JPH0643276A - Fuel assembly - Google Patents

Abstract

PURPOSE:To provide a fuel assembly using no extra portion for a rotation preventing means of short fuel rods, having a simple structure, and widening the application range without increasing the local pressure loss. CONSTITUTION:Short fuel rods 11B are partially used among many fuel rods 11 arranged squarely. A screwed lower end plug 11e is fitted at the lower end and screwed into the pedestal 13a of a lower tie plate 13, a solid column-like upper end plug 11e is fitted at the upper end, and vertical coupling sections 17 such as grooves or slits are provided on both sides of the upper end plug 11d. A spacer spring 16 fitted to a block wall surrounding the unit cell 14c of a spacer 14 arranged at the height position of the coupling sections 17 is coupled with the vertical coupling sections 17.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water nuclear reactor which mainly incorporates a plurality of short fuel rods.

[0002]

2. Description of the Related Art As shown in FIG. 7, a fuel assembly for a boiling water reactor comprises a large number of fuel rods 1 and a small number of water pipes (not shown) arranged in the central portion in 8 rows, 8 columns or 9 rows. Square arrangement in columns,
Spacers 4 are arranged in several stages along the axial direction to keep the distance between the rods constant, and a channel box 5 is fitted on the outer side, and most of the fuel rods 1 and water pipes are placed above and below. The ends are fitted and supported in the through holes of the upper tie plate 2 and the lower tie plate 3 through the upper and lower end plugs 1a and 1b, respectively, and two of the fuel rods on each side of the outermost periphery are attached to the tie rod 1A.
As such, screws are provided on the upper and lower end plugs 1c and 1d, the lower end plug 1d is screwed into the pedestal 3a of the lower tie plate 3, and the upper end plug 1c penetrates the through hole of the upper tie plate 2. The lock nut 8 is screwed into the protrusion to fasten the fuel bundle.

By the way, recently, by reducing the amount of uranium in the upper part of the fuel bundle, the water / uranium ratio is increased to improve the shutdown margin of the nuclear reactor, and at the same time, the pressure loss of the coolant in the upper part is decreased to stabilize the flow. In order to achieve this, some of the parallel fuel rods are made shorter than ordinary long fuel rods, and the lower end plugs 1e are provided with the same screws as the tie rods 1A, and the pedestal 3 of the lower tie plate 3 is provided.
It is screwed into a. (For the sake of clarity in FIG. 7,
Although shown next to the tie rods, most are provided in columns and rows inside the tie rods. )

However, if the lower end plug 1e is simply screwed into the lower tie plate 3, the screwed portion will come loose due to the vibration during transportation of the assembly and the vibration due to the upward flow of the coolant during the operation of the furnace. Because of fear
As this type of fuel assembly, a structure in which the rotation of a short fuel rod is prevented is proposed by JP-A-1-124793 and JP-A-60-125589.

[0005]

By the way, in the former rotation preventing means, the lower end plug of the short fuel rod is provided with a reverse lead screw which is divided into upper and lower halves, and a nut is attached to the upper screw. A washer with a blade is attached to the screw hole of the lower tie plate, and the nut is wrapped by the blade.It requires extra parts, and at the same time the structure is complicated and there is a problem that it takes time to process and join. On the other hand, in the latter, the upper end plug of a short fuel rod is formed in the shape of a square shaft, and this is fitted into a square cell of a grid-like spacer. Although this has a simple structure, it forms a coolant flow path. Since the spacer cells are closed, local pressure loss of the spacer is increased and there is a drawback that the spacer cell cannot be applied to the spacer made of a circular tube element.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a rotation preventing means for a short fuel rod, which does not require extra parts, has a simple structure, does not cause an increase in local pressure loss of the spacer, and has a wide range of application. And

[0007]

In order to achieve the above object, the present invention has a large number of fuel rods and a small number of water pipes arranged in a square array, and the upper and lower ends are connected to the upper tie plate and the lower tie plate through upper end plugs or lower end plugs. It is supported by a tie plate and is inserted through unit cells of spacers arranged in several stages in the middle of the axial direction to hold them at regular intervals, and some of the above fuel rods are used as short fuel rods. As a fuel assembly, the lower end plug of the short fuel rod is screwed into the pedestal of the lower tie plate, while the upper end plug is provided with a vertically-oriented engaging portion, and the vertically-oriented engaging portion has its height position. It is characterized in that a spacer spring attached to a predetermined position of the corresponding unit cell of the spacer disposed in is engaged.

[0008]

1 to 3 show an embodiment of the present invention, in which a large number of fuel rods 11, 1 including long and short ones are used.
1B and a small number of water pipes (not shown) arranged in the center are 8
The long fuel rods 11 and the water pipes are arranged in a square array in 8 rows and the like, and the upper and lower ends are an upper end plug (not shown) and an upper end plug (not shown), and an upper tie plate (not shown) and a lower tie plate. 13 are inserted into and supported by through holes, and are held in the axial direction by the following spacers 14 arranged in several stages so that the mutual intervals are constant, and a channel box 15 is fitted on the outside. ing.

