Special receptacle or package – Shock protection type
Reexamination Certificate
1999-11-22
2001-05-22
Fidei, David T. (Department: 3728)
Special receptacle or package
Shock protection type
C206S584000, C250S507100
Reexamination Certificate
active
06234311
ABSTRACT:
TECHNICAL FIELD
The present invention relates to shock-absorbing systems arranged around containers (or packaging) of radioactive material, in particular those having a weight ranging from a few tonnes to more than 100 or 150 tonnes, generally used for the transport and/or storage of irradiated nuclear fuel or for any other radioactive material; with these systems the said packaging is able to withstand prescribed drop tests under conditions such that they fulfil the safety criteria required by regulations applying to the transport or storage of said radioactive material.
STATE OF THE PRIOR ART
Transport and/or storage containers for irradiated fuel or for any other radioactive material, due to the need for shielding against radiation, often have thick metal walls (for example several centimetres to several tens of centimetres thick) in steel or cast iron whose weight is therefore high ranging from a few tonnes to over 150 tonnes.
Generally these metal containers comprise at least one thick cylindrical sleeve inside which the radioactive material or fuel elements are placed, closed at its two ends by a base and a lid that are also thick. They are usually handled by means of kingpins fixed to the sleeve. The cylindrical sleeve may have a straight, circular or polygonal (rectangular, square . . . ) cross-section.
All these containers must be fitted with shock-absorbing systems to enable them to withstand the tests laid down by applicable regulations, in particular the so-called free-fall test from a height of 9 metres. The shock absorbers must be designed such that they are effective at all possible angles of fall.
In general, these shock-absorbing devices comprise metal casings which cap the ends of the container and project beyond the metal body such as to provide not only against vertical falls along the longitudinal axis of the container, but also against lateral falls (along an axis perpendicular to the previous axis) or oblique falls (at the end corners of the container).
FIG. 1
shows an example of a known shock-absorbing device, capping the end of a container and comprising a sleeve (
1
) closed by a lid (
2
) and handled by means of kingpins (
3
). Said shock-absorbing device comprises a metal casing (
4
) divided into compartments filled with wood pieces (
5
) whose fibres are orientated to provide efficient shock absorption in several directions; it can be seen that the result is limited to obtaining efficient shock absorption only when the stress due to impact is exerted in a direction parallel to the fibres. Therefore, with this shock-absorbing device it is not possible to obtain isotropic shock absorption (that is to say having the same efficiency irrespective of the angle of fall) over the entire surface of the casing.
It is known to replace said partitioned casing filled with wooden pieces by a solid metal cover as soft as aluminium for example according to U.S. Pat. No. 4,806,771. The use of solid metal as a shock absorber has the advantage of being isotropic and of having crush properties that are well identified, reproducible and stable in time. On the other hand, it leads to a significant increase in weight and, since solid metal has high crush resistance, the accelerations transmitted to the container during a fall are also high, generally higher than those obtained with a wood-filled casing, which can limit its area of use.
To have a shock-absorbing system that is less stiff than solid metal and lighter in weight, it is known to use, as for example in U.S. Pat. No. 3,675,746, a plurality of metal tubes arranged and stacked in a larger tube. This kind of system has sufficient resistance to crushing in a direction perpendicular to the major axis of the tubes; on the other hand it is much too high in the axial direction (buckling) when shock absorption is too inflexible and inefficient. Therefore, even by placing these tubes in a partitioned casing and by arranging them in each compartment in a particular orientation, it would at the most only be possible to reduce the anisotropy of shock absorption as is obtained with wood-filled casings having varied fibre orientations as described above.
To improve the isotropy of shock-absorption, patent JP 04042097 is known to use a partitioned casing, each compartment being filled with small metal pieces, in bulk, of Raschig ring type or sectioned pieces of extruded aluminium for example.
Since said small pieces have individual anisotropic behaviour, they can only bring average improvement in the isotropy of shock-absorption and under certain conditions:
firstly random stacking must be made, the orientation of each piece needing to be different to that of neighbouring pieces; the average isotropy obtained in this way, despite the anisotropic behaviour of each piece, cannot exclude all risks of anisotropic stacking:
also, stacking must under all circumstances remain as regular as possible with good cohesion of the pieces, the spaces between them being as small and regular as possible such as to ensure homogeneous distribution of the pieces; this condition for acceptable shock-absorbing isotropy can only be partly achieved, since it is little compatible with the first condition of random piece distribution which leads to each piece having a different orientation; therefore the presence of compartments in the casing is essential to promote, and especially endeavour to maintain, a sufficiently homogeneous distribution of the pieces while limiting their possibility of movement.
Despite these precautions, it can be seen that with such a system it is difficult to prove vis-á-vis regulations in force that shock absorption is intrinsically isotropic, the risks of anisotropic stacking not being entirely eliminated, and that the distribution of the pieces is sufficiently homogeneous in each compartment or from one compartment to another.
Having regard to these disadvantages, the applicant has endeavoured to find a system providing shock absorption in the event of fall of the container that is intrinsically isotropic from every possible angle while remaining homogenous, as light as possible and easy to implement.
DESCRIPTION OF THE INVENTION
The invention is a shock-absorbing system integral with a container, typically a metal transport or storage container for radioactive material, characterised in that it comprises at least one casing covering said container at least in part and forming an enclosed space filled with a stack of elementary pieces having at least three converging axes of symmetry, whose symmetry in rotation is at least 3-fold, that is to say that, from a given point, a rotation of no more than 120° C. must be made to obtain an identical point.
The point of intersection of these axes preferably forms a centre of symmetry of the piece which is therefore a piece with centred symmetry.
Hence these elementary pieces comprise regular polyhedrons such as tetrahedrons with equilateral surfaces, cubes and all regular polyhedrons having a greater number of equal surfaces, but also spheres.
It is particularly advantageous to use a cube, or especially a sphere which have centred symmetry, the sphere also having a simple form and an unlimited number of axes of symmetry, and hence having perfect homogeneity and isotropy.
These pieces may be in varied materials provided that they have sufficient deforming ability, for example ceramic, resin, whether reinforced or not. Generally metal pieces are used, preferably in steel, aluminium, copper or their alloys, which have a good ability to deform while absorbing high energy without breaking under strong impact, as is the case with the fall of a container.
If the elementary pieces are in resin, solid pieces can be used, but if the elementary pieces are in metal it is particularly advantageous for them to be hollowed out, while paying heed to the aforementioned conditions of symmetry so that they may deform more easily.
In general a casing is fixed to each end of the container and therefore covers the ends of the sleeve, the base and lid; its projecting part also protects the ends of the
Fidei David T.
Pearne & Gordon LLP
Transnucleaire SA
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