JPS6331064B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel assembly for a nuclear reactor, and particularly to a structure for restraining fuel rods and configuring the assembly.

A conventional fuel assembly for a boiling water reactor is shown in FIG. The fuel assembly 1 includes a plurality of fuel rods 5, spacers 4 for keeping the horizontal spacing between the fuel rods 5 constant, spacer holding rods 6 for holding the spacers 4, and the fuel rods 4 and the spacer holding rods. It is composed of an upper tie plate 2 and a lower tie plate 3 that support 6.

The spring 8 is installed between the upper tie plate 2 and the fuel rod 5, the combined fuel rod 5a and the spacer holding rod 6, and connects the upper tie plate 2 to the combined fuel rod 5a.
The nuts 7 serve to hold the upper tie plate 2 in place by pressing the fuel rods 5 toward the lower tie plate 3. It plays the role of seating at rate 3.

The fuel rods 5, combined fuel rods 5a, and spacer holding rods 6 are fitted to the upper tie plate 2 and lower tie plate 3 as follows. The upper end plug 9 and the lower end plug 10 attached to the upper and lower ends of the fuel rod 5 and the upper end plug 14 and the lower end plug 15 attached to the spacer holding rod 6 are connected to holes provided in the upper tie plate 2. 11 and a hole 12 provided in the lower tie plate 3, and the combined fuel rod 5a is fitted in the lower tie plate 3 with a connecting screw 13.
and is fixed to the upper tie plate 2 with a nut 7.

Further, a handle 16 is attached to the upper tie plate 2 to be engaged with a handling machine when transferring fuel.

In the structure shown in FIG. 1, the maximum distance between the upper tie plate 2 and the lower tie plate 3 is determined by the length of the combined fuel rods, and
If the fuel assembly is subjected to an axial compressive load, the spring 8 may be compressed and the distance may be reduced by the spring compression distance.

Because of this structure, for example, when handling a fuel assembly, when the handle 16 of the upper tie plate is locked to a handling machine and the assembly is moved (displaced) upward, the handle 16 and the combined fuel rods 5a are The full load of the fuel assembly is applied. This load depends on the acceleration during handling. That is, if the moving amount (displacement amount) of the handle portion is x and the moving speed is v, the acceleration α is expressed as α=d/dtv=d/dt (d/dtx) (1). Here, d/dt indicates time differentiation.

Also, the load F applied to the handle and the combined fuel rod
If the mass of the aggregate is M, then F=M×α+D...(2). Here, D is the weight due to gravitational acceleration (1 g).

From equations (1) and (2), if the moving speed of the handle portion changes discontinuously, the acceleration α and therefore the load M become infinite. Such a situation can occur at the moment when a stationary fuel assembly is moved upward by grasping the handle. Of course, in reality, the moving speed of the handle during handling is
Due to the elastic expansion and contraction of the load transmission mechanism of the handling machine (typically a wire rope), etc., the state does not become completely discontinuous, and therefore, in equations (1) and (2), Although the acceleration α or F is not infinite, it becomes a large value, which places a burden on the handle and the coupled fuel rod.

FIG. 2 schematically shows the above matters. Elements 31, 32 modeled in Fig. 2,
33 and 34 correspond to the upper tie plate handle, upper tie plate, combined fuel rod, and lower tie plate shown in FIG. 1, respectively. Further, 35 is a mass M which is approximately the total mass of the fuel assembly.

In FIG. 2, the stationary elements 31, 3
If 2, 33, 34 and the mass 35 are moved at a constant speed, in principle, the entire body will be instantaneously subjected to infinite acceleration. Elements 31, 32, 33 and 34 supporting mass 35 must therefore support an infinite load.

An object of the present invention is to provide a nuclear fuel assembly that can suppress the axial acceleration generated during movement and reduce the tensile load applied to the combined fuel rods, thereby preventing the combined fuel rods from breaking.

A feature of the present invention is that the upper end of the first fuel rod connecting the upper tie plate and the lower tie plate is inserted into an elastic body located above the upper tie plate, or the lower end of the first fuel rod is inserted into an elastic body located above the upper tie plate. is inserted into an elastic body located below the lower tie plate, and one end of the first fuel rod is pressed in the axial direction of the first fuel rod by the corresponding elastic body.

The principle of the invention is illustrated in FIG. Considering the case where a tensile load is applied to the assembly through the upper tie plate, in order to avoid the instantaneous large acceleration mentioned above, it is necessary to provide a buffer element on at least a portion of the connection from the handle part to the assembly mass M. Just insert . The buffer element smoothes the change in the displacement X' of the mass M with respect to the change in displacement X of the handle portion, and can reduce the maximum value of the load.

