JP3088487B2 - Fuel assembly and fuel channel box - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly and a fuel channel box thereof, and more particularly to a fuel assembly and a fuel channel box which are suitable for improving the life and mechanical integrity.

[0002]

2. Description of the Related Art As shown in FIGS. 20 and 21, a conventional fuel assembly 30 has a plurality of fuel rods 3 and water rods 4 bundled by a plurality of spacers 5, and upper and lower ends of the fuel rods 3 are of an upper type. The fuel bundle supported by the rate 6 and the lower tie plate 7 is housed in a fuel channel box 31, and the upper end of the channel box 31 and the upper tie plate 6 are connected by a channel fastener 9.

[0003] When the fuel assemblies 2 are loaded into the core, the channel fasteners 9 press adjacent fuel assemblies 2 against each other to secure a passage for facilitating the operation of inserting and extracting the control rod.

[0004] The fuel channel box 31 is a long rectangular tube having the following functions. (1) By forming a coolant flow path isolated for each fuel assembly, the flow rate of the coolant flowing in the fuel assembly is secured, and the coolant flows evenly.

(2) To form a guide surface for inserting and removing the control rod between the fuel assemblies.

(3) To ensure the rigidity of the fuel assembly and to facilitate the handling. Also, fuel channel box 3
The upper part of the channel spacer 1 is provided as a backup in the event that the above-mentioned channel fastener 9 is broken.
Is provided.

During operation of the reactor, the fuel assembly 30
Due to the flow pressure of the coolant flowing through the fuel channel box, a pressure difference is generated between the inner and outer surfaces of the fuel channel box 31 and neutron irradiation is performed, so that the fuel channel box 31 is deformed to expand outward. Such a deformation may reduce the gap with the control rod and thus hinder the insertion and withdrawal operation of the control rod. For this reason, conventionally, the deformation is suppressed by making the thickness of the fuel channel box 31 sufficiently large, and the service period of the fuel channel box 31 is limited to cope with the problem.

[0008] However, from the viewpoint of neutron economy,
It is desirable to reduce the thickness of the fuel channel box 31 as much as possible to reduce the volume ratio of the structural material occupying in the core, and also from the viewpoint of reducing waste, the service period of the fuel channel box 31 in the furnace can be increased. It is desirable to reduce the amount of waste disposal by making the length as long as possible and reusing the used fuel channel box 31. In recent years, fuel burnup has been promoted from the viewpoint of improving fuel economy. In this case, the fuel channel box 3
1 is also used for a longer period of time than before.

In view of the above background, Japanese Patent Application Laid-Open No. 57-175286 discloses a method of increasing the life of the fuel channel box 31 without increasing the volume of the fuel channel box 31 and suppressing the deformation of the fuel channel box 31 and improving the service life. As described in the official gazette, the stress is reduced by increasing the thickness of the corner (corner) in the cross section where the stress due to the pressure difference between the inside and outside of the fuel channel box is highest, and the side having the relatively low stress A structure for reducing the thickness of the central region has been proposed.

[0010]

Problems to be Solved by the Invention
The method described in JP-A-86-86 eliminates the disadvantages of the prior art. However, in this prior art, no consideration is given to the contact portion between the fuel channel box and the upper lattice plate when the fuel channel box is loaded in the core and the channel spacer 10 mounted on the upper portion of the fuel channel box. There was a problem.

That is, when the fuel assemblies 30 are loaded in the core, they are pressed against each other by the above-mentioned channel fasteners 9 and installed. Channel spacer 10
Is mounted on the flat portion at the top of the channel box. A reaction force is applied to the upper surface of the fuel channel box 31 which is in contact with the upper lattice plate and the mounting portion of the channel spacer 10 during an earthquake or the like.

The channel box described in Japanese Patent Application Laid-Open No. 57-175286 is characterized in that the thickness of the corner of the cross section is large and the thickness of the center of the side is small. top guide surface in contact of the upper portion of the, with the mounting portion of the channel spacer 10
In consideration of the reaction force at the time of an earthquake or the like, it is preferable that the surface in contact with the upper lattice plate receives a reaction force on the entire side surface of the thick portion and the thin portion. It is necessary to prevent cracks and the like from being generated even when concentrated stress is concentrated.

An object of the present invention is to provide a fuel assembly and a fuel channel box that reduce local stress and deformation due to a reaction force applied to an upper portion of the fuel channel box during an earthquake or the like.

