JPH07218671A - Fuel assembly - Google Patents

Abstract

(57) [Summary] [Purpose] To prevent short fuel rods from moving or rising during transportation of fuel assemblies or during reactor operation. [Structure] A lower end plug 6 of a short fuel rod 2A is provided with a male screw portion 6A for screwing with a screw hole 11 of a lower tie plate 4, and a protrusion 6B is provided on the upper portion of this male screw portion 6A. A washer 12 having a through hole 12A having a notch 12C for inserting the short fuel rod 2A and a through hole 12B for inserting the fuel rod 13 adjacent to the short fuel rod 2A is used, and a lower tie plate 4 is used. Short fuel rod 2A, and this short fuel rod 2
The fuel rod 13 adjacent to A is fitted and assembled. As a result, the rotation of the short fuel rod 2A is controlled and the lifting is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water reactor, and more particularly to a fuel assembly which uses short fuel rods as some of its fuel rods during transportation and reactor operation. The present invention relates to a fuel assembly configured to prevent the short fuel rods from moving and rising during the operation.

[0002]

2. Description of the Related Art In a boiling water reactor, a reactor pressure vessel is filled with a coolant having functions of neutron moderation and fuel cooling. During operation of the reactor, the coolant is heated while moving from the bottom to the top in the core, and voids (vapor bubbles) are generated inside the coolant. Therefore, the volume ratio of voids to water (void ratio) becomes higher toward the upper part of the core.

In the upper part of the core, when the void ratio becomes zero when the reactor is shut down, the change in the moderator (= coolant) density from the time of reactor operation becomes very large, and the moderating effect of thermal neutrons becomes too large. The control rods tend to reduce the margin for keeping the core in a non-marine area (generally called the core shutdown margin).

In order to solve such a problem, a fuel assembly 20 shown in FIGS. 9 and 10 is conventionally known in a boiling water reactor. In FIG. 9 and FIG. 10, the boiling water reactor fuel assembly 20 is covered with a rectangular tube-shaped channel box 1, and the channel box 1 has 9 rows and 9 lines.
The fuel rods 2 bundled in a square lattice are arranged in rows.

Each fuel rod 2 is held at a predetermined interval by a spacer 5, and both upper and lower ends are constrained and fixed by an upper tie plate 3 and a lower tie plate 4, respectively. The cladding of the fuel rod 2 is filled with a large number of fuel pellets obtained by sintering slightly enriched uranium dioxide into pellets. At the center of the cross section of the fuel assembly, a plurality of water rods through which a coolant flows are arranged.

Further, by disposing a plurality of short fuel rods 2A in which some of the fuel rods 2 are shorter than the other fuel rods 2 at appropriate places in the fuel assembly, the flow passage area of the coolant above the fuel assembly is arranged. Has been proposed to reduce the pressure loss of the fuel assembly, and in combination with this, the amount of slightly enriched uranium existing in the upper core region is reduced to improve the shutdown margin of the reactor. There is.

[0007]

However, in such a fuel assembly, since the upper part of the short fuel rod 2A is not fixed and only the lower part is supported by the lower tie plate 4, the fuel assembly is formed. There is a problem that the short fuel rods 2A may be separated from the lower tie plate 4 during transportation due to horizontal acceleration applied during transportation of the body.

Further, during the operation of the reactor, the flow force of the coolant and the force for floating the fuel rod due to the hydraulic vibration due to the coolant act on the fuel rods. There is a problem of possibility of leaving the lower tie plate 4.

The present invention has been made to solve the above problems, and in a fuel assembly using short fuel rods as some of the fuel rods, the short fuel rods are used during transportation of the fuel assemblies and during reactor operation. (EN) Provided is a fuel assembly for a boiling water reactor, which prevents the fuel rod from separating from the lower tie plate and improves the integrity of the fuel rod.

[0010]

SUMMARY OF THE INVENTION The present invention is a fuel assembly in which a portion of a fuel rod is replaced by a short fuel rod, and a screw portion which is screwed into a screw hole of a lower tie plate is provided under a lower end plug of the short fuel rod. And a protrusion is provided on the upper part of the thread.

Further, the lower end plug is provided with a rotation preventing washer having a through hole having a notch for fitting with a projection of the lower end plug and a through hole for fitting with a lower end plug of an adjacent fuel rod. The insertion of the short fuel rod and the fuel rod adjacent to the short fuel rod are prevented from rotating.

