JPH08122472A - End plug of tube and welding method thereof - Google Patents

Abstract

PURPOSE: To enhance a welding strength of an end plug to a tube. CONSTITUTION: An end plug 12 is formed in a virtually columnar shape having a slightly larger outside diameter than the one of a cladding tube 1 and a tapered contact face 14 is provided on the outer peripheral side of a fore end face 12a thereof on the cladding tube 1 side. When the fore end face 12a of the end plug 12 is brought into contact with the cladding tube 1 concentrically substantially, the contact face 14 is brought into ring-shaped linear contact with an outer peripheral edge part 1c of the cladding tube 1. When a pair of electrodes 4 and 5 are brought into pressure contact with the outer peripheral surface of the cladding tube 1 oppositely and electrified, a current (i) at the time of start of welding flows at the shortest distance along the outer peripheral surface of the cladding tube 1 from the electrode and runs to the end plug 12 through a welding part wherein the outer peripheral edge part 1c and the contact face 14 are in pressure contact with each other, so that the welding be performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an end plug of a pipe, particularly an end plug of a fuel rod or the like fixed to the end of a cladding pipe by resistance welding in the process of manufacturing a fuel rod of a nuclear fuel assembly. Regarding the welding method.

[0002]

2. Description of the Related Art A fuel rod used as a fuel for a nuclear reactor has a large number of fuel pellets inserted in a cylindrical cladding tube and each end plug is fixed and sealed at both ends of the cladding tube. In the cladding tube, a plenum spring is interposed between the end plug on one end side and the fuel pellet to press and hold the fuel pellet on the other end side. As a method of welding the end plugs of the fuel rod having such a structure to the cladding tube, a resistance welding method as shown in, for example, FIGS. 4 to 6 has been adopted.

In this welding method, first, as shown in FIG. 4, a taper surface 2 formed at the tip end of a columnar end plug 3 is fitted to the end surface 1a of a coating tube 1. In the fitted state, the tapered surface 2 of the end plug 3 is held in line contact with the inner peripheral edge portion 1b of the end surface 1a of the covering tube 1. still,
Usually, the cladding tube 1 and the end plug 3 are made of a zirconium alloy (zircaloy) or the like. Further, the tip ends of a pair of fixed electrodes 4 and 5, for example, are brought into contact with the outer peripheral surface of the cladding tube 1 at a constant pressure, and the electrodes 4 and 5 are connected to respective output terminals of a welding power source (not shown). There is. Each electrode 4, 5
The end edges 4a and 5a close to the end plug 3 of No. 3 are chamfered, which contributes to prevention of interference when inserting the cladding tube 1 between the electrodes 4 and 5 and suppression of growth of the bulging portion during welding. ing. Then, while the tapered surface 2 of the end plug 3 is brought into pressure contact with the inner peripheral edge portion 1b of the cladding tube 1, a current is passed between the electrodes 4 and 5,
As shown in FIG. 2, between the cladding tube 1 and the end plug 3, as shown by the broken line, from the outer peripheral surface of the cladding tube 1 to the inner peripheral edge 1b of the end surface 1a.
And the tapered surface 2 of the end plug 3 as a path, a current i flows,
The contact surfaces of both are resistance welded. According to this welding method, as shown in FIG. 6, the welded portion 6 between the cladding tube 1 and the end plug 3 is welded.
Bulging portions (swelling) 7 and 8 are formed on the outer peripheral surface and the inner peripheral surface due to the pressure applied during welding. The outer bulging portion 5 will be removed by the subsequent grinding.

[0004]

By the way, in the welding method of the end plug 3 as described above, when the current i flows between the cover pipe 1 and the end plug 3, as shown in FIG. A current path is formed across the tube 1 in an oblique direction from the outer peripheral surface to the inner peripheral edge portion 1b. Therefore, the current path becomes relatively long, and the heat generated by the contact resistance at the contact surface spreads widely over the entire tapered surface 2, so that the progress of melting of the welded portion on the end plug 3 side progresses toward the cladding tube 1 side. However, there is a problem that the bond line 10 forming the boundary of the welding material as shown in FIG. In particular, the welded portion 6 where such a bond line 10 remains has a drawback that the weld strength is not sufficient.

In view of such circumstances, it is an object of the present invention to provide a pipe end plug and a welding method for the end plug, which can achieve resistance welding with high welding strength in a short energizing time. It is a thing.

