JPH11174177A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an output peaking and improve a cooling capacity, by arranging a substance with a smaller deceleration capacity than light water at a region existing the light water between a surface where a gap between fuel rods closest to each surface is winder than others and a second-closest fuel rod to the surface out of four inner surfaces of a channel box. SOLUTION: A region where no fuel rod 5 exists in a channel box 9 is filled with light water 13. A gap between the fuel rods closest to surfaces 9a and 9c out of inner surfaces 9a-9d of a channel box 9 is wider than the gap between the fuel rods closest to the surfaces 9b and 9d. A water elimination rod 17 whose diameter is equal to that of the fuel rod 5 and section is in semicircular shape is arranged over the entire height of the surfaces 9a an 9c at a region where the light water 13 exists between the surfaces 9a and 9c and the second- closest fuel rod to each surface, thus reducing the output of the fuel rod closest to the surfaces 9a and 9c and making uniform the flow of the light water 13 that flows in the channel box 9 as a cooling material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly used for a boiling water reactor.

[0002]

2. Description of the Related Art In a nuclear reactor, a nuclear fission reaction causes the depletion of fissile materials such as uranium-235 and plutonium-239, as well as uranium-238 and plutonium-2.
Conversion of fuel parent material, such as 40, to fissile material has occurred. The ratio between the rate of fissile material production and the rate of fissile material consumption during removal of spent fuel assemblies is called the conversion ratio. As a method of effectively utilizing uranium resources, increasing the conversion ratio is considered. Improvement of the conversion ratio makes the neutron spectrum hard by reducing the water-to-fuel volume ratio in the fuel assembly loaded in the reactor, and promotes neutron capture of fuel-parent substances such as uranium-238 and plutonium-240. This can be achieved by performing

As means for improving the conversion ratio by reducing the water-to-fuel volume ratio in the fuel assembly, Japanese Patent Application Laid-Open No.
-64980 or JP-A-61-275694,
A method is shown in which fuel rods are densely arranged in a triangular lattice in a rectangular channel box.

[0004]

SUMMARY OF THE INVENTION Japanese Unexamined Patent Publication No. Sho 62 (1987) was conceived as means for reducing the water to fuel volume ratio in a fuel assembly.
-64980 or JP-A-61-275694,
Since the fuel rods are arranged in a triangular grid in a rectangular channel box, two opposing surfaces of the four inner surfaces of the channel box have a thicker layer of light water between the fuel rods than the other two surfaces. Exists. Therefore, the water to fuel volume ratio around the fuel rods facing the two surfaces is larger than the water to fuel volume ratio around the fuel rods facing the other two surfaces. In such a region where the water to fuel volume ratio is large, there is a problem that the output of the fuel rod increases and the output peaking increases.

[0005] Further, there is a problem that the coolant flowing in the channel box hardly flows to the densely arranged fuel rods, and more coolant flows in a region where the water-to-fuel volume ratio is large, and the cooling capacity is reduced. Such an increase in output peaking and a decrease in cooling capacity decrease the thermal margin of the fuel assembly. JP-A-62-64980 or JP-A-61-264
No. 75694 does not take any measures against these problems.

SUMMARY OF THE INVENTION It is an object of the present invention to reduce the output peaking by reducing the water-to-fuel volume ratio of the fuel rods facing the inner surface of the channel box, to improve the cooling capacity by making the coolant flow uniform, It is to improve the thermal margin of the fuel assembly.

[0007]

To achieve the above object, the present invention, as shown in FIG. 1, comprises a plurality of fuel rods arranged in a triangular dense grid, and a cross-sectional shape surrounding these fuel rods. In a fuel assembly comprising a quadrangular cylindrical channel box, of the four inner surfaces of the channel box, a surface where the gap between the fuel rods closest to each surface is wider than the other, and a fuel second closest to this surface A substance having a smaller moderating power than the light water is arranged in an area where the light water exists between the rod and the rod.

The present invention relates to a fuel assembly comprising a plurality of fuel rods arranged in a triangular dense grid and a channel box having a rectangular cross section and surrounding the fuel rods.
Of the four inner surfaces of the channel box, in a region where the light water exists between the surface where the gap between the fuel rods closest to each surface is larger than the other and the fuel rod second closest to this surface, A substance having a smaller moderating power than the light water may be disposed over the entire height of the inner surface of the channel box.

