US 3005105 A
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S L A I Dn E M M E 8 5 m m K C S M 7 U I l L AD C R .Mw DO 0 E F .m K l S .l Q M F 5 l l I n* P l. 9 P 3 5 I ITI 6 H l l S a. wu 3 1 2- n. 6 Q 1 y 9 1.. 1 H Il Oct 17,
W) @l I7 i 4 MFV 3,005,105 SHIPPING CASK FOR RADIOACTIVE MATERIALS Elmer C. Lusk, Columbus, Chio, assignor, by mesne assignments, to The Edlow Lead Company, Columbus,
Ghio, a corporation of @bio Filed Oct. 17, 1958, Ser. No. 767,935 6 Claims. (Cl. Z50-108)` This invention relates to shipping casks for radioactive material. More particularly, it relates to a shipping cask for spent reactor fuel elements.
Two problems are encountered in the shipping of spent fuel elements. First, of course, there is the factor of radiation protection, so that shielding, usually lead, must be provided. But there is also the problem of efficiently carrying away the heat being generated by the fuel elements. Aluminum-clad fuel elements must be maintained at a relatively low temperature so that softening and melting of the aluminum is avoided. Also, fuel elements clad with stainless steel must be kept at a low temperature while being shipped, due to the comparatively low melting point of the lead shielding. Thus, should portions of the fuel element shipping container become hot enough to melt the lead shielding in the shipping cask, expansion of the melting lead and possible explosion of the shipping cask may result. For these reasons, the handling of spent fuel elements is now undertaken in one of two ways. Either (l) the fuel elements are allowed to be stored longer before shipping, so that their thermal heat generation becomes low enough for safe shipping, or, (2) if they are shipped While relatively thermally active, a special cooling system, which must be maintained in transit to ensure safety, must be used.
It is an object of the present invention to provide a shipping cask for radioactive material which has the property of good heat conduction between the inside and outside of the cask. It is another object of this invention to increase the utility of containers for carrying spent fuel elements.
A further object is to save expense by decreasing the time in which spent fuel elements must be stored before being shipped.
A further object and feature of the present invention is to provide suciently good heat conduction from the hot inner portions of the cask to the outer surface of the Cask that hot segments and resulting melting of the shielding material are avoided, even when relatively fresh fuel elements are shipped.
Many other objects and features of this invention will become apparent as the description proceeds.
In the drawings:
FIG. l is an elevational view illustrating an embodiment, partially in cross section, of the shipping cask of the present invention;
FIG. 2 is a top-sectional view of the cask of FIG. 1 taken on the plane 2-2 of FIG. 1;
FIG. 3 is a partial expanded view in cross section of a section of the outer periphery of FIG. 2;
FIG. 4 is a view similar to FIG. l, illustrating a second embodiment of the shipping cask of the present invention;
FIG. 5 is an upper cross-sectional View of the cask of FIG. 4, taken on the plane 5 5 of FIG. 4; and
FIG. 6 is a sectional view, taken on the plane 6--6 of FIG. 5, illustrating the heat-conductive lin of FIGS. 4 and 5.
Shipping casks of the general type illustrated in the drawings of the present invention are well known and are made by pouring molten lead into an annular cylindrical void between a relatively thin inner container and ted States atent O ICC an outer shell, the area within the container becoming the holding place for the materials to be carried. It is well known that when such lead-filled casks are made, the molten lead shrinks upon solidifying and cooling. This is due for lthe most part to the relatively high coefficient of thermal expansion of lead as compared to casing materials such as steel. In shrinking, the lead tends to solidify around the middle and pull away from the outer shell of the cask. It has been found that when the lead solidifies, a uniform space or void between the lead and the outer shell exists along the inside periphery of the outer shell. The lack of contact between the outer shell and the lead acts as an insulation, grossly interfering with heat transfer to the outer surface of the cask.
Thus, if spent fuel elements which are producing considerable quantities of heat are carried in such a cask, the heat produced by the fuel elements is not efficiently carried away and portions of the lead, usually along the outside wall of the inner container, may become hot enough for them to melt, creating a very dangerous situation. Of course, if aluminum-clad elements are being shipped, damage to the aluminum might occur even before lead melting, as hereinbefore discussed. Prior attempts to avoid this region of no contact were primarily aimed at bonding the outer container to the lead filling by methods such as soldering. The purpose of the bonding `was to prevent shrinkage from the casing. However, high tension forces are developed in the lead during shrinkage, and the relatively low strength of the lead in tension prevents success for this bonding method and results in fissures of the lead.
