JPH01227990A - Nuclear fuel assembly - Google Patents

Abstract

PURPOSE:To decrease the amt. of leakage by providing a staircase-shaped labyrinth sealing structure to the spacing between a channel box and a lower tie plate. CONSTITUTION:Both the top and bottom ends of plural pieces of fuel rods 1 are supported by an upper tie plate 4 and the lower tie plate 5 and the intermediate parts thereof are arrayed and supported by plural spacers 3 to constitute a fuel bundle 7. The channel box 8 of a square shape to house the fuel bundle 7 is fixed to the upper tie plate 4. The sealing structure of the staircase- shaped labyrinth 14 is provided over the entire circumference of the side face of the lower tie plate 5 in the spacing 12 between the side face 10 in the channel box 8 and the side face 11 of the lower tie plate 15. Since the leakage amt. of the hot water from the spacing 12 is thereby decreased, the stability of the reactor core and the economy of fuel are enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a nuclear fuel assembly, and particularly to a seal structure suitable for reducing the amount of leakage from the side surface in a channel box and the side surface and gap of a lower tie plate.

[Conventional technology]

As shown in Figure 2, a BWR fuel assembly consists of a plurality of fuel rods 1. Water rods 2, spacers 3 for periodically arranging and supporting them in alignment. Upper tie plate 4 supporting the upper end plug of each fuel rod 1 and water rod 2. Lower tie plate supporting the lower end plug5. It consists of a spring 6 attached to the upper end plug part for pressing each fuel rod 1 and water rod 2 downwardly into the lower tie plate 5, and a channel box 8 that houses a fuel bundle 7 assembled with these members. ing.

In addition, the lower end of the lower tie plate 5 is the recirculation flow rate (hot water).
This is the lower nozzle 9 into which the water flows.

FIG. 3 shows a conventional sealing structure for the gap between the channel box and the lower tie plate.

A finger spring 13 fixed to the side surface 11 of the lower tie plate is inserted into a gap 12 between the side surface 10 in the channel box 8 and the side surface 11 of the lower tie plate 5.

(Problem to be solved by the invention) In the conventional fuel assembly described above, the channel box 8
A finger spring 13 is inserted between *12 and the lower tie plate 5, and this finger spring 13 prevents hot water in the channel box 8 (inside the fuel assembly) from leaking out of the channel box 8 from the gap 12. This has the disadvantage that the amount of leakage from the gap 12 cannot be controlled.

An object of the present invention is to provide a seal structure for reducing the amount of leakage from the gap 12 compared to the finger spring 13.

[Means to solve the problem]

The above object is achieved by making the structure of the gap between the side surface inside the channel box and the side surface of the lower tie plate into a stepped labyrinth.

[Effect]

In the gap between the channel box and the lower tie plate, the side surface inside the channel box and the side surface of the lower tie plate are machined into a step-like shape, and the two machined surfaces face each other to form a step-shaped labyrinth flow path. When the shape of the flow path is complicated in this way, when the fluid flows from the high pressure side to the low pressure side, as shown in Fig. 4, ■ Pressure loss due to fluid friction proportional to the length of the flow path, ■ As the flow direction changes, a pressure loss occurs due to deceleration (expansion) and acceleration (reduction) at each step. The sum of these pressure losses is the total pressure loss in the labyrinth.

That is, if the pressure difference between the inside and outside of the channel box is the same, the amount of leakage will decrease by the amount that the total pressure loss increases.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. First, the fuel assembly is roughly divided into two parts. One is a fuel bundle 7, in which a plurality of fuel rods 1 are supported at both upper and lower ends by an upper tie plate 4 and a lower tie plate 5, and the middle part thereof is aligned and supported by a plurality of spacers 3. . The other is a channel box 8 which houses the fuel bundle 7 and has a rectangular tube shape. The channel box 8 is fixed to the upper tie plate 4 and is not in contact with the lower tie plate 5.

The sealing structure of the gap 12 between the side surface 10 in the channel box 8 and the side surface 11 of the lower tie plate 5 according to the present invention is a stepped labyrinth 14 as shown in FIG. A detailed view of this stepped labyrinth seal structure is shown in FIG. This stepped labyrinth 14 is provided around the entire side surface of the lower tie plate 5. Labyrinth structure 1 of this example
4, the above-mentioned gap 12 is Δs=0.2I, -stage pitch P=
12ua, depth of the expansion chamber 15 Δh=o, 5nn.

It is a three-tier stack with a length T=2.5 an.

Now, hot water discharged from the recirculation system (recirculation flow rate) 16
flows into the channel box 8 from the lower nozzle 9 of the lower tie plate 5. Most of the inflowing hot water 16 rises in the axial direction while being heated along the fuel rods, and becomes in a two-phase state of steam and hot water.

The remaining part of the hot water 17 is transferred to the labyrinth seal structure 14.
As a result, a leakage flow 17 flows out of the channel box 8. At this time, the amount of leakage is as follows. The amount of leakage is determined by the total pressure drop in the labyrinth flow path. - The pressure loss in the labyrinth flow path per stage is, as shown by equation (1), a fluid friction loss based on the viscosity of the fluid and a shape loss due to a change in the flow path shape.

ΔPt=ΔP1+ΔPs −(1)Δ
Pressure loss per PT knee stage, ΔPz: *Friction loss.

ΔPs: Shape loss ΔPn and ΔPs are expressed by equations (2) and (3).

λ: Friction coefficient, Q: Seal length @ us: Average flow velocity number. Therefore, in a three-stage seal structure, it becomes as follows. ΔPo: pressure difference between the inside and outside of the channel box. Therefore, the amount of leakage Q is as follows: Q, = A - uyo (5). A: Seal gap area The seal structure of the present invention can reduce the amount of leakage by about 16% compared to the conventional seal structure.

This can improve core stability and fuel economy.

FIG. 6 shows a modification of the labyrinth structure, in which a groove 18 is provided in the side surface 11 of the lower tie plate 5 to prevent the lower end of the channel box 8 from becoming thinner. It is a larger space.

FIG. 7 shows a modification of the stepped labyrinth, in which a diaphragm plate 19 is further provided in the space of the enlarged chamber 15 to increase the labyrinth effect.

〔Effect of the invention〕

According to the present invention, by providing the stage-type labyrinth seal structure in the gap between the channel box and the lower tie plate, it is possible to increase the pressure loss in the labyrinth part compared to the conventional seal structure. This has the effect of reducing the amount of leakage from gaps. Further, by reducing the amount of leakage, it is possible to contribute to improving the stability of the reactor core and the economic efficiency of fuel.

[Brief explanation of the drawing]

Fig. 1 is an axial sectional view showing an embodiment of the present invention, Fig. 2 is an overall view of a fuel assembly, Fig. 3 is an axial sectional view and a horizontal sectional view showing a conventional seal structure, and Fig. 4 is an axial sectional view showing an embodiment of the present invention. An explanatory diagram showing the principle,
FIG. 5 is a detailed view of a seal structure showing an embodiment of the invention, and FIGS. 6 and 7 are axial sectional views showing modifications of the invention. DESCRIPTION OF SYMBOLS 1...Fuel rod, 3...Spacer, 4...Upper tie plate, S...Lower tie plate, 7...Fuel bundle, 8...Channel box, 10...Inside channel box side surface, 11... side surface of the lower tie plate, 12... gap, 14... labyrinth structure,
15... Expansion chamber, 16... Hot water, 17... Leakage flow, 18... Groove, 19... Squeezing plate. Figure 1 Figure 2 Figure 3 Figure 4: l11+X Figure 5 Figure 6

Claims (1)

[Claims] 1. In a nuclear fuel assembly consisting of a nuclear fuel bundle in which a plurality of nuclear fuel rods are connected by an upper tie plate, a spacer, and a lower tie plate, and a channel box that accommodates the bundle, the side surface inside the channel box and this A nuclear fuel assembly characterized in that a step-shaped labyrinth seal structure is provided in the gap between the facing sides of the lower tie plate.