Molten metal filled silicon carbide fuel cladding tube and uniform distribution fabrication method

Abstract

A fuel rod design and technology for encapsulating nuclear fuel pellets (103) in a nuclear fuel rod (105) is provided. The disclosed fuel rod includes a tubular cladding (110) formed of silicon carbide and a metal that melts during the nuclear reaction of the nuclear fuel pellets, and a metallic filler structure (120) positioned inside the tubular cladding to include a metal tube that fills the gap between the nuclear fuel pellets and the inner wall of the tubular cladding, and structured to include a sealed metallic end cap (120A) at one end of the nuclear fuel pellets to leave a space inside the tubular cladding as a reservoir (150) between one end and the sealed metallic end cap of the metallic filler structure.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This patent document claims the benefit of and priority to U.S. Patent Application No. 17 / 079,328, filed in the U.S. Patent and Trademark Office on October 23, 2020. The entire contents of the above-mentioned patent application are incorporated by reference as part of the disclosure of this application.

[0002] This patent document relates to tubes for holding nuclear fuel material, such as fuel pellets. [Background technology]

[0003] Many nuclear reactors use fissile material as fuel to generate electricity through nuclear fission chain reactions. The fuel is usually held within a robust physical container, for example, within fuel rods that can withstand the high operating temperatures and extreme neutron irradiation environment. Summary of the Invention [Problem to be solved by the invention]

[0004] Fuel structures must maintain their shape and integrity within the core for a period of time (e.g., several years) to prevent fission products from leaking into the reactor coolant. Other structures, such as heat exchangers, nozzles, nosecones, flowpath inserts, or related components, also require high temperature performance, corrosion resistance, and specific, non-planar shapes, where a high degree of dimensional accuracy is important. [Means for solving the problem]

[0005] This patent document discloses devices, systems, and methods for providing improved thermal conductivity and encapsulating nuclear fuel materials, such as fuel pellets.

[0006] In one aspect, an apparatus configured to enclose a stack of nuclear fuel pellets is disclosed, the apparatus including: a tubular cladding structured to have a hollow interior having a length, an inner cross-sectional shape, and an outer cross-sectional shape for retaining the nuclear fuel pellets inside the tubular cladding, the tubular cladding comprising silicon carbide; and a metallic packing structure formed of a metal that melts during a nuclear reaction of the nuclear fuel pellets, the metallic packing structure being positioned inside the tubular cladding to include metal tubes that fill gaps between the nuclear fuel pellets and an inner wall of the tubular cladding, and the metallic packing structure being structured to include a sealed metallic end cap at one end of the nuclear fuel pellets to leave a space between one end of the interior of the tubular cladding and the sealed metallic end cap of the metallic packing structure as a reservoir located between an end of the tubular cladding material and the sealed metallic end cap of the metallic packing structure for accumulating fission gases from the nuclear fuel pellets during a nuclear reaction of the nuclear fuel pellets.

[0007] The following features may be included in various combinations: the cladding is monolithic silicon carbide; the cladding is a CMC; the reservoir includes a spring or spacer; the inner cross-sectional shape and the outer cross-sectional shape are annular; the nuclear fuel pellets include U3Si2, UN, or UO2; the gap between the nuclear fuel pellets and the inner wall of the tubular cladding has a thickness between about 50 μm and about 150 μm; and the metal may be tin (Sn). The tubular cladding and metal filler are configured to stop ingress of coolant into the tubular cladding from a microcrack leak through the tubular cladding by chemically reacting the metal filler structure with the coolant at the location of the leak to form a metal oxide that fills the microcrack with the metal oxide.

[0008] In another aspect, the disclosed technology may be implemented to provide a method for encapsulating nuclear fuel pellets inside a nuclear reactor. The method includes the steps of: disposing nuclear fuel pellets inside a hollow interior space in a tubular cladding structured to include SiC to hold the nuclear fuel pellets inside the tubular cladding having a continuous gap between the nuclear fuel pellets and an interior sidewall of the tubular cladding and one interior end of the tubular cladding; and forming a metal packing structure inside the tubular cladding, the metal packing structure including a metal tube formed of a metal that melts during a nuclear reaction of the nuclear fuel pellets and filling the gap between the nuclear fuel pellets and the interior wall of the tubular cladding to provide a seal inside the tubular cladding during the nuclear reaction, and including a sealed metal end cap at one end of the nuclear fuel pellets to leave a space inside the tubular cladding between the one end and the sealed metal end cap of the metal packing structure as a reservoir for accumulating fission gas from the nuclear fuel pellets during the nuclear reaction of the nuclear pellets.

