SiC secondary neutron source cladding tube and secondary neutron source rod

Abstract

A SiC secondary neutron source cladding tube is configured to be of a multi-layer tubular structure and comprises a tubular SiC fiber braid layer, a cracking carbon interface layer, a SiC coating and an Al2O3 tritium-resistant coating which are sequentially arranged from inside to outside. The SiC secondary neutron source cladding tube has good tritium resistance, high-temperature mechanical property and wear resistance, can effectively reduce tritium emission of a nuclear power plant caused by tritium permeation and diffusion in a reactor secondary neutron source, and improves the safety of the nuclear power plant. The utility model also provides a secondary neutron source rod.

Description

Technical Field

[0001] The utility model belongs to the field of nuclear power, and in particular relates to a SiC secondary neutron source cladding tube and a secondary neutron source rod. Background Art

[0002] In the field of nuclear power generation, tritium is a key source of radioactive emissions into the surrounding environment during nuclear power plant operation. With increasing attention to nuclear power plant operational safety, effectively reducing tritium emissions has become a core issue in controlling radioactive emissions from nuclear power plants.

[0003] During nuclear power plant operation, tritium primarily originates from various components within the reactor core. The Sb-Be pellets within the secondary neutron source rods are of particular concern. Under the complex physical effects of neutron irradiation, these pellets generate significant amounts of tritium. Currently, the cladding of secondary neutron sources is typically made of stainless steel. However, stainless steel has significant limitations in its tritium barrier properties. These limitations allow tritium to penetrate more easily, leading to significantly elevated tritium levels in the primary circuit of a nuclear power plant.

[0004] To address the problem of tritium penetration, some existing technical solutions have attempted to apply a tritium-barrier coating to the cladding surface. Theoretically, this is indeed a feasible method. However, most coating materials with tritium-barrier properties are ceramic materials. When ceramic coatings are applied to the metal cladding tube substrate, a series of problems arise. For example, there is a thermal stress mismatch between the ceramic material and the metal cladding tube substrate, which is caused by the difference in thermal expansion coefficients between the two. Moreover, the ceramic coating is susceptible to corrosion in the complex operating environment of nuclear power plants and is prone to peeling after being affected by various factors for a long time. These problems seriously affect the actual effect of the tritium-barrier coating.

[0005] In summary, developing and providing a secondary neutron source cladding with better tritium resistance is of great significance to nuclear power plants. This will have a positive and far-reaching impact on reducing tritium emissions from nuclear power plants, and help further ensure the safety and environmental friendliness of nuclear power plant operations. Utility Model Content

[0006] The purpose of the present invention is to provide a SiC secondary neutron source cladding tube to reduce tritium emission from the secondary neutron source. The present invention also provides a secondary neutron source rod.

[0007] According to an embodiment of one aspect of the present invention, a SiC secondary neutron source cladding tube is provided, wherein the SiC secondary neutron source cladding tube is configured as a multi-layer tubular structure, including a SiC fiber braided layer, a cracked carbon interface layer, a SiC coating and an Al2O3 tritium-barrier coating arranged in sequence from the inside to the outside along the radial direction, wherein the SiC fiber braided layer includes a three-dimensional structure formed by braiding and winding SiC fibers, and the SiC coating is configured as a dense SiC layer without a fiber structure.

[0008] SiC fibers themselves have excellent tritium resistance and a small neutron absorption cross-section. Compared to materials like zirconium alloys, they also possess superior high-temperature mechanical strength, wear resistance, and high-temperature stability. Furthermore, unlike components like control rods, secondary neutron source rods do not require frequent up-and-down motion during reactor operation. The strength of the composite tubes woven from SiC fibers meets the design requirements for secondary neutron source rods. Al2O3 exhibits excellent corrosion resistance and airtightness, and exhibits superior tritium resistance compared to SiC at the rated operating temperature of pressurized water reactors. Furthermore, Al2O3 and SiC have similar thermal expansion coefficients, effectively preventing cracking and spalling caused by thermal mismatch. The Al2O3 coating can also effectively address the airtightness issues of SiC cladding tubes and improve the welding performance between the cladding tubes and the secondary neutron source rod metal end plugs.

[0009] Furthermore, in some embodiments, the SiC fiber braided layer is configured as a tube with a diameter of 7.4 mm-9.6 mm, and the braiding angle of the SiC fiber is 30°-50°.

