Patent Number: 043572987
Section: claims

1. A fuel assembly for use with a plurality of fuel assemblies and a neutron moderator fluid circulating through said fuel assemblies in a nuclear reactor core wherein the neutron flux density varies from relatively low neutron flux densities toward the ends of said core to a higher neutron flux density intermediate the ends of said core, said fuel assembly comprising: a plurality of elongated fuel elements; means for holding said fuel elements in a spaced array; a plurality of fuel element spacers in axially spaced locations along said fuel assembly for laterally supporting said fuel elements, said spacers being of first and second types, said first type of spacers having a relatively low neutron absorption cross section but presenting a relatively high neutron moderator fluid flow resistance, said second type of spacer having a higher neutron absorption cross section but presenting a lower neutron moderator fluid flow resistance, the spacers of said first type being axially located along said fuel assembly in regions of highest neutron flux density and the spacers of said second type being axially located along said fuel assembly in regions of lower neutron flux density. 2. A fuel assembly for use with a plurality of fuel assemblies and a water moderator-coolant in the core of a boiling water reactor having means for directing a flow of said coolant-moderator through said fuel assemblies, said fuel assembly comprising: a plurality of fuel element spacers in axially spaced locations along said fuel assembly for laterally supporting said fuel elements, said spacers being of first and second types, said first type of spacers having a relatively low neutron capture cross section but presenting a relatively high coolant-moderator flow resistance, said second type of spacers having a neutron absorption cross section higher than the spacers of said first type but presenting a coolant-moderator flow resistance lower than the spacers of said first type, the spacers of said first type being axially located along said fuel assembly predominately in regions of highest neutron flux density and the spacers of said second type being axially located along said fuel assembly predominately in regions of lower neutron flux density. 3. The fuel assembly of claim 2 wherein said first type of spacers is formed predominately from a zirconium alloy and wherein said second type of spacer is formed predominately from a nickel alloy. 4. The fuel assembly of claim 2 having seven spacers, the lower four spacers being of said first type and the upper three spacers being of said second type. 5. The fuel assembly of claim 2 having seven spacers, the lower three spacers being of said first type and the upper four spacers being of said second type. 6. The fuel assembly of claim 2 having seven spacers, the lower three spacers and the top most spacer being of said first type and the remaining three spacers being of said second type. 7. A fuel assembly for use with a plurality of fuel assemblies and a water coolant-moderator circulating through said fuel assemblies in the core of a boiling water reactor wherein during operation of said reactor said water is a two-phase mixture of vapor and liquid in an upper boiling region of said core where thermal limits are generally reached first and a single-phase liquid in a lower non-boiling region of said core where thermal limits are generally never exceeded, said fuel assembly comprising: a plurality of elongated fuel elements; means for holding said fuel elements in a spaced array including a plurality of fuel elements in axially spaced locations along said fuel assembly for laterally supporting said fuel elements, each of said spacers providing a passage for each of said fuel elements and in each of said passages providing a resilient means for engaging said fuel elements, said spacers being of first and second types, said first type of spacers having structural members thereof formed of a material having a relatively low neutron capture cross section, the resilient means of said first type of spacers being formed of a material different from the structural members thereof, said first type of spacers being located along the length of said fuel assembly predominantly in the non-boiling region of said core, said second type of spacers having a highly skeletonized structure to provide minimized coolant-moderator flow pressure drop thereacross, the resilient means of said second type of spacers being formed integral with the structural members thereof, said second type of spacers being located along the length of said fuel assembly predominately in the boiling region of said core. 8. The fuel assembly of claim 7 wherein the structural members of said first type of spacer are formed of a zirconium alloy and the resilient means thereof are formed from a nickel alloy. 9. The fuel assembly of claim 7 wherein said second type of spacers, including the structural members and resilient means thereof, are formed from a nickel alloy. 10. The fuel assembly of claim 7 having seven spacers the lower four spacers being of said first type and the upper three spacers being of said second type. 11. The fuel assembly of claim 7 having seven spacers, the lower three spacers being of said first type and the upper four spacers being of said second type. 12. The fuel assembly of claim 7 having seven spacers, the upper most spacer and the lower three spacers being of said first type and the remaining spacers being of said second type. 13. The fuel assembly of claim 7 wherein the number of spacers of either type is no greater than one more than the number of spacers of the other type.