JPH0318482A - Method for welding stainless steels containing boron - Google Patents

Abstract

PURPOSE:To equalize toughness of a weld zone with that of base metal and to improve safety of a welded structure by subjecting the weld zone or the vicinity of the weld zone to forced cooling while stainless steels containing boron being welded. CONSTITUTION:While the stainless steels containing boron being welded, the weld zone or the vicinity of the weld zone is subjected to forced cooling, hence a boride molten by welding heat is cooled quickly. As a result, the growth of particles of the boride is hindered and coarsening is prevented and it is made possible to equalize toughness of the weld zone with that of the base metal and safety of the welded structure can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for welding stainless steel containing boron, and particularly for high-density storage of spent fuel in fuel storage facilities of nuclear power plants. The present invention relates to a method for welding boron-containing stainless steel suitable for improving the strength of welded joints of racks for storage, and a method for manufacturing spent fuel storage racks.

[Conventional technology]

Austenitic stainless steel is produced by supersaturated carbon (carbon
C) is a chromium (Cr)-based carbohydrate (Cr23
C,) precipitates near the grain boundaries, resulting in the formation of a Cr-depleted layer, which causes a phenomenon in which the corrosion resistance of the grain boundaries deteriorates. Conventional water-cooled welding of austenitic stainless steel was aimed at suppressing the precipitation of Cr carbides in the above temperature range due to the heat of the welder (for example, the
6-53881). In addition, in order to prevent stress corrosion cracking in the weld heat affected zone of austenitic stainless steel, for example, in welding a tube, in order to apply compressive stress to the side exposed to the corrosive environment, the inner surface of the tube is cooled and the outer surface is A heating technique is also known (see Japanese Patent Laid-Open No. 75140/1983). However, the above conventional technology is aimed at improving the corrosion resistance of welded parts, and is used to improve the corrosion resistance of materials such as boron-containing stainless steel in the weld heat-affected zone of materials that contain eutectic (borides) that have a lower melting point than the base metal. No consideration was given to preventing the bolide from becoming coarse.

Conventional spent fuel storage racks are known, for example, from U.S. Pat.
As described in No. 19859, rectangular cylinders made of boron-containing stainless steel with a square cross section perpendicular to the axial direction are arranged in a checkered pattern so that the cross section is horizontal,
At this time, the vertical corner of the rectangular cylinder and the corner of the adjacent rectangular cylinder are connected using a metal piece bent into an L-shape or a two-shape. This connection was performed by fillet welding of a groove formed between the end of the metal piece and the side surface of the rectangular cylinder.

[Problem to be solved by the invention]

The above conventional technology uses boron-containing stainless steel as the material. No consideration was given to the presence of coarse boride (Fe, Cr) and B in the heat-affected zone of the weld, which would reduce toughness.

Boron-containing stainless steel used in fuel racks intended for high-density spent fuel storage is a material in which about 0.5 to 0.7 weight percent of boron is added to austenitic stainless steel. For this reason, boron is eutectic (F 6
, cr) 2B (hereinafter referred to as "boride"), which is dispersed in the material in the form of fine particles. This eutectic is known to melt at a relatively low temperature (approximately 1300°C), and when this material is welded, the boride melted in the heat affected zone of welding gradually crystallizes after welding, but at room temperature - Under air cooling conditions, crystal grains become coarser and toughness decreases. In particular, in the conventional technology (US Pat. No. 4,119,859), adjacent rectangular cylinders are connected by welding using L-shaped or double-shaped metal pieces, so when a load is applied due to an earthquake, etc. The load is transmitted to the welded portion as a bending load, and the load must be supported by the heat-affected zone, which has reduced toughness, and the structure must be strengthened.

An object of the present invention is to provide a highly reliable spent fuel storage rack with less deterioration in toughness of the welded heat affected zone. [Means for Solving the Problems] The above problems can be achieved by a method for welding boron-containing stainless steel, which is characterized by forcibly cooling the welded portion or the vicinity of the welded portion during welding.

The above problem is also solved by the fact that rectangular cylinders made of boron-containing 18Cr-8Ni stainless steel are arranged so that the cross section perpendicular to the axial direction of the rectangular cylinder has a checkerboard pattern, and adjacent In the method for manufacturing a spent fuel storage rack in which the square cylinders are connected to each other by welding via metal pieces to form a spent fuel storage rack, the welding part or the vicinity of the welding part is forced during the welding. This is achieved by cooling. Furthermore, the above-mentioned problem includes a tube to which cooling fluid is supplied, and a plurality of nozzles protruding from the outer surface of the tube in the radial direction and along the generatrix of the tube, and the nozzles are connected to each other between rectangular cylinders. It is also achieved by a cooling device used for welding boron-containing stainless steel, which is provided at the same pitch as the welding pitch of the metal pieces connecting the two.

