Patent Number: 047175346
Section: claims

1. A nuclear fuel cladding having a predetermined concentration of burnable absorber integrally incorporated therein, for use in containing a nuclear fuel in a reactor, the cladding formed as a hollow composite tube comprising: an outer tubular layer, having a first thickness of at least 15 mils, consisting essentially of a first zirconium alloy;  an intermediate layer bonded to the inner wall of said outer tubular layer, having a second thickness less than said first thickness, consisting essentially of a mixture of a boron-containing material having a predetermined enrichment of boron and a second zirconium alloy, said second thickness being defined by said predetermined enrichment of boron in said boron-containing material and by the predetermined concentration of burnable absorber incorporated into said fuel cladding; and  an inner layer of zirconium metal, having a third thickness less than said second thickness, bonded to said intermediate layer, said third thickness being sufficient to isolate said intermediate layer from said nuclear fuel.  an outer tubular layer, having a first thickness of at least 15 mils, consisting essentially of a zirconium alloy;  an intermediate layer bonded to the inner wall of said outer tubular layer, having a second thickness less than said first thickness, of about 3-5 mils, consisting essentially of a mixture of a zirconium boride and a zirconium alloy; and  an inner layer of zirconium metal, having a thickness less than said second thickness, of about 1-2 mils, bonded to said intermediate layer. 2. A nuclear fuel cladding as defined in claim 1 wherein said first and said second zirconium alloys are selected from the group consisting of Zircaloy-2, Zircaloy-4 and a zirconium alloy containing about 2.5 percent by weight niobium. 3. A nuclear fuel cladding as defined in claim 1 wherein said boron-containing material is selected from the group consisting of natural boron, enriched boron, zirconium boride, boron nitride, boron carbide, or mixtures thereof. 4. A nuclear fuel cladding as defined in claim 1 wherein said outer tubular layer is at least twice the thickness of the sum of the thicknesses of the intermediate layer and the inner layer. 5. A nuclear fuel cladding as defined in claim 1 wherein said composite cladding has a wall thickness of between 18-22 mils. 6. A nuclear fuel cladding as defined in claim 1 wherein said intermediate layer has a thickness of between 3-5 mils. 7. A nuclear fuel cladding as defined in claim 6 wherein said inner layer has a thickness of between 1-2 mils. 8. A nuclear fuel cladding having a burnable absorber integrally incorporated therein, for use in containing a nuclear fuel in a reactor, the cladding formed as a hollow composite tube comprising: 9. In a nuclear fuel rod comprising a metallic tubular cladding formed from a metal selected from zirconium and a zirconium alloy, containing a nuclear fuel, and having end sealing means thereon to hermetically seal said nuclear fuel within said metallic tubular cladding, the improvement wherein said metallic tubular cladding is a composite, said cladding having integrally incorporated a predetermined concentration of burnable absorber material, said cladding comprising an outer tubular layer having a first thickness, sufficient to provide the mechanical integrity and strength to contain the nuclear fuel, said first thickness being at least about 15 mils, said outer tubular layer consisting essentially of a first zirconium alloy; an intermediate layer bonded to the inner wall of said outer tubular layer, having a second thickness less than said first thickness, consisting essentially of a mixture of a boron-containing material having a predetermined enrichment of boron and a second zirconium alloy, said second thickness being determined by the predetermined concentration of burnable absorber incorporated into said cladding and the predetermined enrichment of boron in said boron-containing material; and an inner layer of zirconium metal, having a third thickness less than said second thickness, said third thickness being between about 1-2 mils, bonded to said intermediate layer. 10. A nuclear fuel rod as defined in claim 9 wherein said first and said second zirconium alloys are selected from the group consisting of Zircaloy-2, Zircaloy-4 and a zirconium alloy containing about 2.5 percent by weight niobium. 11. A nuclear fuel rod as defined in claim 9 wherein said boron-containing material is selected from the group consisting of natural boron, enriched boron, zirconium boride, boron nitride, boron carbide, or mixtures thereof. 12. A nuclear fuel rod as defined in claim 9 wherein said outer tubular layer is at least twice the thickness of the sum of the thicknesses of the intermediate layer and the inner layer. 13. A nuclear fuel rod as defined in claim 9 wherein said composite cladding has a wall thickness of between 18-22 mils. 14. A nuclear fuel rod as defined in claim 9 wherein said intermediate layer has a thickness of between 3-5 mils. 15. In a nuclear fuel rod comprising a metallic tubular cladding formed from a metal selected from zirconium and a zirconium alloy, containing a nuclear fuel, and having end sealing means thereon to hermetically seal said nuclear fuel within said metallic tubular cladding; the improvement wherein said metallic tubular cladding is a composite of an outer tubular layer having a first thickness of at least 15 mils consisting essentially of a zirconium alloy; an intermediate layer bonded to the inner wall of said outer tubular layer having a second thickness less than said first thickness, of about 3-5 mils, consisting essentially of a mixture of zirconium boride and a zirconium alloy; and an inner layer of zirconium metal, having a thickness less than said second thickness, of about 1-2 mils, bonded to said intermediate layer.