[0009] These and other aspects and their implementations are described in more detail in the figures, description, and claims. [Brief explanation of the drawings]

[0010] [Figure 1A] 1 illustrates an exemplary nuclear fuel assembly in accordance with some example embodiments. [Figure 1B] 1 depicts an example of a tin-backfilled silicon carbide (SiC) tube having one or more nuclear fuel pellets, according to some example embodiments. [Figure 2] 1 illustrates another embodiment of a tin-refilled SiC tube having a fuel pellet stack containing one or more types of fuel pellets. [Figure 3]1 illustrates X-ray computed tomography (XCT) images of tin-containing and tin-free SiC coatings, according to some example embodiments. [Figure 4] Some physical properties of Sn are shown. [Figure 5] 29 elements and their associated fission yields over various time periods are shown. [Figure 6] 1 depicts a setup for testing the quality of a tin-refilled coating, according to some example embodiments. [Figure 7] 1 shows an example of 2-D X-ray scans of a SiC tube with a molybdenum (Mo) pellet and a tin bond. DETAILED DESCRIPTION OF THE INVENTION

[0011] The disclosed devices and techniques significantly improve heat transfer between the nuclear fuel pellets and the cladding wall by filling the silicon carbide nuclear fuel cladding with a molten metal, e.g., molten tin. The use of other non-metals (carbon or silicon) may also be possible. However, the use of molten tin is specific to silicon carbide cladding because molten tin undesirably corrodes common metallic cladding, e.g., Zircaloy. The disclosed devices have uses in areas including reactor cladding, heat storage and heat extraction components, heat recovery system components, and nuclear waste processing and storage.

[0012] The nuclear fuel material used in nuclear reactors is typically held in fuel rods capable of withstanding high operating temperatures and extreme neutron irradiation environments. The fuel structures must maintain their shape and integrity within the reactor core over long periods of time, thereby preventing fission products from leaking into the reactor's coolant. FIG. 1A shows an example of a nuclear fuel rod assembly 100 formed from a bundle of fuel rods 101 used in a nuclear reactor. Each rod contains nuclear fuel pellets 103, e.g., uranium-containing pellets, within its hollow interior, and spacer grids are used to hold the rods in the assembly. Reactors are built to hold many nuclear fuel rod assemblies during operation. While some fuel rods use zirconium cladding, the fuel rods in this document use SiC ceramic matrix composites (CMCs) for improved performance.

[0013] Silicon carbide (SiC) can be used for both fission and melting applications and has recently been considered as a candidate material for accident-resistant fuel cladding for light water nuclear reactors. High-purity, crystalline SiC is a stable material under neutron irradiation, experiencing minimal swelling and strength changes above 40 dpa, representing many times the exposure over a typical light water reactor (LWR) fuel life. In addition, SiC retains its mechanical properties at high temperatures and reacts slowly with water vapor compared to Zircaloy, thereby providing improved safety for water-cooled reactors in the event of a loss of coolant capacity (LOCA) and other potential accident conditions. However, various monolithic SiC materials tend to exhibit low fracture toughness alone, making such materials unsuitable for nuclear cladding applications where a fuel containment vessel is essential and a coolable shape must be maintained, especially under transient or abnormal conditions. To address this fragile behavior of such monolithic SiC materials, engineered composite structures can be used, using strong silicon carbide fibers to reinforce the SiC matrix and form a SiC-SiC composite. Compared to monolithic SiC, these composites offer improved fracture toughness, pseudoductility, and follow a more favorable failure process. High-purity, radiation-resistant silicon carbide composites are typically fabricated using chemical vapor infiltration (CVI). While CVI provides the purity required for nuclear applications, it is difficult to achieve very low porosity levels (<5%). As a result, the composite alone may not be sufficient to contain one or more fission gases within the fuel cladding. Ultimately, a SiC-based cladding structure optimized to combine a robust SiC-SiC composite with a monolithic SiC layer is the most promising design for achieving a fully SiC-based accident-resistant fuel cladding design, as the dense, monolithic SiC acts as an impermeable fission gas barrier and provides improved corrosion resistance. Furthermore, additional protection can be achieved using the disclosed technique of using tin as a molten gap filler between the cladding and the fuel pellet.

