JPS61196193A - Nuclear fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to a fuel assembly for a light water reactor, and particularly relates to a lower support structure for a nuclear fuel element or a hollow rod, which is suitable when the upper end of the nuclear fuel element or hollow rod is not supported. [Background of the Invention] A fuel assembly for a light water reactor comprises, for example, as shown in FIG. 2, a plurality of nuclear fuel elements 1 and one or more hollow rods 2 arranged at regular intervals in the horizontal direction. The upper and lower ends of the nuclear fuel element 1 and the hollow rod 2 are bundled in a square grid shape by a plurality of support grids 3, and the upper and lower end plugs 7 and 9 are inserted into the holes of the upper support plate 4 and the lower support plate 5, respectively. It has a well-maintained structure.

An external spring 8 is inserted into the upper end plug 7 of the nuclear fuel element 1 and the hollow rod 2, and the upper end plug 7 has a structure that allows it to slide through the hole in the upper support plate 4, so that it cannot be removed during use. Even if the lengths of the nuclear fuel elements 1 or the hollow rods 2 are uneven due to differences in thermal expansion and elongation due to irradiation, no axial force is applied.

Upper end plugs 7 and lower end plugs 9 of several of the nuclear fuel elements 1
are threaded, and the upper support plate 4 and the lower support plate 5 are connected via the nuclear fuel element 1 by nuts or screws.

At least one of the hollow rods 2 is provided with a protrusion 6 at a portion where the support grid 3 is located, and the protrusion 6 prevents the support grid 3 from moving in the axial direction.

However, in recent years, from the perspective of effectively utilizing uranium resources and reducing fuel cycle costs, attempts have been made to increase the so-called high burnup by prolonging the period in which the fuel is used in the reactor, making it necessary to review the conventional fuel structure. It has become. In particular, as the period of use of the fuel in the reactor becomes longer, the length irregularities due to the difference in irradiation elongation between the nuclear fuel elements 1 and the hollow rods 2 tend to become larger than before, and this phenomenon may occur as a result. As a result, for example, the external spring 8 may come into close contact with the nuclear fuel element 1 and there will be no expansion allowance to absorb the elongation of the nuclear fuel element 1, and unnecessary force will be applied in the axial direction, and the hollow rod 2 with little elongation may be removed from the upper support plate 4 or the lower support plate 5. It is possible that it may come off. Since such a phenomenon is unfavorable in terms of fuel reliability, sufficient consideration must be given to the design when the fuel is to be used in the reactor for a long period of time.

Regarding the problems mentioned above regarding long-term use of fuel in the reactor,
For example, the compression amount of the external spring 8, that is, the nuclear fuel element 1
Another possible method is to increase the amount of expansion of the hollow rod 2 to prevent the external spring 8 from sticking, but this method has the disadvantage that the strength of the spring material must be increased. In addition, as a measure to prevent the end plug from coming off the upper support plate 4 or the lower support plate 5,
In particular, regarding the hollow rod 2 with little irradiation elongation, for example, JP-A No. 56-106183 and JP-A No. 56-11718
However, the former method is more difficult than the conventional method in terms of fuel assembly, and the latter method has drawbacks such as being dependent on the performance of the spring.

[Purpose of the invention]

An object of the present invention is to provide a lower support structure for a nuclear fuel element or hollow rod that is suitable when the upper end of the nuclear fuel element or hollow rod is not supported.

[Summary of the invention]

When fuel is used in a nuclear reactor for a long period of time, as described above, the lengths of the nuclear fuel elements 1 or the hollow rods 2 become uneven due to the difference in elongation due to irradiation, resulting in problems such as tight contact between the external springs 8 and end plugs 7 and 9. Although it is conceivable that unfavorable phenomena may occur from the viewpoint of fuel reliability, such as the possibility of separation from the upper support plate 4 or the lower support plate 5, if a part of the nuclear fuel element 1 or the upper part of the hollow rod 2 is If the structure is not held by the support plate 4, at least there is no concern that unnecessary axial force will be applied to the nuclear fuel element 1 or the hollow rod 2 due to the close contact of the external spring 8, and one of the drawbacks of the prior art is eliminated. become.

However, with such a structure, the end of the nuclear fuel element 1 or the hollow rod 2 will be held only by the lower support plate 5, so some kind of measure should be taken to prevent the lower end plug 9 from coming off the lower support plate 5. It is necessary to securely fix it by some method. In this case, it is necessary to use a method that is easy to fix and does not cause unnecessary damage to the nuclear fuel element 1 or the hollow rod 2 during fixation.

