JPS62110188A - 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 for a boiling water nuclear reactor, and particularly to a fuel assembly having a tie plate (fuel support plate).

[Background of the invention]

JP-A-58-147682 is related to measures to prevent fuel rod breakage when fuel assemblies fall, and this method reduces the impact energy when falling by attaching an elastic body to the lower tie plate. This is to prevent fuel rod breakage by absorbing water, and requires considerable changes to the cast metal fittings of the tie plate during manufacturing, but in this patent, the tie plate supports the combined fuel rods, which are the most easily damaged of the fuel rods. The objective is to easily take measures against fuel rod damage during manufacturing.

[Purpose of the invention]

The purpose of the present invention is to provide a notch in the boss of the tie plate that supports the fuel assembly during fuel handling and is most likely to be damaged when the fuel falls, by fixing it with a screw. The present invention provides a fuel assembly structure that prevents damage to the fuel rods by causing damage to the boss portion when the fuel assembly falls.

[Summary of the invention]

The present invention is aimed at preventing damage to fuel rods in cases where fuel assemblies fall, which may occur during work handling boiling water reactor fuel. In order to prevent damage to the combined fuel rods due to bending stress caused by deformation of the tie plate lattice plate, the strength of the support part of the combined fuel rods of the tie plate is reduced, and the boss part is damaged in the event of a fall to alleviate the bending stress. It is.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG.

As shown in Fig. 1, a fuel assembly used in a boiling water reactor includes a spacer 3 that maintains a constant horizontal spacing between a plurality of fuel rods 4, and a vertical spacing between these spacers 3 that maintains a constant spacing. The fuel rod 4 is composed of an upper tie plate 1 and a lower tie plate 2 that support the water rod 5, the fuel rod 4, and the water rod 5, and maintain a constant spacing between them.

The upper and lower ends of the fuel rods are screwed to the fuel rods 4, and the combined fuel rods 6, which have the function of supporting the weight of the fuel assembly when performing the hand song of the fuel assembly, are used to hold other fuels. The upper and lower end plugs of the rod 4 are connected to the upper and lower tie plates 1.
, 2, and can move freely in the axial direction of the fuel assembly, but is restrained by screws to the upper and lower tie plates. FIG. 2 shows a detailed view of the lower tie plate 2.

Two combined fuel rods 6 are arranged on each side of the outermost periphery of the tie plate, and are screwed and fixed to a boss portion 7 provided on the tie plate. Further, the boss portion 7 has rectangular parallelepiped protrusions called ribs 8 between each boss, thereby supporting each boss portion.

In a fuel assembly having the above structure, if a fall accident occurs when the fuel assembly is handled, the load generated by the acceleration at the time of the fall and the fuel assembly's own weight (approximately 310 kg) will cause the fall to occur. An excessive impact force acts on the lower end of the lower tie plate 2 of the fuel assembly. The impact load deforms the lattice-shaped portion of the lower tie plate, thereby generating bending stress in the lower end plug of the fuel rod 4.

However, the normal fuel rod 4 is connected to the upper and lower tie plates only by inserting the upper and lower end plugs 9 of the fuel rod into the boss portions 7 provided on the tie plates, respectively, and is not screwed. Therefore, it can move freely in the axial direction. Therefore, there is a margin in terms of strength against the above bending stress.

On the other hand, the combined fuel rod 6 has a structure in which the lower end plug is screwed directly into the boss portion 7 of the lower tie plate, and the upper end plug is threaded and tightened with a nut 11.

Therefore, since the above-mentioned bending stress acts properly, the end plug 12 of the combined fuel rod 6 may be damaged by the past load transmitted from the boss portion.

In past cases, the combined fuel rod 6 was damaged.
are often observed.

In view of the above, as a measure to prevent fuel assembly falling accidents, in this embodiment, as shown in FIG. By removing part rib 8, the strength will be strong enough to withstand normal handling, but will it withstand too much when dropped? #The boss part 7 of the tie plate is designed to be easily damaged when an impact force is generated, and by damaging the boss part 7 of the tie plate when the fuel assembly falls, the end plug 12 of the combined fuel rod 6 is removed from the boss part 7. This has the effect of drastically reducing the impact stress propagated to the joint fuel rods 6 and preventing damage to the combined fuel rods 6.

