Patent Number: 053348478
Section: description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Bismuth, in its natural state, consists entirely of the isotope 209, which has a half-life of approximately one pentillion (10.sup.18) years and thus is essentially non-radioactive. Bismuth has a relatively low melting point of 271.degree. C., which is approximately midway between the melting points of tin and lead. The density of bismuth is 9.75 grams/cubic centimeter (86% of the density of lead), making it a good absorber of alpha, beta and gamma radiation; moreover, its gamma-ray absorption spectrum compliments that of uranium in that the absorption "edges", corresponding to ionization thresholds of inner shell electrons, appear at quite different energies. At present, bismuth is relatively inexpensive ($0.10 per gram for 99.5% purity). Despite being brittle and thus difficult to machine, bismuth is easily shaped by casting, since it expands approximately 3% upon solidification. Although hot, concentrated mineral acids will attack it, bismuth is otherwise immune to corrosion under most environmental conditions. Also, since bismuth forms salts that hydrolyze in water to become insoluble, it is virtually non-toxic. Bismuth forms high-melting, intermetallic compounds with both uranium and gadolinium, and thus wets both materials. However, molten bismuth close to its melting point will not dissolve significant amounts of either material, or a compound of the two. As a result, bismuth will "tin" both of these metals. That is, molten bismuth will spread over them when molten and adhere strongly to them when cooled. This is similar to the manner in which tin or its alloys coat and adhere to copper or brass. Since bismuth is itself resistant to environmental corrosion, a coating of bismuth will protect a radiation shield made of less resistant metals, such as uranium and gadolinium, from attack by water, air or soil acids. Referring now to FIG. 1, the composition in its preferred embodiment is a container 10 for transporting or storing radioactive material. Container 10, preferably rectangular in shape, comprises a lid 12 and a hollow body 14 forming a central cavity 16. Lid 12 and body 14 are both formed by machining or otherwise forming depleted uranium 18 into respective shapes that are slightly undersized from the desired final dimensions. The shape of body 14, for example, can be formed from a single piece of uranium 18 or, alternatively, from several pieces of uranium 18 held together by appropriate means, such as machine screws or the like. Preferably, each piece of uranium 18 is coated thinly with bismuth, such as by dipping the pieces of uranium 18 into a bath of molten bismuth prior to assembly. If container 10 is to be used for storing or transporting materials having significant neutron emission, body 14 and lid 12 each are equipped with an outer jacket made of a neutron absorber, such as gadolinium. Preferably, lid 12 is equipped with gadolinium pieces 22, 24, also coated with a thin sheet of bismuth before assembly, and applied on the outer areas of lid 12. Similarly, body 14 is equipped with pre-coated gadolinium pieces, such as pieces 26, 28, formed on the outer surface of body 14. Body 14, with or without a gadolinium outer surface, is then coated with a layer of bismuth 32 by an appropriate means, such as by dipping body 14 into a bath of molten bismuth. Preferably, body 14 is placed in a mold made of a high-melting metal to which bismuth does not adhere, such as stainless steel, and molten bismuth is poured into the mold. Body 14 is positioned within the mold so that molten bismuth poured into the mold covers the entire surface area of body 14. Upon cooling, the mold is removed. A similar process is performed on lid 12 whereby a coating of bismuth 34 is applied to lid 12. In FIG. 2, a cross-section of the composition 40 in its preferred embodiment is shown. Composition 40 comprises a layer of uranium 42, which is preferably depleted uranium, an intermediate layer of gadolinium 44, and an outside layer or coating of bismuth 46. Bismuth layer 46, being corrosion resistant, prevents attacks by water, air, soil acids, and the like (shown generally as arrows 52, 54) on gadolinium layer 44 and uranium layer 42, both of which are less resistant to environmental corrosion. In use, composition 40 is placed between a radiation source (not shown) and the area to be shielded so that uranium layer 42 is closest to the radiation source. As previously stated, bismuth layer 46, which is corrosion resistant, protects gadolinium layer 44 and uranium layer 42 from environmental corrosion 52, 54, thereby prolonging the structural integrity of composition 40 and its use as a radiation shield. Most gamma rays (shown generally as arrow 62) emitted from the radiation source are absorbed by uranium layer 42. Any neutron emission (shown generally as arrow 64) from the radiation source will be absorbed by gadolinium layer 44. Bismuth layer 46 absorbs additional stray gamma rays (shown generally as arrow 66) and the bulk of radiation emitted from uranium layer 42, in addition to protecting gadolinium layer 44 and uranium layer 42 from environmental corrosion. It will be apparent to those skilled in the art that many changes and substitutions can be made to the preferred embodiment herein described without departing from the spirit and scope of the present invention as defined by the appended claims.