[0010] Furthermore, in some embodiments, the thickness of the SiC coating is 50 μm-200 μm.

[0011] Furthermore, in some embodiments, the thickness of the Al2O3 tritium barrier coating is 5 μm-20 μm.

[0012] According to another embodiment of the present invention, a method for manufacturing a SiC secondary neutron source cladding tube is provided, which is used to manufacture the SiC secondary neutron source cladding tube provided in any of the aforementioned embodiments, and includes the following steps:

[0013] Step a): providing SiC fibers, and braiding the SiC fibers around a core rod to form a tubular SiC fiber braided layer;

[0014] Step b): placing the tubular SiC fiber braided layer in a vacuum environment, and depositing a cracked carbon interface layer on the surface of the tubular SiC fiber braided layer using a chemical vapor deposition process;

[0015] Step c): densifying the tubular SiC fiber braided layer using a chemical vapor infiltration process;

[0016] Step d): depositing a SiC coating on the surface of the cracked carbon interface layer using a chemical vapor deposition process;

[0017] Step e): depositing an Al2O3 tritium barrier coating on the surface of the SiC coating using a magnetron sputtering process to obtain a cladding tube preform;

[0018] Step f): sintering the cladding tube preform to obtain a finished SiC secondary neutron source cladding tube.

[0019] Furthermore, in some embodiments, step b) is carried out in a chemical vapor deposition furnace, the vacuum environment pressure is 0.1Pa-0.4Pa, the cracking gas raw material is introduced at a gas flow rate of 10sccm-30sccm, and is heated to the cracking temperature of the cracking gas raw material to perform cracking carbon deposition, wherein the cracking gas raw material includes a gas containing an alkyl, alkenyl or alkynyl group, and the holding time is 1h-2h.

[0020] Furthermore, in some embodiments, the step c) is carried out in a chemical vapor infiltration furnace, and the densification treatment step is: providing a vacuum environment of 0.1Pa-0.4Pa in the chemical vapor infiltration furnace, using hydrogen as a carrier gas and argon as a dilution / protective gas, introducing a densification gas raw material, and heating to 1000℃-1400℃; wherein, the densification gas raw material includes a mixed gas of silicon tetrachloride and methane, trichloromethylsilane or tetramethylsilane, the gas flow rate of the densification gas raw material is 1000sccm-5000sccm, the gas flow rate of hydrogen is 2000sccm-6000sccm, the gas flow rate of argon is 2000sccm-6000sccm, and the holding time is 5h-10h.

[0021] Furthermore, in some embodiments, step d) is carried out in a chemical vapor deposition furnace, a vacuum environment of 0.1 Pa-0.4 Pa is provided in the chemical vapor deposition furnace, hydrogen is used as a carrier gas, argon is used as a dilution / protective gas, a coating gas raw material is introduced, and the temperature is heated to 1000°C-1400°C; wherein the coating gas raw material includes a mixed gas of silicon tetrachloride and methane, trichloromethylsilane or tetramethylsilane, the gas flow rate of the coating gas raw material is 1000sccm-2000sccm, the gas flow rate of hydrogen is 1000sccm-2000sccm, the gas flow rate of argon is 1000sccm-2000sccm, and the holding time is 5h-10h.

[0022] Furthermore, in some embodiments, the step e) is performed in a magnetron sputtering device, the tubular SiC braided layer processed in the step d) is placed in the magnetron sputtering device, and the vacuum is evacuated to 1×10-3 Pa-4×10 -3 Pa, use Al with a purity of not less than 99% as the target material, and set the shaft speed to 0.4rpm-0.6rpm;

[0023] First, introduce argon gas with a gas flow rate of 100 sccm-200 sccm, set the bias voltage to 400V-600V, the target power to 0W, the vacuum degree to 0.2Pa-0.4Pa, and the deposition time to 30min-60min;

[0024] Subsequently, argon gas is introduced with a gas flow rate of 100sccm-200sccm; oxygen gas is introduced with a gas flow rate of 10sccm-20sccm, the bias voltage is set to 50V-80V, the target power is 6000W-7000W, the vacuum degree is 0.2Pa-0.4Pa, and the processing time is 15h-60h; and a dense Al2O3 tritium barrier coating is formed on the surface of the SiC coating.

