JPH09243770A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the sudden rise of reactivity in the occurrence of an earthquake by shifting the position of the central axis of a D-grid reactor core fuel rod bundle in the direction leaving a control rod. SOLUTION: When a fuel assembly in which the position of a fuel rod bundle 12 is shifted in the direction leaving a control rod 4 is charged in a reactor core, the space between the fuel rod bundles 12 in the adjacent fuel assemblies is narrowed in the side opposite to the control rod 4. Even when a channel box 2 is vibrated to extend the gap width at an earthquake, it never becomes an excessive gap width, and sudden rise of reactivity is hardly caused in the individual fuel assemblies. As another method, the thickness dimension of the side surface of the channel box 2 is set larger on the control rod 4 side and shorter on the opposite side to the control rod 4, and the fuel rod bundle 12 may be arranged so that its central axis is conformed to the central axis of a substantially rectangular parallelepiped formed by the inside surfaces of the channel box 2.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a fuel assembly used in a boiling water reactor.

[0002]

2. Description of the Related Art Core structures of boiling water reactors (hereinafter referred to as BWRs) include C-lattice core structures and D-lattice core structures. As shown in FIG. 4A, the C-lattice core structure has a space between the fuel assembly 1 and a fuel assembly adjacent to the fuel assembly 1, that is, a water gap between the channel boxes. The width a, b is the control rod 4
The width of the water gap in which the control rods 4 are arranged as shown in FIG. 4 (b) is the same as that of the D-lattice core structure as shown in FIG. 4 (b). The width a is wider than the width b of the water gap where the control rod 4 is not arranged (a> b).
(Note that the same or corresponding parts are denoted by the same reference numerals in FIG. 4 described above and all the drawings described below.)

Generally, the distribution of the thermal neutron flux inside the fuel assembly is higher on the water gap side, so that the fission reaction of thermal neutrons and uranium occurs in the water gap closer to the outer periphery of the fuel rod bundle than to the inner depth of the fuel rod bundle. Be active on the near side. Therefore, if all the fuel rods forming the fuel rod bundle have the same enrichment, the output of the fuel rod becomes large at the outermost periphery of the fuel rod bundle, and local peaking occurs.

Therefore, conventionally, the concentration of each fuel rod is changed according to the deceleration state of neutrons at that location, and the concentration distribution is given to the fuel rod bundle, thereby suppressing the occurrence of local peaking. There is.

For example, in the fuel assembly having the C-lattice core structure, since the water gap width is the same in the entire circumference, the deceleration state of thermal neutrons over the outermost four-sided area of the fuel rod bundle inside the channel box. Is even. For this reason, the enrichment distribution of the fuel rod bundle arranged inside the channel box is made substantially uniform without any bias.

In the fuel assembly of the D-lattice core structure, the water gap width outside the channel box is wide on the side where the control rods are arranged, and narrow on the side where the control rods are not arranged. The deceleration state of the thermal neutrons in the outermost four-sided area of the is high on the control rod side and low on the side where the control rod is not arranged.

Therefore, by arranging the fuel rods having a low enrichment on the control rod side and the fuel rods having a high enrichment on the side where the control rods are not placed, a biased enrichment distribution is obtained. The output for each bar is flattened.

The BWR core has a flat lattice shape having an upper lattice plate 6 and a plurality of holes 7a as shown in FIG. 5 (a) regardless of whether it has a C lattice core structure or a D lattice core structure. And a core support plate 7.

Each of the holes 7a provided in the core support plate 7 has a control rod insertion hole for inserting the control rod 4 in its center, and four fuel assemblies 1 centered on the control rod 4.
And a fuel support metal fitting 8 having an assembly support hole for symmetric loading. (In addition, as one means for determining whether the core structure is the C lattice or the D lattice described above, adjusting the arrangement of the assembly support holes of the fuel support fitting 8 can be mentioned.)

In the BWR core, as shown in FIG. 5 (b), the fuel assembly 1 is formed of four bodies to form one lattice cell,
This one cell is loaded in one mass of the upper lattice plate 6. 1
A control rod 4 is arranged at the center position of the cell, and a channel fastener 5 at one corner on the upper side of each fuel assembly 1 is arranged.

