Patent Number: 041558073
Section: claims

1. A core for a nuclear reactor comprising: a vertically oriented first fuel assembly including a plurality of generally parallel coextending fuel rods and structures for laterally spacing said rods positioned at first preselected core elevations, said structures including straps bounding the lateral periphery of said assembly at said first core elevations;  a vertically oriented second fuel assembly including a plurality of generally parallel coextending fuel rods and structures for laterally spacing said rods positioned at second preselected core elevations, said structures including straps bounding the lateral periphery of said second assembly at said second elevations, at least one of said second preselected core elevations being different than all of said first preselected core elevations; and  a vertically oriented transition fuel assembly positioned adjacent and between said first and second fuel assemblies, said transition assembly including a plurality of generally coextending parallel fuel rods and structures for laterally spacing said rods, said structures including means for bounding the lateral periphery of said transition assembly at said first and second preselected core elevations, said means being of sufficient size along the assembly length such that any contact among the rodded length of said adjacent assemblies is initially through said structures.  a plurality of first fuel assemblies, each said first assembly including fuel rods laterally spaced by a plurality of first grid structures positioned at first core elevations, said grid structures each forming a boundary about the periphery of each said fuel assembly at said first elevations;  a plurality of second fuel assemblies, each said second assembly including fuel rods laterally spaced by a plurality of second grid structures positioned at second core elevations, at least one of said first core elevations being different than all of said second core elevations, said second structures each forming a boundary about the periphery of each said fuel assembly at said second elevations; and  at least one transition fuel assembly positioned adjacent a first assembly and adjacent a second assembly comprising a plurality of parallel laterally spaced fuel rods and transition grid structures each forming a boundary about the periphery of said transition fuel assembly, said transition grid structures being positioned such that a transition grid structure is at each said first core elevation and a transition grid structure is at each said second core elevation and such that each first and second grid structure is laterally adjacent a transition grid structure. 2. A core for a nuclear reactor comprising: 3. The core of claim 2 wherein at least one of said transition grid structures includes outer straps having a length along the assembly elevation greater than the length of each said first and second grid structures. 4. The core of claim 3 wherein said outer straps include openings adapted to permit the flow of coolant therethrough. 5. The core of claim 2 wherein at least one of said transition grid structures comprises a partial auxiliary grid structure, said partial auxiliary grid structure comprising auxiliary outer straps about the periphery of said transition assembly affixed to periphery straps of a grid structure similar to said first grid structure by extended peripheral straps along the transition assembly length, and a plurality of auxiliary inner straps forming cells about a selected subplurality of said rods of said transition assembly. 6. The core of claim 5 wherein said partial auxiliary grid structure further comprises auxiliary sleeves affixed within a selected plurality of said cells, said auxiliary sleeves extending to and affixed within cells of another grid structure. 7. The core of claim 2 wherein said grid structures are of a material comprising alloys of zirconium. 8. A core for a nuclear reactor, said core comprising a plurality of fuel assemblies, each said assembly comprising a plurality of parallel coextending fuel rods and a plurality of grid structures for laterally spacing said rods, said fuel assemblies including a first plurality of assemblies including first grid structures at first core elevations, a second plurality of assemblies including second grid structures at second core elevations, at least one of said second core elevations being different than all of said first core elevations, and at least one transition fuel assembly positioned adjacent a first assembly and adjacent a second assembly including transition grid structures sized and positioned such that laterally adjacent each first and second grid structure is a transition grid structure, and laterally adacent each transition grid structure is a first grid structure and a second grid structure. 9. A transition fuel assembly for a core of a nuclear reactor, said core including a plurality of first fuel assemblies having parallel and coextending fuel rods laterally spaced at first preselected core elevations by grid structures each including straps forming the lateral periphery of each said first assembly at said first preselected elevations, a plurality of second fuel assemblies having parallel and coextending fuel rods laterally spaced at second preselected core elevations by grid structures including straps forming the lateral periphery of each said second assembly at said second preselected elevations, at least one of said second preselected core elevations being different than all of said first preselected core elevations, said transition fuel assembly comprising parallel and coextending fuel rods laterally spaced by grid structures, said last named grid structures each including straps forming the lateral periphery of said transition assembly at selected elevations, said straps being sized and positioned so that upon placement of said transition assembly in said core laterally adjacent a first and second fuel assembly, each said last-named grid structure is laterally adjacent a first assembly grid structure and a second assembly grid structure and such that laterally adjacent each first and second assembly grid structure is a grid structure of the transition assembly.