Zirconium alloy fuel rod end plug composite welding method and zirconium alloy fuel rod

Abstract

The invention discloses a zirconium alloy fuel rod end plug composite welding method and a zirconium alloy fuel rod, and belongs to the field of welding. The zirconium alloy fuel rod end plug composite welding method comprises the following steps that a zirconium alloy end plug and a zirconium alloy fuel rod cladding are spliced together and fixed, and an inert gas environment is provided after vacuumizing; firstly, a zirconium alloy end plug and a zirconium alloy fuel rod cladding are subjected to TIG welding, and meanwhile ultrasonic stirring and electromagnetic stirring are provided; then, TIG welding is conducted on an inflation hole of the zirconium alloy end plug, and ultrasonic stirring and electromagnetic stirring are provided at the same time; and after welding is completed, a welding area is subjected to polishing and quality detection. According to the method, air holes in the zirconium alloy welding area can be effectively eliminated, uniform distribution of structures and components is promoted, and the welding quality and the corrosion resistance of the welding area are improved.

Description

Technical Field

[0001] The present invention belongs to the field of welding, and particularly relates to a composite welding method for end plugs of zirconium alloy fuel rods and zirconium alloy fuel rods. Background Art

[0002] Due to the characteristics of low thermal neutron absorption cross-section, good corrosion resistance, good thermal strength, and excellent welding performance of zirconium alloy, it is currently widely used as the structural material of fuel rods in commercial pressurized water reactor nuclear power plants. The fuel rod includes a tubular fuel rod cladding and end plugs at both ends. The end plugs are fixedly connected to the fuel rod cladding by welding to enclose the fuel pellets therein. In addition to the welds connected to the fuel rod cladding, inflation holes are provided on the end plugs for filling helium gas into the fuel rod, and the inflation holes also need to be plug-welded after inflation. Since the fuel rod needs to serve in the primary loop cooling water under high temperature, high pressure, and irradiation environments for a long time, the welding quality of the end plugs has an important impact on the safety of the reactor. Due to the constraints of the fuel rod manufacturing process, the end plug welding cannot be carried out under vacuum conditions, which leads to the easy formation of defects such as pores in the zirconium alloy welding structure, and has a significant adverse impact on the welding quality and the corrosion resistance of the welding area. Therefore, providing a zirconium alloy end plug welding method that can improve the welding quality has positive significance for improving the safety of reactor fuels. Summary of the Invention

[0003] The purpose of the present invention is to provide a composite welding method for end plugs of zirconium alloy fuel rods to improve the welding quality of the end plugs. The present invention also provides a zirconium alloy fuel rod.

[0004] According to an embodiment of one aspect of the present invention, there is provided a composite welding method for end plugs of zirconium alloy fuel rods. The composition of the end plugs of the zirconium alloy fuel rods includes, by weight ratio: 1.0% - 1.5% of Sn, 0.18% - 0.24% of Fe, 0.07% - 0.13% of Cr, 0.09% - 0.16% of O, 0.007% - 0.012% of Si, and the balance is Zr and inevitable impurities, wherein 0.25% < Fe + Cr < 0.37%; The method includes the following steps: Step a): Provide a zirconium alloy fuel rod cladding, splice the end plug of the zirconium alloy fuel rod with the zirconium alloy fuel rod cladding and fix it with a fixture, perform an exhaust treatment in a vacuum environment, and then provide an inert gas environment; Step b): Perform TIG welding on the connection position between the zirconium alloy fuel rod cladding and the end plug of the zirconium alloy fuel rod, and simultaneously provide ultrasonic stirring and electromagnetic stirring to the welding molten pool; Step c): Perform TIG welding to plug the inflation hole of the end plug of the zirconium alloy fuel rod, and simultaneously provide ultrasonic stirring and electromagnetic stirring to the welding molten pool; Step d): Grind and perform quality inspection on the welding area.

