Damped storage rack for nuclear fuel assemblies

A storage rack for storing nuclear fuel rod assemblies is provided with an array of cell housings having support elements which are preloaded against the adjacent corner of the adjacent cell housing. The support elements are fixed to the outer corner of one cell housing and have pads which are preloaded or biased against the adjacent corner of the adjacent cell housing. The interface between the pads of the support elements and the adjacent cell housing provide a coulomb damping function which is effective in absorbing vibration from rough handling or seismic events. The cell housings can be located in alternating positions in the array and additional cell locations can be formed from the outer walls of the surrounding cell housings. The cell housings are held together in the array by support bars which are affixed to the top and bottom ends of the cell housings. The support bars may include recesses to align the cell housings.

CROSS-REFERENCE TO RELATED APPLICATIONS 
Not Applicable 
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
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REFERENCE TO MICROFICHE APPENDIX 
Not Applicable 
BACKGROUND OF THE INVENTION 
The invention relates to storage racks for storing nuclear fuel assemblies 
both during transport and during stationary storage. Preferably, the racks 
are highly overdamped, enabling them to best withstand vibrations caused 
by seismic events or rough handling. 
Fuel for nuclear reactors is typically configured in the form of elongated 
fuel rods, which may be separate, stand-alone elements, or may be 
positioned within canisters. Hereinafter, the fuel rods and rod/canister 
combinations are referred to as fuel assemblies. Both before and following 
use, the fuel assemblies must be stored and/or transported with great 
care. To assure that such care is achieved, storage racks are often used 
to support a plurality of fuel assemblies in a generally parallel, 
spaced-apart configuration, while maintaining the fuel assemblies in a 
subcritical array environment. During storage, the racks and the fuel 
assemblies contained therein, may be completely submerged in a pool of 
water. The water provides cooling and additional shielding from nuclear 
radiation. 
The fuel storage racks of the prior art typically consist of an assembly of 
hollow cells, each defined by an array of elongated rectangular 
cross-section boxes or compartments. The boxes are typically made by 
forming sheets of stainless steel into elongated rectangular cross-section 
tubes (typically 9 inches square by 14 feet long and welding the corners 
of the elongated tubes together to form a matrix of elongated hollow 
cells, each adapted the receive a single fuel assembly. Typically, the 
tubes are joined at their corners to common rod segments that are located 
at various positions along the adjacent corners of the tubes. Exemplary 
storage racks are disclosed in U.S. Pat. Nos. 4,695,424, 4,857,263, 
4,948,553, and 4,366,115. Alternatively, the tubes can be held in place by 
support bars that are welded or otherwise affixed to the top and bottom 
ends of each tube such as disclosed in commonly owned U.S. Pat. No. 
5,384,813 entitled Highly Damped Storage Rack for Nuclear Fuel Assemblies, 
which is hereby incorporated by reference. 
A neutron absorbing (or "poison") material, such as borated stainless 
steel, is typically welded or otherwise rigidly affixed to each of the 
walls of the boxes to absorb neutron flux from the fuel assemblies which 
may be positioned within the boxes, thereby avoiding an undesirable 
concentration of neutrons. Alternatively, the neutron absorbing material 
can preloaded against the walls of the tube as described in commonly owned 
U.S. Pat. No. 5,384,813 in order provide a coulomb damping function for 
improved resistance to vibration damage. 
One of the problems associated with assembling prior art storage racks is 
that the tolerance variation of the longitudinal bow for the tubes can 
vary up to .+-.0.1875 inches. After assembling the tubes together at their 
ends, any bow over the fourteen foot length of the tube makes it difficult 
to weld rod segments at positions intermediate to the ends. In addition, 
if the bow were to increase over time, the adjacent corners could bear 
against each other, potentially causing premature failure of the storage 
rack. 
Accordingly, it is an object of the present invention to provide an 
improved storage and/or transport rack for nuclear fuel assemblies. 
Another object of the present invention is to provide an improved storage 
rack for nuclear fuel assemblies which is highly overdamped to enable the 
rack to withstand the vibration of seismic events or rough handling such 
as may be encountered during transportation of the rack. 
It is another object to provide a storage rack for nuclear fuel assemblies 
which has improved torsional and crush strength. 
A further object is to provide an improved storage rack for nuclear fuel 
assemblies which may be easily and inexpensively manufactured. 
It is yet another object to provide a storage rack for nuclear fuel 
assemblies which is resistant to failure due to the bowing of the tubes of 
the storage rack. 
