Patent Number: 053012186
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the fuel element or pin 10 comprises an elongated body of fuel 12, such as a rod sometimes referred to as a pin or slug, containing a fissionable material in the form of metal or metal alloy. The fuel body 12 (or several aligned fuel bodies) is housed within a sealed, tube-like metal container 14, sometimes referred to as "fuel cladding". Cladding 14 isolates the fuel body 12 from the coolant which flows over the exterior surface of the fuel element 10 to transfer heat away therefrom, thereby protecting the fuel from reaction with or contamination from the coolant. The cladding also seals in fission products and precludes their escape into the coolant. The fuel body 12 is designed in configurational dimensions for radial expansion of about 25 to 30 vol. % due primarily to internally generated, fission-produced gases. Thus, the initially produced metal fuel units are of substantially smaller cross-sectional area than the internal cross-sectional area within the fuel cladding 14. As a result, an intermediate space 16 is formed between the exterior surface of the fuel body 12 and the interior surface of the fuel cladding 14. This initial intermediate space 16 is designed to accommodate the expansion of the fuel body 12 attributable to the gases produced during service and to protect the fuel cladding 14 from physical stress and possible rupture which would otherwise arise due to internal pressure of a confined body of expanding fuel. The intermediate space 16 of the fuel element 10 is initially filled during the fuel fabrication process with a liquid metal bonding material such as sodium, potassium, lithium or their alloys in solid form, which becomes molten at normal reactor operating temperatures and is displaced by the fuel as it expands. The bonding material enhances heat transfer from the fuel outward to the cladding while the fuel is expanding to fill the intermediate space. The fuel units of a typical fuel element for service in a liquid metal-cooled nuclear reactor are cylindrical bodies having a diameter of approximately 0.19 inch. The stainless steel cladding which surrounds such fuel units has a wall thickness of about 20 mils and an outside diameter of about 0.26 inch. The barrier in accordance with the invention is a multi-layer expandable body 18 of expendable alloying metal which is positioned between the fuel unit and fuel cladding. As a result, the metal alloy fuel interacts with the barrier material, not the stainless steel of the cladding. The barrier may be composed of any alloying metal having the property of increasing the melting temperature of metallic fuels, such as zirconium, titanium, niobium, molybdenum, vanadium, chromium and the like. Zirconium is a preferred alloying metal for the practice of the invention. In accordance with the preferred embodiment of the invention, the barrier is a multi-layer rolled foil 18 of zirconium or functionally equivalent metal. As best seen in FIG. 2, metal foil is rolled into the shape of a multi-layer tube and then laser tack-welded to hold the foil in the rolled state. The laser welding penetration is adjusted so that a foil weld 20 fuses only two or more of the outermost layers. At the weld point, the rolled foil must comprise at least one more layer than the number of foil layers penetrated by the laser weld. The weld 20 is designed to fail in response to fuel or blanket alloy swelling during irradiation. The fused material holding the outermost and one or more of the middle layers together fails at a level of stress which is less than the level of stress at which the material of said foil would rupture. After weld failure, the overlapping layers slip and the multi-layer foil unrolls, as shown in FIG. 3. For the purpose of illustration, FIG. 3 shows a failed weld area 22 in the outermost layer circumferentially shifted relative to a weld residue 20' still fused on the middle layer. (Obviously the positions of the failed weld area and the weld residue could be reversed.) If the barrier were comprised of only two layers at the point of tack welding, the relative movement of the two foil layers could give rise to a direct "line of sight" for radiation through the failed weld area 22 if the latter cleared the edge of the second layer. However, in accordance with the invention, a third foil layer is provided to assure that a barrier of at least two foil layers is presented along every " line of sight" from the fuel body 12 to the cladding 14. Thus, unrolling of the foil masks defects in the individual foil layers arising from small undetected manufacturing defects or weld failures, thereby providing a barrier which accommodates swelling associated with the metal fuel and blanket alloys. In the initial step, the tube is formed with at least 21/2 turns of the foil and then tack-welded. Laser weld development with 0.002-inch-thick pieces of zirconium foil revealed that the weld power could be adjusted so the weld penetrates the outer layers of foil in a stack without affecting the innermost layer. The geometric arrangement of cladding and barrier must satisfy the following relationship to assure that the foil movement masks the weld defects with one less foil layer than is initially provided at the weld point: EQU (C-2T-B).pi.+3S&lt;L where C is the cladding diameter; T is the cladding thickness; B is the barrier diameter; S is the tack weld diameter; and L is the inside foil lap past the weld area. This expression assures that the inside foil layer laps the weld area enough that the failed weld areas in the unrolled or slipped foil barrier will not provide a direct "line of sight" between the cladding and fuel or blanket alloy. The preferred embodiment shown in FIG. 1 has been disclosed for the purpose of illustration only. It will be readily appreciated that the invention is not limited to a rolled foil barrier having only three foil layers at the point of tack welding. The barrier may have more than three foil layers at the point of tack welding provided that at least two but less than all of the foil layers are fused together by tack welding from the outermost side.