[Effect]

During welding of boron-containing stainless steel, the welded part or the vicinity of the welded part is forcedly cooled, so the boride melted by welding heat is rapidly cooled. Due to rapid cooling,
The lengthening of the boride particles is prevented, and the bolide remains relatively fine-grained after cooling. Therefore, the dispersion strengthening effect of the boride is ensured, and a decrease in toughness is avoided.

When welding a fuel rack made of rectangular cylinders made of boron-containing stainless steel, if welding is performed while the welded part or the vicinity of the welded part is forcedly cooled, the deterioration of toughness in the vicinity of the welded part can be avoided as described above, and the structure As a result, the safety factor against external loads is improved. A plurality of nozzles are provided on the outer surface of one tube in the radial direction and along the generatrix of the tube, and the plurality of nozzles are welded in the longitudinal direction of the tube by welding metal pieces connecting the rectangular cylinders to each other. If the cooling device is provided at the same pitch as the pitch, a plurality of welded portions of the rectangular cylinder can be cooled simultaneously.

Therefore, a plurality of welding parts can be simultaneously welded while being cooled. Further, once the cooling device is placed at a predetermined position within the rectangular cylinder, a plurality of welding locations can be sequentially cooled and welded without moving the cooling device.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 to 6.

Spent nuclear fuel at nuclear power plants is reprocessed,
Or until it is subjected to subsequent processing such as permanent storage,
It is temporarily stored in the spent fuel storage pool within the power plant.

Inside the spent fuel storage pool, there is a spent fuel storage rack (hereinafter referred to as the "fuel rack") that has square pipes as its main components and stores the spent fuel with its longitudinal direction substantially vertical.
However, even after the spent fuel has been stored as planned, it is arranged in an orderly manner so that it has a storage capacity that can accommodate all the fuel in the reactor core. By the way, the capacity for subsequent processing of spent fuel is limited, so in order to carry out such work smoothly and with sufficient margin, it is necessary to change the spent fuel storage capacity of the power plant by changing the dimensions of the spent fuel storage pool. It is necessary to secure as much as possible.

To this end, it is necessary to make the spent fuel storage pitch of fuel racks within the size of the spent fuel storage pool in the power plant as small as possible to improve storage efficiency. A method for improving storage efficiency is to use a square tube made of a material to which an element with a large neutron absorption cross section is added as the constituent material of the fuel rack, and to make the square tube so that its rectangular cross section is horizontal. , the method of arranging them in a checkered pattern is effective in increasing the storage density.

Here, the square tube is made of 18Cr-8Ni stainless steel containing boron as an element with a large neutron absorption cross section (hereinafter referred to as "boron-containing stainless steel").
It is called. ) is used as the material.

Square tubes made of boron-containing stainless steel, which are the constituent members of the fuel rack, are cast into slabs after a predetermined amount of boron is added in the steel manufacturing process, and then sent to the rolling process. In the rolling process, the slab is rolled to a sheet steel having a nominal thickness of, for example, 5 mm. After the above-mentioned steel plate is made into a cylinder by pressing or rolling, the butt portions are welded, and further,
After solid solution heat treatment and strain relief, it is drawn into a square shape. In this case, the boron content is 0.5-1
．． The amount of boron added is adjusted to be 0 weight percent.

A fuel rack 2 is manufactured using the rectangular tube 1, which is a rectangular cylinder manufactured as described above. FIG. 6 shows a part of the fuel rack 2 according to the present invention. The square tubes 1 constituting the fuel rack 2 have a checkered cross section perpendicular to the axial direction in order to increase the storage density of spent fuel. It is assembled while being arranged like this.

During assembly, in order for the fuel rack 2 to have sufficient strength as a structure, the ridge 1 in the horizontal section of the square tube 1 is
a and the square tube 1 facing diagonally in the checkered pattern arrangement.
are connected to the edge 1b via an L-shaped metal piece 3. The end surface 3a of the metal piece 3 is fixed to the side surface 1c of the square tube 1 by fillet welding, as shown in FIG. The metal piece 3 is made of 18Cr-8Ni stainless steel, has a thickness of 3 mm, and is welded over the entire length of the square tube 1. However, if there is sufficient strength, the entire length of the square tube 1 may be divided at appropriate intervals. Hereinafter, a welding method in the method of manufacturing the fuel rack 2 according to the present invention will be explained.

