Patent Number: 044951361
Section: claims

1. In combination with the core for a nuclear pressurized water reactor having a core barrel which encloses a plurality of vertically oriented and closely spaced elongated fuel assemblies, each of said fuel assemblies having a generally square cross-sectional configuration with four axial exterior faces, the combined array of said fuel assemblies having the general configuration of a right circular cylinder to the extent permitted by the square configuration of those fuel assemblies which are positioned at the periphery of said combined fuel assembly array, a portion of said peripherally positioned fuel assemblies of said array having one axial peripheral face which forms a portion of the periphery of said combined fuel assembly array, the remainder of said peripherally positioned fuel assemblies of said array having two axial peripheral faces which form a portion of the periphery of said combined fuel assembly array, a flow baffle closely surrounding said axial peripheral faces of said fuel assemblies which form the outer periphery for said combined fuel assembly array to direct the flow of coolant fluid upwardly through said fuel assembly array during operation of said reactor, those of said fuel assemblies which are positioned interiorly within said array each comprising a plurality of regularly spaced parallel and coextensive relatively small diameter fuel rods each of which comprises fissile pellets stacked within a sealed metallic cladding, said small diameter fuel rods being primarily supported by upper and lower nozzles and by grid structures spaced along the length of said rods, each of said fuel assemblies which are interiorly positioned within said combined array having a high H/U ratio, and a relatively small proportion of control rod thimbles spaced among said small diameter fuel rods; the improved constructions for said fuel assemblies which are peripherally positioned in said combined array; said improved peripherally positioned fuel assemblies each incorporating two different fuel rod designs, the first of said fuel rod designs being the same as the fuel rod designs used in said fuel assemblies which are interiorly positioned within said combined array, and the second of said fuel rod designs differing from said first fuel rod designs in that the diameter of said second fuel rod designs is larger than that of said first fuel rods and the pellets of said second fuel rods comprise fertile material with those portions of said peripherally positioned assemblies which incorporate said second fuel rods having a low H/U ratio, and a relatively small proportion of control rod thimbles spaced among said fuel rods of said peripherally positioned arrays;  said first fuel rod designs and said second fuel rod designs positioned in separate discrete locations within each of said peripherally positioned fuel assemblies, said second fuel rods positioned nearest the peripherally positioned axial faces of said fuel assemblies which form the periphery of said combined array, and said first design fuel rods positioned furthest away from the peripherally positioned axial faces of said fuel assemblies which form the periphery of said combined array.  said improved peripherally positioned fuel assemblies each incorporating two different fuel rod designs, the first of said fuel rod designs being the same as the fuel rod designs used in said fuel assemblies which are interiorly positioned within said combined array, and the second of said fuel rod designs differing from said first fuel rod designs in that the diameter of said second fuel rod designs is larger than that of said first fuel rods and the pellets of said second fuel rods comprise fertile material with those portions of said peripherally positioned assemblies which incorporate said second fuel rods having a low H/U ratio, and a relatively small proportion of control rod thimbles spaced among said fuel rods of said peripherally positioned arrays;  said first fuel rod designs and said second fuel rod designs positioned in separate discrete locations within each of said peripherally positioned fuel assemblies, said second fuel rods positioned nearest the axial faces of said peripherally positioned assemblies which form the periphery of said combined array, said first design fuel rods positioned furthest away from the axial faces of said peripherally positioned fuel assemblies which form the periphery of said combined array, and a minor proportion of fuel rod locations which are designed to be occupied by said first fuel rods being left open to increase the H/U ratio. 2. The combination as specified in claim 1, wherein said peripherally positioned fuel assemblies which have only one axial peripheral face forming a portion of the periphery of said combined fuel assembly array each have said second fuel rods positioned in a discrete location abutting said one axial peripheral face and aligned in rows parallel to said one axial peripheral face. 3. The combination as specified in claim 1, wherein said peripherally positioned fuel assemblies comprise locations for seven rows of said second fuel rods which parallel said one axial face, each of said seven rows having locations for seventeen of said second fuel rods except for a minor proportion of control rod thimble locations, and the remainder of said peripherally positioned fuel assemblies have locations for ten rows of said first fuel rods, and each of said ten rows having locations for seventeen of said first fuel rods except for a minor proportion of control rod thimble locations. 