Patent Number: 047770074
Section: claims

1. A fast breeder incorporating a core having a driver core region containing a fissile material, an external blanket region surrounding said driver core region and containing a fertile material and an internal blanket region disposed within said driver core region and containing a fertile material, characterized in that the axial thickness of said internal blanket region is greater at the central portion of said internal blanket region than at the peripheral portion of the same, and that the outer peripheral end of the peripheral portion of said internal blanket region opposes to said external blanket region with a portion of said driver core region interposed therebetween. 2. A fast breeder as claimed in claim 1, wherein the ratio d.sub.1 /D.sub.2 between the diameter d.sub.1 of said internal blanket region and the diameter D.sub.2 of said driver core region is determined to fall between 0.76 and 0.90. 3. A fast breeder as claimed in claim 1 or 2, wherein said fissile material in said driver core region has a uniform degree of enrichment. 4. A fast breeder as claimed in claim 1 or 2, wherein the axial thicknesses of the portions of said internal blanket region of greater and smaller axial thicknesses are constant. 5. A fast breeder incorporating a core having a driver core region containing a fissile material, an external blanket region surrounding said driver core region and containing a fertile material, and an internal blanket region disposed within said driver core region and containing a fertile material, characterized in that said internal blanket region and said driver core region are constituted by a first zone in which an internal blanket region containing a fertile material is disposed between axially spaced upper and lower driver core regions containing a fissile material, a second zone surrounding said first zone, said second zone having an internal blanket region having an axial thickness smaller than that of said internal blanket region in said first zone and disposed between axially spaced upper and lower driver core regions containing a fissile material, and a third zone surrounding said second zone and having only a driver core region containing a fissile material but having no internal blanket region. 6. A fast breeder as claimed in claim 5, wherein the ratio d.sub.1 /D.sub.2 between the diameter d.sub.1 of said second zone and the diameter D.sub.2 of said third zone is determined to fall between 0.76 and 0.9. 7. A fast breeder as claimed in claim 5 or 6, wherein said first, second and third zones have an equal degree of enrichment of said fissile material. 8. A fast breeder as claimed in claim 7, wherein said internal blanket regions in said first and second zones have constant axial thicknesses along the radius of said core. 9. A fast breeder incorporating a core having a plurality of first fuel assemblies each of which including external blanket regions provided at the axially upper and lower ends thereof and containing a fertile material, an upper and lower driver core regions disposed between said external blanket regions and containing a fissile material and an internal blanket region disposed between said upper and lower driver core regions and containing a fertile material, and a plurality of fourth fuel assemblies each of which having said external blanket region but no driver core region, characterized in that it comprises: a plurality of second fuel assemblies constituting a second zone which surrounds a first zone constituted by said first fuel assemblies, each of said second fuel assemblies having an identical arrangement of said external blanket regions, driver core regions and said internal blanket region to that of said first fuel assembly, said internal blanket region of said second fuel assembly having an axial thickness smaller that that of said internal blanket region of said first fuel assembly; and a plurality of third fuel assemblies constituting a third zone surrounding said second zone, each of said third fuel assemblies having said external blanket regions provided at axially upper and lower ends thereof and said driver core 10. A fast breeder as claimed in claim 9, wherein the ratio d.sub.1 /D.sub.2 between the diameter d.sub.1 of said second zone and the diameter D.sub.2 of said third zone is determined to fall between 0.76 and 0.9. 11. A fast breeder as claimed in claim 9 or 10, wherein said first, second and third zones have an equal degree of enrichment of said fissile material. 12. A fast breeder as claimed in claim 11, wherein all of said first fuel assemblies constituting said first zone have an equal axial height of said internal blanket regions, and all of said second fuel assemblies constituting said second zone have an equal axial height of said internal blanket regions. 13. A fast breeder as claimed in claim 1, wherein the axial thickness of the central portion of said internal blanket region is of a constant thickness and the axial thickness of said internal blanket region varies between the central portion and the outer peripheral end of said internal blanket region. 14. A fast breeder as claimed in claim 13, wherein the axial thickness of said internal blanket region varies in a stepped manner between the central region and the outer peripheral end thereof.