Patent Number: 040594840
Section: claims

1. A hybrid fuel assembly to reduce the maximum and average linear heat generation rates for use in a nuclear reactor having a plurality of control rods comprising a plurality of elongated fuel rods disposed in spaced parallel array, said fuel rods including a first plurality of said fuel rods having a first diameter and a second plurality of said fuel rods having a second diameter greater than said first diameter, said first plurality of fuel rods includes a first fuel enrichment and said second plurality of fuel rods includes a second fuel enrichment different from said first fuel enrichment, said second plurality of fuel rods being divided into clusters of fuel rods forming islands, said islands including guide tubes having diameters substantially the same as said second plurality of fuel rods, one guide tube corresponding to each of said control rods, said islands located within said assembly so that said guide tubes may receive all of said control rods, said islands being symmetrically located relative to the intersecting diagonal centerlines of said fuel assembly. 2. The hybrid fuel assembly of claim 1 wherein said first plurality of fuel rods are located about the periphery of said fuel assembly. 3. The hybrid fuel assembly of claim 1 wherein said second plurality of fuel rods are located internally of said fuel assembly. 4. The hybrid fuel assembly of claim 1, wherein said second plurality of fuel rods are located internally of said fuel assembly in clusters of fuel rods each of which comprises an island wherein said fuel rods have a pitch different from that of said first plurality of fuel rods. 5. The hybrid fuel assembly of claim 4 wherein said fuel assembly comprises four of said islands, each of said islands being symmetrically located relative to the intersecting diagonal centerlines of said fuel assembly. 6. The hybrid fuel assembly of claim 1 wherein said fuel assembly comprises a plurality of islands symmetrically located about the intersecting diagonal centerlines of said fuel assembly, each of said islands comprising said second plurality of fuel rods, said first plurality of fuel rods having a first pitch corresponding to that for said fuel assembly and said second plurality of fuel rods having a second pitch different from said first pitch. 7. The hybrid fuel assembly of claim 1 wherein said fuel assembly includes a plurality of islands comprising a predetermined number of said second plurality of fuel rods, and each of said islands replacing a predetermined number of said first plurality of fuel rods which is greater than the number of fuel rods of said second plurality contained in each of said islands. 8. The hybrid fuel assembly of claim 1 wherein said fuel assembly comprises a plurality of islands of said second plurality of fuel rods, said second plurality of fuel rods having a larger diameter than that of said first plurality of fuel rods, said fuel assembly having maximum power peaking based on volumetric heat generation rate in the ones of said first plurality of fuel rods surrounding each of said islands. 9. The hybrid fuel assembly of claim 1 wherein said second plurality of fuel rods has a diameter greater than that of said first plurality of fuel rods. 10. The hybrid fuel assembly of claim 1 wherein said fuel assembly comprises a plurality of modules wherein each of said modules comprises an island having said second plurality of fuel rods maintained at a pitch different from that of said first plurality of fuel rods, each of said islands being located in the vicinity of said control means for said first plurality of fuel rods, thereby reducing the linear heat generation rates of said second plurality of fuel rods to that normally associated with said first plurality of fuel rods.