JPH0666974A - Fuel assembly - Google Patents

Abstract

PURPOSE:To improve the limit power of peripheral fuel rods and increase the thermal margin of the whole fuel assembly by increasing the coolant flow in the flow path surrounded by the peripheral fuel rods and a channel box. CONSTITUTION:In the lower tie-plate 28, a coolant guide inlet 30 is provided to introduce coolant between each fuel rod 23 and a water rod 24. The shape of the lower tie-plate 28 is so made that the center of fuel holding part 28a projects more than the peripheral part. The fuel rods 23 supported by the fuel support part 28a and arranged in the center is made longer than those arranged in the periphery. And the lower end of the fuel rods arranged in the center is supported by the fuel support part 28a of the lower tie-plate 28 at lower position than the lower end of the peripheral fuel rods.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a boiling water reactor (BW).
The present invention relates to a fuel assembly loaded in a core such as R), and particularly to a fuel assembly having an increased limit output.

[0002]

2. Description of the Related Art FIG. 11 shows the structure of a conventional fuel assembly. This fuel assembly 1 includes a plurality of vertical fuel rods 3 arranged in parallel with each other in a vertically long rectangular tubular channel box 2 having a square cross section, and at least one fuel rod 3 arranged in parallel at a central portion thereof. The water rod 4 is housed therein, and these are arranged in a square lattice pattern.

An upper end plug 5 and a lower end plug 6 are welded and fixed to the upper and lower ends of each fuel rod 3 and water rod 4, respectively, and the upper end plug 5 and the lower end plug 6 are connected to the upper tie plate 7 and the lower part. Supported by tie plate 8.

Each fuel rod 3 is aligned and supported at a plurality of intermediate positions in the axial direction (vertical direction) at appropriate intervals by spacers 9 each having a flat rectangular tube shape and held by the water rods 4, whereby the diameter of the fuel rod 3 is increased. Directional displacement is constrained.

The water rod 4 is for improving the water-to-uranium ratio in the fuel assembly 1 to improve the nuclear characteristics. The shaft portion of the lower end plug 4a of the water rod 4 is
A screw portion is formed, and this screw portion is screwed into the screw hole of the lower tie plate 8.

The lower tie plate 8 has a coolant inlet port 10 that opens at the bottom for introducing a coolant such as light water.
And a cooling channel 11 for guiding the coolant introduced into the lower tie plate 8 into the channel box 2.

The fuel support portion of the lower tie plate 8 is formed in a flat plate shape having a substantially square shape in plan view (thickness of about 20 m).
m), the lower end plugs 6 of the fuel rod 3 and the water rod 6,
It has a boss having a fitting hole that supports 4a, and a web that integrally connects the bosses in a substantially square lattice shape. The space surrounded by each boss and the web serves as a flow path for a coolant such as light water, and the coolant introduced from the coolant introduction port 10 of the lower tie plate 8 passes through the coolant flow passage 11 and the channel box 2 Will be introduced in.

Incidentally, in recent years, in order to obtain more significant nuclear characteristics, it has been proposed to make the water rod 4 thick as shown in FIGS. 12 and 13A. The diameter of the large diameter water rod 4 is larger than that of the fuel rod 3,
The lower end of the water rod 4 is closed by a large-diameter lower end plug 4a fixed to the lower end.

[0009]

By the way, in a nuclear power plant, automatic control and introduction of daily load following operation,
It is desired to expand operational flexibility. In line with this request, for example, in a boiling water reactor, in order to further improve the thermal-hydraulic characteristics of the core, it is necessary to improve the stability of the core, increase the thermal margin (increase the limit output), reduce the core pressure loss, etc. Development is in progress. Under such circumstances, it has been found that the conventional fuel assembly 1 in which the fuel rods 3 are arranged in a square lattice has room for expanding the thermal margin.

That is, the fuel assembly 1 of the boiling water reactor
Generally, in consideration of the effective use of thermal neutrons, for example, Fig. 1
As shown in FIG. 4, the fuel rod output (rod output) on the outer peripheral side facing the channel box 2 is made higher than the rod output on the inner side.

