JP2000162354A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the increase in the leaked flow rate of a fuel assembly, and to improve the generation efficiency of a plant in high combustion by providing bypass holes at four corner parts of a lower tie plate. SOLUTION: A lower tie plate 4 consisting of an inlet nozzle 10 of a ring for allowing a cooling material 9 to flow in, a rectangular lattice part 11 for supporting the lower end of a fuel rod 2, and an expansion channel 12 for connecting the inlet nozzle 10 and the lattice part 11. At four corner parts 13 of the lower tie plate 4, a bypass hole 14 is provided, and the corner part 13 being positioned downstream as compared with the bypass hole 14 is set to a plane part 15. Also, the sectional area of a channel 16 at the gap between a channel box 7 and the lower tie plate 4 is larger than that of the bypass hole 14. One part of the flow rate of a reactor core that flows into the lower tie plate 4 flows out of the bypass hole 14, and an outflow flow 17 collides with the channel box 7 for mostly flowing out of the channel 16. After the collision, outflow flow 19 that flows into one part of a side-surface gap 18 collides with down flow 20 for damming, and the increase in a leakage current is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear power plant, and more particularly to a structure of a fuel assembly suitable for suppressing an increase in leakage flow from a gap between a channel box and a lower tie plate. .

[0002]

2. Description of the Related Art A boiling water nuclear power plant supplies a coolant to a fuel assembly in a core, the coolant receives heat from a fuel rod, boils, and the steam drives a generator to generate electricity.

A fuel assembly includes a fuel bundle assembled from a fuel rod, a water rod, an upper tie plate and a lower tie plate that support the upper and lower ends thereof, spacers that align and support them, and the like. And a channel box forming a coolant flow path. The channel box has an upper end fixed to the upper tie plate of the fuel bundle and a lower end overlapping the lower tie plate so that it can freely expand and contract. The overlap length of the current fuel assembly is about 60 mm. The gap width between the overlapping channel box and the lower tie plate is about 0.25 mm at the beginning of combustion. In addition, recent channel boxes have a thicker area around the overwrap to suppress creep deformation due to higher burnup.

[0004] In the above fuel assembly, the coolant supplied from the recirculation pump mostly flows from the inlet nozzle of the lower tie plate, is heated in the fuel rod region, and flows out of the upper tie plate. A portion of the coolant flows out of the lower tie plate bypass hole and the gap between the channel box and the lower tie plate. These two outflow flows become the leakage flow of the fuel assembly (also referred to as bypass flow). This leakage flow rises between adjacent channel boxes to cool the furnace instrumentation tubes and prevent stagnation of coolant. Therefore, this leakage flow rate is an essential flow rate for ensuring the reliability of the core. The leakage flow rate of the current fuel assembly is about 8% of the coolant flow rate (core flow rate) supplied to the fuel assembly at the beginning of combustion.

However, from the viewpoint of the power generation efficiency of the plant, this leakage flow rate is one of the causes of lowering the power generation efficiency because a part of the core flow rate supplied to the fuel assembly becomes a bypass flow rate and returns to the recirculation pump. Has become. Therefore, it is desired that this leakage flow rate be constant from the beginning of combustion to the end of combustion.

However, the above-mentioned leakage flow rate increases as the burnup of the fuel assembly increases (high combustion). The reason why the leakage flow rate increases is that the channel box undergoes creep deformation with high combustion. The creep deformation of the channel box is caused by the pressure difference between the inside and outside of the channel box, and the entire channel box gradually expands. For this reason, the gap width between the channel box and the lower tie plate is increased, and the leakage flow rate is increased. FIGS. 2A and 2B show the leakage flow rate flowing out of the lower tie plate. The leakage flow rate is the outflow flow rate from the bypass hole and the outflow flow rate from the gap between the channel box and the lower tie plate (hereinafter referred to as the outflow flow rate of the channel box). Looking at the relationship between the burnup and the leakage flow rate shown in FIG. 3, the outflow flow rate of the bypass hole is constant even when the combustion is high, but the outflow flow rate from the channel box increases when the combustion rate is high. The increasing tendency changes greatly because the gap expands over the entire circumference.
The leakage flow rate is as high as 11% of the core flow rate at the end of combustion. As a result, there occurs a problem that the power generation efficiency of the plant is lower than the initial stage of combustion due to the high combustion.