One end surface (upper end surface) in the longitudinal direction of the square tube 1 is chamfered to improve insertion of the fuel assembly. On the other hand, a base 4 is provided with a plurality of circular openings 5 in a grid pattern on the base surface 4A for supporting spent fuel and taking in cooling water, one side of which is in contact with the surface plate, and the base surface 4A is a vertical surface. installed parallel to. Next, the lower end of the chamfered square tube 1 makes the longitudinal axis of the square tube 1 horizontal,
It is arranged so that the axis and the center of the opening 5 at the corner of the base 4 coincide with each other, and are in contact with the base surface 4A. At this time, the rectangular tube 1 is arranged so that any side of the cross section perpendicular to the axial direction is parallel to the side of the base 4 that is in contact with the surface plate. In this state, the square tube 1 is the base 4
The base surface 4A is fixed with a jig so as not to move or rotate, and the entire circumference of the lower end outer surface of the square tube 1 that is perpendicularly in contact with the base surface 4A is fillet seal welded. After fillet seal welding, as shown in FIGS. 3 and 5, a cooling device 6 having a nozzle 7 at the tip is inserted into the square tube 1, and the cooling water spouted from the nozzle 7 is applied to the fillet seal. It is placed in a position that reaches the back side of the weld.

Next, fillet welding is performed while the back surface of the fillet seal welded portion is cooled by the cooling water jetted from the nozzle 7 until the weld leg length of the fillet seal welded portion reaches a predetermined value. After the cooling water absorbs the heat caused by the fillet welding, it is discharged from the opening at the upper end of the square tube 1.

According to the above procedure, every other square tube 1 is brought into contact with the opening 5 along the side of the base 4 that is in contact with the surface plate,
They are sequentially fixed to the base 4 by welding.

Next, every other corner forming the second row of square tubes is placed in the second row of openings 5 counting from the bottom row of openings (the row of openings along the side of the base 4 that is in contact with the surface plate). The lower end of the tube 1 is brought into contact with a position where the edge of the square tube faces the edge of the lowermost square tube, and is restrained and fixed by a jig 8. The square tube on the second stage is installed so that its axis passes through the center of the opening 5 and is parallel to the axis of the square tube 1 on the lowest stage. The jig 8 is used to prevent the right angle formed by the horizontal plane of the lowest square pipe and the vertical plane of the second stage square pipe from becoming an acute angle or an obtuse angle due to welding near the ridges of the opposing square pipes. To prevent this, the square tube 1 is restrained from rotating about its axis. The jig 8 is provided at several locations in the longitudinal direction of the square tube l. FIG. 4 is an example of the jig 8,
It consists of a frame member 8A that restrains the vertical position of the square tube 1, and a positioning jig 8B that restrains the horizontal position and spacing of the screen. It is restrained by both the jig 8B.

Furthermore, in order to ensure the mutual positional accuracy of the square tubes, after the second stage square tubes are placed in a predetermined position and restrained by the jig 8,
The edges of the opposing square tubes are connected and fixed by metal pieces 3 tack welded to each square tube. This metal piece 3 may be L-shaped or may be a rectangular parallelepiped with groove-shaped grooves.

As described above, after measures have been taken to prevent deformation of the structure consisting of the base 4 and the square tube 1 by welding, the base 4 and the second stage square tube 1 are welded. When performing fillet welding between the base 4 and the square tube 1 in the second and subsequent stages, it may not be possible to weld the side of the square tube 1 on the surface plate side, but in this case, fillet seal welding can be performed on the other three sides. After this, water-cooled welding of the fillet is performed in the same way as the welding of the bottom square tube and base 4. Cooling water flows out from between the side of the surface plate side of the rectangular tube that is not sealed welded and the base surface 4A, but there is no problem with welding on the other three sides.