[0005] The zirconium alloy used for the end plug bar has a relatively high content of Sn, which can reduce the sensitivity to impurities introduced during the welding process when welding the cladding and end plug, and offset the harmful effects of impurities such as N, C, and Al on the corrosion resistance. At the same time, appropriate contents of Fe and Cr are used to reduce the number of exchanges between oxygen ions and vacancies during the corrosion process by pinning the extra vacancy concentration generated during the welding temperature rise process, thereby slowing down the exchange speed and improving the corrosion resistance of the weld. Since the end plug welding process of the zirconium alloy fuel rod cannot be carried out in a vacuum, defects such as bubbles will inevitably appear in the weld structure using conventional welding methods. The research of the present invention finds that when using ultrasonic stirring-assisted welding while performing gas shielded welding, the cavitation effect is limited to the surface layer of the molten pool and is not sufficient to eliminate the pores in the deep layer of the zirconium alloy molten pool; only using electromagnetic stirring-assisted welding, the Lorentz force has a low migration efficiency for micron-sized pores and is prone to form large pores; using ultrasonic stirring and electromagnetic stirring-assisted welding simultaneously can effectively eliminate pores and improve the welding quality.

[0006] Further, in some embodiments, in the said step b) and the said step c), after the TIG welding starts, the welding current is controlled to be 50A - 60A.

[0007] Further, in some embodiments, in the said step b) and the said step c), after the TIG welding starts, the ultrasonic vibration frequency is set to 30kHz - 50kHz, and the amplitude is 15μm; the electromagnetic stirring magnetic field intensity is set to 1mT - 10mT, and the frequency is 20Hz - 80Hz.

[0008] Further, in some embodiments, in the said step b) and the said step c), during the TIG welding arc extinguishing stage, the current decreases at a rate of 4.5A / s - 5.5A / s, and at the same time, the ultrasonic amplitude and the electromagnetic field intensity are gradually reduced.

[0009] Further, in some embodiments, in the said step b) and the said step c), during the TIG welding process, a two-color infrared thermometer and a PID controller are used together to perform closed-loop control on the welding process.

[0010] Further, in some embodiments, in the said step b) and the said step c), during the TIG welding process, when the flow rate of the molten pool is lower than 0.3m / s, the magnetic field intensity of the electromagnetic stirring is increased.

[0011] Further, in some embodiments, in the said step a), the inert gas environment is a helium gas environment of 0.5MPa - 1MPa, and the helium gas concentration is not less than 99.99%.

[0012] Further, in some embodiments, the composition of the fuel rod cladding by weight percentage includes: 0.1% - 0.5% of Nb, 0.3% - 0.9% of Sn, 0.2% - 0.4% of Fe, 0.01% - 0.3% of Cr, 0.09% - 0.16% of O, and at least one of 0.015% - 0.03% of Si, 0.01% - 0.2% of V, and 0.01% - 0.06% of Ge, with the balance being Zr and unavoidable impurities.

[0013] The cladding material itself should have good corrosion resistance.

[0014] Further, in some embodiments, the inner diameter of the fuel rod cladding is 4 mm - 15 mm, the outer diameter of the part of the fuel rod end plug inserted into the fuel rod cladding is 3.5 mm - 15 mm, and the mating clearance is 0 - 0.5 mm.

[0015] According to an embodiment of another aspect of the present invention, there is provided a zirconium alloy fuel rod, including a fuel rod cladding and an end plug, welded by the zirconium alloy fuel rod end plug composite welding method provided in any of the foregoing embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a micrograph of the welding area of the zirconium alloy end plug plug hole welding in an embodiment; Figure 2 It is an elemental distribution map of the welding area of the zirconium alloy end plug plug hole welding in an embodiment; Figure 3 It is a micrograph of the welding area of the zirconium alloy end plug plug hole welding in a comparative example; Figure 4 It is the corrosion weight gain test curve of an embodiment and a comparative example; Figure 5 It is a photograph of a zirconium alloy fuel rod sample in an embodiment.

[0017] 1 - air hole; 2 - end plug; 3 - weld seam; 4 - fuel rod cladding.

[0018] The purpose of the above-mentioned drawings is to make a detailed description of the present invention so that those skilled in the art can understand the technical concept of the present invention, rather than aiming to limit the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] The present invention will be further described in detail below through specific embodiments in conjunction with the drawings.

[0020] References to "embodiments" in this document mean that specific features, structures, or characteristics described in connection with the embodiments can be included in at least one embodiment of this document. The phrase appearing at various positions in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive independent or alternative embodiments. Those skilled in the art should be able to understand that the embodiments in this document can be combined with other embodiments without structural conflicts.

