KR100961486B1 - Unit lattice plate, unit lattice support and lattice support of nuclear fuel rod - Google Patents

Abstract

The disclosed nuclear fuel rod support grids have convex portions alternately formed at one side and the other side at regular intervals, and at least one convex portion has a larger diameter than the other convex portions, and a plurality of units arranged in a row at regular intervals. A plurality of unit support grids are laminated and coupled to a support grid plate and a plurality of fixed grids coupled to both ends of the plurality of unit support grid plates to fix the plurality of unit support grid plates. When laminating, it may be laminated by rotating 90 degrees or 180 degrees in one direction. According to this, even if the gap between the fuel rods is narrowed because the outer diameter is increased and the gap between the fuel rods is narrowed compared to the conventional fuel rods, the dual cooling fuel rods can accommodate the change in the vibration characteristics of the fuel rods and prevent the flow of coolant. Excluding the structure can provide an increased effect in terms of thermal hydraulic power, and can also improve the strength of the nuclear fuel rod support grid. In addition, it can be applied to the support grid of the conventional 16X16 nuclear fuel assembly by changing the position and number of the convex portion, there is an advantage that can be applied to the purpose for supporting the structure or the elongated tube in contact with the fluid in the general industry.

Nuclear Fuel Rod, Support Grid

Description

Unit lattice plate, unit lattice support and lattice support of nuclear fuel rod}

The present invention relates to a nuclear fuel rod support grid, and more particularly, to a nuclear fuel assembly used in a light-water reactor core, and to a nuclear fuel rod support grid for supporting a dual fuel rod housed in the nuclear fuel assembly.

The nuclear fuel rod support grid is a main component of the fuel assembly used for the light-reactor core, and the fuel rod constituting the fuel assembly is stably positioned in a constant space until the end of the cycle under severe conditions inside the core.

Meanwhile, a coolant having a high flow rate flows around the fuel assembly, and the coolant may generate a fluid-induced vibration in the fuel rod. The fuel rod support grid may suppress a fluid-induced vibration of the fuel rod. Have

FIG. 1 is a front view schematically showing a conventional fuel assembly, FIG. 2 is a cross-sectional view schematically illustrating the fuel assembly of FIG. 1, and FIG. 3 is a schematic view of a support grid constituting the fuel assembly of FIG. 1. 4 is a plan view schematically illustrating the support grid body of FIG. 3, and FIG. 5 is a perspective view schematically illustrating the unit grid plate of the support grid body of FIG. 3.

1 to 5, the nuclear fuel rod assembly 10 includes a nuclear fuel rod 11, a guide tube 14, a support grid 15, an upper fixture 12, and a lower fixture 13. .

The nuclear fuel rod 11 includes a cylindrical uranium sintered body in the zirconium alloy cladding tube, and by the nuclear fission reaction of the uranium sintered body High temperature heat is generated.

The guide tube 14 is used as a passage of a control rod that moves up and down to regulate the output of the reactor core and stop the fission reaction.

The support grid 15 is usually made of a Zircaloy alloy to form a nuclear fuel rod cell into which a plurality of nuclear fuel rods are inserted, and a guide tube cell into which a plurality of guide tubes are inserted.

Forming the fuel rod cell The support grid itself is located in two grid springs 28, one on each of two surfaces, and the upper and lower sides of the grid springs, respectively, and four dimples 29, two on each of the other two surfaces, protrude. The fuel rods 11 are supported at six support points.

If the spring force of the spring 28 and the dimple 29 is too small, the nuclear fuel rod 11 may not be arranged at a predetermined position, which may cause loss of support integrity of the nuclear fuel rod 11.

If the elastic force is too large, a flaw such as scratches may occur on the surface of the nuclear fuel rod 11 due to excessive frictional resistance when the nuclear fuel rod 11 is inserted into the support grid itself.

In addition, the longitudinal growth of the nuclear fuel rods 11 due to neutron irradiation during the operation of the reactor may not be adequately accommodated, which may cause the nuclear fuel rods 11 to bend, that is, to cause warpage of the nuclear fuel rods 11.

When the fuel rods 11 are bent, the fuel rods 11 are brought into close contact with or in contact with adjacent fuel rods 11 to narrow or block the cooling water channel between the fuel rods.