[0014]

(1) In order to achieve the above object, according to the present invention, a plurality of fuel rods and water rods are bundled with a plurality of spacers, and upper and lower ends are supported by upper and lower tie plates. In a fuel assembly having a square tubular fuel channel box having a side wall and a corner portion, the side wall of the upper portion of the fuel channel box has at least a longitudinal central region of the fuel channel box. Has a thick wall area thicker than the side wall
And the thick region above the fuel channel box
The axial length is less than 25% of the total length of the channel box
It is assumed that. (2) In order to achieve the above object, the present invention provides a fuel bundle comprising a plurality of fuel rods and water rods bundled by a plurality of spacers, and upper and lower ends supported by upper and lower tie plates. In a fuel assembly provided with a square tubular fuel channel box having a side wall and a corner portion, the side walls of the upper portion and the lower portion of the fuel channel box are respectively at least longer than the side wall of the longitudinal center region of the fuel channel box. Also has a thick region having a large thickness. (3) Further, in order to achieve the above object, the present invention stores a fuel bundle in which a plurality of fuel rods and water rods are bundled by a plurality of spacers, and upper and lower ends are supported by upper and lower tie plates. A fuel cell box having a rectangular tubular shape having side walls and corner portions, and at least a longitudinal central region of the fuel channel box,
In a fuel assembly having a corner portion thicker than a side wall, a side wall of an upper portion of the fuel channel box is
Possess the longitudinal central region thick region thickness is thicker than the side wall of thick territory of the fuel channel box upper part
The axial length of the region is less than 25% of the total length of the channel box
It is assumed that (4) In order to achieve the above object, the present invention provides a fuel bundle comprising a plurality of fuel rods and water rods bundled by a plurality of spacers, and upper and lower ends supported by upper and lower tie plates. A fuel cell box having a rectangular tubular shape having side walls and corner portions, and at least a longitudinal central region of the fuel channel box,
In the fuel assembly having the corner portion thicker than the center portion of the side wall, the side walls of the upper portion and the lower portion of the fuel channel box each have a thicker region thicker than the side wall of the longitudinal center region. Shall have. (5) In the above (1) to (4), preferably, the upper portion has a substantially uniform thickness over the entire cross section including the thick region. (6) In the above (2) or (4), preferably, the axial length of the thick region above the fuel channel box is 25% or less of the entire length of the channel box. (7) In the above (1) to (4), the axial length of the thick region above the fuel channel box may be 15% or less of the entire length of the channel box. (8) In the above (1) to (4), the axial length of the thick region above the fuel channel box may be 10% or less of the entire length of the channel box. (9) In the above (2) or (4), preferably, at least a part of the fuel rod is filled with natural uranium in a lower portion, and a thick region below the fuel channel box is filled with the natural uranium. It is located below the upper end of the region that is set. (10) In the above (2) or (4), the axial length of the thick region below the fuel channel box is not less than the distance between the lower end of the fuel channel box and the upper surface of the lower tie plate. . (11) In order to achieve the above object, the present invention provides:
A plurality of fuel rods and water rods are bundled by a plurality of spacers, and the upper and lower ends are supported by upper and lower tie plates to accommodate a fuel bundle, and a square cylindrical shape having a side wall and a corner for forming a fuel assembly. In the fuel channel box,
The side wall of the upper portion of the fuel channel box is to have at least longitudinally central region thick-walled region is thicker than the side walls of the fuel channel box, wherein the fuel Chang
The axial length of the thick region above the
It shall be 25% or less of the entire length of the flannel box . (12) Further, in order to achieve the above object, the present invention provides:
A plurality of fuel rods and water rods are bundled by a plurality of spacers, and the upper and lower ends are supported by upper and lower tie plates to accommodate a fuel bundle, and a square cylindrical shape having a side wall and a corner for forming a fuel assembly. In the fuel channel box,
The side walls of the upper portion and the lower portion of the fuel channel box each have a thick region that is thicker than at least the side wall of the fuel channel box in the central region in the longitudinal direction. (13) In order to achieve the above object, the present invention provides:
A plurality of fuel rods and water rods are bundled by a plurality of spacers, and the upper and lower ends are supported by upper and lower tie plates to accommodate a fuel bundle, and a square cylindrical shape having a side wall and a corner for forming a fuel assembly. A fuel channel box,
In a fuel channel box in which the corner portion is thicker than a side wall at least in a longitudinal central region, a side wall of an upper portion of the fuel channel box is
Possess the longitudinal central region thick region thickness is thicker than the side wall of thick territory of the fuel channel box upper part
The axial length of the region is less than 25% of the total length of the channel box
It is assumed that (14) In order to achieve the above object, the present invention provides:
A plurality of fuel rods and water rods are bundled by a plurality of spacers, and the upper and lower ends are supported by upper and lower tie plates to accommodate a fuel bundle, and a square cylindrical shape having a side wall and a corner for forming a fuel assembly. A fuel channel box,
In a fuel channel box having a corner portion thicker than a side wall at least in a longitudinal central region, a side wall of an upper portion and a lower portion of the fuel channel box is thicker than a sidewall of the longitudinal central region, respectively. Has a thick thick region.