Here, FIG. 6 (a) and FIG. 6 show the channel fastener 5 located in the upper corner of the fuel assembly 1.
This will be briefly described with reference to FIG. The channel fastener 5 is a leaf spring member that is mounted across two surfaces of the channel box 2, and includes a screw member 9 and a fastener holder 10.
It is fixed to the triangular plate 11 on the upper surface of the channel box by.

As described above, since four fuel assemblies 1 are loaded in one mass of the upper lattice plate 6 with the channel fasteners 5 on the control rod side, the channel fasteners of each fuel assembly 1 are loaded. Fives will face each other on the control rod side. Since these channel fasteners 5 are spring members, they are pressed against each other when they face each other, and as a result, the upper portion of the channel box 2 is pressed inside one mass of the upper lattice plate 6 and the fuel assembly 1 is inserted into the core. Is fixed.

That is, the lower part of the fuel assembly 1 is fixed by the assembly supporting hole of the fuel support fitting 8 and the upper part is pressed against the inner wall of the upper lattice plate 6 by the spring force of the channel fastener 5. , Its position in the core has been determined.

[0014]

Generally, a fuel assembly loaded in the core has a longitudinal length of about 4 m, and the upper and lower ends of the fuel assembly are fixed and loaded in the core. Therefore, when an earthquake occurs, Due to the vibration, the middle part of the channel box may be horizontally oscillated, and the water gap width between the channel box and another channel box may be changed.

Therefore, in order to investigate the change in the water gap width when an earthquake occurs, a vibration test is conducted using a full-scale simulated fuel assembly, and the stress of the channel box generated during the earthquake is measured in the fuel assembly. Measured over length.

In this vibration test, strain gauges are attached in advance at seven spacer positions on the outer surface of the channel box in the axial direction, and the stress generated in the channel box when vibration is applied is measured at positions 1 to 7. It was done by the method. FIG. 7 shows the results obtained by the above vibration test. In this graph, the position is plotted on the vertical axis, and the generated stress is plotted on the horizontal axis so that the value increases as the distance from the vertical axis increases.

From FIG. 7, the channel fastener side (that is,
It can be seen that the generated stress is higher on the side where the control rods are arranged) than on the upper lattice side (that is, the side where the control rods are not arranged). That is, it can be said that the channel box has a larger amplitude on the side of the channel fastener than on the side of the upper lattice plate when an earthquake occurs.

Further, FIG. 8 shows the state of the core when vibration occurs. Here, in consideration of the fact that the vibration direction of an earthquake is not uniform, a case is shown where vibration occurs in a diagonal direction, which is the vibration direction that produces the largest amplitude in the fuel assembly.

In FIG. 8, the quadrangle 1a shown by the broken line is the position of the fuel assembly when no vibration is generated, and the quadrangle 1 shown by the solid line is the position of the fuel assembly when the vibration is generated.

According to FIG. 7, since the channel box has a larger amplitude on the channel fastener side (control rod side) than on the upper lattice plate side, the water gap width around the fuel assembly is smaller than that in the normal state on the channel fastener side. It can be said that is narrowed and widened on the side of the upper lattice plate.

In the fuel assembly having the C-lattice core structure, the reactivity to the variation of the water gap width is such that the water gap width around the fuel assembly is uniform over the entire circumference and the reactivity is arranged in the core. The enrichment of fuel rods is not so different, so it does not change much.

However, in a boiling water reactor having a D-lattice core structure, the water gap width around the fuel assembly differs between the channel fastener side and the upper lattice plate side, and the water gap width on the upper lattice plate side is narrower. Since the fuel rods having a relatively high degree of enrichment are arranged in the region along the side, if the gap width on the side of the upper lattice plate widens due to an earthquake, the reactivity will suddenly increase suddenly. That is, the asterisk is the point where the reactivity increased sharply.

Therefore, the present invention aims to obtain a boiling water reactor having a D-lattice core structure, in which even if the fuel assembly oscillates due to an earthquake, a momentary increase in reactivity is unlikely to occur. To aim.

[0024]

In order to achieve the above object,
The invention of claim 1 is a fuel assembly used in a boiling water reactor having a D-lattice core structure, wherein a plurality of fuel rods arranged in parallel are bundled in a square lattice shape and fixed in a channel box. In the fuel assembly for a nuclear reactor, the central axis of the fuel rod bundle is located at a position displaced from the control rod with respect to the central axis of the substantially rectangular parallelepiped formed by the outer surface of the channel box. It is a thing.