Here, the cooling water flow path is enclosed by four nuclear fuel rods 11 or through the sub-channel 25 surrounded by three nuclear fuel rods 11 and one guide tube 14 in the axial direction at the bottom of the core. Flows quickly to the top;

That is, the sub-channel 25 refers to a space surrounded by the nuclear fuel rods 11 and refers to a passage through which a side is open so that fluid can freely move to a neighboring flow path.

As described above, when the cooling water passage is narrowed or blocked, the heat generated from the fuel cannot be effectively transferred to the cooling water, which causes a temperature rise of the nuclear fuel rod locally, thereby resulting in the possibility of occurrence of nuclear boiling escape (DNB). It is the main cause of reducing the output of nuclear fuel.

The upper fixture 12 and the lower fixture 13 serve to fix and support the nuclear fuel rod assembly 10 to the upper and lower structures of the core, and the lower fixture 13 floats the inside of the core. Includes a filtration device (foreign filter, not shown) to filter foreign matter.

FIG. 6 is a schematic cross-sectional view of the dual cooling fuel rod, and FIG. 7 is a plan view schematically illustrating the nuclear fuel assembly in which the dual cooling fuel rod of FIG. 6 is inserted.

6 and 7, the dual cooled fuel rod has an annular structure instead of a cylindrical shape, which is disclosed in US Patent Nos. 3,928,132 and 6,909,765.

The dual-cooled nuclear fuel rod 20 having the annular structure is an sintered body 21 formed in an annular shape and an inner covering tube 22 provided on the inner circumference of the sintered body 21 and an outer covering tube 23 provided on the outer circumference. Is done.

In the case of the above structure, the coolant can flow not only to the outside of the dual cooling nuclear fuel rod 20 but also to the heat transfer is made by the dual, not only to maintain a low center temperature of the dual cooling nuclear fuel rod 20 The heat transfer area is also increased to achieve high combustion and high power.

As described above, when the central temperature of the dual-cooled fuel rod 20 is kept low, the possibility of fuel damage due to the increase in the central temperature of the nuclear fuel rod is lowered, thereby increasing the safety margin of the dual-cooled nuclear fuel rod 20.

However, in order to be structurally compatible with the existing pressurized water reactor-type core, the dual-cooled fuel rod 20 cannot change the position of the guide tube 14 within the fuel assembly 10, and the fuel is increased due to an increase in the fuel rod outer diameter. The gap between the rods is significantly narrower than before.

For example, when the fuel assembly is constructed by inserting dual-cooled fuel rods in a 12 × 12 array so as to be structurally compatible in existing LWR cores, the gap between the fuel rods is about 3.24 mm to about 1.24 mm. Will decrease.

Therefore, due to the narrow gap between the fuel rods, a problem arises in that the support grid developed so far cannot be used as the support grid of the dual cooling fuel rod 20 as it is.

That is, if the thickness of the unit grid of the existing support grid itself is subtracted from the gap between the fuel rods of 1.24 mm and subdivided in half, the distance between the unit grid and the fuel rod is only about 0.383 mm.

In such a narrow gap, the spring design having the spring stiffness and hydraulic characteristics (mainly pressure loss) of the existing support grid is impossible by applying the same shape and support position as the existing spring.

In addition, there is a problem in that the cooling water flow path is reduced by the narrow interval is reduced the cooling function.

The present invention has been made to solve the above problems, and provides an improved fuel rod support grid that can adequately support and cool the fuel rods even at intervals between the fuel rods that narrow as the outer diameter of the fuel rod increases. It is for that purpose.

The unit support grid plate of the present invention protrudes to one side, and a plurality of first convex portions in which the nuclear fuel rod is in contact with the portion protruding to one side, and alternately formed with the first convex portion, protrudes to the other side, protrudes to the other side The plurality of second convex portions contacting the nuclear fuel rods and the plurality of first convex portions and the plurality of second convex portions may be provided at the portions thereof.

Each of the plurality of first convex portions and the plurality of second convex portions is spaced apart from the intermediate support portion and the intermediate support portion continuous with the connecting portions between the plurality of first convex portions and the plurality of second convex portions. The lower support part may be spaced apart from the upper support part connected to one side of the connection part and the intermediate support part, and the continuous support part is connected to the other side of the connection part.

The upper support and the lower support may be point symmetrical with respect to the midpoint of the intermediate support.