[0007] As a known example of reducing the leakage flow rate from the lower tie plate, there is Japanese Patent Application No. 8-315797 (refer to Japanese Patent Application No. 8-315797). In this known example, it is described that a plurality of hemispherical projections are provided along the axial direction at four corners of a lower tie plate to increase the flow path resistance there and suppress the leakage flow rate. However, it seems that this suppression effect becomes smaller as the channel box expands and deforms.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a mechanism for suppressing an increase in leakage flow rate of a fuel assembly in order to improve the power generation efficiency of a plant in high combustion.

[0009]

In order to achieve the above object, the present invention provides bypass holes at corners at four corners of a lower tie plate, and further comprises a channel box and a lower tie plate downstream from the bypass holes. The gap is a passage having a cross-sectional area larger than that of the bypass hole.

That is, FIG. 4 shows a cross section of a corner portion of a lower tie plate according to the prior art and the present invention. One of the present inventions is that bypass holes are provided at four corners near the upper end of the lower tie plate. Another is that the gap between the channel box downstream of the bypass hole and the lower tie plate is a flow path having a cross-sectional area larger than the cross-sectional area of the bypass hole.

Conventionally, the outflow flow rate of the bypass hole is caused to flow out of the side surface of the lower tie plate. However, the present invention allows the outflow flow rate of the bypass hole to flow out of the four corners of the lower tie plate, thereby forming the corner and the side surface. Outflow (downflow) from the channel box descending the gap
To prevent the leakage flow rate from increasing. In the present invention, even if the gap width between the channel box and the lower tie plate is increased due to high combustion, the increase width is smaller than the outflow velocity from the bypass hole, so that the outflow from the bypass hole is blocked. The effect is maintained as it is and at the same time the outflow does not change. That is, the present invention is a fluid dynamics application suppression mechanism using a velocity difference (fluid force) between the outflow of the bypass hole and the downward flow from the channel box.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below. FIG. 5 shows the configuration of the fuel assembly. The fuel assembly 1 includes a fuel bundle 6 assembled by a fuel rod 2, a water rod, an upper tie plate 3, a lower tie plate 4 for supporting the upper and lower ends thereof, a spacer 5 for aligning and supporting them, and the like. 6 and a channel box 7 which forms a coolant flow path.
The upper end of the channel box 7 is fixed to the upper tie plate 3 of the fuel bundle 6, and the lower end overlaps with the lower tie plate 4 so that it can freely expand and contract.

The length of the overlap is about 60 mm, and the gap width 8 between the overlapping channel box 7 and the lower tie plate 4 is about 0.25 mm at the beginning of combustion. FIG. 1 shows a lower tie plate 4 of the present invention. The lower tie plate 4 is an annular inlet nozzle 1 into which coolant 9 flows.
0 and a rectangular grid portion 11 that supports the lower end of the fuel rod 2 and an enlarged flow channel 12 that connects these.

The lattice portion 11 is composed of a number of bosses into which the lower ends of the fuel rods 2 are inserted, and a web connecting the bosses to a square lattice. Further, the space surrounded by the boss and the web becomes a coolant passage through which the coolant passes. Bypass holes 14 are provided in the corner portions 13 at the four corners of the lower tie plate 4. Further, the corner portion is flat 15 downstream of the bypass hole 14, and the flow path 16 in the gap between the channel box 7 and the lower tie plate 4 has a larger cross-sectional area than the cross-sectional area of the bypass hole 14. .

In such a configuration, a part of the core flow rate flowing into the lower tie plate 4 is reduced.
Flows out of the bypass holes 14 provided in the four corners 13. The outflow 17 of this bypass hole collides with the channel box 7, and most of the outflow 17 flows out through the passage 16 in the gap between the channel box 7 and the lower tie plate 4. A part of the effluent spreads right and left with the force colliding with the channel box 7 and flows into the side gap 18 between the channel box and the lower tie plate. The outflow 19 flowing into the plane gap 18 collides with the outflow (downflow) 20 from the upstream channel box to block the downflow 20, but the outflow 19 decelerates as it flows through the side gap 18. And merges with the downflow 20 of the channel box and flows out. The effect of blocking the outflow 17 of the bypass hole is determined by the magnitude relationship between the outflow speed from the bypass hole and the descending speed of the channel box.

The outflow speed of the bypass hole is determined by the pressure difference between the inside and outside of the lower tie plate 4 and the outflow resistance of the bypass hole. Further, the descending speed of the channel box is determined by the differential pressure between the inside and outside of the channel box 7 and the flow path resistance between the channel box and the lower tie plate.

