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(11) | EP 0 233 070 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Impact-absorbing shipping cask for radioactive waste material |
| (57) An individual, discrete impact absorbing member 20 of a solid, soft light metal is
fitted to each of the six corners, at each end of the radioactive waste shipping cask
10 and laps the sides 12, ends 14 and 16, and truncated corners 18 of the cask for
predetermined distances and with predetermined thicknesses to provide a volume of
the metal in excess of the volume of the metal subject to being crushed in specified
drop tests for a Class A shipping package. |
[0002] United States patent application Serial No. 831,970, filed February 21, 1986 discloses an impact protection arrangement particularly useful for type B radioactive waste casks. As noted in that application, the requirements for withstanding impact forces and other conditions such as internal and external heat are relatively severe for the type B casks. Such type B casks are typically cylindrical' and constructed of steel. The essence of the invention of that application is the use of impact skirts in the form of shallow caps fitted to each end of the cask, with each skirt comprising a one-piece member of a solid, soft, light -metal material such as aluminum.
[0003] The invention of the cross-referenced patent application is based in part on the fact that most materials exhibit a pseudo material property sometimes called "dynamic flow pressure" which is defined as the energy necessary to displace a unit volume of the material. The dynamic flow pressure of most light and/or soft metals is relatively constant over a wide range of displacement of the metal and has a value slightly higher than the compressive yield strength of the metal which permits the absorbed energy to be substantially directly related to the displacement of the metal.
[0004] That cross-referenced application also points out that both the soft metal material and the geometry of the skirt are factors in striving for a situation of relatively constant deceleration in which the deceleration forces are a function of crush distance, rather than the deceleration forces being proportional to displacement.
[0005] U.S. Patent Application S.N. 627,896, filed July 5, 1984 discloses a hexagonal concrete cask or module used for on-site storage and off-site disposal of radioactive waste to be transported as type A shipping packages.
[0006] The impact protection requirements for type A casks are not nearly as severe as for the type B casks since the free drop is only from one foot (0.3 m) rather than 30 feet (9 m).
[0007] It is the aim of this invention to provide an impact protection arrangement for type A casks of the general character disclosed in the last noted patent application, and which is relatively lightweight and inexpensive, is reusable, and which uses impact protection blocks which are all of the same design and so can be obtained from a single mold pattern.
[0008] Accordingly, the present invention resides in an impact-absorbing shipping cask for radioactive waste material and of a right regular polygon shape which comprises an individual impact absorbing member fitted to each corner formed between adjacent side faces and the end faces of said cask, each member having an impact absorbing portion consisting of a one-piece, monolithic block of a soft, light metal, configured to lap the cask end face and two adjacent side faces for predetermined distances and having a predetermined thickness, to provide a volume of said metal in excess of the volume of said metal subject to being crushed in specified drop tests.
[0009] In order that the invention can be more clearly understood, a preferred embodiment thereof will now be described, by way of example, with reference to the accompanying drawings in which:
Figure 1 is a schematic side view of an impact-absorbing hexagonal concrete cask;
Fig. 2 is an end view of the cask of Fig. 1;
Fig. 3 is a fragmentary enlarged detail view of a part of the cask end with a single impact protection block applied to a corner of the cask;
Fig. 4 is a cross-section .corresponding to one taken along the line IV-IV of Fig. 3; and
Fig. 5 is an isometric view of a single discrete block with a small portion of the cask to which it is fit, and includes directional arrows to indicate the direction of drop with the cask in different orientations.
[0010] Referring to the drawing, a concrete radioactive waste cask generally designated 10 and of the general type disclosed in U.S. Patent Application S.N. 627,896, filed July 5, 1984, (and to which reference is to be had for details thereof) has sides 12 and opposite ends 14 and 16. As configured as an example for purposes of this application, the cask is a hexagonally-shaped prism with truncated corners 18. That is, at each corner between each two adjacent sides 12, the corner is truncated.
[0011] Discrete impact protection blocks generally designated 20 are fitted to each corner formed between adjacent sides 12 and the ends 14 and 16. Thus, for the hexagonally-shaped cask of Figs. 1 and 2, six blocks are required at each end for a total of 12 blocks.
[0012] The six blocks 20 at each end of the cask are attached to each other by steel L's 22 which, at each block location, are interposed between the block and cask to help spread the impact load (Fig. 4). The blocks and L's in effect form a ring for each end of the cask. The rings at each end of the, cask in turn are secured to each other by the use of rods 24 with turn buckles 26 providing adjustable tension means connecting each block at one end of the cask to the axially aligned block at the opposite end of the cask. This permits the removal of the rings when the casks go into storage, and the subsequent reuse of the rings.
