second article (e.g., the SiC end plug), and the matrix. The sealing layer may partially extend into the matrix. The matrix, the plurality of inclusions, and the sealing layer each may include the same ceramic polymorph. 
       FIG. 2A  shows an exemplary schematic diagram of the process for sealing the first end of the SiC structure in form of a SiC tube. In this particular example, the first opening end  201  has a tapered opening with an opening dimension that gradually decreases from the outmost part of the opening towards the inner part of the SiC tube  205 . A sealing end plug  203  is designed to have the shape and dimension to fit into the tapered shape of the first opening end  201  of the SiC tube  205 . For example, the SiC tube  205  can be a circular tube or a tube of another geometrical shape such as square. For a circular SiC tube  205 , the first opening end  201  in the specific example in  FIG. 2A  is a tapered and tubular opening with a larger opening aperture at the end and gradually decreases towards the inner side of the SiC tube  205 . The sealing end plug  203  has an external shape that is tapered and tubular to fit inside the tapered and tubular opening of the first opening end  201  with some small gap in between to provide some room for accepting a sealing material. 
     A sealing material  202 , such as a preceramic polymer, is applied to form a first joint for filling the slanted interface between the end plug  203  and the opening end of the SiC tube  205  to form a hermetic sealing.  FIG. 2B  is a flowchart representation of a method  220  for sealing two articles (e.g., the first end of a SiC structure  205  and the end plug  203 ) using such a sealing material. The method  220  includes, at  222 , preparing a slurry of preceramic polymer with desired inclusions. Absent inclusions, the slurry may otherwise form numerous cracks and voids during the formation of joint. Inclusions may occupy and/or prevent the development of at least some of such cracks and voids, thus increasing the overall density of joint and improving the joint&#39;s strength and durability. The inclusions may include spheres, flakes, whiskers, fibers, and/or irregular shapes of the material of the articles (e.g., β-SiC) having diameters and/or lengths in the range of nanometers to millimeters. In some embodiments, high aspect ratio inclusions (e.g., having an aspect ratio of 1:2 or greater, or 1:5 or greater, or 1:10 or greater) are believed to be particularly useful for enhancing the mechanical strength and toughness of the joint. For example, matrix having whisker-shaped inclusions may have smaller, rounder voids, and thus more homogeneous. In some embodiments, the preceramic polymer is polycarbosilane (PCS), which is a viscous liquid at room temperature, and the inclusions are in the form of a powder that, in one example, is SiC powder mixed with the liquid via mechanical mixing and ultrasonication. 
     The method  220  then includes, at  224 , applying the slurry to two articles. Here, the two articles are the SiC tube  205  and the sealing plug  203 . In some embodiments, the two articles have substantially similar composition. The ceramic polymorph may be, for example, β-SiC. In some embodiments, the matrix, the plurality of inclusions, and the sealing layer discussed before may comprise more than 99.0 wt % β-SiC, or even more than 99.7 wt % β-SiC. 
     The method  200  also includes curing the slurry at  226  and forming a solid ceramic with inclusions from the slurry at  228 . In some embodiments, the applied slurry can be pyrolyzed to form the solid ceramic. Depending on the particular preceramic polymer being used, such pyrolysis may include one or more intermediate steps. Then the solid ceramic is converted to the desired crystal structure at  230 . 
       FIG. 2C  shows a schematic diagram of intermediate structures during steps  222 ,  226 , and  228  of the method  220 . Ceramic polymer is converted to a ceramic polymorph by the following process: (a) the monomers are polymerized at a relatively low temperature (e.g., 100° C.), (b) the polymer is crosslinked at a high temperature (e.g., 200-400° C.), and (c) the polymer is pyrolyzed at a higher temperature (e.g., 600-850° C.) resulting in formation of an amorphous ceramic. Then, the amorphous ceramic is converted to a crystalline ceramic at a still higher temperature (e.g., greater than 1100° C.). In implementations, the temperature can be selected generate the desired polymorph of the ceramic. 
     The method  200  also includes, at  232 , reinforcing joint with desired material. As shown in  FIG. 2D , in some embodiments, to reinforce and seal the joint, a step of chemical vapor infiltration (CVI) may be performed after the slurry is completely converted to the desired ceramic polymorph, e.g., β-SiC, to form a sealing layer. Indeed, any residual open porosity in the slurry-derived matrix may be used as reactant flow pathways for CVI reactant(s) into the joint and thus partially or fully extend sealing layer into matrix. Such steps are believed to be important for nuclear grade joints, as the ability to retain helium and fission products requires the joint to be structurally sound and substantially impermeable. In some embodiments, a step of chemical vapor deposition (CVD) can further be used to make the joint substantially impermeable. 
