JPH0569195B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a fuel spacer, and is particularly suitable for preventing damage to the cladding surface of fuel rods and water rods (hereinafter referred to as fuel rods, etc.) when assembling a fuel assembly. The present invention relates to a fuel spacer having an elastic support such as a fuel rod.

[Background of the invention]

The conventional fuel spacer is described in Japanese Patent Application Laid-Open No. 58-89181.
As described, for example, an elastic support as shown in FIG. 4 (hereinafter referred to as a lantern spring) is used to support fuel rods of a boiling water reactor fuel assembly. It is installed at the intersection of the spacer grids, and presses the fuel rods, etc. stored in the grids against the dimples and S-bands on the opposing grid plate, so as to keep the spacing between the fuel rods, etc. constant during operation. It is considered.

The above-mentioned lantern spring is designed to prevent the spacing between fuel rods from becoming distorted due to hydraulic vibrations and thermal deformation during operation. In addition to maintaining the same spacing, the structure is designed such that the optimal pressing force is applied by the spring part to avoid excessively pressing the cladding tube of the fuel rod, etc. and generating unnecessary stress. has been done.

In recent years, with the aim of reducing the cost of nuclear power generation,
Rational design of fuel assemblies is underway, and as part of this effort, there are plans to abolish surface autoclaves for cladding of fuel rods, etc.

Compared to a cladding tube with a black surface layer formed by autoclaving, the surface without an autoclave is relatively soft, and the cladding surface scratches that occur during fuel assembly are not a problem with current autoclaved cladding tubes. It is expected that this will occur and its depth will increase. To assemble the fuel assembly, a plurality of spacers with one lantern spring attached to each of the four lattice frames are fixed in a line on a jig, and a fuel rod etc. is inserted from one side. Since the assembly is performed by penetrating all the spacers, the cladding tube of the fuel rod etc. slides on the spring part of the lantern spring or the dimple and S band on the spacer grid plate facing the spring part, and the cladding tube May cause scratches on the surface
In particular, when the surface hardness is low, deep scratches inevitably occur more frequently. For this reason, it is expected that the current shape of the lantern spring will not be sufficient to prevent damage during fuel assembly for future specifications such as fuel rods that do not require an autoclave.

When assembling the fuel assembly, scratches that occur on the surface of the cladding tube are caused by the cladding tube sliding against the spring part of the lantern spring attached to the spacer or the spacer grid plate facing the spring part, as described above. Occur.

The ease with which the above scratches occur is correlated with the pressing force of the lantern spring that presses and fixes the cladding tube toward the opposing grid plate within the spacer, but it is also possible that hydraulic vibration or thermal deformation occurs during operation. However, due to the original purpose of the lantern spring, which is to maintain a constant distance between the fuel rods, etc., and the constraint of not damaging the integrity of the cladding by applying unnecessary stress to the cladding, it is difficult to press the lanternspring. The force is designed to be the optimum value based on numerous demonstration tests, results in commercial reactors, and stress analysis.

The lantern springs currently in use have a structure as shown in Figure 4, and are determined by the thickness of the material and the offset (the amount of displacement of the cladding tube contact part of the spring part when the cladding tube is inserted into the spacer lattice). )
The structure is such that the pressing force is determined by .

Therefore, since it is impossible to reduce the pressing force and prevent scratches from occurring, a sheet with low frictional force is inserted at the contact area between the spring part of the lantern spring and the cladding tube to prevent scratches from occurring due to sliding. Although methods have been considered to prevent this, the diameter of the welded part of the cladding end plug (hereinafter referred to as bead diameter) is usually larger than the diameter of the cladding, so when this part passes through the spacer, In order to avoid excessive bending of the lantern spring and deformation to the plastic range of the spring, the film sheet must be reversed and manufacturing requirements for this bead diameter must be made stricter, but this does not affect the fuel assembly described above. This is not necessarily a good idea as it goes against the rationalization of the body.

Therefore, we focused on the structure of the lantern spring itself, and when assembling, we temporarily recessed the spring part that comes into contact with the cladding tube of the lantern spring by some method, so that at least the lantern spring could have a dimple on the spacer lattice facing the cladding tube. and must not be pressed against the S band.

On the other hand, from the perspective of rationalizing fuel assemblies,
It is necessary to minimize changes to the conventional spacer structure, and the structure must be improved to allow for better backfit.