That is, according to the first aspect of the invention, since the position of the fuel rod bundle inside the fuel assembly is shifted in the direction away from the control rod, the arrangement of the individual fuel assemblies (channel boxes) in the core is changed. Even if not, the space between the fuel rod bundles in the adjacent fuel assemblies in the narrow water gap is substantially narrow,
Even if the channel box oscillates when the earthquake occurs and the water gap width between the fuel assemblies expands, the reactivity does not suddenly increase.

Further, the invention of claim 2 is the fuel assembly for a boiling water reactor according to claim 1, wherein the thickness dimension of the channel box is a side surface facing the control rod rather than a side surface not facing the control rod. Is characterized by being larger.

That is, in the invention of claim 2, the thickness of the side surface of the channel box is changed between the channel fastener side and the upper lattice plate side so that the position of the central axis of the substantially rectangular parallelepiped formed by the inner surface of the channel box is changed. By shifting the fuel rod bundle mounted on the inner surface of the channel box from the position of the central axis of the substantially rectangular parallelepiped formed by the outer surface of the channel box, the fuel rod bundle mounted on the inner surface of the channel box is substantially displaced from the control rod. It is arranged.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in more detail with reference to examples. FIG. 1 illustrates one exemplary embodiment of the present invention. FIG. 1A is a top view of the fuel assembly of the present embodiment, and FIG. 1B is a perspective view of a main part of a lower tie plate portion viewed from the AA surface side in FIG. 1A.

The fuel assembly shown in FIG. 1A has 8 rows and 8 rows.
It has a bundle of fuel rods with a row of fuel rod arrangements, two sides 2 of the channel box on the side of the channel fasteners 5.
The distance from a, 2b to the side surface of the fuel rod bundle is wider than the distance from the two side surfaces 2c, 2d of the channel box on the side of the triangular plate 11b diagonally opposite to the channel fastener 5 to the side surface of the fuel rod bundle. The box and the fuel rod bundle are arranged in parallel.

The arrangement of such fuel rod bundles is determined by the location bosses 3a to 3d provided on each of the upper tie plate U and the lower tie plate L of the fuel assembly. In the present embodiment, first, the location bosses 3a, 3 facing the two side surfaces 2a, 2b of the channel box on the channel fastener 5 side of the upper tie plate U.
The heights of a, 3b and 3b are higher than those of the location bosses 3c, 3c, 3d and 3d facing the other two side surfaces.

The location bosses 3a to 3d have protrusions contacting the inside of the channel box to prevent the channel box from approaching, thereby forming a gap at a constant interval between the fuel rod bundle and the inside of the channel box.

Therefore, in this embodiment, the two sides 2 of the channel box on the side of the channel fastener 5 are provided.
The distance from a, 2b to the side surface of the fuel rod bundle is wider than the distance from the two side surfaces 2c, 2d of the channel box on the side of the triangular plate 11b diagonally opposite to the channel fastener 5 to the side surface of the fuel rod bundle.

In other words, the location bosses 3a, 3b on the side of the channel fastener 5 have a projecting dimension of about 1 mm, which is longer than that of the prior art, and the location bosses 3c, 3d on the diagonal side of the channel fastener 5 are about 1 mm. Is getting shorter.

Further, in this embodiment, as shown in FIG. 1 (b), the leak control plate in which the protrusion height is adjusted in the lower tie plate L is used. The leak control plate is pressure-bonded to the inner surface of the channel box to control the leak flow rate. However, in the present embodiment, in order to maintain good control of the leak flow rate, the channel box 2 and the lower portion of the fuel rod bundle are provided. A leak control plate that corresponds to the deviation of the central axis is used.

That is, the two leak control plates 13a and 13b (only 13a is disclosed in the drawing) facing the two side surfaces 2a and 2b of the channel box on the side of the channel fastener 5 have a large protrusion size, and the diagonal position with respect to the channel fastener 5 is large. Two leak control plates 13c, 13 facing the two side faces 2c, 2d of the channel box on the side of the triangular plate 11b.
The protrusion size of d (only 13d is disclosed in the drawing) is shortened to shift the position of the fuel rod bundle in the direction away from the control rod (that is, the channel fastener).

