Structure assemblable and disassemblable on a racking site for the storage of nuclear reactor fuel elements

An assemblable and disassemblable racking structure for the storage of radioactive fuel elements, comprising the juxtapositioning of vertical, parallelepipedic racks, at least two meshed networks of horizontal bars, one fixed to a base plate and the other assembled with the upper part of the racks, horizontal strips fixed to at least two sides of the rack level with each network and cooperating with the bars of each network in order to fix the racks, the assembly of the bars of the networks to one another and the bars of the lower network to the base plate being brought about by dismantlable fixing means. The invention also relats to the process for assembling the structure.

FIELD OF THE INVENTION 
The present invention relates to a structure which can be assembled and 
disassembled on a racking site for the storage of nuclear reactor fuel 
elements, of the type comprising juxtapositioning of vertical, 
parallelepipedic racks which are open at their ends. 
Such storage rack systems are used for storing the maximum number of 
nuclear reactor fuel elements in a minimum of space. Conventionally, these 
elements are separately placed in metal racks arranged in a support 
chassis. The storage and relative proximity of the fuel elements must be 
such as to prevent them from reaching their criticality level, and for 
this reason there must be a minimum spacing between the individual racks. 
This spacing is dependent on the type of fuel element stored, the 
protection offered by the racks and the ambient storage medium (dry or in 
a pool). The chassis must ensure this minimum spacing and must be 
sufficiently nondeformable and rigid to withstand the stresses due e.g. to 
earth tremors. 
BACKGROUND OF THE INVENTION 
The racking structures for the storage of nuclear reactor fuel elements 
according to the prior art generally comprise a network of metal beams or 
girders which are welded to the racks. For example, they comprise two 
systems of intersecting beams forming two grids into which the racks it. 
Two metal enclosures maintain the peripherally positioned racks in place. 
All the components of these structures are mainly welded. This type of 
construction is described in French Pat. Nos. 2,462,767 and 2,509,899 and 
U.S. Pat. No. 4,187,433. 
However, these structures cause problems. If they are assembled in the 
workshop, their transportation to the site is difficult due to their large 
overall dimensions and weight. If they are assembled on site, welding 
takes a long time and is expensive, while making their final geometry more 
random due to thermal stresses. The quality of the welds is difficult to 
inspect on site. It is also difficult to dismantle the structure without 
seriously damaging it. 
SUMMARY OF THE INVENTION 
The present invention relates to a racking structure for the storage of 
nuclear reactor fuel elements of the type comprising a juxtapositioning of 
vertical parallelelepipedic racks which are open at their ends, making it 
possible to obviate the aforementioned difficulties and comprising: 
a horizontal base plate, 
at least two meshed networks of horizontal bars, one called the lower 
network, fixed to the base plate, the other assembled to the upper part of 
the racks, ensuring the vertical maintenace and transverse rigidity 
thereof by gripping in the meshes of each network, 
horizontal strips, respectively above and below the bars of the network, 
fixed to at least two sides of each rack level with each network, so as to 
cooperate with the bars in order to ensure the locking in vertical 
translation of each rack, 
means making it possible to hold the racks located on the periphery of the 
racking structure against the corresponding bars, 
the assembly of the bars of the networks with one another and the bars of 
the lower network to the base plate being ensured by dismantlable fixing 
means. 
According to a preferred embodiment of the assemblable and disassemblable 
storage racking structure according to the invention, each of the racks 
comprises a tube with four lateral faces, which are parallel to one 
another in pairs and which define two pairs of parallel faces, and wherein 
the networks are in each case constituted by two lines of parallel bars 
defining meshes in which the said racks it, one of the lines, called the 
upper line, resting on the first pair of strips fixed to a first pair of 
parallel faces, while the second or loewr line bears under a second pair 
of strips to the other pair of parallel faces, so that there is a certain 
clearance between the bars of the upper line and the bars of the lower 
line, dismantlable fixing means connecting each of the lines of the same 
network, so as to grip the strips between the corresponding bars and fix 
the racks in the said meshes. 
