Patent Number: 046719270
Section: claims

1. A nuclear reactor fuel rod assembly having a fuel rod containing a hybrid burnable abosorber-containing nuclear fuel, said fuel rod having a predetermined absorption capability, said fuel assembly comprising: a nuclear fuel pellet containing a fissionable material and a hybrid absorber material, said hybrid absorber material comprising gadolinium oxide, and coated boron carbide particles, said boron carbide particles being of a particle size of between 20 to 100 microns in diameter and each coated by 0.5 to 10 micron coating of a helium gas-impervious material, the total weight percent of hybrid absorber material in said fuel pellet being less for a fuel rod having the predetermined absorption capability of said fuel rod assembly than the total weight percent of gadolinium oxide required when used without boron carbide and of the total weight percent of boron carbide required when used without gadolinium oxide.  a nuclear fuel pellet comprising uranium dioxide containing a hybrid absorber comprising 2 to 10 weight percent of the pellet of gadolinium oxide, and 0.02 to 1.0 weight percent of coated boron carbide particles, said boron carbide particles being of a particle size of between 20 to 100 microns in diameter and each coated by a 0.5 to 10 micron coating of a helium gas-impervious material, the addition of a predetermined weight percent of boron carbide particles to said fuel rod acting to reduce the amount of gadolinium oxide required in said fuel rod to achieve said predetermined burn out rate by an amount greater than said predetermined weight percent of boron carbide particles. 2. A nuclear reactor fuel rod assembly as defined in claim 1 wherein said nuclear fuel pellet contains 1 to 20 percent by weight of said gadolinium oxide. 3. A nuclear reactor fuel rod assembly as defined in claim 2 wherein said gadolinium oxide is present in an amount of between 2 to 10 percent by weight of the pellet. 4. A nuclear reactor fuel rod assembly as defined in claim 2 wherein said boron carbide particles are present in an amount of between 0.02 to 1.0 percent by weight of said pellet. 5. A nuclear reactor fuel rod assembly as defined in claim 4 wherein said coating is a helium gas-impervious material selected from the group consisting of nickel, zirconium, niobium, graphite and alumina. 6. A nuclear reactor fuel rod assembly as defined in claim 4 wherein said fissionable material is uranium dioxide. 7. A nuclear reactor fuel rod assembly as defined in claim 6 wherein said pellet is cylindrical in shape with a length of between 0.4 to 0.6 inch, and a length to diameter ratio of less than 1.7:1. 8. A nuclear reactor fuel rod assembly having a fuel rod containing a hybrid burnable absorber-containing nuclear fuel, said fuel rod having a predetermined burn out rate, said fuel rod assembly comprising: 9. A nuclear fuel rod, for use in a nuclear reactor fuel rod assembly, the fuel rod having a predetermined burn out rate and having a plurality of hybrid burnable absorber-containing fissionable fuel pellets contained within a metallic elongated tubular cladding, said hybrid burnable absorber-containing fuel pellets consisting essentially of a fissionalbe material, gadolinium oxide in an amount of between 1 to 20 percent by weight, and 0.02 to 1.0 percent by weight of boron carbide particles, said boron carbide particles being of a particle size of between 20 to 100 microns in diameter and each coated by a 0.5 to 10 micron coating of a helium gas-impervious material, the addition of a predetermined weight percent of boron carbide particles to said fuel pellets acting to reduce the amount of gadolinium oxide required to achieve said predetermined burn out rate in said fuel rod by an amount greater than said predetermined weight percent of boron carbide particles. 10. A nuclear fuel rod as defined in claim 9 wherein said fissionable material is uranium dioxide. 11. A nuclear fuel rod as defined in claim 10 wherein said gadolinium oxide is present in an amount of 2 to 10 weight percent. 12. A nuclear fuel rod as defined in claim 11 wherein said helium gas-impervious material is selected from the group consisting of nickel, zirconium, niobium, graphite and alumina. 13. A nuclear fuel rod as defined in claim 9 wherein said pellet is in the form of a cylindrical pellet having a length of between 0.4 to 0.6 inch, and a length to diameter ratio of less than 1.7:1. 14. An improved nuclear fuel rod containing gadolinium oxide absorber material, for use in a nuclear reactor fuel rod assembly, the fuel rod having a plurality of hybrid burnable absorber-containing fissionable fuel pellets contained within a metallic elongated tubular cladding, said hybrid burnable absorber-containing fuel pellets consisting essentially of pellets of uranium dioxide containing between 2 to 10 weight percent of gadolinium oxide and 0.02 to 1.0 weight percent of coated boron carbide particles, said boron carbide particles being of a particle size of between 20 to 100 microns in diameter and each coated by a 0.5 to 10 micron coating of a helium gas-impervious material, the addition of a predetermined weight percent of said boron carbide particles to the gadolinium oxide containing fuel pellets acting to delay the burn out rate of said fuel rod to a burn out rate which would require the addition of a greater weight percent of gadolinium oxide than the predetermined weight percent of boron carbide particles added. 15. A nuclear reactor fuel composition, containing an hybrid burnable absorber, for use in fuel rod assemblies of a nuclear reactor, consisting essentially of a fissionable material, gadolinium oxide in an amount of between 1 to 20 percent by weight of the composition, and 0.02 to 1.0 percent by weight of boron carbide particles, said boron carbide particles being of a particle size of between 20 to 100 microns in diameter and each coated by a 0.5 to 10 micron coating of a helium gas-impervious material, the addition of boron carbide particles as well as gadolinium oxide absorber material to said fissionable material resulting in a fuel composition having less weight percent of absorber material than a fuel composition having the same absorption capabilities but having only gadolinium oxide or only boron carbide absorber material. 16. A nuclear fuel composition as defined in claim 15 wherein said fissionable material is uranium dioxide. 17. A nuclear fuel composition as defined in claim 16 wherein said gadolinium oxide is present in an amount of 2 to 10 weight percent. 18. A nuclear fuel composition as defined in claim 17 wherein said helium gas-impervious material is selected from the group consisting of nickel, zirconium, niobium, graphite and alumina. 19. A nuclear fuel composition as defined in claim 15 wherein said composition is in the form of a cylindrical pellet having a length of between 0.4 to 0.6 inch, and a length to diameter ratio of less than 1.7:1. 20. An improved nuclear fuel composition, containing a hybrid burnable absorber, for use in fuel rod assemblies of a nuclear reactor, consisting essentially of a pellet of uranium dioxide containing between 2 to 10 weight percent of gadolinium oxide and 0.02 to 1.0 weight percent of coated boron carbide particles, said boron carbide particles being of a particle size of between 20 to 100 microns in diameter and each coated by a 0.5 to 10 micron coating of a helium gas-impervious material, the addition of a predetermined weight percent of said boron carbide particles to said fuel composition acting to delay the burn out rate of said fuel composition by an amount which would require the addition of a greater weight percent of gadolinium oxide than the weight percent of boron carbide particles added.