In the current spacer, as shown in Figure 5a and b, the lantern spring length is slightly longer than the height of the spacer band indicated by 8 in the figure, and the upper and lower ends of the lantern spring are located at the upper and lower parts of the spacer. Since the lantern spring protrudes from the spacer grid and has a structure with considerable play in the vertical direction, the only parts to operate the lantern spring when assembling the fuel assembly are the upper and lower ends of the lantern spring. Therefore, with the current structure, vertical movement (deformation) of the spacer lattice is only possible.

However, in the currently used lantern spring shown in FIG.
Therefore, the contact portion 3 with the cladding tube can be lowered only by pulling the upper and lower end portions 1 of the lantern spring up and down, and this vertical tension is not sufficient at the initial stage of fuel assembly assembly. Good, but it is technically almost impossible to pull the fuel rods up and down with more than half of the fuel rods (30 rods) inserted into the 8x8 lattice spacer, and even if it were possible, the operability would be extremely poor. I have no choice but to do so.

Therefore, the only remaining method of operating a lantern spring is to press the upper and lower end portions of the lantern spring against each other from above and below, but in the structure of the lantern spring currently in use, conversely the peaks of the spring portion are raised high. The intended purpose cannot be achieved because the object is deformed in the direction of

[Purpose of the invention]

The present invention was made in view of the actual situation described below, and employs the only remaining method of operating a lantern spring, in which the upper and lower end portions of the lantern spring are pressed against each other from above and below.
The purpose is to reduce the occurrence of scratches on the surface of the cladding tube when assembling the fuel assembly by recessing the spring portion of the lantern spring. That is, an object of the present invention is to provide a cladding tube of a fuel rod, etc., which has not undergone surface autoclaving, etc., with an elastic support (lantern spring) that reduces the possibility of surface scratches occurring during assembly of a fuel assembly. An object of the present invention is to provide a fuel spacer with improved performance.

[Summary of the invention]

In order to achieve the above object, the present invention provides a fuel spacer comprising a spacer lattice forming a plurality of spaces into which fuel rods are inserted, and elastic supports provided at the intersections of the spacer lattice. The spring portion of the elastic support has a contact portion with the fuel rod, and the spring portion is recessed inward from the upper and lower ends of the upper and lower ends of the elastic support, and the spring portion contacts the upper and lower ends at the contact portion. It is characterized by projecting outward from the end.

[Embodiments of the invention]

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

First Embodiment A first embodiment of the present invention is shown in FIG. In this embodiment, the spring part 2 of the lantern spring comes out with an inclination that is once concave toward the inside of the lantern spring with respect to the upper and lower end parts 1 of the lantern spring that form the attachment parts to the spacer grid, and then goes out from the outside of the lantern spring. It is formed into a mountain shape with an overhang. The chevron-shaped protruding portion 3 of the spring portion 2 forms a contact portion with a cladding tube such as a fuel rod, and the spring portion 2 is configured vertically symmetrically. By pressing the upper and lower end portions 1 of the lantern spring configured as described above against each other from above and below, a moment is generated in the spring portion 2 of the lantern spring that causes the chevron-shaped protruding portion 3, which is to become the cladding tube contact portion, to dent inward. work. The behavior of the spring portion 2 at this time is shown by the broken line in FIG.

In addition, the results of analysis of the offset of the lantern spring of the above example (the amount of displacement of the contact part of the cladding tube of the spring part when the cladding tube is inserted into the spacer grid) and the pressing force against the cladding tube are shown in the solid line in Fig. 6. show. The dashed line in the figure represents the offset and pressing force of a conventional lantern spring.

For the same amount of offset as the conventional lantern spring, the lantern spring of the present invention has a smaller pressing force. In other words, it becomes a soft spring, but as explained earlier, this pressing force is optimized, and excessive force is unnecessary from the perspective of the integrity of the cladding, so this optimized Since it is sufficient to secure the amount of offset that generates the pressing force, it is understood from FIG. 6 that the same pressing force as the conventional one can be obtained by increasing the offset amount by about 6% compared to the conventional offset amount.

Another possible method is to increase the thickness of the spring itself to increase its rigidity and thereby increase the pressing force for the same amount of offset. In this case, it is sufficient to increase the plate thickness by about 8% compared to the plate thickness rigidity.

All of the above methods are fully applicable to current processing techniques and can be applied to current spacers.

