Patent Number: 048225592
Section: claims

1. A nuclear fuel element for use in power-producing nuclear reactors, comprising a circular metal cladding tube containing sintered cylindrical oxide fuel pellets and an inert filler gas, the tube being formed internally with a substantially cylindrical inner surface having a plurality of longitudinal ribs forming axially extending channels and axial channel space volumes, the fuel pellets having chamfered endfaces creating toroidal space volumes at the adjoining ends of the fuel pellets, the ratio of the axial channel space volumes to toroidal space volumes including any dish volumes at the pellet end faces being substantially equal to or less than one (1). 2. The nuclear fuel element according to claim 1, wherein the chamfer extends towards the centre of the pellet by at least about 10% of the pellet radius. 3. The nuclear fuel element according to claim 1, wherein the fuel pellets have a length/diameter ratio of from about 0.8 to about 1.2. 4. The nuclear fuel element according to claim 2, wherein a heavier inert gas is added to the helium filler gas of the fuel element and wherein a plenum space is arranged at at least one end of the element for accommodating released fission gases. 5. The nuclear fuel element according to claim 1, wherein each of said ribs, in cross-section has an inward contour at least part of which lies inside a circle segment formed between a circle circumscribing the deepest points of channels and a chord connecting adjacent deepest points. 6. The nuclear fuel element according to claim 5, wherein said part has a circular shape approximately matching that of the juxtaposed fuel pellet. 7. The nuclear fuel element according to claim 6, wherein said circular part occupies about one fifth to one half of the rib centre spacing. 8. The nuclear fuel element according to claim 1, wherein said ribs are evenly distributed over the inside surface of the cladding tube so as to form an inner contour of the cladding tube conforming to a regular polygon. 9. The nuclear fuel element according to claim 1, wherein a thin layer of graphite is present between the pellet column and the cladding. 10. The nuclear fuel element according to claim 1, wherein the said ratio of axial channel to toroidal space volumes is at most about 0.5. 11. A process for the manufacture of a nuclear fuel element according to claim 5 comprising the steps of forming a cylindrical cladding tube by tube drawing using a polygonal mandrel, characterized by selecting the mandrel dimension in relation to the tube dimension in such a manner that plastic deformation of the tube occurs only at the corners of the polygonal form, whereas in the intermediate parts between said corners the deformation is elastic resulting in the desired rib configuration. 12. The nuclear fuel element according to claim 2, wherein the fuel pellets have a length/diameter ratio of from about 0.8 to about 1.2, such as approximately one (1). 13. The nuclear fuel element according to claim 3, wherein the said ratio of axial channel to toroidal space volumes is at most about 0.5. 14. A process for the manufacture of a nuclear fuel element according to claim 6, comprising the steps of forming a cylindrical cladding tube by tube drawing using a polygonal mandrel, characterized by selecting the mandrel dimension in relation to the tube dimension in such a manner that plastic deformation of the tube occurs only at the corners of the polygonal form, whereas in the intermediate parts between said corners the deformation is elastic resulting in the desired rib configuration. 15. A process for the manufacture of a nuclear fuel element according to claim 7, comprising the steps of forming a cylindrical cladding tube by tube drawing using a polygonal mandrel, characterized by selecting the mandrel dimension in relation to the tube dimension in such a manner that plastic deformation of the tube occurs only at the corners of the polygonal form, whereas in the intermediate parts between said corners the deformation is elastic resulting in the desired rib configuration. 16. A process for the manufacture of a nuclear fuel element according to claim 8, comprising the steps of forming a cylindrical cladding tube by tube drawing using a polygonal mandrel, characterized by selecting the mandrel dimension in relation to the tube dimension in such a manner that plastic deformation of the tube occurs only at the corners of the polygonal form, whereas in the intermediate parts between said corners the deformation is elastic resulting in the desired rib configuration. 17. A process for the manufacture of a nuclear fuel element according to claim 9, comprising the steps of forming a cylindrical cladding tube by tube drawing using a polygonal mandrel, characterized by selecting the mandrel dimension in relation to the tube dimension in such a manner that plastic deformation of the tube occurs only at the corners of the polygonal form, whereas in the intermediate parts between said corners the deformation is elastic resulting in the desired rib configuration. 18. A process for the manufacture of a nuclear fuel element according to claim 10, comprising the steps of forming a cylindrical cladding tube by tube drawing using a polygonal mandrel, characterized by selecting the mandrel dimension in relation to the tube dimension in such a manner that plastic deformation of the tube occurs only at the corners of the polygonal form, whereas in the intermediate parts between said corners the deformation is elastic resulting in the desired rib configuration. 19. The nuclear fuel element according to claim 1, wherein the inert filler gas is helium.