Method for producing a supporting grid

Abstract

To provide a manufacturing method of a support grid capable of further improving strength and rigidity.SOLUTION: A method for manufacturing a support grid having an intersection formed by meshing slits of a plurality of straps with each other includes a step of forming an end welded portion extending over four regions defined by the intersection by irradiating an end of the intersection with a laser, and a step of forming an inner welded portion extending over the four regions by irradiating an inner region separated from the end of the intersection with a laser.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a support grid. [Background technology]

[0002] A fuel assembly applied to a light water reactor includes a bottom nozzle, a fuel assembly body, an upper nozzle, and a support grid (see, for example, Patent Document 1 below). The bottom nozzle has a plurality of holes (guide tube mounting holes) for inserting a plurality of guide tubes included in the fuel assembly body, and a plurality of holes (coolant flow holes) for circulating light water as a coolant. The coolant flows through the coolant flow holes in the bottom nozzle around the guide tubes, i.e., the fuel rods, cooling them, and then flows away through the upper nozzle to the upper plenum.

[0003] The fuel assembly main body mainly comprises a plurality of guide tubes that guide the control rods in the forward and backward directions, and a plurality of fuel rods. The guide tubes and fuel rods extend in the vertical direction and are arranged in a grid pattern in a plan view, spaced apart from one another in the horizontal direction. Support grids are provided at midpoints in the vertical direction of the guide tubes and fuel rods. The support grids are arranged primarily for the purpose of supplementing the rigidity of the guide tubes and other long objects. The support grids are formed by combining metal plate materials called straps in a grid pattern, and spaces (compartments) are formed between the straps through which the guide tubes and other objects are inserted.

[0004] For example, in the configuration disclosed in Patent Document 1 below, straps are joined together by welding, with the welded portions formed at both ends in the vertical direction of the intersection of the straps. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 8-327766 Summary of the Invention [Problem to be solved by the invention]

[0006] The support grid is an important element that determines the seismic performance of the fuel assembly, and therefore is required to have a suitable strength and rigidity. However, there is a risk that sufficient strength and rigidity may not be obtained by simply forming welds only at the upper and lower ends of the straps or in their vicinity, as described above.

[0007] The support grid is an important element that determines the seismic performance of the fuel assembly, so it must have a certain strength and rigidity. However, changes in external factors, such as an increase in the level of earthquake motion, are expected in the future. For this reason, there is a risk that sufficient strength and rigidity cannot be obtained by forming welds only at both ends in the vertical direction as described above.

[0008] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a method for manufacturing a support grid that can further improve strength and rigidity. [Means for solving the problem]

[0009] In order to solve the above problems, the method for manufacturing a support grid according to the present disclosure is a method for manufacturing a support grid having intersections formed by interlocking slits in a plurality of straps, and includes the steps of: irradiating a laser onto the ends of the intersections to form end welds across four regions defined by the intersections; and irradiating a laser onto an internal region at the intersections that is spaced from the ends to form internal welds across the four regions. [Effects of the Invention]

[0010] According to the present disclosure, a method for manufacturing a support grid that can further improve strength and rigidity can be provided. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a perspective view illustrating a configuration of a fuel assembly according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is an enlarged perspective view of a main portion showing the configuration of a support grid according to an embodiment of the present disclosure. [Figure 3] FIG. 10 is a perspective view illustrating a state before straps are combined together in a method for manufacturing a support grid according to an embodiment of the present disclosure. [Figure 4] 10 is a perspective view showing a state after the straps are combined to form welds in a method for manufacturing a support grid according to an embodiment of the present disclosure. FIG. [Figure 5] 3 is a cross-sectional view showing the configuration of each welded portion of a support grid according to an embodiment of the present disclosure. FIG. [Figure 6] 1 is a flowchart illustrating steps in a method for manufacturing a support grid according to an embodiment of the present disclosure. [Figure 7] FIG. 2 is a plan view showing the configuration of a jig used in a method for manufacturing a support grid according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] A method for manufacturing a support grid 40 according to an embodiment of the present disclosure will now be described with reference to FIGS.

[0013] (Fuel assembly 1) The fuel assembly 1 is used as a fuel (heat source) for a light water reactor, for example. As shown in FIG.