[0021] In the description of this document, "a plurality of" means at least two.

[0022] Pressurized water reactors rely on the chain fission reaction of uranium oxide fuel pellets in the fuel assemblies to generate heat. The primary coolant transfers the heat to the secondary coolant water in the steam generator to produce steam to drive the steam turbine to do work and output electricity. In this process, the fuel rods need to be in a high-temperature and high-pressure liquid environment for a long time and maintain stable strength and complete structure to effectively contain the fuel pellets and prevent fission products from leaking into the primary coolant. Therefore, the reliability of the fuel rod cladding is crucial for the safety of pressurized water reactors. Zirconium alloys have a low thermal neutron absorption cross-section, good corrosion resistance, and good high-temperature mechanical properties and welding properties. Therefore, they are currently the most commonly used fuel rod cladding materials. During the manufacturing process of fuel rods, welding is required to seal the fuel pellets inside the fuel rods. The welding between the tubular fuel rod cladding and the upper and lower end plugs is a circumferential weld. An inflation hole is also provided on the upper end plug for filling helium into the fuel rod before sealing. Finally, the inflation hole needs to be sealed by plug welding. During the welding process, if the welded area shows a coarse-grained structure, it will seriously affect the corrosion resistance of the zirconium alloy; on the other hand, as Figure 3 shown, since the welding needs to be carried out in a helium atmosphere, directly performing TIG welding on zirconium alloy fuel rods is likely to result in pores 1 in the welded structure, affecting the welding quality.

[0023] To overcome the above problems, an embodiment of one aspect of the present invention provides a zirconium alloy end plug composite welding method, which can effectively improve the welding quality of the welded structure of zirconium alloy fuel rods, avoid defects such as pores, and improve the corrosion resistance of the welded structure.

[0024] In a preferred embodiment, the zirconium alloy fuel rod end plug composite welding process is as follows: A end plug is provided. The components of the end plug by weight percentage include: 1.0% - 1.5% of Sn, 0.18% - 0.24% of Fe, 0.07% - 0.13% of Cr, 0.09% - 0.16% of O, 0.007% - 0.012% of Si, and the balance is Zr and inevitable impurities, where 0.25% < Fe + Cr < 0.37%; after the end plug is formed, it is annealed at 550°C - 780°C for 1h - 8h to form a fully recrystallized structure, in which Zr(Fe,Cr)2 second-phase particles with an average size of 100nm - 230nm are distributed. In a preferred embodiment, the components of the fuel rod cladding by weight percentage include: 0.1% - 0.5% of Nb, 0.3% - 0.9% of Sn, 0.2% - 0.4% of Fe, 0.01% - 0.3% of Cr, 0.09% - 0.16% of O, and at least one of 0.015% - 0.03% of Si, 0.01% - 0.2% of V, 0.01% - 0.06% of Ge, and the balance is Zr and inevitable impurities; after the fuel rod cladding is formed, it is annealed at 450°C - 600°C for 1h - 8h to form a partially recrystallized structure, in which second-phase particles with an average size of 50nm - 180nm are distributed. According to different specific components, the types of second-phase particles include Zr(Fe,Nb,Cr)2, Zr2(Fe,Nb,Cr), and / or Zr(Fe,Nb,V)2, Zr2(Fe,Nb,V), and / or Zr(Fe,Nb,Cr,Ge)2, Zr2(Fe,Nb,Cr,Ge).

[0025] The fuel rod cladding and the end plug are cleaned and dried to remove surface oxides, oil stains, dust, etc. The fuel rod cladding and the end plug are spliced together so that the welding surfaces cooperate with each other, and are fixed using a special fixture to prevent deformation or displacement during the welding process. Next, a vacuum treatment is carried out in a sealed welding chamber to remove the air inside the fuel rod cladding, and then an inert gas is backfilled. In a preferred embodiment, helium with a purity of more than 99.99% and a pressure of 0.7MPa is backfilled into the welding chamber. After the pressure stability is verified through a pressure stabilization test, subsequent welding operations are carried out.