[0015]

[0016]

(1) By providing a thick region thicker than the side wall of the longitudinal center region of the fuel channel box on the side wall of the upper part of the fuel channel box, the upper end of the fuel channel box can be used in the event of an earthquake or the like. The applied reaction force is moderated, and local stress and deformation are reduced. Also,
Axial length of thick region above fuel channel box
To less than 25% of the total length of the channel box
More warpage in the fuel channel box
The thicker area above the fuel channel box
Is more control rod than the axial center of the fuel channel box
The control rod is inserted between the fuel assemblies.
It does not affect the function of pulling out. (2) By providing a similar thick region on the side wall of the lower portion of the fuel channel box, the amount of coolant leaking from between the lower portion and the lower tie plate is suppressed, and the mechanical integrity of the channel box is reduced. Improve the performance. (3) In a fuel channel box in which a corner portion is thicker than a side wall at least in a longitudinal center region of the fuel channel box, a wall thickness of an upper portion of the fuel channel box is larger than a side wall of the longitudinal central region. By providing a thick thick region, only the corners ( corners ) where the stress generated by the internal and external differential pressure is maximum in the central region in the longitudinal direction are thickened. In addition to widening the gap with the control rod at the position where the amount of deformation is maximum in the cross section, the reaction force received at the upper end of the fuel channel box during an earthquake or the like is reduced, and local stress and deformation are reduced. . Also, the thick region above the fuel channel box
Of the channel box is less than 25% of the total length of the channel box.
By doing so, the fuel channel box
Even if warpage occurs, the upper part of the fuel channel box
Thick region is the axial center of the fuel channel box.
No longer protrudes to the control rod side, and the control rod is
It does not affect the function of inserting and removing the cable. (4) At least in the longitudinal center region of the fuel channel box, a similar thick region is provided on the side wall of the lower portion of the fuel channel box, so that the stress generated by the internal / external differential pressure is maximized in the longitudinal center region. Since only the corners ( corners ) are thickened, the increase in volume is suppressed by reducing the thickness at the center of the side, and the gap between the control rod at the position where the amount of deformation is maximized in the cross section is widened. The amount of coolant leakage between the lower part and the lower tie plate is suppressed, and the mechanical integrity of the channel box is improved. (5) By making the upper portion have a substantially uniform thickness over the entire cross-section including the thick region, the effects of the above-mentioned (1) to (4) regarding the upper portion can be reliably obtained. (6) By setting the axial length of the thick region in the upper portion of the fuel channel box to 25% or less of the entire length of the channel box, even if warpage of the fuel channel box occurs within an allowable range, The thick region no longer protrudes toward the control rod than the axial center of the fuel channel box, and does not affect the function of inserting and removing the control rod between the fuel assemblies. (7) By making the axial length of the thick region above the fuel channel box 15% or less of the entire length of the channel box, in addition to the effect of (6) above, the control rods are fully inserted between the fuel assemblies. The control rod roller at the time does not interfere with the thick region above the fuel channel box. (8) By making the axial length of the thick region above the fuel channel box 10% or less of the entire length of the channel box, in addition to the effects of (6) and (7), the thickness of the upper portion of the fuel channel box can be increased. The meat region does not overlap with the fuel effective upper end position of the fuel rod, so that neutron economy and fuel economy are improved. (9) When at least a part of the fuel rods is filled with natural uranium in the lower part, the thick region of the lower part of the fuel channel box is located below the upper end of the region filled with natural uranium, In the area above the natural uranium on the fuel rods, the moderator is reduced by the effect of eliminating the water area by the thick-walled area in the natural uranium area without causing parasitic absorption of neutrons necessary for heat generation. Pu239
Can promote the transition to (10) By setting the axial length of the thick region of the lower part of the fuel channel box to be equal to or more than the distance between the lower end of the fuel channel box and the upper surface of the lower tie plate, the lower tie plate rotates during an earthquake and the fuel channel box rotates. Reduces the stress generated in the fuel channel box even when it hits the inner surface of the fuel channel box, and improves the mechanical integrity of the fuel channel box. (11) When the fuel channel box is invented,
With the configurations of (11) to (14) of the above solution, the operations described in (1) to (4) can be obtained.