[0014] In various nuclear reactor applications, in addition to providing the desired strength or toughness at the high temperatures generated by nuclear reactions, it is desirable for SiC-based fuel cladding to meet a range of material property and performance requirements, exhibit stability under irradiation, and exhibit reduced oxidation compared to other reactor cladding materials, such as Zircaloy. These requirements are primarily driven by the differences in properties between silicon carbide structures and Zircaloy tubes and the implications of these differences for performance. Specifically, the properties of SiC-based cladding are highly dependent on the processing route used, particularly for any fiber-reinforced composite layers. Additionally, while SiC-SiC composites undergo quasi-ductile fracture rather than brittle fracture, large-scale microcracks develop during this process, which can result in loss of hermeticity. These microcracks develop at strain levels in the 0.1% range, where Zircaloy cladding still does not exhibit any plastic deformation. Therefore, attention to characterization and careful development of SiC-based cladding designs is needed to mitigate microcracking and ensure hermeticity. Another consideration is that silicon carbide has a lower irradiated thermal conductivity than Zircaloy, but like Zircaloy, does not undergo irradiation-induced creep at LWR operating temperatures, which has the advantage of retarding pellet cladding mechanical interactions and associated stresses.

[0015] Therefore, achieving controllable cladding circularity, roughness, and straightness is crucial for predictable heat transfer through the cladding. The lower thermal conductivity of SiC-based cladding leads to higher temperature gradients through the cladding for a given linear heat generation rate. These temperature gradients can result in significant stresses due to thermal expansion and irradiation-induced, temperature-dependent swelling. These stresses (and the corresponding likelihood of failure) can be reduced by reducing the cladding wall thickness, thus lowering the temperature gradient. In addition, the cladding structure (a combination of composite and monolithic SiC layers) can significantly affect the stress distribution as well as accident scenarios, regardless of cladding thickness during normal operating conditions. With careful design, stresses on critical layers within the cladding structure can be reduced. However, fabrication and handling challenges exist associated with both reducing wall thickness for long fuel cladding tubes and producing specially designed tube structures.

[0016] The introduction of SiC-based accident-tolerant cladding in light water reactors not only requires optimized structural design and the development of consistent, scalable fabrication methods, but also a thorough understanding and characterization of the resulting materials. Among other performance metrics, mechanical and thermal properties must be measured, and permeability must be evaluated. While a limited collection of test standards is generally accepted by the community (ASTM C28.07 Ceramic Matrix Composites Subgroup), the development of additional characterization tools is required.

[0017] PCT Application No. PCT / US2018 / 055704, filed October 12, 2018, entitled "JOINING AND SEALING PRESSURIZED CERAMIC STRUCTURES," and PCT / US2017 / 045990, filed August 8, 2017, entitled "ENGINEERED SIC-SIC COMPOSITE AND MONOLITHIC SIC LAYERED STRUCTURES," contain technical information related to the technology disclosed in this patent document and are incorporated by reference in their entireties as part of the disclosure of this patent document.

[0018] Currently, LWR cladding contains high-pressure helium to provide heat transfer between the nuclear fuel and the cladding. The thermal conductivity of high-pressure helium around the fuel pellets is much lower than that of liquid metals, such as tin (Sn). In some implementations, the disclosed tin-filled SiC cladding can be structured to provide an approximately 200-fold improvement in thermal conductivity between the fuel and the cladding. The higher efficiency of the disclosed technology reduces fuel temperatures by approximately 500 degrees Celsius (C or °C), providing a greater margin for accident prevention. Higher efficiency also increases fuel utilization and reduces waste. Tin-filled SiC cladding has the advantage of mitigating microcracks in the SiC cladding, which, by forming tin-oxides, limits coolant ingress into the cladding and interaction between the fuel and the leaked coolant. If sufficient molten tin is available after a leak, the tin can cause the SiC cladding to self-heal by refilling the leak location.