[0014] (Bottom nozzle 10) The bottom nozzle 10 supports the fuel assembly body 20 from below. The bottom nozzle 10 has a bottom nozzle body 11 and multiple legs 12. The bottom nozzle body 11 is plate-shaped and extends horizontally. The bottom nozzle body 11 has a square shape in plan view. Note that the planar shape of the bottom nozzle body 11 is not limited to a square, and can be changed to a rectangle, polygon, or circle as appropriate depending on the design and specifications.

[0015] The lower nozzle body 11 has a plurality of holes formed therein. These holes include guide tube mounting holes and coolant flow holes (not shown). Guide tubes 61, which will be described later, are inserted into the guide tube mounting holes. Light water, serving as a coolant (primary coolant), is supplied to the coolant flow holes from a lower plenum provided further below the lower nozzle 10. These holes penetrate the lower nozzle body 11 in the vertical direction. The diameter of the holes is constant throughout the entire area in the vertical direction. Furthermore, these holes are arranged in a lattice pattern at intervals from one another in a plan view. The diameter of the guide tube mounting holes is larger than the diameter of the coolant flow holes. Furthermore, the number of guide tube mounting holes is smaller than the number of coolant flow holes.

[0016] The diameter and arrangement of the coolant flow holes are set so that the pressure drop of the coolant flowing therethrough is appropriate. The number and arrangement of the guide tube mounting holes are appropriately set in accordance with the guide tubes 61 that are aligned with the arrangement of the control rods, which is set in consideration of the nuclear characteristics and thermal fluid characteristics within the core.

[0017] Legs 12 are integrally provided on the underside of each of the four corners of the bottom nozzle body 11. In other words, a plurality of these leg parts 12 are provided at intervals in the circumferential direction on the outer periphery of the bottom nozzle body 11. Each leg part 12 has a columnar shape with a rectangular cross section that protrudes downward from the bottom nozzle body 11. The four leg parts 12 have the same length in the vertical direction. The lower end surfaces of these leg parts 12 abut against reactor internal structures (not shown). That is, the load of the fuel assembly 1 is supported by these leg parts 12.

[0018] (Fuel assembly body 20) The fuel assembly main body 20 includes multiple guide tubes 61, multiple fuel rods 62, and multiple support grids 40. The guide tubes 61 are cylindrical and extend vertically. Control rods and other components for controlling nuclear fission reactions are inserted into the guide tubes 61 as needed. The guide tubes 61 guide the insertion direction of the control rods. The insertion depth of the control rods is adjusted appropriately depending on the operating state of the light-water reactor. The fuel rods 62 are formed by loading, for example, pellet-shaped (cylindrical) nuclear fuel pellets stacked vertically. Although not shown in detail, the fuel rods 62 mainly include a cylindrical cladding tube, the nuclear fuel pellets contained within the cladding tube, an upper end plug closing the upper opening, a lower end plug closing the lower opening, and a spring that presses each fuel pellet downward from the inner surface of the upper end plug. The guide tubes 61 and the fuel rods 62 have the same vertical length.

[0019] (Top nozzle 30) The upper nozzle 30 has a plurality of upper coolant flow holes 31 and upper guide pipe mounting holes 32 through which guide pipes 61 are inserted. An upper plenum (not shown) is provided above the upper nozzle 30 to discharge light water as a coolant.

[0020] (Support grid 40) The support grid 40 is a frame through which the fuel rods 62 and guide tubes 61 are inserted. The support grid 40 is provided to supplement the rigidity of these components. A plurality of support grids 40 are arranged at intervals in the vertical direction. The support grid 40 has a plurality of openings (compartments S, described later) formed therein for inserting the fuel rods 62 and guide tubes 61. The arrangement shape of these openings is the same as that of the holes in the bottom nozzle 10.

[0021] Next, the configuration of the support grid 40 will be described in detail with reference to Figures 2 to 5. As shown in Figure 2, the support grid 40 has a plurality of straps 41, end welds 42, and internal welds 43.

[0022] The straps 41 are strip-shaped plate materials with the horizontal direction as the longitudinal direction. A plurality of straps 41 are combined with each other so as to form a lattice shape when viewed from above and below, thereby forming the support grid 40. The straps 41 are joined to each other by end welds 42 and internal welds 43.