4. The combination as specified in claim 1, wherein said peripherally positioned fuel assemblies which have two axial peripheral faces forming a portion of the periphery of said combined fuel assembly array have said second fuel rods positioned in discrete locations aligned in rows abutting both of said axial peripheral faces. 5. The combination as specified in claim 4, wherein said peripherally positioned fuel assemblies comprise locations for seven rows of said second fuel rods positioned parallel to both of said axial peripheral faces, each of said seven rows having locations for seventeen of said second fuel rods except for a minor proportion of control rod thimble locations, and the remainder of said peripherally positioned fuel assemblies have locations for ten rows of said first fuel rods, and each of said ten rows having locations for ten of said first fuel rods except for a minor proportion of control rod thimble locations. 6. In combination with the core for a nuclear pressurized water reactor having a core barrel which encloses a plurality of vertically oriented and closely spaced elongated fuel assemblies, each of said fuel assemblies having a generally square cross-sectional configuration with four axial exterior faces, the combined array of said fuel assemblies having the general configuration of a right circular cylinder to the extent permitted by the square configuration of those fuel assemblies which are positioned at the periphery of said combined fuel assembly array, a portion of said peripherally positioned fuel assemblies of said array having one axial peripheral face which forms a portion of the periphery of said combined fuel assembly array, the remainder of said peripherally positioned fuel assemblies of said array having two axial peripheral faces which form a portion of the periphery of said combined fuel assembly array, a flow baffle closely surrounding said axial peripheral faces of said fuel assemblies which form the outer periphery for said combined fuel assembly array to direct the flow of coolant fluid upwardly through said fuel assembly array during operation of said reactor, those of said fuel assemblies which are positioned interiorly within said array each comprising a plurality of regularly spaced parallel and coextensive relatively small diameter fuel rods each of which comprises fissile pellets stacked within a sealed metallic cladding, said small diameter fuel rods being primarily supported by upper and lower nozzles and by grid structures spaced along the length of said rods, each of said fuel assemblies which are interiorly positioned within said combined array having a high H/U ratio, and a relatively small proportion of control rod thimbles spaced among said small diameter fuel rods; the improved constructions for said fuel assemblies which are peripherally positioned in said combined array; 7. The combination as specified in claim 6, wherein said peripherally positioned fuel assemblies which have only one axial peripheral face forming a portion of the periphery of said combined fuel assembly array each have said second fuel rods positioned in a discrete location abutting said one axial peripheral face and aligned in rows parallel to said one axial peripheral face. 8. The combination as specified in claim 7, wherein said peripherally positioned fuel assemblies comprise locations for seven rows of said second fuel rods which parallel said one axial face, each of said seven rows having locations for seventeen of said second fuel rods except for a minor proportion of control rod thimble locations, and the remainder of said peripherally positioned fuel assemblies have locations for ten rows of said first fuel rods, and each of said ten rows having locations for seventeen of said first fuel rods except for a minor proportion of control rod thimble locations. 9. The combination as specified in claim 6, wherein said peripherally positioned fuel assemblies which have two axial peripheral faces forming a portion of the periphery of said combined fuel assembly array have said second fuel rods positioned in discrete locations aligned in rows abutting both of said axial peripheral faces. 10. The combination as specified in claim 9, wherein said peripherally positioned fuel assemblies comprise locations for seven rows of said second fuel rods positioned parallel to both of said axial peripheral faces, each of said seven rows having locations for seventeen of said second fuel rods except for a minor proportion of control rod thimble locations, and the remainder of said peripherally positioned fuel assemblies have locations for ten rows of said first fuel rods, and each of said ten rows having locations for ten of said first fuel rods except for a minor proportion of control rod thimble locations.