[0019] The cladding temperature in a light water reactor (LWR) during normal operation is approximately 343 degrees Celsius (C). Tin has a melting point of 232 C and is therefore in its liquid phase at LWR operating temperatures, so tin or tin eutectic is a suitable molten metal at these operating temperatures.

[0020] The tin-filled SiC cladding is also simple to fabricate and reduces costs by eliminating pressure seals, spring components, and smoothing the cladding's inner surface after fabrication. A smooth inner surface is desirable for safe fuel pellet packing. The tin-refilled cladding containing the fuel pellets protects the fuel pellets, making transport before use (pre-irradiation) safer. Post-irradiation benefits include more rapid cooling of the fuel rods than He-filled fuel rods due to the increased thermal conductivity of the tin-refilled rods. For example, experimental results of the disclosed device show an improvement in the thermal conductivity of tin-filled fuel rods to approximately 60 watts per meter Kelvin (W / m K) from approximately 0.2 W / m K for helium.

[0021] In addition to tin, various other metals with low melting points can be used to implement the disclosed technology. For example, metals such as lead (Pb) or bismuth (Bi) and other metals located near Sn on the periodic table can be used. In various fuel rod designs for reactor applications, tin has another property in the event of a rod leak: it has the added advantage that when tin reacts with water, it can form stable tin oxide, SnO2, which is insoluble in water and can be used to stop the leak. Recharge of liquid metal (e.g., Sn) promotes the water impermeability of SiC ceramic matrix composite (CMC) tubes by providing an internal seal against water ingress. If a small hole develops in the cladding that begins to leak coolant or water through the SiC cladding, Sn reacts with the coolant / water to form tin oxide at the leak location. Tin oxide has a melting point above 1600°C, which is higher than the temperature of tin or the cladding. Tin oxide effectively self-heals or fills leaks, thereby protecting the uranium silicide pellets from contact with the coolant. Use of the disclosed tin recharge eliminates the need for a high-pressure He recharge, thereby simplifying the sealing process. The Sn recharge also stabilizes the pellets during transport and storage.

[0022] Disclosed advantages of Sn refilling include: Sn is a better thermal conductor than He; fuel rods containing fuel pellets and Sn refilling have no initial internal pressure (unlike current high-pressure He refilling); sealing the ends of the cladding is easier than if He were used; Sn refilling reduces the probability of gas leakage during operation, which will be repaired by the rapid oxidation of Sn; Sn-refilled cladding has a simpler internal structure than conventional high-pressure He-refilled tubes because it does not require high-pressure gas seals; Sn-refilled tubes do not require any springs; fuel pellet loading is improved; molten Sn acts as a lubricant in operational systems; Sn is easy to transport because it is solid at transport temperatures; and pellets are protected.

[0023] 1B illustrates an example tin-refilled SiC tube 105 with fuel pellets 130, according to some example embodiments. The fuel tube 105 includes a tubular cladding 110 made from silicon carbide (SiC) ceramic matrix composite (CMC), monolithic SiC, other materials including SiC, or other high-temperature ceramics or materials. The interior space inside the tubular cladding 110 is filled with nuclear fuel pellets 130, whose volume or size is smaller than the interior size of the tubular cladding 110, thereby forming a gap between the interior wall of the tubular cladding 110. This can gap may be about 50 μm to about 150 μm in some fuel tube designs. The gap between the fuel pellets 130 and the inside of the tubular cladding 110 is filled with a suitable metallic filler structure 120, e.g., tin (Sn), to provide a sealing interface with the inner wall of the tubular cladding 110, fill any cracks in the tubular cladding 110, and bond the fuel pellets 130 from the SiC tubular cladding. The metallic filler structure 120, as illustrated, forms a tubular structure with a tubular end 120A proximate the top of the nuclear fuel pellets 130 and is spaced a distance from the top inner end of the tubular cladding 110 to enclose the interior space as a reservoir 150, allowing fission gas to accumulate during operation. The reservoir 150 includes a volume open to gas and may include a spring and / or spacer, e.g., a SiC spacer. Fission gas from the fuel pellets diffuses through the molten tin and accumulates in the reservoir 150 until the pressure inside the cladding equilibrates. As fission gases accumulate in the liquid tin and form gas bubbles, the gas bubbles float and drift to the reservoir.