[0026] Next, welding is carried out. First, TIG welding is performed on the connection position between the fuel rod cladding and the end plug. After welding is completed, plug welding is carried out on the gas filling hole of the upper end plug. Specifically, an arc starter dedicated to zirconium alloy fuel rod welding is used to start the arc, overcoming the problems of high ionization potential and high excitation potential of helium gas to ensure stable ignition of the arc. After the TIG welding arc is started, the welding current is controlled at 50A - 60A; after the arc is started, ultrasonic stirring and electromagnetic stirring are simultaneously provided to the welding molten pool area. In a preferred embodiment, the ultrasonic vibration frequency is set at 30kHz - 50kHz, and the amplitude is 15μm; the electromagnetic stirring magnetic field strength is set at 1mT - 10mT, and the frequency is 20Hz - 80Hz. During the welding process, ultrasonic vibration can stir the molten pool, promoting full melting and flow of the metal and more precisely controlling the energy input during the welding process; electromagnetic stirring can enhance the convection of the molten pool, transporting inclusions in the molten pool to the edge of the molten pool, effectively eliminating weld porosity and improving the uniformity of element distribution in the welding area; further, the present invention finds that when ultrasonic stirring is used to assist welding during gas shielded welding, the cavitation effect is limited to the surface layer of the molten pool and is not sufficient to eliminate the pores in the deep layer of the zirconium alloy molten pool; only using electromagnetic stirring to assist welding, the Lorentz force has a low migration efficiency for micron-sized pores and is prone to form large pores; while using ultrasonic stirring and electromagnetic stirring to assist welding simultaneously can effectively eliminate pores and improve the welding quality.

[0027] In a preferred embodiment, a two-color infrared thermometer and a PID controller are used together to perform closed-loop control on the TIG welding process, dynamically adjusting the arc length, helium gas flow rate, and welding speed to achieve accurate control of welding parameters. In a further preferred embodiment, when it is detected that the flow velocity of the molten pool is lower than 0.3m / s, the magnetic field strength of the electromagnetic stirring is increased to further refine the grains and promote the discharge of bubbles in the molten pool.

[0028] In the arc extinguishing stage, the welding current is reduced at a rate of 5A / s, and the ultrasonic vibration amplitude and the electromagnetic stirring magnetic field strength are gradually reduced synchronously to avoid microcracks at the weld caused by sudden cooling of the molten pool.

[0029] After welding is completed, the welding area should be fully cooled, and the cooling time is not less than 20s. Subsequently, the weld is polished, and the parts with excessive surface undulation of the weld structure are polished and trimmed. The quality of the circumferential weld and the plug weld is inspected by means of visual inspection, go / no-go gauge inspection, X-ray inspection, corrosion performance inspection, and metallographic sampling inspection.

[0030] In a preferred embodiment when the welding operation is correct, the macroscopic morphology of a fuel rod welding sample (not full length) is as Figure 5 shown, where a uniform and defect-free weld 3 is formed between the end plug 2 and the fuel rod cladding 4. The plug welding area in the end plug 2 is as Figure 1As shown, it can be seen that the tissue morphology is uniform, without defects such as pores, cracks, and inclusions. The element distribution in the welding area is as Figure 2 shown. It can be seen that the elements such as N, O, Fe, and Sn are evenly distributed without segregation and do not aggregate at the welding interface.

[0031] The fuel rod provided in the embodiment of the present invention and the comparative sample using a ZIRLO alloy fuel rod cladding and a Zr-4 alloy end plug are jointly placed in a steam environment at 427 °C and 10.3 MPa for 100 days of corrosion. The sample weight gain curve is as Figure 4 shown. It can be seen that compared with the comparative example, the corrosion resistance of the embodiment is improved by 28.8%.

[0032] The composite welding method provided in the embodiment of the present invention utilizes the coupling effect of high-frequency ultrasonic stirring and electromagnetic stirring, making the input energy more focused on the welding area, improving the energy utilization efficiency, improving the dimensional accuracy and welding quality of the end plug circumferential welding and plug hole welding, promoting the full flow of the molten metal in the molten pool, effectively eliminating the bubbles generated during the welding process, promoting the uniform distribution of elements in the welding area, and reducing defects such as pores and cracks after welding. It should be understood that during the welding process, the frequency, amplitude of the ultrasonic vibration, and the magnetic field strength of the electromagnetic stirring need to be reasonably restricted, otherwise it will cause deformation damage to the material or welding arc deflection, resulting in welding defects.

