Storage rack for the storage of fuel elements of nuclear reactors

Storage rack for nuclear fuel elements, consisting of a lattice construction, constituted by a first arrangement of parallel, vertical and equally spaced relatively long sheet elements, having sharply shaped contours and a second arrangement of parallel, vertical equally spaced relatively short aligned sheet elements, having sharply shaped contours, each bridging the distance between the relatively long sheet elements, such that a plurality of adjoining vertical storage cases is formed, having a rectangular horizontal cross section, each for receiving one nuclear fuel element. The relatively short sheet elements are provided with lugs at their opposite long edges engaging the surface of the relatively long sheet elements, said lugs fit in accurately formed slots in the relatively long sheet elements for connecting the sheet elements by means of a keying.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a storage rack for the storage of fuel elements of 
nuclear reactors consisting of a sheet metal lattice arrangement 
constituting a plurality of abutting similar vertical storage cases or 
tubes having in general a rectangular cross section. In anyone of the 
cases of this storage rack a rod-shaped fuel element may be stored. 
2. Description of the Prior Art 
Within or in the neighbourhood of the reactor building there are available 
storage pools for storing spent or new thermal reactor fuel elements 
during a long or only a short period. Therein the fuel elements are placed 
in racks below water in such an arrangement that the necessary heat 
dissipation is warranted and no nuclear chain reaction may occur. 
Generally for such a design there are included very broad margins with 
respect to the mutual distances between the elements. When the storage 
capacity of such pools has been exhausted and the transfer of the spent 
fuel elements to a regeneration plant is not yet possible a temporary 
solution is found by placing the rods closer to each other in the pool in 
which case however, much more attention should be given to warrant 
sufficient criticality margins for the ensemble. 
A possibility for reducing the mutual distance or the pitch with which the 
fuel elements are being placed is the inclusion in the construction of a 
so-called neutron poison, i.e. a material having a very high effective 
cross section value for the absorption of neutrons. Such a material is the 
boron isotope B.sup.10 that may be included as an alloying component in 
the stainless steel, mostly used for the manufacture of these racks. When 
utilising such a material for the manufacture of the storage racks as a 
whole or part thereof the possibility arises of a much smaller pitch of 
the fuel elements and consequently of a much more compact storage than in 
the conventional racks. 
From nuclear physic calculations it is apparent that dependent on the 
chosen construction, the geometry of the ensemble and the specific fuel 
element, there may be found an optimum ratio of the construction material 
and the surrounding water with respect to the fuel elements. In case of 
the storage racks of the above-mentioned type there exists therefore a 
certain interstice between the cases determining the final storage 
capacity. 
At the found optimum any further increase of the amount of boron, i.e. the 
increase of the thickness of the borated sheet steel of the cases does not 
lead to an increase of the absorption capacity and consequently not to a 
decrease of the water gap or the mutual distance between the elements; 
that is to say that when the "saturation value" of the boron content is 
reached a decrease of the mutual distance between the elements will bring 
the ensemble closer to the criticality. Hence the effective multiplication 
factor will more closely approach the limit value usually assumed at 0.95. 
In case of storage rack arrangements including a neutron poison the water 
gap between the fuel rods play an important role as a moderator for the 
neutrons leading to a relatively high peak of the neutron flux between the 
said cases. This "surge" of the neutron flux at the location of the gap 
and consequently also in the case walls of borated steel leads to an 
increased capture of neutrons and consequently to a decrease of the 
multiplication factor of the ensemble. 
Also in case of a rack not provided with a neutron poison a decrease of the 
mutual distance between the elements leads to an increase of the 
multiplication factor after all. 
In case of a given rack construction and given fuel elements a theoretical 
minimum distance or gap width constituted by metal and water should be 
maintained between the fuel elements in order to satisfy the criterion for 
the multiplication factor. 
However, theoretical gap widths may only be applied when the storage rack 
may be manufactured within very close tolerances. 
Up till now these storage racks have been made as a welded construction. 
