CN111451633A - Welding and packaging method for molybdenum alloy accident fault-tolerant fuel rod - Google Patents

Abstract

The invention discloses a welding and packaging method for a molybdenum alloy accident-tolerant fuel rod. The whole welding and packaging process includes laser fusion welding in a negative pressure inert gas atmosphere and rotary friction welding without upsetting in a high pressure gas atmosphere. In the present invention, a metal layer is added at the assembly interface of the cladding tube and the solid lower end plug, and the cladding tube and the hollow upper end plug, and laser welding is performed in a negative pressure inert gas atmosphere, so that the ring of the molybdenum alloy fuel rod is improved. Seam welding strength, toughness, and can effectively avoid porosity defects. Since the sealing and welding of the hollow upper end plug needs to be carried out at a high pressure of 2-3MPa, the hollow upper end plug and the rod-shaped sealing material are welded by means of rotary friction welding without upsetting to ensure the weld quality of the welded joint, welding Strength, and avoid the impact of the impact of the upsetting force when using traditional rotary friction welding, so as to crush the thin molybdenum alloy cladding tube.

Description

A welding and packaging method for molybdenum alloy accident-tolerant fuel rods

technical field

The invention relates to the technical field of welding, in particular to a welding and packaging method for a molybdenum alloy accident-tolerant fuel rod.

Background technique

For a long time, zirconium alloy materials have been mainly used for nuclear reactor fuel cladding components. However, in the 2011 Fukushima nuclear power plant accident in Japan, zirconium alloys reacted with water/water vapor at a temperature of about 1200 °C, and eventually caused an explosion. The explosion of the Fukushima nuclear power plant unit has made the world's nuclear industry and scientific community realize that it is necessary to develop a system that can resist severe accident conditions for a long time, delay the deterioration of the situation, and buy more precious time for people to take emergency measures. A novel fuel system for the risk of leakage of radioactive materials, namely Accident Tolerant Fuel (ATF). Molybdenum has the advantages of small neutron absorption cross section, good high temperature strength, good thermal conductivity, and good corrosion resistance of linear expansion coefficient small box. Therefore, molybdenum alloy is listed as the main candidate material for ATF cladding by the world nuclear industry.

At present, the traditional welding process of zirconium alloy fuel rod cladding assemblies mainly includes the following steps: (1) performing circumferential seam welding between the lower end plug and the cladding tube in a normal pressure atmosphere, (2) performing a circular seam welding in a normal pressure atmosphere Circumferential seam welding between the upper end plug and the cladding tube, (3) in a high-pressure inert gas atmosphere, the upper end plug seals the welding point of the hole, so as to realize the helium filling in the sealed box cladding tube; in the above welding process, the resistance is usually used welding, argon arc welding, laser welding, etc. However, the above welding process is not applicable to the welding and packaging of molybdenum alloy fuel rod cladding components, such as:

Due to the large size of the weld seam and the heat-affected zone of the molybdenum alloy after welding, and the serious coarsening of the grains, the joint strength and toughness are very poor under the combined action of the intrinsic brittleness of the material and the weakened grain boundaries of the impurity segregation. Since the high electrical conductivity of molybdenum makes the weldability of molybdenum and molybdenum alloys worse, resistance welding is not suitable for molybdenum alloy fuel rod cladding welding. The friction stir welding tool is seriously worn, and there are also problems such as a difficult-to-repair key hole in the weld when the stirring head lifts the workpiece at the end of welding, the corrosion resistance of the weld is reduced, and the thin-walled pipe fittings are not easily clamped. Not suitable for molybdenum alloy fuel rod cladding welding. The brazing seam strength and heat resistance of brazing are lower than that of the base metal, and its high-temperature service performance is usually not as good as that of fusion welded joints, so brazing is not suitable for molybdenum alloy fuel rod cladding welding. During fusion welding, after the high-performance molybdenum alloy undergoes a process of melting and solidification (or recrystallization), its strength and toughness advantages will disappear, and serious embrittlement and porosity defects will appear.

