JPS61256283A - 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] The present invention relates to a fuel assembly, and particularly to a fuel assembly having a water rod shape suitable for improving the soundness of a large water rod.

[Background of the invention]

FIG. 4 shows a fuel assembly used in a boiling water nuclear reactor, which has a large water rod in the center of the assembly that occupies four fuel rods.

Figure 4 li! The collection of 8 families is published in Japanese Patent Application Laid-Open No. 59-65792.
This is shown in the number. Divided into four diagrams, 1 is a fuel rod, 2 is a water rod, 4 is a lower tie plate, 9a is a lower end plug of water rod 2, and 9b is a lower end plug of fuel rod 1. A plurality of fuel rods 1.

It is held by an upper tie plate and a lower tie plate 4, and the water rod 2 is installed on the lower tie plate 4 by a lower end plug 9a. Also, JP-A-5
As described in Japanese Patent No. 9-65792, a structure in which the water rod 2 also has an upper end plug and the water rod 2 is held by an upper tie plate and a lower tie plate 4 in the same way as the fuel rod 1 is also considered.

7jlcs the lower structure of the fuel ill and water rod 2 and lower tie plate 4 for these fuel assemblies.
This will be explained with a diagram. In FIG. 4, 21 is a coolant passage hole provided in the lower tie plate 4. In FIG. The coolant coming from below the lower tie plate 4 flows through the passage hole 21.
The fuel rods 1 are guided through the fuel assembly to cool the fuel rods 1.

Further, 41a is a water rod boss, and 41b is a fuel rod boss.

As shown in FIG. 4, conventional water rod 2
Since the water rod boss 41a is made larger according to the outer diameter of the water rod, the area of the coolant passage hole 21 in the center of the lower tie plate 4 becomes smaller, and the flow of water into the lower tie plate 4 below the water rod 2 becomes smaller. There was a problem in that the resistance increased and the pressure loss increased.

To deal with this, 1. For example, as shown in FIG. 5, the tr area of the water rod boss 41a is decreased and the area of the coolant passage hole 210 is increased. As a result, the integrity of the quarter rod 2 against flow vibration or fretting corrosion becomes a problem.

Let us consider a case where a large displacement occurs in the fuel assembly during an earthquake. As shown in FIG. 4 or FIG. 5, the 'F' end plugs 9a, l! of the water rod 2 are shown. : If the fitting part with the lower tie plate 4 is thin like the lower end plug 9b of the fuel rod 1, the outer diameter of the water rod 2 is large and the cross-sectional area is extremely large compared to the fuel rod 1. Therefore, since the water rod 2 itself has high rigidity, if a large displacement occurs between the water rod 2, the fuel rod 1, and the lower tie plate 4.

A large bending moment is applied to the F end frame 9a of the water rod 2, which poses a major problem in terms of mechanical soundness.

[Purpose of the invention]

An object of the present invention is to provide a fuel assembly that has low flow resistance and that can always ensure the mechanical integrity of the lower end plug of the water rod.

[Summary of the invention]

A feature of the present invention is that in the hollow tube constituting the water rod, the outer diameter of the lower part of the hollow tube is narrower than the outer diameter of the central portion of the hollow tube; The range is 401H or more from the upper surface of the lower tie plate of the hollow rod to a range not exceeding the lowermost spacer position.

This makes it possible to reduce flow resistance and ensure the mechanical integrity of the lower end plug of the water rod during earthquakes and the like.

Due to flow characteristics, the smaller the cross-sectional area of the flow path, the greater the pressure loss. In order to reduce pressure loss, the cross-sectional area of the flow path may be increased. The flow resistance t in the large automatic rod section can be reduced by setting the outer diameter of the hollow tube at the bottom of the water rod to a<f.

On the other hand, stress applied to the end plug of the water rod during an earthquake will be explained using FIGS. 6 and 7.

Figure 6 is a schematic diagram of the water rod when the displacement or season changes. In Figure 6, 22 is the water rod tube (
(hollow tube), 41a indicates the boss of the lower tie plate. Further, the symbol Δ indicates the amount of displacement of the water rod at a height L from the upper surface of the lower tie plate, and X indicates the height from the upper surface of the lower tie plate to the point at which the outer diameter of one water rod becomes thicker f.

The stress σ applied to the lower end plug of the water rod is.

It is calculated from the following formula.

σ=M/Z here Bending moment of mid water rod lower end plug.

E is the Young's modulus of the water rod, I+ is the moment of inertia of the small diameter part of the water rod, E2 is the moment of inertia of the large diameter part of the water rod, Δ is the earthquake displacement of the lower part of the water rod, L is the water rod Distance from the lower tie plate until the rod produces a displacement Δ.

X is the length of the small diameter part of the water rod.

Z is the section modulus of the water rod lower end plug, and σ is the stress of the water rod lower end plug.

FIG. 7 shows the relationship between the length X of the narrow diameter portion of the water rod and the stress σ applied to the lower end plug.

As can be seen from FIG. 7, by increasing the length X of the narrow portion of the water rod, the stress on the lower end plug can be reduced.

As can be seen from the above considerations, at the bottom of one water rod, the outer diameter of the hollow rod of the water rod is
By making the central portion of the water rod thinner than the outer diameter, it is possible to reduce the stress applied to the one-quarter rod lower end plug at the same time as improving the flow characteristics, and to ensure the mechanical integrity of the lower end plug.

[Embodiments of the invention]

A preferred embodiment of the present invention will be explained based on FIGS. 2 and 3. FIG. It shows a.

1 is the fuel group, 3 is the natural spacer, 4 is the lower tiebrae)
, 5td upper tiebrae), 6 is a channel box, 7 is a channel fastener, 8 is a hanging metal fitting, 9a and 9
b is "F part Zuirei, ioa, a and 10b are upper end plugs, 22
is quarter rod tube (hollow tube), 231I'i water rod, 41a is water rod boss, 41b
is the fuel rod boss, 21 is the coolant passage hole, 101 is U 0
2 Berets are shown.

The fuel rod 1 has a large number of UOz pellets 101 filled in a cladding tube sealed with an upper end plug 10b and a lower end plug 9b. The water rod 23 seals the lower end of the water rod tube 22 with the lower end plug 9a,
The upper end of the water rod tube 22 is connected to the upper end plug 10a.
It is sealed. Water rod tube 22
has a coolant inlet 23A on the side wall at the lower end and a coolant outlet 238t on the side wall at the upper end. The water rod 23 is arranged at the center of the cross section of the fuel assembly, as shown in FIG. 2, and is surrounded by the fuel rods 1. The outer diameter of the water rod 23 is larger than the outer diameter of the fuel rod 1 at its central portion. However, the outer diameter of the water rod 23 is
It becomes thinner at the bottom and top as shown in Figure J1. Namely.

The outer diameter of the central water rod tube 22a is the same as that of the water rod tubes 22b located at its upper and lower portions.
The outer diameter of the upper and lower water rod tubes 22b is approximately the same as the outer diameter of the fuel rod l.

As shown in FIG. 3, the lower portion of the water rod 23 employs a water rod tube 22b having an outer diameter smaller than that of the water rod tube 22a in the central portion. Therefore, by making the water rod boss 41a smaller,
Even if the flow path area of the coolant passage hole 21 around the water rod 23 is increased.

There is no need to worry about flow vibration of the water rod 23, fretting corrosion, etc. In addition, since the outer diameter of the water rod tube 22b in the F section is made thinner, a lateral force is applied to the fuel assembly during an earthquake, causing deformation of the fuel assembly such that the axial center of the fuel assembly curves. Also, the outer diameter of the lower water rod tube 22b is thin and easy to bend, and the lower end plug 9
The stress applied to a can be reduced. Therefore, the stress applied to the lower end plug 9a during an earthquake can be reduced.

Mechanical integrity of the fuel assembly during an earthquake can be ensured.

Thin water tube 22 at the bottom of the water rod 23
The length b needs to be 40W or more considering the stress applied during an earthquake. When the length is shorter than 40 m, excessive stress is applied to the lower end plug 9a.

The mechanical integrity of the fuel assembly during an earthquake is impaired.

In this embodiment, the upper part of the water rod 23 is also used.

Because it uses a thin water tube 22b.

The pressure loss at the void generation part can be reduced.5 Furthermore, since the lower part of the water rod 23 is easy to bend, the load (reaction force) that the fuel spacer 3 receives from the water rod 23 during an earthquake can be reduced.

〔Effect of the invention〕

According to the present invention, the flow resistance at the bottom of the fuel assembly can be reduced, and the stress generated in the lower end plug of the water rod during an earthquake can be alleviated to ensure the mechanical integrity of the fuel assembly. can do.

[Brief explanation of drawings]

FIG. 1 is a side view of a fuel assembly that is a preferred embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along the line -■ in FIG. Figure 3 is a detailed longitudinal sectional view of the lower end of the fuel assembly in Figure 1;
Figure 4 is a vertical sectional view of the lower tie plate of the conventional fuel assembly, Figure dc5 is a vertical sectional view of the lower end of the improved fuel assembly, and Figure 6 is a schematic diagram of the displacement of the water rod during an earthquake. , FIG. 7 is a characteristic diagram showing the relationship between the length of the diameter portion of the lower part of the water rod and the stress of the lower end plug. 1...Fuel rod, 4...Lower tie plate, 9a...
- Water rod lower end plug, 22a, 22b...Water rod tube, 23...Water rod. Also 1 mouth 3 0 horse 4 ro high 5 mouth ’7 low diameter lily (χ) = 5 length 3 (η 笥)

Claims (1)

[Claims] 1. An upper tie plate, a lower tie plate, a plurality of fuel rods whose ends are held by the upper tie plate and the lower tie plate, and a plurality of fuel rods arranged between the fuel rods and having an outer diameter larger than that of the fuel rods. In a fuel assembly consisting of a large water rod, the hollow tube of the water rod becomes thin at the upper and lower parts, and the lower part of the water rod extends from the upper surface of the lower tie plate within a range not exceeding the lowermost spacer. A fuel assembly characterized in that the height of the hollow rod is reduced by 40 mm or more compared to the outer diameter of the central portion of the hollow rod.