FIG. 2 shows the positional relationship between the channel box 2 and the fuel rod bundle 12 when a fuel assembly in which the position of the fuel rod bundle 12 is displaced in the direction away from the control rod 4 is loaded in the core. It is a schematic diagram.

As can be seen from FIG. 2, when a fuel assembly in which the position of the fuel rod bundle 12 is displaced in the direction away from the control rod 4 is loaded into the core, the fuel rod bundles 12 in the adjacent fuel assemblies are loaded.
The distance between them becomes narrower on the side opposite to the control rod. Therefore, even if the channel box oscillates and the gap width expands when an earthquake occurs, the gap width does not become excessively large, and the reactivity of individual fuel assemblies does not rapidly increase.

The above has described one embodiment in which the fuel rod bundle is arranged at a position displaced from the control rod in the channel box. However, the present invention is not limited to this, and the thickness of the side surface of the channel box is controlled. It is large on the rod side (that is, the channel fastener side) and short on the side opposite to the control rod (that is, the upper lattice plate side), and the center axis of the fuel rod bundle and the center axis of the substantially rectangular parallelepiped formed by the inner surface of the channel box are May be arranged so as to coincide with each other.

FIG. 3 is a cross-sectional view showing the arrangement of the channel box and the fuel rods at this time. In this case, since the central axis of the substantially rectangular parallelepiped formed by the inner surface of the channel box is displaced from the position of the central axis of the substantially rectangular parallelepiped formed by the outer surface of the channel box in the direction away from the control rod 4, The position of the fuel rod bundle arranged in alignment with the central axis of the substantially rectangular parallelepiped formed by the side faces is also shifted in the direction away from the control rod 4.

Therefore, when such a fuel assembly is loaded in the core, the fuel rod bundles 1 in the fuel assemblies substantially adjacent to each other are loaded.
The distance between the two becomes narrower on the side opposite to the control rod. Therefore, even if the channel box oscillates when the earthquake occurs and the gap width expands, the gap width does not become excessively large, and the reactivity of individual fuel assemblies does not rapidly increase.

[0041]

As described above, in the boiling water nuclear reactor fuel assembly of the present invention, when the fuel rod bundles in the adjacent fuel assemblies are opposed to each other when loaded in the core, the distance between the control rods is opposite. Since the width becomes narrower on the side, even if the channel box oscillates and the gap width widens when an earthquake occurs, there is an advantage that the gap width does not become excessive and the reactivity of individual fuel assemblies does not suddenly increase.

Further, since the outer shape of the fuel assembly of the present invention is the same as that of the conventional one, there is an advantage that it can be loaded into the BWR core which has been conventionally used.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an outline of one embodiment of the present invention.

FIG. 2 is a schematic diagram showing a positional relationship between a channel box and a fuel rod bundle when a fuel assembly of the present invention is loaded in a core.

FIG. 3 is an explanatory diagram showing an outline of another embodiment of the present invention.

FIG. 4 is a schematic diagram showing a C-lattice core structure and a D-lattice core structure of a boiling water reactor.

FIG. 5 is a partial explanatory view showing a core structure of a general BWR.

FIG. 6 is an explanatory view briefly showing the structure of a channel fastener.

FIG. 7 is a diagram showing the results of a vibration test.

FIG. 8 is an explanatory diagram showing a state of the core when vibration occurs in a diagonal direction of the core lattice.

[Explanation of symbols]

1 Fuel Assembly 2 Channel Box 3a-3d Location Boss 4 Control Rod 5 Channel Fastener 6 Upper Lattice Plate 7 Core Support Plate 7a Hole 8 Fuel Support Metal Fitting 9 Screw Member 10 Fastener Retainer 11 Triangular Plate 12 Fuel Rod Bundle 13a, 13d Leak Control Plate U Upper tie plate L Lower tie plate

Claims (2)

[Claims] 1. A fuel assembly for use in a boiling water reactor having a D-lattice core structure, wherein a plurality of fuel rods arranged in parallel are bundled in a square lattice shape and fixed in a channel box. In the fuel assembly for a reactor, the central axis of the fuel rod bundle is located at a position displaced from the control rod in a direction away from the central axis of the substantially rectangular parallelepiped formed by the outer surface of the channel box. Type nuclear reactor fuel assembly. 2. The thickness dimension of the channel box is
The boiling water reactor fuel assembly according to claim 1, wherein the side surface facing the control rod is larger than the side surface not facing the control rod.