Patent Number: RE0298760
Section: summary

The present invention relates to an improved container or packaging assembly for transporting radioactive materials. More specifically, the present invention relates to a container or packaging assembly for transporting heat emitting radioactive materials which has improved heat dissipating features and which, by virute of its unique structure, has reduced weight. Due to the radiation hazard associated with radioactive materials, it is generally necessary that these materials be packaged in sturdily constructed containers consisting of materials and design which will readily attenuate the radiation and dissipate the heat emitted by the radioactive material being transported. Radiation attenuation is necessary to biologically shield personnel and the public from radiation in the course of transport under both normal conditions of transport and under accident conditions. Heat dissipation is often necessary to protect the container from damage and to protect personnel from serious burns while carrying heat emitting radioactive materials. In the past it has been the practice to construct heavy shipping containers for transporting highly radioactive heat emitting materials with a central cavity considerably larger than that necessary to obtain the radioactive materials to be transported. This void space is usually filled with a fluid, such as water, which circulates through the cavity to an external heat exchanger to provide cooling. This fluid also frequently serves to absorb neutrons. The cavity is normally lined with a corrosion resistant material, such as stainless steel, which is relatively resistant to decontaminating solutions. The material surrounding the liner is usually of high density because the ability to attenuate beta gamma radiation is proportional to the density of the shielding material. The outer structure of the container normally includes an external wall surrounding the liner together with heat dissipating and impact fins integral with the wall. The outer surface including the external wall and fins are normally made of the same material, such as stainless steel or conventional steel coated with stainless steel. The fins serve to dissipate the heat emitted by the contained radioactive materials and protect the package against impact in the event of an accident. The surface of the outer structure is normally constructed of stainless stell or other acid resistant material in order to avoid reaction with corrosive decontaminating solutions, such as nitric acid. Where highly radioactive materials are to be shipped, such as irradiated fuel elements or the waste resulting from the recovery of the fuel values found in irradiated fuel elements, the thick shielding and heat dissipating requirements for the walls and fins of the shipping container may result in a container which when loaded consists of as much as 98 percent shielding and heat dissipating structure and as little as 2 percent radioactive material payload to be transported. Unfortunately, containers of this type, while providing the necessary protection against emitted radiation, structural integrity, and heat dissipation, are so extremely heavy that the cost of transporting the radioactive material is extraordinarily high. Further, transport is often limited to one fuel assembly on a single legal weight road trailer at a time. It is, accordingly, an object of the present invention to provide a container for transporting radioactive materials which has improved means for dissipating heat generated by the radioactive materials. It is another object of the present invention to provide such a container for transporting radioactive materials which in addition to having an improved assembly for dissipating emitted heat is significantly lighter than previously known containers of this type. Still another object of the present invention is to significantly lessen the time and materials required, compared to present practice, to remove road dirt and other contamination from the container proper after transport and to scrub or treat the container to remove radioactive contamination after loading and unolading operations. In accordance with the present invention, these and other objects are achieved by providing a container for transporting radioactive materials in which the system of heat dissipating fins externally located on the container, having generally the configuration of heat dissipating fins employed in prior structures, is readily removable or detachable from the external wall of the container. Numerous and unexpected advantages are realized by providing detachable cooling fins which are removable from the outer casing of the container rather than being an integral part thereof. By employing removable external fins in accordance with the present invention, it is possible to construct these fins of not only lighter materials, but materials which also have superior heat conducting properties, such as aluminum or aluminum alloys, rather than the heavier metals such as stainless steel. Stainless steel has heretofore been the preferred material for the fins in order that they will be resistant to corrosive cleansing solutions, such as nitric acid, with which the casing must be decontaminated. By providing detachable fins that are removable, they can be taken off the container prior to loading and unloading operations and thereby avoid the necessity of scrubbing or treating the fins with the decontaminating solutions. Thus, the fins may be constructed of materials which would not otherwise be non-corrosive to the decontaminating solution. Accordingly, the fins can be made of lighter, more heat conductive materials. An additional advantage of the present invention is that due to the improved heat dissipation accomplished by using fins made of a substantially higher heat conducive material, it is not necessary to provide for a cooling liquid to circulate within the chamber. Thus, in addition to permitting a more simplified and therefore less costly device, the present invention also dispenses with the need for a central cavity which is substantially larger than the size of the radioactive materials to be contained therein. According to this invention, the central cavity into which the radioactive material is to be placed is substantially no larger than that necessary to accommodate the largest contemplated load of radioactive material since there is no requirement for a space through which to circulate a cooling fluid. A further advantage of the present invention is that it permits easier cleaning. The container proper must be meticulously clean for loading and unloading of the radioactive material since this operation is performed visually under several feet of water which must be sparkling clear to permit observation of operations. Consequently, after detaching the heat dissipation system, the container proper requires only a minimum of cleaning time and materials in order to remove dirt, etc. picked up by the outer shell in transport before it can be loaded or unloaded. What cleaning is required is done to an essentially smooth, flat exterior wall surface instead of one containing the many crevices found in "premanently" finned structures. Similarly, decontaminating procedures are drastically simplified since it is no longer necessary to submerge the cooling fins into the contaminating water during loading and unloading operations. Hence, only the smooth, flat exterior wall surface of the container, and not the fin structure, need be scrubbed or treaded for decontamination. Still another advantage of the heat dissipation system of the present invention is that the cooling fins may be designed for optimum heat transfer with little regard for strength because they can remain on the carrying vehicle and are thus protected from the handling injury they would be subjected to were they part of the container proper which must be moved around in the cleaning, unloading and loading processes. This design thus permits even lower weight in the heat dissipation system than would be practicable were the heat dissipataion system permanently affixed to the container. Still another advantage of the detachable fin system design of the present invention is that although the outer surface of the container proper may become contaminated with radioactivity from exposure to the loading or unloading area and this contamination may exceed the external surface limits for transport permitted by regulatory authority, the container may still be used since it is enveloped by the uncontaminated fin system, and the contaminatin is therefore not accessible on the outside surface of the package as presented for shipment.