Further, according to the present invention, in a fuel assembly in which a part of a fuel rod is replaced by a short fuel rod, a screw portion screwed into a lower tie plate is formed under a lower end plug of the short fuel rod, and an upper portion of the screw portion. It is characterized in that a square portion is provided in the. Further, a quadrilateral anti-rotation washer is fitted into the square portion of the lower end plug, and a screw hole into which the screw portion of the lower end plug is screwed into the lower tie plate and a washer having the same shape as the washer fitted. It is characterized by the provision of counterbores.

[0013]

When the short fuel rod is mounted on the lower tie plate, the protrusion or the square portion of the lower end plug, the washer and the counterbore are fitted with each other. As a result, it is possible to prevent the short fuel rod from rotating and to prevent the short fuel rod from rising.

Further, by mounting a compression coil spring on the rectangular portion of the lower end plug, the rotation preventing washer is crimped in a spot-tipped manner to control the rotation of the short fuel rod and prevent the short fuel rod from rising. Therefore, by suppressing the rotation of the short fuel rod, it is possible to prevent the short fuel rod from being separated from the fuel assembly.

[0015]

EXAMPLE A first example of a fuel assembly according to the present invention will be described with reference to FIGS. The overall structure of the fuel assembly according to the embodiment of the present invention is essentially the same as that shown in FIGS. 9 and 10 except for the main part where the lower end plug 6 of the short fuel rod 2A is attached to the lower tie plate 4. Not a thing.

That is, in the fuel assembly of this embodiment, as described with reference to FIGS. 9 and 10, the fuel rods 2 are arranged in a square lattice in 9 rows and 9 columns in the rectangular tubular channel box 1, and the fuel is Fuel bundles in which the upper and lower ends of the rods 2 are supported by the upper and lower tie plates 3 and 4 and the intermediate portion is held by a plurality of spacers 5 to keep the distance between the fuel rods 2 are incorporated.

A part of the fuel rods 2 is
It is replaced by a short fuel rod 2A lacking only the upper part. The arrangement of the short fuel rods 2A changes depending on the nuclear design of the fuel assembly, but FIG. 10 is only one example. .

That is, in FIG. 10, in the channel box 1 of the fuel assembly 20, long fuel rods 2 are arranged at positions indicated by white circles, and short fuel rods 2A are arranged at positions indicated by diagonal lines. The structure of this short fuel rod 2A is shown in FIG. In FIG. 3, the cladding tube 7 that constitutes the short fuel rod 2A
A large number of fuel pellets 8 are loaded therein, and a plenum spring 9 is arranged above the fuel pellets 8,
An upper end plug 10 and a lower end plug 6 are welded to the upper and lower ends of the covering pipe 7.

The structure of the lower end plug 6 of the short fuel rod 2A is shown in FIG.
And shown in FIG. 1 and 2, the lower end plug 6
A male screw 6A for attaching the short fuel rod 2A to the lower tie plate 4 by screwing into the female screw hole 11 formed in the lower tie plate 4 is formed in the lower small diameter portion of the.

Further, above the male screw 6A of the lower end plug 6, a cylindrical portion 6C having a diameter larger than that of the male screw 6A, and a taper portion 6D formed between the male screw 6A and the cylindrical portion 6C.
And a projection 6B formed on the tapered portion 6D.

In the figure, reference numeral 12 is a rotation preventing washer for stopping rotation of the short fuel rod 2A. The washer 12 is for inserting the short fuel rod 2A in a predetermined position, and is also adjacent to the short fuel rod 2A. The fuel rod 13 to be inserted is also inserted. The washer 12 is an oval thin plate, and the lower end plug 6 is provided on the right side of the thin plate surface.
Through hole 12A provided with a notch 12C for fitting with the protruding portion 6B of
On the left side, there is provided a through hole 12B that fits with the lower end plug 13A of the fuel rod 13 that fixes the washer 12 at a predetermined position.

Next, referring to FIGS. 1 and 2, the fuel rod 13
An assembly means for mounting the short fuel rod 2A on the lower tie plate 4 will be described. When assembling the short fuel rod 2A into the lower tie plate 4, a through hole is formed in the lower end plug 6 of the short fuel rod.
The notch 12C of 12A is passed through the washer 12 so as to fit with the protrusion 6B of the lower end plug 6, and at this stage, while holding the washer 12 with a dedicated jig, the lower end plug 6 of the short fuel rod 2A is held.
Is screwed into the screw hole 11 of the lower tie plate 4 by a predetermined amount.