Notwithstanding the use of welding jigs the accurate gage maintenance and 
the attainable straightness of the cases and other construction parts 
constituting the storage rack are limited by the deformations inherent to 
each welding process. Therefore in case of a welded construction the 
theoretically permissible minimum gap width has to be increased with 
additional safety margins in view of the necessary manufacture tolerances. 
SUMMARY OF THE INVENTION 
The invention now provides a storage rack of the above-mentioned type, the 
manufacture of which may be carried out within very close tolerances, 
whereby the minimum distance between the fuel elements will nearly not be 
affected by the manufacturing process. If for a certain fuel element 
having a length of for instance 4 m the width of the water gap may be for 
instance 25 mm, the tolerance in case of a welded construction with 
respect to the case walls will be .+-.3 mm. Due to both the adjoining 
cases the tolerance with respect to the water gap will consequently be 
.+-.6 mm or about 25%. 
In accordance with the present invention for fuel elements having the same 
length, these tolerances will be .+-.5 mm with respect to the case walls 
and 1 mm with respect to the water gap, this corresponding with 4%. 
In the storage rack according to the present invention the lattice 
arrangement has been constructed of a plurality of mutually substantially 
perpendicular sheet elements having sharply shaped contours, all sheet 
elements in one of the two perpendicular directions completely or 
partially traversing the lattice arrangement at a mutual distance 
substantially corresponding to the width of a storage case and all sheet 
elements in the other of the two perpendicular directions each extending 
only substantially over the width of one storage case, the latter narrow 
sheet elements at the opposite edges thereof having been provided with 
lugs fitting in accurately formed openings in the traversing sheet 
elements, said lugs having been immobilized by means of a keying, the 
arrangement being such that the long edges of the narrow sheet elements 
may be pressed tightly against the traversing sheet elements. 
In accordance with one preferred embodiment of the invention all storage 
cases within the lattice arrangement have been separated from each other 
by a wall consisting of two parallel sheet elements at a short distance 
from each other inclusive the interstice between these sheet elements, the 
said interstice housing the lugs and the keyings. 
The keyings may then be embodied in such a manner that of the narrow sheet 
elements constituting together a partition wall, the lugs have been 
provided at the same level, said lugs having been connected with each 
other by means of a connecting strip running parallel to the traversing 
sheet elements and that between this connecting strip and the opposite 
wall of the cooperating traversing sheet element a key has been pinched. 
The connecting strips between the lugs may be secured by welding. These 
welds are no such critical welds that these will cause a deformation of 
the rack construction. 
Moreover each lug may be provided with an opening having an edge running 
parallel to the wall of a cooperating traversing sheet element while a key 
may be pinched between this edge and the wall of the traversing sheet 
element. In general the lugs will consist of an extended part of the sheet 
elements and slidably fit into slots in the cooperating sheet elements. 
For securing after assembling, all keys may be welded to the traversing 
sheet elements, the connecting strips and the lugs, respectively. These 
welds are not critical either. Such securing may also be attained by 
deformation of the keys. 
In a rectangular lattice arrangement the storage cases at each of the four 
outer walls of the lattice may be enclosed by a traversing sheet element 
extending over the entire side wall, while the sheet elements of the side 
walls extending parallel to the narrow sheet elements have been provided 
with slots for slidably receiving lugs formed on the edges of the 
traversing sheet elements perpendicular thereto, which lugs may then be 
fastened by means of a keying as described above. 
For the present invention it is in particular of importance that there are 
manufactured sheet elements having very accurate contours and openings for 
the lugs, for instance with an accuracy up to 0.1 mm. Hereby and by the 
use of keyings instead of welding combined with bending there may be 
obtained particularly dimensionally stable constructions throughout the 
entire height and width of the storage rack. The said accuracy applies in 
particular to the longitudinal edges of the sheet elements. 
By the use of the lattice arrangement according to the present invention in 
combination with a particular locking construction for the top plate and 
the bottom plate of the lattice, wherein no welding joints are employed 
either, there is provided a storage rack having a very stable 
construction. 
