Patent Number: 062721976
Section: claims

1. A fuel assembly for a nuclear reactor including: a plurality of fuel pins having respective axes extending substantially parallel to each other; at least two structural grids spaced apart from each other, the structural grids being in contact with said fuel pins and maintaining said fuel pins substantially mutually parallel and preventing contact therebetween, wherein the fuel assembly further comprises at least one mixing grid situated intermediate said at least two structural grids, said mixing grid having turbulence inducing means to promote turbulence in a coolant flowing through said fuel assembly in use, wherein said mixing grid is positioned and fixedly located out of contact with said fuel pins and wherein the mixing grid is formed from a single sheet of metal wherein the plane of the metal sheet from which the mixing grid is formed lies in a plane which is transverse to the axes of the fuel pins. 2. A fuel assembly according to claim 1, wherein the thickness of the sheet material is lies in the range from 0.5 mm to about 1 mm. 3. A fuel assembly according to claim 1 wherein the mixing grid is in the form of a framework having an array of apertures of predetermined size and shape. 4. A fuel assembly according claim 1 wherein the turbulence inducing means include vanes attached to the mixing grid. 5. A fuel assembly according to claim 1 wherein the vanes are integrally formed with the mixing grid. 6. A fuel assembly according to claim 3 wherein the framework includes framework members surrounding each aperture and wherein at least some of the framework members surrounding each aperture are twisted out of the plane of the sheet so as to form turbulence inducing features. 7. A fuel assembly according to claim 1 wherein the mixing grid does not extend to and encompass an outer peripheral ring of fuel pins. 8. A fuel assembly according to claim 1 wherein the mixing grid is located and held in position within the fuel pin assembly by thimble tubes in which moderator control rods run. 9. A fuel assembly according to claim 8 wherein the mixing grid is located by swaging to the thimble tubes. 10. A fuel assembly according to claim 8 wherein the mixing grid is first joined to short tubes which are fitted over the thimble tubes and are fixed to the thimble tubes. 11. A fuel assembly according to claim 1 wherein the mixing grid is located at a position between two adjacent structural grids where, during operation of the reactor in use, the hottest region of the fuel pins is located. 12. A mixing grid for use in a fuel pin assembly including a plurality of fuel pins having respective axes extending substantially parallel to each other, at least two structural grids spaced apart from each other, the structural grids being in contact with the fuel pins and maintaining the fuel pins substantially mutually parallel and preventing contact therebetween, said mixing grid including turbulence inducing means to promote turbulence in a coolant flowing through the fuel assembly in use and wherein: said mixing grid is formed from a single sheet of metal;  said mixing grid is sized and configured to be situated intermediate said at least two structural grids, positioned and fixedly located out of contact with said fuel pins, and such that, when said mixing grid is so positioned, the plane of the metal sheet from which the mixing grid is formed lies in a plane which is transverse to the axes of the fuel pins. 13. A fuel assembly according to claim 8 wherein the mixing grid is located by welding to the thimble tube.