EP1005049A1 - Shock absorbing device for a cask for the containment of radioactive material - Google Patents

Abstract

The shock absorber comprises one or more hollow end caps or rings (4,7) covering at least part of the container (1,2) and forming closed chambers filled with elementary components (6) with at least three axes of symmetry, and with a symmetry of rotation of at least 3. The shock absorber comprises one or more hollow end caps or rings (4,7) covering at least part of the container (1,2) and forming closed chambers filled with elementary components (6) with at least three axes of symmetry, and with a symmetry of rotation of at least 3. The elementary components have a center of symmetry where the axes of symmetry meet and are in the shape of spheres or regular polyhedrons, preferably cubes. They are preferably hollow with a constant wall thickness, are made from a metal, preferably steel, aluminum, copper or alloys of these, and have a mean diameter of 20-80 mm.

Description

TECHNICAL AREA

The invention relates to shock absorbing devices arranged around radioactive material containers (or packaging), especially those with a mass ranging from a few tonnes to more than 100 or 150 tonnes, generally used for the transport and / or storage of spent nuclear fuel or for all other radioactive materials; these devices allow the so-called packaging to withstand regulatory drop tests under conditions as they meet the safety criteria required by regulations applicable for the transport and storage of said radioactive material.

STATE OF THE ART

Containers for transporting and / or storing irradiated fuel or all other radioactive material often involves, due to the need for a shielding against radiation, thick metal walls (e.g. several centimeters to several tens of centimeters) made of steel or cast iron iron, and therefore have a high mass which can range from a few tonnes to over 150 tonnes.

Generally these metal containers have at least one ferrule thick cylindrical, inside which the radioactive material takes place or the combustible elements, closed at its two ends by a bottom and a also thick cover. They are usually handled using pins attached to the ferrule. The cylindrical shell can have a cross section circular or polygonal (rectangular, square ...).

All these containers must be equipped with shock absorbing devices to enable them to withstand the tests prescribed by the applicable regulations, in particular the so-called free fall tests from a height of 9 meters. The shock absorbers must be designed to be effective in all possible angles of fall.
In general, these damping devices include metal covers which cover the ends of the container projecting beyond the metal body, which allows to take into account not only vertical falls along the longitudinal axis of the container, but also lateral (along a perpendicular axis above) or oblique (on the end angles of the container).

Figure 1 shows an example of a known shock absorbing device, covering the end of a container comprising a ferrule (1) closed by a cover (2) and can be manipulated using the pins (3). Said damping device comprises a metal cover (4) divided into compartments filled with pieces of wood (5) including fibers have a chosen orientation to provide effective cushioning in several directions; we see that the result is limited to obtaining amortization effective only when the stress caused by the shock is exerted in one direction parallel to the fibers. So with this shock absorbing device it is not possible to obtain isotropic damping (that is to say of the same efficiency whatever the angle of fall) over the entire surface of the hood.

It is known to replace said compartmentalized cover filled with pieces of wood, by a soft solid metal cover such as aluminum, for example according to US patent 4806771. The use of solid metal as shock absorber has the advantage of being isotropic and having good crushing properties identified, reproducible and stable over time. On the other hand it leads to a significant weight increase and as solid metal presents a high crushing stress, the accelerations transmitted to the container during falls are also high, usually higher than those obtained with a hood filled with wood, which can limit its field of use.

To have a damping device less stiff than solid metal and more lightweight, it is known to use, for example in US Pat. No. 3,675,746, a plurality of metal tubes stored and stacked in a larger tube. Such a device has sufficient crush resistance in one direction perpendicular to the long axis of the tubes, on the other hand in the axial direction (buckling) it is much too high, the damping then being too steep and ineffective. Thus, even by placing these tubes in a compartmentalized cover and by storing them in each compartment according to a particular orientation, it would be possible at best only to reduce the damping anisotropy, as this is achieved with the covers filled with pieces of wood having orientations of various fibers, mentioned above.

To improve the damping isotropy, it is finally known from patent JP 04042097 to use a compartmentalized cover, each compartment being filled with small loose metal parts, like Raschig rings or pieces of cut parts, for example extruded aluminum.