The storage racks according to the present invention entail the further 
advantages of enabling a very accurate prefabrication of all sheet 
elements, an easy cleaning of all parts of the storage rack separately, 
this being a necessity for the storage of nuclear fuel elements. Further 
the absence of welds, always resulting in uncertain material properties, 
excludes all worries about structural changes when exposed to the ionising 
radiation of the fuel elements. 
By keyings a very accurate assembly is possible. For the maintenance of the 
structural shape of the arrangement there is no necessity to use expensive 
assembly jigs as are required when welding. The keyings and the special 
construction of the storage rack by means of traversing and narrow sheet 
elements furthermore open the possibility of a rapid and simple exchange 
of parts of the rack. 
Moreover separate parts of a rack may easily be controlled with respect to 
the cleanness thereof. There are no critical welding constructions whereby 
expensive weld controls may be omitted and possible faults are eliminated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1 there is shown a plan view of a number of storage 
racks indicated with the Roman FIGS. I-XI, respectively, arranged in a 
water pool enclosed in a tank 1 of concrete. In anyone of the spaces 
traced by a square drawn in the racks a fuel element may be stored. 
FIG. 2 shows a portion of a plan view of the rack indicated in FIG. 1 by 
XI, in which the top plate of the rack has been removed. The rack has been 
constructed of long traversing sheet elements 3 and narrow sheet elements 
2 arranged between the aforesaid sheet elements 3. Hence the walls between 
the several cases consist of two parallel sheets including an interstice, 
said interstice as well as the case per se being filled with water. These 
two parallel sheets including the water gap determine the minimum safety 
distance between the fuel elements. Preferably the sheet elements consist 
of stainless steel having a certain boron content. The sheets of borated 
stainless steel absorb neutrons whereas the water gap constitutes the 
moderator for the neutrons. 
As is apparent from FIG. 2 the sheets 3 traverse the entire length of the 
storage rack arrangement. Between these sheets 3 there are shown keyings 
generally indicated by the reference numeral 4 which keyings will be 
described below in further detail. 
At their extremities the traversing sheets 3 have been screwed on bars or 
fillets 5. These fillets 5 extend over the entire height of the rack and 
have been screwed at the extremities thereof to a bottom plate and a top 
plate not shown in FIG. 2. 
Outside the outer wall 6 of the rack the narrow sheet elements extending 
through this outer wall have been screwed by means of lugs 8 to 
corresponding bars or fillets 5 likewise extending over the entire height 
of the storage rack. For that purpose the lugs 8 run through slots in the 
outer wall 6. This connection is likewise described below in further 
detail. 
As shown in FIG. 2 the outer wall 7 consists of separate narrow sheet 
elements provided with lugs slidably fitting into slots close to the outer 
edges of the traversing sheet 3. However, this outer wall 7 may also 
consist of a single long traversing sheet element in which case the 
traversing sheet elements 3 have been provided with lugs 8 slidably 
fitting into slots in the sheet element 7. 
Referring to FIGS. 3 and 5 there as shown details of an example of a keying 
4, by means of which the narrow sheet elements 2 may be fastened to the 
traversing sheet elements 3. At the same level along the longitudinal edge 
all narrow sheet elements have been provided with lugs 9 slidably fitting 
into slots 10 of which there has been shown only one in FIG. 3 at the left 
hand bottom part. With reference to the above description it will be clear 
that each intermediate wall consists of two parallel sheet elements 2 and 
3. As shown in the plan view of FIG. 5 the lugs present at the same level 
on the narrow sheet elements 2 extend to the slots at the same level in 
the traversing sheet elements 3. As shown in FIG. 3 these lugs have been 
arranged on one line so that only one lug of each keying has been shown. 
Between each two lugs 9 of a wall there have been provided connecting 
strips which may be slid through the slots 12 in the lugs. These may then 
be secured by means of an edge weld or by deformation. It is evident that 
the separate small welds do not affect the shape of the rack arrangement 
in total. Keys 13 have been slid and secured between these connecting 
strips 11 and the adjoining wall of the traversing sheets 3. These may be 
likewise secured by means of welds or deformation. 
