Patent Number: 
Section: claims

1. A method of implementing an economical first core for a pressurized water nuclear reactor, said method comprising:providing energy output and discharge burn-up data for substantially all of a plurality of regions that make up a target equilibrium reload cycle core, each of said regions comprising a plurality of equilibrium reload cycle core fuel assemblies;providing a plurality of new fuel assemblies which when configured in a first core of a pressurized water reactor emulate the energy output and discharge burn-up of the corresponding regions that make up the target equilibrium reload cycle core, each of said new fuel assemblies having a vertical length and having not been previously irradiated in a reactor core, a majority of said new fuel assemblies having a different elemental make up than a corresponding one of the equilibrium reload cycle core fuel assemblies, said new fuel assemblies having different average enrichments of uranium 235 including high enrichment fuel assemblies and low enrichment fuel assemblies; andarranging said plurality of new fuel assemblies in a first core configuration of a pressurized water reactor based upon said data, in order to emulate said target equilibrium reload cycle core in terms of spatial reactivity distribution, said first core including an interior and a periphery,wherein said first core is entirely made up of said new fuel assemblies,wherein substantially all of said high enrichment fuel assemblies are loaded toward the interior of said first core, andwherein an advanced lattice design and associated structure are provided for at least some of said plurality of new high enrichment fuel assemblies of said first core in which there is no extended barrier which would prohibit cross flow of a coolant over the vertical length of said fuel assemblies, in order that lateral circulation of said coolant across said fuel assemblies is provided as said coolant moves vertically, with the advanced lattice having a peripheral row of fuel rods extending around the sides of the fuel assembly and a number of interior rows of fuel rods wherein most of the fuel rods in the peripheral row have a lower enrichment than most of the fuel rods in the interior rows. 2. The method of claim 1 including providing bundles of said plurality of new fuel assemblies, wherein said bundles include fuel batches each having a number of fuel assemblies with each fuel assembly having an average enrichment of uranium; wherein said fuel batches include high enrichment fuel batches having fuel assemblies substantially each having a high average enrichment of uranium, medium enrichment fuel batches having fuel assemblies substantially each having a medium average enrichment of uranium and low enrichment fuel batches having fuel assemblies substantially each having a low average enrichment of uranium. 3. The method of claim 2 wherein said equilibrium cycle reload core includes batches of feed fuel assemblies, once-burned fuel assemblies and twice-burned fuel assemblies; wherein said batches of feed fuel assemblies, said batches of once-burned fuel assemblies, and said batches of twice-burned fuel assemblies each have an average enrichment of uranium 235, and a size; wherein the size of a particular one of said batches is defined by the quantity of fuel assemblies within the batch; wherein the average enrichment of uranium 235 of said feed fuel assemblies is highest, the average enrichment of uranium 235 of the once-burned fuel assemblies is less than the average enrichment of uranium 235 of said feed fuel assemblies but greater than the average enrichment of uranium 235 of said twice-burned fuel assemblies, and the average enrichment of uranium 235 of the twice-burned fuel assemblies is the least; and wherein said high enrichment fuel batches of said first core are generally the same size and average enrichment as said batches of feed fuel assemblies of said equilibrium cycle reload core. 4. The method of claim 3 wherein each of said once-burned fuel batches and said twice-burned fuel batches of said equilibrium cycle reload core further includes a beginning of cycle burnup, an initial enrichment, and a reactivity; and wherein said method further comprises approximating the reactivity of said once-burned fuel batches and said twice-burned fuel batches of said equilibrium cycle reload core in said first core through use of fuel batches in said first core which have initial average enrichments based upon the reactivity of the beginning of cycle burnup and initial enrichment of said once and twice-burned fuel batches of said equilibrium reload cycle core. 5. The method of claim 3 wherein one or more of said fuel batches of said first core include one or more sub-batches comprising fuel assemblies of substantially the same average enrichment. 6. The method of claim 5 including providing as said high enrichment fuel batches, a plurality of high enrichment sub-batches sized and enriched to emulate said feed fuel batches of said known equilibrium reload cycle core. 7. The method of claim 6 wherein said high enrichment sub-batches are primarily loaded at the interior of said first core; and wherein the low enrichment fuel batches are primarily loaded at the periphery of said core. 8. The method of claim 1 wherein the resulting average enrichment within said first core ranges from about 1.5 percent weight of uranium 235 to about 5.0 percent weight of uranium 235. 9. The method of claim 2 wherein at least one of said high enrichment fuel batches is disposed adjacent at least one of said low enrichment fuel batches within said first core configuration; and wherein said method further comprises the step of employing radial zoning of said fuel assemblies in order that said lattice design is structured to compensate for a large thermal neutron flux peak which results from said highly enriched fuel batches within said first core configuration being disposed adjacent less enriched fuel batches within said first core configuration. 10. The method of claim 9 wherein said advanced lattice design includes a generally square pattern of adjacent rows of fuel rods; wherein said fuel rods include at least six different fuel rod types ranging in average enrichment from least enriched to most enriched; wherein said six different fuel rod types ranging in average enrichment from least enriched to most enriched comprise very low enrichment fuel rods, low enrichment fuel rods, medium enrichment fuel rods, medium enrichment fuel rods with integral fuel burnable absorbers, high enrichment fuel rods, and high enrichment fuel rods with integral fuel burnable absorbers; wherein said generally square pattern has four corners; and wherein said advanced lattice disposes said very low enrichment fuel rods at the four corners of said generally square pattern, said low enrichment fuel rods at the outermost rows of said generally square pattern and said high enrichment fuel rods and said high enrichment fuel rods with integral fuel burnable absorbers toward the center of said generally square pattern. 11. The method of claim 1 wherein each of the new fuel assemblies has one of at least three different average enrichments comprising a highest enrichment, a lower enrichment and a lowest enrichment, wherein the advanced lattice is employed in most of the new fuel assemblies having the highest enrichment that are positioned adjacent at least one of the new fuel assemblies with the lowest enrichment. 12. The method of claim 1 wherein each of the new fuel assemblies has one of at least three different average enrichments comprising a highest enrichment, a lower enrichment and a lowest enrichment, wherein the first core has a peripheral row that extends around the first core that has no fuel assemblies with the highest enrichment. 13. The method of claim 1 including the step of providing the advanced lattice design for at least some of the new high enrichment fuel assemblies that are adjacent to low enrichment fuel assemblies of the initial core, employing radial zoning of the fuel assembly to reduce the enrichment of the fuel rods at the edges of the fuel assembly wherein the lowest enrichment fuel rods are disposed at the corners of the lattice with the next lowest enrichment rods disposed along the sides or edges of the lattice.