• d'une part, l'empilement doit être aléatoire, chaque pièce devant avoir une orientation différente de celle de ses voisines; l'isotropie moyenne ainsi obtenue malgré un comportement anisotrope de chaque pièce ne peut pas écarter tout risque d'empilement anisotrope;
• d'autre part, l'empilement doit, dans toutes les circonstances, demeurer le plus régulier possible, avec une bonne cohésion des pièces et des espaces entre elles aussi réduits et réguliers que possible, de façon à assurer une répartition homogène des pièces; cette condition pour avoir une isotropie acceptable de l'amortissement ne peut qu'être imparfaitement réalisée car elle est peu compatible avec la première condition d'une distribution aléatoire des pièces qui entraíne une orientation différente de chacune d'elles; ainsi la présence de compartiments dans le capot est essentielle pour favoriser et surtout tenter de maintenir un répartition suffisamment homogène des pièces en limitant leur possibilité de mouvements.

Said small parts having an anisotropic individual behavior, they can only improve the isotropy of the damping only on average and under certain conditions:

• on the one hand, the stacking must be random, each piece having to have a different orientation from that of its neighbors; the average isotropy thus obtained in spite of an anisotropic behavior of each part cannot rule out any risk of anisotropic stacking;
• on the other hand, the stack must, in all circumstances, remain as regular as possible, with good cohesion of the parts and spaces between them as small and regular as possible, so as to ensure a homogeneous distribution of the parts; this condition to have an acceptable isotropy of the damping can only be imperfectly realized because it is not very compatible with the first condition of a random distribution of parts which results in a different orientation of each of them; thus the presence of compartments in the cover is essential to favor and above all to try to maintain a sufficiently homogeneous distribution of the parts by limiting their possibility of movements.

Despite these precautions, we see that it is difficult, with such a device, to justify with regard to the regulations in force that amortization is intrinsically isotropic, any risk of anisotropic stacking not being ruled out, and that the distribution of parts is sufficiently homogeneous in each compartment or from one compartment to another.

In view of these drawbacks, the applicant sought a device providing shock absorption which is intrinsically isotropic in the event of a fall container at all possible angles, while being homogeneous, the lightest possible and simple to carry out.

DESCRIPTION OF THE INVENTION

The invention is a shock absorbing device secured to a container typically metallic for transporting or storing radioactive materials, characterized in that it comprises at least one cover covering at least in part of said container and forming a closed enclosure filled with a stack elementary parts having at least three concurrent axes of symmetry, the rotational symmetry is at least of order 3, that is to say that from a point it is necessary rotate no more than 120 ° to obtain an identical point.

The point of intersection of these axes preferably constitutes a center of symmetry of the part which is thus a part with centered symmetry.

So these elementary pieces include regular polyhedra like tetrahedron with equilateral faces, the cube and all regular polyhedra having a greater number of equal faces, but also the sphere.

It is particularly advantageous to use a cube or especially a sphere which have centered symmetry, the sphere also having a simple shape and a number unlimited axes of symmetry and therefore perfect homogeneity and isotropy.

These parts can be of various materials provided they have a capacity of sufficient deformation, for example ceramic, resin, reinforced or not. We use usually metal parts, preferably steel, aluminum, copper or their alloys, which have a good capacity to deform by absorbing a high energy without breaking during violent shocks, as is the case in a fall from a container.

When the elementary parts are resin we can use massive parts, whereas in the case of elementary metal parts, it is particularly advantageous that they are hollowed out, respecting the conditions of symmetry above, so that they can deform better.

In general, a cover is attached to each end of the container and therefore covers the ends of the shell, the bottom and the cover; it also overlaps the ends of the side wall of the shell. The cover may cover the end of the container in whole or only in part; in the latter case it typically has the shape of a crown of L-shaped cross section covering the end angle of the container and leaving partly visible the center of the lid or of the bottom. It is possible to install intermediate covers filled with elementary parts according to the invention, encircling the shell between its ends.
The covers are generally metallic and made of sheet steel having a thickness sufficient not to be deformed by the load of the spheres under the usual conditions of handling and positioning of the cover and however sufficiently thin to be able to deform without breaking in the event of a fall. The thickness of the sheet is typically between 2 and 8 mm depending on the mass of the container to be absorbed. They can be made of other materials, for example plastic.

