JP2001349973A - Lower nozzle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lower nozzle being rapidly manufacturable at a low cost and having good quality. SOLUTION: The lower nozzle 1 of a fuel assembly is integrally formed by mechanical working. The lower nozzle 1 has a lower plate 3 with a plurality of hole parts 9 bored therein, a plurality of leg parts 5, and an internal hollow part 7 communicating with the hole parts. Positioning holes 13 for locking pins are provided at the four leg parts 5, and the internal corners 7a of the internal hollow part are cut with a T-slot cutter by means of the positioning holes 13. The internal hollow part 7a is communicated with drain holes 9a located outside the positioning holes 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lower nozzle incorporated in a nuclear fuel assembly used in a nuclear reactor.

[0002]

2. Description of the Related Art Conventionally, for example, a nuclear fuel assembly used for a nuclear reactor of a light water reactor has a plurality of support grids arranged at predetermined intervals between an upper nozzle and a lower nozzle, and includes an instrumentation tube and a plurality of control rod guides. A tube is fixed to each support grid, the upper nozzle and the lower nozzle, and a number of fuel rods are inserted and held in the grid space of each support grid. The lower nozzle is composed of a substantially rectangular plate-shaped lower plate and four legs hanging down from the four corners, and the lower plate has a large number of drain holes for flowing a coolant such as cooling water. A hole is formed in the entire surface, and a hole for attaching a control rod guide tube or an instrumentation tube is formed. The drain hole is formed off the mounting position of the fuel rod. Further, on the back surface which is the leg side surface of the lower plate, a space for distributing cooling water to each drain hole is formed as a substantially square inner space, and the four corners of the inner space go around each leg. Is formed. Further, two of the four legs in the diagonal direction have positioning holes for engaging the positioning pins projecting from the lower core plate to position the fuel assembly and extend from the bottom surface to the lower plate side. It is provided.

[0003]

Such a lower nozzle is usually manufactured by forming the lower plate and the leg as separate parts and then welding the legs to the four corners of the lower plate. It is necessary to sensitize the heat-affected zone to corrosion and cope with welding defects. Therefore, it is necessary to strictly control the welding conditions and the like. Is required to confirm. Therefore, there is a problem that the welding operation requires skill. Therefore, a technology for integrally forming a lower nozzle by welding and combining machining and electric discharge machining without manufacturing the same is proposed, for example, in Japanese Patent No. 3008070. In this technique, a rectangular stainless steel block body is formed with a cutting tool to form a lower plate and four legs, and a recessed portion corresponding to the inner space inside the legs is formed by electric discharge machining. The company says it will perform detailed processing such as drilling and chamfering of drainage holes, control rod guide tubes, and fixing holes for instrumentation tubes. However, the electric discharge machining has a disadvantage that the machining speed is slow and the production cost is high as compared with the machining using a cutting tool or the like.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a lower nozzle of a fuel assembly which can be manufactured at low cost and with high quality without using welding or electric discharge machining.

[0005]

In the lower nozzle of the fuel assembly according to the present invention, a plurality of support grids are arranged at predetermined intervals between the upper nozzle and the lower nozzle, and the instrumentation tube and the control rod guide tube are arranged in the upper portion. Fixed to the nozzle and the lower nozzle, respectively
In a fuel assembly in which a large number of fuel rods are held in each support grid, the lower nozzle has a lower plate having a plurality of holes, a plurality of legs, and an inner space communicating with the holes. A positioning hole is provided in each of the legs, and the lower nozzle is integrally formed by machining. According to the present invention, when manufacturing the lower nozzle, the machining speed is high, and the manufacturing cost is reduced at a lower cost as compared with the conventional production method using electric discharge machining by producing the lower nozzle by integral cutting by machining without using electric discharge machining. Since it can be manufactured and has no welded parts and is integrally formed, there is no occurrence of welding defects or corrosion and the quality is high.