Advantageously, the means making it possible to hold the racks located on 
the periphery of the structure incorporate securing means fixed to the two 
ends of the bars of each network.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a storage assembly grouping several storage racking structures 
S.sub.1, S.sub.2, S.sub.3, S.sub.4, etc., according to the invention. For 
example, during storage in a pool, several structures according to the 
invention are fitted and can easily be secured. They are then arranged by 
juxtapositioning them, so as to form the storage assembly. FIG. 1 more 
specifically constitutes an exploded view of an assemblable and 
disassemblable structure S.sub.1 for the storage racking system of nuclear 
reactor fuel elements according to the invention. 
It firstly comprises a juxtapositioned arrangement of vertical, 
parallelepipedic metal racks 1, which are open at their ends. The main 
object of the structure according to the invention is to arrange said 
racks 1 and to rigidly secure the same, so that the connections between 
the racks and the networks are embedded or tailed-in structures. In order 
to satisfy this objective, the structure comprises a horizontal base plate 
2, to which is fixed a lower network 4 of horizontal, parallel bars. At 
least one further network 6 of horizontal, parallel bars is assembled to 
the upper part of the racks. Thus, the racks are located in the meshes 8 
defined by the networks 4, 6, the spacing between the racks being ensured 
by the dimensions of the bars. 
Retaining parts 10a, 10b are fixed to the end of each horizontal, parallel 
bar, so that the racks located on the periphery of the structure can be 
applied to the corresponding bars. By way of example, two of such 
retaining parts 10a, 10b are shown in FIG. 1. 
The means used for fixing the racks to the lower network 4 and base plate 2 
and for fixing the other network 6 to the racks are shown in FIG. 2, in 
accordance with the preferred embodiment of the invention. FIG. 2 shows 
one rack 1. It is formed from a tube with four lateral faces, which are 
parallel to one another in pairs and which define two pairs of parallel 
faces. Its upper end widens in such a way that the fuel element can easily 
be placed in the rack. 
Two meshed networks 4, 6 are shown, namely, a lower network 4 and an upper 
network 6. Each of them comprises two pairs 12, 14 of horizontal, parallel 
bars, one being called the lower line 12 and the other the upper line 14. 
The parallel bars of the lower line 12 are orthogonal with respect to the 
bars of the upper line 14, thus, defining the rectangular meshes 8 in 
which are placed the racks. 
Level with each network 4, 6 in FIG. 2, the parallel bars of the upper 
lines 14 rest on a first pair 16 of strips fixed to a first pair of 
parallel faces of the rack. In the same way, a second pair 18 of strips 
fixed to a second pair of parallel faces of the rack bear against the 
parallel bars of the lower line 12. The two pairs 16, 18 of strips are 
fixed in such a way that there is a certain clearance between the bars of 
the upper line 14 and the bars of the lower line 12. 
Assembly screws 20 pass through the bars of the same network level with 
their intersections. For simplicity reasons, only one screw 20 is shown in 
FIG. 2. The connecting stresses exerted by these screws 20 are applied in 
the following way. The parallel bars of the upper line 14 bear against the 
first pair 16 of strips which is itself integral with the second pair 18 
of strips via the rack, said second pair 18 of strips bearing on the 
parallel bars of the lower line 12. 
At the lower network 4, the assembly screws 20 also pass through base plate 
2, on which rests the lower line 12 of parallel bars. This prevents 
vertical translation of the rack, whose connection with base plate 2 is of 
the tailed-in type. 
At the other networks, in this case the upper network 6, the pairs 16, 18 
of strips are gripped between lines 12, 14 of corresponding parallel bars, 
thus preventing vertical translation of the upper network 6 with respect 
to the rack and thus producing an embedding or tailing-in effect between 
them. 
Advantageously, the structure is rigidified, by fixing to each end of the 
parallel bars a securing element 10a, 10b, which makes it possible to 
secure the racks located at the periphery of the structure and to lock 
them to the corresponding bars (FIG. 1). 
