Patent Number: 040295451
Section: claims

1. A nuclear fuel element comprising (a) a central core of a body of nuclear fuel material selected from the group consisting of compounds of uranium, plutonium, thorium and mixtures thereof and (b) an elongated composite cladding container comprising an outer layer of a material selected from the group consisting of zirconium or a zirconium alloy having two coatings bonded on the inside surface with the first coating being an undeformed diffusion barrier of constant thickness selected from the group consisting of chromium and chromium alloys and the second coating being an undeformed metal layer of constant thickness selected from the group consisting of copper, nickel, iron and alloys thereof, said diffusion barrier being about 0.00005 to about 0.001 inch in thickness, said metal layer being about 0.0001 to about 0.002 inch in thicknes, each of said coatings having an impurity content of less than about 1 weight percent, and said cladding container enclosing said core so as to leave a gap between said core and said container during use in a nuclear reactor. 2. A nuclear fuel element of claim 1 which has in addition a cavity and a nuclear fuel material retaining means in the form of a helical member positioned in the cavity. 3. A nuclear fuel element of claim 1 in which the metal layer is copper. 4. A nuclear fuel element of claim 1 in which the metal layer is nickel, 5. A nuclear element of claim 1 in which the metal layer is iron. 6. A nuclear fuel element of claim 1 in which the outer layer is a zirconium alloy. 7. A nuclear element of claim 1 in which the diffusion barrier is chromium. 8. A nuclear fuel element of claim 1 in which the diffusion barrier is a chromium alloy. 9. A nuclear fuel element of claim 1 in which each of said coatings has an impurity content of less than about 1000 parts per million. 10. A nuclear fuel element of calim 1 in which the nuclear fuel material is comprised of uranium dioxide. 11. A nuclear fuel element of claim 1 in which the nuclear fuel material is a mixture comprising uranium dioxide and plutonium dioxide. 12. A nuclear fuel element which comprises an elongate composite cladding container of an outer layer of a material selected from the group consisting of zirconium or a zirconium alloy having two coatings bonded on the inside surface with the first coating on the outer layer being an undeformed diffusion barrier of constant thickness selected from the group consisting of chromium and chromium alloys and the second coating on the first coating being an undeformed metal layer of constant thickness selected from the group consisting of copper, nickel, iron and alloys thereof, said diffusion barrier being about 0.0005 to about 0.001 inch in thickness, said metal layer being about 0.0001 to about 0.002 inch in thickness each of said coatings having an impurity content of less than about one weight percent, a central core of a body of nuclear fuel material selected from the group consisting of compounds of uranium, plutonium, thorium and mixtures thereof disposed in and partially filling said container and forming an internal cavity in the container, an enclosure integrally secured and sealed at each end of said container, a nuclear fuel material retaining means positioned in the cavity, and said cladding container enclosing said core so as to leave a gap between said core and said container during use in a nuclear reactor.