[0013] One of the single discrete blocks 20 is shown in somewhat more detail in Fig. 3 and includes a center part 28 which fits the truncated corner, and opposite wing parts 30 and 32 which fit to the sides 12.
[0014] Fig. 5 is intended to aid in understanding the orientation of the cask 10 and blocks 20 for various drops. The numbered arrows indicate the direction of drop with the cask in the various orientations. Arrow 34 is for a flat end drop. Arrow 36 is for a side drop on a flat side. Arrow 38 is for a corner drop on a side corner. Arrow 40 is for a corner drop on a corner. Arrow 42 is for a side drop on a corner.
[0015] Various values related to the impact protection can be determined mathematically using the same general approach found in the cross-referenced patent application, which is hereby incorporated by reference and to which reference should be had as to the method of going about the determination of the mathematical values. The general approach is to first determine the potential energy to be absorbed upon impact, based upon the gross weight of the package and the fall distance, the crush volume and the displacement distance of the crushed volume, the crush area, and from this the deceleration force and the deceleration may be determined, and then the percent displacement relative to the thickness of the material which absorbs the impact is found.
[0016] As an example of the design basis for a specific set of blocks 20, it is assumed that the gross package weight is 50,000 lbs. (2.27 E + 4 kg) and it is 'subjected to a free-drop from one foot (0.3 m). The potential energy is 58,333 foot-pounds (79,100 J) which includes an allowance of two inches (0.05 m) for crush distance. The blocks are of cast aluminum, having a dynamic flow pressure of about 15,000 psi (103 E + 6 Pa). This results in a crush volume of 46.7 inches (3.01 E - 2 m2) in every case and irrespective of the orientation of the package for the drop.
[0017] The dimensions of the block for the example are as follows. All parts of the block 28, 30, 32 overlap the side 12 of the block in the area designated 44 in Fig. 4 by two inches (0.05 m) and has a three inch (0.07 m) depth from the outer side of the cask to the outer side of the block. The part 46 of the block overlapping the end 14 overlaps for 0.75 inches (0.02 m) and has a depth of three inches (0.07 m) from the end of the cask to the outer end. Each of the wings 30 and 32 have a lateral dimension along their longest sides of 4 inches (0.1 m) while the lateral dimension of the center part 28 at its longest side is 7.86 inches (0.2 m).
[0018] With the specified blocks and weight and drop distance, the values in the following table have been determined for drops in the specified orientations.
[0019] It will be seen that the arrangement according to the invention provides deceleration values in g's which range from 8.4 to 17.2. This is accomplished using impact protection blocks 20 which do not project more than 3 inches (0.07 m) from the ends and sides. The system alsc provides such impact protection for the entire module by only protecting the twelve corners. Finally, the weight of both rings with the specified blocks plus the tie rods and turn buckles is only in the order of about 500 lbs. (22E kg).
1. An impact absorbing shipping cask for radioactive waste material and of a right
regular polygon shape which comprises an individual impact absorbing member fitted
to each corner formed between adjacent side faces and the end faces of said cask,
each member having an impact absorbing portion consisting of a one-piece, monolithic
block of a soft, light metal, configured to lap the cask end face and two adjacent
side faces for predetermined distances and having a predetermined thickness, to provide
a volume of said metal in excess of the volume of said metal subject to being crushed
in specified drop tests.
2. A cask according to claim 1, characterized in that the cask is of hexagonal shape
and includes side corners truncated between the adjacent side faces, and the members
are fitted to the corners formed between the end faces, two adjacent side faces and
the intermediate truncated face.
3. A cask according to of claim 1 or 2, characterized in that adjustable tension means
connect at least some of the corner members at one end of said cask to the axially
aligned corner members at the opposite end of said cask.
4. A cask according to claim 1, 2 or 3 characterized in that means connect each member
at an end of said cask to its two adjacent members at the same end of said cask so
as to form a ring-like structure of members and connecting means.
5. A cask according to claim 4, characterized in that the connecting means comprise
steel angle members extending along the corners at each end of said cask and interposed
between the blocks and said cask.
6. A cask according to any of claims 1 to 5, characterized in that each member laps
the side face for a greater distance than it laps the end face.
7. A cask according to any of claims 1 to 6, characterized in that the cask is made
of concrete and has the form of an hexagonally shaped prism.
8. A cask according to any of claims 1 to 7, characterized in that the soft light
metal is aluminum.