Further, the lower nozzle is characterized in that an inner space in the leg is formed by cutting using a positioning hole.
At the time of machining, the lower nozzle consisting of the lower plate and multiple legs is roughly cut, and when cutting the corners of the inner space, the legs are cut with a cutting tool from the inside of each leg and then to the legs. The depth of the leg may be further cut outward by inserting a cutting tool through the drilled positioning hole. Thus, even if a hole is further provided outside the positioning hole overlapping each corner of the lower plate in a plan view, the hole can be communicated with the inner space, and even at the corner, the cooling fluid can be spread further outside. As a result, the cooling effect can be exerted more uniformly on the many fuel rods arranged. In that regard, in the prior art, in order to form an inner space portion in the leg portion, if the cutting tool attempts to cut the recessed portion of the leg from the inside of the leg to the outside with the cutting tool, the leg holds the cutting tool. Machining is not possible because it interferes with machine tool entry,
Therefore, it has to be performed by electric discharge machining or welding.

[0007]

FIG. 1 is a block diagram showing an embodiment of the present invention.
This will be described with reference to FIGS. FIG. 1 is a plan view showing a quarter of a lower nozzle according to an embodiment of the invention, FIG. 2 is a side view of the lower nozzle, FIG. 3 is a bottom view of the lower nozzle, and FIG. 5 is a side view of a main portion of a T-slot cutter used for machining an inner corner portion of FIG. 5, FIG. 5 is a flow sheet showing a cutting process of a lower nozzle, and FIGS.
(C) is a figure which shows the inside space processing process of a leg part. The lower nozzle 1 according to the embodiment of the present invention has a shape shown in FIGS. 1 and 2, and has leg portions 5 at four corners on the back surface of a substantially square (preferably square) plate-like lower plate 3.
Are suspended and integrally formed. An inner space 7 for passing a cooling fluid such as cooling water is formed on the back surface of the lower plate 3 on the side of the leg 5, and the inner space 7 is a substantially cross-shaped main inner space 7 a having no leg 5. And four inner corners 7b of each of the four leg portions 5 with the inside facing outward. The inner corners 7b are formed on the outer peripheral ridge of each corner of the lower plate 3. The outer surface 5a of the leg portion 5 is formed along a substantially right angle while leaving the outer surface 5a thin. Note that FIG. 1 shows a quarter including one corner of the lower plate 3.

A plurality of drain holes 9 are formed in the lower plate 3 as a plurality of holes for flowing cooling water, and the inner space 7 communicates with the area of the plurality of drain holes 9 in the lower plate 3. Thus, the cooling water flowing upward from below can be reliably flowed upward from the inner space 7 through all the drain holes 9. Each of the four legs 5 has a substantially prismatic shape having a quadrangular cross section, and the inner corners are largely chamfered by a chamfered portion 5b to form an outer shape. 4 legs 5
The positioning hole 13 which penetrates the thick lower part 5A from the bottom surface 5c to the inner space corner part 7b of the inner space part 7 is perforated, for example, in two leg parts 5 and 5 in one diagonal direction. The positioning holes 13 provided are engagement holes for positioning fuel assemblies by engaging with positioning pins projecting from a lower core plate (not shown).

The lower nozzle 1 according to the present embodiment has the above-described configuration. Next, a method for processing the lower nozzle 1 will be described with reference to FIGS. In FIG. 5, first, a square plate or rectangular parallelepiped material made of a suitable material such as stainless steel is cut with a rolling tool such as an end mill or a cutting tool to hang from a substantially square plate-shaped lower plate 3 and four back corners thereof. The four legs 5 to be formed are integrally formed (step 101). In this state, a substantially cross-shaped portion excluding the legs 5 on the back surface of the lower plate 3 forms a main inner space 7.
a. Next, the inner hollow corner portion 7b of each leg 5 is cut, which will be described with reference to FIGS. 6 (a), 6 (b) and 6 (c). FIG. 6 is a sectional view taken along the line AA of the leg 5 shown in FIG. 2, and a rolling tool as shown in FIG. 4, for example, a T-slot cutter 14 is used as a cutting tool. The T-slot cutter 14 includes a large-diameter blade portion 15 having a cutting edge (not shown) protruding from a distal end surface and an outer peripheral surface, and a shank portion 16. The cutting process is performed by moving while rotating around O.