On the other hand, looking at the flow rate distribution of the coolant in the fuel assembly 1, as shown in the figure, the flow rate of the flow path surrounded by the channel box 2 and the fuel rods 3 on the outer peripheral side is the internal flow rate. It is lower than the flow rate of the road.

Then, as the output of the fuel assembly 1 is increased, the fuel rods 3 on the outer peripheral side facing the channel box 2
In (3a), the temperature of the surface region (region B shown by hatching in FIG. 13B) rapidly increases, and the limit output is easily reached. As shown in FIG. The thermal margin of the fuel on the outer peripheral side is smaller than that of the fuel, and it can be pointed out that the expansion of the thermal margin of the entire fuel assembly and thus the increase of the limit output is hindered from this point. .

In view of the above circumstances, the present invention increases the coolant flow rate of the flow passage surrounded by the channel box and the fuel rods on the outer peripheral side, thereby improving the limit output for the fuel rods on the outer peripheral side. The purpose of this is to increase the thermal margin of the entire fuel assembly.

[0014]

In order to achieve the above object, a fuel assembly according to the present invention comprises a plurality of vertical fuel rods arranged in parallel, and a central position of the fuel rods arranged in parallel with the fuel rods. At least one water rod, an upper tie plate and a lower tie plate supporting upper and lower ends of the fuel rod and the water rod, and a space between the upper tie plate and the lower tie plate. A spacer for holding the fuel rods and the water rods apart from each other in a horizontal direction, and a channel box for accommodating the spacers, and introducing a coolant into the lower tie plate between the fuel rods and the water rods. In the fuel assembly provided with a mouth, the fuel support portion of the lower tie plate is shaped such that the central portion thereof projects below the outer peripheral portion. In addition, among the fuel rods supported by the fuel support portion, those arranged on the center side are made longer than those arranged on the outer peripheral side, and the lower end portion of the fuel rod on the center side is formed on the outer peripheral side. It is characterized in that it is supported by the fuel support portion of the lower tie plate at a position lower than the lower end portion of the rod.

[0015]

According to the present invention, since the fuel support portion of the lower tie plate is shaped such that the center side is lower than the outer peripheral side, the coolant flowing from the coolant introduction port is diverted at the lower surface of the fuel support portion and It is possible to increase the flow rate of the flow path surrounded by the fuel rods and the channel box, which is the most thermally demanding part. Therefore, the cooling effect of the fuel rods on the outer peripheral side is improved as compared with the conventional fuel assembly.

On the other hand, since the lower end portion of the fuel rod on the center side is located below the lower end portion of the fuel rod on the outer peripheral side, the bubble generation position on the central side of the fuel assembly is lower than that on the outer peripheral side. descend. As a result, it becomes difficult for the coolant to flow in the central side flow path due to the influence of the generation of bubbles, which reduces the flow rate. Therefore, by increasing the amount of cooling water flowing into the outer peripheral side flow passage in the fuel assembly, it is possible to improve the limit output of the outer peripheral side fuel rods and consequently increase the thermal margin.

Further, since the fuel rods on the center side are longer than the fuel rods on the outer peripheral side in the downward direction, the amount of fuel loaded increases, which can also contribute to the increase in the output of the nuclear reactor. In other words, the reaction between neutrons and fuel is a factor that greatly affects the power output of a nuclear reactor, but the degree of this reaction (reactivity) is high in areas with a lot of water and low in areas with a lot of steam. Therefore, the increase in the fuel loading amount in the water-rich portion below the fuel rod effectively acts on the increase in the power output of the reactor.

[0018]

Embodiments of the present invention will be described below with reference to FIGS. 1 shows a sectional structure of the entire fuel assembly, FIG. 2 shows a planar shape of a lower tie plate thereof, and FIG. 3 shows an AA sectional shape of FIG.

As shown in FIG. 1, a fuel assembly 21 of this embodiment has a plurality of vertical fuel rods 2 arranged in parallel in a vertically long rectangular tubular channel box 22 having a square cross section.
3 and two water rods 24 arranged in parallel at the center thereof.