     Sealing the first joint may be implemented in various ways, including, e.g., techniques and materials disclosed in U.S. Pat. No. 9,132,619 B2 entitled “High durability joints between ceramic articles, and methods of making and using same” and granted to General Atomics. The entire disclosure in the U.S. Pat. No. 9,132,619 B2 is incorporated by reference as part of this patent document. 
     After the first joint at the first end is formed, the SiC tube  205  may be loaded with materials, such as nuclear pellets and retaining springs, or heat exchanger components, via its second opening on the opposite end of the tube  205 . The second end of the SiC tube  205  is then joined and sealed with a second SiC end plug in a similar manner to form a second hermetic joint and sealing with the sealing material  202 . 
     There remain some challenges for creating the second joint for the SiC structure. First, for applications in the nuclear field, components such as the nuclear pellets and retaining springs are placed inside of the cladding before the second end is sealed. These components may not withstand the high operating temperature if the sealing of the second end requires heating up the entire cladding structure. Second, it is normal to fill the gap within the cladding with helium gas or other gas compositions to permit a better thermal contact between the nuclear fuel and the cladding. Therefore, it is desirable for the sealing and joining method to hermetically seal the cladding while maintaining an elevated internal pressure in the cladding. 
     As mentioned previously, several methods can be used for joining ceramic components, including brazes, preceramic polymers, glasses, and ceramics deposited using CVI and/or CVD. However, it can be difficult to use those methods to create a hermetic sealing that maintain an internal pressurization. It is challenging to create the sealing interface that can offer the same advantages, such as such as corrosion, temperature, or irradiation resistance, as the SiC ceramic material. This patent document describes a multi-step joining process that produces a joint exhibiting a combination of these advantages and can make a hermetic resilient joint. 
     As part of the mechanism for maintaining an elevated internal pressure in the ceramic structure, a ceramic end plug with a fill hole can be used for sealing the second end of the cladding structure.  FIG. 3A  shows an example of such a ceramic end plug  301 . The end plug  301  has a fill hole  303  of a small diameter that enables gas composition to go through the end plug  301 . The end plug  301  can be made of the same material as the SiC composite structure. It is shaped to include a top surface  305  having a first opening  309  and a bottom surface  307  having a second opening  302 . The first opening  309  and the second opening  302  form the fill hole  303 . The first opening  309  and the second opening  302  can have substantially the same shape so that fill-hole  303  has a uniform cross-section. In some embodiments, the second opening  302  is smaller than the first opening  309  so that they form a tapered fill-hole  303 . In some embodiments, the body of the end plug  301  is also tapered to allow a slanted interface between an end plug  301  and an opening end of the SiC structure  205 . 
     With this fill hole  303 , the sealing is performed in two steps. First, the end plug  301  is inserted into the opening end of the SiC composite structure, such as the SiC structure  205  as shown in  FIG. 2 , and the sealing material  202  is used to seal the interface between the end plug  301  and the SiC structure  205 . The presence of the fill hole  303  provides a gas conduit between the interior of the SiC structure  205  and the exterior so that a gas system can be coupled to the SiC structure  205 . The fill hole  303  can be sealed by applying a high melt-temperature material to flow into the fill hole  303  and to re-solidify in the fill hole  303 . Such configuration allows the internal pressure of the cladding to be adjusted after the end plug  301  is placed on the cladding structure and before it is entirely sealed. In some embodiments, the sealing process also includes treating the joined ceramic structure and the end plug at a temperature of at least 1350° C. to further strengthen the seal. 
       FIGS. 3B-3C  show some schematic diagrams of an end plug with a fill hole.  FIG. 3B  shows a schematic diagram of an alternative ceramic end plug  311 . The end plug  311  includes a tubular neck section  312  that leads to a wider body  313 . In some embodiments, the body  313  has two sections: an upper section  314  and a lower section  315 . Each of the sections has a tapered shape so that the two sections together form a mirrored frustum. The frustum shape allows a slanted interface between the end plug  311  and an opening end of the SiC cladding  205 . The end plug  311  also includes a first opening  316  at the top of the tubular neck section  312  and a second opening  317  at the bottom of the lower section  315  of the body  313 . The first opening and the second opening form a fill hole  318 . The first opening  316  and the second opening  317  can have substantially the same shape so that fill-hole  318  has a uniform cross-section. In some embodiments, the second opening  317  is smaller than the first opening  316  so that they form a tapered fill-hole  318 . In this example, an additional SiC pin  319  is placed in the fill hole  318  to facilitate the sealing process, which will be discussed in connection with  FIGS. 7A-7D . 
       FIG. 3C  shows a schematic diagram of an alternative ceramic end plug  321 . The end plug  321  has a tubular neck section  322  that leads to a wider body  323 . In some embodiments, the body  323  also has two sections: an upper section  324  and a lower section  325 . In this example, the upper section  342  has a tapered shape while the lower section  325  has a uniform cross section. The end plug  321  also includes a first opening  326  at the top of the tubular neck section  322  and a second opening  327  at the bottom of the lower section  325  of the body  323 . The first opening and the second opening form a fill hole  328 . The first opening  326  and the second opening  327  can have substantially the same shape so that fill-hole  328  has a uniform cross-section. In some embodiments, the second opening  327  is smaller than the first opening  316  so that they form a tapered fill-hole  328 . In this example, an additional SiC pin  329  is placed in the fill hole  328  to facilitate the sealing process, which will be discussed in connection with  FIGS. 7A-7D . It is also noted that while the end plugs are coupled to SiC tubes in the above examples, they can also be coupled to other types of SiC composite structures suitable for a variety of high temperature applications. 
       FIG. 4A  shows an exemplary fill-hole. The fill hole  401  can be created using laser drilling or electrical discharge machining (EDM) of the end plug  403 . The fill hole  401  can also be formed integrally by hot pressing the end plug  403  so that geometry of the fill hole is incorporated in the end plug without any extra machining. Sizes of the fill hole depend on properties (e.g. viscosity) of the sealing material and the operating temperature. In some embodiments, the diameter of the holes ranges from 1 to 2 mm. The fill hole  401  can have a substantially uniform cross-section, such as shown in  FIG. 3  and  FIG. 4A . Alternatively, the fill hole  401  can have a tapered cross-section that narrows as the fill hole  401  gets deeper in the end plug  403  to allow better control of the re-solidification process.  FIG. 4B  shows an exemplary cross section image of a partial hole  405  in an end plug  407  that was obtained by using X-ray computed tomography (XCT). 
       FIG. 5  shows an exemplary schematic diagram for the multi-step approach of sealing the second end of the SiC composite structure using the end plug with a fill hole in  FIG. 3 . The approach utilizes a sequence of steps to join and seal the composite structure, such as ceramic cladding  205 , and the second ceramic end plug  301  that contains a fill hole  303 . The ceramic cladding tube  205  has two opposite openings and the first opening end is sealed with the first ceramic end plug  203  as shown in  FIG. 2 .  FIG. 5  shows the steps of using the end plug with a fill hole in  FIG. 3  to seal the second opening end. In the first step  501 , the second ceramic end plug  301  is placed on the second opening of the cladding after components, such as nuclear pellets (not shown), are loaded into the cladding  205 . The second step  502  of the process is similar to the step used to form the first joint as shown in  FIG. 2 . This step  502  uses a sealing material  202 , such as the preceramic polymer as discussed above, to form a joint between the end plug  301  and the cladding  205 . In the second step  502 , procedures such as chemical vapor infiltration (CVI) or chemical vapor deposition (CVD) can be performed to strengthen the joint interface. In some embodiments, the existing gases in the cladding tube  205  are vacuumed out to create a low internal pressure for CVD to complete successfully. Then the ceramic cladding tube  205  can be filled with a desired gas composition  509  via open fill hole  304 . Then, at an elevated pressure level caused by the desired gas composition  509 , a molten high melt-temperature material  510  (e.g. oxide, silicon, transition metal, etc.) is directed to flow into the fill hole  304  and re-solidify within the fill hole, thereby sealing the fill gas  509  in the cladding  205 . In step  504 , an additional CVD step can also be performed so that the final surface is substantially the same as the parent material. 