Thus, in addition to the objects and features set forth above, it is a primary object and feature of the present invention to provide a lead shipping Cask in which dependable heat conductivity to the outer shell of the cask is ensured.
Referring to FIGS. 1 and 2 of the present invention, the embodiment 20 of the shipping cask shown has an inner stainless steel container 10 and an outer stainless steel shell 11. Between the inner and outer cylindrical shells 10 and 11 is an annular section of lead 12, which has been poured into the shielding cavity of the cask to lill the space between the inner and outer shells 10 and 11. Also illustrated, for the sake of clarity, are the upper plug 13, the carrying handles 14, the base plate 15, and the heavy carrying base 16.
According to this preferred embodiment of the present invention, iron tins 17 are welded in circumferentially spaced relationship to the inner side of the outer shell 11. Each iin 17 is welded substantially along the entire vertical height of the stainless steel shell 11. The iron tins 17, as illustrated, are short and angled. FIG. 3 illustrates in detail a cross section of the n 17. The iin is shown welded to the outer shell 11 by weldment 18. Stainless steel weld rod is used in the welding. Also shown, and exaggerated for the sake of clarity, is the void 19 between the shell 11 and the lead mass 12 caused by shrinkage of the lead 12 away from the shell 11. It is apparent that the attachment of the fins 17 between the shell 11 and the lead mass 12 provides a heat-conductive bridge from the lead mass 12 to the shell 11. The number of lins 17 which may be Welded around the inner side of the outer shell 11 is, of course, dependent upon the heat-conductive efficiency which is desired and the size of the tins. FIG. 2 illustrates a suitable spacing of tins 17 for most applications. Further preferred ranges for iin construction are hereafter discussed. The fins 17 are angled away from the radial direction, so that the fins 17 do not interfere too greatly with the shielding effect of the lead 12 against straight-line radiation. The tins 17 are further yangled inwardly at their inner ends to provide a hook, ensuring good heat-conductive contact between the fins 17 and the lead mass 12 during and after cooling and shrinkage of the lead 12 and during subsequent changes in temperature. In the present invention, it is preferred that the fins 17 be Welded to the outer shell 11 before the lead 12 is poured, so that the final major step in manufacturing the cask 20 is the pouring of the lead 12.
Another preferred embodiment of the present invention is illustrated in FIGS. 4, 5, `and 6. 'I'he shipping cask 21 is similar to the first embodiment in that it comprises an inner stainless steel container 22, an outer stainless steel shell 23, and a monolithic poured annular body of lead 24. In addition, a plurality of copper tins run from the inner container 22 to the outer shell 23 and are evenly spaced as shown around the shell circumference and throughout the lead 24. The iins 25 are silver-soldered along the length of the outer shell 23. They are at a substantial angle from the radius and are in tangential contact with the inner shell 22. The fins 25 need not be iixedly attached to the inner shell 22. FIG. 6 illustrates a iin 25 in detail. As illustrated, a plurality of small holes 26 are situated throughout each iin 25. In this embodiment also, the iins 25 are attached to the outer shell 23 and thereafter the molten lead 24 is poured between the shells 22 and 23 to form the lead mass 24 which solidifes around the tins 25.
As was mentioned, the purpose of having the tins at an angle from the radius is to avoid substantial interference in the shielding ability of the cask against the strong straight-line radiation. It is seen that this embodiment also provides, in the fins 25, many heat-conductive paths between the outer shell 23 and the lead mass 24, even though the lead mass will shrink from the outer shell 23. The purpose of the perforations 26 in the fins 25 is to ensure greater contact for heattransfer purposes between the lead mass 24 and the tins 25, even upon the shrinkage of the lead mass 24 which takes place. The shipping cask 21, although requiring more fm material than shipping cask 20, has the added advantage of having heat conducting tins all the way in to the center shell 22 to aid the lead 24 in conducting heat to the outer shell 23.
Copper is preferred for the ns of the embodiment of FIG. 4 of the present invention for several reasons. Not only is it relatively inexpensive and a good conductor, but it adheres well to lead. The heat conductivity of copper is .941 cal./cm.2/cm./sec./ C. at 20 C. In comparison, the heat conductivity of lead is only .082 cal./ cm.2/cm./sec./ C. at 20 C. Thus, the existence of the copper fins 25 within the lead mass 24 aids greatly the conducting of heat through the lead mass from the center shell. In addition, of course, the contact between the fins 25 and the outer shell 23, which is ensured, provides a heat-conductive path to the outer shell 23.