[0023] The end welds 42 and the internal welds 43 are respectively formed at intersections 44 that form an X-shape when the straps 41 are combined together. As shown in FIG. 2, the end welds 42 are formed at both ends of the intersections 44 in the vertical direction. The end welds 42 are formed so as to straddle four sections S formed by the intersections 44 (see FIG. 5). The "sections S" referred to here are the spaces formed between the pair of straps 41 that form the intersections 44 when viewed from the vertical direction.

[0024] A plurality of (two) internal welds 43 are formed between the end welds 42. As an example, one internal weld 43 is formed at each of the positions one-third of the length between the end welds 42. Note that the term "weld" here refers to the portion of the material formed by solidification of the molten pool produced by welding. These welds are most preferably formed by laser welding. However, arc welding or brazing can also be used. Furthermore, it is preferable that both the end welds 42 and the internal welds 43 are formed by spot welding. In other words, these welds are not linear, but rather point-like. This is because forming linear welds makes it easier for cracks to occur within the material due to aging, which may lead to brittle fracture.

[0025] The fuel rods 62 and guide tubes 61 are inserted into the rectangular cross-section openings surrounded by the straps 41. A leaf spring 45 for biasing these components toward the central axis is formed integrally with the strap 41 at the vertical center. Dimples 46 for holding the fuel rods 62 and the like are formed on both vertical sides of the leaf spring 45. Furthermore, vanes 47 for adjusting the flow of cooling water are provided at the upper end of the wall surface of each compartment S.

[0026] (Details of Strap 41 configuration) The straps 41 are metal plates formed by punching and have a band-like shape with the horizontal direction as the longitudinal direction. As shown in FIG. 3 , a slit 48 extending in the vertical direction is formed in the strap 41 at a position midway along its longitudinal extension. As will be described in detail later, a portion of the support grid 40 is assembled by combining a pair of straps 41 from above and below so that they cross each other. At this time, the slits 48 are interlocked from above and below. That is, of the pair of straps 41, the slit 48 of the upper strap 41 is open downward. The slit 48 of the lower strap 41 is open upward. The open end of the slit 48 is referred to as the open end 71. Both slits 48 extend to the center (referred to as the terminal end 72) of the strap 41 in the width direction (vertical direction).

[0027] A first opening 81 is formed in the center between the terminal end 72 and the open end 71 of the slit 48, straddling the slit 48. The first opening 81 penetrates the strap 41 in the thickness direction. The first opening 81 is preferably circular when viewed in the thickness direction of the strap 41. In other words, the first opening 81 consists of a pair of semicircles formed on both sides of the slit 48 in the width direction. However, the first opening 81 may be only a semicircle formed on only one side of the slit 48 in the width direction.

[0028] Furthermore, a second opening 82 is formed in the center between the terminal end portion 72 and the edge of the strap 41 on the side where the slit 48 is not formed. The second opening 82 has a circular shape when viewed in the thickness direction. The second opening 82 penetrates the strap 41 in the thickness direction. The first opening 81 and the second opening 82 serve as positioning targets for forming the internal weld 43 described above.

[0029] (Method of manufacturing the support grid 40) Next, each step of the manufacturing method of the support grid 40 will be described mainly with reference to FIG. 6. First, in step S1, the strap 41 described above is prepared. The strap 41 is formed by punching or laser processing a plate material. It is desirable that the slits 48 described above are formed in this state. In the following step S2, the first opening 81 and the second opening 82 are formed in the strap 41. Note that this step S2 may be performed simultaneously with the punching process, etc., in step S1.

[0030] Furthermore, in step S3, the straps 41 are combined from above and below so that the slits 48 of the straps 41 interlock (see FIG. 3). As a result, the first openings 81 and the second openings 82 of the combined straps 41 overlap each other. In the following step S4, the assembly of straps 41 is fixed using a jig 90. Here, as shown in FIG. 7, the jig 90 is plate-shaped and has welding holes 91 arranged in a grid pattern at positions corresponding to the intersections 44 of the support grid 40. The jig 90 is placed over the assembly of straps 41 so that the welding holes 91 align with the intersections 44. In this state, laser welding is performed on the intersections 44 through the welding holes 91 of the jig 90. This forms the end welds 42 on one side in the vertical direction. The assembly is then turned upside down, and the jig 90 is placed over it again, after which the end welds 42 on the other side are formed. In the final step S5, the jig 90 is removed, and laser welding is performed on the first opening 81 and the second opening 82 from any one of the four sections S, thereby forming the internal weld 43. This completes all steps in the manufacturing method of the support grid 40.