First, in FIG. 6 (a), a point P which is equidistant from the outer surfaces 5a, 5a of the legs 5 is set on a diagonal line of the lower plate 3 and the T-slot cutter 14 is rotated on its axis as a rotation axis. While moving in the diagonal direction and moving in the direction of the central axis O, the area between the lower part 5A and the lower plate 3 is cut away leaving the lower part 5A of the leg 5 to remove the quarter-arc wall surface 17a in plan view. A first inner back portion 17 is formed (step 102). Next, the leg portion 5 is drilled using a drill or the like so as to communicate from the center of the lower surface 5c to the first inner deep portion 17, thereby forming a positioning hole 13 as shown in FIG. 6B (step 103). And this positioning hole 1
3, a T-slot cutter 14S having a smaller diameter than its inner diameter.
Is inserted into the first inner back portion 17 to further cut the wall surface 17a (see FIG. 2). For this purpose, in FIG. 6C, for example, the rotation axis of the T-slot cutter 14S is set as appropriate on a virtual concentric circle R1 having a smaller diameter than the inner diameter of the positioning hole 13 and horizontally turned, so that the T-slot cutter 14 is rotated along the concentric circle R1. the wall 17a of the leg portion 5 to each cutting pressurized Engineering outward angle of the leg 5 14S of the rotating shaft by cutting while moving between Q2-Q1-Q3, the wall surface 17a of the leg portion 5 Cutting is performed on the outer side surfaces 5a and 5a, respectively. In this manner, the second inner deep part 19 having an arc shape in a plan view is formed (Step 104).

As a result, the inner hollow corners 7b composed of the first inner deep part 17 and the second inner deep part 19 are stolen, respectively, to form four inner hollow parts 7 connected to the main inner hollow part 7a, The thickness of the leg portion 5 shown in FIG. 6C is reduced by the second inner back portion 19 into a thin portion of the outer peripheral surfaces 5a and 5a and a remaining portion 21 having a slightly thicker wall surface 19a having an arc-shaped wall surface 19a at a corner. Will be left behind. Then, the lower plate 3 is cut in detail such as by drilling holes for fixing the drain holes 9... And fixing holes 11 such as control rod guide tubes and instrumentation tubes. In this way, the inner space 7 extends to the vicinity of the outer corner in each leg 5, and the drain holes 9 located outside the respective positioning holes 13 formed in the lower plate 3 (codes for convenience thereof). 9a)...

Therefore, when a fuel assembly having such a lower nozzle 1 is mounted in a nuclear reactor and cooling water flows upward from below the fuel assembly during operation, the cooling water flows from the inner space 7 of the lower nozzle 1. , 9... Of the lower plate 3 can be made to pass through, and even if the drain holes 9 a. The cooling efficiency of the body can be increased and averaged.

As described above, according to the present embodiment, the lower nozzle 1 can be manufactured by machining with a cutting tool when manufacturing the lower nozzle 1. Therefore, since no electric discharge machining is used, the machining speed is rapid and the heat-affected zone in the welded portion is improved. It is not necessary to deal with corrosion of the lower nozzle 1 and it is possible to manufacture the lower nozzle 1 of good quality at low cost. In particular, when processing the first and second inner deep portions 17 and 19 for forming the inner hollow corner portion 7b in the leg portion 5, the first inner deep portion 17 is formed from the inside of the leg portion 5 to the outside. After the cutting process, the second inner back portion 19 is further utilized by using the positioning holes 13.
Are cut off, so that the drain holes 9a communicating with the inner space 7 outside the positioning holes 13 of the lower plate 3 are formed.
Can be formed, and the cooling efficiency of the fuel assembly can be increased to more uniformly control the fuel assembly.