It is yet a further object to provide a storage rack for nuclear fuel 
assemblies which is resistant to failure due to the bowing of the 
individual cell tubes of the storage rack by providing support elements 
that are positioned between adjacent cell tubes of the storage rack in 
order to facilitate alignment of the cell tubes and resist bowing by 
biasing the support elements against adjacent cell tubes to distribute the 
forces attributable to bowing over the adjacent cells. 
Other objects of the invention will in part be obvious and will in part 
appear hereinafter. 
SUMMARY OF THE INVENTION 
According to the present invention, a rack structure is provided for long 
term storage and/or transport of nuclear fuel assemblies. The storage rack 
includes an array of individual storage cells. The cells of the array are 
defined by a plurality of substantially round or polygonal cross-section, 
elongated cell housings, each extending along an elongated central axis, 
wherein the central axes are substantially parallel to each other. In 
accord with an important aspect of the invention, a support element 
includes a first portion adapted to be fixed to one of the cell housings 
and a second portion adapted to press or be biased against an adjacent 
cell housing. A stiffener wall may be welded to the adjacent cell housings 
along the perimeter of the rack to enclose the open cells along the 
perimeter. The cell housings and the stiffener walls are held in parallel 
alignment by support bars affixed thereto, for example by welding, at both 
the top and bottom ends of the array of cell housings. Preferably, the 
support bars are positioned at the top and bottom ends between each row of 
cell housings and along the outer perimeter of the rack. The support bars 
may be recessed on one side or on alternating sides to provide positioning 
of the cell housings prior to affixation. A base plate is affixed to the 
bottom of the array to define the lower boundary of the respective cells 
and to support the fuel assemblies therein. To facilitate water flow for 
cooling of the nuclear fuel assemblies, the base plate may include holes 
at positions within each cell. Pedestals extending from the base plate may 
be used to raise the rack above a floor. 
Preferably, the cell housings are substantially square in cross-section and 
positioned in alternate points of a rectangular grid configuration, so 
that each cell housing defines one cell in its interior and so that the 
outer walls of three or more adjacent cell housings define one cell. The 
first portion of the support element is adapted to be fastened to one 
corner of the cell housing and the second portion of the support element 
is adapted to engage and bear against the corner of an adjacent cell 
housing. 
The second portion of the support element is adapted to be pressed or 
biased against the outer wall of the adjacent cell housing. The resulting 
friction between the second portion and cell wall results in a coulomb 
damping function that is effective in damping vibration. The support 
elements also serve to align the cells during assembly and help to make 
the overall assembly more resistant to bowing by individual cell housings. 
The support elements can be fastened to one of the cell housings by 
welding, brazing or mechanical fastening. The second portion of the 
support element is merely adapted to engage and bear against an adjacent 
object or an adjacent cell housing. The second portion may also be further 
adapted to enhance the friction with the adjacent cell housing in order to 
improve coulomb damping performance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1-4 show two embodiments of a storage rack 10 of the invention, which 
forms a close packed array, 3 rows by 5 columns of elongated cells C1-C15. 
In these embodiments, the odd numbered or primary cells C1, C3, C5, C7, 
C9, C11, C13, C15 are formed from rectangular cell housings 3 which extend 
along housing axes 3'. The even numbered or secondary cells C2, C4, C6, 
C8, C10, C12, C14 are formed by the walls of the surrounding cells. It is 
noted that the secondary cells along the perimeter of the array C2, C4, 
C6, C10, C12, C14 are not completely surrounded by primary cells. If it is 
necessary that one of these secondary cells is to be utilized, a stiffener 
wall 3A is inserted substantially flush with exterior walls of the 
adjacent primary cell housings to completely enclose the secondary cell as 
shown in FIG. 3. 
In the embodiment of FIGS. 1 and 2, the individual cell housings 3 are 
assembled into a rack assembly by fixturing the individual cell housings 3 
and welding adjacent corners of the cell housings 3 to rod segments 5. The 
rod segments 5 can have various shapes and sizes including round, square, 
triangular or wedge. The major tolerance variation in the longitudinal bow 
of the cell housings 3 is from approximately .+-.1/8 inches to .+-.3/16 
inches and thus the rod segments 5 can vary in diameter from zero to 5/16 
inches in diameter. The stiffener wall 3A, if required, can be welded to 
the adjacent cell housings 3. 