[0024] Figure 2 shows another embodiment 200 of a tin-refilled SiC tube containing fuel pellets. The outer layer is a SiC cladding, the two ends of the tube are sealed with two sealing modules, and an internal reservoir is formed inside one of the left sides. Inside the cladding is a stack of fuel pellets, and tin (Sn) bonds the fuel pellet stack to the SiC cladding at temperatures below the Sn melting point of 232°C. The encapsulated fuel pellet stack is mechanically stable and supported by the SiC cladding and Sn bonding.

[0025] FIG. 3 shows X-ray computed tomography (XCT) images of SiC CMC cladding with and without Sn, according to some example embodiments. Image 310 shows an XCT image of SiC cladding 325 and molybdenum (Mo) fuel pellets 335, where the cladding is Sn-free and refilled with He 330. Image 320 shows an XCT image of SiC CMC cladding 325 and molybdenum (Mo) fuel pellets 335, where the cladding is He-free and refilled with Sn 340. Exemplary locations where Sn fills the voids in the SiC CMC cladding are shown at 342. By filling the voids in the SiC CMC with Sn, thermal conductivity is enhanced, and if there is a microcrack leak through the SiC cladding, water ingress is stopped by reaction of Sn with the coolant at the leak location to form Sn oxides that fill the microcracks. Note that while 342 identifies only two locations where Sn fills the voids, there are many other locations along the length of the cladding in the image.

[0026] Figure 4 shows some properties of Sn, whose melting and boiling points are compatible with LWR.

[0027] Figure 5 shows 29 elements and their associated fission yields over various time periods, including 1 year, 10 years, 100 years, and 1000 years. Elements with low fission yields are elements that are stable for use in LWRs. Sn has a very low fission yield, making it a good candidate for refilling nuclear fuel pellet tubes.

[0028] In one installation, molybdenum (Mo) pellets were used and the molten metal completely filled the gap between the fuel pellets and the inner surface of the monolithic SiC cladding.

[0029] Based on the enthalpy (H), entropy (S), and heat capacity (C) of the HSC simulations, no liquid tin-induced corrosion / reaction or corrosion occurs in SiC cladding. HSC simulations confirm that no liquid tin-induced corrosion / reaction occurs with uranium dioxide (UO2) up to at least 1500°C. HSC simulations confirm that no liquid tin-induced corrosion / reaction occurs with U3Si2 up to at least 1500°C, confirming that tin is compatible with U3Si2 fuel.

[0030] The majority of fission products do not chemically react with Sn. Iodine (I) reacts with Sn according to the reaction: Sn + I2(g) = SnI2, but due to the presence of large amounts of cesium (Cs) in the fission gases, CsI is formed rather than SnI2. Therefore, I is compatible with the disclosed technology.

[0031] As described above, the reservoir, which includes the open space above the fuel stack, accumulates fission gases, which diffuse through the Sn due to the pressure gradient until equilibrium is reached. The fission gases do not significantly affect heat transfer.

[0032] Xenon-135( 135 There are several pathways for the production of Xe. In the first pathway, 135 Xe is a stable material with a high cross section of 2.65E6 Barn. 136 The second pathway involves neutron capture by becoming Xe, with a half-life of 9.17 hours. 135 Beta decay to Cs occurs. If the fuel tube is filled with He, the first pathway is preferred. If the tube is filled with liquid tin, 135 The Xe bubbles up to the top of the reservoir location. 135The chances of Xe capturing neutrons are reduced, and the second pathway is preferred. Neutron control is different. Using tin, 135 It is possible to avoid the low neutron density problem caused by Xe.