[0017]

[0018]

Embodiments of the present invention will be described below with reference to the drawings. First, a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a bird's-eye view of the channel box of the present embodiment, FIG. 2 is a sectional view of a part A in FIG. 1, and FIG. 3 is a sectional view of a part B in FIG.

The fuel channel box 1 according to the embodiment shown in FIG. 1 has a rectangular cylindrical shape having four side walls and four corners ( corners ), and has a cross-sectional shape of an upper part 1.
Except for a, the corner portion as shown in FIG. 2 (a corner portion) 1b
Is thicker than the thickness of the center in the width direction of the side wall (hereinafter, simply referred to as the center of the side wall) 1c. Thick corner 1b
With respect to the difference in wall thickness between the thin wall portion 1c and the center portion 1c of the thin wall, an extremely thick corner portion increases the volume of the fuel channel box 1 and is not desirable from the viewpoint of neutron economy. For this reason, the relative ratio of the corner thickness is substantially determined from the relationship between the stress / deformation amount and the volume (or cross-sectional area) of the fuel channel box 1, and the thickness of the corner is approximately 1% of the thickness of the central portion of the side wall. It is preferably from 0.4 to 1.7 times.

The upper portion 1a of the fuel channel box 1 includes a region that comes into contact with an upper grid plate (not shown) and a channel spacer mounting region. And the same thickness as the thick corner portion ( corner portion) 1b. Here,
The terms "upper" and "lower" refer to "downward" when the coolant that removes heat from the fuel assembly flows into the fuel assembly during reactor operation, and "upward" when the coolant flows out.

FIGS. 4 and 5 show a fuel assembly 2 constructed using the fuel channel box 1 described above. The fuel assembly 2 bundles a plurality of fuel rods 3 and two water rods 4 arranged in the center with a plurality of spacers 5 and supports the upper and lower ends of the fuel rods 3 with an upper tie plate 6 and a lower tie plate 7. The fuel bundle thus obtained is stored in the channel box 1, and the upper end portion of the fuel channel box 1 and the upper tie plate 6 are connected by a channel fastener 9.

The channel fastener 9 presses the adjacent fuel assemblies 2 when the fuel assemblies 2 are loaded on the core, and secures a passage through which the control rod can be easily inserted and pulled out. Further, as shown in FIG. 5, a channel spacer 10 is provided at the upper part of the fuel channel box 1 as a backup in case the channel fastener 9 is broken.

According to this embodiment, since the thick region is provided in the upper portion 1a of the fuel channel box 1 which is in contact with the upper lattice plate, the reaction force received by the fuel channel box 1 during an earthquake or the like is reduced by the upper portion 1a. Because we receive on the entire side wall,
There is an effect that extreme stress and deformation can be moderated. Also,
Even in the channel spacer mounting region, the local stress can be reduced to 以下 or less.

Next, the concept of selecting the axial length of the thick region of the upper portion 1a in the fuel channel box of the above embodiment will be described. First, a case in which the warpage of the fuel channel box is considered will be described with reference to FIG.
FIG. 6 shows the axial section of one side of the fuel channel box in the axial direction and the control rod 13. The fuel channel box 1 has a function of forming a guide surface when the control rod 13 is inserted between and withdrawn from the fuel assembly. Further, since the fuel channel box 1 is a long object, the warpage C is allowed at the time of manufacturing or the like.

In consideration of the above, the thick region (thickness of the corner portion = thickness of the flat portion) of the upper portion 1a of the fuel channel box 1 is larger than that of the control rod in the central portion in the axial direction of the fuel channel box 1. It is necessary to make the area not protrude to the side.

In general, the warp of a long object is allowed to be 0.1% of the entire length of the long object. If this is applied to the fuel channel box 1, the warp of the thick portion of the upper portion 1a can be reduced. The axial length Lu of the thick region for preventing the lower end portion 14 from protruding toward the control rod side from the axial center portion of the fuel channel box is determined by the following equation.

[0027]

## EQU1 ## Z = Lu / L
... (1) Z = (1/2) ± (1/2) (B / C) 1/2?
... (2) At present, in a general BWR fuel, a fuel channel box having a length of about 4 m is used. In this case, C is about 4 mm. The difference between the thickness of the corner portion and the thickness of the flat portion is generally about 1 mm. In this case, B is about 1 mm.

From the above equations (1) and (2), the axial length Lu of the upper thick region of the fuel channel box 1 is:
If the total length L of the channel box 1 is within the range of 25%, the function of inserting and removing the control rod 12 between the fuel assemblies will not be affected. In addition, it does not substantially reduce the original function of the uneven shape. However,
As described above, the allowable% varies depending on the thickness specification / length, but it is preferable to arrange the upper thick region as far as possible on the upper side, based on another selection criterion described below.