In the above-described embodiment, the positioning hole 13 is formed in the leg 5 after forming the first inner deep portion 17 when forming the inner hollow corner 7b of the leg 5. Instead, the first inner deep portion 17 may be formed after the positioning hole 13 is formed. In short, the positioning hole 13 may be formed before the second inner deep portion 19 is formed. Of the positioning holes 13 provided in the four legs 5, two diagonal holes 13, 13 are used as locking holes for engaging positioning pins, and the other two positioning holes 13, 13 are used. It does not have to be used for positioning. In addition, instead of the above-described embodiment, the positioning holes 13 are respectively formed in the four legs 5, and as shown in FIG. 7, each of the corners of the lower plate 3 overlaps with the positioning holes 13 in a plan view. The drain hole 9 (9a) may be formed. Also in this case, the cooling fluid can be passed over a wider range than the conventional lower nozzle in which the drain hole 9 is not formed at the position overlapping the leg 5 of the lower plate 3, and the cooling efficiency can be improved. In this case, the processing surface shape of the inner space corner 7b of the inner space 7 may be different from the shape shown in FIG. The shape of the inner hollow corner 7b shown in FIG.
Only a small arc-shaped machined surface shape is left in plan view.

[0015]

As described above, the lower nozzle according to the present invention has a lower plate having a plurality of holes, a plurality of legs, and an inner space communicating with the holes. Are provided with positioning holes, respectively, and can be manufactured by integral molding by machining without using electric discharge machining.The machining speed is high and the production cost is low compared to the conventional method using electric discharge machining. Since there is no welded portion and it is integrally formed, there is no need to deal with welding defects and corrosion.

In the lower nozzle, since the inner space in the leg is formed by cutting using the positioning hole, the inner space is connected to each corner of the lower plate, that is, the hole outside the positioning hole. The cooling fluid can be distributed to uniformly cool many fuel rods.

[Brief description of the drawings]

FIG. 1 shows the lower nozzle according to an embodiment of the present invention.
FIG.

FIG. 2 is a side view of a lower nozzle.

FIG. 3 is a bottom view of a lower nozzle.

FIG. 4 is a side view of a main part of a T-slot cutter used for machining an inner corner of a leg of a lower nozzle.

FIG. 5 is a flow sheet showing a cutting process of a lower nozzle.

6 (a), (b), and (c) are cross-sectional views taken along the line AA of FIG. 2 and are diagrams illustrating a step of processing the inner space of the leg.

FIG. 7 is a plan view showing a quarter of a lower nozzle according to another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Lower nozzle 3 Lower plate 5 Leg 7 Inner space 7b Inner space corner 9, 9a Drain hole 13 Positioning hole 14, 14ST slot cutter

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kohei Suzuki 622-1, Funaishikawa, Tokai-mura, Naka-gun, Ibaraki Prefecture Inside Mitsubishi Nuclear Fuel Co., Ltd. (72) Inventor Kenji Suga 622, Funaishikawa, Tokai-mura, Naka-gun, Ibaraki Prefecture 1 Inside Mitsubishi Nuclear Fuel Co., Ltd. (72) Inventor Hiroaki Kadono 622-1, Funaishikawa, Tokai-mura, Naka-gun, Ibaraki Prefecture Inside Mitsubishi Nuclear Fuel Co., Ltd. (72) Kazuyuki Oizumi 3-1-1 Koganemachi, Niigata City, Niigata Prefecture No. 1 Mitsubishi Materials Corporation Niigata Works (72) Inventor Takashi Koike 1-1-1 Wadasaki-cho, Hyogo-ku, Kobe City, Hyogo Prefecture Inside Kobe Shipyard, Mitsubishi Heavy Industries, Ltd.

Claims (2)

[Claims] 1. A plurality of support grids are disposed at predetermined intervals between an upper nozzle and a lower nozzle, and an instrumentation pipe and a control rod guide pipe are fixed to the upper nozzle and the lower nozzle, respectively. In a fuel assembly in which a fuel rod is held, the lower nozzle has a lower plate having a plurality of holes, a plurality of legs, and an inner space communicating with the holes. The lower nozzle is provided with a positioning hole, and the lower nozzle is integrally formed by machining. 2. The lower nozzle according to claim 1, wherein the lower nozzle is formed by cutting an inner space inside the leg through a positioning hole.