Although a dedicated jig is not shown, a jig having a function of holding a thin flat object by inserting two thin flat plates at the tip of the jig and sandwiching between the two thin flat plates is inserted. . After that, the through hole 12B and the lower tie plate hole 14 into which the fuel rod 13 is inserted are reversely rotated in the range of 0 to 360 °.

In this state, the jig is removed to remove the fuel rod.
2 by inserting 13 into the lower tie plate hole 14.
As shown in, the short fuel rod 2A and the fuel rod 13 are fitted into the rotation preventing washer 12, respectively.

Here, in such a screw-in structure, the only possible reason for lifting the short fuel rod 2A is to cause the horizontal and vertical vibrations generated during the transportation of the fuel assembly or the hydraulic power generated by the coolant during the reactor operation. Due to vibration or the like, torque is generated in the direction opposite to the direction in which the short fuel rod 2A is screwed into the lower tie plate 4, and the short fuel rod 2A
This is the case when it acts on.

However, in the first embodiment of the present invention, even if the torque for rotating the short length fuel rod 2A acts, the short length is shortened via the protrusion 6B of the lower end plug 6 and the notch 12C of the through hole 12A. Since the fuel rod 2A and the rotation-preventing washer 12 are fitted to each other, the rotation of the short fuel rod 2A is controlled by the washer 12, and the washer 12 is fitted to the fuel rod 13 through the through hole 12B. Since it is fixed by this, the rotation of the short fuel rod 2A can be suppressed.

Next, a second embodiment of the present invention will be described with reference to FIG. The second embodiment shown in FIG. 4 is different from the first embodiment shown in FIG. 1 in that instead of the protrusion 6B, the taper portion 6D at the upper portion of the male screw 6A formed on the lower end plug 6 has a quadrangular square shape. That is, the portion 15 is formed, and the shape of the through hole 12D of the oval washer 12 to be inserted into the square portion 15 is formed into a quadrangle that matches the cross-sectional shape of the square portion 15.

In the present embodiment, it is not necessary to limit the shapes of the rectangular portion 15 and the through hole 12D to a quadrilateral, and a short fuel rod may be used as long as it has a shape other than a perfect circle such as a polygon. The rotation of 2A can be suppressed, and the detachment from the lower tie plate can be prevented.

The operation and effect of this second embodiment is the first
Although the same as in the first embodiment, the prismatic portion 15 is easier to manufacture and easier to assemble than the protrusion 6B of the first embodiment.

Next, referring to FIGS. 5A and 5B, third and fourth embodiments of the present invention will be described. The third and fourth embodiments shown in FIG. 5 differ from the first embodiment shown in FIG. 1 in that a through hole for fixing a washer 12 with a fuel rod 13 adjacent to a short fuel rod 2A is a through hole. 12E and two through holes 12F are provided.

That is, in the third embodiment of FIG. 5 (a), the washer 12 is L-shaped and has a through hole for inserting the short fuel rod 2A in the center.
12A and through holes 12E and 12F into which the fuel rods 13 are inserted are formed on the left and right sides of the through hole 12A.

In the fourth embodiment of FIG. 5B, three through holes 12E and 12F are formed in series in the elongated oval washer 12. The number of through holes for fixing the fuel rods adjacent to the short fuel rods 2A provided on the washer 12 is one and two.
There is no need to limit it to one.

Also, by combining these embodiments, it is possible to suppress the rotation of the short fuel rod 2A and prevent the short fuel rod 2A from separating from the lower tie plate.

In the fuel assemblies according to the third and fourth embodiments, even if the short fuel rod 2A rotates in the direction of separating from the lower tie plate, the protrusion 6 of the lower end plug 6 is formed.
The short fuel rod 2A and the rotation-preventing washer 12 are fitted through B and the notch 12C of the through hole 12, and the washer 12 is fixed by being fitted with the fuel rod 13 through the through hole 12B. Therefore, the rotation of the short fuel rod 2A is suppressed, and the short fuel rod 2A
It is possible to prevent separation from the lower tie plate.

Next, a fifth embodiment of the present invention will be described with reference to FIGS. The overall structure of the fuel assembly according to the present invention is essentially the same as that shown in FIG. 9 except the mounting portion of the lower end plug 6 and the lower tie plate 4 of the short fuel rod 2A, and therefore the description thereof will be given. It is omitted and only the main part will be described.

