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 for successive cycles with, between each cycle, steps for replacing spent nuclear fuel assemblies with fresh nuclear fuel assemblies, including:operating the nuclear reactor for an initial cycle for starting the nuclear reactor, during which the core contains initial nuclear fuel assemblies, at least some of the initial nuclear fuel assemblies only comprising nuclear fuel rods, before irradiation, containing uranium oxide and not containing any plutonium oxide; thenoperating the nuclear reactor for transition cycles, at least some of the initial nuclear fuel assemblies being progressively replaced during the replacement steps preceding the transition cycles, with transition nuclear fuel assemblies or with plutonium-equilibrium nuclear fuel assemblies, the plutonium-equilibrium nuclear fuel assemblies only 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 nuclear fuel isotope composition and a same nominal total plutonium mass content; and thenoperating the nuclear reactor for at least one plutonium-equilibrium cycle during which the core only contains plutonium-equilibrium nuclear fuel assemblies. 2. The method as recited in claim 1 wherein at least some of the transition nuclear fuel assemblies comprise poisoned nuclear fuel rods, the poisoned nuclear fuel rods containing, before irradiation, at least one consumable neutron poison. 3. The method as recited in claim 2 wherein at least some of the poisoned nuclear fuel rods do not contain, before irradiation, any plutonium. 4. The method as recited in claim 1 wherein the nuclear fuel rods of at least some of the initial nuclear fuel assemblies have, before irradiation, nominal plutonium fissile isotope contents less than those of the nuclear fuel rods of the plutonium-equilibrium nuclear fuel assemblies. 5. The method as recited in claim 1 wherein at least some of the transition nuclear fuel assemblies comprise nuclear fuel rods, before irradiation, containing uranium oxide and not containing any plutonium oxide. 6. The method as recited in claim 1 wherein, at least during the replacement step preceding at least one of the transition cycles or the at least one plutonium-equilibrium cycle, transition nuclear fuel assemblies are replaced with plutonium-equilibrium nuclear fuel assemblies. 7. The method as recited in claim 1 wherein at least some initial nuclear fuel assemblies are zoned nuclear fuel assemblies, the nuclear fuel rods of the zoned nuclear fuel assemblies containing, before irradiation, uranium and plutonium mixed oxide, the zoned nuclear fuel assemblies each comprising several zones in which the nuclear fuel rods have, before irradiation, different nominal plutonium isotope contents. 8. The method as recited in claim 7 wherein at least some of the zoned nuclear fuel assemblies each comprise:a first central zone comprising nuclear fuel rods having a first nominal plutonium fissile isotope content; anda second zone extending along outer faces of the zoned nuclear fuel assembly and consisting of nuclear fuel rods having a second nominal plutonium fissile isotope content strictly less than the first nominal plutonium fissile isotope content. 9. The method as recited in claim 8 wherein at least some of the zoned nuclear fuel assemblies each further comprise:a third zone positioned in corners of the zoned nuclear fuel assembly and consisting of nuclear fuel rods having a third nominal plutonium fissile isotope content strictly less than the second nominal plutonium fissile isotope content. 10. The method as recited in claim 7 wherein, during the initial cycle for starting the nuclear reactor, the zoned nuclear fuel assemblies are located in an outer peripheral layer of the core. 11. The method as recited in claim 10 wherein at least some of the zoned nuclear fuel assemblies each comprise, during the replacement step preceding a first transition cycle, transition nuclear fuel assemblies loaded into the layer immediately adjacent to the outer peripheral layer of the core. 12. The method as recited in claim 11 wherein, during the replacement step preceding the first transition cycle, the zoned nuclear fuel assemblies are displaced immediately inside the layer located immediately inside the outer peripheral layer of the core. 13. The method as recited in claim 10 wherein transition nuclear fuel assemblies loaded during the replacement step preceding a first transition cycle comprise nuclear fuel rods, before irradiation, containing uranium oxide and not containing any plutonium oxide. 14. The method as recited in claim 13 wherein, during the replacement step preceding the first transition cycle, the zoned nuclear fuel assemblies are displaced immediately inside the layer located immediately inside the outer peripheral layer of the core. 15. The method as recited in claim 10 wherein, during the replacement steps preceding the first transition cycle, a second transition cycle, a third transition cycle and the equilibrium cycle, plutonium-equilibrium nuclear fuel assemblies are loaded. 16. The method as recited in claim 1 wherein, in the initial cycle for starting the nuclear reactor, all the nuclear fuel rods of the initial nuclear fuel assemblies, before irradiation, contain uranium oxide but do not contain any plutonium oxide. 17. The method as recited in claim 1 wherein in the plutonium-equilibrium cycle, the nuclear fuel rods of all the plutonium-equilibrium nuclear fuel assemblies have a same nuclear fuel isotope composition and a same nominal total plutonium mass content.