At the same time, the plugging of the upper end plug also needs to be carried out in a high pressure (2-3MPa) environment, although a high power density heat source can be welded at a very low heat input, and a small size weld can be obtained. Molybdenum and molybdenum Alloy welding has great advantages, but the application of vacuum electron beam welding is limited due to the high pressure environment when the upper end plugs the hole. When welding under normal pressure, especially negative pressure, due to the high energy density of the laser welding heat source, the metal material is rapidly heated and melted and the metal is violently evaporated. The violent evaporation will generate recoil pressure on the surface of the molten pool. , the liquid surface of the molten pool is recessed downward, forming a small hole surrounded by liquid metal. The laser beam is reflected multiple times inside the small hole and converges at the bottom of the small hole, causing the metal at the bottom of the molten pool to evaporate strongly, and so on, forming a narrow and deep weld. The recoil pressure is closely related to the environmental pressure. When laser welding is performed under the condition of 2-3MPa high pressure, the penetration depth will be greatly reduced, and the drop rate will be as high as 80%. The advantages of small size, narrow and deep seam lead to deterioration of joint performance, so in the high pressure environment required for nuclear fuel cladding packaging, laser welding method cannot be used to seal the upper end plug. The rotary friction welding method is to use the relative motion between the contact surfaces of the workpieces to be welded to generate friction heat and plastic deformation under the action of pressure, so that the temperature near the contact surface is raised to a temperature close to the melting point, and the material is The deformation resistance is reduced, the plasticity is improved, the oxide film at the interface is broken, and the material produces appropriate flow and macroscopic plastic deformation, and welding is achieved through atomic diffusion and recrystallization at the interface. However, the traditional rotary friction welding process will use upsetting measures. Under the impact of the upsetting force, the molybdenum alloy thin-walled cladding tube will be crushed, so the traditional rotary friction welding process is not suitable for the welding of molybdenum alloy fuel rods.

Since molybdenum is listed as the main candidate material for ATF cladding by the world nuclear industry due to its excellent properties, it is urgent to propose a molybdenum fuel cladding package that is not only suitable for molybdenum alloys, but also can improve weld quality and welding strength. Welded encapsulation method for alloy accident-tolerant fuel rods.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for welding and encapsulating a molybdenum alloy accident-tolerant fuel rod, so as to solve the problems existing in the above-mentioned prior art, so that the cladding tube and the solid lower end plug, the cladding tube and the hollow upper end plug are assembled at the interface. Circumferential seam welding has high strength, good toughness, and can effectively avoid pore defects; at the same time, the sealing effect of the hollow upper end plug is good, and the joint quality is high, which solves the problems of welding seam quality and welding strength when molybdenum alloy is used as fuel rod cladding. .

For achieving the above object, the present invention provides the following scheme:

The invention provides a welding and packaging method for a molybdenum alloy accident-tolerant fuel rod. The fuel rod comprises a solid lower end plug, a cladding tube, a heat insulation block, a nuclear fuel pellet, a spring, a hollow upper end plug and a rod-shaped sealing block, wherein the The solid lower end plug, the cladding tube, the hollow upper end plug and the rod-shaped sealing block are all made of molybdenum alloy, and the packaging method includes the following steps:

a) One end of the cladding tube is assembled with the solid lower end plug, and a metal layer is added at the assembly interface of the cladding tube and the solid lower end plug, and the melting point of the metal layer is lower than that of the cladding shell The melting point of the tube and the solid lower end plug, in the negative pressure inert gas atmosphere, the circular seam between the solid lower end plug and the cladding tube is welded by laser fusion welding. During the welding process, the assembly interface is The solid lower end plug and the base material of the cladding tube and the metal layer are melted, and the solid lower end plug and the base material of the cladding tube form a fusion connection at the assembly interface, and the metal layer is formed at the assembly interface. metallurgical bonding;

b) sequentially filling the heat insulating block, a plurality of the nuclear fuel pellets, the heat insulating block and the spring into the cladding tube, and assembling the hollow upper end plug at the other end of the cladding tube to complete internal assembly;

c) adding the metal layer at the assembly interface of the cladding tube and the hollow upper end plug, the melting point of the metal layer is lower than the melting point of the cladding tube and the hollow upper end plug, and is inert under negative pressure In a gas atmosphere environment, laser fusion welding is used to weld the annular seam between the hollow upper end plug and the cladding tube. During the welding process, the hollow upper end plug and the base metal of the cladding tube at the assembly interface are welded. and the metal layer is melted, the hollow upper end plug and the base material of the cladding tube form a fusion connection at the assembly interface, and the metal layer forms a metallurgical bond with the assembly interface;

d) In a high-pressure inert gas atmosphere, the upsetting-free rotary friction welding method is used to weld the hollow upper end plug and the rod-shaped sealing block. Transient axial upsetting load.