[0031] (Action and effect) Here, the support grid 40 is an important element that determines the seismic performance of the fuel assembly 1, and therefore the support grid 40 is required to have a suitable strength and rigidity. However, conventionally, welds have generally been formed only at the top and bottom ends of the straps 41 or in their vicinity. This has led to the problem that sufficient strength and rigidity cannot be obtained. To solve this problem, the present embodiment employs the above-described configurations and methods.

[0032] According to the above method, the internal welds 43 are formed in addition to the end welds 42, further improving the strength and rigidity of the support grid 40. On the other hand, if the support grid 40 were constructed solely with the end welds 42, the end welds 42 would be unable to resist external forces caused by earthquakes or other events, potentially causing the support grid 40 to deflect. This could result in misalignment of the fuel rods 62 and guide tubes 61 supported by the support grid 40. In particular, if the guide tubes 61 are misaligned or damaged, the control rods would not be able to move smoothly back and forth, posing a significant risk. In contrast, according to the above method, the additional internal welds 43 ensure stable connection strength between the straps 41 even at intermediate positions in the vertical direction. This allows the support grid 40 to withstand loads even when the above-described external forces are applied. This further improves the seismic performance of the fuel assembly 1.

[0033] According to the above method, a plurality of internal welds 43 are provided at intervals in the direction in which the intersections 44 extend. This makes it possible to further increase the bonding strength between the straps 41. Therefore, even when an external force is applied, the straps can withstand the load. Therefore, the seismic resistance of the fuel assembly 1 can be further improved.

[0034] According to the above method, the first opening 81 and the second opening 82 are formed in advance in the strap 41. These first openings 81 and the second openings 82 are provided at positions corresponding to the internal welds 43, respectively. When the straps 41 are assembled, the first openings 81 or the second openings 82 of the straps 41 overlap each other. That is, their vertical positions are aligned. Because the first openings 81 and the second openings 82 are formed in this manner, when the straps 41 are assembled, regardless of which section S around the intersection 44 is used for welding, the molten metal or brazing filler metal will be uniformly distributed to all four sections S through the first openings 81 or the second openings 82. In other words, it is possible to form uniform welds in all four sections S with a single welding operation. This significantly improves the efficiency and smoothness of the operation. Furthermore, because the first openings 81 or the second openings 82 can be used as positioning targets during welding, the possibility of variations or deviations in the welding position for each strap 41 can be minimized. Therefore, it is possible to ensure uniformity in strength and rigidity throughout the entire support grid 40. As a result, it is possible to eliminate the possibility that only a portion of the support grid 40 will deform or bend when an external force is applied.

[0035] According to the above method, the assembled straps 41 are fixed by a jig 90, and then welding is performed through holes formed in the jig 90 to form the end welds 42. Because the straps 41 are fixed by the jig 90, the possibility of the straps 41 being dislodged or broken during sequential welding operations can be minimized. This allows the assembly work of the support grid 40 to proceed more stably and efficiently. It also ensures uniformity in the finish of the end welds 42. As a result, the seismic resistance of the support grid 40 can be further improved.

[0036] According to the above method, the end welds 42 and the internal welds 43 are not linear but rather point-like. In other words, when forming these welds, it is important to focus the laser on a single point (focus) and irradiate it without moving it, rather than scanning it. Forming linear welds rather than point-like welds is likely to lead to cracks due to aging within the welds, potentially resulting in brittle fracture. However, with the above-described point welding, the volume of the welds formed by the welds is minimized, thereby avoiding the risk of aging, such as cracks. This further improves the seismic resistance of the support grid 40.

[0037] (Other embodiments) Although the embodiments of the present disclosure have been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and design changes and the like are also included within the scope of the gist of the present disclosure. For example, the numbers of guide tubes 61, fuel rods 62, and coolant flow holes in the drawings referred to in the above embodiments are merely examples, and can be changed as appropriate depending on the size and design of the reactor.