According to the present invention, there is provided a fuel assembly comprising a plurality of fuel rods arranged in a triangular dense grid and a channel box having a rectangular cross section and surrounding the fuel rods.
Of the four inner surfaces of the channel box, in a region where the light water exists between the surface where the gap between the fuel rods closest to each surface is larger than the other and the fuel rod second closest to this surface, The substance having a smaller moderating power than the light water may be arranged at least from a lowermost section where the fuel rod is present to a height of a half of the entire length of the fuel rod.

[0010]

FIG. 1 shows a first embodiment of the present invention. The fuel assembly 1 includes fuel rods 5 arranged in a triangular dense lattice and a channel box 9 surrounding the fuel rods. A region in the channel box 9 where the fuel rods 5 do not exist is filled with light water 13. Inner surface of channel box 9, 9
Among the fuel rods a, 9b, 9c, and 9d, the gap between the fuel rods closest to the face 9a and the face 9c is wider than the gap between the fuel rods closest to the face 9b and the face 9d. These surfaces 9a and 9c
In the region where the light water 13 exists between the fuel rods and the second closest fuel rod on each surface, a water exclusion rod 17 having the same diameter as that of the fuel rod 5 and a semicircular cross section is provided on the surfaces 9a and 9c. Place over the entire height. The cross-sectional shape of the water removal rod 17 is not limited by the present embodiment as long as the purpose of removing the light water 13 is satisfied.

FIG. 2 shows a conventional fuel assembly for a boiling water reactor as a comparative example of this embodiment. The fuel assembly 2 includes fuel rods 6 arranged in a triangular dense lattice and a channel box 10 surrounding the fuel rods. The area in the channel box 10 where the fuel rods 6 do not exist is filled with light water 14.
In the fuel assembly 2 of the prior art, since a thick layer of light water 14 exists between the surfaces 10a and 10c and the fuel rod closest to each surface, the water-to-fuel volume ratio is large around this fuel rod. . Therefore, the output of these fuel rods increases, and the output peaking is large. Further, the light water 14 flowing in the channel box 10 as a coolant flows more in these regions where the water-to-fuel volume ratio is large, and hardly flows around the densely arranged fuel rods 6.

In the first embodiment of the present invention, the water drainage rod 17 is used to determine the water-to-fuel volume ratio around the fuel rod closest to the surfaces 9a and 9c and the water around the fuel rod closest to the surfaces 9b and 9d. It can be reduced to the same as the fuel volume ratio. According to this embodiment, the output of the fuel rod closest to the surfaces 9a and 9c can be reduced, and the output peaking can also be reduced. Light water 1 flowing through the channel box 9 as a coolant
3 can be made uniform.

FIG. 3 shows a second embodiment of the present invention. FIG. 4 shows a cross-sectional view taken along the line AA ′ of the fuel assembly 3. The fuel assembly 3 includes fuel rods 7 arranged in a triangular dense lattice and a channel box 11 surrounding the fuel rods 7. Channel box 1
1 is filled with light water 15 in a region where the fuel rods 7 do not exist. Inner surface of channel box 11, 11a, 1
Among 1b, 11c, and 11d, the gap between the fuel rods closest to the surfaces 11a and 11c is wider than the gap between the fuel rods closest to the surfaces 11b and 11d. This face 11
a, in a region where the light water 15 exists between the surface 11c and the fuel rod second closest to each surface, a water exclusion rod 18 having the same diameter as the fuel rod 7 and having a semicircular cross section, The fuel rods 7 are arranged from the cross section at the lower end where the fuel rods 7 exist to half the height of the fuel rod 7.

In the boiling water reactor, since the lower area of the fuel assembly has a smaller void fraction, the water to fuel volume ratio is larger than that of the upper area of the fuel assembly. Therefore, the effect of reducing the water to fuel volume ratio by the water exclusion rod is greater in the lower region of the fuel assembly than in the upper region of the fuel assembly. By arranging the water exclusion rod 18 in the lower region of the fuel assembly, the water-to-fuel volume ratio can be effectively reduced, and by shortening the water exclusion rod 18 compared with the first embodiment, the fuel can be reduced. The pressure loss in the upper region of the assembly can be reduced.

