JPS63275990A - Fuel assembly - Google Patents

Abstract

PURPOSE:To safely and easily disassemble a fuel assembly, by providing three or more tab-like projections to the part of the inlet edge of the support engaging hole provided to a lower tie plate in which a lower end plug is inserted at an equal interval in the circumferential direction thereof. CONSTITUTION:Three or more tab-like projections 9 are provided per the edge surface 8a of one support engaging hole. Therefore, the taper part 6a of a lower end plug 6 is seated on each of the tab-like projections 9 and, moreover, a fuel rod 3 is well-balanced without inclination. Further, since the tab-like projections 9 are positioned at an equal interval in the circumferential direction, the edge part 8b of the support engaging hole 8a of a tie plate 8 and the taper part 6a of the lower end plug 6 become spot contact and the contact area of both of them is reduced. By this constitution, the bonding strength of the lower end plug due to the adhesion of fur or an oxide film to the tie plate can be reduced and the lower end plug can be easily pulled off from the support engaging hole.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly loaded into the core of a light water reactor such as a boiling water reactor (hereinafter referred to as BWR).

(Prior Art) The general configuration of a fuel assembly loaded into a BWR core is as shown in FIG. 5, in which a fuel bundle 2 is housed in a rectangular cylindrical channel box 1.

In the fuel bundle 2, fuel rods 3 are arranged in a square lattice of, for example, 8 rows by 8 columns, and are bound into a bundle by spacers 4 in the shape of flat rectangular cylinders arranged in multiple stages in the axial direction.

The fuel rod 3 is constructed as shown in FIG. 6, a fuel cladding tube 3a is filled with a plurality of fuel pellets 3b, an upper end plug 5 is provided at the upper end of the fuel cladding tube 3a, and an upper end plug 5 is provided at the lower end of the fuel cladding tube 3a. Each of the lower end plugs 6 is stopped. The upper end plug 5 is supported by the upper tie plate 7, and the lower end plug 6 is supported by the lower tie plate 8.

In conventional fuel, the lower end plug 6 is fitted into the support fitting hole 8a of the lower tie plate 8, as shown in FIG.
A portion of the smooth outer circumferential surface of the tapered portion 6a of the lower end plug 6 is supported by coming into line contact with an edge surface 8b formed by chamfering the upper end opening periphery of the supporting fitting hole 8a.

(Problems to be Solved by the Invention) However, in such conventional fuel assemblies, as the residence time in the reactor becomes longer, the contact area between the lower end plug 6 and the supporting fitting hole 8a of the lower tie plate 8 is reduced. When the fuel assembly is dismantled, the adhesion force becomes strong, and when the fuel assembly is disassembled, an excessive load may be applied to the fuel rod, which may damage its integrity.

That is, because the fuel assembly stays in reactor water for a long time, water scale adheres to the outer periphery of the fuel rods 3, such as the fuel cladding tube 3a, the upper end plug 5, the lower end plug 6, etc., and an oxide film is formed. Ru.

In particular, since the outer circumferential surface of the tapered portion 6a of the lower end plug 6 is smooth, the edge surface 8b of the upper end opening of the supporting fitting hole 8a of the lower tie plate 8 that contacts the tapered portion 6a is linear. Since the contact area is large, there is a risk that this contact area may become firmly fixed due to limescale or oxide film.

As long as the fuel burn-up is relatively small, the staying time in the reactor is short, so the sticking force is weak and does not pose a particular problem.

However, as fuel assemblies spend longer in the reactor due to the increase in fuel combustion rates in recent years, the water scale that adheres to the outer periphery of the fuel rods 3 and the thickness of the oxide film formed from it also grow and become thicker. Become.

For this reason, the adhesion force of the line contact portion between the lower end plug 6 and the lower tie plate 8 is further strengthened, and when the fuel assembly is disassembled, the lower end from the support fitting hole 8a of the lower tie plate 8 is removed. It becomes difficult to pull out the plug 6, and if a pulling force that exceeds the fixing force between the lower end plug 6 and the supporting fitting hole 8a is applied to both, the integrity of the fuel rod 3 may be damaged.

SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel assembly that can reduce the adhesion force between the lower tie plate and the lower end plug and that can be easily disassembled without impairing the integrity of the fuel rods.

[Structure of the invention]

(Means for Solving the Problems) The present invention solves the problems of the conventional example in that the edge surface 8b of the upper end opening of the fitting hole of the lower tie plate is formed as a smooth surface, so that the taper of the lower end plug is This was done by focusing on the fact that the contact area with the outer circumferential surface of the lower tie plate is large, and the adhesion force between the two is large. A tab-shaped protrusion is formed on the edge surface of the upper opening to reduce the contact area, and is constructed as follows.

That is, the present invention provides a fuel assembly in which a lower end plug of a fuel rod containing a plurality of fuel pellets is fitted into a support fitting hole of a lower tie plate to be supported, in which a tapered portion of the lower end plug is supported. A tab-shaped protrusion is formed on the edge surface of the upper end opening of the support fitting hole of the lower tie plate into which the lower tie plate is fitted.

(Function) The tabular surface of the lower end plug contacts the tab-shaped protrusion provided on the edge surface of the upper end opening of the support fitting hole of the lower tie plate at a point, so the contact area is reduced. The adhesion force of this contact portion is reduced.