[0025] Furthermore, in some embodiments, in step f), the sintering step includes: placing the cladding tube preform in an adapted graphite mold, sealing both ends, heating to 1000° C.-1200° C. and maintaining the temperature for 2 h-4 h to complete sintering.

[0026] According to another embodiment of the present invention, a secondary neutron source rod is provided, comprising an end plug, a cladding tube and a pellet, wherein the cladding tube adopts the SiC secondary neutron source cladding tube provided in any of the aforementioned embodiments, the end plug is welded at both ends of the cladding tube, and the neutron source pellet is enclosed in the cladding tube. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic structural diagram of a secondary neutron source assembly in one embodiment;

[0028] Figure 2 This is a schematic diagram of the cross-sectional structure of a secondary neutron source rod in one embodiment;

[0029] Figure 3 Schematic diagram of the cross-sectional structure of the SiC secondary neutron source cladding tube in one embodiment.

[0030] 1-Secondary neutron source rod; 2a-Upper end plug; 2b-Lower end plug; 3-Neutron source pellet; 4-Secondary neutron source cladding tube; 5-SiC fiber braided layer; 6-Cracked carbon interface layer; 7-SiC coating; 8-Al2O3 tritium barrier coating.

[0031] The purpose of the above embodiments is to provide a detailed description of the present invention in conjunction with the accompanying drawings so that those skilled in the art can understand the technical concept of the present invention, and is not intended to limit the present invention. For the sake of simplicity, the above drawings only schematically illustrate structures related to the technical features of the present invention and do not strictly depict the complete structure and all details according to actual scale. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below through specific embodiments in conjunction with the accompanying drawings.

[0033] References to "embodiments" herein mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment herein. The appearance of such phrases in various places in the specification does not necessarily refer to the same embodiment, nor does it limit mutually exclusive independent or alternative embodiments. Those skilled in the art will appreciate that an embodiment herein may be combined with other embodiments as long as no structural conflicts arise.

[0034] In the description herein, unless otherwise specified or limited, the technical terms "installed," "connected," and "connected" should be understood broadly, and may refer to, for example, a movable connection, a fixed connection, or an integral connection. Those skilled in the art will understand the specific meanings of these terms in the embodiments of this application based on the specific circumstances.

[0035] In the description of this document, terms indicating orientation or positional relationships, such as "up", "down", "left", "right", "horizontal", "vertical", "height", "length", and "width", are intended to accurately describe the embodiments and simplify the description, but are not intended to limit the parts or structures involved to having a specific orientation, being installed or operated in a specific orientation, and should not be understood as limiting the embodiments in this document.

[0036] In the description herein, terms such as "first" and "second" are used only to distinguish different objects and should not be understood to indicate relative importance or to limit the quantity, specific order, or primary and secondary relationship of the described technical features. In the description herein, "plurality" means at least two.

[0037] As a clean, efficient, and sustainable energy source, nuclear power is an important option for addressing climate change and achieving carbon neutrality. With increasing concern about the safety of nuclear power, reducing the total amount of radioactive emissions from nuclear power plants and minimizing their environmental impact has become a key issue in the nuclear power sector.

[0038] Tritium is the primary source of radioactive emissions during normal nuclear power plant operation. With a half-life of 12.3 years, its presence in the natural environment can have harmful effects on humans and organisms. Therefore, limiting tritium emissions is a key area of ​​safety improvement for nuclear power plants.

[0039] During the operation of pressurized water reactor (PWR) nuclear power plants, tritium is generated from a variety of sources, including fission reactions in fuel rod pellets, neutron capture by burnable poisons and coolant benzene, and activation of coolant lithium. Neutron irradiation also generates significant amounts of tritium in the Sb-Be pellets of the reactor's secondary neutron source (SSN). However, the stainless steel cladding used for the SNS rods has limited tritium barrier properties, which can easily lead to significant tritium permeation, making it a significant source of tritium in the primary coolant. Currently, the primary approach to improving the tritium barrier properties of stainless steel cladding is to apply a coating with high tritium barrier properties, such as oxide ceramic coatings like aluminum oxide and titanium oxide, to the cladding to reduce tritium permeation. However, the physical and chemical properties of ceramic coatings differ significantly from those of stainless steel cladding tubes. Under high-temperature irradiation conditions, ceramic coatings applied to stainless steel are prone to cracking and flaking due to thermal mismatch.