With reference to FIG. 3 it will be evident that the traversing sheets 3 at 
the lower edges rest into slots 16 in the bottom plate 14. The lower edges 
of the narrow sheet elements 2 do not engage the bottom plate 14. At the 
upper edges each two traversing sheet elements of a partition wall include 
part of a beam belonging to the top plate or the upper frame 15. 
FIG. 4 shows the manner in which the bottom plate 14 and the upper frame 15 
are secured to the narrow sheet elements 2. If desired however, this 
arrangement may also be applied to the traversing sheet elements 3. In 
each of the narrow sheet elements 2 there has been provided a slot 25 in 
the lower part as well as in the upper part thereof. Lugs extending from 
the blocks 17 fit into this slot 25. The said blocks have been provided 
with a boring having an internal thread in which a screw 18 may be 
screwed. By means of these screws 18 the narrow sheet elements 2 may be 
drawn onto the bottom plate 14 and onto the upper frame 15. Consequently 
the traversing sheet elements 3 inclusive the lower edges thereof are 
firmly pressed into the slot 16, whereas the upper edges thereof are going 
to rest against the shoulders 19 of an upper frame beam 15. The force thus 
exerted on the narrow sheets 2 is transferred by means of the keyings 4 to 
the traversing sheets 3. 
FIGS. 5 and 6 also show the pulling bars or fillets 5 outside the lattice 
arrangement. Onto these fillets 5 there have been screwed eyes of narrow 
sheet elements 2 extending to slots through the side wall of the rack 
consisting of a single traversing sheet element 3. On the other hand edges 
of the traversing sheets 3 have been screwed through the beams 5. These 
beams 5 have been provided with short bores at both the lower part and the 
upper part thereof, by means of which these beams or fillets 5 may be 
screwed against the bottom plate 14 and the upper frame 15, vide FIG. 6. 
This moreover contributes to a very stable rack arrangement. 
In accordance with the embodiment of FIGS. 3 and 5 each side wall 7 of a 
single case consists of a single narrow sheet element. This side wall 7 
may, however, also consist of a traversing sheet element through which 
lugs will extend formed on the upper edges of the traversing sheet 
elements 3. The lugs have been fastened to the fillet 5 by means of 
screws. 
In accordance with the storage rack arrangement of FIG. 7 the intermediate 
walls have different widths. This possibility is created by embodying the 
keyings of the lugs of the narrow sheet elements 2 onto the traversing 
sheet elements 3 in different ways. In accordance with this arrangement 
there are employed narrow sheet elements 2 having lugs in the interstice 
between two sheet elements 3 not confronting each other but offset with 
respect to each other as shown in FIG. 8. These lugs 8 have been provided 
with slots also parallel to the traversing sheet elements 3, whereas an 
edge of these slots extends to the interior between the two parallel sheet 
elements 3. Between this edge and the opposite wall of the sheet element 3 
a wedge 13 is pinched. Upon securing these wedges 13 they may be fastened 
by means of an edge weld to the puncheon 9 or by deformation. 
The traversing sheets 3 again rest in a slot in the bottom plate at 16. 
Where in this case the distance between these plates 3 is much smaller 
there is employed a single broad traversing slot instead of two separate 
slots as shown for the embodiment of FIG. 2. 
The way of securing the bottom plate 14 and the top plate not shown in 
these figures may be carried out in the same manner as shown for the 
embodiment in accordance with FIGS. 2 through 6 inclusive. With reference 
to FIGS. 8 and 9 there have been shown metal blocks 17 between the narrow 
sheet elements 2 the extending lugs of the blocks running to the opposite 
slots 25 provided on the lower part and, if necessary, also on the upper 
part of the narrow sheet elements 2 together constituting the intermediate 
wall. These blocks 17 on their turn have been provided with threaded bores 
in which a screw 18 may be screwed by means of which the sheet elements 2 
may be pulled against the bottom plate 14, and, if required, against the 
top frame 15. Then the protruding edges of the traversing sheet elements 3 
will come to rest into the slots 16 of the bottom plate and if necessary, 
into the slots of the top plate or against the shoulders of a top frame 
constituted by traverse means. 