Preferably, the metal layer is a metal layer attached to the assembly interface or a metal layer added independently at the assembly interface, and the material of the metal layer is one or more of Zr, Ti, Ni, Hf and Al.

Preferably, in step a) and step c), it also includes preheating the workpiece to 400-450°C before the laser fusion welding, setting the defocusing amount of the laser beam to be -1mm to +1mm, and the welding speed to be 2m/ min to 20 m/min, and repeated welding is performed on the annular seam for many times.

Preferably, both the laser fusion welding and the non-upset rotary friction welding are performed in a hyperbaric chamber, and the hyperbaric chamber is evacuated to a pressure of less than ^10-9 MPa before welding, and then filled with an inert gas, so that Repeat vacuuming and filling of inert gas to ensure that the oxygen content of the gas in the hyperbaric chamber is ≤1.5PPM, the nitrogen content is ≤50PPM, the total carbon content is ≤5PPM, and the water content is ≤3PPM. Welding is completed in a negative pressure inert gas atmosphere or a high pressure inert gas atmosphere.

Preferably, when the non-upset rotary friction welding method is used, the fuel rods that have completed two circumferential seam welding are clamped on the hydraulic clamp, the clamping contact surface is completely located in the area of the hollow upper end plug, and the rod-shaped sealing block is clamped On the rotary chuck connected to the motor, the hollow upper end plug is aligned with the rod-shaped sealing block; during welding, the motor drives the rotary chuck to rotate, thereby driving the rod-shaped sealing block to rotate, and the hydraulic clamp passes through the hydraulic cylinder A constant load is applied along the axial direction, that is, welding pressure; the rod-shaped sealing block is in contact with the end face of the hollow upper end plug, and the relative rotation generates friction under the welding pressure, and the welding is completed by the heat generated by the frictional force. The application is the whole process from the start of welding to the end of welding, and the applied welding pressure and the rotation speed of the motor are constant.

Preferably, the welding pressure is 80±10MPa, the rotation speed is 2000±200r/min, and the welding time is 3±1s.

Preferably, the range of negative pressure in the negative pressure inert gas atmosphere is 0.01-0.1 MPa, and the range of high pressure in the high-pressure inert gas atmosphere is 2-3 MPa.

Preferably, after step d), it also includes step e) under the protection of normal pressure inert gas, to remove the flash produced by the rotary friction welding process without upsetting.

Preferably, a machining method is used to remove the burrs generated during the non-upset rotary friction welding process. During the removal process, the fuel rods are stationary and the processing head rotates to remove the burrs.

Preferably, the inert gas is argon or helium.

The present invention has achieved the following technical effects with respect to the prior art:

1. The present invention adds a metal layer at the assembly interface of the cladding tube and the solid lower end plug, and the cladding tube and the hollow upper end plug, and adopts the form of laser fusion welding for welding, so that during the welding process, the solid lower end at the assembly interface is The plug (or hollow upper end plug), the base material of the cladding tube and the metal layer are all melted, and the metal layer forms a metallurgical bond with the assembly interface position, which improves the toughness and strength of the weld position, and avoids the traditional fusion welding process. After the melting-solidification (or recrystallization) process, its strength and toughness advantages will disappear, and serious embrittlement problems will occur.

2. When the traditional zirconium alloy fuel rod is welded and packaged, the circumferential seam welding between the upper end plug (or lower end plug) and the cladding tube is carried out under normal pressure, and the laser fusion welding adopted in the present invention is performed under negative pressure. In an inert gas atmosphere, in a negative pressure environment, the gas in the welding pool can escape in time, which can effectively avoid porosity defects and improve the strength of the welded joint. , thereby forming a narrow and deep weld, further increasing the joint strength.

3. The hollow upper end plug and the rod-shaped sealing material are welded by means of rotary friction welding without upsetting in a high-pressure inert gas atmosphere, which ensures the welding seam quality and welding strength of the welded joint, and avoids the use of traditional rotary During friction welding, an impactful upsetting force is applied, thereby squeezing the thin molybdenum alloy cladding tube.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings required in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some of the present invention. In the embodiments, for those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative labor.