Therefore, as a fixing method, for example, a threaded portion is provided in a portion of the lower end plug 9 that does not fit into the lower support plate 5, and the lower end plug 9 By providing a threaded portion on the outer surface of the portion so as to protrude toward the placement side, the threaded portion of the lower end plug 9 and the threaded portion of the lower support plate 5 are fixed via a nut.

With this fixing method, the fixing operation is performed on the upper surface of the lower support plate 5, so fixing is easy. In addition, when fixing, the nuclear fuel element 1 or the hollow rod 2 is not rotated, but only the identification nut 10 is rotated and fixed, so the nuclear fuel element 1 or the hollow rod 2 and the support grid 3 rub against each other, causing scratches. There is no need to worry about it.

The structure described above eliminates the drawbacks of the prior art when using fuel in a nuclear reactor for a long period of time.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 1 and 3.

FIG. 1 is a diagram schematically showing an embodiment of the present invention. 1st
In the figure, elements on the left represent features of the present invention, and elements on the right represent conventional elements.

The element on the left shows a hollow rod 2 as an example, the upper end of which is not held by the upper support plate 4, and only the lower end of which is fixed to the lower support plate 5 via a nut 10.

Further, the nut 1o for fixing the upper end of the hollow rod 2 is prevented from turning by a washer 11 for preventing rotation of the nut, and is generally designed to prevent the fixing nut 10 from loosening due to vibration or the like.

With this structure, the upper end can expand freely, so that unnecessary axial force is not generated in the nuclear fuel element 1 or the hollow rod 2 due to the close contact of the external spring 8 as in the prior art. Also.

Since the lower end plug 9 and the lower support plate 5 are securely fixed at the lower end via the fixing nuts 10, the nuclear fuel element 1 or the hollow rod 2 will not come off the lower support plate 5. Therefore, the nuclear reactor A structure suitable for use as a fuel that can be used for a long period of time can be obtained.

An example of a specific method of fixing the lower end of the nuclear fuel element 1 or hollow rod 2 to the lower support plate 5, which is a feature of the present invention, will be explained with reference to FIG.

In FIG. 3, the upper part of the lower end plug 9, that is, the part that does not fit with the lower support plate 5, is threaded.

Further, the fitting portion of the lower support plate 5 with the lower end plug 9 is made to protrude from the upper surface of the lower support plate 5 and has a thread cut on its outer surface. The threaded portion of the lower end plug 9 and the threaded portion of the lower support plate 5 are connected and fixed by a fixing nut 10. Further, a washer 11 for preventing rotation of the fixing nut is provided between the fixing nut 10 and the lower support plate 5.
This is to prevent loosening. As the fixing nut rotation prevention washer 11, for example, one shown in FIG. 6, which has a shape similar to that widely used in conventional nuclear fuel assemblies, can be considered. In the example shown in FIG. 6, the rotation of the fixing nut 10 can be prevented by bending the protrusion 12 into the shape shown in FIG. 3 after the fixing nut 10 is tightened.

The fixing procedure as shown in FIG. After inserting the end plug 9 into the fitting hole of the lower support plate 5, rotate the fixing nut 10 with a tool such as a spanner in the direction of removing the fixing nut 10 from the lower end plug 9, and insert it into the lower support plate 5. Tighten the fixing nut 10.

By doing so, the lower end plug 9 can be reliably connected and fixed to the lower support plate 5 via the fixing nut 10. Finally, the protrusion of the anti-rotation washer 11 is bent to prevent the fixing nut 10 from rotating.

In addition, when fixing the lower end portion of the hollow rod 2 for holding the support grid 3 by the method described above, it is recommended to take the following considerations to be sure.

That is, as shown in the conventional example in FIG. 2, the hollow rod 2 for holding the support grid 3 is provided with protrusions 6 for holding the support grid 3 at the positions where the support grid 3 is arranged. After inserting the holding hollow rod 2 into the support grid 3, the hollow rod 2 is rotated, for example, by 45 degrees, so that the projections 6 are caught on the frame of the support grid 3. Therefore, if the hollow rod 2 were to rotate, there is a possibility that the support lattice 3 would come off the protrusion 6 and the support lattice 3 would move in the axial direction. Therefore, as is conventionally known,
For example, as shown in FIG. 4, the cross-sectional shape of the lower end plug 9 may be square, polygonal or semicircular, and the lower support plate 5 may have a cross-sectional shape.
If the holes are also shaped in the same way, rotation of the hollow rod 2 can be prevented, and a lower holding structure for the hollow rod 2 suitable for holding the support grid 3 can be obtained.