[0040] In order to solve the above problems, an embodiment of one aspect of the present invention provides a SiC secondary neutron source cladding tube, such as Figure 3 As shown, the cladding tube adopts a multi-layer tubular structure, the matrix is ​​a SiC fiber braided layer 5, the SiC fiber braided layer 5 includes a three-dimensional structure formed by weaving and winding SiC fibers, and the whole is tubular; the outside of the SiC fiber braided layer 5 is a cracked carbon interface layer 6, which is formed by cracked carbon deposited on the surface of the SiC fiber braided layer 5; the outside of the cracked carbon interface layer 6 is a SiC coating 7, which is a dense SiC layer formed by chemical vapor deposition, and the inside of the SiC coating 7 is a dense SiC tissue without fiber structure; the outside of the SiC coating 7 is an Al2O3 tritium barrier coating, which is a dense Al2O3 layer formed by magnetron sputtering.

[0041] SiC has good resistance to tritium penetration, and SiC has a small neutron absorption cross-section and good neutron permeability, making it a good choice as a core component structural material. SiC also has good high-temperature mechanical properties, is stable at high temperatures, and is not prone to reacting with water. Moreover, after the secondary neutron source rod is loaded into the reactor, it will basically not move during operation and will usually remain inserted in the fuel assembly guide tube. Therefore, the brittleness of the SiC material itself is acceptable when used as a secondary neutron source cladding tube material. However, although the SiC material itself has good tritium resistance, it is limited by the manufacturing process and macroscopic structure, and the airtightness of the SiC tube is insufficient; moreover, Figure 2 As shown, the two ends of the secondary neutron source tube need to be welded with metal end plugs to seal them, but the welding performance of SiC and metal end plugs is poor. Therefore, it is necessary to modify the surface of the SiC material by providing an Al2O3 tritium barrier coating 7.

[0042] The advantage of Al2O3 is that it has good thermal stability and certain mechanical strength, is easy to prepare into a dense coating and has good corrosion resistance and air tightness. Furthermore, Al2O3 has good tritium resistance (10 -8 molH2 m -1 s -1 MPa -1 / 2 ). At the same time, compared with 304 stainless steel (thermal expansion coefficient 15.98×10 -6 / k) compared to other metal materials, Al2O3 (thermal expansion coefficient (5.3-5.8) × 10 -6 / k) and SiC (thermal expansion coefficient (4.0-4.8) × 10 -6 / k) have closer thermal expansion properties.

[0043] At the same time, to enhance the integrity and airtightness of the SiC braided layer 5, a dense SiC coating 7 is provided on the outside of the SiC braided layer 5. However, the overall brittleness of the single SiC component structure is relatively obvious, so it is necessary to provide a cracked carbon interface layer 6 between the SiC braided layer 5 and the SiC coating 7 to improve the quasi-ductility of the composite structure composed of the SiC braided layer 5 and the SiC coating 7.

[0044] The SiC secondary neutron source cladding tube provided in the above embodiment can be used to manufacture Figure 1 The secondary neutron source rod 1 in the secondary neutron source assembly is shown. The structure of the secondary neutron source rod 1 is as follows Figure 2 As shown, the secondary neutron source rod 1 includes an upper end plug 2a and a lower end plug 2b, respectively, which are fixed to the ends of the secondary neutron source cladding tube 4 by welding, forming a closed structure within the secondary neutron source cladding tube 4. The secondary neutron source pellet 3 (Sb-Be pellet) is enclosed within the secondary neutron source cladding tube 4. The upper end plug 2a and the lower end plug 2b can be made of materials with good high-temperature mechanical properties and radiation damage resistance, such as nickel alloy, zirconium alloy, or stainless steel. Al2O3 has better welding performance, and the Al2O3 tritium-barrier coating 7 enables a good welded connection between the upper end plug 2a, the lower end plug 2a, and the secondary neutron source cladding tube 4. The secondary neutron source rod 1 has excellent tritium-barrier performance, which can effectively reduce the penetration and diffusion of tritium generated by the secondary neutron source pellet 3 under the influence of neutron irradiation into the primary circuit coolant, thereby reducing tritium emissions from the secondary neutron source.