Figure 1 is a structural diagram of a molybdenum alloy accident-tolerant fuel rod;

Fig. 2 is the schematic diagram of laser fusion welding joint adding Zr element metal layer for the present invention;

Among them, 1-solid lower end plug, 2-cladding tube, 3-insulation block, 4-nuclear fuel pellet, 5-spring, 6-hollow upper end plug, 7-rod-shaped sealing block.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

The purpose of the present invention is to provide a kind of welding and packaging method of molybdenum alloy accident fault-tolerant fuel rod, so as to solve the problems existing in the above-mentioned prior art, so that the cladding tube and the solid lower end plug, the cladding tube and the hollow upper end plug are assembled The circumferential seam welding at the interface has high strength and toughness, and can effectively avoid pore defects; at the same time, the sealing effect of the hollow upper end plug is good, and the joint quality is high, which solves the problem of welding seam quality and welding when molybdenum alloy is used as fuel rod cladding. issue of strength.

In order to make the above objects, features and advantages of the present invention more clearly understood, the present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

Example 1

This embodiment provides a welding and packaging method for a molybdenum alloy accident-tolerant fuel rod. The fuel rod includes a solid lower end plug 1, a cladding tube 2, a heat insulation block 3, a nuclear fuel pellet 4, a spring 5, a hollow upper end plug 6 and a rod seal Block 7. The length of the cladding tube 2 is 200-800mm, the outer diameter is 9.5mm, and the wall thickness is 0.5mm. 40mm. Among them, the solid lower end plug 1, the cladding tube 2, the hollow upper end plug 6 and the rod-shaped sealing block 7 are all made of molybdenum alloy.

In this embodiment, the entire welding and packaging process of the fuel rod includes laser fusion welding performed in a negative pressure inert gas atmosphere and rotary friction welding without upsetting performed in a high pressure inert gas atmosphere. Among them, the laser welding equipment is: IPGYSL-4000 fiber laser, YASKAWAHP20 welding robot and welding tooling. Among them, the maximum output power of the fiber laser can reach 4000W, the spot diameter is 0.2mm, the laser wavelength is 1.07μm, and the beam focal length is 150mm. The welding tooling includes a gas shielding chamber, a rotating chuck, and a temperature-controlled heating device, which can realize the functions of pre-welding, girth welding and gas shielding during the laser welding of molybdenum alloy components. In this embodiment, the welding parameters of laser fusion welding are selected as the laser power of 1000-2500W, the defocus of the laser beam from -1mm to +1mm, and the welding speed from 2m/min to 20m/min. Under these welding parameters, the quality of the welded joint Better, higher strength, better metallographic structure around the weld.

During the welding process, the cladding tube 2 and the solid lower end plug 1 were first polished with sandpaper, and then the sample was immersed in acetone for ultrasonic cleaning. A metal layer is added at the assembly interface of the cladding tube 2 and the solid lower end plug 1. The material of the metal layer can be one or more of Zr, Ti, Ni, Hf and Al. 1 is assembled, placed in a high pressure chamber, and then the assembled cladding tube 2 and the solid lower end plug 1 are clamped by a rotary chuck. Before welding, the high pressure chamber is evacuated until the pressure in the chamber is less than 10 ^-9 MPa, and then filled with 99.999% pure argon gas, and then evacuated to the pressure in the chamber less than 10 ^-9 MPa, and then filled with 99.999% pure argon gas The argon gas in the hyperbaric chamber is repeatedly evacuated and filled with argon gas to ensure the purity of the gas in the hyperbaric chamber, so that the oxygen content in the hyperbaric chamber is ≤1.5PPM, the nitrogen content is ≤50PPM, the total carbon content is ≤5PPM, and the water content is ≤ 3PPM, fill with argon for the last time before welding, so that the pressure reaches the pressure of "negative pressure inert gas atmosphere" (0.01-0.1MPa). Use a heating device to preheat the assembled cladding tube 2 and the solid lower end plug 1 before welding. The preheating temperature is 400℃-450℃. The preheating before welding can reduce the welding residual stress of the molybdenum alloy laser welded joint. . During the welding process, the laser head is kept fixed, and the laser spot is focused on the butting area of the cladding tube 2 and the solid lower end plug 1, and then the rotating chuck drives the cladding tube 2 and the solid lower end plug 1 to rotate, and the cladding tube 2 and the solid lower end plug are completed. 1 Circumferential seam welding. During the above laser fusion welding process, the base material of the solid lower end plug 1 and the cladding tube 2 and the metal layer at the assembly interface melt, and the solid lower end plug 1 and the base material of the cladding tube 2 form a fusion connection at the assembly interface. The metal layer forms a metallurgical bond with the assembly interface location.

