Patent Number: 046738142
Section: summary

FIELD OF THE INVENTION The invention relates to a container for receiving and safely storing radioactive materials and other materials damaging to living organisms. The container is especially suited for storing vitrified radioactive fission products or irradiated nuclear reactor fuel elements. The container includes a vessel and a sealing cover. One end of the vessel is provided with a circular opening into which the sealing cover can be placed to tightly seal the container. BACKGROUND OF THE INVENTION Radioactive materials such as vitrified radioactive fission products or irradiated nuclear fuel elements have to be placed in special containers for the purpose of transport and storage. The containers have a high shielding to radiation and a sufficient cooling surface as well as a high stability. Metal containers guarantee a safe enclosure for radioactive waste products. The metal containers are open only at one end and it is this opening through which the materials to be stored are passed. After filling a container destined for terminal storage with glass from highly radioactive fission products, it has been conventional practice to place a cylindrical cover in the open end of the vessel and to weld the cover to the abutting container rim. By means of the tight seal of the vessel with the sealing cover, it was believed that the radioactive materials or materials damaging to living organisms could be safely separated from the ambient. The results obtained up until now with this procedure have been unsatisfactory. The welding activity has to be carried out in a so-called "hot cell." Accordingly, it was necessary to conduct the welding operation from a remote location with the aid of remotely-controlled apparatus. Up to now, the desired impermeability of the seal to gas of 10.sup.-3 Torr Liter/Second could not be obtained with certainty and reproducibility. The operating person carrying out the welding operation must necessarily perform each welding operation individually and always with a different quality. SUMMARY OF THE INVENTION It is an object of the invention to provide a container of the kind described above wherein a safe closure of the container is achieved by the sealing cover with the required impermeability to gas. It is a further object of the invention to provide method for tightly sealing the container. The container includes a vessel and a sealing cover for closing the vessel off from the ambient. According to a feature of the invention, the open end of the inner bore of the vessel is widened to define a conical surface whereat the sealing cover is seated when pressed into the vessel, the sealing cover having an outer peripheral conical surface formed to converge toward the interior of the vessel and the outer peripheral conical surface having a taper corresponding to the taper of the conical surface of the vessel. The wall of the vessel and the sealing cover are welded together by means of a fused-mass extending about the periphery of the sealing cover. After the vessel is filled, the conical sealing cover is pressed into the conical opening of the vessel. As the conical sealing cover is pressed into the vessel, the sealing cover exerts a radial force on the vessel wall surface so that irregularities of this surface surrounding the cover are minimized and even eliminated. The surface of the vessel wall adapts itself well to the outer peripheral surface of the sealing cover. By means of the conical configuration of the two surfaces which are pressed against each other, namely, the conical surface of the vessel and the peripheral surface of the sealing cover, a considerable improvement in the seal between the inner wall of the vessel and the sealing cover is obtained. The conical sealing surfaces are responsible for the improvement in the seal of the connection between sealing cover and vessel. After completing the fused-mass welding operation, a safe and tight closure of the container is obtained. In an advantageous further embodiment of the invention, the upper outer edge of the sealing cover lies beneath the rim of the vessel whereby the vessel rim and the upper edge of the sealing cover are welded together with a fillet weld extending about the periphery of the sealing cover. If the conical sealing cover is pressed so deep into the conical section of the container so that there is sufficient room between the upper edge of the sealing cover and the rim of the vessel to accommodate a fillet weld, then the required impermeability to gas and mechanical joint between the vessel and sealing cover can be obtained. According to another feature of the invention, the upper portion of the outer peripheral surface of the cover can be bevelled so that the same diverges away from the remainder of this peripheral surface. With this short cylindrical bevel of the cover, a welding starter gap of wedge-shaped configuration is obtained between the inner wall of the vessel and the peripheral surface of the sealing cover. After pressing the sealing cover into the conical seat of the vessel, the sealing cover is welded to the vessel by means of a fused-mass weld under the application of shielding gas. In this way, the shielding gas can blow the melt into the annular wedge-shaped gap. This embodiment is especially suited for a remotely-controlled welding process which can be conducted automatically in a hot cell. The quality of the weld joint is higher than if an operating person individually conducted each welding operation. Further, the weld contemplated by this embodiment of the invention is reproducible every time. In still a further embodiment of the invention, the sealing cover is provided with a valve accessible from the outside and which valve communicates with the interior of the vessel; this arrangement permits the valve to be connected to a test gas source. By holding the valve open, an equalization of pressure is possible during the welding process between the space defined by the sealing cover and vessel and the space surrounding the system. On the other hand, after the welding process has been terminated, a simple test of the impermeability to gas of the weld joint is possible. A further advantageous embodiment of the invention is achieved by mounting a valve in a recess of a projection formed on the sealing cover. The recess defined by the projection constitutes a protective chamber for the valve so that the latter will not become damaged should the container be inadvertently dropped. Another advantageous embodiment of the invention is to provide a plug for closing off the recess. The plug can, for example, be configured to threadably engage an internal thread of the recess. Where the parts are made of metal and the container receives radioactive waste materials, it is preferable to weld the plug with the projection. According to a further embodiment of the invention, the projection can be configured as a cylinder and adapted to threadably engage the sealing cover in a centrally disposed threaded bore formed in the latter. The projection can furthermore be given the shape of a knob. This embodiment permits the operation of sealing the container to be conducted without difficulty by remotely-controlled programmed machines. The invention also is directed to a method for tightly sealing a container for receiving and safely storing radioactive materials and other materials dangerous to living organisms especially such materials as vitrified radioactive fission products or irradiated nuclear reactor fuel elements, and wherein the container includes a vessel having a circular opening at one end thereof for receiving the materials to be stored and which opening is closed off from the ambient with a sealing cover placed therein. The method of the invention includes the steps of: conically widening the inner bore of the vessel at the opening end thereof to define a conical seating surface; turning the outer peripheral surface of the cover to have a conical surface having the same taper as the taper of the seating surface; turning the upper portion of the outer peripheral surface of the cover to define a cylindrical surface; pressing the cover down onto the conical seating surface to a depth below the rim of the vessel after filling the vessel with the materials to be stored thereby defining an annular groove of wedge-shaped section; joining the cover to the vessel by means of a gas-shielded arc weld while maintaining an equalization of pressure between the interior of the container and the ambient, the flow of shielding gas being directed from above into the annular gap of wedge-shaped section and, the weld formed in this manner including: a first portion defining an annular bevel weld filling in the annular groove of wedge-shaped section; and, a second portion defining an annular fillet weld disposed in the fillet defined by the top peripheral edge surface of the cover and the remainder of the conical seating surface of the vessel above the bevel weld; and discontinuing the maintenance of the equalization of pressure after completing the welding step. With the aid of the invention, containers for receiving material which is radioactive or dangerous to living organisms can, after they have been filled, be safely sealed with a high impermeability to gas and again be tested as to the integrity of the seal. The invention permits the utilization of remotely-controlled programmed robots and automatic welding equipment to produce connections of a reproducible high quality.