In the embodiment of FIGS. 3 and 4, the individual cell housings 3 and the 
stiffener walls 3A are held in parallel alignment by upper and lower 
support bars 1, 7, and 8 which extend transverse to the longitudinal axis 
of the cell housings 3. The support bars 1, 7 and 8 are located between 
each of the rows of cells and along the perimeter of the array as shown in 
FIG. 3. Support bars 1, 7 and 8 are provided at both the top and bottom 
end portions of the rack. The cell housings are held in parallel alignment 
by welding the individual cell housings 3 to the upper and lower support 
bars 1, 7 and 8. The support bars also add strength to the upper and lower 
ends of the cell housings to resist damage during inserting and removal of 
the nuclear fuel rod assemblies. 
As shown in FIGS. 2 and 4, a base plate 2 is welded to the bottom of the 
rack to close the bottom of the cells and support the nuclear fuel 
assemblies. The base plate 2 may also be provided with holes 12 (shown in 
FIGS. 1 and 3) at locations within each cell and pedestal feet 9 to 
facilitate the flow of water for enhanced cooling. 
Each cell housing 3 is an elongated tube, typically having a square or 
rectangular cross-section. The housing is constructed from suitable 
material, for example, 0.090 inch thick stainless steel tubing. 
Preferably, the tubes are square in cross-section, approximately nine 
inches along each side and 14 feet long. Each outer surface of the housing 
is planar, to which is applied, with or without a preload force, an 
elongated slab 4 constructed of a damping material. This damping material 
can also be a neutron absorbing material, such as borated stainless steel, 
borated aluminum, boral (such as manufactured by Brooks & Perkins, 
Minneapolis, Minn.), or other neutron absorbing materials may be used. The 
damping material can be fixed to outside of the cell housings (such as by 
welding or brazing) or the damping material can be preloaded against the 
outer surface by retainer clips (not shown) which are welded to the 
outside of the housings along the perimeter of each surface as disclosed 
in commonly owned U.S. Pat. No. 5,384,813. 
As shown in FIGS. 2 and 4, each of the cell housings 3 can include one or 
more support elements 14 fixed at one corner on the outside of the cell 
housing 3 which are adapted to bear against the adjacent corner of an 
adjacent cell housing 3. The support element 14 serves to align the cell 
housings 3 during assembly and further functions to help the cell housing 
resist bowing after assembly. In addition, the portion of the support 
element that bears against the adjacent corner of the adjacent cell 
housing 3 can provide coulomb damping of vibration in the individual cell 
housings 3. 
FIG. 5A shows a diagrammatic layout view of a support element 14 (prior to 
forming) in accordance with one embodiment of the present invention. 
Support element 14 includes a first portion adapted to engage a first cell 
housing and at least one second portion adapted to bear against an 
adjacent cell housing. Preferably, the support element 14 has a second 
portion which includes upper tabs 22 and a third portion which includes 
lower tabs 24. The upper tabs 22 and the lower tabs 24 are connected by 
upper arm 28 and lower arm 30, respectively, to the first portion which 
includes central tabs 26. In the preferred embodiment, the support element 
14 is formed from 0.060 inch thick stainless steel that is approximately 3 
inches long. The upper and lower tabs 22 and 24 are approximately 0.25 
inches wide and extend from approximately 0.22 inches from the upper and 
lower arms 28, 30. The central tabs 26 are approximately 0.50 inches wide 
and extend from approximately 0.22 inches from the upper and lower arms 
28, 30. The width of each of the upper and lower arms 28, 30 is 
approximately 0.180 inches. 
FIGS. 5B and 5C shows diagrammatic views of a formed support element 14 in 
accordance with one embodiment of the present invention. Upper and lower 
tabs 22 and 24 are bent, toward the same side of the upper and lower arms 
28, 30, to a 90 degree angle to form an inside corner that is adapted to 
engage the outside corner of an adjacent cell housing 3. The central tabs 
26 are bent, toward the opposite side from the upper and lower tabs 22, 
24, to a 90 degree angle to form an inside corner that is adapted to 
engage and be fastened the outside corner of a cell housing 3. The central 
tabs 26 can be fastened to the cell housing 3 by any fastening method 
including welding, brazing or the use of mechanical fasteners or 
adhesives. 