After completing the above-mentioned circumferential seam welding, put the heat insulating block 3, several nuclear fuel pellets 4, heat insulating blocks 3, and springs 5 in sequence into the cladding tube 2, and then assemble the hollow upper end plug 6. A metal layer is added at the assembly interface of the hollow upper end plug 6, and the metal layer is of the same material as the metal layer added at the assembly interface of the cladding tube 2 and the solid lower end plug 1, and then the assembled whole is put into the high-pressure chamber for welding. The process is the same as the welding process of the cladding tube 2 and the solid lower end plug 1, and will not be repeated here.

After the above process is completed, the hollow lower end plug 6 and the rod-shaped sealing material 7 are welded, and this process needs to be carried out by means of rotary friction welding without upsetting in a high pressure (2-3MPa) environment. The specific operation process is as follows: put the fuel rod assembly and the rod-shaped sealing block 7 that have been welded with two circular seams before into the high-pressure chamber, and assemble the rod-shaped sealing block 7 and the hollow upper end plug 6 on the fixture respectively in the high-pressure chamber. , the fuel rod assembly that has completed two circumferential seam welding is clamped by the hydraulic clamp, the clamping contact surface is completely located in the area of the hollow upper end plug 6, and the rod-shaped sealing block 7 is clamped on the rotating chuck connected to the motor. After the clamping is completed, the high pressure chamber is evacuated until the pressure in the chamber is less than 10 ^-9 MPa, and then filled with 99.999% pure argon gas. The oxygen content of the gas in the hyperbaric chamber should be ≤1.5PPM, the nitrogen content should be ≤50PPM, the total carbon content should be ≤5PPM, and the water content should be ≤3PPM. Fill with argon gas for the last time before welding, so that the pressure reaches the pressure of the high-pressure package gas of the fuel rod (2-3MPa). Then, non-upset rotary friction welding is performed. During this process, the motor drives the rotary chuck to rotate, thereby driving the rod-shaped sealing block 7 to rotate. The hydraulic fixture applies a constant load in the axial direction through the hydraulic cylinder, that is, the welding pressure; the rod-shaped sealing block 7 and the The hollow upper end plug 6 is in contact with the end faces, and the relative rotation generates friction under the welding pressure. The welding is completed by the heat generated by the frictional force. The application of the welding pressure is the whole process from the beginning of the welding to the end of the welding. Set the welding process parameters as follows: the welding pressure is 80±10MPa, the motor speed is 2000±200r/min, and the welding time is 3±1s. In traditional rotary friction welding, in order to make the two parts to be welded contact, the rotary chuck rotates, and the rotary friction welding starts. At the end of the welding, the hydraulic cylinder pushes the hydraulic fixture to apply the upsetting force. The hydraulic oil is supplied to drive the hydraulic clamp to quickly apply an impactful axial force forward to complete the welding. In the present invention, the impactful upsetting force applied at the end of welding is eliminated, and a constant welding pressure is applied throughout the welding process. Adopting the form of non-upset rotary friction welding avoids the need to apply impact forging force in the welding process of the traditional rotary friction welding process, resulting in the collapse of the cladding tube due to the thin wall of the cladding tube 2. The phenomenon.

After the non-upset rotary friction welding process is completed, the pressure in the high pressure chamber is adjusted to the normal pressure level, which is about 0.101MPa. Under the protection of argon, the flash generated during the non-upset rotary friction welding process is removed by machining. During the processing, the fuel rod is clamped on the fixture and does not move, and the processing head rotates to cut off the flash. After the flash removal is completed, the fuel rods are taken out from the high pressure chamber, and the whole welding and packaging process is completed.

In the whole welding and packaging process, the argon gas filled in the high pressure chamber can also use other inert gases such as helium gas.