The intermediate support part, the upper support part and the lower support part may have a wave formed in a long axis direction.

Each of the plurality of first convex portions and the plurality of second convex portions includes a center plane and a pair of inclined surfaces inclined to be linearly symmetrical to the long axis of the center surface, wherein each of the pair of inclined surfaces includes the plurality of first convex surfaces. At least one sub-convex portion may protrude in a protruding direction of each of the convex portion and the plurality of second convex portions.

Sub-convex portions protruding from each of the pair of inclined surfaces may be line symmetric with respect to the center surface.

Only one side of the sub-convex portions may be continuous to the inclined surface.

At least one of the plurality of first convex portions and the plurality of second convex portions may have a larger diameter than other convex portions.

The unit support grid of the present invention may include a plurality of unit support grids disposed at regular intervals from each other and a pair of fixed grids coupled to both ends of the plurality of unit support grids to fix the plurality of unit support grids. Can be.

A plurality of slits may be formed in the pair of fixed gratings, and each of the plurality of unit support gratings may be inserted into and coupled to each of the plurality of slits.

In the fuel rod support grid of the present invention, a plurality of unit support grids are stacked to form a nuclear fuel rod support grid, and the plurality of unit support grids may be stacked by being rotated at a predetermined angle with each other.

The predetermined angle may be any one of 90 degrees and 180 degrees in one direction whenever the plurality of unit support grids are stacked.

The plurality of unit support grids may be stacked and coupled by a fixing pin.

The fixing pins may be inserted into any one of each of four corners of each of the plurality of unit support grids or central portions of four surfaces.

Nuclear fuel rod support grid of the present invention is a convex portion alternately formed on one side and the other side at regular intervals, at least one convex portion is formed larger diameter than the other convex portion, a plurality arranged in a row at regular intervals A plurality of unit support grids are laminated and coupled to each of the unit support grids of the plurality of unit support grids and a plurality of fixed grids for fixing the plurality of unit support grids to be coupled to both ends of the unit support grids; The itself may be laminated by rotating 90 degrees or 180 degrees in one direction when laminating.

The convex portion is divided into an intermediate support portion, an upper support portion above the intermediate support portion and a lower support portion below the intermediate support portion, wherein the upper support portion has one end floating in the space, and the lower support portion has the other end floating in the space. The upper support and the lower support may be point symmetrical with respect to the midpoint of the intermediate support.

The convex portion may include a center surface and an inclined surface inclined to both sides of the center surface, respectively, and each of the inclined surfaces may protrude at least one sub convex portion in a convex direction of the convex portion.

One side of the at least one sub-convex portion may be floating in space.

A fixing pin may be inserted into one portion of each of four corners of each of the plurality of stacked unit support grids or each of the central portions of the four surface of the stacked unit support grids to couple the plurality of unit support grids.

According to the nuclear fuel rod support grid of the present invention, the dual-cooled fuel rods are accommodated so that the outer diameter is increased compared to the conventional fuel rods so that the vibration characteristics of the dual-cooled fuel rods can be accommodated even if the gap between the fuel rods is narrowed. It can be formed of a stack of unit support grids.

Accordingly, the number and shape of the nuclear fuel rod support that can be formed in the nuclear fuel rod support lattice, ie, the convex portions that function as conventional springs and dimples, can be freely changed, and the heat-hydraulic side can be eliminated by excluding the structure that obstructs the flow of cooling water. Can also provide an increased effect.

In addition, when the fuel rod support grid is formed by stacking the unit support grids, the strength of the nuclear fuel rod support grids may be improved.

On the other hand, the fuel rod support grid of the present invention can be applied to the support grid of the conventional 16X16 fuel assembly by changing the position and number of the convex portion, the purpose for supporting a structure or a long elongated tube in contact with the fluid in the general industry There is an advantage that can be applied to.

Hereinafter, the nuclear fuel rod support grid according to the present invention will be described in detail with reference to the accompanying drawings.

8 is a view showing a unit support grid in accordance with an embodiment of the present invention.

Referring to FIG. 8, the unit support grid plate 100 may have a shorter length than a conventional one, and include a plurality of first convex portions 110, a plurality of second convex portions 120, and a plurality of connecting portions 130. It may be provided.