Another example of a specific method of fixing the lower end of the nuclear fuel element 1 or hollow rod 2 to the lower support plate 5, which is a feature of the present invention, will be explained with reference to FIG.

The method of fixing the lower end plug 9 and the lower support plate 5 in FIG. 5 is basically the same as the embodiment shown in FIG. 3.

One embodiment shown in FIG. 5 is characterized by having an adapter 13 screwed into the lower support plate 5. The upper part of the adapter has a protrusion on which the fixing nut 10 is caught, and the fixing nut 10 is held by the protrusion.
is held by the lower support plate 5.

In the case of this embodiment, the fixing operation may be performed, for example, by the following procedure. That is, after fitting the fixing nut 10 and the anti-rotation washer 11 into the adapter 13, the adapter 13 is screwed into the lower support plate 5, and then the nuclear fuel element 1 is screwed into the lower support plate 5.
Alternatively, the hollow rod 2 is inserted into the hole of the lower support plate 5, and the fixing nut 10 is rotated to connect and fix the lower end plug 9 and the fixing nut 10. Finally, as in the embodiment shown in FIG.
What is necessary is to prevent the rotation of o.

〔Effect of the invention〕

According to the present invention, in a nuclear fuel assembly having one or more nuclear fuel elements 1 or hollow rods 2 whose upper ends are not supported by the upper support plate 4, the lower ends of the nuclear fuel elements 1 or hollow rods 2 are It can be securely fixed to the lower support plate 5. Moreover, since there is no external spring 8 on the upper part of the nuclear fuel element 1 or the hollow rod 2 and it is not held by the upper support plate 4, the length due to the difference in irradiation elongation between the nuclear fuel elements 1 or the hollow rod 2 during use. Even if misalignment occurs, no unnecessary force is generated on the nuclear fuel element 1 or the hollow rod 2, so a structure suitable for fuel used for a long period of time in a reactor can be obtained.

Furthermore, if the structure according to the present invention is applied to the hollow bar 2 for holding the support grid 3, the lower end plug 9 of the hollow bar 2 can be attached to the lower support plate 5.
Since the support grid 3 can be reliably fixed and prevented from rotating, it is possible to reliably hold the support grid 3 at a predetermined axial position.

Furthermore, the method of fixing the lower end plug 9 to the lower support plate 5, which is a feature of the present invention, is based on an operation on the upper surface of the lower support plate 5, and there is no operation of rotating the nuclear fuel element 1 or the hollow rod 2 during fixation. From things like that.

It is easy to assemble and can be assembled without causing unnecessary damage.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing the structure of a conventional nuclear fuel assembly, Fig. 3 is a vertical sectional view showing details of an embodiment of the invention, and Fig. 4 The figure shows the lower end plug M-M
FIG. 5 is a longitudinal sectional view showing details of an embodiment of the present invention, and FIG. 6 is a view showing an example of a washer for preventing rotation of a nut. DESCRIPTION OF SYMBOLS 1...Nuclear fuel element, 2...Hollow rod, 3...Support grid, 4...Upper support plate, 5...Lower support plate, 6...
- Support grid holding protrusion, 7... Upper end plug, 8... External spring. 9... Lower end plug, 10... Nut, 11... Nut rotation prevention washer, 12... Nut rotation prevention washer protrusion, 13... Screw-in adapter.詩／E7 3rd fEJ 萬4/El

Claims (1)

[Claims] 1. In a fuel assembly for a light water reactor, in which a plurality of nuclear fuel elements and one or more hollow rods are bundled by a plurality of support grids, and the upper and lower ends of the nuclear fuel elements and hollow rods are held by upper and lower support plates. , having one or more nuclear fuel elements or hollow rods whose upper ends are not held by an upper support plate; A thread is cut in the part that does not fit with the plate, and the fitting part of the lower support plate that fits with the lower end plug protrudes upward from the other parts and has a thread cut on the outer surface, and the lower end plug threaded part A nuclear fuel assembly characterized in that the threaded portion of the lower support plate is connected to the threaded portion of the lower support plate through a nut.