Patent Number: 
Section: description

FIG. 1 illustrates a storage basket for radioactive materials according to a first preferred embodiment of the invention. This storage basket comprises several tubes 10, several cross pieces 12 (FIG. 2) placed between the tubes 10, and assembly means 14 making the assembly fit together and making it compact. As already mentioned, the radioactive materials that could be transported in the basket may be of any nature whatsoever. In particular, they may be fuel elements from nuclear reactors with a square or hexagonal cross section. The tubes 10 are straight metallic tubes and are all identical. In particular, all tubes 10 have the same section and the same length and they are made of the same material. The assembly means 14 contain tubes 10 in a bundle, parallel to each other, along a regular network. More precisely, the bundle of tubes 10 is designed to be placed vertically. The result is several adjacent compartments 16, each of which can contain conditioned radioactive materials, for example such as fuel elements from nuclear reactors, for transport and/or storage purposes. Each of the tubes 10 materialises a first wall of the corresponding compartment. This first wall surrounds the compartment, almost without any discontinuity, over its entire height and around its entire periphery. The cross pieces 12 are metallic parts formed of a number of plane plates 18 called xe2x80x9cflangesxe2x80x9d, usually without any openings. All flanges 18 for a single cross piece 12 are connected to each other by a straight common edge and are distributed at equal angular intervals around the said edge. In the embodiment illustrated in FIG. 1, the length of the cross pieces 12 is approximately equal to the length of the tubes 10. In one variant embodiment (not shown) each cross piece 12 is formed from several segments of cross pieces placed end to end. The total length of these segments is then approximately equal to the length of the tubes 10. Each of the cross pieces 12 is associated with a group of adjacent tubes 10, the number and layout of which depends on the prismatic cross section of tubes and the shape of the network formed by the tube bundle. The common edge of the cross piece is placed at the centre of the group of tubes 10 and is laid out parallel to the centre lines of the tubes. The number of the flanges 18 of the cross piece 12 is equal to the number of the tubes 10 in the said group. Thus, a flange 18 of the cross piece 12 is placed between each pair of adjacent tubes in the group of tubes 10. The flanges 18 of each cross piece 12 extend between the tubes 10 such that the outside edge of each flange 18 opposite the common edge is parallel to the said edge and is in contact with the outside edge of the flange of the adjacent cross piece 12 placed between the same pair of tubes 10. The cross pieces 12 thus materialize a second approximately continuous wall around each of the compartments 16. Furthermore, the assembly produced by assembly means 14 is such that each of the flanges 18 of the cross pieces 12 is in contact with each of the tubes 10 placed on each side of this flange, over practically the entire height of the basket. The second wall is materialized by cross pieces 12 around each compartment 16 is therefore at least in partial contact with the first wall materialized by the tubes 10. In the embodiment illustrated in more detail in FIGS. 1 and 2, each of the tubes has a prismatic square section. The regular network formed by the bundle of tubes 10 is then a network with a square pitch, such that each group of tubes is formed from four adjacent tubes 10. According to the rule mentioned above, each cross piece 12 then comprises four flanges 18, along two directions orthogonal to each other. This layout, as clearly shown in FIGS. 1 and 2, enables a complete surface contact between the flanges 18 of the cross pieces 12 and the adjacent faces of the tubes 10. As mentioned above, different layouts are possible to enable a reliable contact between the outside edges of the flanges 18 opposite the common edges of the cross pieces 12. According to a first layout illustrated in FIG. 3A, the outside edges of the flanges 18 are provided with flats 20, approximately parallel to the planes of the flanges. More precisely, the flats 20 are located in the median plane of the flanges and oriented to face each other when two adjacent cross pieces 12 are in position. The flats 20 are then in contact with each other over most of the length of their surfaces. This layout is a means of increasing the contact surfaces between the flanges 18 of the cross pieces 12, and consequently facilitating transfer of heat flux from one cross piece to the next. According to a second layout illustrated in FIG. 3B, the outside edges of the flanges of the cross pieces 12 opposite the central edge have complementary shapes on adjacent cross pieces, such that they fit into one another. These complementary shapes may for example be a tenon shape 22 and a mortise shape 24. This layout provides the same advantages as the above, plus a nesting effect that further increases contacts and the propagation of heat flux. In the basket according to the invention, the main function of the tubes 10 forming the first walls of the compartments is to provide the basket with mechanical strength, both under normal conditions of use and under accidental conditions (for example when the basket is dropped). The geometry of the basket is thus maintained under all circumstances, which helps to maintain control over nuclear criticality. To enable them to perform this function efficiently, the tubes 10 are advantageously made of a resistant material, preferably chosen among the group comprising stainless steel, carbon steel, aluminium and aluminium alloys with good mechanical properties, and titanium. This list of materials is in no way limitative. The tubes 10 may be obtained using any manufacturing technique, for example by rolling or folding a plate to the required shape, and then closing it with a longitudinal weld. In the case of aluminium tubes, usual extrusion