[0038] <Additional Notes> The manufacturing method of the support grid 40 described in each embodiment can be understood as follows, for example.

[0039] (1) A first embodiment of the method for manufacturing a support grid 40 includes a step of irradiating a laser onto the end of the intersection 44 to form an end weld 42 across four regions S defined by the intersection 44, and a step of irradiating a laser onto an internal region at the intersection 44 that is spaced apart from the end to form an internal weld 43 across the four regions.

[0040] According to the above method, since the inner welds 43 are formed in addition to the end welds 42, the strength and rigidity of the support grid 40 can be further improved.

[0041] (2) The manufacturing method of the support grid 40 according to the second aspect is the manufacturing method of the support grid 40 of (1), in which the internal welds 43 are arranged at intervals in the direction in which the intersections 44 extend.

[0042] According to the above method, the bonding strength between the straps 41 can be further increased.

[0043] (3) A manufacturing method of a support grid 40 according to a third aspect is a manufacturing method of a support grid 40 according to (1) or (2), and includes the steps of forming the strap 41 having the slit 48 before the step of forming the end weld 42, and forming a first opening 81 that opens across the slit 48 at a position corresponding to the internal weld 43 in the slit 48, and a second opening 82 that opens on an extension of the slit 48 and at a position spaced apart from the first opening 81, and in the step of forming the internal weld 43, welding is performed on the first opening 81 and the second opening 82.

[0044] According to the above method, it is possible to significantly improve the efficiency and smoothness of the work, and also to ensure uniformity in the strength and rigidity of the support grid 40 as a whole.

[0045] (4) A manufacturing method of a support grid 40 according to a fourth aspect is a manufacturing method of a support grid 40 according to any one of aspects (1) to (3), and in the step of forming the end welds 42, the plurality of straps 41 with the slits 48 interlocked are held by a plate-shaped jig 90 from the direction in which the intersections 44 extend, and the end welds 42 are formed through holes formed in the jig 90.

[0046] According to the above method, it is possible to ensure uniformity in the finish of the end welded portion 42.

[0047] (5) The manufacturing method of the support grid 40 according to the fifth aspect is a manufacturing method of the support grid 40 according to any one of the aspects (1) to (4), in which the end welds 42 and the internal welds 43 are point-shaped.

[0048] According to the above method, the volume of the material formed by welding is minimized, thereby avoiding the risk of cracks and other deterioration over time. [Explanation of symbols]

[0049] DESCRIPTION OF SYMBOLS 1...fuel assembly 10...bottom nozzle 11...bottom nozzle body 12...leg 20...fuel assembly body 30...top nozzle 31...upper coolant flow hole 32...upper guide tube mounting hole 40...support grid 41...strap 42...end weld 43...internal weld 44...intersection 45...leaf spring 46...dimple 47...vane 48...slit 61...guide tube 62...fuel rod 71...open end 72...terminal end 81...first opening 82...second opening 90...jig 91...welding hole S...compartment

Claims

1. A method for manufacturing a support grid having intersections formed by interlocking slits of a plurality of straps, comprising: forming end welds across four regions defined by the intersections by irradiating the ends of the intersections with a laser; forming an internal weld across the four regions by irradiating an internal region at the intersection spaced from the ends with a laser; A method for manufacturing a support grid comprising:

2. The method for manufacturing a support grid according to claim 1 , wherein a plurality of the internal welds are arranged at intervals in the direction in which the intersections extend.

3. before the step of forming the end weld, forming the strap with the slit; forming a first opening that straddles the slit at a position corresponding to the internal weld in the slit, and a second opening that opens at a position on an extension line of the slit and spaced apart from the first opening; Including, The method for manufacturing a support grid according to claim 1 or 2, wherein in the step of forming the internal weld, welding is performed on the first opening and the second opening.

4. 3. A method for manufacturing a support grid as described in claim 1 or 2, wherein in the step of forming the end welds, the plurality of straps with the slits interlocked are held by a plate-shaped jig from both sides in the direction of extension of the intersection, and the end welds are formed through holes formed in the jig.

5. The method for manufacturing a support grid according to claim 1 or 2, wherein the end welds and the internal welds are in the form of points.

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