Patent Number: 051620979
Section: claims

1. A nuclear reactor comprising: a reactor vessel having a longitudinal centerline axis extending upwardly relative to gravity;  a reactor core having a first level and a second level, said first level having a first set of fuel units arranged in a first two-dimensional array, said second level having a second set of fuel units arranged in a second two-dimensional array, each fuel unit of said second set being disposed directly above a fuel unit of said first set;  circulation means for circulating a heat transfer fluid axially through said core from said first level toward said second level;  said heat transfer fluid being initially in the form of water, and said first level being configured for boiling said water to generate saturated steam, said saturated steam being channeled upwardly to said second level; and  said second level being configured for receiving said saturated steam from said first level for cooling said second level and heating said saturated steam for forming superheated steam.  said reactor core has a neutron spectrum from said first level to said second level varying from relatively soft to hard, respectively;  said second set of fuel bundles includes fertile fuel which forms fissile fuel by conversion due to said hard neutron spectrum; and  said first set of fuel bundles includes fissile fuel which is fissionable due to said soft neutron spectrum.  each of said lower and upper fuel bundles includes said respective fuel rods spatially joined together by a pair of axially spaced apart tie plates;  each of said lower fuel bundles being disposed in a flow baffle which channels through said baffle and over said fuel rods a portion of said water for forming said saturated steam; and  each of said upper fuel bundles being disposed over a respective one of said lower fuel bundles for receiving said saturated steam and forming said superheated steam.  a plurality of stationary poison curtains disposed between adjacent ones of said upper fuel bundles; and  a plurality of axially movable control rods disposed between adjacent ones of said lower fuel bundles.  at a design point for said superheated steam formed in said core second level, said poison curtains are essentially transparent to said hard neutron spectrum and allow said core second level to remain critical; and  at off-design points wherein said core second level is in a flooded state or density of said superheated steam is below density at said design point, said poison curtains automatically cause said core second level to become subcritical. 2. A nuclear reactor according to claim 1 wherein each of said fuel units is a fuel bundle containing vertically extending fuel rods, each of said fuel rods having a fuel section containing fissile fuel and a plenum section for accumulating gaseous fission byproducts, said fuel rods in fuel bundles of said first set having first plenum sections below their fuel sections, said fuel rods in fuel bundles of said second set having second plenum sections above their fuel sections. 3. A nuclear reactor according to claim 2 wherein: 4. A nuclear reactor according to claim 3 wherein said first set of fuel bundles includes preburned fuel bundles previously disposed in said second set which have undergone said conversion for forming fissile fuel, said preburned fuel bundles undergoing additional burning in said first level. 5. A nuclear reactor according to claim 4 wherein said preburned fuel bundles previously disposed in said second set are disposed in said first set in an inverted position. 6. A nuclear reactor according to claim 5 wherein said first plenums are disposed in said water and extend vertically upwardly therein. 7. A nuclear reactor according to claim 6 wherein said fuel bundles of said first and second sets are identical in configuration. 8. A nuclear reactor according to claim 3 wherein: 9. A nuclear reactor according to claim 8 wherein said flow baffles are disposed solely around said lower fuel bundles and not around said upper fuel bundles. 10. A nuclear reactor according to claim 8 further including: 11. A nuclear reactor according to claim 10 wherein: 12. A nuclear reactor according to claim 11 wherein said poison curtains contain boron and in said flooded state are effective for absorbing neutrons. 13. A nuclear reactor according to claim 12 wherein said upper fuel bundles have a configuration for increasing fast neutron leakage at said off-design points below said design point steam density. 14. A nuclear reactor according to claim 13 wherein said upper fuel bundle configuration includes a tighter pitch and smaller fuel rod diameter than in a conventional boiling water reactor. 15. A nuclear reactor according to claim 14 wherein said lower and upper fuel bundles are each configured in a hexagonal configuration. 16. A nuclear reactor according to claim 15 wherein said lower and upper fuel bundles are identical in configuration including having the same axial length. 17. A nuclear reactor according to claim 16 wherein said first set of fuel bundles includes preburned fuel bundles previously disposed in said second set which have undergone said conversion for forming fissile fuel, said preburned fuel bundles undergoing additional burning in said first level. 18. A nuclear reactor according to claim 17 wherein said preburned fuel bundles previously disposed in said second set are disposed in said first set in an inverted position. 19. A nuclear reactor according to claim 18 further characterized by the absence of a steam separator disposed above said core second level, said core second level being effective for providing said superheated steam with substantially little water therein. 20. A nuclear reactor according to claim 3 further characterized by the absence of a steam separator disposed above said core second level, said core second level being effective for providing said superheated steam with substantially little water therein.