       FIG. 6  illustrates an exemplary configuration to seal the fill hole  303 . This configuration utilizes a steep temperature profile  601  presented near the end of the end plug  301  for achieving the sealing. The steep temperature profile  601  maintains a very high temperature a few inches above the end plug  301  to allow the sealing material  510  to stay in a liquid form. The steep temperature profile  601  also keeps a much lower temperature around the fill hole  303  to allow the sealing material  510  to successfully solidify in the fill hole  303 . 
     First, the sealing material  510  is melted, using a heat source, to a liquid form at a high temperature. Then, as the sealing material  510  flows from a few inches above the end plug  301  and down the much cooler fill hole  303 , the temperature profile quickly changes from high temperature to low temperature. The sealing material  510  then re-solidifies and seal the fill hole  303 . In some embodiments, the sealing material solidifies as it flows in the fill hole  303 . In some embodiments, the sealing material may remain molten until the removal of the heat source to further decrease the temperature around the fill hole  303 . 
       FIG. 7A  shows an exemplary configuration to seal the fill hole with a pin. In this example, an end plug  701  having a mirrored frustum shape is used. Similar to step  501  in the process shown in  FIG. 5 , after components  706 , such as nuclear pellets, are loaded into the composite structure  705 , the end plug  701  and the composite structure  705  can be joined using a sealing material (not shown), such as the preceramic polymer as discussed above. Another type of sealing material  702  can be placed into the fill hole  703  before the sealing of the fill hole starts. The end plug  701  also includes a pin  704  that is made of SiC or other materials. In order to seal the fill hole  703 , localized heat can be applied to areas around the end plug  701  to heat up the sealing material  702 . Specificity regarding localized heat will be discussed detail in connection with  FIGS. 11-13 . Pressure can be applied to the pin  704  at the same time so that the pin  704  can press the heated sealing material  702  to form a seal. In some embodiments, as shown in  FIG. 7A , the pin  704  has a diameter substantially similar to the diameter of the fill hole  703  so that the sealing material  702  is pressed to the bottom of the end plug  701  to form an internal seal. In some implementations, the pin  704  can have a smaller diameter than the diameter of the fill hole  703  so that the sealing material  702  can be pressed around the pin  704  to seal the fill hole  703 . 
       FIG. 7B  shows another exemplary configuration to seal the fill hole with a pin. In this example, an end plug  711  having a tapered shape is used. Similar to step  501  in the process shown in  FIG. 5 , after components  706 , such as nuclear pellets, are loaded into the composite structure  705 , the end plug  711  and the composite structure  705  can be joined using a sealing material (not shown), such as the preceramic polymer as discussed above. Another type of sealing material  702  can be placed into the fill hole  713  before the sealing of the fill hole starts. The end plug  711  also includes a pin  714  that is made of SiC or other materials. In order to seal the fill hole  713 , localized heat can be applied to areas near the end plug  711  to heat up the sealing material  702 . Specificity regarding localized heat will be discussed detail in connection with  FIGS. 11-13 . Pressure can be applied to the pin  714  at the same time so that the pin  714  can press the heated sealing material to form a seal. In some embodiments, as shown in  FIG. 7B , the pin  714  has a smaller diameter than the diameter of the fill hole  713  so that the sealing material  702  can be pressed around the pin  714  to seal the fill hole  713 . In some implementations, the pin  714  can have a diameter substantially similar to the diameter of the fill hole  713  so that the sealing material  702  is pressed to the bottom of the end plug  711  to form an internal seal. 
       FIG. 7C  shows another exemplary configuration to seal the fill hole with a pin. In this example, an end plug  721  is used. The end plug  721  has a wide first opening  722  at the top and a small second opening  723  at the bottom. The fill hole  726  thus has two sections: a cone-shaped, wide first section  724  and a narrow second section  725 . Similar to step  501  in the process shown in  FIG. 5 , the end plug  721  and the composite structure  705  can be joined using a sealing material (not shown), such as the preceramic polymer as discussed above. Another type of sealing material  702  can be placed into the wide section  724  of the fill hole  726  before the sealing of the fill hole starts. The end plug  721  also includes a pin  727  that is made of SiC or other materials. In this example, the pin  727  has a corresponding cone shape. In order to seal the fill hole  726 , localized heat can be applied to areas around the end plug  721  to heat up the sealing material  702 . Specificity regarding localized heat will be discussed detail in connection with  FIGS. 11-13 . Pressure can be applied to the cone-shaped pin  727  at the same time so that the pin  727  can press the heated sealing material  702  to form a seal in the fill hole  726 . 