Embodiment 2

In this embodiment, the solid lower end plug 1 , the cladding tube 2 , the hollow upper end plug 6 and the rod-shaped sealing block 7 are all made of 0.5wt% La ₂ O ₃ nano-particle doped reinforced molybdenum alloy, wherein the La ₂ O ₃ nano-particles are The particle form is dispersed in the molybdenum matrix. High melting point refractory metals such as molybdenum alloy are difficult to be processed by smelting, and are often prepared by powder metallurgy. During the preparation process of powder metallurgy, the gas in the powder embryo cannot fully overflow, and is compressed to bridge the micro-holes inside the material, resulting in The density of the material has certain deficiencies, so the La ₂ O ₃ nanoparticle-doped reinforced molybdenum alloy used in this embodiment is a high gas content material. The welding and packaging process of the fuel rod is the same as that of the first embodiment.

Embodiment 3

As a preferred embodiment, the metal layer added at the assembly interface between the cladding tube 2 and the solid lower end plug 1, and the cladding tube 2 and the hollow upper end plug 6 is Ti or Zr, and the welding parameters of the laser fusion welding are set to be laser power 1000～ 2500W, defocus amount 1mm, welding speed 5~12m/min, welding parameters of non-upset rotary friction welding are welding pressure 80MPa, motor speed 2000r/min, welding time 2s, the rest of the operation process is the same as that of Example 1. The welded joint in which Zr element is added is shown in Figure 2.

Embodiment 4

In this embodiment, the cladding tube 2 is wrapped around a passive alkali metal molybdenum alloy heat pipe for heat exchange, the heat pipe material is 0.5wt% La ₂ O ₃ nanoparticle doped reinforced molybdenum alloy, the outer diameter is φ16mm, and the length is 2-8m , the wall thickness is 1.2mm.

The entire welding and packaging process is similar to that of the first embodiment, except that the welding process parameters of the laser fusion welding are adjusted to the laser power of 3000-4000W, the defocus amount of 0mm, and the welding speed of 2-6m/min. The welding process parameters of rotary friction welding without upsetting are adjusted to welding pressure of 80 MPa, motor speed of 2000 r/min, and welding time of 3 s. The rest of the process is the same as the first embodiment.

Adaptive changes made according to actual needs are all within the protection scope of the present invention.

In the present invention, specific examples are used to illustrate the principles and implementations of the present invention, and the descriptions of the above embodiments are only used to help understand the method and the core idea of the present invention; There will be changes in the specific implementation manner and application scope of the idea of the invention. In conclusion, the contents of this specification should not be construed as limiting the present invention.

Claims (10)

1. A welding and packaging method for a molybdenum alloy accident fault-tolerant fuel rod is characterized by comprising the following steps: the fuel rod comprises a solid lower end plug, a cladding tube, a heat insulation block, a nuclear fuel pellet, a spring, a hollow upper end plug and a rod-shaped sealing block, wherein the solid lower end plug, the cladding tube, the hollow upper end plug and the rod-shaped sealing block are all made of molybdenum alloy, and the packaging method comprises the following steps:

a) assembling the solid lower end plug at one end of the cladding tube, adding a metal layer at an assembling interface of the cladding tube and the solid lower end plug, wherein the melting point of the metal layer is lower than those of the cladding tube and the solid lower end plug, welding a circular seam between the solid lower end plug and the cladding tube by adopting laser fusion welding in a negative pressure inert gas atmosphere, melting the solid lower end plug at the assembling interface, a cladding tube base material and the metal layer in the welding process, forming fusion connection between the solid lower end plug and the cladding tube base material at the assembling interface, and forming metallurgical bonding between the metal layer and the assembling interface;

b) sequentially filling the heat insulation block, a plurality of nuclear fuel pellets, the heat insulation block and the spring into the cladding tube, and assembling the hollow upper end plug at the other end of the cladding tube to finish internal assembly;

c) adding the metal layer at an assembly interface of the cladding tube and the hollow upper end plug, wherein the melting point of the metal layer is lower than the melting points of the cladding tube and the hollow upper end plug, welding a circular seam between the hollow upper end plug and the cladding tube by adopting laser fusion welding in a negative pressure inert gas atmosphere environment, melting the hollow upper end plug, a cladding tube base material and the metal layer at the assembly interface in a welding process, forming fusion connection between the hollow upper end plug and the cladding tube base material at the assembly interface, and forming metallurgical bonding between the metal layer and the assembly interface;

d) and welding the hollow upper end plug and the rod-shaped sealing block by adopting an upsetting-free rotary friction welding method under a high-pressure inert gas atmosphere, wherein an instantaneous axial upsetting load with impact is not applied in the rotary friction welding process.