Therefore, as fuel combustion progresses, water scale or oxide film adheres to the uneven surface of the edge surface of the upper end opening of the support fitting hole of the lower tie plate and the tapered surface of the lower end plug, and the film thickness gradually decreases. Even if they grow and become thicker, these oxide films only grow in the recesses of the uneven surface of the lower tie plate, and do not strengthen their adhesion.

As a result, the lower end plug can be pulled out from the lower tie plate with a relatively small pulling force, and the fuel assembly can be easily dismantled without damaging the integrity of the fuel rods.

(Example) Hereinafter, an example of the present invention will be described based on FIGS. 1 to 3.

FIG. 1 shows a main part of an embodiment of the present invention, and this embodiment is characterized by an edge surface 8b of an upper end opening of a supporting fitting hole 8a of a lower tie plate 8. As shown in FIG.

That is, a tab-shaped projection 9 is provided on the edge surface 8b of the upper end opening of the support fitting hole 8a of the lower tie plate 8. The tapered portion 6θ of the lower end plug is seated on the tab-shaped projection 9. In order to balance the lower end plug 6, that is, the fuel rod 3, without tilting, the tab-shaped protrusion 9 is
Three or more support fitting holes are provided per edge surface 8b.

FIG. 2 is an enlarged view of the support fitting hole 88 of the lower tie plate 8 provided with three tab-shaped protrusions 9 according to one embodiment. - Tab-shaped protrusions 9 are positioned at equal intervals in the circumferential direction. Therefore, the edge portion 8b of the support fitting hole 8a of the lower tie plate 8 and the tapered portion 6a of the lower end plug 6 come into point contact, and the contact area is reduced.

That is, when the number of tab-shaped protrusions 9 is three and the thickness of the tab-shaped plate is about 1 m, the contact area between the lower end plug 6 and the lower tie plate 8 is equal to that of the tab-shaped protrusions 9 as in the conventional example. It is approximately 1 compared to the case where the protrusion 9 is not provided and the lower end plug 6 and the support fitting hole 8a of the lower tie plate 8 are in line contact. The height of the tab-shaped protrusion 9 is set to be approximately 1 m or more, with consideration given to making it larger than the thickness of the water scale that adheres and the oxide film that is formed. FIG. 3 shows an example of the shape of the tab-shaped protrusion 9. As shown in FIG.

FIG. 4 shows another embodiment of the tab-shaped protrusion 9. The following three methods can be considered for attaching the tab-shaped protrusion 9. First of all, when making the lower tie plate by casting, there is a method to make this tab-shaped protrusion at the same time. The second method involves machining the lower tie plate using a casting machine. A third method is to attach a tab-shaped protrusion by welding to the edge of the upper opening of the support fitting hole, which has been made smooth by spinning 9 machining of the lower tie plate.

Whichever method is used, the function of reducing the contact surface between the lower end plug and the lower tie plate is the same.

Even if an oxide film grows and limescale adheres, the tapered surface 6a of the lower end plug formed by the tab-shaped protrusion 9, the support fitting hole 8a of the lower tie plate, and the edge 8b of the upper end opening. The gap between the two can be maintained.

Therefore, the tapered surface 6a of the lower end plug and the support fitting hole 8a of the lower tie plate can maintain a point contact state even after the required fuel combustion, reducing the contact surface roughness. .

Therefore, according to this embodiment, it is possible to reduce the adhesion force of the lower end plug to the lower tie plate due to the adhesion of water scale and oxide film, and to facilitate the removal of the lower end plug from the support fitting hole. be able to.

This prevents damage to the integrity of the fuel rod by applying excessive external force to the fuel rod when the lower end plug is pulled out of the support fitting hole.

It becomes possible to dismantle the focal body while maintaining the integrity of the fuel rod.

〔Effect of the invention〕

As explained above, in the present invention, a protrusion is provided on the edge surface of the upper end opening of the support fitting hole of the lower tie plate into which the lower end plug is fitted, thereby forming an uneven surface. The contact area with the fitting hole can be reduced, and the adhesion force between the two can be reduced.

Therefore, even if an oxide film forms and grows around the lower end frame, it is possible to prevent the VAm force between the lower end plug and the support fitting hole from increasing, and to support the lower end plug with a relatively small external force. The fuel assembly can be easily and soundly disassembled by pulling it out from the fitting hole.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view showing a main part of an embodiment of a fuel assembly according to the present invention, and FIG. 3 is a sectional view taken along the line A-A in FIG. 2, FIG. 4 is a longitudinal sectional view showing another embodiment of the present invention, and FIG. 5 is a longitudinal sectional view of a conventional fuel assembly. FIG. 6 is a longitudinal sectional view of a conventional fuel rod, and FIG. 7 is a longitudinal sectional view of a lower tie plate portion of a conventional fuel assembly. 3. Fuel rod 6. Lower end plug 6a, tapered part 8. lower tie plate 8a,
Fitting hole for support 8b, edge surface 9, protrusion Patent attorney Norihiro Ken Ken Daishimaru Figure 1 Figure 2 8b 8α Figure 3 8α Figure 4 Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] 1. In a fuel assembly in which the lower end plug of a fuel rod containing a plurality of fuel pellets is fitted into a support fitting hole of a lower tie plate for support, the lower tie plate into which the lower end plug is fitted is used to support the lower tie plate. A fuel assembly characterized in that three or more tab-shaped protrusions are provided at equal intervals in the circumferential direction on the edge of the entrance of the fitting hole.