       FIG. 7D  shows yet another exemplary configuration to seal the fill hole with a pin. In this example, an end plug  731  is used. The end plug  731  has a narrow first opening  732  at the top and a wide second opening  733  at the bottom. The fill hole  736  thus has two sections: a narrow first section  734  and a cone-shaped, wide second section  735 . Similar to step  501  in the process shown in  FIG. 5 , the end plug  731  and the composite structure  705  can be joined using a sealing material (not shown), such as the preceramic polymer as discussed above. Another type of sealing material  702  can be placed into the wide section  735  of the fill hole  736  before the sealing of the fill hole starts. The end plug  731  also includes a pin  737  that is made of SiC or other materials. In this example, the pin  737  has corresponding two sections: a narrow first section  738  and a cone-shape section  739 . The ceramic composite structure  705  can be filled with a desired gas composition  740 . In order to seal the fill hole  736 , localized heat can be applied to areas around the end plug  731  to heat up the sealing material  702 . Heat can also be applied to other sections of the composite structure  705  so that the pressure level of the desired gas composition  740  increases. Then, at an elevated pressure level caused by the desired gas composition  740 , the pin  737  presses the heated sealing material  702  to form a seal in the fill hole  736 . Specificity regarding localized heat will be discussed detail in connection with  FIGS. 11-13 . 
       FIG. 8  shows an exemplary flow chart for a method  800  of joining and sealing ceramic structures. The method  800  includes: at  802 , forming a joint of a ceramic structure and an end plug by a sealing material, wherein the end plug has a hole that goes through a top surface and a bottom surface of the end plug; at  804 , filling the ceramic structure with a desired gas composition through the hole; at  806 , heating a material into a molten form using a heat source; and, at  808 , directing the material to flow into the hole, wherein the material solidifies to seal the end plug. 
       FIG. 9  shows an exemplary flow chart for a method  900  of joining and sealing ceramic structures. The method  900  includes: at  902 , forming a joint of a ceramic structure and an end plug using a sealing material, wherein the end plug has a body including a hole that goes through a top surface and a bottom surface of the end plug, and a pin positioned in the hole; at  904 , placing a material in the hole of the end plug; at  906 , applying heat to a section of the ceramic structure near the end plug to heat the material; and, at  908 , applying pressure to the body or the pin so that the pin presses the heated material in the hole to seal the end plug. 
     In some embodiments, the end plug  203 , as demonstrated in  FIG. 2 , can be implemented using a tapered design to allow easier placement of the plug  203  at one end of the SiC structure  205 . The taper angle in such a tapered design can vary between 0 to 45 degrees. In some embodiments, a 7° taper angle is used. However, the tapered design also makes the sealing process more complex because it requires a non-uniform application of the sealing material  202  to achieve an irradiation resistant and thermo-mechanically sound hermetic seal between the end plug  203  and the cladding  205 . This patent document also describes a transition metal collar that can be used as an insert between the inner surface of the SiC structure and the end plug to provide a mechanically strong and thermal expansion and radiation resistant hermetic seal. The insert can also be used to join SiC structures in various configurations, e.g. forming a large assembly of SiC tubes having T-shaped or elbow-shaped joints. 
     A transition metal insert can be used between the inner surface of the SiC structure and the end plug. The transition metal insert includes a top surface. The diameter of the top surface is substantially the same as the inner diameter of the structure so that the insert can fit securely into the structure. The top surface has a first opening. The shape and size of the first opening match the shape and size of the larger end of a tapered end plug. The insert also includes a bottom surface that has a diameter substantially the same as the diameter of the top surface. The bottom surface has a second opening that has a shape and size substantially same as the shape and size of the smaller end of the tapered end plug. The top and bottom surfaces are connected by one or more side walls to form a solid body. The first and second opening on the top and bottom surfaces form a large hollow space in the solid body that allows the end plug to be tightly coupled to the insert. 
       FIG. 10A  shows a side view of an exemplary insert  1000 . The diameter  1001  of the insert is substantially the same as the inner diameter of the corresponding SiC structure. The SiC structure can have a variety of shapes for various high temperature applications.  FIG. 10B  shows a cross-section of an exemplary insert  1000 . A hollow space  1003  is formed within the solid body  1005  of the insert  1000  to allow an end plug to be positioned there. 