FIG. 6 is an exploded view of the fifth embodiment, and FIG.
8 is a vertical cross-sectional view showing a state where the short fuel rod 2A is assembled to the lower tie plate 4, and FIG. 8 is an elevational view of the short fuel rod used in this embodiment.

6 to 8, a male screw 6A for screwing and fitting the female screw hole 11 of the lower tie plate 4 is formed in the lower portion of the lower end plug 6. Also,
On the upper part of the male screw 6A portion of the lower end plug, there is formed a quadrangular rectangular portion 15 having a solid cross section larger than the diameter of the male screw 6A. In the figure, reference numeral 17 is a washer for preventing rotation of the short fuel rod 2A, and 18 is a compression coil spring 18 for pressing the washer 17 to a predetermined position of the lower tie plate 4.

Here, the washer 17 is made of a quadrilateral thin plate,
A quadrangular through hole 17A, which is slightly larger than one side of the rectangular portion 15 of the lower end plug 6, is provided at the center of the thin plate. Further, a compression coil spring 18 is provided between the lower end of the short fuel rod 2A and the washer 17 so that the washer 17 does not float up even when some external force acts on the washer 17 during transportation of the fuel assembly or operation of the reactor. Is installed.

On the other hand, the lower tie plate 4 is formed with a quadrangular counterbore 16 into which a washer 17 is fitted, and one side of the counterbore 16 is slightly longer than one side of the outer frame of the washer 17, It has the same depth as the thickness of the washer 17.

Further, a screw hole 11 for screwing the male screw 6A of the lower end plug 6 of the short fuel rod is formed in the center of the counterbore 16 for supporting the short fuel rod 2A on the lower tie plate 4.

Next, the assembling means of the fifth embodiment will be described with reference to FIG. When the short fuel rod 2A is assembled in the channel box 1 of the fuel assembly, the compression coil spring 18 is first attached to the lower end plug 6 of the short fuel rod 6A, and then the through hole 17A of the rotation preventing washer 17 is formed as described above. The washer 17 is inserted so as to match the square portion 15 of the lower end plug 6.

At this stage, a dedicated jig (a thin flat plate-shaped jig having a slit in the center of the jig whose length allows the rectangular portion 15 of the lower end plug to rotate) is applied to the washer 17. ,
The jig is pushed downward or upward until the bottom surface of the jig is located above the lower end of the rectangular portion 15 of the lower end plug.

While confirming the direction so that the washer 17 and the spot facing 16 of the lower tie plate 4 are correctly aligned during this pushing up, the lower end plug 6 of the short fuel rod 2A is moved to the lower tie plate 4.
A predetermined amount is screwed into the screw hole 11. When the jig is pulled out in this state, the washer 17 is fitted to the counterbore 16.

Here, in such a screw-in structure, the only possible cause for lifting the short fuel rod 2A is to cause the horizontal or vertical vibration generated during transportation of the fuel assembly or the coolant during the operation of the reactor. It is conceivable that a torque opposite to the direction in which the short fuel rod 2A is screwed into the lower tie plate 4 due to hydraulic vibration or the like acts on the short fuel rod 2A, causing the short fuel rod 2A to rotate.

However, in the fifth embodiment, even if the torque for rotating the short fuel rod 2A acts, the square portion 15 of the lower end plug 6 of the short fuel rod 2A and the rotation preventing fitting to this are prevented. Since the washer 17 and the through hole 17A have a quadrilateral shape and the counterbore 16 into which the rotation preventing washer 17 is fitted is also a quadrilateral shape, the short fuel rods 2A You can control the rotation of.

In addition, since the compression coil spring 18 is mounted, the washer 17 causes the lower tie plate 4 by this spring force.
Since the washer 17 is crimped and fixed to the spot facing 16, the washer 17 does not float up.

In this embodiment, it is not necessary to limit the shapes of the rectangular portion 15, the washer 17 (including the through hole 17A) of the lower end plug and the spot facing 16 of the lower tie plate to a quadrilateral shape. If it has a polygonal shape, the rotation of the short fuel rod 2A can be controlled and the lifting can be prevented.

In the boiling water reactor fuel assembly according to the present embodiment described above, even if the short fuel rods 2A are rotated in the floating direction, each of them is a square tube portion of the lower end plug 6 having a quadrilateral shape. 15, anti-rotation washer 17 (including through hole 17
A) and the counterbore 16 are fitted to each other, and the compression coil spring 18 is attached to prevent the washer 17 from floating, so that the rotation of the short fuel rod 2A is suppressed and the lifting can be prevented.