[0006]

SUMMARY OF THE INVENTION A pipe end plug according to the present invention is an end plug to be welded to the end of the pipe by a resistance welding method, the end plug being attached to the outer peripheral edge of the end face of the pipe during welding. It is characterized in that a contact surface for contact is formed. The outer surface of the contact surface of the end plug is equal to or slightly larger than the diameter of the outer peripheral edge of the end surface of the tube, and at least the contact area with the outer peripheral edge of the end surface of the tube is tapered. It is a feature. The tube is a cladding tube of a fuel rod, and the contact surface of the end plug is in line contact with the outer peripheral edge of the end surface of the tube. A method for welding an end plug according to the present invention is a method for welding an end plug to an end portion of a pipe by a resistance welding method, wherein the end plug has an outer peripheral edge of the end face of the pipe due to its abutting surface. It is characterized in that, while being pressure-welded to the electrode, the electrode abutted on the outer peripheral surface of the tube is energized for resistance welding. The outer diameter of the contact surface of the end plug is equal to or slightly larger than the diameter of the outer peripheral edge of the end surface of the tube, and at least the contact area with the outer peripheral edge of the end surface of the tube is tapered. The tube may be a cladding tube of a fuel rod, and the abutting surface of the end plug may be welded while being in line contact with the outer peripheral edge of the end surface of the tube.

[0007]

In the end plug according to the present invention, when the welding electrode is pressed against the outer peripheral surface of the pipe when the contact surface of the end plug comes into contact with the outer peripheral edge of the end face of the pipe, the current flows when the welding electrode is energized. Flows along the outer peripheral surface of the pipe almost parallel to the axis, the current path between the pipe and the end plug becomes short, and a large current flows in a short time, and at the same time, the contact part on the end plug abuts. Since it is near the tip edge of the surface, the heat generated by the contact resistance at the contact portion is concentrated not only on the pipe side but also on the end portion side. Therefore, the bond line is hard to remain in the welded part. At least the contact area of the end plug with the outer peripheral edge of the end surface of the pipe is formed in a tapered shape, so that the stroke of the end plug in the pressure contact state during welding becomes large, and a sufficient bulge portion is formed. Secured and highly airtight welding is performed. Therefore, it is highly useful as an end plug of a fuel rod that requires high welding strength and airtightness.

In the welding method of the end plug according to the present invention, resistance welding is performed by energizing the electrode in contact with the outer peripheral surface of the pipe in a state where the end plug is pressed against the outer peripheral edge of the end face of the pipe by the contact surface. Since the current flows along the outer peripheral surface of the pipe, the current path between the pipe and the end plug is short, and a large current flows in a short time, and at the same time, the heat generated by the contact resistance at the contact portion causes heat generation on the pipe side and the end. Since it concentrates on the plug side, especially at the initial stage of welding, the weld portion of the pipe and the end plug is melted well, and the bond line is unlikely to remain at the weld portion. Further, due to the shape of the contact surface of the end plug, the stroke of the pressure contact of the end plug during welding becomes large, so that a sufficient bulging portion is secured and welding with high airtightness is performed. Therefore,
It is highly useful as a welding method for a fuel rod end plug having high welding strength and high airtightness over the entire circumference of the welding region between the cladding of the fuel rod and the end plug.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 to 3, but the same parts or members as those in the above-described conventional art will be designated by the same reference numerals and the description thereof will be omitted. FIG. 1 is a sectional view of a main part showing a fitting structure of a cladding tube and an end plug before welding, FIG.
3 is an enlarged view of a B portion in FIG. 1, and FIG. 3 is a view similar to FIG. 2 after welding. At the stage before welding the cladding tube 1 and the end plug 12 shown in FIG. 1, the end plug 12 has a substantially cylindrical shape with an outer diameter slightly larger than the outer diameter of the cladding tube 1 (for example, about 4 mm in diameter). A truncated cone-shaped recess is formed in the tip end surface 12a on the cladding tube 1 side, and is constituted by a central circular bottom surface 13 and a tapered ring-shaped contact surface 14 on the outer peripheral side thereof. Then, with the tip end surface 12a of the end plug 12 being brought into contact with the cladding tube 1 in a substantially concentric manner, the contact surface 14
Is in contact with the outer peripheral edge portion 1c of the cladding tube end surface 1a at a position slightly inside the outer peripheral edge thereof, and is substantially in line contact in a ring shape. The cladding tube 1 and the end plug 12 are each made of a zirconium alloy (zircaloy) or the like as in the prior art.