Next, the selection of the axial length of the upper thick region relating to the interference with the control rod roller will be described with reference to FIG. FIG. 7 shows an axial connection diagram when the control rods 13 are fully inserted between the fuel assemblies, and an upper lattice plate 20 is shown above. Fuel channel box 1
Has a function of forming a guide surface when the control rod 13 is inserted and withdrawn between the fuel assemblies.
When the control rod roller 15 is completely inserted between the fuel assemblies,
It is preferable not to interfere with the thick region of the upper portion 1a of the fuel channel box 1.

That is, the axial length Lu of the upper thick region of the fuel channel box 1 may be set within the distance D from the upper end of the fuel channel box 1 to the control rod roller 15 when fully inserted. That is, the axial length Lu of the upper thick region is preferably within 15% of the total length L of the fuel channel box 1.

Another selection criterion of the axial length of the upper thick region will be described with reference to FIG. From the viewpoint of improving neutron economy and fuel economy, it is preferable to make the thickness of the fuel channel box 1 as thin as possible. Therefore, as described above, in the portion excluding the upper portion 1a of the fuel channel box 1, The thickness of the corners of the cross section is made thicker than the thickness of the center of the side wall, so that the cross-sectional area does not increase. In consideration of this, the axial length Lu of the thick region of the upper portion 1a of the fuel channel box 1 may be set so as not to overlap the fuel effective upper end position of the fuel rod 3 shown in FIG. It is desirable that the axial length Lu of the upper thick region of the fuel channel box 1 so as not to overlap the upper end position of the effective fuel length is within 10% of the total length L of the fuel channel box 1.

Another embodiment of the present invention will be described with reference to FIGS. FIG. 8 shows a fuel assembly 2A of the present embodiment. An area 17 in which natural uranium is arranged is provided below the fuel rod 3. Fuel rod 3 and two water rods 4 arranged in the center
Are supported by an upper tie plate 6 and a lower tie plate 7, and spacers 5 are arranged in the axial direction in order to suppress lateral vibration of the fuel rods 3 due to coolant. The fuel assembly 2 is covered with a rectangular cylinder fuel channel box 1A, and the fuel channel box 1A is fixed to a post of the upper tie plate 6 by a channel fastener (not shown). The channel box 1A has an upper portion 1a
And except for the lower portion 1d, the corners as shown in FIG. 9 (co
(Wall portion) 1b is thicker than the thickness of the side wall center portion 1c. The ratio of the thickness of the flat side wall central portion 1c to the thickness of the corner portion 1b is 1.4 to 1.7.

The upper portion 1a of the fuel channel box 1 has a uniform thickness in the cross section as in the first embodiment and has the same thickness as the thick corner portion ( corner portion) 1b.
In the present embodiment, the lower portion 1d of the fuel channel box 1 also has a uniform thickness in the transverse section and the same thickness as the thick corner portion ( corner portion) 1b, similarly to the upper portion 1a. The upper end 18 of the lower portion 1d is located above the upper surface of the lower tie plate 7 and below the natural uranium region 17 located below the fuel rod 3. The conventional finger spring is not used.

The upper part 1a of the fuel channel box 1
Is the same as that of the first embodiment, and the concept of selecting the length in the axial direction is the same as that of the first embodiment. The lower portion 1d of the fuel channel box 1 has an effect of suppressing the amount of coolant leakage and an effect of improving the mechanical integrity of the channel box, as described later.

Next, some ideas regarding the selection of the axial length Ld of the thick region of the lower portion 1d of the fuel channel box 1A will be described. First, the upper end 1 of the lower portion 1d
The reason why the length Ld is determined so that the length 8 is located below the natural uranium region 17 disposed below the fuel rod 3 will be described with reference to FIGS. 10 and 11.

The fuel rod 3 is provided with the natural uranium 17 at the lower portion as described above. The main purpose of this natural uranium is (1)
(2) Reduction of unburned uranium during fuel removal. That is, fast neutrons generated in the fuel assembly and thermal neutrons decelerated by the moderator leak from the upper and lower portions of the fuel assembly. This quantity is proportional to the slope of the neutron flux distribution. FIG. 11 is a view for explaining the present effect of the natural uranium region. For fuel rods without natural uranium, Fig. 11
As shown by the solid line, the slope of the neutron flux distribution is as large as θ0, but in the fuel rods on which natural uranium is arranged, the slope of the neutron flux distribution decreases as θ1 as shown by the broken line in FIG. Leakage is also reduced.