The welding of the metal pieces 3 that connect the square tubes to each other at the edges is performed on the square tubes 1.
This is performed intermittently at a pitch of 200 mm in the longitudinal direction. During this welding, a cooling device 6 equipped with a nozzle 7 that spouts cooling water is inserted into the square tube 1 as shown in FIG. Welding is performed while the area or the vicinity of the welded area is cooled from the inner surface of the square tube 1. Fig. 1 shows a device for cooling the welded part of the metal piece 3 that connects a pair of square tubes. Welding of the metal pieces to be connected may be done at the same time. The cooling device N6 includes a tube 6 and a plurality of nozzles 7 attached to the outer circumferential surface of the tube 6 along the generatrix at intervals of 2oOmm in the longitudinal direction of the tube 6 and with their axes in the radial direction of the tube 6.
It is equipped with The mounting interval of the nozzles 7 matches the pitch of the welding described above, so that the entire length of the welded portion of one metal piece 3 with one of the rectangular tubes can be cooled by one nozzle 7. A plurality of nozzles may be used to cool the welded portion of l metal pieces. This cooling device 6 is connected to a water source containing cooling water, and the supply of cooling water is started before the start of welding, and the cooling water is constantly supplied during welding. After confirming that the cooling water is properly sprayed onto the back surface of the welding part to be welded, the fillet welded part is formed by the square tube 1 and the entire length of the end face 3A of the metal piece 3 in the longitudinal direction. were welded by tungsten inert gas arc (TIG) welding. For welding materials. The same boron-containing stainless steel as square tube 1 or Y308L was used. In addition, in order to make the change in the restraint state of the structure due to the progress of welding as uniform as possible in the longitudinal direction of the square tube 1, welding should be performed first near both ends and then at the center in the longitudinal direction of the square tube 1. were welded and then carried out alternately symmetrically to the central part. Alternatively, all welding lines may be welded at the same time. In this example, 20
Although the cooling device 6 in which the nozzles 7 are installed at 0 mm intervals has been described, the nozzles 7 are installed only at one location on the tube 6, and the T
It may be moved to the part to be welded in conjunction with the IG welding torch.

Further, in the above embodiment, the case where the welded part is cooled from the back side has been described, but it is also possible to cool it from the torch side as shown in the example shown in FIG. The welding torch 9 shown in FIG.
A nozzle 9 formed around A and spouting shielding gas
B, and a cooling nozzle 9c formed concentrically around the nozzle 9B and spouting cooling water, the heat generated by welding is cooled by the cooling water spouting from the cooling nozzle,
Temperature rise near the weld is suppressed.

In the same manner as the installation and welding of the square tubes in the second stage, the square tubes in the third and subsequent stages are installed and welded, and a fuel rack 2 having a predetermined storage capacity is assembled.

According to this embodiment, since boron-containing stainless steel is welded without injecting cooling water near the welding area, the boride in the heat-affected zone melted by welding heat is rapidly cooled and becomes fine particles before it becomes coarse. It is possible to obtain the same toughness in the heat-affected zone as the base metal by releasing it as is, which has the effect of improving the safety factor against the load of the weld heat-affected zone.

In addition, the welded part is cooled immediately after welding. This has the effect of reducing contact deformation compared to air cooling.

〔Effect of the invention〕

According to the present invention, since boron-containing stainless steel is welded while cooling the weld zone, coarsening of the boride in the weld heat affected zone can be prevented and the toughness of the weld zone can be made equal to that of the base metal. This has the effect of improving the safety of welded structures using boron-containing stainless steel.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing one embodiment of the present invention. Figure 2 is the 6th
3 and 4 are perspective views showing one embodiment of the present invention. FIG. 5 is a perspective view showing an embodiment of the present invention. FIG. 6 is a perspective view showing the embodiment of the present invention. FIG. 7 is a sectional view showing another embodiment of the present invention; FIG. 7 is a sectional view showing still another embodiment of the present invention. l... Square cylinder (square tube), 2... Spent fuel storage rack, 3... Metal piece, 6... Pipe, 7... Nozzle.

Claims (1)

[Claims] 1. A method for welding boron-containing stainless steel, which comprises forcibly cooling the welded part or the vicinity of the welded part during welding. 2. A rectangular cylinder made of boron-containing 18Cr-8Ni stainless steel is arranged so that a cross section perpendicular to the axial direction of the rectangular cylinder forms a checkerboard pattern, and the rectangular cylinders are adjacent on the diagonal of the cross section. In a method for manufacturing a spent fuel storage rack in which the spent fuel storage rack is constructed by welding together metal pieces, the welded part or the vicinity of the welded part is forcibly cooled during the welding. A method of manufacturing a spent fuel storage rack characterized by: 3. A tube to which cooling fluid is supplied, and a plurality of nozzles protruding from the outer surface of the tube in the radial direction and along the generatrix of the tube, and the nozzles are metal pieces that connect the rectangular cylinders to each other. A cooling device used for welding boron-containing stainless steel, which is installed at the same pitch as the welding pitch of.