The end plug of the fuel rod according to this embodiment has the above-mentioned structure, and the welding method will be described below. First, when the end plug 12 is brought into pressure contact with the end surface 1a of the cladding tube 1 in a substantially concentric manner, the vicinity of the tip edge of the tapered contact surface 14 of the tip surface 12a of the end plug 12 is shown in FIGS. Thus, the outer peripheral edge portion 1c of the cladding tube end surface 1a is held in line contact (may be surface contact). For example, a pair of electrodes 4 and 5 are pressed against the outer peripheral surface of the end of the cladding tube 1 at positions facing each other, and these electrodes 4 and 5 are connected to the output terminals of a welding power source (not shown), respectively. It is similar to the prior art. In this state, when current is applied between the electrodes 4 and 5 under the same conditions as the conventional welding method (welding current value, energizing time, pressurizing force, etc.), the current i at the start of welding is, for example, as shown in FIG. , Flows from the electrode 4 along the outer peripheral surface of the cladding tube 1 in a direction that is substantially parallel to the axis of the cladding tube 1, and the outer peripheral edge portion 1c
The end plug 12 is energized through the welding portion where the contact surface 14 and the contact surface 14 are in contact with each other to perform welding.

According to this embodiment, the current path between the cladding tube 1 and the end plug 12 is shortened to the shortest distance as compared with the current path of the above-mentioned prior art, and the conventional method can be used in a short time. Since a larger amount of current can be passed, it is suitable for resistance welding.
In particular, at the initial stage of welding, the current is concentrated on the outer peripheral edge 1c of the end face on the cladding side, and the current does not flow on the end plug side in the vicinity of the front edge of the contact surface 14, so that heat generation does not diffuse and contact resistance is reduced. The heat generation part due to the heat is concentrated in a narrow contact portion between the cladding tube 1 side and the end plug 12 side, so that not only the metal on the cladding tube 1 side but also the metal on the end plug 12 side rapidly progresses. It will be. In addition, since the welding between the coating pipe 1 and the end plug 12 proceeds from the outer peripheral side toward the inner peripheral side, melting proceeds efficiently and well on both the coating pipe 1 side and the end plug 12 side. . Therefore, a bond line does not appear in the fusion zone 6 and high welding strength can be obtained. Also, end plug 1
On the second side, the abutting surface 14 forming the fusion portion is formed in a tapered shape on the inner peripheral surface side so as to form a substantially triangular cross section having a contact point with the outer peripheral edge portion 1c of the cladding tube 1 as an apex. From
A stroke of pressure welding at the time of welding becomes large, and a sufficient volume is secured in the bulging portion. Therefore, the welded part 6 having higher airtightness can be obtained. In this way, the end plug 12 is resistance-welded to the covering pipe 1, and after cooling, the bulging portion 7 on the outer peripheral side is removed by grinding, and preferably the outer peripheral region larger than the outer diameter of the covering pipe 1 of the end plug 12 is also ground. Remove by.

As described above, according to this embodiment, the welding current path between the electrodes 4, 5 and the cladding tube 1 and the end plug 12 is shortened to the shortest distance, so that a large current can be supplied in a short time. It is possible to flow in a concentrated manner, especially at the initial stage of welding, the heat generated by the contact resistance at the contact portion is concentrated on the pipe side and the end plug side, so the metal in the weld portion can be efficiently and quickly melted. hand,
There are advantages that a bond line is difficult to form and the strength of the welded portion 6 is high. Moreover, since the melting of the end plug 12 proceeds from the tapered tip end side of the tapered contact surface 14, that is, from the outer peripheral side to the inner peripheral side, the stroke of the pressure contact of the end plug 12 increases, and the airtightness is further improved. A welded portion 6 having a high height can be obtained. Further, since the outer diameter of the contact surface 14 of the end plug 12 is formed to be slightly larger than the outer diameter of the covering pipe 1, the end plug 12 can be easily pressure-contacted to the covering pipe 1 with high accuracy.