Each of the spacers 14 is formed in a grid pattern by partition plates 14b (FIG. 3) extending vertically and horizontally on the insides of four outer plates 14a, and a square wall is formed on one side at the center of the partition walls on four sides surrounding the unit cells 14c. The elastic protrusion 16a is provided and the fixed protrusion 16 is provided on the other side.
A clip-shaped spacer spring 16 provided with b is attached, and the fuel rods 11 and 11B and the water pipe penetrating each unit cell 14c are supported by the spacer spring 16 so as to keep constant intervals from four directions.

Two of the outermost fuel rods 11 are tie rods (not shown), the lower end plugs are screwed into the pedestal 13a of the lower tie plate 13, and the upper end plugs are provided with lock nuts. Fuel bundles are fastened,
Also, the second shortest fuel rods 11B (four in total) are arranged from the both sides in rows and columns that are one inward from the outermost periphery, and their lower end plugs 11e are the pedestals of the lower tie plate 13 like the tie rods. It is screwed into the screw hole of 13a.

Further, as shown in FIG. 2, a solid cylindrical upper end plug 11d is attached to the upper end of the short fuel rod 11B, and the upper end plug 11d has key groove-shaped vertically engaging portions on both sides. The vertical engaging portion 17 is provided with a corresponding unit cell 1 of the spacer 14 arranged at the height position.
The mountain-shaped elastic portion 16a of the spacer spring 16 attached to the partition wall of 4c is engaged so that the short length fuel rod 11B does not rotate unintentionally. In this case, the fixed protrusion 16b may be engaged with the vertically-oriented engaging portion 17 on one side as shown by a dotted line.

FIG. 4 shows the upper end plug 11d of the short fuel rod 11B.
FIG. 6 shows a modified example in which the vertical cylindrical engaging portion 17 formed of a long hole or a slit is provided on both sides thereof, and the chevron elastic portion 16a of the spacer spring 16 is engaged with the engaging portion 17. is there.

FIGS. 5 and 6 show another modification. In this case, circular tube elements 14b 'which form a unit cell 14c' into which the fuel rods 11 and 11B are to be inserted are joined vertically and horizontally. And the angled elastic portion 16a 'of the spacer spring 16' attached to the joint portion of the adjacent circular tube element 14b 'is engaged with the vertical engaging portion 17 provided on the upper end plug 11d of the short fuel rod 11B. It is made to unite and to prevent the rotation.

In the illustrated example, the short fuel rod 11B is the second fuel rod from the both sides in the row and column which is one inward from the outermost periphery of the fuel bundle. However, the present invention is not limited to this, and the fuel rod is selected at a required position. be able to.

[0015]

As described above, the present invention provides a fuel assembly for a boiling water reactor in which a part of parallel fuel rods is a short fuel rod, and the lower end plug of the short fuel rod is a lower tie plate. On the other hand, the upper end plug is provided with a vertically engaging portion such as a groove and a slit, and the vertical engaging portion is provided with a predetermined unit cell corresponding to the spacer arranged at the height position. Since the spacer spring attached at the position of is engaged, even if vibration or swing acts on the assembly during transportation or operation of the furnace,
Since the rotation of the short fuel rod is reliably prevented and there is no fear of coming out of the lower tie plate, and as the rotation preventing means, the spacer spring normally provided in the spacer for holding the fuel rod is used. It does not require extra members as before, has a simple structure, and does not block the spacer unit cell. It can be applied to a spacer composed of a circular tube element, and its application range can be expanded.

[Brief description of drawings]

FIG. 1 is a partially cut front view showing an embodiment of the present invention.

FIG. 2 is a partially cut front view showing a supporting state of an upper end plug of a short fuel rod.

FIG. 3 is a plan view of the portion shown in FIG.

FIG. 4 is a partially cut front view showing a supporting state of a modified example of an upper end plug of a short fuel rod.

FIG. 5 is a partially cut front view showing a supporting state of a short fuel rod when a spacer is changed.

FIG. 6 is a plan view of the portion shown in FIG.

FIG. 7 is a partially cut front view of a conventional fuel assembly.

[Explanation of symbols]

 11 Fuel Rod 11B Short Fuel Rod 11d Upper End Plug 11e Lower End Plug 13 Lower tie Plate 13a Pedestal 14 Spacer 14a Outer Plate 14b Partition Plate 15 Channel Box 16 Spacer Spring 16a Mountain-Shaped Elastic Part 17 Longitudinal Engagement Part

Claims (1)

[Claims] 1. A large number of fuel rods and a small number of water pipes are arranged in a square, and upper and lower ends are supported by an upper tie plate and a lower tie plate via upper end plugs or lower end plugs, and there are several stages along the axial direction. A lower end of the short fuel rod in a fuel assembly that is inserted through the unit cells of the arranged spacers and holds them at regular intervals and uses some of the fuel rods as short fuel rods. While the stopper is screwed into the pedestal of the lower tie plate, a vertical engaging portion is provided on the upper end stopper, and the vertical engaging portion has a predetermined unit cell corresponding to the spacer arranged at the height position. A fuel assembly characterized in that a spacer spring attached at the position of is engaged.