In this way, a rigid, assemblable and disassemblable storage racking 
structure is obtained, which is able to withstand earth tremors, and it is 
possible to precisely position the racks in accordance with the imposed 
spacing. 
Obviously, many modifications are possible. For example, the number of 
racks and networks shown is in no way limitative and is dependent on the 
storage requirements. It is also possible to use other means for securing 
the racks to the periphery of the structure, e.g. a metal belt gripping 
said racks in a detachable manner. In addition, each horizontal strip can 
comprise one or more pieces. In the same way, the parallel bars of the 
lower line 12 of the lower network 4 fixed to base plate 2 can be 
subdivided into several parts. It is also possible to use bars with 
different shapes, e.g., solid, hollow, U-shaped, etc. 
The structure according to the invention offers the following advantages. 
In the normal case of racks welded to the base plate, the connection 
obtained is not of the tailed-in type, although one of the main features 
of the structure according to the invention is to bring about a tailing-in 
of the racks on to the base plate via the lower network of parallel bars, 
pairs of strips and fixing means. In the same way, there is a tailing-in 
effect between the racks and the upper network of parallel bars by means 
of pairs of strips and corresponding fixing means. Thus, the structure 
according to the invention makes it possible for the racks to effectively 
participate in rigidifying the storage racking system. Assembly can be 
carried out on site, which makes it possible to transport the parts in the 
unassembled state, to save space and weight and facilitate transport. 
There is no need for specialized assembly buildings. Cleaning and 
passivation are facilitated, because they can be carried out on small 
volume components (bars and plates) and not on a previously assembled 
structure. There are better guarantees regarding the final quality 
obtained. Thus, because no welding operations are carried out, there is no 
need to X-ray the welds as is normally the case, and in addition the 
physical and chemical characteristics of the materials used are not 
deteriorated by overheating due to welding. The absence of welding 
operations eliminates thermal stresses and prevents the dimensional 
accumulation of assembly errors and expansions. The bars according to the 
invention are previously accurately perforated in the workshop, which 
makes it possible to respect the desired dimensions of the racking system, 
the minimum spacing between each rack and the overall dimensions of the 
racking system. It is possible to dismantle and work on one or more 
damaged racks without any deterioration to the overall structure. 
An assembly phase of the stroage racking system according to the invention 
is shown in FIG. 3, and the assembly process comprises: 
installing assembly jigs (not shown) for the vertical positioning of base 
plate 2 and each lower line 12 of vertically straightened parallel bars, 
positioning the base plate 2 and the lower lines 12 of parallel bars, 
horizontally fitting a first row of racks 1. 
FIG. 4 shows that the second pair 18 of strips of each rack 1 is applied to 
the corresponding parallel bars of the lower lines 12 by 
placing the corresponding bar of the upper line 14 level with each network 
4, 6, it being applied to the first pair 16 of pairs of each rack 4 (FIG. 
4); 
fixing by means of screws 2 the parallel bars of the upper lines 14 to the 
parallel bars of the lower lines 12 by gripping the pairs 16, 18 of strips 
between them, the parallel bars of the lower line 12 of the lower network 
4 also being fixed to the base plate 2, 
repeating the operation of installing a row of racks and fixing the bars of 
the corresponding upper line until the desired number of rows is obtained; 
fixing the securing parts 10a, 10b to each end of the bars of the 
structure, so as to obtain a non-deformable., rigid structure (FIG. 1). 
It is then possible to vertically straighten the structure and place it at 
its intended location in the storage assembly on the site. Disassembly of 
the structure according to the invention takes place by performing the 
assembly operations in precisely the reverse order. 
Compared with known procedures, this assembly process has the advantage of 
being fast, respecting the dimensional characteristics imposed and 
consequently permitting the production of a storage racking system at a 
cost well below that conventionally encountered in such cases. 
A description has been given hereinbefore of an assembly process in which 
the racks are installed horizontally. However, it is possible to 
vertically assemble the structure, by carrying out the same operations as 
described hereinbefore. The assembly jigs are then modified to keep the 
upper network of parallel bars horizontal.