techniques are advantageously used, such that seamless tubes of any shapes and dimensions can be obtained. In the basket according to the invention, the function to evacuate heat produced by the radioactive material is achieved mainly by the cross pieces 12. This function is particularly important when the radioactive materials are fuel elements that have been irradiated in nuclear reactors since these elements emit high thermal power. In the layout according to the invention, the heat flux produced by the radioactive materials is transmitted firstly to the metal in the cross pieces 12 by the metal in the tubes 10 forming the first walls. This transmission is facilitated by the fact that the flanges 18 of the cross pieces 12 forming the second walls of the compartments are held immediately adjacent to the walls of the tubes 10 forming the first walls of the compartments, in order to facilitate heat transfer. The efficiency of this thermal transfer is advantageously increased by choosing the material used for the cross pieces from good heat conducting metals such as copper and its alloys, or aluminium and its alloys. The heat flux then passes into the material of the cross pieces 12 that are in contact with each other through their outside edges, from the inside towards the outside of the basket. In particular, due to the contact made between the outside edges of the flanges of the cross pieces, the heat flux passes towards the outside of the basket almost without interruption and therefore without encountering any high thermal resistances that could cause an excessive increase in the inside temperature of the basket. The heat flux is then dissipated into the atmosphere or into the structure of a transport or storage container in which the basket is placed. When the basket is full of fissile radioactive materials that could cause a chain reaction, its components must also fulfil a third essential function, which is to control nuclear criticality. This control is achieved firstly by the mechanical strength of the basket, obtained due to the mechanical strength of the tubes 10 forming the first walls of the compartments as described above. Nuclear criticality is also controlled partly by adding neutron poisons such as boron or cadmium into the structure of the basket. These poisons may be incorporated in dispersed form, either in the metal of the tubes 10 or in the metal of the cross pieces 12 or in both of these components at the same time. Alternately, the poison may also be added in the form of a layer of material, such as a sintered material based on boron carbide, plated on the flanges 18 of the cross pieces 12. When the neutron poison is boron, it is advantageously enriched in boron 10 which is the isotope of boron that is effective as a neutron absorber. This characteristic is a means of not excessively modifying the mechanical and metallurgical strength of the materials used, while reducing their total boron content. In one example embodiment, the flanges of the cross pieces 12 are composite partitions comprising bringing a metal that is a very good conductor of heat such as copper or a copper alloy into contact with a material with a high boron content such as sintered material based on carbon boride B4C. As illustrated in the above example, the same component of the basket may simultaneously fulfil several of the above mentioned functions in order to optimise performances. In the embodiment of the invention shown in FIG. 1, the assembly means 14 are materialized by encircling structures surrounding the bundle of tubes 10 and cross pieces 12 at different levels. The number of encircling structures 14 is equal to at least two. In FIG. 1, this number is equal to three. Each encircling structure comprises an encircling strip 26 surrounding the tube bundle and a tension system (not shown). Advantageously, the surrounding strips 26 are made of a metal different from the metal used for the tubes 10. This metal is chosen to give a coefficient of expansion less than or possibly equal to that of the coefficient of expansion of the metal in the tubes, such that the cohesion and contact between the tubes and the cross pieces remain unchanged or improve when the temperature increases. The efficiency of heat transfer is preserved. The tightening force of the surrounding structures is initially adjusted to the required value by means of tension systems (not shown). As shown also in FIG. 1, the basket according to the invention may comprise a rigid bottom plate or lower plate 28, usually metallic, in addition to the components described above. The tubes 10 and the cross pieces 12 connected by assembly means 14 are supported on the lower plate 28. This lower plate is particularly useful to retain radioactive materials, for example when the basket has to be handled separately. The basket according to the invention may also be fitted with a head plate or a top plate (not shown). This plate may then be fitted with devices for handling the basket. In one variant embodiment, not shown, the tubes 10 with a prismatic square cross section are replaced by tubes with a rectangular cross section. The cross pieces 12 then comprise two small coplanar flanges with a width equal to half the length of the short side of the rectangle, and two large flanges orthogonal to the first two flanges with a width equal to half the length of the long side of the rectangle. In another variant embodiment (not shown), the tubes 10 have a prismatic circular section. The flanges of the cross pieces 12 are then tangent to the tubes at their ends. Therefore, this is the location at which heat flux is transmitted from the tubes to the cross pieces. The second embodiment of the invention shown in FIGS. 4 and 5 is essentially different from the first embodiment by the shape of the tubes 10. Thus, instead of having a prismatic square section, in this case the tubes 10 have a prismatic hexagonal section. The bundle of tubes 10 then forms a triangular network. Application of the principles mentioned above means that each cross piece 12 has three flanges 18 laid out at 120xc2x0 to each