An upper end plug 25 and a lower end plug 26 are welded and fixed to the upper and lower ends of each fuel rod 23 and water rod 24, and the upper end plug 25 and the lower end plug 26 are in the upper tie plate 27 and the lower part. Tie plate 2
It is supported by 8.

Each of the fuel rods 23 is aligned and supported at a plurality of positions at appropriate intervals at intermediate positions in the axial direction (vertical direction) by spacers 29 having a flat rectangular tube shape held by the water rods 24. Directional displacement is constrained. Lower end plug 24a of the water rod 24
A threaded portion is formed on the shaft portion of, and this threaded portion is screwed into the screw hole of the lower tie plate 28.

The lower tie plate 28 has a coolant inlet port 30 that opens at the bottom for introducing a coolant such as light water.
And a cooling passage 31 for guiding the coolant introduced into the lower tie plate 28 into the channel box 22.

The fuel support portion of the lower tie plate 28 is formed in a flat plate shape having a substantially square shape in plan view (thickness: about 20 m).
m) has a boss having fitting holes for supporting the lower end plugs 26, 24a of the fuel rod 23 and the water rod 24, and a web integrally connecting the bosses in a substantially square lattice shape.
The space surrounded by the bosses and the web serves as a coolant channel for light water or the like, and the coolant introduced from the coolant inlet port 30 of the lower tie plate 33 passes through the coolant channel 31 and enters the channel box 22. Will be introduced to.

Therefore, in this embodiment, as shown in FIG. 3, the fuel support portion 28a of the lower tie plate 28 has a shape in which the central portion thereof projects downward from the outer peripheral portion and is also shown in FIG. Of the fuel rods 23 supported by the fuel support portion 28a, which are arranged on the center side 2
3a is longer than the one 23b arranged on the outer peripheral side, and the lower end portion of the fuel rod 23a on the central side is located on the outer peripheral side.
It is configured to be supported by the fuel support portion 28a of the lower tie plate 28 at a position lower than the lower end portion of b. According to this embodiment having such a configuration, the fuel support portion 28a of the lower tie plate 28 has a shape in which the central side is lower than the outer peripheral side, so that the coolant flowing from the coolant introduction port 11 is generated in the fuel support portion 28a. It becomes possible to increase the flow rate of the flow path which is divided at the lower surface and which is surrounded by the channel box 22 and the fuel rod 23, which are the most thermally severe in the outer peripheral portion. Therefore,
The cooling effect for the fuel rods 23b on the outer peripheral side is improved as compared with the conventional fuel assembly.

On the other hand, since the lower end portion of the fuel rod 23a on the center side is located lower than the lower end portion of the fuel rod 23b on the outer peripheral side, the bubble generating position on the central side of the fuel assembly 21 is on the outer peripheral side. Lower than that. As a result, it becomes difficult for the coolant to flow in the central side flow path due to the influence of the generation of bubbles, which reduces the flow rate. Therefore, by increasing the amount of cooling water flowing into the outer peripheral side flow passage in the fuel assembly, it is possible to improve the marginal output of the outer peripheral side fuel rods 23b and thus increase the thermal margin.

Further, since the fuel rod 23a on the center side is longer downward than the fuel rod 23b on the outer peripheral side, the amount of fuel loaded increases, which can also contribute to an increase in the output of the nuclear reactor. In other words, the reaction between neutrons and fuel is a factor that greatly affects the power output of a nuclear reactor, but the degree of this reaction (reactivity) is high in areas with a lot of water and low in areas with a lot of steam. Therefore, the increase in the fuel loading amount in the water-rich portion below the fuel rod effectively acts on the increase in the power output of the reactor.

4 and 5 show a modification of the lower tie plate. That is, in the lower tie plate 28 of FIG. 3 described above, the fuel support portion 28a is lowered in four steps from the outer peripheral side to the center side, but may be lowered in three steps as shown in FIG. Alternatively, as shown in FIG. 5, the configuration may be lowered in two steps.

Further, FIG. 6 shows a structural example in which the water rod 24 has a large diameter. Even in this case,
Other configurations are the same as the above, such as the fuel support portion 28a having a high and low configuration, and the description thereof will be omitted.