[0045] In a preferred embodiment, the secondary neutron source cladding tube 4 provided in the above embodiment can be manufactured by the following method:

[0046] Step a): braiding a tubular SiC fiber braided layer 5 .

[0047] SiC fiber raw material is provided. Specifically, third-generation continuous SiC fiber that meets the requirements for high-purity cubic β-SiC can be used, such as Cansas-3303, KD-S, and KD-SA. SiC fiber can also be produced using methods such as polymer impregnation pyrolysis (PIP). A tubular SiC fiber layer 5 is braided around a glass rod with a diameter of 7.4 mm to 9.6 mm, forming a three-dimensional structure with a braiding angle of 30° to 50°.

[0048] Step b): Deposition of a cracked carbon interface layer 6 .

[0049] The braided SiC fiber tube is placed in the heating section of a chemical vapor deposition furnace (CVD furnace). The CVD furnace is evacuated to 0.1 Pa-0.4 Pa, and an alkyl, alkenyl, or alkynyl gas containing carbon and hydrogen elements is introduced as a cracking gas feedstock, with a gas flow rate of 10 sccm-30 sccm. The CVD furnace is heated to the cracking temperature of the cracking gas feedstock (for example, 1000°C-1100°C for methane and 700°C-900°C for acetylene) to promote the dehydrogenation reaction of the introduced cracking gas feedstock, thereby depositing a cracked carbon (PyC) cross-section layer on the surface of the SiC fiber braid layer 5. The total deposition time (holding time) of the cracked carbon is 1 hour to 2 hours.

[0050] Step c): densification of the SiC matrix.

[0051] Chemical vapor infiltration (CVI) is used to achieve densification between the SiC fiber braid 5 and the cracked carbon interface layer 6. The SiC fiber tube, after cracked carbon deposition, is placed in the heating section of a CVI furnace. The CVI furnace is evacuated to 0.1 Pa-0.4 Pa. Trichloromethylsilane, tetramethylsilane, or silicon tetrachloride + methane is used as the densification gas raw material, hydrogen is used as the carrier gas, and argon is used as the dilution / shielding gas. The gas is introduced into the CVI furnace and heated to 1000°C-1400°C for chemical vapor infiltration. As the SiC matrix gradually infiltrates, the gap between the SiC fiber braided layer 5 and the cracked carbon interface layer 6 decreases, the infiltration rate decreases, and the gas flow rate of the densification gas raw material gradually increases from 1000sccm-2000sccm to 3000sccm-5000sccm. The hydrogen flow rate is controlled at 2000sccm-6000sccm, the argon flow rate is 2000sccm-6000sccm, and the infiltration time (holding time) is 5h-10h.

[0052] Step d): preparing a SiC coating.

[0053] A dense SiC coating 7 is deposited on the densified SiC fiber braided layer 5 and the cracked carbon interface layer 6 using a CVD process. The CVI-infiltrated SiC fiber tube is placed in the heating section of a CVD furnace, which is evacuated to 0.1 Pa to 0.4 Pa. Trichloromethylsilane, tetramethylsilane, or silicon tetrachloride + methane are used as coating gas raw materials, with hydrogen as the carrier gas and argon as the dilution / protective gas. The coating gas is introduced into the CVD furnace and heated to 1000°C to 1400°C to deposit a dense SiC coating 7 on the surface of the cracked carbon interface layer 6. The coating gas raw material flow rate is 1000 sccm to 2000 sccm, the hydrogen flow rate is 1000 sccm to 2000 sccm, and the argon flow rate is 1000 sccm to 2000 sccm. The vapor deposition time (holding time) is 5 hours to 10 hours. Finally, a SiC coating with a thickness of 50 μm to 200 μm is obtained.

[0054] Step e): preparing an Al2O3 tritium barrier coating.