[0033] 6 illustrates a setup for testing the quality of tin-refilled coatings according to some example embodiments. A chamber 610 is surrounded by a heating element 650. Valves 616 and 621 control the vacuum 620 or compressed argon 615 connecting to the chamber. Inside the chamber 610 is a SiC tube 625, and inside the tube 625 are Mo pellets 635 and Sn 630. Graphite 640 is at the bottom of the SiC tube 625. A thermocouple 645 measures the temperature inside the SiC tube 625.

[0034] The following steps are performed to produce a SiC cladding tube with Mo fuel pellets having Sn bonds. In the first step, a vacuum is drawn in chamber 610 by opening valve 621 and closing valve 616. Next, heating element 650 heats the chamber and contents to a temperature greater than 350°C, melting the Sn. Next, the chamber is pressurized, forcing the liquid tin into the gap between the Mo fuel pellets and the inner wall of the SiC tube.

[0035] The inspections include adjusting the vacuum level, inspecting for Sn oxidation, which involves adding H to Ar as an O getter, and inspecting Sn quality. The inspections also include inspecting the uniformity of Sn refill.

[0036] Figure 7 shows an example of a 2-D X-ray scan of a SiC tube with Mo pellets and Sn bonding. In the example of Figure 7, the tube is monolithic SiC with an inner diameter of 8.20 mm. There are five Mo pellets, each 7.76 mm in diameter. Using the setup of Figure 6, the vacuum drawn was 60 mTorr, 80 psi N2 was used, the thermocouple temperature was 500°C (minimum), the pre-pressurization period was 30 minutes, and the pressurization period was until the thermocouple measured room temperature. The 2-D X-ray scan shows that the gap was filled with Sn, with a gap uniformity of 25 micrometers.

[0037] In some exemplary embodiments, a SiC tube with fuel pellets and metallurgical bonds can be fabricated using the following fabrication steps: 1) packing fuel pellets into one end of a sealed cladding tube with tin particles or strips between the pellets and the cladding inner diameter; 2) adding additional tin on top of the pellets in the fission gas reservoir region so that the total amount of tin equals the total gap volume; 3) placing the tube in a vacuum / pressurization chamber and pumping the cladding tube to a vacuum level of around 10 mTorr; 4) heating the tube to a temperature above the tin melting point (230°C) so that the tin both in the gap and on top is molten; 5) turning off the vacuum pump and applying argon pressure from the top to force the liquid tin downward and fill the gap; and 6) cooling to solidify the tin. In some exemplary embodiments, the pellets are Mo pellets and the metal is tin.

[0038] While this patent document contains many details, these should not be construed as limiting the scope of any invention or what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of a particular invention. Certain features described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Furthermore, while features may be described above as operative in certain combinations and even initially claimed as such, one or more features of a claimed combination can optionally be deleted from the combination, and the claimed combination can also be directed to subcombinations or variations of subcombinations.

[0039] Similarly, although operations are shown in the figures in a particular order, this should not be understood as requiring such operations to be performed in the particular order or sequential order shown, or that all illustrated operations must be performed to achieve a desirable result. Furthermore, the separation of various components of the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.

[0040] Although only a few implementations and examples are described, enhancements and variations of other implementations can be made based on what is described and illustrated in this patent document. [Explanation of symbols]

[0041] 100: Nuclear fuel rod assembly, 101: Fuel rod, 103: Nuclear fuel pellet, 105: Fuel tube (SiC tube), 110: Tubular cladding, 120: Filler structure, 130: Fuel pellet, 150: Reservoir