In FIG. 3a, the buffer element 21 is an elastic body.
is arranged between the element 32 (upper tie plate) and the element 33 (combined fuel rod), and the acceleration α' of the mass 35 relative to the acceleration of the upper tie plate is such that it can prevent the combined fuel rod from breaking. becomes smaller. The function is the same even if the damping element 21 is placed between the element 33 (combined fuel rod) and the element 34 (lower tie plate) as shown in FIG. 3b. In the conventional configuration, when the fuel assembly is suspended in a refueling machine during movement, the combined fuel rods are subjected to a tensile load, and in this state are subjected to acceleration in the axial direction as described above. Therefore, in the conventional example,
There is a risk that the coupled fuel rods will break due to the action of tensile loads and axial accelerations. However, by arranging the shock absorbing element 21, which is an elastic body, as shown in FIGS. Breakage of the combined fuel rods can be prevented when the assembly is moved.

A nuclear fuel assembly which is a preferred embodiment of the present invention will be described below with reference to FIG. Portions that differ from the configuration in FIG. 1 will be explained. In this embodiment, the upper end plug of the combined fuel rod 5a is made to protrude longer than the conventional one above the upper tie plate 2, and the buffer spring 17 is fitted into the protruding portion of the upper end plug. . The buffer spring 17 is connected to the coupled fuel rod 5a.
It is arranged between the nut 7 attached to the upper end and the upper surface of the upper tie plate 2. The combined fuel rods 5a support the weight of the lower tie plate 3, the fuel rods 5, the spacer support rods 6, and the fuel spacers 4, and the combined fuel rods 5a are held on the upper tie plate 2 by buffer springs 17. .

When the fuel assembly of this embodiment is suspended in a fuel exchange machine during exchange, the tensile load applied to the combined fuel rods 5a is significantly reduced by compressing the buffer spring 17. Further, even if an upward displacement is applied to the handle 16 due to movement during replacement, a portion of this displacement is absorbed by the compression of the buffer spring 17, so that the handle 16 and the upper tie plate 2 Even if the acceleration becomes infinite, the magnitude of the axial acceleration applied to the coupled fuel rods 5a can be suppressed to a finite value. These actions can prevent the combined fuel rods 5a from breaking when the fuel assembly is moved.

In another embodiment of the fuel assembly pair shown in FIG. 5, a buffer spring 17 is disposed between the lower tie plate 3 and a nut attached to the lower end plug of the combined fuel rod 5a. The buffer spring 17 is inserted into the lower end plug of the combined fuel rod 5a. In this embodiment as well, the combined fuel rods 5a support the weight of the lower tie plate 3, the fuel rods 5, the spacer support rods 6, and the fuel spacers 4. Therefore, even when the fuel assembly of this embodiment is suspended in a fuel exchanger, the tensile load applied to the combined fuel rods 5a is significantly reduced by compressing the buffer spring 17. Further, even if an upward displacement is applied to the handle 16 due to movement during replacement, a portion of this displacement is absorbed by the compression of the buffer spring 17, so that the handle 16 and the upper tie plate 2 Even if the acceleration of the coupled fuel rod 5a becomes infinite,
The magnitude of the axial acceleration applied to can be suppressed to a finite value. These actions can prevent the combined fuel rods 5a from breaking when the fuel assembly is moved.

In the above embodiments, a spring was used as the buffer element, but any other structure may be used as long as it has elasticity.

According to the present invention, the upper end of the first fuel rod connecting the upper tie plate and the lower tie plate is fitted above the upper tie plate, or the lower end of the first fuel rod is fitted below the lower tie plate. Since most of the mass of the fuel assembly is supported by the elastic body fitted with the first fuel rod, it is possible to significantly reduce the tensile load applied to the first fuel rod, and suppress the acceleration applied in the axial direction of the first fuel rod. Breakage of the first fuel rods during movement of the fuel assembly can be prevented.

[Brief explanation of the drawing]

Fig. 1 is a conceptual diagram showing the prior art, Fig. 2 is a schematic diagram showing the principle of the prior art, Fig. 3 is a schematic diagram showing the principle of the present invention, and Fig. 4 is a partial concept showing an embodiment of the present invention. FIG. 5 is a partial conceptual diagram showing another embodiment of the present invention. 1...Fuel assembly, 2...Upper tie plate,
3... Lower tie plate, 4... Spacer, 5...
... Fuel rod, 6 ... Spacer holding rod, 7 ... Nut, 8 ... Spring, 9 ... Upper end plug, 10 ... Lower end plug, 11, 12 ... Hole, 13 ... Connection screw, 1
4... Upper end plug, 15... Lower end plug, 16... Handle, 5a... Combined fuel rod.

Claims (1)

[Claims] 1. an upper tie plate having a handle, a lower tie plate, and a first fuel rod connecting the upper tie plate and the lower tie plate;
In a nuclear fuel assembly comprising a second fuel rod and a spacer support rod whose ends are held by the upper tie plate and the lower tie plate, and a fuel spacer held by the spacer support rod, the first fuel rod an upper end of the first fuel rod is inserted into an elastic body located above the upper tie plate, or a lower end of the first fuel rod is inserted into an elastic body located below the lower tie plate; A nuclear fuel assembly characterized in that one end of the fuel rod is pressed in the axial direction of the first fuel rod by the corresponding elastic body.