We can plan to improve the rigidity of the covers by any type of reinforcement exterior or interior, for example spacers connecting two walls of said cover and arranged between the filling spheres; they can participate in shock absorption. Such a hood is particularly efficient while being simple to carry out, the presence of compartments is not essential.

The enclosure formed by the cover also has a height (or thickness) included generally between 10 and 100 cm; it is all the more important as the desired depreciation is high (for example for the most heavy) or that the elementary parts are more easily deformable.

In addition, the fact of using symmetrical elementary parts according to the invention makes it easy to achieve regular, compact and homogeneous stacking in the entire enclosure without the need to take any special precautions; in particular the spheres are set up in a random way then are ordered automatically; stacking presents no risk of segregation. So the use of symmetrical elementary parts, such as spheres with symmetry centered, therefore isotropic and leading to an isotropic stacking, provides a isotropic damping by construction, whatever the angle of fall.

The elementary parts advantageously have an average diameter of between 20 and 80 mm. When they are too small, their manufacture and in particular their recess leading to thin parts can cause problems, and when they are too large the distribution of the homogeneity of the crushing forces can be affected.
It is advantageous that the ratio between the height of the enclosure of the cover and the diameter of the elementary parts is between 2 and 20%.

When the elementary parts are hollowed out, in particular the metallic spheres, they are preferably hollow parts with constant wall thickness; But they can also be obtained from solid parts in which there are drilled several identical holes of constant diameter, which can pass through them part by part, the distribution of which always respects the conditions of symmetry mentioned above.

The vacuum rate (ratio between the vacuum volume and the volume of the part) is adapted to the resistance to crushing that one wants to obtain. It is generally between 30 and 90% and preferably between 40 and 80%. For hollow parts with constant wall thickness, the ratio between the wall thickness and the average diameter, based on the largest dimension or the circumscribed circle, is typically between 0.03 and 0.3, which is complies with the above vacuum rate ranges.
The elementary parts according to the invention, in particular the hollowed-out parts, are deformable during impacts and it is remarkable to note that, unlike the use of tubular parts, they have, due to their specific symmetry characteristics, the property to deform in an identical or very similar manner whatever the direction of the force applied and that thus they provide the shock-absorbing device according to the invention, an isotropic and effective shock absorption whatever the angle of fall.

In addition we see that by combining the diameter of the elementary parts and their rate the device according to the invention can be adapted to all types of vacuum containers while retaining the essential property of isotropic behavior.

Thus for the same type of hood, having for example a space requirement constant and adaptable on containers with a constant outside diameter and variable lengths, it is possible to vary the dimension and / or the rate of empty parts filling said cover so as to adapt the characteristics amortization of the device according to the invention to the mass of the container, variable depending on its length and the load it contains.

In general, the elementary parts are all identical; however we can use parts of different diameters or vacuum rates in the same hood, for example bunk beds, to obtain characteristics progressive amortization.

One can also advantageously introduce into the cover, after installation elementary parts, a binder (for example cement, glue, resin) spreading in the interstices between said parts; this allows, after solidification, to improve their cohesion, in particular when they are not all identical, or to avoid their dispersion in the event of partial tearing of the cover, while retaining their depreciation capacity.

It can be seen that the device according to the invention can easily be used for all container types ranging from heavier to lighter; just adjust the size and the vacuum rate of elementary metal parts to give them the crushing characteristics necessary for cushioning the container considered.

• La figure 1 illustre un dispositif amortisseur de l'art antérieur comportant un capot compartimenté rempli de bois.
• La figure 2 illustre un conteneur équipé à une de ses extrémités d'un dispositif amortisseur de choc selon l'invention.
• La figure 3 illustre différents types de pièces évidées à symétrie centrée.

It may be noted that the symmetry of the parts according to the invention is not considered to be affected by the presence of defects or leftovers, linked for example to the method of manufacturing said parts (such as beads, access holes to the interior cavity, traces of machining etc.), which would not have the symmetry according to the invention, insofar as said defects are not such as to significantly call into question the isotropic behavior of the parts. In other words the parts with at least order 3 symmetry comprising this type of fault are part of the invention.

• Figure 1 illustrates a shock absorber device of the prior art comprising a compartmentalized cover filled with wood.
• FIG. 2 illustrates a container fitted at one of its ends with a shock absorbing device according to the invention.
• Figure 3 illustrates different types of hollowed parts with centered symmetry.

