Patent Number: 053501616
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to metal components and more particularly relates to metal components suitable for use in or near nuclear reactors. The properties of metal components in a nuclear reactor are affected by radiation exposure. For safety reasons, the extent of irradiation-induced change in reactor parts can be a significant factor in reactor design. One component of a nuclear reactor system in which consideration of radiation-induced changes is particularly important is in the fuel assembly. In a conventional reactor, the fuel is contained in rods which are grouped together and held in place by fuel assembly grids. The grids are structured to provide an individual channel for each rod. Coolant is circulated through the channels along the outer surface of the fuel rods. Adequate flow of coolant is needed in order to keep the fuel rods from overheating. Each rod in a fuel assembly grid is held in place within a particular channel by springs, usually cantilever or arched springs. The springs are specially designed to result in minimal disruption to the flow of coolant around the rods, while supporting the rods strongly enough to prevent vibration or longitudinal displacement due to flow forces. Cantilever springs frequently are preferred over arched springs because grids containing cantilever springs have less blockage of coolant flow, and can be made shorter, than grids containing arched springs. Both of these factors contribute to the desirable result of a relatively low pressure drop across grids having cantilever springs. The springs often are made of zircaloy, a zirconium-tin alloy. Cantilever springs made from zircaloy have a disadvantage, however, in that they tend to relax after only a short period of irradiation. Frequently, the load on the springs decreases to zero during one operational cycle of the fuel assembly. While the use of a high initial load may result in a somewhat longer retention period for a positive spring force, the degree of improvement is not substantial. In any event, the use of springs having a high initial load can be disadvantageous because the fuel rods must be inserted very carefully in order to avoid scoring the cladding of the rods. SUMMARY OF THE INVENTION An object of the invention is to provide a metal component which will undergo differential growth upon exposure to a neutron flux. Another object of the invention is to provide a metal component with improved fatigue resistance and a reduced tendency to crack due to radiation exposure. Another object of the invention is to provide a treated metal spring which has a smaller loss in spring force when subjected to a neutron flux than an untreated spring of otherwise similar structure and composition. Yet another object of the invention is to provide a nuclear fuel assembly grid spring made from zircaloy which will allow for relatively easy insertion of a fuel rod against an initial preload, and will maintain sufficient spring force during in-reactor use. Broadly stated, the present invention is a metal component having non-uniform material characteristics which cause the component to undergo differential growth upon exposure to a neutron flux. The metal component can be straight or have any number of folds or curves. The component preferably is a curved or bent sheet or rod suitable for use in a nuclear reactor, and more preferably is a cantilever spring. The type of metal and the thickness or cross section of the component are selected such that the shape of the component will change as a result of its differential growth. "Non-uniform material characteristics" of the component include differences in physical and/or chemical properties of various portions or layers of the component. These differences may be incorporated deliberately in the component at the time of manufacture, or may be brought about by physically and/or chemically treating selected portions of a component which previously had uniform material properties. Preferably, the non-uniformity is obtained by cold working selected portions of a component which previously had uniform material characteristics. The invention is based upon the inventor's recognition of the potential usefulness of the phenomenon of radiation-induced differential growth, i.e., that portions or layers of a metal component which have different material characteristics undergo different rates of growth upon exposure to a neutron flux. This differential growth occurs whether or not irradiation occurs at a constant temperature. By selecting an appropriate pattern of material characteristics for a particular component, the size and shape of the component can be caused to change in a desired way upon exposure to radiation. The change usually is gradual, and often begins immediately when irradiation commences. For example, when a selectively cold worked metal component is exposed to radiation, the portions of the metal component that have been cold worked "grow" faster than the portions that have not been cold worked. Thus, when a layer on the convex side of a curved metal component is cold worked, but a layer on the directly opposite concave side is not cold worked, the degree of curvature of the component increases upon exposure to radiation. On the other hand, if a layer on the concave side of the curved metal component according to the invention is cold worked, a layer on the directly opposite convex side is not cold worked, the degree of curvature of the component decreases upon exposure to radiation. Changes in shape other than changes in degree of curvature also can be effected according to the invention. In a preferred embodiment, a conventional cantilever spring is cold worked along the convex side of the curve at the base of the spring, while the directly opposite concave side of the curve is not cold worked. If the selectively cold worked spring is in an unloaded state and is exposed to radiation, the degree of curvature of the spring increases (the radius of curvature decreases) because the cold worked layer on the convex side of the spring "grows" faster than the untreated layer on the concave side of the spring. Along these same lines, if the selectively cold worked spring is loaded with a fuel rod and is exposed to radiation, the spring will exert a force against the loaded object which is due to the differential growth between the adjacent cold worked and untreated layers. This force will at least partially compensate for any relaxation of the spring which may occur due to radiation exposure. Another feature of the invention is that as a result of the cold working, the fatigue resistance of the component is increased, and cracking of the metal is substantially prevented.