       FIG. 11A  shows a side view of another exemplary insert  1100 . In this embodiment, the insert  1100  also includes an outer lip  1101 . The outer lip  1101  includes a raised part  1103  that extends from the top surface and protrudes from one or more side walls of the insert  1100 . The raised part  1103  provides support for the insert  1100  so that the insert  1100  can be placed at the top of an end of the SiC structure without sliding down the structure during the joining or sealing process. The length  1103  of the raised part is substantially the same as the thickness of the structure so that a uniform appearance of the insert and the structure can be achieved at the sealed joint.  FIG. 11B  shows a cross-section view of an exemplary insert  1100 . A hollow space  1105  is formed within the solid body  1107  of the insert  1100  to allow an end plug to be positioned there. 
     An insert as illustrated in the examples in  FIGS. 10A-11B  can be fabricated with any transition metal, such as scandium, titanium, chromium, etc. In some embodiments, molybdenum is used. After the insert is fabricated, the fabricated part is cleaned. In some embodiments, the fabricated part can be cleaned using an ultrasonic bath. The cleaned part is then polished to reduce surface blemish. In some embodiments, the cleaned part is electro-polished in an acid bath. The polished insert then can be placed between the end plug and the structure. 
       FIG. 12A  shows a side view of an exemplary insert  1000  placed in a SiC structure  1007  with an end plug  1005 . The end plug  1005  is tightly coupled to the insert  1000 , which is positioned securely within the SiC structure  1007 .  FIG. 12B  shows a perspective view of an exemplary insert  1000  placed in the structure  1007  with an end plug  1005 . The use of the insert  1000  allows a mechanically strong and thermal expansion and radiation resistant hermetic seal to be formed at the end of the structure  1007 . 
       FIG. 13A  shows a side view of another exemplary insert  1100  placed in a structure  1007  with an end plug  1005 . In this embodiment, the outer lip  1101  ensures that the insert  1100  does not slide down the structure  1007  during the sealing process.  FIG. 13B  also shows a perspective view of an exemplary insert  1100  placed in the structure  1007  with an end plug  1005 . 
     In some embodiments, the assembly of the insert, the end plug, and the SiC structure is hot pressed in an inert atmosphere to temperatures beyond 1500° C. with pressures varying between 0.5 to 5 kN. The inert atmosphere can be helium, for example. The hot-pressed assembly is then annealed for durations ranging between 2 and 4 hours to relieve interfacial residual stresses.  FIG. 14  shows exemplary macro- and microstructures of an insert  1401  and a SiC plug  1005  after an annealing process. In this embodiment, the insert  1401  does not have an outer lip. The microstructure at the interface of the SiC plug  1005  and the insert  1401  demonstrates excellent thermal stitch and anchoring that indicates plastic deformation and annealing. 
       FIG. 15  shows an exemplary flow chart for a method  1500  of manufacturing an insert for sealing a ceramic structure with an end plug. The method  1500  comprises: at  1502 , fabricating a part that comprises: a body that is shaped to fit between one opening end of the ceramic structure and the end plug to seal the opening end with the end plug, wherein the body includes a first surface having a first opening, a second surface having a second opening, and a side wall connecting the first surface and the second surface, and wherein the first opening and the second opening form a hollow space in the body to enable the end plug to be coupled to the part; at  1504 , cleaning the fabricated part; and, at  1506 , polishing the cleaned part to reduce surface blemish. 
       FIG. 16  shows an exemplary flow chart for a method  1600  of sealing or joining a ceramic structure with an end plug and an insert. The method  1600  comprises: at  1602 , placing the insert between the ceramic structure and the end plug to form an assembly; at  1604 , positioning the assembly in an inert gas composition; at  1606 , hot-pressing the assembly under a first temperature and a first pressure; and, at  1608 , annealing the hot-pressed assembly for a duration of time to relieve interfacial residual stress. 
       FIG. 17  shows an exemplary fuel rod  1700  after joining and sealing both ends of a ceramic structure using processes in accordance with one or more embodiments of the present technology. The fuel rod  1700  now includes a SiC tubular structure  1701 , a first end plug  1703  joined with the tubular structure  1701  using a sealing material  1705 , and a second end plug  1707  joined with the tubular structure  1701  using the same sealing material  1705 . The sealing material  1705  can be the preceramic polymer as discussed above. The fill-hole for the second end plug  1707  is sealed with a second sealing material  1709  (e.g., oxide, silicon, transition metal, etc.) so that all the components now form a sealed nuclear fuel rod  1700  that contains one or more nuclear pellets  1711 . 