The iins of the present invention illustrated and described above successfully increase the ability of the cask to withstand high fuel element heat generation. In addition, there results substantially no loss in radiation protection, due to the very minor replacements of tin material for lead. For example, for the shipping cask 21 there is only the equivalent of one eighth inch per eleven inches of lead thickness lost through use of the ns, and in the shipping cask 20, this loss is negligible.
The actual dimensions of the cask illustrated herein are as follows:
It may also be mentioned that, for this size cask, approximately 18,000 pounds of lead are required to fill the shielding cavity.
Thus, it is seen that the cask of the present invention comprises an inner container, an outer shell, a monolithic cast-lead shielding material situated therebetween, and is characterized by a plurality of heat-conductive ns attached to the inside of the outer shell and extending into the shielding material.
For fuel elements which are shipped in water in the inner container of the present invention, it is important that the water temperature be maintained at not much more than 200 F., that is, that the water not be allowed to boil. It is preferred that, for a circulating-air ternperature of about F. outside the outer shell of the shipping cask in combination with a temperature of about 200 F. for the water around the fuel elements, the area of contact between the iron ns and the shell of the cask of FIG. 1 should be at least as large as three percent of the total inside area of the shell adjacent the lead shielding. It was found that when this area of contact was about 3-4 percent the heat-carrying eiciency (or kilo- Watt ratio) of the cask was improved approximately three to one over a similar cask without the iron iins, the gap 19 in these casks being about one tenth of an inch Wide. Doubling this percentage area of contact by enlarging the tins or increasing their number results in only about an added l0 percent increase in eiiiciency.
Of course, as the internal temperature of the cask goes up, the effectiveness of the ins increases. Also, if the gap between the lead and outer shell is larger, the iins are more important.
For the cask of FIG. 4, it was found that a copper cross section of 2 inches around a circumference of the cask increased the heat ow through the lead annulus about one third. Only about ten percent in effectiveness is gained by use of copper fins instead of iron fins to span the gap between the lead and the outer shell. The efficiency of the cask of FIG. 4 may be increased by adding `fins to the outside of the stainless steel outer shell to promote better contact of the shell with the outside air.
It will be understood, of course, that the preferred embodiments described and shown herein are intended to be illustrative rather than limiting, and that the various minor changes which may be made by those skilled in the art to secure specilic advantages under particular circumstances are intended to be encompassed in the claims below.
What is claimed is:
l. A carrying cask for radioactive materials which comprises: a thin, inner cylindrical container; an outer cylindrical shell, spaced outwardly from said inner container; and a monolithic cast-lead shielding material situated therebetween; characterized by a plurality of heatconductive fins attached to said outer shell and extending into said lead mass, said fins providing a heat-conductive bridge from said lead mass to said shell across the void therebetween inherent in such a cast-lead carrying cask.
2. The carrying cask of claim 1 wherein the tins extend to the inner container.
3. The carrying cask of claim 2 wherein the tins consist essentially of copper.
4. 'Ihe carrying cask of claim 1 wherein the fins extend into the lead mass at a substantial angle from the radius of the cask.
5. The carrying cask of claim l wherein the ns are perforated and extend into the shielding material in a direction such that substantial interference in the shielding ability of the shielding material to straight-line radiation emanating from the container is avoided.
6. A carrying cask for radioactive materials which comprises: a thin, inner cylindrical container; an outer cylindrical shell, spaced outwardly from said inner container; and a monolithic cast-lead shielding material sit- 6 uated therebetween; characterized by a plurality of heatsmall so that substantial interference in the shielding conductive ns attached to said Outer shell and extendability of the shielding material to straight-line radiaing into said lead mass, said fins providing a heat-contion emanating from the container is avoided. ductive bridge from said lead mass to said shell across the void therebetween inherent in such a cast-lead car- 5 References Cited in the le of this patent rying cask, the area of contact between said ns and said UNITED STATES PATENTS shell being at least as large as 3 percent of the total inside area of said shell adjacent said shielding material 2,600,390 Bayer June 17 1952 and the size and number of said Ifins being suiciently 2,702,613 Walther Feb. 22, 1955