Patent Number: 042886988
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS This application is related to the commonly assigned copending applications naming as inventors one or more of the present applicants: Ser. No. 875,079 filed Feb. 3, 1978 (U.S. Pat. No. 4,229,316 issued Oct. 21, 1980), Ser. No. 903,093 filed May 5, 1978 (U.S. Pat. No. 4,235,739 issued Nov. 25, 1980), Ser. No. 940,098 filed Sept. 6, 1978 (U.S. Pat. No. 4,234,798 issued Nov. 18, 1980), Ser. No. 940,856 filed Sept. 8, 1978 and Ser. No. 966,951 filed Dec. 6, 1978. FIELD OF THE INVENTION The present invention relates to a container or vessel for the transport and storage of radioactive materials, especially radioactive wastes from nuclear-reactor installations and particularly wastes which arise in nuclear power-plant operations. BACKGROUND OF THE INVENTION As described in the above-identified copending applications and the literature referred to or cited therein and in the same class, the problem of disposing, storing and transporting radioactive materials such as radioactive wastes obtained in nuclear power plant operations, such as irradiated fuel elements, generally requires that a vessel, canister or container be provided which can be hermetically sealed and is of a sufficient thickness to block the emission of radioactivity from the contents of the vessel to the ambient environment. It has been proposed, for example, to provide relatively thick-walled vessels which can be cast from high-density materials providing a gamma-radiation shield, e.g. from cast iron, cast steel or spherolitic (i.e. spheroidal-graphite or nodular) cast iron, the cast structure having sufficient structural strength to withstand rough handling, little tendency to fracture, rupture or crack, and excellent gamma-radiation shielding effectiveness. The generally upright containers, e.g. containers formed in one piece with a bottom and vertical walls, can be provided with a plug-type cover to prevent the escape of radiation in the end of the container which is closed and means can be provided to serve as a neutron absorber or moderator. In general the latter means can consist of a material having a higher absorption cross section for neutrons. For example, some of the above-identified copending applications describe constructions of a container for the purposes set forth in which passages are provided at least in the vertical walls of the vessel, e.g. during the casting thereof, into which a moderating material is introduced. When reference is made herein to a cast vessel wall of cast iron or spherolytic cast iron, however, it should be understood that it does not exclude a matrix of the cast metal in which gamma-radiation absorbers are embedded. Any gamma-radiation absorbers conventional in the art, therefore, may be disposed in such a metal matrix within the purview of the present invention. Neutron moderators can be identified herein as materials capable of braking the energy of neutrons to velocities which render the neutrons incapable of detrimentally affecting living organisms and nonliving systems. In a transport and storage vessel which has been found to be particularly effective, e.g. as described in German patent document (Utility Model-Gebruuchsmuster) No. 77 27 690, the moderator passages extend vertically and are spaced apart about the periphery of the vessel with as close a spacing as possible and are given an elongated cross section to form, to the greatest extent feasible, a closed shield around the contents of the vessel. The elongated cross section was of oval or rectangular type and extended so that the major dimension of the cross section, the major axis of the ellipse, was tangential, secantial or otherwise offradial so that projections of the passages in the radial direction generally overlapped. Even when the passages had other cross sections, the prevailing principle was to provide the moderator cross section as large as possible consistent with structural stability of the vessel walls and the space between the passages as close as possible also consistent with such structural stability. As the passages are positioned more closely, or the volume of the passages is increased for a given wall thickness, the strength of the wall diminishes and hence thicker walls must be used for a given set of parameters in terms of passage cross section and spacing. Furthermore, in the earlier vessel construction, the moderator-containing passages were generally located in a row separating inner and outer layers of the cast material from one another so that the connection between these layers was effected with relatively thin webs. This again was detrimental to the mechanical stability of the device. The primary disadvantage was the inability of the vessel to withstand sudden shock as must be tolerated in transport and other handling of the container. Here again, the remedy was to increase the mass, wall thickness and size of the container to counter the decreased stability and thereby increase the handling complexity, transport cost and fabrication cost. OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved container for the transport and storage of radioactive materials having excellent strength and neutron moderating characteristics, a reasonable weight and the ability to withstand significant mechanical stress. Yet another object of the invention is to provide a container for the purposes described which is free from the disadvantages of earlier systems and yet can be fabricated at low cost and with reduced material consumption for a given weight of mechanical and radiation-shielding criteria. It is also an object of this invention to provide a transport and storage vessel extending the principles set forth in the above-identified applications. SUMMARY OF THE INVENTION We have now discovered, most surprisingly, that it is possible to overcome the disadvantages enumerated above and provide a structurally sound container for the storage and transport of radioactive materials, if one conceives of a cast vessel surrounded by an imaginary layer of the neutron-moderating material whose thickness and type is sufficient to attenuate the neutron velocity so as to render it environmentally safe for the particular contents of the vessel. Such a thickness will have a given volume, depending upon the material stored and the neutron moderator material and the layer will, moreover, have an area in a cross section transverse to the vessel walls, e.g. perpendicular to the upright axis of the vessel. When the same moderator material is disposed in circular cross-section bores or passages formed in the wall and the cross sections of these passages in the same plane as that mentioned above is totaled, the total cross-sectional area of the bores should be at least equal to the cross-sectional area of the imaginary layer in the same plane and the spacing of the bores should be at least twice their diameter in accordance with the present invention. Thus the volume of the moderator material actually used, i.e. in the passages, should be equal to or greater than the volume of the material of the imaginary layer. What is surprising and totally unexpected, is that under the conditions set forth the vessel has a moderating effect, using spaced apart passages, which would be at least equal to that which would be associated with a vessel of the same wall thickness to which a uniform layer was applied of the same moderator material in spite of the fact that the passages are spaced apart in a grate-like pattern. Because of the relatively large spacing which can be provided, the amount of wall material between passages is large so that the overall strength of the cast iron or spherulitic cast iron body is extremely high, the vessel is far less susceptible to rupture or cracking than heretofore and it is not necessary to unduly increase the wall thickness. In fact, experience has shown that the wall, instead of being weakened by the passages can be analogized to a multi-cell girder structure or like structurally stable compartmented body with usually high strength and resistance to impact. While practically any neutron moderator material conventional in the art can be used, water has been found to be preferred. According to another feature of the invention, the passages are closed by the cover which has a plug-like configuration and fits into a recess in the body, the cover and the bottom being likewise formed with channels or passages containing the moderator material. The latter channels may have a somewhat elongated cross section and preferably have semicylindrical bottoms and parallel flanks. These channels can open at the surface of the cover and bottom and can be closed, in turn, by sealing plates.