The first convex portion 110 and the second convex portion 120 may be alternately formed with the connection portion 130 interposed therebetween. If the first convex portion 110 protrudes in one direction, the The second convex portion 120 may protrude in the other direction, which is the opposite direction of the first convex portion 110.

The protruding portions of the first convex portion 110 and the second convex portion 120 may function as a spring, and may be elastically supported in contact with a nuclear fuel rod, particularly a dual cooling nuclear fuel rod.

At least one portion 120a having a larger diameter than other portions may be formed in the plurality of first convex portions 110 and the plurality of second convex portions 120 so that the guide tube or the measuring tube is inserted.

The convex parts 110 and 120 to be described below are concepts including the first convex part 110 and the second convex part 120, and the first convex part 110 will be described as an example.

FIG. 9 is a view illustrating a first embodiment of the convex portion in the unit support grid of FIG. 8.

Referring to FIG. 9, the convex part 110 may be formed of an intermediate support part 111, an upper support part 112, and a lower support part 113.

The intermediate support part 111 may be formed at the center of the convex part 110, and may be continuously formed at the connection parts 130 between the convex parts 110.

The upper support portion 111 is formed on the upper portion of the intermediate support portion 111, only one side may be formed continuously in the connecting portion 130.

That is, the upper support part 112 has a shorter horizontal length than the intermediate support part 111, and thus cannot connect between the connection part 130 and the adjacent connection part 130 like the intermediate support part 111, but the connection part 130 and the adjacent connection part ( The other side may be floating in the space between 130).

The lower support part 113 may be formed below the intermediate support part 111, and only the other side thereof may be continuously formed on the connection part 130.

That is, if the upper support part 112 is connected to the connection part 130 on one side of the convex part 110, the lower support part 113 may be connected to the connection part 130 on the other side of the convex part 110. Can be.

The lower support portion 113 also has a horizontal length shorter than that of the intermediate support portion 111, like the upper support portion 112, and thus cannot connect between the connection portion 130 and the adjacent connection portion 130 like the intermediate support portion 111. One side may be floating in a space between the 130 and the adjacent connection unit 130.

The intermediate support part 111, the upper support part 112, and the lower support part 113 may be formed by cutting the upper and lower parts of the convex part 110 in a “a” or “b” shape. .

The upper support 112 and the lower support 113 may have a point symmetric relationship with respect to the midpoint of the intermediate support 111.

In the structure as described above, the convex portion 110, in particular the intermediate support portion 111, the upper support portion 112 and the lower support portion 113 is in surface contact with the fuel rod even in a narrow space between the nuclear fuel rods. In line contact, the vibration characteristic of the nuclear fuel rod can be sufficiently accommodated.

In particular, since the upper support portion 112 and the lower support portion 113 is a part of the floating state in space, the elastic bearing force of the nuclear fuel rod can be increased.

10 is a view illustrating a modified example of the convex portion of FIG. 9.

Referring to FIG. 9, the convex portion 110, in particular, the intermediate support portion 111, the upper support portion 112, and the lower support portion 113 may be wavy in the form of alternating digging and plungers in a long axis direction (horizontal direction). Can be.

In particular, a portion of the intermediate support 111, the upper support 112 and the lower support 113 in contact with the nuclear fuel rod may be a wave shape of the wave shape to surround the outer peripheral surface of the nuclear fuel rod.

Forming the wave shape as described above can improve the elastic support of the convex portion 110.

11 to 13 are diagrams illustrating a second embodiment of the convex portion in the unit support grid of FIG. 8.

Referring to FIG. 11, the convex portion 110 may include a central plane 114 and a pair of inclined surfaces 115 that are inclined to be linearly symmetrical to the long axis of the central plane 114.

Each of the pair of inclined surfaces 115 may protrude the sub-convex portion 116 in the protruding direction of the convex portion 110.

The sub-convex portion 116 is formed by forming a pair of slits parallel to the inclined surface 115 and then protruding a surface between the pair of slits by drawing or the like. can do.

Referring to FIG. 12, the sub convex portion 116 may be cut in an upper portion or a lower portion thereof so that only one side thereof is connected to the inclined surface 115, and the other side thereof may float in a space.

Referring to FIG. 13, a plurality of sub-convex portions 116 may be formed on each of the pair of inclined surfaces 115, and one side, that is, the center surface 114 is cut away, and only the other side is inclined surface 115. ), One side may be floating in space.

According to the structure as described above, the sub-convex portion 116 formed in the convex portion 110, in particular the inclined surface 115 in the narrow space between the nuclear fuel rods are in surface contact or line contact with the fuel rods of the fuel rods The vibration characteristic can be sufficiently accommodated.

In particular, since the sub convex portion 116 is partially floating in space, the elastic bearing force of the nuclear fuel rod may be increased.

FIG. 14 is a plan view illustrating a unit support grid formed by the unit support grids of FIG. 8.

Referring to FIG. 14, in the unit support grid 200, a pair of fixed grating plates 210 are coupled to both ends of the plurality of unit support grids 100 in a state in which the plurality of unit support grids 100 are arranged at regular intervals. Can be formed.

A plurality of slits (not shown) may be formed in each of the pair of fixed grating plates 210, and end portions of the unit support grid 100 may be inserted into and coupled to each of the plurality of slits.

Welding may be added to the coupling portion to reinforce the coupling between the plurality of unit support grid plates 100 and the pair of fixed grating plates 210.

Meanwhile, in the case of the unit support grid plates 100 disposed at both ends of the plurality of unit support grid plates 100 arranged in a row, the unit support grid plates 100 may be formed to be convex inward.

That is, the unit support grid plate 100 protrudes so that the first convex portion 110 and the second convex portion 120 alternately face each other in the opposite direction. In the case of the unit support grid plate 100 disposed at both ends, Only one of the first convex portion 110 and the second convex portion 120 may be formed so that the convex portion faces the adjacent unit support grid plate 100.

15 and 16 are perspective views illustrating a nuclear fuel rod support grid formed by stacking the unit support grids of FIG. 14.

15 and 16, the nuclear fuel rod support grid 300 may be formed by stacking a plurality of unit support grids 200.

The plurality of unit support grids 200 may be stacked by being rotated at a predetermined angle with each other.

That is, the plurality of unit support grids 200 may be stacked by being rotated by 90 degrees or 180 degrees every time they are stacked.

When the plurality of unit support grids 200 are stacked by being rotated every 90 degrees, the nuclear fuel rod may be elastically supported by the convex portion 110 at four points, and rotated every 180 degrees to support the plurality of unit support grids. When itself 200 is stacked, the nuclear fuel rod may be elastically supported by the convex portion 110 at two points.

In the stacked plurality of unit support grids 200, fixing pins 310 may be inserted into each of four corners or central portions of four surfaces to reinforce the stack coupling.

The nuclear fuel rod support lattice 300 having the structure as described above has a dual cooling nuclear fuel rod accommodated so that the outer diameter is increased compared to the conventional nuclear fuel rod, so that even if the interval between the nuclear fuel rods is narrowed, the vibration characteristics of the dual cooling nuclear fuel rod are changed. It may be formed of a stack of the unit support grid 200 that can accommodate.

Therefore, the number and shape of the nuclear fuel rod support that can be formed in the nuclear fuel rod support grid 300, that is, the convex portion 110 that functions as a conventional spring and dimple, can be freely changed, and the flow of cooling water is hindered. Excluding the structure can provide an increased effect in terms of thermal hydraulics.

In addition, when the nuclear fuel rod support grid 300 is formed by stacking the unit support grids 200, the strength of the nuclear fuel rod support grid 300 may be improved.

On the other hand, the nuclear fuel rod support grid 300 of the present invention can be applied by changing the position and the number of convex portions in the conventional grating itself of the 16X16 fuel assembly, and supports a structure or a long elongated tube in contact with the fluid in the general industry There is an advantage that can be applied to the purpose.

1 is a front view schematically showing a conventional fuel assembly.

2 is a plan sectional view schematically showing the nuclear fuel assembly of FIG.

Figure 3 is a plan view schematically showing a support grid applied to the fuel assembly of Figure 1;

Figure 4 is a perspective view schematically showing a support grid applied to the nuclear fuel assembly of Figure 1;

5 is a perspective view schematically showing a unit support grid of the support grid of FIG. 4.

Figure 6 is a schematic cross-sectional view showing a conventional dual cooling nuclear fuel rods.

7 is a plan view schematically showing the dual cooling fuel assembly containing the dual cooling fuel rod of FIG.

8 is a perspective view schematically showing a unit support grid in accordance with an embodiment of the present invention.

9 is a perspective view showing a first embodiment of the convex portion in the unit support grid of FIG.

10 is a perspective view schematically showing a modified form of the convex portion of FIG. 9;

FIG. 11 is a perspective view illustrating a second embodiment of the convex portion in the unit support grid of FIG. 8. FIG.

12 and 13 are perspective views schematically showing a modified form of the convex portion of FIG. 11.

14 is a plan view schematically showing a unit support grid according to an embodiment of the present invention.

15 is a perspective view schematically showing a nuclear fuel rod support grid according to an embodiment of the present invention.

FIG. 16 is a perspective view schematically illustrating a fixing pin coupled to the nuclear fuel rod support grid of FIG. 15; FIG.

<Description of the symbols for the main parts of the drawings>

100 ... unit support grid 110 ... first projection

111 ... intermediate support 112 ... upper support

113 ... Lower support 114 ... Center plane

115 ... Slope 116 ... Sub Convex

120 ... 2nd projection 130 ... Connection

200 ... unit support grid 210 ... fixed grating

300 ... nuclear fuel rod support grid 310 ... fixed pin

Claims (20)

A plurality of first protrusions protruding to one side and contacting a nuclear fuel rod to a portion protruding to the one side; A plurality of second convex portions formed alternately with the first convex portion, protruding to the other side, and the nuclear fuel rods contacting the protruding portion to the other side; And A connecting portion connecting the plurality of first convex portions and the plurality of second convex portions, Each of the plurality of first convex portions and the plurality of second convex portions may be: An intermediate support portion continuous with the connecting portions between the plurality of first convex portions and the plurality of second convex portions; An upper support part spaced apart from the intermediate support part and connected to one side connection part of the connection part; And The unit support grid is spaced apart from the intermediate support, characterized in that it comprises a lower support portion continuous to the other side of the connection portion. delete The method according to claim 1, And the upper support and the lower support are point symmetrical with respect to the midpoint of the intermediate support. The method according to claim 3, The unit support grid, characterized in that the intermediate support portion, the upper support portion and the lower support portion is a wave formed in the long axis direction. The method according to claim 1, Each of the plurality of first convex portions and the plurality of second convex portions includes a center plane and a pair of inclined surfaces that are inclined to be linearly symmetrical to the long axis of the center plane. And at least one sub-convex portion protrudes from each of the pair of inclined surfaces in a protruding direction of each of the plurality of first convex portions and the plurality of second convex portions. The method according to claim 5, And the sub convex portions protruding from each of the pair of inclined surfaces are linearly symmetrical with respect to the center surface. The method according to claim 5, The sub convex portion of the unit support grid, characterized in that only one side is continuous to the inclined surface. The method according to any one of claims 1 and 3 to 7, The unit support grid of claim 1, wherein at least one of the plurality of first convex portions and the plurality of second convex portions has a larger diameter than other convex portions. delete delete delete delete delete delete Convex portions are alternately formed at one side and the other side at regular intervals, and the at least one convex portion has a larger diameter than the other convex portions, and a plurality of unit support grid plates arranged in a line at regular intervals; And  A plurality of unit support grids having a plurality of fixed grids coupled to both ends of the plurality of unit support grids to fix the plurality of unit support grids are laminated and coupled, The plurality of unit support grids are stacked by rotating 90 degrees or 180 degrees in one direction when stacked, The convex portion is divided into an intermediate support, an upper support above the middle support and a lower support below the intermediate support, The upper support portion has one side end floating in the space, The lower support portion has the other end floating in the space, And the upper support and the lower support are in point symmetry with respect to the midpoint of the intermediate support. delete The method according to claim 15, The intermediate support portion, the upper support portion and the lower support portion nuclear fuel rod support grid, characterized in that the wave in the longitudinal direction. The method according to claim 15, Wherein the convex portion is made of the inclined surface inclined toward both sides of the center plane and the center plane, Each of the inclined surfaces, the nuclear fuel rod support grid, characterized in that at least one sub-convex projecting in the convex direction. 19. The method of claim 18, The at least sub-convex portion of the nuclear fuel rod support grid, characterized in that one side is floating in the space. The method according to claim 15, The nuclear fuel rod support grid, characterized in that the fixing pin is inserted into any one portion of each of the four corners or each of the central portion of the four surface of the stacked plurality of unit support grid body to couple the plurality of unit support grid body.

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