The arrangement of the thick lower portion 1d of the channel box 1A in the natural uranium region 17 has a function of increasing the effect of natural uranium, but does not produce an adverse effect. On the other hand, if the upper limit 18 of the thick lower portion 1d is raised above the natural uranium region 17, a phenomenon of parasitic absorption of neutrons necessary for heat generation will occur in the future. Therefore, in the region where the neutron flux is high, the thickness of the channel box must be reduced.

The arrangement of the thick lower portion 1d of the channel box 1A in the natural uranium region 17 effectively burns U235 contained in natural uranium and U238 becomes Pu.
It does not prevent the conversion effect of converting to 239 and burning. In other words, the conversion of U238 can be expected to reduce the moderator due to the effect of eliminating the water region by the thick part of the channel, and rather increase the fast neutrons. Therefore, it is expected that uranium burning due to the conversion will increase, albeit slightly.

Except for the lower and upper natural uranium regions, the channel box is made flat and thin to reduce parasitic absorption of neutrons.

Next, the effect of suppressing the amount of coolant leakage will be described quantitatively. In FIG. 12, the coolant entering from the lower tie plate triple bridge section 19 flows through the fuel assembly and exits from the upper tie plate 6, but the pressure outside the channel box is low and the channel box has
Internal pressure is applied. This size is maximum at the upper surface position of the lower tie plate 7 and becomes smaller as going upward. The channel box creeps outward due to the pressure difference between inside and outside. This deformation increases as the burnup of the fuel assembly advances. This increase in the amount of deformation increases the amount of coolant leaking from the gap between the channel box and the lower tie plate. It is desirable that the bypass flow rate flowing outside the channel box be as constant as possible,
This is because it affects the limit output of the fuel rod, which is a problem in designing the fuel assembly.

FIG. 13 shows the amount of change in the leakage flow rate (difference between the beginning of the cycle and the end of the cycle). This figure shows the result of calculating the amount of increase in the leakage flow rate using the height of the thick portion under the channel box as a parameter. Fuel bundle assembly when the channel box and the lower tie plate fitting portion length, immediately
Bottom and bottom of fuel channel box 1A in assembled state
The distance from the upper surface of the section tie plate 7 is indicated by H (see FIG. 10) by a relative value. As can be seen from FIG. 13, if the length of the thick portion is 1.5H or more, 90% of the effect of the case where the entire length is thick can be obtained. Therefore, it is desirable that the thickness of the thick portion is 1.5H or more. Thick part is H
In this case, about 80% of the effect can be obtained. When the thickness is less than H, the effect suddenly decreases. In FIG. 10, it is almost 3H.

Next, how the mechanical integrity of the fuel channel box is improved will be described with reference to FIGS. At the time of the earthquake, the lower tie plate 7 rotates as shown in FIG. At this time, the upper surface position of the lower tie plate 7 applies a large force to the channel box. Conversely, the lower end of the channel box receives a force in the opposite direction, and the channel box rotates while fitted to the lower tie plate 7. FIG. 15 shows the result of analyzing this state over the entire length of the fuel assembly (illustration up to the first spacer). The shearing force acting on the channel box is generated locally on the inner surface of the channel box, and causes an excessive force on the channel box.
In order to reduce this stress, it is necessary to provide a thick portion above the upper surface of the lower tie plate 7.

FIG. 10 shows the connection with the control rod 13. This figure shows a state in which the control rod 3 has descended to the lowermost end, and the thick portion 1d is arranged at a position where the fuel rod roller 15 does not interfere with the upper limit 18 of the thick portion 1d. Therefore, it is understood that the current control rod can be used as it is.

FIGS. 16 and 1 show still another embodiment of the present invention.
7 will be described. In FIG. 16, a fuel channel box 1B is formed in the shape of a long rectangular tube by joining long halves 20 and 21 each having a long U-shaped cross section.
The thickness of the lower portion 1d near the joint 22 at the center of the side wall is the same as the thin thickness of the center portion 1c of the side wall in the axial center region. As a result, the joining portion 22 has a constant thickness over the entire length, and it is easy to determine joining conditions such as welding.

FIG. 17 shows an embodiment of a channel clip attached to the upper end of the channel box 1A. Upper part 1a of fuel channel box 1A
As described above, a region having a uniform thickness in the transverse cross section and the same thickness as the thick corner portion ( corner portion) 1b is provided. This area includes the upper end of the fuel channel box 1A, and thus the upper end of the fuel channel box 1A.
The thickness of the corner portion and the central portion of the side wall are the same, and when joining the channel clip 23 to the upper end of the fuel channel box 1A, the joining can be performed without changing welding conditions and the like.