7 and 8 show how the flow rate distribution inside the fuel assembly changes. For reference, in the figure, the change in the flow rate distribution in the lower tie plate of the conventional structure is also indicated by a broken line.

According to the fuel assembly of this embodiment, as shown in FIG. 7, immediately after the lower tie plate, the flow rate in the flow passage on the outer peripheral side (channel side) is considerably larger than the flow rate inside. However, since the hydraulic diameter of the flow passage surrounded by the channel box and the fuel rod is smaller than the hydraulic diameter of the internal flow passage, and the flow passage on the outer peripheral side is less likely to flow, the flow rate distribution is gradually made uniform. Further, as shown in FIG. 8, the flow rate at the position where the fuel rod temperature rises is increased by about 20% as compared with the conventional structure. The increase in the limit output due to this is 5%. FIG. 9 shows the relationship between the protrusion ratio of the lower tie plate and the increase ratio of the fuel loading amount, and FIG. 10 shows the axial power distribution.

As shown in FIG. 9, in the present invention, since the fuel rod located at the center has a longer portion than the fuel rods on the outer peripheral side, the fuel loading amount is increased by about 1% at the maximum. The output of the reactor can be increased as compared with the structure.

As a factor that greatly affects the power output of the nuclear reactor, the reaction between neutrons and fuel is increased. The degree of this reaction (reactivity) is high in a portion where water is abundant and is large in a portion where steam is abundant. It has the property of becoming lower. Therefore, as shown in FIG. 10, the increase in the fuel loading amount in the water-rich portion below the fuel rod effectively acts on the increase in the power output of the reactor.

[0033]

As described above, according to the present invention, by increasing the flow rate of the coolant in the flow passage surrounded by the channel box and the fuel rods on the outer peripheral side, the limit output for the fuel rods on the outer peripheral side can be obtained. And the thermal margin of the entire fuel assembly can be increased.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a fuel assembly according to the present invention.

FIG. 2 is a plan view showing a lower tie plate in the embodiment.

3 is a sectional view taken along the line AA of FIG.

FIG. 4 is a sectional view showing a modified example of the lower tie plate.

FIG. 5 is a sectional view showing a modified example of the lower tie plate.

FIG. 6 is a cross-sectional view showing a modified example using a large diameter water rod.

FIG. 7 is a graph showing a flow rate characteristic according to the embodiment.

FIG. 8 is a graph showing flow rate characteristics at different positions according to the example.

FIG. 9 is a graph showing a protrusion rate of a lower tie plate and an increase rate of a fuel loading amount.

FIG. 10 is a diagram showing output characteristics according to the embodiment.

FIG. 11 is a sectional view showing a conventional example.

FIG. 12 is a cross-sectional view showing another conventional example.

13A is a cross-sectional view showing a conventional example, and FIG. 13B is a partially enlarged view of FIG. 13A.

FIG. 14 is a characteristic diagram showing a coolant flow rate, a thermal margin, a rod output and the like in a conventional example.

[Explanation of symbols]

 21 fuel assembly 22 channel box 23 fuel rod 24 water rod 27 upper tie plate 28 lower tie plate 28a fuel support 29 spacer 30 coolant inlet

Claims (1)

[Claims] 1. A plurality of vertical fuel rods arranged in parallel, at least one water rod arranged in parallel with the fuel rod at its central position, and upper and lower ends of these fuel rods and water rods. Upper and lower tie plates that support the parts, spacers that are arranged between the upper and lower tie plates at intervals, and that horizontally maintain the fuel rods and the water rods, and channels that accommodate these spacers. In a fuel assembly including a box, the lower tie plate is provided with a coolant inlet for introducing a coolant between the fuel rods and the water rods, a fuel support portion of the lower tie plate is It has a shape projecting downward from the outer peripheral portion and is arranged on the center side of the fuel rods supported by the fuel supporting portion. The fuel rods of the lower tie plate are supported at a lower position than that of the fuel rods on the outer peripheral side, and the lower end of the fuel rods on the central side is lower than the lower end of the fuel rods on the outer peripheral side. A fuel assembly characterized by the following.