With reference to FIGS. 10 and 11 there have been shown outside views of a 
lower edge and of the narrow lower part, respectively, of a rack in 
accordance with the embodiment of FIG. 7. 
The bottom plate 14 rests on a metal base 20 or has been united therewith 
to a single structure. In view of the fact that in accordance with this 
embodiment the traversing sheet elements 3 are closer to each other than 
in accordance to the embodiment of FIG. 2 there are employed in this case 
narrow pull rods 5 instead of the broad traversing side beams of the 
embodiment of FIG. 2. These rods have been provided at their bottom and 
the top ends with an external screw thread. These extremities run through 
openings in the bottom plate and in the top plate, which extremities are 
fastened by means of nuts 21. The narrow pull bars are provided between 
the lugs 8 of the narrow and long side walls, respectively. The lugs 
present on the two extremities of a pull bar 5 belong to the two parallel 
sheet elements of an interior wall. In view of the fact that in accordance 
with this embodiment the narrow sheet elements 2 are much further apart 
than the traversing sheet elements 3 there have been provided as is 
evident from FIG. 11, strips 22 instead of pull bars 5 on the narrow side 
wall. The same applies to the pull bars 5 which may be welded to the 
extremities of the lugs 8. In that case the pull bars 5 at the broad side 
walls of the rack arrangement will then preferably also have a rectangular 
cross section excluding the extremities thereof provided with a screw 
thread. The strips 22 may then be pulled against the bottom plate 14 by 
means of a hook joint. This hook joint may also be employed for the entire 
rack arrangement. 
With reference to FIG. 12 there has been shown another detail of the rack 
arrangement in accordance with FIG. 7, that is a view partially in cross 
section of the lower left hand part of the rack arrangement including the 
carrier construction. In each storage case the bottom 14 has been provided 
with a circular opening 23. In the other parts of this bottom plate 14 
there have been provided slots 16 wherein the lower edges of the 
traversing sheet elements 3 will rest in case they are pulled against the 
bottom plate 14 by means of the screws 18 and blocks 17 of FIGS. 8 and 9. 
It goes without saying that the present invention is not limited to the 
embodiments shown in FIGS. 2 through 12 inclusive and that modifications 
and alterations are possible without surpassing the scope of the 
invention. In this respect FIG. 13 schematically shows a plan view of 
another embodiment of the rack arrangement according to the invention. In 
accordance with this embodiment the intermediate walls between the several 
cases are solely provided by a single sheet. In this case these cases are 
constituted by the traversing sheet elements 3 and the narrow sheet 
elements 2 mounted transversely thereto. 
Between the several traversing sheet elements 3 the narrow sheet elements 2 
have been shifted half a pitch with respect to each other, i.e. half the 
width of the rectangular case. By means of lugs 9 the narrow sheet 
elements 2 run through slots in the traversing sheet elements 3. As 
described with respect to the above embodiments the lugs 9 also possess 
slots into which keys may be slid and pinched. In accordance with this 
embodiment the above described interior wall consists of only one sheet, 
although the distance between the several fuel elements, one of which has 
schematically between indicated at 24, remains sufficiently large, that is 
on the one hand because this rod rests against the outer edge of the lugs 
9 of the narrow sheet elements 2 running through the slots in the 
traversing sheet elements 3 into the interior of the case. On the other 
hand on the walls of the narrow sheet elements 2 there have been provided 
strips or pins thus securing that the fuel element will be supported on 
the four longitudinal sides. When employing this embodiment the bottom 
plate and the top plate may be provided with slots for all sheet elements. 
Also in this case a particularly firm rack arrangement is warranted 
serviceing the minimum safety requirements. 
Contrary to a welded rack the storage rack in accordance with the invention 
may be assembled of prefabricated cleaned parts at the site of the storage 
pool within the reactor building, thus eliminating the transport of the 
complete voluminous rack arrangement from factory to the pool.