Patent Number: 043364608
Section: summary

DESCRIPTOIN TECHNICAL FIELD The present invention relates generally to shipping containers, and more particularly relates to shipping containers used to contain spent nuclear fuel assemblies during transport. BACKGROUND In the operation of a nuclear reactor, the fissionable nuclear fuel is typically contained in a fuel rod bundle or assembly. The fuel in the reactor is useful for a period of time and then becomes depleted and is no longer useful for a fission reaction in its present form. The nuclear fuel may, however, be reprocessed by techniques known in the art to yield fissionable material once again. Since reprocessing facilities are few and far between, spent nuclear fuel assemblies must often be transported for great distances. Although the spent nuclear fuel is no longer useful in fission reactions, it is nevertheless still "hot" in terms of both radioactivity and temperature. In order to render the fuel assembly safe for handling outside the reactor vessel, the fuel assembly is usually loaded into a shipping cask. U.S. Pat. No. 3,962,587 (incorporated herein by reference) discloses such a shipping cask. The shipping casks generally comprise a body for containing the spent nuclear fuel assembly therein. The body of the cask is typically made of a heavy shielding material to absorb radiation from the material inside. The shipping cask is usually loaded by lowering into a storage pool at the nuclear power plant. When the cask is removed from the pool, it contains both the fuel assembly and water from the pool, which may or may not be drained off later. The loaded cask is extremely heavy and since it contains highly radioactive material, measures must be taken to insure that the cask is not dropped. In order to provide the desired extra measure of safety required, the trunnions attached to the cask for lifting the cask are usually oversized to withstand several times the normal load levels. Inspection is usually required against material failure and/or attachment failure, such as a defect in a weld. Thorough inspection is often difficult or impossible, and is always a costly expedient. Another expedient is to provide a redundant number of trunnions, along with a hoisting yoke which attaches to all trunnions and which transfers the load to the remaining trunnions if one trunnion should fail. That expedient is also expensive, and the added presence of the yoke impedes the loading and unloading of spent fuel assemblies. The shipping cask is typically transported by railroad car or by truck. Since the possibility of a crash is always present, precautions must be taken to minimize the probability of a cask rupture upon impact with an obstacle. Impact limiters have been used in the past to minimize this probability. An impact limiter generally comprises a cap-like structure filled with a crushable material which fits on the ends of the cask. Upon impact the crushable material absorbs a large portion of the impact energy, thereby reducing the forces applied to the body of the cask. Controlled absorption of the impact is a desirable feature of such impact limiters. SUMMARY OF THE INVENTION The present invention relates to a shipping cask for spent nuclear fuel, the cask having multiple element trunnions attachable thereto to provide load path redundency in case of a material failure and to provide for ease of inspection. The present invention also includes a multiple element lifting yoke compatible with the trunnions on the cask and impact limiters for the ends of the cask. The shipping cask generally comprises an elongate body for containing spent nuclear fuel assemblies. A pair of trunnions are detachably disposed on the outer surface of the body adjacent the top end thereof. The trunnions each comprise a first element attached to the body to provide a first load path and a second element attached to the body to provide a second load path. The lifting yoke generally comprises a pair of lifting plates assemblable about a conventional crane hook and a pair of side arms attachable to the lifting plates. The lower end of each side arm is engagable with the trunnions of the shipping cask. Both the lifting plates and the side arms are comprised of a plurality of elements detachably laminated to one another. The impact limiters generally comprise a cap-like structure filled with a crushable substance and engagable with the ends of the cask. The impact limiters have an impact surface which is convex to thereby distribute impact forces gradually over the impact surface. Accordingly, it is an object of the present invention to provide am improved cask for transporting spent nuclear fuel. It is another object of the present invention to provide an improved lifting apparatus for lifting spent nuclear fuel casks. A further object of the present invention is to provide improved impact limiters for spent nuclear fuel casks. Yet another object of the present invention is to provide a spent nuclear fuel cask having lifting trunnions which provide separate and distinct load paths between the trunnions and the cask. Another object of the present invention is to provide multiple element trunnions which are removable from a spent nuclear fuel cask to provide easy inspection for defects. Still another object of the present invention is to provide lifting apparatus which is multiply redundant and disassemblable for easy inspection. These and other objects, features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiment and the appended drawing and claims.