Upper and lower tabs 22, 24 form pads that bear against the adjacent corner 
of an adjacent cell housing. The interface between the upper and lower 
tabs 22, 24 and the outer surfaces that form the adjacent corner of the 
adjacent cell housing is such that they establish a coulomb damping 
function that damps vibration in the storage rack. The upper and lower 
arms 28, 30 preferably act as leaf springs to press or bias the upper and 
lower tabs 22, 24 respectively against the adjacent corner of the adjacent 
cell housing 3. In addition, the support elements 14 facilitated alignment 
of the cell housings 3 in the array during assembly and serve to maintain 
the array integrity by opposing bowing of individual cell housings. Thus, 
when all the cell housings are aligned via support elements 14, any 
tendency of an individual cell housing to bow is opposed by the support 
elements in contact therewith and the adjacent cell housings which 
distribute the load due to bowing over the adjacent cell housings. As 
shown in FIG. 5B, One or both of the upper and lower arms 28, 30 can be 
curved or formed prior to installation to enhance the bias force of the 
pads of upper and lower tabs 22, 24 against the adjacent cell housing. For 
example, in FIG. 5B, lower arm 30 is curved or bent away from central tab 
26 in order to increase the pressure that lower tab 24 applies on the 
adjacent cell housing after it is installed. 
FIG. 5C shows a top view of a support element 14 installed between adjacent 
corners of two cell housings 3 and 3'. Tabs 26 of support element 14 are 
fastened the adjacent sides that form the corner of cell housing 3 and 
tabs 22 bear against the adjacent sides that form the corner of cell 
housing 3'. In the preferred embodiment, two support elements are 
installed between adjacent cell housings. Preferably, the two support 
elements are fastened at positions that are equally spaced from the ends 
of the cell housings and from each other. In one preferred embodiment, 
each cell housing is fabricated with two support elements on each of two 
adjacent corners and each of the cell housings is positioned in the array 
that forms the storage rack wherein the two corners that carry the support 
elements are oriented in the same direction. 
FIG. 6 shows a top view of storage rack 100 in accordance with an 
alternative embodiment of the invention. In this embodiment, the cell 
housings 103 are hexagonal in cross-section as opposed rectangular or 
square and the sides of the cell housings, as opposed to the corners of 
the cell housings are adjacent to one another. As one having ordinary 
skill in the art will appreciate, the cell housings 3, 103 can have any 
cross-sectional shape including round or circular cross-sections and 
polygonal cross-sections. In accordance with the invention, one or more 
support elements 114 are disposed between two adjacent cell housings. In 
this embodiment, the support element 114 is formed substantially as shown 
in FIG. 5A and the tabs are not bent or formed. The central tabs 126 are 
fastened to one side of cell housing 103 and the upper tab 122 and the 
lower tab (not shown) are pressed or biased against an adjacent side of 
cell housing 103'. In this configuration, the support elements 114 provide 
a coulomb damping function to reduce vibration and help align the cell 
housings in the matrix as described above. 
Depending upon the cross-sectional shape of the cell housings and geometry 
of the array, the features (e.g. a side or corner) of one cell may be 
adjacent to a different feature of an adjacent cell. Preferably, the first 
portion of the support element is adapted to conform to the surface 
contour and engage the feature or portion of the cell housing that it is 
to be fixed to. Thus, for example if the first portion is adapted to be 
fixed to an outside corner, the first portion is conformed with a 
complementary inside corner shape. Alternatively, if the first portion is 
adapted to be fixed to a flat surface or a curved surface, the first 
portion is substantially flat or curved, respectively to facilitate 
engagement. Similarly, the second portion is adapted to conform to the 
surface contour of the feature to which it is to engage. In addition, 
either the second portion or the feature (or both) may be further adapted 
to change the friction between them in order to change the coulomb damping 
performance. For example the surface of the second portion or the feature 
may be textured and/or coated and/or plated with a material to change the 
frictional characteristics and the coulomb damping performance. 
Alternatively, a friction enhancing pad may be fixed to either the second 
portion or the feature (or both). 
One of ordinary skill will appreciate that in all embodiments of the 
invention including those shown in FIGS. 1-6, that the cell housings 3, 
103 can be provided with neutron absorbing or "poison" material 4, 104. 
The poison material 4, 104 can be either fixed to the cell housing or the 
poison material 4, 104 can be held in place by clips or a cover plate as 
disclosed in commonly owned U.S. Pat. No. 5,348,813. In addition, as one 
of ordinary skill will appreciate, the number, size and configuration of 
support elements 14, 114 can be readily determined as a function of the 
desired level of vibration damping. Furthermore, while specific 
embodiments have disclosed having either corner bearing or side bearing 
support elements, one of ordinary skill in the art will appreciate that 
both corner bearing and side bearing support elements can be used in the 
same rack assembly, depending upon the level of vibration damping 
required. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in respects as illustrative and 
not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of the equivalency of the claims 
are therefore intended to be embraced therein.