It should be noted that the outer diameter of the end plug 12 in the above-described embodiment is formed to be larger than the outer diameter of the cladding tube 1 only at the tip end surface 12a, and the outer diameter of the rear region thereof may be the same as the outer diameter of the cladding tube 1. Well, in this case, grinding after welding becomes easy. Alternatively, the outer diameter of the entire length of the end plug 12 may be the same as the outer diameter of the cladding tube 1. Further, the shape of the tip end surface 12a of the end plug 12 is not limited to the truncated conical recess, and may be a conical recess without the bottom surface 13. Alternatively, it may be formed as a spherical recess.

Although the end plugs 12 of the fuel rods of the fuel assembly have been described in the above embodiments, the present invention is not limited to this and can be used for welding end plugs of various pipes.

[0015]

As described above, according to the end plug of the pipe of the present invention, the end plug is formed with the contact surface that comes into contact with the outer peripheral edge of the end face of the pipe during welding. Since the current flows along the outer peripheral surface of the tube, the current path between the tube and the end plug becomes short, a large current flows in a short time, and heat due to contact resistance is generated at the contact part on the tube side and the end plug side. Because it occurs intensively,
Especially in the initial stage of welding, the fusion of the welded part of the pipe and the end plug progresses well,
Welding progresses from the outer peripheral side to the inner peripheral side, bond lines are less likely to remain in the welded portion, and the weld strength is high. The contact surface of the end plug has an outer diameter that is the same as or slightly larger than the outer diameter of the outer peripheral edge of the end surface of the pipe, and the contact area with the outer peripheral edge of the end surface is tapered. The accuracy of pressure welding to the pipe is high, and in welding, the stroke of pressure welding of the end plug is increased, a bulging portion is sufficiently secured, and at the same time, welding with high airtightness is performed, and welding strength is increased. The tube is a cladding tube for fuel rods, and the contact surface of the end plug makes a line contact with the outer peripheral edge of the end surface of the tube. Therefore, it is useful as a cladding tube for fuel rods that requires high welding strength and airtightness. The nature is great.
Further, according to the welding method of the end plug of the present invention, the end plug is energized to the electrode abutting on the outer peripheral surface of the pipe in a state of being pressed against the outer peripheral edge of the end surface of the pipe by the abutting surface thereof. Since resistance welding is used, the current flows along the outer peripheral surface of the tube, so the current path between the tube and the end plug is shortened, a large current flows in a short time, and heat generation due to contact resistance occurs. Since it occurs intensively at the contact part on the pipe side and the end plug side, melting of the weld part of the pipe and end plug progresses particularly well at the initial stage of welding, welding progresses from the outer peripheral side to the inner peripheral side, and bond to the weld part. There is an advantage that the welding strength is high without the appearance of lines.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a main part of a fuel rod according to an embodiment of the present invention showing a fitting state between an end portion of a cladding tube and an end plug before welding.

FIG. 2 is an enlarged view of a B part in FIG.

FIG. 3 is a view similar to FIG. 2 after welding.

FIG. 4 is a cross-sectional view of a main part of a conventional fuel rod showing a fitted state of an end portion of a cladding tube and an end plug before welding.

5 is an enlarged view of a portion A in FIG.

FIG. 6 is a view similar to FIG. 5 after welding.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Clad tube 1c Outer peripheral edge 4,5 Electrode 6 Welded portion 12 End plug 12a Tip surface 14 Tapered surface

Claims (4)

[Claims] 1. An end plug to be welded to an end of a pipe by a resistance welding method, wherein the end plug is formed with an abutting surface that abuts an outer peripheral edge of an end face of the pipe during welding. End plug to do. 2. The contact surface of the end plug is formed such that the outer diameter thereof is the same as or slightly larger than the outer diameter of the outer peripheral edge of the end surface of the tube, and at least the contact area with the outer peripheral edge of the end surface of the tube is tapered. The end plug according to claim 1, wherein the end plug is formed in 3. The end according to claim 1, wherein the tube is a cladding tube for a fuel rod, and the contact surface of the end plug makes line contact with the outer peripheral edge of the end surface of the tube. plug. 4. A welding method of an end plug, wherein an end plug is welded to an end of a pipe by a resistance welding method, wherein the end plug is pressed against an outer peripheral edge of the end face of the pipe by a contact surface thereof. In this state, energize the electrode that is in contact with the outer peripheral surface of the tube,
A method for welding end plugs, characterized in that resistance welding is performed.