Comparing the two, the pressure difference is such that the differential pressure between the inside and the outside of the lower tie plate 4 is greater than the differential pressure between the inside and outside of the channel box 7, and the loss resistance of the bypass hole is smaller than the flow resistance of the gap. Thereby, the outflow speed from the bypass hole becomes higher than the descending speed of the channel box. Therefore, the outflow 17 of the bypass hole
Can block the downflow 20 of the channel box.

The outflow flow rate of the bypass hole is determined by the product of the cross-sectional area of the bypass hole 14 and the outflow velocity.
The cross-sectional area of the flow passage between the channel box and the lower tie plate is made larger than the cross-sectional area of the bypass hole so that the outflow resistance of the bypass hole becomes a rule in order to make the outflow flow rate of the bypass hole constant. FIG. 6 shows the relationship between the burnup and the leakage flow rate.

Most of the leakage flow rate in the initial stage of combustion in the present invention is the flow rate of the flow out of the bypass hole 14, and the remaining part is the flow rate of the flow out of the channel box 7. Therefore, the leakage flow rate at the beginning of combustion is adjusted to a predetermined flow rate mainly by adjusting the hole diameter of the bypass hole 14.

At the end of combustion, the channel box 7
The creep deformation causes the gap width 8 between the channel box and the lower tie plate to increase. As a result, the flow path resistance of the gap between the channel box and the lower tie plate and the flow path resistance of the channel box downstream of the bypass hole and the flow path 16 of the lower tie plate become small, which causes an increase in leakage flow rate. However, the outflow flow rate of the bypass hole does not change because the outflow resistance of the bypass hole 14 is governed.

In the gap between the channel box and the lower tie plate, the flow rate of the bypass hole does not change, so that the flow path resistance decreases and the leakage flow increases.
The increasing tendency becomes smaller because the length of the side gap 18 flowing out is shorter than in the conventional case. As a result, an increase in the leakage flow rate can be suppressed even at a high burnup. The leakage flow rate in the present invention is about 9% of the core flow rate at the end of combustion.

[0022]

According to the present invention, the power generation efficiency of the plant in high combustion can be improved by suppressing an increase in the leakage flow rate of the fuel assembly.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating a function showing an overview structure of a lower tie plate according to an embodiment of the present invention.

2 (A) and 2 (B) are a cross-sectional view showing an outflow situation of a leakage flow rate in a conventional lower tie plate and a side cross-sectional view of FIG. 2 (B).

FIG. 3 is a characteristic diagram showing a relationship between burnup and leakage flow rate in a conventional lower tie plate.

FIG. 4 is a sectional view showing a corner portion of a lower tie plate according to the present invention and a conventional one;

FIG. 5 is a cross-sectional view showing a configuration of a fuel assembly of the present invention.

FIG. 6 is a characteristic diagram showing a relationship between burnup and leakage flow rate in a lower tie plate of the present invention.

[Explanation of symbols]

1 ... fuel assembly, 2 ... fuel rod, 3 ... upper tie plate,
4 ... Lower tie plate, 5 ... Spacer, 6 ... Fuel bundle, 7 ... Channel box, 8 ... Gap width between channel box and lower tie plate, 9 ... Coolant, 10 ... Inlet nozzle, 11 ... Lattice, 12 ... Enlarged flow path, 13: Four corners of lower tie plate, 14: Bypass hole, 15: Flat part of corner, 16: Flow path between channel box and lower tie plate, 17: Outflow of bypass hole, 18 ... side gap between the channel box and the lower tie plate, 19 ... outflow from the side gap between the channel box and the lower tie plate, 20 ... downflow from the channel box, 21 ... outflow from the bypass hole, 22 ... from the channel box Outflow of 23, ...
Leakage flow rate.

Claims (2)

[Claims] 1. A fuel bundle assembled from a fuel rod, a water rod, an upper tie plate and a lower tie plate that support the upper and lower ends thereof, and a spacer or the like that aligns and supports the fuel rod and the water rod. And a channel box forming a coolant flow path, wherein bypass holes are provided at four corners of the lower tie plate. 2. The fuel assembly according to claim 1, wherein the corners at the four corners of the lower tie plate have a larger cross-sectional area between the channel box downstream from the bypass hole and the lower tie plate than the cross-sectional area of the bypass hole. A fuel assembly, comprising: