Patent Number: 
Section: claims

1. A method for improving the energy generating output of a nuclear reactor while satisfying a maximum subcritical banked withdrawal position (MSBWP) reactivity limit, comprising:ranking enrichments of individual fuel rods by evaluating an uranium enrichment of fuel rods at a two top-most nodes of each fuel rod of a fuel bundle, a highest ranked fuel rod being a fuel rod with the lowest enrichment of uranium at said two top-most nodes, the evaluation of the two top-most nodes being performed by discounting existence of any natural uranium blanket,replacing a highest ranked fuel rod with a fuel rod containing natural uranium in either the top node, or the two top-most nodes,performing a core simulation to determine whether there is any margin to a MSBWP reactivity limit,repeating the replacing and performing function for each lower ranked fuel rod until no fuel rods violate the MSBWP reactivity limit, so as to achieve a desired lattice design for the top of the fuel bundle, andoperating the core of the reactor having fuel bundles configured with the desired lattice design at the top of the fuel bundle. 2. The method of claim 1, wherein the top node of each fuel rod in the fuel bundle is natural uranium, thereby providing the fuel bundle with a 6-inch natural uranium blanket at the top of the fuel bundle. 3. The method of claim 1, wherein the two top-most nodes of the fuel rods of the fuel bundle are between 138 and 150 inches from the bottom of the fuel bundle, and the top-most node between 144 and 150 inches from the bottom of the fuel bundle is natural uranium. 4. The method of claim 1, wherein the top-most node is located between 144 and 150 inches from the bottom of the fuel bundle, and a lower of the two top-most nodes is located between 138 and 144 inches from the bottom of the fuel bundle.