In Figure 1 we see, as has already been said, the thick metallic ferrule (1) container, closed at one end by the thick cover (2). The container is manipulated by the pins (3). A cover (4) covers the entire end of the container and overflow the end of the outer wall of the shell (1).

This cover is divided into compartments by walls (4a), each of the compartments containing a piece of wood whose fibers are oriented judiciously. It can be seen that the damping at a determined location depends both on the direction of the wood fibers and on the direction of the impact with respect to said fibers.
Similar observations would be made by replacing the wood with a stack of arranged tubes whose orientation would be that of the fibers.

In FIG. 2, which illustrates the invention, it can be seen that the cover (4) is filled with hollow spheres (6) all identical (only a few are shown) and that it covers the entire end of the container. The cover has internal stiffeners (8). It might not cover it entirely and thus leave part of the cover (2) visible, it would then form a crown of straight section in the shape of an L.
We also see that the shell is equipped with an intermediate cover (7) surrounding it, according to the invention. It is filled with hollow spheres (6a) different from that of the end cover, because the crushing stresses sought in this area are different.

In Figure 3 which illustrates elementary parts hollowed out according to the invention we see first of all in Figure 3a spheres, in profile and exploded, in which have were drilled holes (10) so as not to destroy the centered symmetry of the room. It is thus noted that there is a hole (10) opening on the surface at each of the ends of a system of 3 perpendicular axes of symmetry and that each holes centered on one of the axes of symmetry cross the sphere right through passing through its center. The sphere with its holes retains a symmetry of order 4.

We could also make 4 holes arranged at the vertices of a tetrahedron equilateral inscribed in the sphere which would cross the tetrahedron from its vertices to its center or to the center of opposite faces.

In Figure 3b we see, in profile and exploded, an elementary piece in the form of hollow sphere. This type of part can include a trace of its process of manufacturing in the form of a hole whose diameter can for example reach about 10 mm for hollow spheres with a diameter of 60 to 80 mm.

In Figure 3c we see, in profile and exploded, an elementary part in the form of cube having a hole (11) centered on the axis of symmetry of each of its faces, said hole passing right through the cube passing through its center. These holes do not alter the symmetry of the cube.

Claims (13)

Shock absorber device secured to a transport container or storage of radioactive materials characterized in that it comprises at least a cover (4,7) at least partially covering said container (1,2) and forming a closed enclosure filled with a stack of elementary parts (6) having at least three concurrent axes of symmetry including rotational symmetry is at least order 3. Device according to claim 1 characterized in that the parts elementary have a center of symmetry which is the intersection point of the axes of symmetry. Device according to either of Claims 1 and 2, characterized in that that the elementary parts (6) are chosen from either preferably the spheres, that is, regular polyhedra, preferably cubes. Device according to any one of Claims 1 to 3, characterized in that that the elementary parts are metallic, preferably steel, aluminum, copper or their alloys. Device according to any one of Claims 1 to 4, characterized in that that the elementary parts (6) are hollowed out, preferably parts hollow with constant wall thickness or parts in which have been drilled holes of constant diameter arranged symmetrically without destroying the symmetry of said elementary parts at least of order 3. Device according to Claim 5, characterized in that the vacuum rate of the elementary pieces (6) hollowed out, defined as the ratio between the volume of vacuum and the volume of the room, is between 30 and 90% and preferably between 40 and 80%. Device according to either of Claims 5 and 6, characterized in that that the hollow elementary parts (6) with constant wall thickness have a ratio between their material thickness and their average diameter between 0.03 and 0.3. Device according to any one of Claims 1 to 7, characterized in that that the average diameter of the elementary parts (6) is between 20 and 80 mm. Device according to any one of Claims 1 to 8, characterized in that that the enclosure formed by the cover (4) has a height of between 10 and 100 cm. Device according to any one of Claims 1 to 9, characterized in that that the cover (4) has reinforcements on the outside or preferably in the enclosure in the form of spacers. Device according to any one of Claims 1 to 10, characterized in that that the elementary parts (6) are embedded in a binder. Transport or storage container for radioactive materials characterized by what it includes at least one shock absorbing device of one any of claims 1 to 11. Container according to claim 12 characterized in that it comprises a shock absorbing device at each of its ends and optionally at least one shock absorbing device around the ferrule (1).

Images