Patent Number: 042740070
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS This application is related to our copending application Ser. No. 966,951, filed Dec. 6, 1978 and entitled "Radioactivity-Shielding Transport and Storage Receptacle for Radioactive Wastes". The application is also related to the copending application Ser. No. 940,856, filed Sept. 8, 1978 and entitled "Transport-Storage Vessel for Radioactive Materials". The latter application makes reference to our application Ser. No. 940,098 (see also Ser. No. 107,276 filed Dec. 26, 1979). The prior art best known to applicants is the art of record in said applications, which are included herein by reference in their entireties. FIELD OF THE INVENTION Our present invention relates to transport and storage vessels or canisters for nuclear materials, especially nuclear wastes such as irradiated or spent fuel elements for nuclear reactors and, more particularly, to an improved radiation-shielding transport or storage vessel. BACKGROUND OF THE INVENTION As will be apparent from our prior applications mentioned above and the developments in the transport and storage of nuclear wastes referred to by the art of record in these applications, it is recognized that the storage of radioactive materials, such as spent nuclear fuel rods or other materials, can be effected with canister receptacles or vessels, which are sealed after the nuclear waste is introduced and which have radiation-shielding properties as a result of the wall thickness of the vessel body and/or radiation absorbing characteristics of the material, from which the body is composed. For example, the body may be made of a material having a high neutron cross section or materials with a high neutron cross section can be incorporated in the body. Alternatively, or in addition, passages or spaces may be formed in the body and gamma-ray or neutron absorptive or moderating materials can be introduced. Obviously the vessel must have excellent structural integrity as well as a capacity to act as a radiation shielding material. The canister basically comprises a chamber-forming body or receptacle, advantageously upwardly open, with a relatively thick vertical wall and a closed bottom and a cover for the upper end or mouth of the chamber. The vertical walls are usually comprised in one piece with the bottom of cast iron, preferably spherolytic cast iron, or cast steel, while a recess is formed in the upper end of the body so that the cover structure can be recessed therein with a plug-like fit and an upper surface flush with the upper surface of the body. The earlier systems also provided for sealing the cover to the body and even monitoring the state of the seal to ascertain whether any leakage may have occurred. Such monitoring is relatively simple because, in addition to the radioactive materials filling the chamber, the latter receives a control gas blanket and any failure of the seal can be detected by monitoring the composition of this control gas or detecting the presence thereof. It has also been noted that the cast iron or steel alloy can include radiation-absorbing alloying components and/or radiation-absorbing inclusions. The term "radiation shielding" as used to describe a canister or vessel, thus defines a vessel whose thickness at any point (wall, cover or bottom) is sufficient to prevent escape of radiation, whether the radiation be of the gamma or neutron type and which also is able to withstand the mechanical stresses to which the vessel may be subjected in handling the in-transport or the like, both during normal manipulations and in the event of a disaster, such as a crash of a transporting vehicle. The most general application utilizes flange-type covers which are bolted in place to the body, and it has been recognized that these systems may be problematic because the seal is maintained only as long as the bolts are tight or intact. In transport accidents, however, the bolted flange receptacles run the risk of shearing or loosening of the bolts and release of radioactive materials into the environment. Thus it is possible to "sniff out" a failure of the seal by monitoring the presence of the control gas outside of the edge of the cover and utilizing an appropriate gas detector or analyzer. If a failure of the seal is observed, components of the radioactive waste or gaseous substances, which are formed by radioactive decay of the waste, can be found in the gas and may wander out of the receptacle. Repair of the defective seam or seal is not a simple matter because removal of the cover can result in a serious contamination of the surrounding space by release of radioactive materials from the interior. The repair thus can only be carried out in a so-called hot-cell at considerable cost. OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved radiation-shielding storage or transport container which allows more effective monitoring of the seal between the cover and the container body, and thereby prevents the escape of radioactive substances more reliably. Another object of the invention is to provide a method whereby a defective seal in a device of the type described can be detected and repaired without the problems heretofore encountered. It is another object of this invention to provide an improved transport vessel and sealing system for such vessel which simplifies mounting of the seal and repair without requiring a hot-cell or creating the danger of contamination in the event of an interruption of the seal. SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter are attained in accordance with the present invention in a system in which the plug-type cover is mounted in a stepped recess at the mouth of the container body and is overlain by a safety cover which peripherally overhangs the first-mentioned cover and is likewise recessed in the end of the container. Sealing means is provided in gaps between the two covers and respective juxtaposed surfaces or seats formed by the recesses so that these seals are substantially complete peripherally, i.e. uninterrupted, and define them control spaces to which passages run and which can be connected by these passages to gas-monitoring devices responsive to the control gas. The passages can be formed in the security cover, the plug-type cover and/or in the wall of the body as may be required and advantageously a plurality of such seals can be provided with each pair of spaced seals defining a respective control space between them. In a preferred embodiment of the invention, the complementary portions of the plug-type cover and the recess in the body have an outward conical convergence, i.e. are slightly tapered downward and have a frustoconical configuration, a plurality of seals being provided axially (vertically) spaced between the frustoconical portions. This arrangement prevents damage to the seals upon insertion of the plug-type cover, especially when seals are carried by grooves in the plug-type cover. According to another feature of the invention, the conical annular seals are O-rings, partially trapped in outwardly open circumferential grooves on the conical plug member of the cover. According to yet another feature of the invention, the receptacle body is formed with an inwardly extending shoulder confronting the lower end of the plug member, while a further annular seal is provided between the shoulder and the end of the plug member. The container walls can be provided along the upper edge of the recess with a lip which lies along a lip of the cover so that the two can be selded together as described in application Ser. No. 966,951. It is apparent that the system of the present invention has the advantage that a multiplicity of seals can be provided, thereby increasing reliability and safety. However, the control possibilities are also increased with this arrangement, for example, if it is assumed that a leakage past one of the seals is detected upon monitoring or analysis of the control gas by the gas detector, the safety cover can be simply removed and replaced with a fresh seal. Only a small amount of radioactive material can accumulate in any annular space between two seals so that the danger of large-scale contamination is excluded. If the passage is so designed that the successive control spaces can be monitored individually, it is possible to evaluate the progress of seal failure and counter this by welding shut the container in the event one of the inner seals appears to develop a leak. Furthermore, it is possible to seal the inner cover by welding and then replace the safety cover when the monitoring operation shows that one of the earlier seals in the path of the gas is beginning to leak.