     As shown in the embodiments illustrated in  FIGS. 7A-D , localized heat play an important role of joining and sealing ceramic structures. Applying conventional sealing or joining methods, however, is insufficient to provide localized heating to create a hermetic joint under a desired internal pressure. This patent document also describes a furnace-type apparatus that facilitates localized heating of the composite structures. The apparatus can sustain a controlled internal pressure, allowing hermetic joints to be made to contain a desired gas composition, which is not feasible using conventional joint processing equipment. 
       FIG. 18  shows an exemplary schematic diagram of a furnace that can be used for joining and sealing ceramic parts. Examples of the processes used in such a furnace include pyrolysis of preceramic polymers, chemical vapor deposition (CVD), and melt infiltration. The furnace  1800  includes a pressure vessel  1801  that are robust against the operating temperature and pressure. The pressure vessel  1801  is coupled to an inlet tubing mechanism  1811  to allow a gas composition to flow into the pressure vessel. The inlet tubing mechanism  1811  can have a variety of arrangements with regard to the number of tubes and their respective placements to accommodate different sealing requirements. In some embodiments, the inlet tubing mechanism  1811  can be arranged as several evenly-spaced tubes near the bottom of the composite structure  1809  to allow the gas composition to flow into the vessel in parallel. The furnace also includes a narrow tubular section  1803  that is coupled to a gas exhaust. The tubular section is coupled to an inner chamber  1805  that holds the composite structure to be sealed. In some embodiments, the inner chamber  1805  is a Quartz sleeve or a Quartz bell. A heating mechanism can be placed outside of the inner chamber  1805  to heat up an end of the composite structure  1809 . For example, as shown in  FIG. 18 , induction coils  1807  are positioned outside of an upper section of the inner chamber  1805  using electrically-isolating feedthroughs (not shown). The induction coils  1807  are operated by receiving an oscillating electric current at an RF frequency to heat up a section of the composite structure  1809  near an end plug  1804  via induction heating where the varying electric current in the induction coils  1807  causes a varying magnetic field that induces an eddy current in the composite structure, causing heating. The inner chamber  1805 , the narrow tubular section  1803  and the gas exhaust are used to direct the flow of reactive gasses used in processes such as CVD. They can also be used for the removal of the excess heat generated in the process. The height of the furnace is determined by the length of the composite structure to be sealed. In some embodiments, the height of the vessel is around 540 mm. In some embodiments, the height of the vessel can be adjusted (e.g., by adding modular sections of pipe to the vessel) to accommodate different lengths of composite structures. 
       FIG. 19  shows an exemplary schematic diagram of the inner chamber  1805 , the narrow tubular section  1803 , and coils  1807 . The inner chamber  1805  isolates the gas compositions from the walls of the furnace  1800 . In some embodiments, the inner chamber  1805  also directs the flow of the gas compositions across the composite structure to be coated. In this embodiment, the inner chamber  1805  is a quartz sleeve having non-uniform cross sections along the sleeve body. The diameter of the quartz sleeve near the coils  1807  is around 70 mm, while the diameter of other sections of the quartz sleeve is around 110 mm. 
       FIG. 20  shows another exemplary scheme diagram of the inner chamber  1805 , the narrow tubular section  1803 , and coils  1807 . In this particular embodiment, the inner chamber  1805  has a uniform cross section along the body. In this example, a composite structure  2001  is placed inside of the inner sleeve. The composite structure  2001  has an end plug  2003  positioned at one end of the structure  2001 . 
       FIG. 21  shows an exemplary configuration of a furnace system for implementing a CVI process. In this particular embodiment, a pump  2105  is connected to the furnace  1800  to control the pressure within the furnace  1800 . A heating mechanism is positioned within the furnace  1800  to provide localized heating to an end of the composite structure. For example, the heating mechanism can be implemented using RF coils. A current is directed through the RF coils  1807  to heat up a portion of the quartz sleeve  1805  by induction heating to allow localized sealing of the composite structure  2111 . In this exemplary setup, a susceptor block  2101  is placed over the SiC end plug  2103 . In operation, the susceptor block  2101  absorbs the electromagnetic energy emitted by the RF coils  1807  and converts the absorbed energy into heat that facilitates the heating of the SiC seal plug  2103  underneath the susceptor block  2101  and the heating of adjacent regions of the composite structure  2111  for joining the SiC seal plug  2103  and the composite structure  2111 . Upon completion of this heated joining process, the joint junction between the SiC seal plug  2103  and the composite structure  2111  so formed exhibits a good mechanical strength that is beneficial to the composite fabrication process. After the joining/sealing process completes, the top part of the susceptor block  2101  may be removed, e.g., being cut or machined away. 
