Patent Number: 056152388
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention comprises three different designs. The first design, designated generally as 10 and partially depicted in an elevated view as FIG. 1, consists of an inner tube 12, a sheet or foil 22 of fissionable material wrapped around said inner tube 12, and an outer tube 26 in turn encircling the foil-wrapped hollow inner tube 12. The inner tube 12 has a raised first end 14 and raised second end 16, each end raised to the same predetermined height relative to the surface of the inner tube 12, thereby effecting a relatively depressed center section 18. A narrow welding rib 20 integrally attached longitudinally to the inner tube 12 is further integrally attached to the first raised end 14 and second raised end 16. The depressed center section 18 is adapted to receive a sheet or foil of low enriched fissionable material 22 having a thickness ranging between approximately 0.025 millimeter and 0.25 millimeters, said thickness not to exceed the difference in the height of the raised ends 14, 16. The sheet or foil 22 is generally rectangular and constructed so that two opposite sides of the foil 22, have a length equal to that of the depressed center section 18 of the inner tube 12. The sheet or foil 22 is further configured so that when it is wrapped circumferentially around the depressed center section 18, its remaining two opposite sides will abut against the raised rib 20. The foil 22 remains substantially in place once set into the depressed center section 18. As depicted in FIG. 2, which is a cross sectional view of FIG. 1 taken along line 2--2, the first embodiment 10 further consists of an outer hollow tube 26 or sleeve, having a slit opening extending longitudinally the entire length of the tube 26. The outer hollow tube 26 is fitted tightly over the inner hollow tube 12 and foil 22 such that the split is positioned over the welding rib 20. The outer hollow tube 26 is then compressed against the foil to assure good mechanical contact and the edges of the slit of the tube 26 are then abutted together by a weld 28 or other suitable means. Compression can be done mechanically with hose-clamp devices. The ends of the outer hollow tube 26 are subsequently sealed by welds 29, or other suitable means, to the raised first end 14 and second end 16 so as to prevent foil exposure to ambient atmosphere when the target is cycled. Typically, welding is performed in an inert atmosphere, such as in a glove box permeated with nitrogen or argon. Compression of the assembled target also can be effected hydraulically whereby the inner tube is plastically deformed just prior to welding the ends of the outer hollow tube 26 to the raised first end 14 and raised second end 16 of the inner hollow tube 12 to seal the tube. Any method of compression can be employed, so long as close mechanical contact between the inner hollow tube 12 and outer hollow tube 26 and the foil 22, as depicted in FIG. 2, is achieved to ensure good thermal conduction. In a second embodiment of the invention, depicted as 100 in FIG. 3, a two-tube configuration is also used wherein an outer tube 126 slidably receives an inner tube 112. As with the inner hollow tube 12 of the first embodiment 10, the inner tube 112 of this second embodiment 100 has a raised first end 114 and raised second end 116 so as to provide a depressed center section 118 to receive a sheet or foil of fissionable material 122. Said fissionable material is wrapped circumferentially around the inner tube 112, with the now wrapped inner tube then being inserted into the outer tube 126. Unlike the first embodiment 10, the outer tube 126 of the second embodiment 100 is not longitudinally split from end to end. Therefore, to assure good mechanical contact between the outside surface of the inner tube 112, the sheet or foil of fissionable material 122, and the inside surface of the outer tube 126, the two tubes are tapered to assure a snug fit. Pressure is applied to further assure a tight fit in methods similar to those outlined above for the first embodiment 10. In one such process, the outer tube 126, is first closed at one end with a plug of similar metal 130. During compression, the top closure plug 132 is pressed down on the inner tube 112 and welded to the outer tube 126 under load to ensure maximum tightness in the assembly. The end plugs 130, 132, are received in a male-female fashion by the ends of the outer tube 126. The mechanical bond between the foil 122 and the tubes is further enhanced when the temperature of the device increases during