Another embodiment of the present invention will be described with reference to FIGS. In FIG. 18, the fuel channel box 1c has a structure in which the fuel channel box 1A described in the other embodiment of the present invention described above further has a vertical groove 24. The longitudinal groove 24 is shown in FIGS.
As shown in the figure, two on each side wall, a total of eight,
The material is removed from the area where the bending moment and the stress of the fuel channel box 1C are lowest. In these areas, the removal of a significant amount of material may significantly increase the increase in the deflection of the fuel channel box wall under pressure. According to the present embodiment, the thickness of the fuel channel box 1 </ b> C is
A and 1B are further thinned and the material is reduced, so that neutron economy improvement and fuel economy improvement are further promoted.

It is preferable that one of the eight vertical grooves 24 extends along the position where the guide roller provided on the control rod moves up and down. This is because when the channel box is deformed outward, the adjacent channel boxes do not restrict the vertical movement of the guide roller. Although the vertical groove 24 described in this embodiment is shown in FIG. 18 as being provided in the fuel channel box 1A, it can be applied to the fuel channel boxes 1 and 1B of other embodiments.

[0048]

According to the present invention, extreme stress and deformation due to the reaction force applied to the upper portion of the fuel channel box during an earthquake or the like can be reduced.

The lower part of the channel box has the effect of reducing the stress, improving the mechanical soundness, reducing the deformation of the channel box, and suppressing the leakage flow rate of the coolant.

[Brief description of the drawings]

FIG. 1 is a bird's-eye view of a fuel channel box according to one embodiment of the present invention.

FIG. 2 is a sectional view of a portion A in FIG.

FIG. 3 is a sectional view of a portion B in FIG. 1;

FIG. 4 is a longitudinal sectional view of a fuel assembly constituted by using the fuel channel box shown in FIG.

FIG. 5 is a top view of the fuel assembly shown in FIG.

FIG. 6 is a diagram showing an axial connection of the fuel channel box shown in FIG. 1;

FIG. 7 is a view showing another axial connection of the fuel channel box shown in FIG. 1;

FIG. 8 is a longitudinal sectional view of a fuel assembly formed by using a fuel channel box according to another embodiment of the present invention.

9 is a bird's-eye view of the fuel channel box shown in FIG.

FIG. 10 is a view showing an axial arrangement of the fuel channel box shown in FIG. 8;

FIG. 11 shows the effect of natural uranium.

FIG. 12 is a diagram showing an axial change of a differential pressure between inside and outside of a fuel channel box.

FIG. 13 is a diagram showing an increase amount of a leakage flow rate using a height of a thick portion of a channel box as a parameter.

FIG. 14 is a diagram showing the movement of the lower tie plate during an earthquake.

FIG. 15 is a diagram showing a shear force acting on a channel box by rotation of a lower tie plate.

FIG. 16 is a bird's-eye view of a fuel channel box according to still another embodiment of the present invention.

FIG. 17 is a bird's-eye view of a fuel channel box according to still another embodiment of the present invention.

FIG. 18 is a bird's-eye view of a fuel channel box according to still another embodiment of the present invention.

FIG. 19 is a sectional view of the fuel assembly shown in FIG. 18;

FIG. 20 is a longitudinal sectional view of a fuel assembly formed using a conventional fuel channel box.

FIG. 21 is a top view of the conventional fuel assembly shown in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Fuel channel box 1a Upper part (thick area) 1b Corner part ( corner part) 1c Side wall center part 1d Lower part (thick area) 2 Fuel assembly 3 Fuel rod 6 Upper tie plate 7 Lower tie plate 17 Natural uranium area 20, 21 long U-shaped member (half body) 22 welded portion 23 channel clip 24 longitudinal groove

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Junjiro Nakajima 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Hitachi, Ltd. Inside the Hitachi Plant (72) Inventor Yasuhiro Aizawa 3-2-2, Sachimachi, Hitachi-shi, Ibaraki No. 1 Hitachi Engineering Co., Ltd. (56) References JP-A-55-55284 (JP, A) JP-A 1-292791 (JP, A) JP-A 1-269085 (JP, A) 125293 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) G21C 3/324

Claims (14)