     However, the CVI process can take a long time and be costly. In some embodiments, due to the pressure requirement for the CVI process, it may be difficult to seal a desired gas composition, such as He, inside of the composite structure.  FIG. 22  shows an exemplary configuration of a furnace system for implementing a liquid silicon flow process that allows the use of a desired gas composition. In this embodiment, a SiC plug  2203  is first coated with resin, which becomes a porous material (e.g., carbon) at a high temperature. Then a thin layer of Si  2205  is placed between the coated SiC plug  2203  and the susceptor block  2201 . The Si layer  2205  melts at a high temperature (e.g. 1450° C. or higher) and reacts with porous material (e.g., carbon) to form SiC. In this particular embodiment, the gas inlet and narrow tubular section  2211  are couple to the quartz sleeve  1805  from the bottom side. The RF coils  1807  heats up a portion of the quartz sleeve  1805  to allow localized sealing of the composite structure  2215 . A pump  2213  is also connected to the furnace  1800  to control the pressure within the furnace  1800 . One advantage of this type of configuration is its short processing time. Also, the Si layer undergoes a liquid to solid transition and expands during this transition, so there is almost no structural void once the SiC plug is sealed. This configuration also allows a desired gas composition, such as He or Ar, to be sealed inside of the composite structure. However, because there could be some unreacted Si in the gap, the mechanical strength of the sealed end may not be as good as the ones manufactured using the CVI configuration shown in  FIG. 21 . 
     During the sealing process, the operating temperatures and pressures of the furnace system can be monitored and controlled with the use of a variety of monitors and regulators. For example,  FIG. 23  shows an exemplary schematic diagram of the monitoring mechanism for the furnace system  2300 . Several temperature monitors are used in this embodiment. A thermocouple  2301  is used to monitor the temperature of the composite structure. A pyrometer, through a quartz sight glass  2305 , provides temperature reading of an area  2303  near the joint area. The area  2303  can be the susceptor block, the area of the composite structure adjacent to the susceptor block, the end plug, or the area of the composite structure adjacent to the end plug. Temperature monitoring can be also conducted at the quartz sight glass  2305 , the electrical feedthrough for the coils  2307 , and/or the top flange of the furnace  2309  with contact thermocouples. The system also includes a gas storage  2315  that supplies a gas composition to the furnace. To monitor the internal pressure of the furnace, a pressure monitor  2311  can be used. Another pressure monitor  2313  can be used to monitor post-filter gas pressure. Additional monitoring can also be implemented. For example, in some embodiments, various aspects of the RF coils are monitored, including the electric current, frequency, and an adequate coolant flow to prevent overheating. 
       FIG. 24  shows another exemplary schematic diagram of the exhaust monitoring mechanism for a furnace system. The exhaust monitoring mechanism includes a back pressure regulator (BPR)  2401  that can control the pre-filter gas pressure, and two pressure transducers  2403  and  2405 . Monitoring both pre-filter gas pressure and post-filter gas pressure using the pressure transducers  2403  and  2405 , allows the system to determine, based on pressure drop across the filter  2407 , whether there is some level of obstruction at the filter  2407 . 
       FIG. 25  shows an exemplary flow chart for a method of sealing ceramic structures used in nuclear reactors. The method includes: at  2502 , positioning the ceramic structure into a chamber of a sealing device; at  2504 , disposing a plug at an end of the ceramic structure, wherein a sealing material is positioned between the ceramic structure and the plug; at  2506 , placing a susceptor block over the plug; and, at  2508 , driving a plurality of induction coils arranged outside of the chamber with a varying electric current to heat the end of the ceramic structure and the susceptor block to a high temperature to join the plug and parts of the ceramic structure in contact with the plug at the end of the ceramic structure, thus sealing the end of the ceramic structure. 
     While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments. 
     Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.