Patent Number: 044951396
Section: summary

FIELD OF THE INVENTION The present invention relates to a container for the storage and shipment of radioactive waste such as spent nuclear-reactor fuel rods. More particularly this invention concerns such a container which is provided with means for monitoring leakage from its interior. BACKGROUND OF THE INVENTION It is standard practice to ship and store spent nuclear-reactor fuel rods in large metallic containers formed normally of vessels and covers both made of spherulitic cast iron or even steel. Such a container is quite large, having wall thickness of 0.2 m to 0.6 m and an overall height of several meters. The vessel can be made as described in copending patent application Ser. No. 379,890 filed 5/1982 of Friedrich Werner, and may have inclusions of shielding metal such as lead or even lead bars imbedded in its walls. The cover of such a container is formed with a plug that fits within the mouth of the vessel. For best sealing action the vessel mouth and plug are complementarily formed with at least one interfitting shoulder bordered by an annular nonplanar--usually cylindrical or frustoconical--surface. Seals, typically O-rings, are set in the confronting surfaces to form several seal barriers. Typically the material inside is stabilized by concrete, but even so radioactive material is quite active. In fact the vessels are often formed with cooling fins for the figuratively and literally hot contents. In order to monitor whether any of the seals has failed, German patent document No. 2,905,094 filed Feb. 10, 1979 with no priority claim by Henning Baatz proposes a system wherein the vessel is formed with several passages that open between the seals. Such a vessel can be pressurized with a tracer gas, or the chambers themselves can be thus pressurized. In this manner a sniffer connected to the other end of any of these passages can detect the presence or absence of this tracer gas as well as any leaked radioactivity. In addition a pressure reading of each of these chambers can often provide valuable information. To this end the upper rim of the vessel is formed with recesses in which the valves for the other ends of the passages open. Thus this rim must be provided with a safety cover to protect these elements. The provision of this extra cover, normally in addition to the above-described cover and a so-called second safety cover overlying it, represents an noticeable manufacturing expense. In addition the passages in the vessel, which may weigh over a ton empty, must be made in situ, that is they cannot be easily conveyed to a shop. This again adds to costs. OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide an improved radioactive-waste container. Another object is the provision of such a radioactive-waste container which overcomes the above-given disadvantages. A further object is to provide an inexpensive such container which is provided with a superior leak monitor. SUMMARY OF THE INVENTION These objects are attained according to the instant invention in a container whose massive metallic vessel, much as in the prior art, has an interior adapted to receive radioactive waste and a mouth formed with inner and outer spaced generally planar and annular vessel shoulders and formed therebetween with a nonplanar intermediate annular vessel surface. A massive metallic cover formed with a plug fits in the mouth and has respective inner and outer plug shoulers closely juxtaposed with the vessel shoulders and a nonplanar intermediate annular plug surface complementary to the intermediate vessel surface. An inner ring seal engages snugly between the inner shoulders. A pair of generally concentric and spaced outer ring seals engage snugly between the outer shoulders and forming an annular outer chamber therebetween. An intermediate ring seal engages snugly between the intermediate surfaces and forms therebetween and with the inner ring seal an annular inner chamber and therebetween and with the outer ring seals an intermediate chamber. The cover is formed with respective inner, intermediate, and outer passages each having one end opening into the respective chamber and another end. Means is provided on the cover at the other ends of the passages for sampling gases therein and in the respective chambers. Thus with the system of this invention the relatively small cover is formed with the passages and is provided with the monitoring means. In fact according to another feature of this invention all the seal rings, which may be of any standard elastic or metallic construction, are received in respective grooves in the cover. The provision of a third chamber on the shoulder at the flange of the cover eliminates the necessity of an additional hermetically tight cover to form an outermost chamber for monitoring leaks. A simple cover serving only to prevent physical damage to the covered structure is all that is needed. All of the passages terminate in respective recesses or pockets formed in the top of the cover and also covered, for safety's sake, by respective bolted-on plates. Obviously these leak monitors are not used a lot; typically they are useful in the event of an accident, such as during transport, when the integrity of the containers might be doubted. According to another feature of this invention the covers are secured by means such as bolts to the vessel at its mouth. Such connection is inexpensive and very strong. The shoulders according to this inventjion are planar and parallel. The intermediate surfaces are surfaces of revolution, normally cylindrical. A body of tracer gas at above-ambient pressure in the vessel makes the system of this invention particularly easy to use to detect leaks. The gas can be in the vessel or in some or all of the chambers, and may be at different pressures in the different chambers so any leakage can be detected. According to this invention the other passage ends are provided with valves of the one-way type, or of the type that only open when connected to an appropriate fitting. Thus leakage at this end of each passage is made impossible.