JP2001133590A - Cask - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve thermal conductivity of decay heat. SOLUTION: Plural square pipes 7 are arranged like a grid, and a support pipe 101 having a rectangular section is passed through each space between the square pipes 7. Subsequently, plural support pipes 101 are passed to intersect perpendicularly to the support pipe 101 array, and the pipes are thus alternately stacked to restrict the square pipes 7. The end face 102 of the support pipe 101 is curved to fit to the inner surface shape of a cavity 5 by machining. Since the square pipe 7 and the support pipe 101 are brought into face contact with each other, decay heat of a spent nuclear fuel assembly can be easily transmitted to the support pipes 101 so as to improve thermal conductivity for that.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cask for accommodating and storing spent fuel assemblies after combustion, and capable of improving the thermal conductivity of decay heat.

[0002]

2. Description of the Related Art A nuclear fuel assembly that has been burned at the end of a nuclear fuel cycle and has become unusable is called spent nuclear fuel. Since spent nuclear fuel contains high radioactive materials such as FP, it must be thermally cooled. Therefore, the spent nuclear fuel is cooled in a cooling pit of a nuclear power plant for a predetermined period (for 3 to 6 months). Then, it is stored in a cask which is a shielding container, transported to a reprocessing facility by a truck or the like, and stored. In storing the spent fuel assemblies in the cask, a holding element called a basket is used. The spent fuel assemblies are stored in cells, which are a plurality of storage spaces formed in the basket.
It is inserted body by body, thereby ensuring an adequate holding force against vibrations during transportation.

[0003] Here, in the above-mentioned cask, not only the function of sealing and shielding radioactive materials and the function of preventing criticality, but also the maintenance of the integrity of spent fuel assemblies and structural members, the spent fuel assemblies are required. An important safety factor is the heat removal function that appropriately removes the decay heat of ash. In order to remove such heat, measures are usually taken to fill the inside of the cask with a helium gas having a high thermal conductivity, or to provide heat transfer fins inside a resin having a low thermal conductivity.

FIG. 12 is a radial sectional view showing an example of a conventional cask. FIG. 13 is a partial sectional view in the axial direction of the cask shown in FIG. This cask 500
A trunk body 1 which is a forged product and shields γ-rays from the spent fuel assembly, an outer cylinder 2 provided outside the trunk body 1, and a neutron charged between the trunk body 1 and the outer cylinder 2 is provided. Resin 3 for shielding, internal fins 4 for heat radiation provided in resin 3, and basket 6 provided in cavity 5 of body 1
It is composed of

[0005] In addition, although not shown, a bottom plate enclosing a resin is provided below the cask 500, and a primary lid and a secondary lid enclosing the resin are provided above the bottom plate. The body 1 and the bottom plate are made of stainless steel or carbon steel to shield gamma rays. The resin 3 is made of a polymer material containing a large amount of hydrogen. The primary lid and the secondary lid are made of stainless steel or carbon steel.

The basket 6 has a structure in which a plurality of aluminum square pipes 7 are supported by a plurality of support plates 8.
The support plate 8 is made of aluminum from the viewpoint of improving the thermal conductivity and reducing the weight, and is provided with a plurality of square holes through which the square pipe 7 is passed in a disk-shaped plate material. The square pipe 7 accommodates a spent fuel assembly (not shown),
It is manufactured by bending an aluminum plate by press working and attaching a neutron absorbing boron plate to the inner surface.
The square pipe 7 is supported and fixed at a plurality of portions in the cavity 5 by the support plate 8. A slight gap G exists between the outer periphery of the support plate 8 and the inner surface of the cavity 5 in consideration of assemblability.

FIG. 14 is an explanatory view showing a support portion between the square pipe and the support plate. The square pipe 7 passes through the hole 9 of the support plate 8 and is fixed by TIG welding. Hole 9 in support plate 8
Is set to be slightly larger than the size of the square pipe 7 in consideration of the deformation of the square pipe 7. For this reason, the square pipe 7 and the support plate 8 are connected by the welding beat 10, and heat conduction of decay heat is mainly performed through the welding beat 10. The decay heat transmitted to the support plate 8 is transmitted to the trunk main body 1 from a contact portion between the support plate 8 and the inner surface of the cavity 5, is transmitted mainly to the inner fins 4 and the outer cylinder 2, and is released to the outside.

[0008]

However, in the above-mentioned conventional cask 500, although the heat of the decay heat is transmitted by the plurality of supporting plates 8, the welding beat 10 is formed between the square pipe 7 and the supporting plate 8. Heat is conducted through the support plate 8 and the small gap G between the support plate 8 and the inner surface of the cavity 5, so that heat conduction is mainly performed in a portion where they are in contact with each other. There is a problem that the conductivity is low.

In the conventional cask 500 described above, since the square pipe 7 is twisted or bent, a gap S exists between the square pipe 7 and the hole 9 of the support plate 8. Therefore, it is necessary to perform welding while adjusting this. Therefore, the cask 500
There was a problem that it took time and effort to manufacture the product.

[0010] Therefore, the present invention has been made in view of the above, and an object of the present invention is to provide a cask that can improve the thermal conductivity of decay heat generated from a spent fuel assembly. In addition, preferably, it is an object to provide a cask that can be easily manufactured.

[0011]

According to a first aspect of the present invention, there is provided a cask for distributing a plurality of square pipes accommodating a spent fuel assembly at regular intervals, and providing the square pipes with the square pipes. A basket is formed by supporting the square pipe by a support member having a heat conductive surface that is in surface contact.

Conventionally, heat conduction between the square pipe and the support member is performed via a welded portion, and the heat conduction portion is about the thickness of the support member (support plate). Therefore, in the present invention, a heat conducting surface that is in surface contact with the square pipe is provided on the support member, and decay heat generated from the spent fuel assembly is transmitted to the support member via the heat conducting surface. Thereby, the thermal conductivity of the cask is improved. As shown in the embodiment, the heat conducting surface is structurally included in a support pipe, a bent plate, or the like.

According to a second aspect of the present invention, there is provided a cask, wherein a plurality of square pipes accommodating a spent fuel assembly are respectively distributed at a fixed interval, and the square pipe is in surface contact with the square pipe in a direction perpendicular to the square pipe. A plurality of support members are passed through, and a support member is further passed between the square pipes in a direction orthogonal to the support members, and the baskets are formed by alternately stacking and restraining the square pipes. is there.

[0014] By restraining the square pipe by the support member that comes into surface contact with the square pipe, the decay heat generated from the spent fuel assembly is transmitted from the square pipe to the support member via the contact surface. Thereby, the thermal conductivity of the decay heat is improved, and the heat removal function of the cask can be enhanced. Here, the term “surface contact” does not mean that the square pipe is in contact only with the thickness of the support member, but that the contact surface is positively provided and brought into contact with the square pipe. Furthermore, since the supporting member is passed between the square pipes and alternately piled up at right angles to each other to restrain the square pipe, welding of the square pipe and the supporting member is not always necessary as in the conventional case. And the cask is easily assembled.
The support member is, for example, a support pipe.

A cask according to claim 3 is the cask, wherein the support member further comprises a support pipe having a plurality of holes and a water passage formed inside the pipe.
Alternatively, it is a support pillar having a groove having an H-shaped cross section or the like and further having a plurality of holes.

In the configuration in which the support members are alternately stacked in the orthogonal direction, water hardly penetrates and circulates hardly.
By providing a plurality of holes to form a passage, water can easily enter and circulate. Also, for example, in a support pillar having an H-shaped cross section, water enters and circulates in the groove. Further, in the case where a hole is provided in the support column, entry of water and the like becomes easier. Further, the weight of the cask can be reduced by using an aluminum material for the support member (claim 4).

According to a fifth aspect of the present invention, the cask further comprises a supporting member made of a material having high thermal conductivity such as aluminum and copper, and a material having high mechanical strength such as stainless steel and carbon steel. It is used in combination with a support member.

Since the spent fuel assembly inserted into the square pipe itself is quite heavy, it is necessary that the support member constituting the basket maintain a heat removal function and maintain mechanical strength. Therefore, a support member made of a material having high thermal conductivity and a support member made of a material having high mechanical strength are used in combination. For example, decay heat is mainly transmitted by an aluminum support member, and the weight of the spent fuel assembly is supported by a stainless steel support member. In this way, it is possible to satisfy the requirements of both the heat removal function and the mechanical strength.

In the cask according to the sixth aspect, the inner surface of the cavity of the trunk body is tapered in the axial direction, and a plurality of square pipes for accommodating the spent fuel assemblies are distributed at regular intervals. The basket is constructed by welding and fixing the square pipe through the hole of the support plate,
The peripheral edge of the support plate is brought into contact with the inner surface of the cavity by inserting the basket into the cavity.

The inner surface of the cavity of the body is tapered,
If a basket is inserted into this cavity, the inner diameter of the cavity is reduced in the axial direction, so that the peripheral edge of the support plate surely contacts the inner surface of the cavity. Thereby, the thermal conductivity between the support plate and the trunk main body is improved.

According to a seventh aspect of the present invention, in the cask, the cask is further provided with a tapered surface on the periphery of the support plate. If the inner surface of the cavity of the body is tapered and a basket with a tapered surface is further inserted around the periphery of the support plate into this cavity, the inner diameter of the cavity becomes smaller in the axial direction. Make sure it contacts the inner surface. This further improves the thermal conductivity between the support plate and the trunk main body.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a cask according to the present invention will be described below in detail with reference to the drawings.
The present invention is not limited by the embodiment.

(Embodiment 1) FIG. 1 is an axial sectional view showing a cask according to Embodiment 1 of the present invention.
FIG. 2 is a radial sectional view of the cask shown in FIG.
FIG. 3 is a perspective view showing the structure of the basket shown in FIG. The cask 100 is characterized in that the square pipe 7 is fixed and fixed by the support pipe 101. In addition, since the configuration of the trunk main body 1 and the like is the same as the above-described conventional one, the description thereof is omitted, and the same components are denoted by the same reference numerals.

As shown in FIG. 3, a plurality of square pipes 7 are arranged in a grid, and a support pipe 101 having a rectangular cross section is passed between the square pipes 7. Subsequently, the support pipe 1
A plurality of support pipes 101 are passed so as to be orthogonal to the 01 row, and the support pipes 101 are alternately stacked to restrain the square pipe 7. The end surface 102 of the support pipe 101 is machined into a curved shape so as to conform to the inner surface shape of the cavity 5.
Further, from the viewpoint of assemblability, the end face 10
A slight gap G is provided between 2 and the inner surface of the cavity 5.

The square pipe 7 and the support pipe 101 are in a state where they can come into contact with a certain margin when inserted into the cavity 5.
, Heat expansion occurs due to the decay heat or radiant heat, and an appropriate surface contact state can be obtained. In addition, square pipe 7
The support pipe 101 and the support pipe 101 may be fixed by welding even in an assembled state. As a material of the square pipe 7, an aluminum composite material or an aluminum alloy obtained by adding B or a B compound powder having a neutron absorption performance to Al or an Al alloy powder is used. In addition, the square pipe 7 converts the mixed powder into a HIP (Hot Isostatic Press).
It is preformed by using such a method, and is manufactured by hot extruding the formed billet.

The Al or Al alloy includes pure aluminum ingot, Al-Cu-based aluminum alloy, Al-Mg
Aluminum alloy, Al-Mg-Si aluminum alloy, Al-Zn-Mg aluminum alloy, Al-F
An e-based aluminum alloy or the like can be used. Further, B ₄ C, B ₂ O ₃ or the like can be used as the B or B compound. Here, the amount of boron added to aluminum is preferably 1.5% by weight or more and 7% by weight or less. If the content is less than 1.5% by weight, a sufficient neutron absorbing ability cannot be obtained, and if the content is more than 7% by weight, the elongation with respect to tension is reduced.

The support pipe 101 is also provided with the square pipe 7.
It is manufactured by extrusion molding in the same manner as described above. However, it is not necessary to add boron for absorbing neutrons, and Al or A
1 alloy is used. Also, all support pipes 1
If the 01 is made of aluminum, the weight of the cask can be reduced. However, in order to obtain sufficient strength to support the weight of the spent fuel assembly, some support pipes 101 are made of stainless steel or carbon steel. You may. further,
Copper support pipe 1 as long as strength requirements are met
01 may be used.

What kind of material is used for the plurality of support pipes 101 is appropriately selected based on the weight of the spent fuel assembly and the calorific value thereof. For example, the support pipes 101 made of stainless steel for securing strength and the support pipes 101 made of aluminum for securing heat conductivity are alternately used in the same stage. When the support pipe 101 made of stainless steel or carbon steel is used, the shielding property of γ-ray can be improved.

The storage of the spent fuel assemblies is performed in a pool. The water in the pool enters through the open end face 102 of the support pipe 101 and the gap between the square pipe 7 and the support pipe 101. The pool water slows down the neutrons generated from the spent fuel assemblies and prevents criticality during storage. The decay heat of the stored spent fuel assembly reaches the outer surface of the square pipe 7 from the contact portion between the spent fuel assembly and the inner surface of the square pipe 7, and is transmitted to the support pipe 101 via the contact surface with the support pipe 101. I do.

The heat of the support pipe 101 is transferred to its end face 102
And is discharged from the outer cylinder 2 to the outside via the inner fins 4 and the resin 3. In particular, the support pipe 101 made of aluminum or copper performs an efficient heat removal action. Further, the square pipe 7 and the support pipe 10
1 is in planar contact, the thermal conductivity is improved, and the heat removal function of the cask 100 is enhanced. In addition, there is a heat removal route for releasing decay heat to the outside via the helium gas filled in the cask 100.

In the cask 100, since the support pipes 101 are alternately stacked to restrain the square pipes 7 to form the basket 6, the assemblability is improved. Furthermore, the weight of the cask 100 can be reduced by using an aluminum support pipe. Although the support pipe 101 is extruded in the above description, it may be formed by press working. Further, instead of the support pipe 101, a solid square bar may be used.

Further, the support pipe 101 has improved thermal conductivity by being in surface contact with the square pipe 7. From this viewpoint, the support pipe 101 is replaced with the support pipe 101 and has an H-shaped cross section as shown in FIG. May be used. According to the support pillar 103, since the cross section is H-shaped, when the spent fuel assembly is accommodated in the pool, water easily enters the periphery of the square pipe 7 so that the heated water Circulation is also easy. This support column 103 can also be manufactured by extrusion molding. Also,
In addition to the H-shaped cross-section, any shape that can secure a contact surface with the square pipe 7 may be, for example, a “Japanese” cross-section, an S-shape, or a U-shape. Furthermore, if holes are drilled in the support columns, water can more easily enter and circulate.

(Embodiment 2) FIG. 5 is an explanatory view showing a basket of a cask according to Embodiment 2 of the present invention. This basket 200 is characterized in that a water passage is provided in the support pipe 201. Since the ambient temperature when the spent fuel assemblies are accommodated in the square pipe 7 is extremely high and the pressure in the cask 200 is low, it is necessary to appropriately circulate the internally heated water. However, when the support pipes 201 are orthogonally assembled and alternately assembled, the space R surrounded by the surfaces of the support pipes 201 and the square pipes 7 is formed.
(Shown by a dotted line in the figure), and water hardly enters the space R from the outside, and the space R is not sufficiently circulated. Therefore, a hole 202 is formed in the support pipe 201 so as to easily penetrate and circulate water, so that a water passage (hole 202 → inside the support pipe 201 → hole 202) is secured.

More specifically, a plurality of holes 202 are formed in the support pipe 201, and water is allowed to enter through the open end face 203 and the holes 202 on the end face of the support pipe 201, thereby forming the support pipe 201.
Water penetrates into the space R through the inside of 01 (an example is indicated by an arrow in the figure). Further, the holes 202 are formed in various places of the support pipe 201 to form a plurality of passages communicating the outside and the space. In addition, the support pipe 201
Water also intrudes from between the pipe and the square pipe 7, so that the water in the cask 200 can be smoothly circulated. Note that the arrangement and number of the holes 202 need to be suppressed to the extent that the strength of the support pipe 201 is not impaired, but there is also an advantage that the provision of the holes 202 can reduce the weight. It is particularly effective for carbon steel and stainless steel.

Third Embodiment FIG. 6 is an explanatory view showing a square pipe and a support plate of a cask according to a third embodiment of the present invention. FIG. 7 is a sectional view of the square pipe and the support plate shown in FIG. In FIG. 1, one square pipe 7 and the support plate 8 are shown, but the other square pipes 7 have the same configuration. As described in the above conventional example, the hole 9 of the support plate 8 is somewhat larger in size than the square pipe 7. In the basket 301, the plate material 302 bent at a right angle to the gap S is sandwiched. I have.
One edge of the plate 302 is welded on the support plate 8 (weld beat 303), and the other edge and a corner of the plate are welded to the square pipe 7 (weld beat 304).
The plate members 302 are provided on four surfaces of the square pipe 7.

The decay heat of the spent fuel assembly is transmitted from the square pipe 7 to the support plate 8 via the plate 302. The difference from the conventional basket 500 is that the square pipe 7 and the support plate 8 are in surface contact via the plate member 302. According to this configuration, the thermal conductivity of the decay heat is improved, and the heat removal function of the cask can be enhanced.

(Embodiment 4) FIG. 8 is an axial sectional view showing a cask according to Embodiment 4 of the present invention.
FIG. 9 is a radial sectional view of the cask shown in FIG.
FIG. 10 is an enlarged view of a portion A in FIG. This cask 40
0 is characterized in that the inner surface of the cavity 405 of the body 401 is tapered. Further, a tapered surface 403 corresponding to the taper of the inner surface of the cavity 405 is provided also on the peripheral edge of the support plate 402. The diameter of the support plate 402 is larger as it is located above the cask 400. The connection between the support plate 402 and the square pipe 7 is performed by welding in the same manner as in the conventional case. Other configurations are similar to those of the conventional cask 50 described above.
Since it is the same as 0, the description is omitted, and the same components are denoted by the same reference numerals.

When the support plate 402 is inserted into the cavity 405 while supporting the plurality of square pipes 7, the tapered surface 403 of each support plate 402 comes into contact with the gradually narrowing inner surface of the cavity 405 and is fixed by the wedge effect. . Thereby, the gap between the support plate 402 and the inner surface of the cavity 405 can be filled, and the cavity 40
5, the contact area between the inner surface and the support plate 402 also increases, so that the thermal conductivity of the cask 400 can be improved. In addition, in order to prevent the movement of the entire basket 6 in the axial direction, it is possible to attach a primary lid to the trunk main body 401 and hold down the primary lid (not shown).

The inner surface of the cavity 405 is tapered by machining. FIG. 11 is a schematic configuration diagram showing a taper machining device for a trunk main body. The taper processing device 450 includes a rotation support base 452 for rotatably mounting the trunk main body 401 on the roller 451, a driving device 453 for rotating the trunk main body 401 on the rotation support base 452, and a trunk main body 40.
1, a fixed table 454 inserted in the axial direction and supported on both sides of the body 401, a moving table 455 positioned on the fixed table 454, and a saddle positioned on the moving table 455 in the radial direction. 456
And a cutting tool 457 attached to the tool post on the saddle 456
It is composed of

In processing the inside of the cavity 405, first, the fixed table 454 is disposed so as to penetrate the trunk body 401, and the rotation support base 45 is driven by the driving device 453.
The upper torso body 401 is rotated. In this state, the saddle 456 is moved in the radial direction to position the cutting tool 457 with a predetermined cutting amount, and the moving table 455 is moved to the body 4.
It is moved while being numerically controlled in the axial direction 01. Then, the position of the saddle 456 is controlled in the radial direction with a constant moving amount with respect to the moving amount of the moving table 455, and the cutting amount is gradually reduced. Thus, the inner surface of the cavity 405 can be tapered.

[0041]

As described above, in the cask according to the present invention (claim 1), a plurality of square pipes accommodating the spent fuel assemblies are distributed at regular intervals, and come into surface contact with the square pipes. Since the square pipe is supported by the support member having the heat conductive surface to form the basket, the heat conductivity of the cask is improved.

Further, in the cask according to the present invention (claim 2), a plurality of square pipes accommodating the spent fuel assemblies are distributed at regular intervals, and the square pipes are provided between the square pipes in a direction orthogonal to the square pipes. A plurality of support members that are in surface contact with the pipe are passed, and further, the support members are passed between the square pipes in a direction orthogonal to the support members, and the baskets are alternately stacked to restrain the square pipes. With this configuration, the thermal conductivity of the square heat generated in the spent fuel assembly is improved, and the heat removal function of the cask is enhanced. Further, the cask can be easily assembled.

In the cask according to the present invention (claim 3), the support member has a support pipe having a plurality of holes and a water passage formed inside the pipe, or a plurality of grooves having an H-shaped cross section. Since the supporting pillars may have holes, the water can sufficiently enter or circulate in the pool.

In the cask according to the present invention (claim 4), the weight of the cask can be reduced by using an aluminum material for the support member.

Further, in the cask according to the present invention (claim 5), the support member made of a material having high thermal conductivity such as aluminum and copper, and the support member made of a material having high mechanical strength such as stainless steel and carbon steel. Since both are used in combination, it is possible to maintain both the heat removal function and the mechanical strength of the cask.

Further, in the cask according to the present invention (claim 6), the inner surface of the cavity of the trunk body is tapered in the axial direction, and a plurality of square pipes for accommodating the spent fuel assemblies are formed at regular intervals. A basket was formed by distributing and welding each square pipe through a hole in the support plate to form a basket. The basket was inserted into the cavity to bring the peripheral edge of the support plate into contact with the inner surface of the cavity. For this reason, since the peripheral edge of the support plate surely comes into contact with the inner surface of the cavity, the heat conductivity between the support plate and the trunk body is improved.

Further, in the cask according to the present invention (claim 7), since the tapered surface is also provided on the peripheral edge of the support plate, the tapered surface of the support plate surely comes into contact with the inner surface of the cavity, and the support plate and the trunk body are connected to each other. The thermal conductivity during the period can be further improved.

[Brief description of the drawings]

FIG. 1 is an axial sectional view showing a cask according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing a radial direction of the cask shown in FIG.

FIG. 3 is a perspective view showing the structure of the basket shown in FIG. 1;

FIG. 4 is a perspective view showing a modification of the cask shown in FIG.

FIG. 5 is an explanatory view showing a basket of a cask according to a second embodiment of the present invention.

FIG. 6 is an explanatory view showing a square pipe and a support plate of a cask according to a third embodiment of the present invention.

FIG. 7 is a sectional view showing the square pipe and the support plate shown in FIG. 6;

FIG. 8 is a sectional view showing an axial direction of a cask according to a fourth embodiment of the present invention.

FIG. 9 is a sectional view showing a radial direction of the cask shown in FIG. 8;

FIG. 10 is an enlarged view of a portion A shown in FIG.

FIG. 11 is a schematic configuration diagram showing a taper processing device for a trunk main body.

FIG. 12 is a radial sectional view showing an example of a conventional cask.

FIG. 13 is a partial sectional view showing an axial direction of the cask shown in FIG.

FIG. 14 is an explanatory diagram showing a support portion between a square pipe and a support plate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1,401 Body 2,402 Outer cylinder 3 Resin 4 Inner fin 5,405 Cavity 6 Basket 7 Square pipe 100,400 Cask 101 Support pipe 102 End face

Claims (7)

[Claims] A plurality of square pipes accommodating a spent fuel assembly are distributed at regular intervals, and a supporting member having a heat conducting surface in surface contact with the square pipe supports the basket while supporting the square pipe. A cask characterized by comprising. 2. A plurality of square pipes accommodating a spent fuel assembly are respectively distributed at a fixed interval, and a plurality of support members which are in surface contact with the square pipe between the square pipes and in a direction orthogonal thereto are passed through. And a basket formed by passing the support members between the square pipes in a direction orthogonal to the support members and stacking the support members alternately to restrain the square pipes. 3. The supporting member, wherein the supporting member has a supporting pipe having a plurality of holes and a water passage formed inside the pipe, or a groove having an H-shaped cross section, and may further have a plurality of holes. The cask according to claim 2, wherein the cask is a pillar. 4. The cask according to claim 2, wherein an aluminum material is used for the support member. 5. The method according to claim 1, wherein a supporting member made of a material having high thermal conductivity such as aluminum or copper and a supporting member made of a material having high mechanical strength such as stainless steel or carbon steel are used in combination. The cask according to claim 2 or 3. 6. An inner surface of a cavity of a trunk main body is tapered in an axial direction, a plurality of square pipes accommodating spent fuel assemblies are distributed at regular intervals, and each square pipe is inserted into a hole of a support plate. And forming a basket by welding and fixing the basket, and inserting the basket into the cavity to bring the peripheral edge of the support plate into contact with the inner surface of the cavity. 7. The cask according to claim 6, wherein a tapered surface is further provided on a peripheral edge of said support plate.