radiation, particularly when the material comprising the inner tube 112 is selected to have a higher coefficient of thermal expansion than the outer tube 126. For example, as aluminum has approximately a 2.5 fold higher coefficient of thermal expansion than zircaloy, any heating of an embodiment having an aluminum inner tube and a zircaloy outer tube will result in a tighter mechanical bond of the two tubes and the foil sandwiched between the tubes. The target is disassembed by cutting off the top plug 132 and pulling out the inner tube 112 with the foil 122. The taper will assist in this operation. In a third embodiment of the present invention the inside surface of an outer tube is lined with a sheet or foil of fissionable material. A first surface of the foil is held securely against the inside surface of the outer tube with a metal cylinder (solid, tubular or sectioned) that contacts and is mechanically expanded against a second surface of the foil. As with the previous two embodiments, the tight contact ensures that fission heat from the foil can be transferred through the tube wall to a coolant material. Substrate Detail A myriad of materials can be utilized as the substrate material for the above-described embodiments. For example, nonfissionable metal materials selected from the group consisting of steel, stainless steel, nickel, nickel alloy, zirconium, zircaloy, aluminum, or zinc coated aluminum can be employed. A variety of zircaloys are suitable, including, but not limited to reactor grade zirconium (UNS #R60001), Zirconium-tin alloy (UNS #R60802), Zirconium-tin alloy (UNS #R60804), and Zirconium-niobium alloy (UNS #R60901). A myriad of substrate shapes are also suitable, including cylinders, plates, spheres and ovals. When dealing with arcuate-shaped substrates, mechanical bonding between substrates and foil is enhanced when a first substrate having the usable convex surface has a higher coefficient of thermal expansion relative to the mating substrate having the concave surface. Upon cycling (and therefore, heating), the convex surface will expand against a first surface of the foil to enhance mechanical bonding. For example, the inventors have determined that with the thermal expansion coefficient of zircaloy of 6-10.times.10.sup.-6, and the thermal expansion coefficient of aluminum at 25.times.10.sup.-6, approximately 3 millimeters of interference occurs at 100.degree. C. if the first substrate consists of aluminum and the second substrate consists of zircaloy. The general dimensions of the target are limited only by reactor design. When working with cylinder-shaped targets, production runs typically require 18 inch lengths. Outer diameters of said cylindrical targets can vary from 2.5 cm to 5.8 cm (1 inch to 2 inches). For example, outer diameters of cylinders used by the inventors was approximately 3.8 cm. for aluminum and 3.2 cm for stainless steel. Cylinder wall thicknesses can vary, but generally range from approximately 0.025 to 0.060 inches. Generally, wall thicknesses are not critical, provided that proper heat conductance is achieved. Preparation of the receiving surfaces of the substrates are crucial, as an advantage of the invention is easy removal of the irradiated foil from the target after cycling. Sticking of the foil, even after compression and cycling, is to be avoided. To avoid such diffusion bonding between the foil and the substrate surfaces, the receiving substrate surfaces are either anodized (to provide an oxide over the metal), or nitrided (whereby the substrate is first subjected to pack-nitriding and then fired). The invented fabrication process and targets provide for target operation between the ranges of approximately 100.degree. C. and 500.degree. C. Foil Detail The method for fabricating the targets, and the targets themselves, utilize low enriched uranium metal or plutonium metal. An advantage of the invention is that a relatively low percent of the total weight of these metals is radioactive isotope. For example, low enriched uranium foil has approximately 19.8 percent .sup.235 U. Foil thicknesses can vary, depending on the target configuration. Thicknesses can range from between approximately 0.001 inches to 0.01 inches. It is the design and fabrication of the invented targets that accommodates the heretofore restrictively high foil thicknesses of more than 0.002 inches, therefore providing an advantage over current state of the art. A supplier for these foils is Marketing Services Corporation, Oak Ridge, Tenn. While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention as defined in the appended claims.