In the above embodiment, the fuel rods are arranged in 9 rows and 9 columns. However, the arrangement is not limited to this, and 8 rows and 8 columns or other arrangements can be used.

[0050]

EFFECTS OF THE INVENTION According to the present invention, it is possible to prevent the short fuel rods from rising and prevent them from being detached from the lower tie plate. be able to.

[Brief description of drawings]

FIG. 1 is an exploded view showing an assembling means of a first embodiment of a fuel assembly according to the present invention.

FIG. 2 is a vertical cross-sectional view showing a partially side view of the assembled state in FIG.

FIG. 3 is an elevational view showing a partial cross section of the short fuel rod in FIG.

FIG. 4 is an exploded view showing an assembling means of a second embodiment of the fuel assembly according to the present invention.

FIG. 5 (a) is an exploded view showing an assembly means of a third embodiment of the fuel assembly according to the present invention, and FIG. 5 (b) is an exploded view showing the assembly means of the fourth embodiment.

FIG. 6 is an exploded view showing an assembling means of a fifth embodiment of the fuel assembly according to the present invention.

7 is a vertical cross-sectional view showing a part of the assembled state in FIG. 6 on a side surface.

FIG. 8 is an elevational view showing a partial cross section of the short fuel rod in FIG.

FIG. 9 is a vertical cross-sectional view showing a part of a conventional fuel assembly in a side view.

10 is a cross-sectional view of the fuel assembly in FIG.

[Explanation of symbols]

1 ... Channel box, 2 ... Fuel rod, 2A ... Short fuel rod, 3 ... Upper tie plate, 4 ... Lower tie plate, 5
... Spacer, 6 ... Lower end plug, 6A ... Male screw part, 6B ... Projection part, 6C ... Cylindrical part, 6D ... Tapered part, 7 ... Fuel cladding tube, 8 ... Fuel pellet, 9 ... Plenum spring, 10 ...
Upper end plug, 11 ... Screw hole, 12 ... Anti-rotation washer, 12A ... Through hole (for short fuel rod), 12B ... Through hole (for adjacent fuel rod), 12C ... Notch, 12D ... Square through hole, 13 … Fuel rods (for adjacent fuel rods), 14… tie plate holes, 15… prismatic parts,
16 ... counterbore, 17 ... washer, 18 ... compression coil spring, 19 ... missing number, 20 ... fuel assembly.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical display location G21C 3/30 GDB L (72) Inventor Ichiro Tsukahara 66-2 Horikawa-cho, Kawasaki-shi, Kanagawa Prefecture Toshiba Inside Engineering Co., Ltd.

Claims (7)

[Claims] 1. A fuel rod is arranged in a square lattice in a square tubular channel box, the upper and lower sides of which are supported by an upper tie plate and a lower tie plate, and a plurality of spacers are provided at an intermediate portion to maintain the distance between the fuel rods. A bundle of fuel
In a fuel assembly in which a part of the fuel rod is replaced with a short fuel rod, a male screw for coupling with a female screw hole of the lower tie plate is provided on a lower end plug of the short fuel rod, and a protrusion or a square shape is provided on an upper portion of the male screw. A fuel assembly characterized by being provided with a portion. 2. A washer having a through hole provided with a notch for fitting with a projection of a lower end plug of the short fuel rod and a through hole for inserting a lower end plug of a fuel rod other than the short fuel rod. The fuel assembly according to claim 1, wherein the fuel assembly is fitted to the lower end plug. 3. A washer having a square through hole that fits into a square portion of a lower end plug of the short fuel rod and a through hole for inserting a lower end plug of a fuel rod other than the short fuel rod, the washer having the lower end. The fuel assembly according to claim 1, wherein the fuel assembly is fitted in a plug. 4. The fuel assembly according to claim 1, wherein a polygonal washer having a through hole having substantially the same shape as the outer shape of the square portion is fitted to the square portion of the lower end plug. . 5. The fuel assembly according to claim 1, wherein a compression spring is mounted on the rectangular portion of the lower end plug, and the compression spring is located above the washer. 6. The fuel assembly according to claim 1, wherein the lower tie plate is provided with a polygonal counterbore having substantially the same shape as the washer. 7. When the short fuel rod is mounted on the lower tie plate, the square portion of the lower end plug, the washer and the counterbore fit with each other, and the washer is pressed by the spring force of the compression spring. The fuel assembly according to claim 1, wherein the fuel assembly is pressure-bonded to the counterbore of the tie plate.