[0055] The dense Al2O3 tritium barrier coating 8 was prepared by magnetron sputtering process. The SiC fiber tube with SiC coating 7 was placed at the center of the rotating shaft of the magnetron sputtering coating equipment and vacuumed to 1×10 -3 Pa-4×10 -3 Pa. Use two pieces of metal Al as targets (purity: 99.0%-100.0%) and place them in parallel on both sides of the turntable. Set the shaft speed to 0.4-0.6rpm. First, introduce argon gas for sputtering cleaning, with a gas flow rate of 100-200sccm, a bias voltage of 400-600V, a target power of 0W, a gas vacuum of 0.2-0.4Pa, and a time of 30-60min. Then carry out Al2O3 coating deposition, with a gas flow rate of Ar: 100-200sccm, O2: 10-20sccm, a bias voltage of 50-80V, a target power of 6000-7000W, a gas vacuum of 0.2-0.4Pa, and a time of 15-60h. A cladding tube preform with a dense Al2O3 tritium barrier coating 8 of 5μm-20μm deposited on the surface is obtained. Step f): sintering.

[0056] The cladding tube preform is sintered using high-temperature isostatic pressing. The preform is placed in a suitable graphite mold, sealed at both ends, and heated to 1000-1200°C for 2-4 hours to obtain the finished SiC secondary neutron source cladding tube. The finished SiC secondary neutron source cladding tube is then machined to obtain the secondary neutron source cladding tube 4 used to manufacture the secondary neutron source rod 1. The graphite sintering mold prevents SiC deformation at high temperatures, improving the quality of the finished SiC secondary neutron source cladding tube 4. After high-temperature sintering, the finished secondary neutron source cladding tube 4 exhibits excellent hardness and wear resistance.

[0057] In different embodiments, during the CVD and CVI treatments, the two ends of the tube body surrounded by the SiC fiber braided layer 5 can be sealed so that the cracked carbon interface layer 6, the SiC coating 7, and the Al2O3 tritium barrier coating 8 are sequentially formed only on the outside of the SiC fiber braided layer 5. Alternatively, the two ends of the tube body surrounded by the SiC fiber braided layer 5 may not be sealed so that the inside of the SiC fiber braided layer 5 is also formed. Figure 3 As shown, a cracked carbon interface layer 6 and a SiC coating layer 7 can be formed in sequence.

[0058] The SiC secondary neutron source cladding tube provided in the above-mentioned embodiment has a hydrogen (tritium) permeability coefficient 9-13 orders of magnitude lower than that of stainless steel, effectively reducing radioactive emissions caused by tritium permeation from the secondary neutron source and improving the safety of nuclear power plants. Furthermore, the SiC and Al2O3 composite structure has well-matched thermal expansion coefficients between different layers, making it less susceptible to cracking and spalling, effectively extending the service life and reliability of the secondary neutron source cladding tube. Furthermore, the Al2O3 material and metal materials have a mature brazing process, which reduces the manufacturing difficulty of the secondary neutron source rod.

[0059] The purpose of the above embodiments is to provide a further detailed description of the present invention in conjunction with the accompanying drawings so that those skilled in the art can understand the technical concept of the present invention. Within the scope of the present invention, optimization or equivalent replacement of the structures or method steps involved, as well as combination of implementation methods in different embodiments without causing any structural or principle conflicts, all fall within the scope of protection of the present invention.

Claims

1. A SiC secondary neutron source cladding tube, characterized in that: The SiC secondary neutron source cladding tube is configured as a multi-layer tubular structure, including a SiC fiber braided layer, a cracked carbon interface layer, a SiC coating and an Al2O3 tritium barrier coating arranged in sequence from the inside to the outside along the radial direction, wherein the SiC fiber braided layer includes a three-dimensional structure formed by braiding and winding SiC fibers, and the SiC coating is configured as a dense SiC layer without a fiber structure.

2. The SiC secondary neutron source cladding tube according to claim 1, characterized in that: The SiC fiber braided layer is configured as a tube with a diameter of 7.4 mm to 9.6 mm, and the braiding angle of the SiC fiber is 30° to 50°.

3. The SiC secondary neutron source cladding tube according to claim 1 or 2, characterized in that: The thickness of the SiC coating is 50 μm-200 μm.

4. The SiC secondary neutron source cladding tube according to claim 1 or 2, characterized in that: The thickness of the Al2O3 tritium barrier coating is 5 μm-20 μm.

5. A secondary neutron source rod, comprising an end plug, a cladding tube and a neutron source pellet, characterized in that: The cladding tube is the SiC secondary neutron source cladding tube according to any one of claims 1 to 4, the end plugs are welded at both ends of the cladding tube, and the neutron source pellets are enclosed in the cladding tube.

Images