[0033] The purpose of the above embodiments is to further elaborate on the present invention in combination with the drawings, so that those skilled in the art can understand the technical concept of the present invention. Within the scope disclosed by the present invention, optimizing or equivalently replacing the involved method steps, and combining the implementation manners in different embodiments without conflict in structure and principle all fall within the protection scope of the present invention.

Claims

1. A composite welding method for the end plug of a zirconium alloy fuel rod, characterized in that the composition of the end plug of the zirconium alloy fuel rod by weight percentage includes: 1.0% - 1.5% of Sn, 0.18% - 0.24% of Fe, 0.07% - 0.13% of Cr, 0.09% - 0.16% of O, 0.007% - 0.012% of Si, and the balance is Zr and unavoidable impurities, wherein 0.25% < Fe + Cr < 0.37%; the composite welding method for the end plug of the zirconium alloy fuel rod includes the following steps: Step a): Provide a zirconium alloy fuel rod cladding, splice the end plug of the zirconium alloy fuel rod with the zirconium alloy fuel rod cladding and fix it with a fixture, perform exhaust treatment in a vacuum environment, and then provide an inert gas environment; Step b): Perform TIG welding on the connection position between the zirconium alloy fuel rod cladding and the end plug of the zirconium alloy fuel rod, and simultaneously provide ultrasonic stirring and electromagnetic stirring for the welding molten pool; Step c): Perform TIG welding to plug the gas injection hole of the end plug of the zirconium alloy fuel rod, and simultaneously provide ultrasonic stirring and electromagnetic stirring for the welding molten pool; Step d): Grind and perform quality inspection on the welding area.

2. The zirconium alloy fuel rod end plug composite welding method according to claim 1, characterized in that In the said step b) and step c), after the TIG welding arc is struck, the welding current is controlled to be 50A - 60A.

3. The zirconium alloy fuel rod end plug composite welding method according to claim 2, characterized in that, In the said step b) and step c), after the TIG welding arc is struck, the ultrasonic vibration frequency is set to 30kHz - 50kHz, and the amplitude is 15μm; the electromagnetic stirring magnetic field intensity is set to 1mT - 10mT, and the frequency is 20Hz - 80Hz.

4. The zirconium alloy fuel rod end plug composite welding method according to claim 2 or 3, characterized in that, In the said step b) and step c), in the TIG welding arc extinguishing stage, the current decreases at a rate of 4.5A / s - 5.5A / s, and at the same time, the ultrasonic amplitude and the electromagnetic field intensity are gradually reduced.

5. The zirconium alloy fuel rod end plug composite welding method according to claim 2 or 3, characterized in that, In the said step b) and step c), during the TIG welding process, a two-color infrared thermometer and a PID controller are used together to perform closed-loop control on the welding process.

6. The zirconium alloy fuel rod end plug composite welding method according to claim 5, characterized in that, In the said step b) and step c), during the TIG welding process, when the flow rate of the molten pool is lower than 0.3m / s, the magnetic field intensity of the electromagnetic stirring is increased.

7. The zirconium alloy fuel rod end plug composite welding method according to claim 1, characterized in that, In the said step a), the inert gas environment is a helium gas environment of 0.5MPa - 1MPa, wherein the helium concentration is not less than 99.99%.

8. The zirconium alloy fuel rod end plug composite welding method according to claim 1, characterized in that, The composition of the fuel rod cladding by weight percentage includes: 0.1% - 0.5% of Nb, 0.3% - 0.9% of Sn, 0.2% - 0.4% of Fe, 0.01% - 0.3% of Cr, 0.09% - 0.16% of O, and at least one of 0.015% - 0.03% of Si, 0.01% - 0.2% of V, 0.01% - 0.06% of Ge, and the balance is Zr and unavoidable impurities.

9. The zirconium alloy fuel rod end plug composite welding method according to claim 1, characterized in that The inner diameter of the fuel rod cladding is 4mm - 15mm, the outer diameter of the part of the end plug of the fuel rod inserted into the fuel rod cladding is 3.5mm - 15mm, and the fit clearance is 0 - 0.5mm.

10. A zirconium alloy fuel rod, comprising a fuel rod cladding and end plugs, characterized in that, The fuel rod cladding and the end plug are welded by the composite welding method for the end plug of the zirconium alloy fuel rod as described in any one of claims 1 to 9.

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