FIG. 5 shows a third embodiment of the present invention. The fuel assembly 4 includes fuel rods 8 arranged in a triangular dense lattice and a channel box 12 surrounding the fuel rods 8. The area in the channel box 12 where the fuel rods 8 do not exist is filled with light water 16. Inner surface of channel box 12, 12
a, 12b, 12c and 12d, the surface 12a and the surface 12a
The gap between the fuel rods closest to c is equal to the surface 12b and the surface 12d.
It is wider than the gap between the fuel rods closest to. In the region where the light water 16 exists between the surfaces 12a and 12c and the fuel rod closest to each surface, the cross-sectional shape of the fuel rod 8 having the same diameter and filled with carbon is semicircular. The water exclusion rods 19 are arranged in all sections where the fuel rods 8 are present. According to this embodiment, the water-to-fuel volume ratio around the fuel rod closest to the surface 12a and the surface 12c can be reduced by filling the water exclusion rod 19 with carbon having smaller deceleration ability than light water.

[0016]

According to the present invention, in a boiling water reactor, there is provided a fuel assembly which realizes a reduction in power peaking and a uniform flow of coolant by reducing the water to fuel volume ratio. be able to.

[Brief description of the drawings]

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

FIG. 2 is a cross-sectional view showing a fuel assembly for a conventional boiling water reactor as a comparative example with respect to the first embodiment of the present invention.

FIG. 3 is a longitudinal sectional view showing a fuel assembly according to a second embodiment of the present invention.

FIG. 4 is a sectional view taken along line AA of FIG. 3;

FIG. 5 is a cross-sectional view showing a fuel assembly according to a third embodiment of the present invention.

[Explanation of symbols]

1, 2, 3, 4 ... fuel assembly, 5, 6, 7, 8 ... fuel rod, 9, 10, 11, 12 ... channel box, 9a,
9b, 9c, 9d ... inner surface of channel box 9, 1
0a, 10b, 10c, 10d ... channel box 10
, 11a, 11b, 11c, 11d... Inside surface of the channel box 11, 12a, 12b, 12c,
12d: Inside surface of the channel box 12, 13, 1
4, 15, 16 ... light water, 17, 18, 19 ... water removal rod.

Claims (4)

[Claims] 1. A plurality of fuel rods which are loaded in a core of a nuclear reactor using light water as a coolant and a moderator, contain a fuel material containing a fissile material, and are arranged in a triangular dense lattice. In a fuel assembly including a rod-shaped channel box having a cross-sectional shape of a quadrangular cylinder, a surface of the four inner surfaces of the channel box, in which a gap between the fuel rods closest to each surface is wider than the other surface, A fuel assembly for a boiling water reactor, wherein a substance having a smaller moderating power than that of the light water is present in a region where the light water exists between the fuel rod and a fuel rod closest to a surface. 2. A plurality of fuel rods which are loaded in a core of a nuclear reactor using light water as a coolant and a moderator, contain a fuel material containing a fissile material, and are arranged in a triangular dense lattice. In a fuel assembly including a rod-shaped channel box having a cross-sectional shape of a quadrangular cylinder, a surface of the four inner surfaces of the channel box, in which a gap between the fuel rods closest to each surface is wider than the other surface, A boiling water type in which a substance having a moderating power smaller than that of the light water is present over the entire height of the inner surface of the channel box in a region where the light water exists between the fuel rod and a fuel rod closest to the surface. Reactor fuel assemblies. 3. A plurality of fuel rods which are loaded in a core of a nuclear reactor using light water as a coolant and a moderator, contain a fuel material containing a fissile material, and are arranged in a triangular dense lattice. In a fuel assembly including a rod-shaped channel box having a cross-sectional shape of a quadrangular cylinder, a surface of the four inner surfaces of the channel box, in which a gap between the fuel rods closest to each surface is wider than the other surface, In a region where the light water exists between the fuel rod and the second closest fuel rod to the surface, a substance having a smaller moderating power than the light water has at least one-half of the entire length of the fuel rod from the lowest cross section where the fuel rod is present. 2
A fuel assembly for a boiling water reactor, wherein the fuel assembly is present up to the height of. 4. A fuel for a boiling water reactor according to claim 1, wherein at least one of said fuel rods contains a uranium-plutonium mixed oxide fuel as said fissile material. Aggregation.