Acceleration that acts when handling a fuel assembly occurs when the fuel assembly is lifted from the reactor by a refueling machine during fuel inspection, or when it is seated on the fuel support inside the reactor, and is caused by fuel design. In this case, an acceleration of approximately 3 to 4 g is considered, including 1 g of its own weight.

The relationship between these accelerations and the stress generated in the lower end plug 12 of the aforementioned combined fuel rod 6 is shown in FIG.

On the other hand, the stress generated in the boss portion of the tie plate also increases as the acceleration generated in the fuel assembly increases, and the stress generated in the boss portion is further increased by a stress concentration factor corresponding to the shape of the notch 10 provided according to the present invention. varies as a function of α.

Therefore, let α0 be the acceleration at which the stress generated in the combined fuel rod 6 reaches the yield stress σy1 of the end plug 12.

As shown in Fig. 5, the stress generated in the boss portion 7 of the tie plate due to the acceleration α0 is the yield stress σy of the tie plate.
If a notch shape of α is provided in the boss part of the tie plate so that the stress concentration coefficient becomes 2 or more, the boss part 7 of the tie plate will always be damaged first when the fuel assembly falls, and the stress from the boss part 7 will be damaged first. The bending stress propagated to the end plug of the combined fuel rod 6 is alleviated, and damage to the combined fuel rod 12 can be prevented.

As a result, the fuel rods will not be damaged even when the fuel assembly falls, making it possible to avoid releasing fissile product gas and the like within the fuel rods into the reactor water.

Regarding the present invention, the above-mentioned publicly known Japanese Patent Application Laid-Open No. 58-14768
In No. 2, an elastic body is attached to the lower tie plate to absorb the impact energy when the fuel assembly falls and prevent damage to the fuel rods.

In manufacturing the tie plate, it is easier to manufacture casting molds, etc. compared to the tie plate structure of the prior art.

On the other hand, in the present invention, the above-mentioned notch 10 is simply provided in the boss portion 7 of the tie plate, and the manufacturing process is not particularly beautiful compared to the prior art.

In the embodiment shown in FIG. 3, a notch 10 is provided in the end plug boss 7 of the combined fuel rod 6 of the lower tie plate, but the upper end plug of the combined fuel rod 6 fixed with a nut 11 has a notch 10. Providing a notch in the fuel rod is also effective in preventing damage to the combined fuel rod.

〔Effect of the invention〕

According to the present invention, by making a relatively simple structural change, even if an accident occurs in which a fuel assembly falls while handling the fuel assembly, the connecting fuel rod end plug can prevent the excessive impact force generated at the time of the fall from occurring. This makes it possible to prevent the leakage of fission product gas and the like into the reactor due to damage to the combined fuel rods.

Furthermore, since the notches are provided in the boss portions of the upper and lower tie plates, the cross-sectional area of the coolant flow path within the fuel assembly becomes slightly larger, which reduces the pressure loss of the coolant flowing within the fuel assembly. You can also expect good results.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a fuel assembly according to an embodiment of the present invention, and FIG.
The figure is a detailed view of the lower tie plate, Figure 3 is a cross-sectional plan view of an embodiment of the present invention, Figure 4 is a diagram showing the relationship between acceleration and stress generated in the end plug, and Figure 5 is a diagram showing the relationship between acceleration and stress generated at the tie plate boss. It is a relationship diagram of stress. DESCRIPTION OF SYMBOLS 1... Upper tie plate, 2... Lower tie plate, 3... Spacer, 4... Normal fuel rod, 5... Water rod, 6... Combined fuel rod, 8... Rib Department,
10... Notch.

Claims (1)

[Claims] 1. In a fuel assembly for a boiling water reactor in which a plurality of fuel rods are arranged and supported in a square grid by a plurality of fuel spacers, the upper part that supports the upper and lower end plugs of the fuel rods is A fuel assembly characterized in that a notch is provided in a boss portion of one of a tie plate and a lower tie plate that holds a combined fuel rod.