Patent Number: 
Section: claims

1. A method for operating a pressurized water nuclear reactor comprising a core containing nuclear fuel assemblies comprising nuclear fuel rods, the method comprising:operating the nuclear reactor during successive cycles with between each cycle, steps for replacing spent nuclear fuel assemblies with fresh nuclear fuel assemblies, including:operating the reactor for at least one plutonium-equilibrium cycle during which the core contains plutonium-equilibrium nuclear fuel assemblies, the plutonium-equilibrium nuclear fuel assemblies comprising, before irradiation, nuclear fuel rods exclusively based on uranium and plutonium mixed oxide, the nuclear fuel rods of each plutonium-equilibrium nuclear fuel assembly having a same isotope composition of nuclear fuel and a same nominal total plutonium mass content; thenoperating the reactor for transition cycles, at least some of the plutonium-equilibrium nuclear fuel assemblies being progressively replaced, during the replacement steps preceding transition cycles, with zoned transition nuclear fuel assemblies and then uranium-equilibrium nuclear fuel assemblies, the zoned transition nuclear fuel assemblies each comprising a central zone comprising nuclear fuel rods containing, before irradiation, uranium oxide and not containing any plutonium oxide, and a peripheral zone extending along outer faces of the zoned transition nuclear fuel assembly, the peripheral zone only comprising, before irradiation, nuclear fuel rods exclusively based on uranium and plutonium mixed oxide, the uranium-equilibrium nuclear fuel assemblies only comprising, before irradiation, nuclear fuel rods containing uranium oxide and not containing any plutonium oxide; and thenoperating the reactor for at least one uranium-equilibrium cycle in which the core contains uranium-equilibrium nuclear fuel assemblies, the uranium-equilibrium nuclear fuel assemblies only comprising, before irradiation, nuclear fuel rods containing uranium oxide and not containing any plutonium oxide. 2. The method as recited in claim 1 wherein during the uranium-equilibrium cycle, the core only contains uranium-equilibrium nuclear fuel assemblies comprising only, before irradiation, nuclear fuel rods containing uranium oxide and not containing any plutonium oxide. 3. The method as recited in claim 1 wherein, during the plutonium-equilibrium cycle, the core only contains plutonium-equilibrium nuclear fuel assemblies. 4. The method as recited in claim 1 wherein, during the plutonium-equilibrium cycle, the plutonium-equilibrium nuclear fuel assemblies only comprise before irradiation nuclear fuel rods exclusively based on uranium and plutonium mixed oxide. 5. The method as recited in claim 1 wherein, during the plutonium-equilibrium cycle, the nuclear fuel rods of all the plutonium-equilibrium nuclear fuel assemblies have a same isotope composition of nuclear fuel and a same nominal total plutonium mass content. 6. The method as recited in claim 1 wherein at least some of the zoned transition nuclear fuel assemblies each comprise in the central zone thereof poisoned nuclear fuel rods, the poisoned nuclear fuel rods containing before irradiation, at least one consumable neutron poison. 7. The method as recited in claim 1 wherein, in at least some of the zoned transition nuclear fuel assemblies, the nuclear fuel rods of the peripheral zones have nominal plutonium fissile isotope contents of less than those of nuclear fuel rods of plutonium-equilibrium nuclear fuel assemblies. 8. The method as recited in claim 1 wherein during the replacement step preceding a first transition cycle, first zoned transition nuclear fuel assemblies are loaded into the core, wherein during the replacement step preceding a second transition cycle, second zoned transition nuclear fuel assemblies having central zones including nuclear fuel rods having, except for possible poisoned nuclear fuel rods, uranium 235 enrichments different from those of the nuclear fuel rods of the central zones of the first zoned transition nuclear fuel assemblies, are loaded into the core. 9. The method as recited in claim 8 wherein, except for the possible poisoned nuclear fuel rods, the nuclear fuel rods of the central zones of the second zoned transition nuclear fuel assemblies have substantially a same uranium 235 enrichment as the nuclear fuel rods of the uranium-equilibrium nuclear fuel assemblies. 10. The method as recited in claim 1 wherein the zoned transition nuclear fuel assemblies are not loaded into an outer peripheral layer of the core and at least some of the zoned transition nuclear fuel assemblies are loaded in a layer immediately adjacent to the outer peripheral layer of the core.