(57) [Claims] 1. A square tubular fuel channel having a plurality of fuel rods and water rods bundled by a plurality of spacers, a fuel bundle having upper and lower ends supported by upper and lower tie plates, and having a side wall and a corner portion. a fuel assembly provided with a box, the side wall of the upper portion of the fuel channel box is to have at least longitudinally central region thick-walled region is thicker than the side walls of the fuel channel box, wherein the fuel channel box above The axial direction of the thick region of the part
A fuel assembly having a direction length of 25% or less of the entire length of a channel box . 2. A square tubular fuel channel having a plurality of fuel rods and water rods bundled by a plurality of spacers, a fuel bundle having upper and lower ends supported by upper and lower tie plates, and having a side wall and a corner portion. In a fuel assembly including a box, the upper and lower portions of the fuel channel box each have a thick wall region having a thickness greater than that of at least a longitudinal central region of the fuel channel box. Characteristic fuel assembly. 3. A square tubular fuel channel having a plurality of fuel rods and water rods bundled by a plurality of spacers, a fuel bundle having upper and lower ends supported by upper and lower tie plates, and containing a side wall and a corner portion. A fuel assembly having a box, and having a corner portion thicker than a side wall at least in a longitudinal central region of the fuel channel box, wherein a side wall of an upper portion of the fuel channel box is formed of the longitudinal central region. Has a thick wall area thicker than the side wall
And the axial direction of the thick region above the fuel channel box.
A fuel assembly having a direction length of 25% or less of the entire length of a channel box . 4. A square tubular fuel channel having a plurality of fuel rods and water rods bundled by a plurality of spacers, a fuel bundle having upper and lower ends supported by upper and lower tie plates, and containing a side wall and a corner portion. In a fuel assembly having a box and having the corner portion thicker than the center portion of the side wall at least in the longitudinal central region of the fuel channel box, the upper portion and the lower portion of the fuel channel box have side walls, respectively. A fuel assembly having a thicker region than the side wall of the central region in the longitudinal direction. 5. The fuel assembly according to claim 1, wherein the upper portion includes the thick region.
A fuel assembly having a substantially uniform wall thickness over its entire cross section. 6. The fuel assembly according to claim 2 , wherein an axial length of the thick region above the fuel channel box is not more than 25% of the entire length of the channel box. . 7. The fuel assembly according to claim 1, wherein the axial length of the thick region above the fuel channel box is 15% of the entire length of the channel box.
% Or less. 8. The fuel assembly according to claim 1, wherein the axial length of the thick region above the fuel channel box is 10% of the entire length of the channel box.
% Or less. 9. The fuel assembly according to claim 2, wherein at least a part of the fuel rod is filled with natural uranium in a lower part, and a thick region below the fuel channel box is formed of the natural uranium. A fuel assembly, wherein the fuel assembly is located below an upper end of a filled area. 10. The fuel assembly according to claim 2, wherein the axial length of the thick region below the fuel channel box is greater than the distance between the lower end of the fuel channel box and the upper surface of the lower tie plate. A fuel assembly, characterized in that: 11. A side wall and a corner portion for bundling a plurality of fuel rods and water rods with a plurality of spacers, storing a fuel bundle having upper and lower ends supported by upper and lower tie plates, and forming a fuel assembly. in rectangular tubular fuel channel box having side walls of the upper portion of the fuel channel box is to have at least longitudinally central region thick-walled region is thicker than the side walls of the fuel channel box, wherein the fuel channel box The axial direction of the thick region in the upper part
A fuel channel box having a direction length of 25% or less of the entire length of the channel box. 12. A side wall and a corner portion for bundling a plurality of fuel rods and water rods with a plurality of spacers, storing a fuel bundle having upper and lower ends supported by upper and lower tie plates, and forming a fuel assembly. In the quadrangular cylindrical fuel channel box having, the side wall of the upper part and the lower part of the fuel channel box each has a thick region thicker than the side wall of at least the longitudinal central region of the fuel channel box. A fuel channel box. 13. A side wall and a corner portion for bundling a plurality of fuel rods and water rods with a plurality of spacers, storing a fuel bundle having upper and lower ends supported by upper and lower tie plates, and forming a fuel assembly. A square tubular fuel channel box having at least a longitudinal central region,
In the fuel channel box having a corner portion thicker than a side wall, a side wall of an upper portion of the fuel channel box has a thick region thicker than a side wall of the longitudinal central region.
And the axial direction of the thick region above the fuel channel box.
A fuel channel box having a direction length of 25% or less of the entire length of the channel box. 14. A plurality of fuel rods and water rods are bundled by a plurality of spacers, and a side wall and a corner portion for accommodating a fuel bundle having upper and lower ends supported by upper and lower tie plates and forming a fuel assembly are provided. A square tubular fuel channel box having at least a longitudinal central region,
In the fuel channel box in which the corner portion is thicker than a side wall, the side walls of an upper portion and a lower portion of the fuel channel box each have a thick region thicker than the side wall of the longitudinal central region. A fuel channel box, characterized in that: