High density spent fuel storage rack

A high density spent fuel storage rack including a plurality of neutron absorbing containers each designed to perimentrically encircle a fuel assembly, the containers being arranged so that the rack can store a plurality of fuel assemblies disposed vertically in an array with predetermined spaces between the adjacent fuel assemblies. Each of the containers is provided with enlarged upper and lower end portions respectively abutting against enlarged upper and lower end portions of the adjacent containers along their outer peripheries. The abutting end portions are joined by edge-welding to one another whereby the containers are formed into a unitary structure providing predetermined spaces between the adjacent containers.

LIST OF PRIOR ART REFERENCES 
The following reference is cited to show the status of the art: 
Japanese Patent Kokai (Laid-Open) No. 114600/76 Frank Bevilacqua, Oct. 8, 
1976 (Corresponding to U.S. Pat. No. 4,004,154 issued on Jan. 18, 1977) 
BACKGROUND OF THE INVENTION 
This invention relates to spent fuel storage racks, and more particularly 
to a spent fuel storage rack capable of storing at high density a 
plurality of fuel assemblies which have been used in nuclear reactors. 
In nuclear power plants, the spent fuel withdrawn from the reactor cores is 
temporarily stored in storage racks immersed in a pool of water or other 
radiation shielding liquid before being reprocessed. In recent years, 
there has been an increase in the amount of spent fuel removed from the 
reactor cores, but existing facilities have no sufficient capacity to 
reprocess the spent fuel at once. In order to cope with this situation, 
proposals have been made to use high density spent fuel storage racks. For 
example, Japanese Patent Kokai (Laid-Open) No. 114600/76 filed by Frank 
Bevilacqua discloses a storage rack of the type described. This storage 
rack comprises a plurality of neutron absorbing containers arranged to 
receive a plurality of fuel assemblies disposed vertically in an array 
with predetermined spaces between the adjacent fuel assemblies, each 
container being designed to perimentrically encircle a fuel element. The 
neutron absorptive containers are each rectangular in cross section. In 
order to form these containers into a unitary structure while maintaining 
the containers in predetermined spaced relationship, spacers or U-shaped 
channels are welded to upper end portions and lower end portions of the 
containers in such a manner that the spacers extend in a grid pattern 
between the rows of the containers and the columns of the containers. Some 
disadvantages are associated with this type of spent fuel storage rack. 
First, it is complex in construction and has many portions where the 
containers are joined by welding to the spacers. A difficulty is 
encountered in carrying out welding, and assembling of this type of rack 
is a time-consuming operation and requires a lot of labor. The containers 
are preferably hexagonal in cross section for storing fuel assemblies 
which are also hexagonal in cross section, in order that high density 
storage can be achieved in a spent fuel storage rack. However, it is 
practically impossible to use spacers for containers of a hexagonal cross 
section in the same manner as spacers are used for the containers of the 
rack of the type referred to above, because the spacers used will increase 
in number and yet the containers will be located relatively sparsely. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a high density spent 
fuel storage rack wherein portions to be joined by welding are small in 
number and welding can be carried out with ease. 
Another object is to provide a high density spent fuel storage rack which 
can be fabricated even if the containers are of any polygonal shape in 
cross section other than square and which is easy to assemble. 
According to the invention, there is provided, in a high density spent fuel 
storage rack including a plurality of neutron absorbing containers each 
designed to perimentrically encircle a fuel assembly, said containers 
being arranged so that the rack can store a plurality of fuel assemblies 
disposed vertically in an array with predetermined spaces between the 
adjacent fuel assemblies, the improvement wherein each of said containers 
is provided with enlarged upper and lower end portions which respectively 
abut against enlarged upper and lower end portions of the adjacent 
containers along their outer peripheries and said abutting end portions 
are joined by edge-welding to one another whereby said containers are 
formed into a unitary structure providing predetermined spaces between the 
adjacent containers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the invention will now be described with reference to the 
accompanying drawings. In FIGS. 1 to 3, a high density spent fuel storage 
rack comprising a first embodiment of the invention is generally 
designated by the reference numeral 2 and includes a plurality of neutron 
absorbing containers 4 made of stainless steel in the form of elongated 
hollow members of a square cross section. The containers 4 are arranged 
vertically to provide an array thereof so that the containers are arranged 
in rows and columns. In this embodiment, there are 30 containers 4 in the 
rack 2 which are arranged in 10 columns and 3 rows. By inserting a fuel 
assembly 5 in each of the containers 4 in such a manner that the former is 
perimentrically encircled by the latter (See FIG. 5), it is possible to 
store the fuel assemblies 5 at high density in the rack 2. 
Each of the containers 4 has an upper end portion 6 and a lower end portion 
8 which are enlarged in size. The enlarged upper and lower end portions 6 
and 8 consist of enlarged portions 10 and 12 of a constant cross-sectional 
area and tapering portions 14 and 16 respectively. Enlarging of the end 
portions of the containers 4 can be effected by inserting a shaping 
element (not shown) into each of the containers 4 by means of the force of 
a hydraulic cylinder and expanding the end portions transversely. 
The enlarged upper end portions 6 and lower end portions 8 of the 
containers 4 are located in a manner to abut against the enlarged upper 
end portions 6 and lower end portions 8 of the adjacent containers 4 at 
outer peripheries 22 and 23 thereof, and the abutting upper end portions 6 
and lower end portions 8 are joined by edge-welding as designated at 22 
and 23. By edge-welding the enlarged upper end portions 6 and lower end 
portions 8 of the adjacent containers 4 which are maintained in abutting 
relation, the containers 4 are formed into a unitary structure and at the 
same time predetermined spaces 24 are provided between the adjacent 
containers 4. The enlarged upper and portions 6 play the role of 
facilitating the insertion of the fuel assemblies 5 into the containers 4. 
A fuel assembly support member 26 made of a stainless steel plate is welded 
to inner wall surfaces of the enlarged lower end portions 8 of each 
container 4 as indicated by the reference numeral 28 in FIG. 3. As clearly 
shown in FIG. 4, each support member 26 is shaped such that it is formed 
with a central opening 32 which provides a seat 30 for the lower end 
portion of each fuel assembly 5 inserted in one of the containers 4, and 
defines passages 34 between the inner wall surfaces of the lower end 
portion 8 and the support member 26. The passages 34 permit a coolant to 
flow from one side of each support member 26 to the other side thereof. 
Each support member 26 can be produced by die pressing a disk having a 
hole in the center so as to form the seat 30 at the upper edge of the 
central opening 32 and a skirt 36 extending downwardly from the seat 30 to 
define the central opening 32 by its inner peripheral surface, and then 
cutting off portions of the disk in four positions in such a manner that 
the passages 34 can be formed and welded portions 38 conforming to the 
inner wall surfaces of each container 4 can be provided. In place of 
providing the passages 34 between each support member 26 and each 
container 4, at least one opening may be directly formed in each support 
member 26. 
The numeral 40 designates reinforcing frames joined by welding to the outer 
peripheral surfaces of the containers formed into a bundle. 
The manner in which neutrons attenuation in the rack according to the 
invention will now be described. The spent fuel in the fuel assemblies 
stored in the rack emits fast neutrons which collide against the stainless 
steel plates constituting the containers 4 for the assemblies 5. The fast 
neutrons have energy of a high intensity such that they pass through the 
stainless steel plate. The fast neutrons that have passed through the 
stainless steel plate have their energy level reduced while passing 
through a coolant or water in the spaces between the adjacent containers, 
with the result that they become slow neutrons. Finally, the slow neutrons 
collide against the stainless steel plate of the adjacent container or are 
reflected by the water in the spaces between the containers to collide 
against the stainless steel plate through which they have first passed. 
Thus the energy level of the slow neutrons is reduced such that they are 
absorbed by a neutron absorbing substance. 
The high density spent fuel storage rack in accordance with the invention 
is of the aforementioned construction. The containers are joined by means 
of edge-welding effected between the abutting enlarged upper and lower end 
portions of the adjacent containers. It will be appreciated that the 
portions to be joined by edge-welding are relatively small in number and 
the welding operation can be carried out with ease. 
FIG. 5 shows a second embodiment of the invention in which the enlarged 
upper end portion 44 and lower end portion 46 of each container 42 consist 
only of tapering portions 48 and 49 respectively. Other parts are similar 
to those of the first embodiment so that they are designated by like 
reference characters. By adopting the enlarged upper and lower end 
portions of the aforementioned shape for each container, there is offered 
the advantage of forming of the enlarged portions being facilitated. 
The essentials of the present invention lie in the fact that the enlarged 
upper and lower end portions of the adjacent containers maintained in 
abutting relation are joined by edge-welding. It should be noted, 
therefore, that the high density spent fuel storage rack according to the 
invention can be fabricated even if the containers are of any polygonal 
shape other than square. FIG. 6 is similar to FIG. 2 but shows a third 
embodiment in which the containers 50 are hexagonal in cross section. The 
rack of this embodiment is substantially similar in construction to the 
rack of the first embodiment except for the fact that the containers 
differ from each other in cross-sectional shape. Therefore, the embodiment 
shown in FIG. 6 will be explained in brief. The enlarged upper end 
portions 52 and the lower end portions (not shown) of the adjacent 
containers 50 maintained in abutting relation are joined to one another by 
edge-welding as designated at 56, and each fuel assembly support member 58 
is welded to the inner wall surfaces of the enlarged lower end portions of 
one of the containers 50. Each support member 58 is formed therein with 
the central opening 62 which provides the seat 60 for supporting the lower 
end portion of each fuel assembly, and defines the coolant passages 64 
between the inner wall surfaces of the lower end portion of each container 
50 and the support member 58. It will be appreciated that the rack of this 
embodiment is convenient for storing fuel assemblies of a hexagonal cross 
section at high density.