An assembly jig for compressing a lantern spring from both ends according to the present invention is shown in FIGS. 3a and 3b.

In this assembly jig, 4 and 5 in the same figure are the upper and lower compression rods, respectively, and there are a total of 8 (upper...4, lower...4).
The upper and lower ends of the lantern springs, which are attached to each of the four lattice frames of the spacer in pairs, are sandwiched between the upper and lower ends of the lantern springs, so that they can each rotate around the rotating shaft 6. It's summery. After holding the lantern spring from both ends using the lever principle, the fuel rod or water rod is inserted into the spacer grid. Since all the lantern springs are recessed inward by the same amount, damage to the surface of the cladding tube such as the fuel rod during the insertion process is completely prevented.

Second Embodiment A second embodiment of the present invention is shown in FIG. The lantern spring according to the second embodiment is different from the first embodiment in that the lantern spring has a chevron-shaped spring portion with two contact portions with the cladding tube as shown in the figure. The lantern spring part 2 emerges from the upper and lower end parts 1 of the lantern spring with an inclination that is once concave toward the inside of the spring, and then projects outward from the spring and climbs up to the mountain-shaped protruding part 3 that becomes the contact part with the cladding tube. The shape is the same as that of the first embodiment of the invention. However, in this second embodiment, there are two chevron-shaped protruding parts 3, 3, and there is a recess between the two protruding parts, so that the stress generated in the spring when displacement in the pressing direction occurs is different from that in the first embodiment. Compared to a single peak shape like this, the spring part covers a wider range, making it easier to design from the standpoint of SCC (stress corrosion cracking).

〔Effect of the invention〕

According to the present invention, while maintaining the basic shape of the conventional spacer, the inclination state of the spring portion of the elastic support body (lantern spring) provided at the intersection of the spacer lattice is slightly changed. By compressing both ends of the lantern spring using the assembly jig described above, the overhanging part of the spring part of the lantern spring that contacts the cladding tube is deformed inwardly, and the fuel assembly is assembled. When a fuel rod or water rod is inserted into the spacer grid during assembly, the cladding tube must be easily inserted without being pressed by the lantern spring against the dimples or S-bands on the spacer grid plate and without unnecessary sliding. become. This reduces the possibility of scratches on the surface of the cladding compared to the conventional case where the lantern spring slides in contact with the cladding with the same pressing force as during reactor operation. Reduce to a large extent. Recently, there have been plans to reduce nuclear power generation costs, and as part of this plan, it is planned to abolish the use of autoclaves on the surface of fuel cladding tubes. Since scratches are likely to occur, it can be said that the elastic body for holding fuel rods according to the present invention has a remarkable effect on fuel rods and the like that have been rationally designed.

[Brief explanation of the drawing]

FIG. 1 is an elevational view of an elastic support for holding fuel rods (lantern spring) used in a fuel spacer according to a first embodiment of the present invention, and FIG. 2 is an elevational view of a lantern spring according to a second embodiment. FIGS. 3a and 3b show an assembly jig used for inserting fuel rods and the like into a fuel spacer according to the present invention, with FIG. 3a being a side view and FIG. 3b being a plan view. Figure 4 is an elevational view of a conventional lantern spring, Figure 5 a, b
4 shows a spacer to which the lantern spring shown in FIG. 4 is attached; FIG. 4A is a side view and FIG. 4B is a plan view. FIG. 6 is an analytical diagram showing the relationship between offset and pressing force in a lantern spring according to an embodiment of the present invention and a conventional lantern spring. 1... Upper and lower end parts of the lantern spring, 2...
... Spring portion of the lantern spring, 3... Mountain-shaped protruding portion of the lantern spring, 4... Upper compression rod of the assembly jig, 5... Lower compression rod of the assembly jig,
6... Rotating shaft of the assembly jig, 7... Hook of the assembly jig, 8... Spacer band.

Claims (1)

[Claims] 1. A fuel spacer comprising a spacer lattice forming a plurality of spaces into which fuel rods are inserted, and elastic supports provided at intersections of the spacer lattice, in which a spring portion of the elastic support supports the fuel rods. The spring portion has a contact portion with the upper and lower ends of the elastic support, and the spring portion is recessed inward from the upper and lower ends of the elastic support and protrudes outward from the upper and lower ends at the contact portion. Features fuel spacer.