Patent Number: 059490845
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in FIG. 1 through FIG. 2. It will be appreciated that the apparatus may vary as to configuration and as to details of the parts without departing from the basic concepts as disclosed herein. Referring to FIG. 1, a radioactive material storage apparatus 10 in accordance with the present invention is generally shown. Apparatus 10 generally comprises an inner vessel 12, an outer casing 14, a particulate matrix 16, a cover 18 and impact limiters 20, 22. Inner vessel 12 forms a chamber 24 structured and configured to receive and contain radioactive material therein. Inner vessel 12 is preferably composed of a metallic material such as stainless steel or like material possessing structural rigidity, yield strength and the capability of effectively shielding photon radiation, such as gamma rays, that are emitted from radioactive material stored therein. An opening 26 on inner vessel 12 provides external access to chamber 24. Outer casing 14, which provides photon radiation shielding capability, is also preferably composed of a metallic material such as stainless steel or like material and surrounds inner vessel 12 such that a space 28 is defmed between the inner vessel 12 and the outer casing 14. Space 28, which is preferably annular, completely envelops chamber 24, with the distance between inner vessel 12 and outer casing 14 being generally uniform around chamber 24. Space 28 is filled with particulate matrix 16 which is a neutron attenuator and absorber. Outer casing 14 also serves as a protective covering for inner vessel 12. Cover 18 is placed over opening 26 and completely seals chamber 24. Cover 18 includes an outer wall 30, an inner wall 32 and a cavity 34 disposed between outer wall 30 and inner wall 32. Outer wall 30 and inner wall 32 are preferably stainless steel. Cavity 34 is also filled with particulate matrix 16 to provide neutron attenuation and absorption. Referring also to FIG. 2, particulate matrix 16 comprises a plurality of metallic particles 36 tightly packed together. Metallic particles 36 are typically composed of depleted uranium or lead and are preferably but not necessarily spherical as shown, however, the use of spherical particles assures a maximum packing density so that the thickness of space 28 can be minimized. The advantage gained by minimizing the thickness of space 28 is that heat generated by the radioactive contents stored within chamber 24 is more easily dissipated by thermal conduction. Depleted uranium and lead both have the capability of attenuating photon radiation from radioactive materials, but by themselves cannot absorb or attenuate neutrons. Therefore, the voids or interstices 38 present between metallic particles 36 is filled with a neutron absorbing material 40. Neutron absorbing material 40 provides both moderation and absorption of neutrons emitted from radioactive materials stored therein. Conventionally known hydrogenous and neutron poison materials are used to formulate neutron absorbing material 40. Preferably, neutron absorbing material 40 is an aggregate mixture composed primarily of calcium sulfate, boric acid and water. Approximately, two parts calcium sulfate is mixed with one part boric acid and one part water to form a pourable aggregate mixture containing the neutron moderator and absorber. The mixture is poured or injected into annular space 28 formed between the inner vessel 12 and the outer casing 14. Metallic particles 36 are then added to the mixture which settle to the bottom 42 of space 28 and begin filling up to the top 44 of space 28. Vibrating the mixture during filling is helpful for settling metallic particles 36 properly within space 28. The mixture containing the metallic particles 36 generally harden within an hour, thus forming particulate matrix 16 which shields against both photon and neutron penetration. The hydrogen present in the water is known to be a neutron moderator, which has the effect of slowing the neutrons as they pass through the water. Borated water, which contains boron, can also be used in the aggregate mixture. Since boron is a natural neutron absorber, borated water increases the ability of the aggregate mixture to absorb neutrons, in addition to its effectiveness as a gamma shield. The combination of hydrogen and boron within the mixture, therefore provides the neutron moderation and absorption. The use of particulate matrix 16 in space 28 eliminates the requirement for a separate neutron shield, and the thermal conductivity of apparatus 10 need not be degraded by the application of such a separate neutron shield. The packing efficiency of particulate matrix 16 is preferably approximately 62.5%. The density of neutron particulate matrix 16 using depleted uranium spheres is preferably approximately 60% of the weight of solid lead or approximately 7.09 g/cm.sup.3, and its photon shielding capabilities would be equivalent to that of lead occupying the same volume, however, the use of depleted uranium in particulate matrix 16 provides the added advantage of incorporating hydrogen atoms for neutron moderation and boron atoms for neutron absorption. The size of metallic particles 36 must be large enough to eliminate the possibility of pyrophoric ignition. The volume of metallic particles 36 is typically in a range between approximately 50% to 70% of the volume of particulate matrix 16. Preferably, the minimum diameter of the metallic particles 36 must be at least 0.5 mm, and the maximum diameter is selected so that the packing efficiency is maximized. Other factors determining maximum sphere diameter are the practically and availability of manufacturing methods and associated production costs. Particulate matrix 16 adds strength and stiffness to apparatus 10 while also providing an effective impact limiter. Kinetic energy due to physical impact on apparatus 10, which may occur during transportation of apparatus 10, would be dissipated by the crushing of particulate matrix 16 at the point of impact. Therefore, outer casing 12 only requires sufficient ductility and tensile strength to deform without rupturing. The addition of impact limiters 20, 22, which are typically fabricated from wood, provide further impact or shock absorption to apparatus 10. Retrieval and/or recycling of metallic particles 36 in particulate matrix 16 is possible using conventional techniques. For instance, particulate matrix 16 would be broken up into small pieces by mechanical agitation, such as vigorous shaking, air blasting or the like. Accordingly, it will be seen that this invention provides for a vessel which allows for storing and/or transporting radioactive materials, which minimizes space requirements due to its "thin wall" design, and which has metallic components that can be recycled using conventional techniques. Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus the scope of this invention should be determined by the appended claims and their legal equivalents.