2. The welding and packaging method for the molybdenum alloy accident fault-tolerant fuel rod as claimed in claim 1, wherein the welding and packaging method comprises the following steps: the metal layer is attached to the assembly interface or independently added at the assembly interface, and the material of the metal layer is one or more of Zr, Ti, Ni, Hf and Al.

3. The welding and packaging method for the molybdenum alloy accident tolerance fuel rod as claimed in claim 1 or 2, wherein: in the step a) and the step c), the method further comprises the steps of preheating the workpiece to 400-450 ℃ before the laser fusion welding, setting the defocusing amount of a laser beam to be-1 mm, setting the welding speed to be 2 m/min-20 m/min, and repeatedly welding the circular seam for many times.

4. The welding and packaging method for the molybdenum alloy accident tolerance fuel rod as claimed in claim 1 or 2, wherein: the laser fusion welding and the upset-free rotary friction welding are both carried out in the high-pressure cabin, and the high-pressure cabin is vacuumized until the pressure in the cabin is less than 10 before the welding starts^-9And (4) MPa, then filling inert gas, repeatedly vacuumizing and filling the inert gas in such a way to ensure that the oxygen content of the gas in the hyperbaric chamber is less than or equal to 1.5PPM, the nitrogen content is less than or equal to 50PPM, the total carbon content is less than or equal to 5PPM and the water content is less than or equal to 3PPM, finally filling the inert gas into the hyperbaric chamber, and completing welding in a negative pressure inert gas atmosphere environment or a high pressure inert gas atmosphere environment.

5. The welding and packaging method for the molybdenum alloy accident fault-tolerant fuel rod as claimed in claim 1, wherein the welding and packaging method comprises the following steps: when the upsetting-free rotary friction welding method is adopted, the fuel rod which completes two circular seam welding is clamped on a hydraulic clamp, the clamping contact surface is completely positioned in the hollow upper end plug area, the rod-shaped sealing block is clamped on a rotary chuck connected with a motor, and the hollow upper end plug is aligned with the rod-shaped sealing block; during welding, the motor drives the rotary chuck to rotate so as to drive the rod-shaped sealing block to rotate, and the hydraulic clamp applies constant load, namely welding pressure, in the axial direction through the hydraulic oil cylinder; the rod-shaped sealing block is in contact with the end face of the hollow upper end plug, friction is generated by relative rotation under the welding pressure, welding is completed through heat generated by friction force, the welding pressure is applied in the whole process from the beginning to the end of welding, and the applied welding pressure and the rotating speed of the motor are constant.

6. The welding and packaging method for the molybdenum alloy accident tolerance fuel rod as claimed in claim 5, wherein the welding and packaging method comprises the following steps: the welding pressure is 80 +/-10 MPa, the rotating speed is 2000 +/-200 r/min, and the welding time is 3 +/-1 s.

7. The welding and packaging method for the molybdenum alloy accident fault-tolerant fuel rod as claimed in claim 1, wherein the welding and packaging method comprises the following steps: the negative pressure value range in the negative pressure inert gas atmosphere environment is as follows: 0.01-0.1MPa, wherein the high pressure value range in the high-pressure inert gas atmosphere environment is as follows: 2 to 3 MPa.

8. The welding and packaging method for the molybdenum alloy accident fault-tolerant fuel rod as claimed in claim 1, wherein the welding and packaging method comprises the following steps: after the step d), the method also comprises a step e) of removing flash generated in the upsetting-free rotary friction welding process under the protection of the inert gas under normal pressure.

9. The method for welding and packaging the molybdenum alloy accident tolerance fuel rod according to claim 8, wherein: and removing the flash generated in the non-upsetting rotary friction welding process by adopting a mechanical processing method, wherein in the removing process, the fuel rod is static, and the processing head rotates to remove the flash.

10. The welding and packaging method for the molybdenum alloy accident tolerance fuel rod as claimed in claim 1 or 8, wherein: the inert gas is argon or helium.

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