other. All other characteristics and properties described for the first embodiment of the invention and its variants are also applicable in this case. FIGS. 6 and 7 show the third embodiment of the invention. As in the second embodiment in FIGS. 4 and 5, the tubes 10 have a prismatic hexagonal section. However, instead of being composed of surrounding structures, in this case the assembly means 14 comprise perforated metallic plates 30 rigidly fixed to each other by connecting devices 32a, 32b to form a rigid structure to hold the tubes 10 and cross pieces 12 in place. More precisely, there are at least two plates 30 and they are uniformly distributed over the height of the basket, perpendicular to the centre line of the tubes 10. The plates 30 are thin, (from a few millimeters to a few centimeters) and comprise a network of holes 34, with shapes and dimensions corresponding to the shapes and dimensions of the tubes 10. Thus, in the example shown in which the tubes 10 are hexagonal, the holes 34 are also hexagonal and their dimensions are slightly larger than the dimensions of the tubes. Therefore a tube 10 fits into each of the holes 34 in each of the plates 30 with a small clearance. Since the plates 30 are fixed to each other by connecting devices 32a and 32b, the tubes 10 are then held in place between the supports consisting of the plate walls. As shown particularly in FIG. 7, the thickness of the walls 36 formed between adjacent holes 34 in the same plate 30 is adjusted to be slightly greater than the thickness of the flanges 18 of the cross pieces 12. This layout leaves sufficient space between the tubes 10 to insert the cross pieces 12, while reducing the assembly clearance to a sufficiently low value to keep the flanges of the cross pieces and the walls of the tubes very close to each other. Each of the cross pieces 12 is then formed from several segments of cross pieces 38, each segment having a length approximately equal to the distance separating two consecutive plates 30 or the distance separating each end of the basket from the closest plate. This layout means that a second approximately continuous wall can be arranged around each compartment. As shown diagrammatically in FIG. 6, the plates 30 advantageously include head and bottom plates at the top and bottom ends of the basket respectively. In this case, the head plate is preferably fitted with devices for handling the basket such as eye bolts. The perforated bottom plate may also be replaced or doubled up by a solid plate as described with reference to FIG. 1. As shown particularly in FIG. 6, the connecting devices 32a and 32b are placed outside the bundle of tubes 10 and they extend parallel to these tubes over the entire height of the basket. At plates 30 other than the head and bottom plates, the connecting devices 32a and 32b comprise notches 40a and 40b respectively. These notches 40a, 40b fit into a part with a complementary shape and cut out in the peripheral edge of the corresponding plates 30. One or several attachment devices such as screws 42 passing through the connecting devices 32a and 32b fix these devices onto each of the plates 30. When top and bottom plates are provided, they are fixed to the ends of the connecting devices 32a and 32b by attachment devices such as screws 44. The layout that has just been described can make the frame formed by plates 30 and connecting devices 32a and 32b into a rigid structure. In the embodiment illustrated in FIG. 6, the connecting devices 32a and 32b are of two different types depending on the position at which they are located at the periphery of the basket. This is due to the fact that an assembly of tubes 10 with a prismatic hexagonal cross section inside an envelope with a prismatic circular cross section forms firstly recessed parts with a prismatic semi-hexagonal cross section and secondly recessed parts with a prismatic saw tooth cross section, alternately around the periphery of the basket. The connecting devices 32a have an inner face complementary to the first recessed parts and connecting devices 32b have an inner face complementary to the second recessed parts. The outer faces of the connecting devices 32a and 32b form sectors of a cylinder and they all have the same radius of curvature which is the same as the radius of curvature of the outer cylindrical envelope of the basket. Consequently, when all connecting devices 32a and 32b have been installed around the bundle of tubes 10, their outer faces materialize the outer cylindrical envelope of the basket. The layout that has just been described is a means of maintaining a uniform clearance between the basket and the container when the basket is placed inside a container. This characteristic improves the transmission of heat flux between the outside of the basket and the container structures. Obviously, the cylindrical shape of the outer contour of the basket is only given as an example. The prismatic rectangular, square or other section could also be obtained by using appropriate shapes for the connection devices. The materials used for making the plates 30 and the connection devices 32a and 32b are preferably metals with good mechanical strength such as metals chosen from the group of stainless steels, carbon steels and aluminium alloys with good mechanical properties. It will be understood that the basket according to the third embodiment that has just been described also has characteristics and properties similar to the characteristics and properties of other embodiments and their variants. The embodiments and variants described above clearly demonstrate the many advantages of the invention. In particular, it is quite clear that the invention is equally suitable for making baskets with hexagonal compartments and for making baskets with compartments with more standard shapes such as square or rectangular. Furthermore, the compact assembly of the tubes and cross pieces achieved by the different assembly means described facilitates heat transfers. The use of attachment devices such as screws or welds is minimized. Therefore manufacturing costs are optimised.