Claims

1. An apparatus configured to enclose a nuclear fuel pellet, comprising: a tubular cladding structured to have a hollow interior with a length, an inner cross-sectional shape, and an outer cross-sectional shape for holding the nuclear fuel pellet inside the tubular cladding, the tubular cladding containing silicon carbide; a metal filler structure formed of a metal that melts during the nuclear reaction of the nuclear fuel pellet and further melts into a liquid state, and has no wettability to the surface of silicon carbide when in the liquid state, the metal filler structure being located inside the tubular cladding to include a metal tube that fills a gap between the nuclear fuel pellet and the inner wall of the tubular cladding, and being located between the end of the tubular cladding and the sealed metal end cap of the metal filler structure, and including a sealed metal end cap at one end of the nuclear fuel pellet so as to leave a space between one end of the interior of the tubular cladding and the sealed metal end cap of the metal filler structure to accumulate fission gas from the nuclear fuel pellet during the nuclear reaction of the nuclear fuel pellet as a reservoir;

2. The apparatus according to claim 1, wherein the tubular cladding and the metal filler are configured to stop the entry of the coolant into the tubular cladding from microcrack leakage through the tubular cladding by forming metal oxides that fill the microcracks with metal oxides by a chemical reaction between the metal filler structure and the coolant at the leakage location.

3. The apparatus according to claim 1, wherein the tubular cladding contains monolithic silicon carbide.

4. The apparatus according to claim 1, wherein the tubular cladding contains one or more silicon carbide ceramic matrix composites.

5. The apparatus according to claim 1, wherein the metal of the metal filler structure that has no wettability to the surface of silicon carbide when in the liquid state has a material composition showing a contact angle with silicon carbide of 165°.

6. The tubular cladding and metal filler structure is U 3 Si 2 , UN, or UO 2 The apparatus according to claim 1, which is configured to be suitable for containing a nuclear fuel pellet containing

7. The apparatus according to claim 1, wherein the metal filler structure is structured such that the gap filled with the metal filler structure has a thickness between about 50 μm and about 150 μm.

8. The apparatus according to claim 1, wherein the metal for forming the metal filler structure contains tin (Sn).

9. The apparatus according to claim 1, wherein the metal for forming the metal filler structure contains a tin eutectic.

10. The apparatus according to claim 1, wherein the metal for forming the metal filler structure contains lead (Pb).

11. The apparatus according to claim 1, wherein the metal for forming the metal filler structure contains bismuth (Bi).

12. The apparatus according to claim 1, wherein the metal for forming the metal filler structure contains a metal located near Sn in the periodic table.

13. A method for encapsulating a nuclear fuel pellet, comprising: placing the nuclear fuel pellet inside a hollow internal space within a tubular cladding structured to contain SiC to hold the nuclear fuel pellet inside the tubular cladding having a gap between the nuclear fuel pellet and the inner wall of the tubular cladding and one inner end of the tubular cladding; forming a metal filler structure that melts during the nuclear reaction of the nuclear fuel pellet inside the tubular cladding and is a metal liquid that has no wettability to the surface of silicon carbide when further melted into a liquid state, and has a density lower than the density of the nuclear fuel pellet in the liquid state, and is structured to include a metal tube that fills the gap between the nuclear fuel pellet and the inner wall of the tubular cladding to provide a seal inside the tubular cladding during the nuclear reaction, and is structured to include a sealed metal end cap at one end of the nuclear fuel pellet so as to leave a space between one end of the inner wall of the tubular cladding and the sealed metal end cap of the metal filler structure as a reservoir for accumulating fission gases from the nuclear fuel pellet during the nuclear reaction of the nuclear fuel pellet.

14. The method according to claim 13, wherein when microcrack leakage occurs through the silicon carbide coating, water ingress is stopped by forming a metal oxide that fills the microcrack by a chemical reaction between the metal filler structure and the coolant at the leakage location.

15. The method according to claim 13, wherein the tubular cladding contains monolithic silicon carbide.

16. The method according to claim 13, wherein the tubular cladding contains one or more silicon carbide ceramic matrix composites.

17. The method according to claim 13, wherein the metal filler structure contains tin (Sn).

18. The method according to claim 13, wherein the metal filler structure contains a metal located near tin (Sn) in the periodic table.

19. The nuclear fuel pellet is U 3 Si 2 , UN, or UO 2 The method according to claim 13, comprising

20. The metal of the metal filler structure that has no wettability to the surface of silicon carbide when melted into a liquid state has a material composition showing a contact angle with silicon carbide of 165°, the method according to claim 13.

Images